Accepted Manuscript

Experimental Study on the Effect of Magnetic Field on Critical Heat Flux of

Ferrofluid Flow Boiling in a Vertical Annulus

Habib Aminfar, Mousa Mohammadpourfard, Rasool Maroofiazar

PII: S0894-1777(14)00164-2

DOI: http://dx.doi.org/10.1016/j.expthermflusci.2014.06.023

Reference: ETF 8259

To appear in: Experimental Thermal and Fluid Science

Received Date: 10 April 2014

Revised Date: 23 June 2014

Accepted Date: 26 June 2014

Please cite this article as: H. Aminfar, M. Mohammadpourfard, R. Maroofiazar, Experimental Study on the Effect

of Magnetic Field on Critical Heat Flux of Ferrofluid Flow Boiling in a Vertical Annulus, Experimental Thermal

and Fluid Science (2014), doi: http://dx.doi.org/10.1016/j.expthermflusci.2014.06.023

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Experimental Study on the Effect of Magnetic Field on Critical

Heat Flux of Ferrofluid Flow Boiling in a Vertical Annulus

Habib Aminfara, Mousa Mohammadpourfardb, and Rasool Maroofiazara,*

a Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran b Department of Mechanical Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran

* Corresponding author.Tel.:/fax: +98 411 3392467 E-mail addresses: hh_aminfar@tabrizu.ac.ir (H.Aminfar),

Mohammadpour@azaruniv.edu (M.Mohammadpourfard), Maroofiazar@tabrizu.ac.ir (R.Maroofiazar).

ABSTRACT

In the present work, effects of using magnetic nanofluid and also applying an external

magnetic field on the critical heat flux (CHF) of subcooled flow boiling has been studied

experimentally. The experiments have been applied in upward flow direction in a 12 mm

I.D., 19 mm O.D. and 0.75 m length annular test section. Inlet subcooling was kept constant

and the mass flux was varied in the range of 0-150 kg/m2.s while the exit was at

atmospheric pressure. Ferrofluids with water as a base fluid and 0.01 and 0.1% volume

fractions of Fe3O4 nanoparticles were utilized. The results indicates that the CHF of

subcooled flow boiling was increased by using nanofluid as a working fluid, which was

mainly due to the deposition of the nanoparticles on the surface of inner tube, and

consequently, increasing the surface wettability. Furthermore, an external magnetic field

by utilizing quadrupole magnet was applied on the subcooled boiling flow at the near exit

of the test section. The obtained results indicated that applying magnetic field caused an

enhancement in CHF values of both pure water and ferrofluids. The main reasons for such

effect of magnetic field can be justified to changing water properties under action of the

magnetic field, single-phase convection heat transfer enhancement, suppression of

nucleate boiling, and stabilization of boiling flow.

2

Keywords: Critical Heat Flux, Ferrofluid, Subcooled Flow Boiling, Non-uniform magnetic

Field, Annular flow.

Nomenclature

DB vapor clot diameter (m) UB vapor clot velocity (m/s)

Heated diameter (m) Voltage

Hydraulic diameter y* thickness of superheated layer (m)

Inner diameter (m) Greek symbols

Outer diameter thickness of the liquid sublayer (m)

d bubble departure diameter (m) surface roughness (m)

f friction factor contact angle ( )

function of the contact angle Density (kg/m3)

Liquid mass flux (kg/(m2.s)) surface tension (N/m)

function of the contact angle Subscripts

hfg vaporization heat (kJ/kg) L liquid

Electric current V vapor

Length (m) Abbreviations

LB vapor clot length (m) Average particle size

Outlet pressure (kPa) Critical heat flux

heat flux BET Surface are analysis

1. Introduction

Due to the large heat transfer coefficient, the subject of boiling in horizontal and vertical

ducts under condition of forced convection is an important issue in many industries. The

main limiting condition for the safe operation of boiling systems is critical heat flux (CHF)

which is an important parameter in pool and flow boiling conditions. Extensive studies

were performed for high flow and high pressure conditions, while most of them were

examined with tubes [1-4]. For safety view of the boiling water reactors, CHF at a low flow

and low pressure is very important. Notable experimental researchs have been performed

on CHF measurement for a low flow and low pressure conditions in the vertical annuli [5-

10].

3

Mishima and Ishii [7] have performed CHF tests for a very low flow conditions of water

in a vertical annulus. Their measured CHF, was much lower than those predictions for

conventional correlations. However, it was well estimated by churn to annular flow regime

transition criterion.

Schoesse et al. [8] measured CHF in a vertical annulus at a low mass flux and low

pressure conditions of 20–280 kg/m2.s and 128 kPa. They concluded that the CHF at low

pressures and low velocities can be divided into low, transient and high mass flux regions,

respectively. For mass fluxes lower than 140 kg/m2.s they reported flooding and chugging

at the exit in CHF conditions. These fluctuations were not significant for mass fluxes higher

than 140 kg/m2.s. The influence of inlet subcooling on CHF was remarkable only in the

transition region and high mass flux region.

Park et al. [9] conducted an experimental study on CHF and two-phase flow

visualization for water flow in the internally-heated, vertical, concentric annuli under near

atmospheric pressure. They concluded that by increasing the flow rate from zero to higher

values, the CHF mechanism was changed in the order of flooding, churn-to-annular flow

transition and local dryout under a large bubble in churn flow.

Chun et al. [10] performed CHF experiments in a vertical annular test section where the

inner and outer tubes were made of Inconel-600 tube and only the inner tube was heated

indirectly. Their experimental conditions were in the rangeof pressure 0.57 to 15.01 MPa,

mass fluxes of 0 kg/m2.s and also from 200 to 650 kg/m2.s, and inlet subcoolings from 85 to

413 kJ/kg. Most of the CHFs were identified to the dryout of the liquid film in the annular-

mist flow. The CHF data under a zero mass flux condition indicated that both the effects of

pressure and inlet subcooling on the CHF were smaller, compared with those for the CHF

4

with a water up-flow. Table 1 summarizes the experimental conditions of above

mentioned reference studies.

One method for changing hydro-thermal behavior of heat transfer systems is utilizing

nanofluids as a working fluid. Ferrofluids are special types of nanofluids which are

synthesized using colloidal mixtures of non-magnetic carrier liquid containing magnetized

nanoparticles. The ferrofluids behave as a fluid that is affected by an external magnetic field

and it is applicable in various fields. Magnetic nanofluids can be exploited as a coolant

(thermal management applications) and/or a heat transfer medium in energy conversion

systems [11]. The later application is in the embryonic stage. Some of the current

applications of magnetic fluids can be categorized as follows [12]:

The first group utilizes the property of fluid that it is absorbed and positioned by a

magnetic field. Samples for this group of applications are seals,

inclinometer/accelerometer, printers, etc.

The second group of applications utilizes the magnetically induced levitation force

on magnetic and nonmagnetic bodies. Some examples are: bearing, damper,

grinding, polishing, etc.

The third group of application is the heat transfer devices. The main factors

distinctive to the magnetic fluids would be the thermo-magnetic convection and the

magnetic buoyant force to the bubble formed by boiling. Samples for this group of

applications are boiling in heat pipes [13-17], thermosyphons [18], quenching [19],

new energy conversion systems [20], microgravity applications[21], heat

exchangers [22], magnetically-driven heat transport device [23], and bubble

absorption [24], etc.

