Heat Exchanger Design

To increase the rate of heat transfer, what can be done?

Q = U A Tln

U = (hi, kwall and ho)

1

o

o

ln

o

w

ti

i

o

ii

o

o h

1R

d

d

kR

d

d

h

1

d

d

U

1

h

f

d

kNuh

f

h

k

dhNu

Nanofluids (Method – 3)

How?, Then answer may be micro-channels (Method – 4)

Fins (Method – 1)

Flow rate (velocity) (Method – 2)

Microfluidics – channel sizes

2

Microchannel Technology

3

44

Flow channel classification

• Channel classification based on hydraulic diameter is intended to

serve as a simple guide for conveying the dimensional range

under consideration.

• Channel size reduction has different effects on different

processes.

• Deriving specific criteria based on the process parameters may

seem to be an attractive option,

but considering the number of processes and parameters that

govern transitions from regular to microscale phenomena (if

present),

a simple dimensional classification is generally adopted in

literature.

55

Fig. 1.1. Ranges of channel diameters employed in various

applications, Kandlikar and Steinke (2003).

66

Table 1.1 Channel dimensions for different types of flow for

gases at one atmospheric pressure.

• The classification proposed by Mehendale et al. (2000)

divided the range:

from 1 to 100 μm as microchannels,

100 μm to 1mm as meso-channels,

1 to 6 mm as compact passages, and

greater than 6 mm as conventional passages.

77

Table 1.2: Channel dimensions for different types of flow for

gases at one atmospheric pressure.

Channel dimensions (μm)

Gas Continuum

flow

Slip flow Transition

flow

Free molecular

flow

Air > 67 0.67–67 0.0067–0.67 < 0.0067

Helium > 194 1.94–194 0.0194–1.94 < 0.0194

Hydrogen > 123 1.23–123 0.0123–1.23 < 0.0123

88

Table 1.3: Channel classification scheme.

Channel Description Value

Conventional channels > 3mm

Minichannels 3 mm ≥ D > 200 μm

Microchannels 200 μm ≥ D >10 μm

Transitional Microchannels 10 μm ≥ D > 1 μm

Transitional Nanochannels 1 μm ≥ D > 0.1 μm

Nanochannels 0.1 μm ≥ D

D: smallest channel dimension

99

Non-circular channels

• In the case of non-circular channels,

it is recommended that the minimum channel dimension;

for example, the short side of a rectangular cross-section

should be used in place of the diameter „D‟.

• We will use the above classification scheme for defining

minichannels and microchannels.

• This classification scheme is essentially employed for ease in

terminology;

the applicability of continuum theory or slip flow conditions

for gas flow needs to be checked for the actual operating

conditions in any channel.

10

Table: Fanning friction factor and Nusselt number for fully developed

laminar flow in ducts, derived from Kakac et al. (1987).

10

1111

Basic heat transfer and pressure drop

considerations• The effect of hydraulic diameter on heat transfer and pressure drop

is illustrated in Figs. 1.2 and 1.3 for water and air flowing in a square channel under constant heat flux and fully developed laminar flow conditions.

• The heat transfer coefficient „h‟ is unaffected by the flow Reynolds number (Re) in the fully developed laminar region, since the “Nu” is constant in laminar flow regime.

• It is given by: Eq. 1

• where k is the thermal conductivity of the fluid and Dh is the hydraulic diameter of the channel.

• The Nusselt number (Nu) for fully developed laminar flow in a square channel under constant heat flux conditions is 3.61.

hD

kNuh

1212

• Figure 1.2 shows the variation of h for flow of water and air

with channel hydraulic diameter under these conditions.

• The dramatic enhancement in h with a reduction in channel size

is clearly demonstrated.

Fig. 1.2. Variation of the heat transfer coefficient with channel

size for fully developed laminar flow of air and water.

1313

• On the other hand, the friction factor f varies inversely with Re,

since the product „f · Re‟ remains constant during fully

developed laminar flow.

• The frictional pressure drop per unit length for the flow of an

incompressible fluid is given by:

Eq (2)

where „ pf /L’ is the frictional pressure gradient, „f’ is the

Fanning friction factor, G is the mass flux, and ρ is the fluid

density.

• For fully developed laminar flow, we can write:

f · Re = C Eq (3)

where Re is the Reynolds number, Re=GDh/μ, and C is a

constant, C =14.23 for a square channel.

D

Gf2

L

p 2

f

1414

• Figure 1.3 shows the variation of pressure gradient with the channel size for a square channel with G =200 kg/(m2 s), and for air and water assuming incompressible flow conditions.