5

Large number of researches has been performed lately on the effect of nanofluids on

the boiling characteristics. Most of the studies are done for pool boiling. On the other hand,

researches regarding to convective flow boiling in nanofluids has become more popular in

the recent years. The most important parameter, which has been considered in previous

studies, is critical heat flux which is a serious limiting condition on safe operation of boiling

equipment. Kim et al. [25] studied flow boiling of a water based alumina, zinc oxide and

diamond nanofluids experimentally. Their results indicate that CHF values of nanofluids

were enhanced with respect to the pure water. Kim et al. [26] have conducted CHF

experiments at a low flow and low pressure in vertical tube utilizing Al2O3 nanofluids. The

results show CHF enhancement of nanofluids in flow boiling for all experimental

conditions. Ahn et al. [27, 28] experimentally investigated the effect of nanofluid in flow

boiling on a short heated surface. It is drawn from their studies that the nanofluid flow

boiling CHF was distinctly enhanced under the forced convective flow compared with that

in pure water.

One of the ways for altering the fluid flow and heat transfer characteristics in both

single and two-phase flows is application of external fields such as magnetic field. There

are several studies in the field of magnetic field effect on heat transfer characteristics

which:

Most of them are for the single-phase flow and fewer studies have been performed

on two-phase flows specially boiling heat transfer. For example, Aminfar et al. [29-

31] investigated the effects of non-uniform axial magnetic fields and uniform

transverse magnetic fields on the hydrodynamic and thermal behavior of a

ferrofluid flow in various ducts. They reported similar effects of the negative

6

gradient axial field and uniform transverse field, and enhancement of Nusselt

number in single-phase flow under application of these magnetic fields.

In the subject of magnetic field’s effect on the boiling characteristics, pool boiling

has been studied more than flow boiling. A literature survey shows that some

interesting results have been obtained from previous works. Nucleate boiling

suppression [32-35] and stability of the boiling two-phase flow [36-38] under

application of external magnetic fields are the most important reported results of

the previous studies.

In the flow boiling studies, stability has drawn more attention and only one paper

deals with critical heat flux of subcooled flow boiling (ref [39]). Lee et al. [39]

performed experiments at the atmospheric pressure and low mass flow conditions

on the CHF of magnetic nanofluid. Based on their experimental data, it was

concluded that the use of magnetic nanofluid improves the flow boiling CHF

characteristics, i.e., the flow boiling CHF was enhanced for the magnetic nanofluid.

Also, the effects of magnetic field on the flow boiling CHF for the magnetic nanofluid

were investigated and the results indicated an additional enhancement in flow

boiling CHF. According to their statement, the permanent magnets absorbed the

magnetic nanoparticles and local concentration of the nanofluid near the tube wall

became higher compared with the bulk region, and therefore the higher flow boiling

CHF was measured.

As stated above, in the ref [39], subcooled flow boiling was considered in vertical

tube and only one data for the magnetic field effect on CHF has been provided. Also,

the observed CHF enhancement has been related only to absorption of the

7

nanoparticles to the inner surface of tube and hence, increasing nanoparticle

deposition and surface wettability.

There are some new aspects in our study regarding to the previous studies and specially to

Lee et al. [39]:

Magnetic field’s effect on CHF in vertical annulus (instead of vertical tube) has

been studied. In our study in contrast to ref [39], magnetic field causes reduction

of nanoparticle deposition on the heater surface.

A series of experimental data for the magnetic field effect in both ferrofluids (i.e.

0.01 and 0.1% vol.) and at the different mass fluxes (0-150 kg/m2.s) have been

presented. It means that flow boiling CHF trend with increasing the mass flux

have been considered and two different regions of mass fluxes for the effect of

magnetic field on CHF have been identified.

The effect of magnetic field on the pure water CHF has been presented in this

study which can be a new data in this field. This result is very useful for large

systems, that using nanofluids is impossible due to several constraints such as

economic issues. In these cases and for increasing the safety margins, application

of magnetic field on the pure water could increase the CHF of flow boiling.

Effects of various parameters such as single-phase heat transfer enhancement,

flow stability, changing bubble detachment diameter and frequency have been

attributed to the observed CHF enhancement, while in the work of Lee et al. [39],

deposition of nanoparticles is the main reason for their observed CHF

enhancement.

8

Two main parts of the present experimental work are as follows: Firstly, the effect of

using the nanofluids with 0.01 and 0.1 vol.% of magnetic nanoparticles on flow boiling CHF

is studied. The used test section is a vertical annulus with a transparent Pyrex outer tube.

Secondly, a quadrupole magnet is used for creating an external magnetic field and the effect

of applying non-uniform magnetic field on the flow boiling CHF is investigated.

2. Experiments

2.1 Ferrofluids

Magnetic fluids (or ferrofluids) are special types of nanofluids which are synthesized

using colloidal mixtures of non-magnetic carrier liquid containing magnetized

nanoparticles [40]. In the present work, ferrofluids were prepared by dispersing 0.01 and

0.1 vol.% of Fe3O4 nanoparticles into the water as a base fluid. The preparation process of

the ferrofluids is as follows:

Firstly, the mass of Fe3O4 nanoparticles were weighed with a digital electronic balance and

then the nanoparticles were added into the weighed water and the Fe3O4/water mixture

was prepared. Finally, the mixture was sonicated continuously for three hours with a

sonicator of the bath type to obtain a uniform dispersion of Fe3O4 nanoparticles in the

water. Through this preparation, the temperature of the ferrofluids increased from 20 to 60

C. Properties and thermophysical properties of the used nanoparticles are summarized in

Table 2 and 3, respectively. Also, a picture of prepared Fe3O4 nanofluids is shown in Fig. 1.

After preparing nanofluids, a prediction of uniform dispersion and stability of the

colloidal suspension is necessary. The thermal characteristics of the nanofluids can be

changed by agglomeration of nanoparticles. There are different approaches for this

9

propose, for example, pH value measurement, zeta potential measurement, and TEM image

of the prepared suspension. Two techniques for stability assurance of the nanofluids have

been used.

Firstly, the pH value of the ferrofluid with higher volume concentration of nanoparticles

was measured which was approximately in the range of 3.5-4. This value is far from the iso-

electric point (IEP) of magnetite (approximately 7). IEP is a condition which an equal

number of positively and negatively charged particles exist in the colloid and the

nanoparticles can agglomerate and settle near this point. Based on this measurement, the

prepared ferrofluid would be stable.

Secondly, the zeta potential, which is a well-known standard for maintaining colloidal

stability of nanofluids, was measured. This measurement was performed for the ferrofluid

with higher volume concentration of nanoparticles at two times: firstly, as soon as the

colloid had been prepared and secondly, after 12 h of nanofluid preservation. Generally, a

zeta potential above 30mV reflects physical stability. Measurements show zeta potential

values of 37.94 and 36.02 for the two above mentioned cases.

According to these measurements, well dispersed ferrofluids with 0.01 and 0.1 vol.%

concentrations is used in this study.

2.2 Experimental setup

The experimental loop was constructed in order to obtain CHF values for water and

ferrofluids flow boiling under the conditions of low pressure and low flow (LPLF

conditions). A schematic diagram of the experimental loop is shown in Fig. 2. The loop

10

consists of a test section, a preheater, a heat exchanger, a centrifugal pump, and a turbine

flow meter.

The test section in this study is an annulus which consists of an internally heated

stainless steel tube placed inside an unheated transparent Pyrex tube. Inner diameter,

outer diameter and heated length of the inner tube are 10 mm, 12 mm and 75 cm,

respectively (see Fig. 3). Copper rods with 12 mm O. D. and 0.27 m long were installed to

the both ends of the inner tube and copper electrodes were attached to these rods for

connecting to a 24kW DC power supply (30V and 800A). Electric heating power and the

corresponding heat flux are calculated by measuring the electric current and the electric

potential difference between two electrodes as follows:

(1)

where V and I are measured for potential difference between two electrodes and electric

current, respectively. is the outer diameter and L is the heated length of the inner heated

tube. For detecting the onset of CHF, K-type thermocouples were attached and fixed to the

inner surface of the heated tube at different locations. A schematic figure of exact position

of thermocouples is illustrated in Fig. 4. A Teflon rod with high melting point was machined

for making a support for thermocouples. The outer diameter of this Teflon tube is

approximately equal to the inner diameter of the annulus heater. Afterward the

thermocouples were installed on the Teflon tube and it was inserted in the heater tube, as

shown in Fig. 4. A sheath and also silicon paste was used to prevent the effect of electric and

magnetic fields to measurements. Although there were small oscillations in the

temperature data of thermocouples, it must be mentioned that the goal of using

11

thermocouples in the present study is detecting CHF point to trip the electric current for

preventing any damage in the test loop. It means that we do not need very accurate

temperature measurements.

It must be noted that for preventing the effect of boiling time to the results, it was planned

that the boiling time for all of the experiments would be the same and it was about 45

minutes for each test.

To make the strong external magnetic field inside the annulus, permanent magnets were

used. The magnets were located where that the quadrupole magnet was formed. The

quadrupole magnets consist of a group of four magnets and are useful as they create

a magnetic field whose magnitude grows rapidly with the radial distance from its

longitudinal axis and its maximum magnitude is about 200 mT in our experiments. The

installation position of the magnet pairs is near the exit of the annulus, at which the CHF

would occur. The installed quadrupole magnet and its location are illustrated in Fig. 3.

Different measurement devices were installed in the test loop in order to control the

system state and to determine the relevant experimental parameters. The mass flow rate

was measured with a turbine flow meter, fluid temperature and pressure were determined

with K-type thermocouples and Omega PXM209 (0–6 bar) absolute pressure transducers,

respectively. The relevant devices were checked in the calibration tests to determine the

measurement accuracies. Measurement uncertainties of power, heat flux, flow rate, and

temperature are less than 4%, 4%, 5%, and 5 ◦C, respectively.

In this study, the uncertainties of the measured parameters were analyzed by the error

propagation method. For example, uncertainty of the heat flux was calculated as follows:

12

Heat flux is calculated using Eq. 1. Thus the main source of heat flux uncertainty is found as

voltage (V), current (I), outer diameter of heater ( ) and length (L). Heat flux uncertainty

can be calculated using the following equation:

The elemental percentage uncertainty of V, I, and L is less than 2.7%, 2.7%, 0.1%, and

1% respectively. Thus:

Therefore, the measurement uncertainty on the calculated heat flux is . For other

parameters, a similar approach was used for calculating the measurement uncertainty.

2.3 Experimental procedure

The CHF experiments were performed by the following procedures. The loop was filled

with the working fluid (water or ferrofluids) and in the case of the ferrofluids, the working

fluid flows through the experimental loop for 30 min with a relatively high mass flux to mix

and disperse the working fluid further. Then, the fluid was degassed by boiling in the warm

up period of the experiments (about half an hour). The fluid which exited from the test

section was entered in a heat exchanger and it was cooled to 20 and returned to the main

tank, again (see Fig. 2).

The water was preheated by the pre-heater to get the desired inlet subcooling of about

106 kJ/kg. After setting the fluid flow rate and inlet subcooling at the desired values, the

electric power to the inner tube is increased gradually. At each power level, the measured

13

parameters are allowed to stabilize for several minutes to achieve a steady-state condition

before raising the power level again. This process continues until a sharp increase of the

wall temperature is observed in the inner tube surface. A CHF condition is determined to

occur when one of the wall temperatures of the tube shows a continuous sharp increase

and then reaches 250 . Whenever the CHF was detected, the electric current was

automatically or manually tripped to prevent any damage to the tubes. After performing

CHF experiments in each case, the used inner tube was replaced with the newer one in

order to prevent the influence of changed surface characteristics on the next experiment

results.

3. Results and discussion

3.1 Pure water CHF validation

For validation of the results, pure water CHF comparison was made with the results of

references [7, 9]. Fig. 5 shows that the results of the present work are in favorable

agreement with the references.

For detecting CHF mechanism at the exit of the annular test section, visual observations

were made with a high speed camera by 1200 frame per second. By applying variable heat

flux at a constant mass flux, typical flow regime at the exit of the test section changed as

follows as shown in Fig. 6: nucleate boiling, bubbly flow, slug flow, churn flow, flooding, and

annular flow. It should be noted that the CHF mechanism was flooding at lower mass fluxes

and it changes to dryout in the annular flow by increasing the mass flux.

3.2 Results of CHF for ferrofluids

14

According to our measurements, Fig. 7 shows a comparison between pure water and

ferrofluids CHF values. As shown, the CHF values are enhanced by dispersing nanoparticles

in the base fluid and this enhancement increases with volume fraction of the nanoparticles.

The maximum enhancement of CHF was about 18% and 33% for 0.01 and 0.1 vol.%

ferrofluids, respectively.

A change in CHF can be related to either changes in the thermophysical properties of the

fluid or changes in the surface characteristics of the boiling surface. Since the volume

fraction of the nanoparticles is very low, CHF enhancement is not due to the change in the

thermophysical properties and hence, the CHF enhancement must come from a surface

effect [41]. There are two ways that we can prove insignificant effect of thermo-physical

properties of the ferrofluids on the results: In the first one, experiments were performed

for pure water flow boiling in an annulus with nanoparticles deposited on the heater

surface. For this propose and in the first step, ferrofluid with higher volume fraction of

nanoparticles (0.1 vol.%) was boiled in the annulus for a specified time and the

nanoparticles deposited on the heater surface (i.e., outer surface of inner tube in annulus).

Afterward the working fluid was changed and the loop was filled with pure water. CHF

experiments were performed with this approach for several times and the obtained results

were approximately the same as the results of ferrofluid flow boiling on clean surface. A

typical diagram for these results is illustrated in Fig. 8. Therefore, it is concluded that the

most important parameter in CHF enhancement of nanofluids is surface parameters rather

than ferrofluid thermo-physical properties.

The second approach is calculating ferrofluids thermo-physical properties using both the

existent theoretical correlations and or experimental measurements. According to the

15

following well-known correlations, thermo-physical properties of ferrofluid with higher

concentration of nanoparticles were calculated and were given in table 3:

(2)

(3)

(4)

(5)

As seen in table 3, the changes in the thermophysical properties of base fluid by addition of

0.1 vol.% of nanoparticles is very low. It means that thermophysical properties of

nanofluids have negligible effect on the observed CHF enhancement. The experimental

measurement for some of the thermophysical properties such as density and thermal

conductivity confirmed the calculated values with above correlations.

The most important surface parameters are surface roughness and contact angle. The

surface roughness is related to the number of micro-cavities available for bubble

nucleation and contact angle is related to the wettability of the surface. According to

previous studies [25, 41], surface roughness is approximately the same for water and

nanofluid boiled surfaces. Therefore, it can be concluded that the CHF enhancement is

mostly due to the change in contact angle. For detail study of this subject, SEM images of

the heater surface (outer surface of inner tube) are illustrated in Fig. 9. It is evident that by

using ferrofluids as working fluid, nanoparticles deposit on the heater surface and change

the surface characteristics of the heater (see Fig. 9b,d). Also, this deposition increases with

increasing the nanoparticles volume fraction.

16

One of the parameters which is affected by deposition of the nanoparticles on the heater

surface is the contact angle of the fluid droplet on the heater surface. To study the effect of

contact angle on CHF enhancement, static contact angle measurements for water droplet

on three different surfaces were performed and the results are shown in Fig. 10 and

compared with reference [25] in Table 3. The measurements shows that water droplet

contact angle changed from 72 on water boiled surface to 25 and 19 on 0.01 and 0.1

vol.% ferrofluid boiled surfaces, respectively. Accordingly, surface wettability was

improved as nanoparticles are deposited on the surface and consequently led to more

stability of liquid film on the heated surface and improvement of flow boiling CHF

characteristics. The relationship between contact angle and CHF for pool boiling is well

known; for example, Kandlikar’s model [42] for vertical heater is as follows:

(6)

where is the contact angle. According to Eq. (6), decrease in the contact angle causes

to increase and this leads to the CHF enhancement. Models for relating the contact angle to

CHF in flow boiling is less developed. One of the models that has been validated against a

vast database is the model of Celata et al. [43]. Celata’s model is based on the liquid

sublayer dryout mechanism, i.e. the dryout of a thin liquid layer beneath an intermittent

vapor blanket due to the coalescence of small bubbles. For available geometric and inlet

thermal-hydraulic conditions and local pressure, the CHF was predicted by a procedure as

follows:

(7)

17

the important parameters which must be calculated are , and which are the length

of vapor blanket, velocity of vapor blanket and the thickness of liquid sub-layer.

(8)

The velocity of vapor blanket can be obtained by a force balance, i.e. buoyancy and drag

forces:

(9)

where is the drag coefficient and is calculated by:

(10)

is the vapor blanket equivalent diameter:

(11)

is a function that depends only on a contact angle. The value of can be calculated

for pure water and ferrofluid boiled surface with contact angles of 72 and 19 ,

respectively. is friction factor and its expression is as follows:

(12)

The solution of this equation for the friction factor requires iteration. For calculation of the

thickness of liquid sublayer the following equation is used:

(13)

where is the superheated layer thickness. When this model is applied to the flow

conditions of our loop, and the contact angle is again changed from 72 to 19 , Eqs. (7–13)

18

(and the accompanying detailed models in Ref. [43]) predict a CHF enhancement about

35% which is approximately equal to our experimentally observed CHF enhancement

(which was about 33%).

For visual comparison between pure water and ferrofluid flow boiling characteristics,

Fig. 11 shows bubbly flow region at a constant mass flux and two different heat fluxes. Due

to opaqueness of 0.1 vol.% ferrofluid, only the pure water and 0.01 vol.% ferrofluid were

compared in this figure. As it is evident, in the case of 0.01% ferrofluid, the mean bubble

diameter is less than pure water’s one and this is due to decrease in the bubble departure

diameter and increase in bubble departure frequency because of nanoparticles deposition

on the heated surface.

3.3 Effect of magnetic field on CHF

By considering the outcomes in Figs. 9, 10 and 11, it was concluded that the deposition

of nanoparticles was the most important reason for CHF enhancement. Accordingly, it is

suggested to use an external magnetic field for changing the nanoparticles behavior at the

surface and study the effect of an external magnetic field on CHF values. For this propose,

quadrupole magnet was used to create an external magnetic field near the exit of the

annulus. The effects of applying this external magnetic field on the CHF values for

ferrofluids are illustrated in Figs. 12 and 13.

These figures show that applying a non-uniform magnetic field reduces the CHF for zero

and very low mass flux (G< 40 kg/m2.s) conditions for both nanofluids with respect to the

no magnetic field case. The main reason of this phenomenon, as expected, is absorption of

nanoparticles to the magnets and decreasing of deposition of the nanoparticles on the

heated surface. For visual confirmation of this statement, SEM images of heater surface

19

before and after application of magnetic field are illustrated in Fig. 9. This figure and also

Fig. 14 show that the nanoparticles were absorbed into the Pyrex tube walls in the presence

of magnetic field (Fig. 14a) and consequently, few amount of nanoparticles deposited on the

heater surface (Fig. 9 c,e and Fig. 14 b,c).

By increasing the mass flux, unexpected results were obtained: the CHF under influence

of magnetic field were increased gradually with respect to no-magnetic field case. The first

thing that comes into mind is that by increasing mass flux, the flow inertia increases and

the effect of magnetic field on the nanoparticles decreases and consequently, higher

number of nanoparticles deposit on the heater surface. But in this case, the upper bond for

the number of deposited particles will be the same as no magnetic field case and according

to this interpretation, the maximum CHF value under application of magnetic field must be

equal to the no magnetic field case at higher mass fluxes. But according to our finding, it is

seen that the application of magnetic field at higher mass fluxes increased the CHF with

respect to the no magnetic field case. The maximum CHF enhancement under application of

the magnetic field with respect to no field cases was 20% for 0.01 vol.% and 23% for 0.1

vol.% ferrofluids. Thus for 0.1 vol.% nanofluid, the total CHF enhancement is as follows:

In this study the maximum values of each term in the above statement are 33% and 23% respectively and hence:

For detail study and finding main reasons of such CHF enhancement under application of

the magnetic field, we should study the effect of magnetic field on the pure water CHF

characteristics. For the first time, interesting results were obtained as shown in Fig. 15. CHF

20

for pure water were increased under the influence of non-uniform transverse magnetic

field. In this case the maximum enhancement was about 15% with respect to no magnetic

field case. This result can somewhat explain the effect of magnetic field on ferrofluids CHF

at higher mass fluxes. Because, as seen in Fig. 15, effect of magnetic field on pure water CHF

increases with increase in the mass flux and the observed CHF trend in Fig. 12 and Fig. 13

can be attributed to this finding. Also, some other mechanisms exist and influence the

magnetic field effect on CHF which some of these mechanisms are mentioned below.

Although the actual mechanisms behind the observed CHF enhancement of both pure

and ferrofluids were not well understood, based on our findings and other researches [29-

31, 34-38, 44-67] following mechanisms could be the most probable reasons for increase in

CHF under the magnetic field effect:

3.3.1 Effect of magnetic field on pure water

Reference to the previous studies, magnetic field could affect both physical and two-

phase flow characteristics of pure water. These effects will be reviewed separately in the

following parts.

A) Physical Properties:

Most important parameters which were affected by application of external magnetic fields

are: surface tension [46, 50, 51, 55], contact angle on surface of materials [50], viscosity

[51, 54], enthalpies [51], vaporization rate [45], etc. The measurements for the change in

above mentioned parameters are related to the effect of magnetic field on hydrogen

bonding of water molecules [51-53].

In the case of boiling of water, the above mentioned parameters have significant effects

on flow boiling characteristics. Therefore, we can expect that the application of magnetic

21

field will also influence the behavior of flow boiling, especially CHF. According to the

observations in [50] for pure water droplet, application of magnetic field decreases contact

angle which is related to the change of surface tension which is one of the above mentioned

physical properties. This increases surface wettability and according to Eqs. (3-6) could

enhance flow boiling CHF.

B) Two-phase flow characteristics:

Effect of magnetic field on two-phase flow characteristics of pure water has been less

studied previously. Matsushima et al. [56-58] investigated gas bubble evolution during the

water electrolysis in a magnetic field. Their results indicated that MHD convection

influences the bubble motions, i.e. bubble detachment, as an additional pumping effect.

This should result in significant reductions of detachment diameter of nucleated bubbles,

void fraction and surface coverage by gas bubbles. A similar phenomenon could occur at

the flow boiling of water under influence of the applied magnetic field in the present study,

and consequently could lead to the observed delay on CHF occurrence.

3.3.2 Effect of magnetic field on Single-phase heat transfer in ferrofluids

There are many studies on the effect of various magnetic fields on the single-phase

forced convective heat transfer [29-31, 59-62]. The applied magnetic fields are transverse

and axial as well as uniform and non-uniform. The majority of these experimental and

numerical studies indicated an enhancement of single-phase heat transfer by applying

external magnetic field. Various reasons justify aforementioned increase in the heat

transfer such as: changing thermo-physical properties of magnetic fluid under application

of external magnetic field for example increasing thermal conductivity [61, 62], reduction

of hydraulic and thermal boundary layer thicknesses due to formation of chain-like clusters

22

[60], formation of vortices with transverse magnetic fields [31, 59], etc. By considering the

above reason, delay in occurrence of CHF in our experimental observations can be justified.

3.3.3 Effect of magnetic field on nucleate boiling characteristics

Based on the literature, magnetic field could affect various parameters in the nucleate

boiling regime. In this section, effect of magnetic field on nucleate boiling suppression,

bubble departure diameter and frequency is explained.

A) Suppression of nucleate boiling

Based on the presented results in [35], the main factors which influence incipient boiling

superheat may be summarized as follows: thermal properties of the liquid coolant and wall

and the corresponding contact angle, heater size and orientation, system pressure, fluid

subcooling, surface roughness and additional fields such as gravitational field and magnetic

field, etc (see Fig. 16). As shown, fluid subcooling, surface roughness, electric field and

acoustic field can activate vapor embryos on the heater surface and consequently, the

incipient boiling superheat decreases. On the other hand, the other factors such as system

pressure, magnetic field and gravitational field suppress the bubble nucleation process.

Accordingly, a portion of CHF enhancement in the present work could be as a result of

nucleate boiling suppression under influence of the applied magnetic field.

B) Bubble departure diameter

Junhong et al. [63] experimentally studied pool boiling of water-based magnetic fluid in the

presence of a magnetic field and derived an expression for bubble departure diameter:

(14)

23

where d is the bubble departure diameter, is the function of the contact angle, is the

surface tension, is the density of liquid, is the density of vapor, is the contact angle,

and is due to the external magnetic field. According to the Eq. (14), bubble

departure diameter decreases by applying magnetic field.

C) Bubble departure frequency

It was stated in the literature [34, 64-66] that applying magnetic field can change the

bubble departure frequency and this can lead to CHF changing under application of an

external magnetic field.

3.3.4 Effect of magnetic field on the stability of flow boiling

For two-phase flow systems, several types of instabilities have been observed and

reported by different authors. Bergles [67] proposed one classification for these

instabilities which is divided into two categories: static instability (such as Ledinegg

instability, boiling crisis, bumping, geysering or chugging) and dynamic instability (such as

acoustic waves, pressure drop oscillations or thermal oscillations). In the case of the boiling

flow, the main factor for the generation of the unstable flow state is related to the rapid

change and transition of the flow pattern as well as void wave phenomena. The resultant

instabilities are undesirable, as they can disturb the heat transfer characteristics so that the

heat transfer surface may burn-out. In the other words, instability in boiling flows is an

important phenomenon which could be an agent of CHF deterioration.

There are a few studies on the effect of magnetic field on the stabilization of the boiling

flows. Most important results of these works are as follows [36-38]:

24

Two-phase flow characteristics were strongly influenced by the magnetic field and

the precise control and stabilization of the boiling two-phase flow in a magnetic

fluid are possible by using the magnetic force of the fluid effectively [36].

A stability diagram was obtained for boiling flow and the effect of magnetic field on

stabilization was studied (see Fig. 17) [37]. As seen, the two-phase flow state can be

stabilized and homogenized by practical use of the magnetization of the fluid. The

important remark is that vapor bubbles can be minutely produced by effective use

of the magnetic body force.

Magnetic body force acts as the suppression effect to the amplitude of the linear

void wave in boiling two-phase flow of magnetic fluid. Also, the magnetic body force

play a role as the dispersion effect for the nonlinear pressure wave in gas-liquid

two-phase mixture flow. Consequently, precise control of the wave propagation is

possible by the effective use of magnetic body force which acts on the fluid [38].

Therefore, it is concluded from the above mentioned studies that a portion of the

observed CHF enhancement in the present study, is due to the stabilization of boiling flow

in the presence of the applied magnetic field.

The results of this experimental study is useful for such current industrial applications

which use boiling as the main heat transfer mechanism, for example, heat pipes,

thermosyphons, quenching, heat exchangers (macro and micro-scales), new fluid driving

systems, micro-gravity applications, etc. A potential application of this study is in the

micro-gravity (space) applications. As we know, boiling phenomenon is affected by the

absence of gravity. Consequently, the replacement of the gravitational field by another

external field (such as magnetic field) appeared to be the solution of this problem in micro-

25

gravity conditions [21]. Also, as mentioned before, one of the important results of this

study is the effect of external magnetic field on pure water flow boiling characteristics. As

we know, pure water is used as working fluid in most of the industrial applications, for

example, various heat exchangers or boilers in power plants. Therefore the results of this

study might be useful in these applications.

It should be noted that in this study we have used a magnetic field with low intensity

(~0.2 T) and a total increase of 56% (33% because of adding Fe3O4 nanoparticles+23%

because of applying magnetic field) have been obtained. According to the available studies,

it is clear that by increasing the intensity of magnetic field, its effect will also be increased

and consequently, the return is increased. Authors conclude that using of the proposed

technique depends on the size of the systems, i.e., smaller systems (e.g., micro heat

exchangers) will be more affected than larger systems for the same applied magnetic field.

According to the obtained result which indicates the effect of magnetic field on two-phase

flow of pure water, one can conclude that the proposed technique can be also utilized in

similar phenomena such as cavitation. This means that by the application of magnetic field,

the cavitation phenomenon can be controlled. New research is in progress by the authors

for application of the ferrofluids and magnetic field in these fields.

It is worth to mention that the proposed technique can be considered as “state of the

art” subject. It means that in addition to the above mentioned applications, by future

progress in nuclear reactors it is possible to select such a material for spacer-grids and or

claddings (or other components) which can be magnetized with magnetic induction. Then,

a similar condition to this study will be obtained. This study is a scientific research and its

application depends on the various working conditions of a different industries. It is

26

concluded that for safety applications, which is very important issue, application of an

external magnetic field could be reasonable option. It should be mentioned that relatively

weak magnetic field have been used by the authors and the observed enhancement of pure

water CHF (about 20%) can be very promising. It means that by application of the magnetic

fields with higher intensity, higher CHF enhancement will be obtained.

4. Conclusion

Critical heat flux behavior in the subcooled flow boiling of the ferrofluids was

investigated experimentally. It was concluded that dispersing nano-sized ferrimagnetic

particles in pure water (i.e. ferrofluid) can enhance the upper limit of applying heat flux in a

flow boiling. Based on the obtained results, deposition of nanoparticles on the heated

surface and changing contact angle and surface wettability was proposed to be the main

mechanisms of the ferrofluids CHF enhancement. Also, CHF enhancement for pure water

and ferrofluids were observed under application of an external magnetic field. In the case

of the ferrofluids, there was a CHF reduction at zero flow condition and very low mass

fluxes which was mainly due to the absorption of the nanoparticles into the magnets and

reduction of the deposited nanoparticles amounts on the heater surface. Some reasons for

CHF enhancement under application of the magnetic field were addressed such as changing

effective thermophysical properties and two-phase flow parameters of pure water, single-

phase convection heat transfer enhancement and delay on boiling incipience, suppression

of nucleate boiling, and stability of two-phase flow under application of the external

magnetic field.

27

References

[1] Katto, Y. 1994. CRITICAL HEAT FLUX. International journal of Multiphase Flow. 20:53-90.

[2] Chang, S., and Baek, W.-P. Understanding, predicting, and enhancing critical heat flux. in: Proceedings of Nuclear Reactor Thermal Hydraulics (NURETH-10). Seoul, Korea2003.

[3] Collier, J.G., and Thome, J.R. Convective Boiling and Condensation: Clarendon Press; 1994.

[4] Lahey, R.T., and Moody, F.J. The thermal-hydraulics of a boiling water nuclear reactor: American Nuclear Society; 1993.

[5] Rogers, J., Salcudean, M., and Tahir, A. Flow boiling critical heat fluxes for water in a vertical annulus at low pressure and velocities. in: Proceedings of the Seventh International Heat Transfer Conference1982. p. 339-344.

[6] El-Genk, M.S., Haynes, S.J., and Sung-Ho, K. 1988. Experimental studies of critical heat flux for low flow of water in vertical annuli at near atmospheric pressure. International Journal of Heat and Mass Transfer. 31:2291-2304.

[7] Mishima, K., and Ishii, M. 1982. Critical heat flux experiments under low-flow conditions in a vertical annulus. NUREG/ CR-2647, ANL-82-6.

[8] Schoesse, T., Aritomi, M., Kataoka, Y., Lee, S.-R., Yoshioka, Y., and Ki Chung, M. 1997. Critical Heat Flux in a Vertical Annulus under Low Upward Flow and near Atmospheric Pressure. Journal of Nuclear Science and Technology. 34:559-570.

[9] Park, J.-W., Baek, W.-P., and Chang, S.H. 1997. Critical heat flux and flow pattern for water flow in annular geometry. Nuclear Engineering and Design. 172:137-155.

[10] Chun, S.-Y., Chung, H.-J., Moon, S.-K., Yang, S.-K., Chung, M.-K., Schoesse, T., et al. 2001. Effect of pressure on critical heat flux in uniformly heated vertical annulus under low flow conditions. Nuclear Engineering and Design. 203:159-174.

[11] Nkurikiyimfura, I., Wang, Y., and Pan, Z. 2013. Heat transfer enhancement by magnetic nanofluids—A review. Renewable and Sustainable Energy Reviews. 21:548-561.

[12] Nakatsuka, K. 1993. Trends of magnetic fluid applications in Japan. Journal of Magnetism and Magnetic Materials. 122:387-394.

[13] Chiang, Y.-C., Kuo, W.-C., Ho, C.-C., and Chieh, J.-J. 2014. Experimental study on thermal performances of heat pipes for air-conditioning systems influenced by magnetic nanofluids, external fields, and micro wicks. International Journal of Refrigeration.

[14] Chiang, Y.C., Chieh, J.J., and Ho, C.C. 2012. The magnetic-nanofluid heat pipe with superior thermal properties through magnetic enhancement. Nanoscale research letters. 7:322.

[15] Ming, Z., Zhongliang, L., Guoyuan, M., and Shuiyuan, C. 2009. The experimental study on flat plate heat pipe of magnetic working fluid. Experimental Thermal and Fluid Science. 33:1100-1105.

28

[16] Jeyadevan, B., Koganezawa, H., and Nakatsuka, K. 2005. Performance evaluation of citric ion-stabilized magnetic fluid heat pipe. Journal of Magnetism and Magnetic Materials. 289:253-256.

[17] Ueno, S., Iwaki, S., Tazume, K., and Ara, K. 1991. Control of heat transport in heat pipes by magnetic fields. Journal of Applied Physics. 69:4925.

[18] Heris, S.Z., Salehi, H., and Noie, S.H. 2012. The effect of magnetic field and nanofluid on thermal performance of two-phase closed thermosyphon (TPCT). International Journal of the Physical Sciences. 7:534-543.

[19] Habibi Khoshmehr, H., Saboonchi, A., Shafii, M.B., and Jahani, N. 2014. The study of magnetic field implementation on cylinder quenched in boiling ferro-fluid. Applied Thermal Engineering. 64:331-338.

[20] Kamiyama, S., Okubo, M., and Fujisawa, F. 1992. Recent Developments of Technology in Magnetic Fluid Experiments. Experimental Thermal and Fluid Science. 5:641-651.

[21] Stoian, F.D. 2003. A Fundamental Study Regarding the Control of Nucleate Boiling in a Complex Magnetizable Fluid By an Applied Magnetic Field, in Microgravity Conditions. 654:149-156.

[22] Bahiraei, M., and Hangi, M. 2013. Investigating the efficacy of magnetic nanofluid as a coolant in double-pipe heat exchanger in the presence of magnetic field. Energy Conversion and Management. 76:1125-1133.

[23] Iwamoto, Y., Yamaguchi, H., and Niu, X.-D. 2011. Magnetically-driven heat transport device using a binary temperature-sensitive magnetic fluid. Journal of Magnetism and Magnetic Materials. 323:1378-1383.

[24] Wu, W.-D., Liu, G., Chen, S.-X., and Zhang, H. 2013. Nanoferrofluid addition enhances ammonia/water bubble absorption in an external magnetic field. Energy and Buildings. 57:268-277.

[25] Kim, S.J., McKrell, T., Buongiorno, J., and Hu, L.-W. 2009. Experimental Study of Flow Critical Heat Flux in Alumina-Water, Zinc-Oxide-Water, and Diamond-Water Nanofluids. Journal of Heat Transfer. 131:043204.

[26] Kim, T.I., Jeong, Y.H., and Chang, S.H. 2010. An experimental study on CHF enhancement in flow boiling using Al2O3 nano-fluid. International Journal of Heat and Mass Transfer. 53:1015-1022.

[27] Ahn, H.S., Kim, H., Jo, H., Kang, S., Chang, W., and Kim, M.H. 2010. Experimental study of critical heat flux enhancement during forced convective flow boiling of nanofluid on a short heated surface. International Journal of Multiphase Flow. 36:375-384.

[28] Ahn, H.S., Kang, S.H., Jo, H., Kim, H., and Kim, M.H. 2011. Visualization study of the effects of nanoparticles surface deposition on convective flow boiling CHF from a short heated wall. International Journal of Multiphase Flow. 37:215-228.

[29] Aminfar, H., Mohammadpourfard, M., and Narmani Kahnamouei, Y. 2011. A 3D numerical simulation of mixed convection of a magnetic nanofluid in the presence of non-uniform magnetic field in a vertical tube using two phase mixture model. Journal of Magnetism and Magnetic Materials. 323:1963-1972.

[30] Aminfar, H., Mohammadpourfard, M., and Mohseni, F. 2012. Two-phase mixture model simulation of the hydro-thermal behavior of an electrical conductive ferrofluid in the presence of magnetic fields. Journal of Magnetism and Magnetic Materials. 324:830-842.

29

[31] Aminfar, H., Mohammadpourfard, M., and Ahangar Zonouzi, S. 2013. Numerical study of the ferrofluid flow and heat transfer through a rectangular duct in the presence of a non-uniform transverse magnetic field. Journal of Magnetism and Magnetic Materials. 327:31-42.

[32] Osamu, T., Masaaki, N., Nobuyuki, T., and Itaru, M. 1980. Pool boiling heat transfer from horizontal plane heater to mercury under magnetic field. International Journal of Heat and Mass Transfer. 23:27-36.

[33] Wagner, L.Y., and Lykoudis, P.S. 1981. Mercury pool boiling under the influence of a horizontal magnetic field. International Journal of Heat and Mass Transfer. 24:635-643.

[34] Bertodano, M.A.L.d., Leonardi, S., and Lykoudis, P.S. 1998. Nucleate pool boiling of mercury in the presence of a magnetic field. International Journal of Heat and Mass Transfer. 41:3491-3500.

[35] Zhou, D.W. 2005. A Novel Concept for Boiling Heat Transfer Enhancement. Strojniški vestnik - Journal of Mechanical Engineering. 51:366-373.

[36] Kamiyama, S., and Ishimoto, J. 1995. Boiling two-phase flows of magnetic fluid in a non-uniform magnetic field. Journal of Magnetism and Magnetic Materials. 149:125-131.

[37] Ishimoto, J. 2007. Stability of the Boiling Two-Phase Flow of a Magnetic Fluid. Journal of Applied Mechanics. 74:1187.

[38] Ishimoto, J., and Kamiyama, S. 1997. Wave propagation in two-phase flow of magnetic fluid under a nonuniform magnetic field. Nuclear Engineering and Design. 175:87–96.

[39] Lee, T., Lee, J.H., and Jeong, Y.H. 2013. Flow boiling critical heat flux characteristics of magnetic nanofluid at atmospheric pressure and low mass flux conditions. International Journal of Heat and Mass Transfer. 56:101-106.

[40] Rosensweig, R.E. Ferrohydrodynamics: Dover Publications; 1997.

[41] Kim, S.J., McKrell, T., Buongiorno, J., and Hu, L.-W. 2009. Enhancement of Flow Boiling Critical Heat Flux (CHF) in Alumina/Water Nanofluids. Advanced Science Letters. 2:100-102.

[42] Kandlikar, S.G. 2001. A Theoretical Model to Predict Pool Boiling CHF Incorporating Effects of Contact Angle and Orientation. Journal of Heat Transfer. 123:1071.

[43] CELATA, G.P., CUMO, M., MARIANI, A., SIMONCINI, M., and ZUMMO, G. 1994. Rationalization of existing mechanistic models for the prediction of water subcooled flow boiling critical heat flux. International Journal of Heat and Mass Transfer. 37:347-360.

[44] Colic, M., and Morse, D. 1999. The elusive mechanism of the magnetic ‘memory’ of water. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 154:167-174.

[45] Nakagawa, J., Hirota, N., Kitazawa, K., and Shoda, M. 1999. Magnetic field enhancement of water vaporization. Journal of Applied Physics. 86:2923-2925.

[46] Amiri, M.C., and Dadkhah, A.A. 2006. On reduction in the surface tension of water due to magnetic treatment. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 278:252-255.

[47] Deng, B., and Pang, X.F. 2007. Variations of optic properties of water under action of static magnetic field. Chinese Science Bulletin. 52:3179–3182.

[48] Holysz, L., Szczes, A., and Chibowski, E. 2007. Effects of a static magnetic field on water and electrolyte solutions. Journal of Colloid and Interface Science. 316:996-1002.

30

[49] Pang, X.-F., and Deng, B. 2008. The changes of macroscopic features and microscopic structures of water under influence of magnetic field. Physica B: Condensed Matter. 403:3571-3577.

[50] Pang, X., and Deng, B. 2008. Investigation of changes in properties of water under the action of a magnetic field. Science in China Series G: Physics, Mechanics and Astronomy. 51:1621-1632.

[51] Toledo, E.J.L., Ramalho, T.C., and Magriotis, Z.M. 2008. Influence of magnetic field on physical–chemical properties of the liquid water: Insights from experimental and theoretical models. Journal of Molecular Structure. 888:409-415.

[52] Iwasaka, M., and Ueno, S. 1998. Structure of water molecules under 14 T magnetic field. Journal of Applied Physics. 83:6459-6461.

[53] Inaba, H., Saitou, T., Tozaki, K.-i., and Hayashi, H. 2004. Effect of the magnetic field on the melting transition of H2O and D2O measured by a high resolution and supersensitive differential scanning calorimeter. Journal of Applied Physics. 96:6127-6132.

[54] Ghauri, S.A., and Ansari, M.S. 2006. Increase of water viscosity under the influence of magnetic field. Journal of Applied Physics. 100:066101-066101-066102.

[55] Cho, Y.I., and Lee, S.-H. 2005. Reduction in the surface tension of water due to physical water treatment for fouling control in heat exchangers. International Communications in Heat and Mass Transfer. 32:1-9.

[56] Iida, T., Matsushima, H., and Fukunaka, Y. 2007. Water electrolysis under a magnetic field. Journal of the Electrochemical Society. 154:E112-E115.

[57] Matsushima, H., Iida, T., and Fukunaka, Y. 2012. Observation of bubble layer formed on hydrogen and oxygen gas-evolving electrode in a magnetic field. Journal of Solid State Electrochemistry. 16:617-623.

[58] Matsushima, H., Iida, T., and Fukunaka, Y. 2013. Gas bubble evolution on transparent electrode during water electrolysis in a magnetic field. Electrochimica Acta. 100:261-264.

[59] Yang, L., Ren, J., Song, Y., Min, J., and Guo, Z. 2004. Convection heat transfer enhancement of air in a rectangular duct by application of a magnetic quadrupole field. International Journal of Engineering Science. 42:491-507.

[60] Motozawa, M., Chang, J., Sawada, T., and Kawaguchi, Y. 2010. Effect of magnetic field on heat transfer in rectangular duct flow of a magnetic fluid. Physics Procedia. 9:190-193.

[61] Lajvardi, M., Moghimi-Rad, J., Hadi, I., Gavili, A., Dallali Isfahani, T., Zabihi, F., et al. 2010. Experimental investigation for enhanced ferrofluid heat transfer under magnetic field effect. Journal of Magnetism and Magnetic Materials. 322:3508-3513.

[62] Azizian, R., Doroodchi, E., McKrell, T., Buongiorno, J., Hu, L.W., and Moghtaderi, B. 2014. Effect of magnetic field on laminar convective heat transfer of magnetite nanofluids. International Journal of Heat and Mass Transfer. 68:94-109.

[63] Junhong, L., Jianming, G., Zhiwei, L., and Hui, L. 2004. Experiments and mechanism analysis of pool boiling heat transfer enhancement with water-based magnetic fluid. Heat and Mass Transfer.

[64] Xu, L., and Peng, X.F. 2002. Fundamental Analysis of Boiling Heat Transfer of Magnetic Fluids in a Magnetic Field. Heat Transfer Asian Research. 31:69-75.

[65] Kobozev, M.A., and Simonovskii, A.Y. 2007. Formation rate of vapor bubbles in magnetic fluid boiling at a single vaporization center: Measuring technique and experimental setup. Technical Physics. 52:1422-1428.

31

[66] Stoian, F.D., Holotescu, S., and Vékás, L. 2010. Bubbles generation mechanism in magnetic fluid and its control by an applied magnetic field. Physics Procedia. 9:216-220.

[67] Bergles, A.E. 1976. Review of instabilities in two-phase systems, In: Two-phase flows & heat transfer. NATO advanced study institute, Istanbul.383–423.

32

Table 1. Working conditions of literature and present studies on CHF in vertical annuli

Do

(mm) Di

(mm) L

(mm) Dhydraulic (mm)

Dheated

(mm) L/Dheated

Pout (kPa)

G (kg/(m2.s))

in (kJ/kg)

Rogers et al.[5] 22–30.2 13.1 480 8.9–17.1 23.85–56.5 8.5–20.2 156 60–648 180–389

El-Genk et al.[6] 20–25.4 12.7 500 7.3–12.7 18.8 _38.1 13.1–26.6 118 0–260 182–312

Mishima and Ishii [7] 25.96 20.45 596.9 5.51 12.5 47.8 101 0-35.8 160-330

Schoesse et al.[8] 22 10 1000 12 38.4 26.0 128 20–280 30–218

Park et al.[9] 29 19 600 10 25.26 23.75 110 0-198.8 295-337

Present work 19 12 750 7 18.1 41.5 85 0-150 106

Table 2. Properties of Fe3O4 nanoparticles

Purity 99.5+%

APS 15-20 nm

BET 81.98 m2/g

Morphology Spherical

Bulk density 0.85 g/cm3

True density 4.8-5.1 g/cm3

Table 3. Thermo-physical Properties of used fluids and nanoparticles.

Property Density

Thermal conductivity

Specific heat

Viscosity

Pure water 978 0.663 4190 0.000404

Fe3O4 nanoparticles 5200 6 670 -------

0. 1% ferrofluid 982 (+0.4 %) 0.6644 (+0.2%) 4172 (-0.43%) 0.000405 (0.25%)

Table 4. Contact angle comparison with reference [25]

Test section surface condition Kim et al. [25]

(with Al2O3 nanoparticles) Present study

(with Fe3O4 nanoparticles)

Boiled in water 83 72

Boiled in 0.01 vol% nanofluid 31 25

Boiled in 0.1 vol% nanofluid 20 19

33

Fig. 1 Used working fluids (a) pure water, (b) 0.01 vol%, and (c) 0.1 vol% ferrofluid.

Fig. 2 Schematic diagram of experimental loop.

34

Fig. 3 (a) Annular test section, (b) the used quadrupole magnet, and (c) cross sectional view of test section.

35

Fig. 4 Schematic view of thermocouples support and locations: (a) Teflon tube and thermocouples locations on it, (b) inserting Teflon tube into the heater Steel tube, and (c) cross section view of the test section and thermocouple attachment.

36

Fig. 5 CHF results comparison of present study with some works in literature.

37

Fig. 6 Flow regimes in a constant mass flux with increasing applied heat flux (a) nucleate boiling at the exit, (b) extension of nucleate boiling to lower parts, (c) bubbly flow, (d) initializing slug flow, (e) slug flow, (f) churn flow, (g) counter-current annular flow (flooding), and (h) annular flow.

38

Fig. 7 CHF comparison between pure water and ferrofluids

Fig. 8. Comparison of CHF between two cases

39

Fig. 9 SEM images of heater surface after boiling in: (a) pure water, (b) 0.01% ferrofluid, (c) 0.01% ferrofluid with magnetic field, (d) 0.1% ferrofluid, and (e) 0.1% ferrofluid with magnetic field.

40

Fig. 10 Contact angle of pure fluid droplet on three heated surfaces (a) water boiled ( ), (b) 0.01% ferrofluid boiled ( ), and (c) 0.1% ferrofluid boiled surface ( ).

Fig. 11 Comparison of pure and ferrofluid boiling characteristics at a constant mass flux; (a) pure water with heat flux of 100 W/m2, (b) 0.01% ferrofluid with heat flux of 100 W/m2, (c) pure water with heat flux of 170 W/m2, and (d) 0.01% ferrofluid with heat flux of 170 W/m2.

41

Fig. 12 CHF of 0.01% ferrofluid with an without magnetic field.

Fig. 13 CHF of 0.1% ferrofluid with and without magnetic field.

42

Fig. 14 Effect of magnetic field on nanoparticle deposition in zero flow condition (a) inner tube without magnetic field, (b) inner tube with magnetic field, and (c) outer Pyrex tube with magnetic field.

43

Fig. 15 CHF of pure water with and without magnetic field.

Fig. 16 Effect of various factors on incipient boiling superheat [35]

44

Fig. 17 Application of stability diagram to experimental data [37]

45

Highlights

Ferrofluid flow boiling CHF are measured experimentally in vertical annulus.

An external magnetic field is applied at near exit of the test section.

Using ferrofluid and applying magnetic field enhances flow boiling CHF.

Pure water is also affected by the applied magnetic field.