• These plots are for illustrative purposes only, as the above assumptions may not be valid for the flow of air, especially in smaller diameter channels.

• It is seen from Fig. 1.3 that the pressure gradient increases dramatically with a reduction in the channel size.

• The balance between the heat transfer rate and pressure drop becomes an important issue

in designing the coolant flow passages for the high-flux heat removal encountered in microprocessor chip cooling.

1515

Fig. 1.3. Variation of pressure gradient with channel size for

fully developed laminar flow of air and water.

Delphi Micro Channel Evaporator

16

Aquaforce Aircooled chiller microchannel coil

Microchannel Reactor Concept

17

Close integration of the exothermic synthesis and steam generation

18

Micro-scale heat exchangers - Introduction

• Micro-scale heat exchangers

or micro structured heat exchangers are heat exchangers

in which a fluid flows in a lateral direction in a confined area

such as a tube or small cavity that dimensions are below the size of 1mm.

• Typically the fluid flows through a cavity which is called a mirochannel.

• This technology exploits enhanced heat transfer resulting

from structurally constraining streams to flow in microchannels,

which reduces thermal resistance to transferring heat.

19

• Fluid flowing through the channels on a plate evaporates or

condenses, and heat is transferred.

• Micro heat exchangers have been demonstrated with

high convective heat transfer coefficients ranging form

10,000 to 35,000 W/(m2-°C), or

about one order of magnitude higher than typically seen

in conventional heat exchangers

with very low pressure drops, typically 1 or 2 psi.

• The basic operating principle of these devices goes back to the

convective heat transfer within the flows of the microchannels.

20

• The convective heat transfer equation is

(1)

• In this equation

„h‟ is the heat transfer coefficient of the microscale heat exchanger,

„Nu‟ is the Nusselt number which is about 3.66 (for circular channels),

„kf‟ is the thermal conductivity of the working fluid, and

„de‟ is the equivalent diameter of the microchannel which the fluid flows

through.

• From this equation one can tell see how

the size of the channel directly affects the heat transfer coefficient of the

heat exchanger,

as the diameter is decreased, the heat transfer coefficient increases.

e

f

f

e

f

e

D

kNuh,or

k

Dh

k

DhNu

21

Different Types of Microscale Heat Exchangers

• The different types of microscale heat exchangers are the same as the different classifications of conventional heat exchangers.

• They have either one or two passages for the fluid to flow through.

• One fluid:

When there is only one fluid and one passage in the heat exchanger the fluid is used to transfer the heat to another location.

Application of this kind of heat exchangers is usually found in electronics to transfer heat into the fluid and out of the electronic device.

• Two Fluids:

When there are two fluids and two passages they are usually classified by the direction in which the fluids flow by each other.

Microscale heat exchangers can either be cross flow or counter flow heat exchangers.

22

• Counter Flow

Counter flow micro scale heat exchangers work the same way as

macro-scale counter flow heat exchangers.

In a counter flow heat exchanger the two fluids flow in opposite

directions of each other.

The fluids enter the heat exchanger at opposite ends.

The cooler fluids exits the counter flow microscale heat exchanger

at the end where the hot fluid enters therefore the cooler fluid will

approach the inlet temperature of the hot fluid.

Counter flow microscale heat exchangers are more efficient than

cross flow microscale heat exchangers.

23

Figure 2: Schematic of Counter Flow Heat Exchanger

24

• Cross Flow

Cross Flow microscale heat exchangers work the same way

as cross flow macro-scale heat exchangers.

In a cross flow heat exchanger one fluid flows perpendicular

to the second fluid.

One fluid flows through tubes or channels and the second

fluid passes around the tubes or channels at a 90° angle.

Cross flow micro heat exchangers are usually found in

applications where one of the fluids changes state therefore

having a two-phase flow.

25

Figure 4: Schematic of a Cross Flow Heat Exchanger

26

• To concurrently achieve the goals of high mass flow rate, low pressure drop, and high heat transfer rates,

the microscale cross flow heat exchanger comprises numerous parallel, but relatively short microchannels.

• The performance of these microscale heat exchangers is superior to the performance of previously available macro-scale heat exchangers.

• Typical channel heights are from a few hundred micrometers to about 2000 micrometers, and

typical channel widths are from around 50 micrometers to a few hundred micrometers.

• The use of microchannels in a cross flow microscale heat exchanger decreases the thermal diffusion lengths, allowing substantially greater heat transfer per unit volume or per unit mass than has been achieved with other heat exchangers.

• The cross flow microscale heat exchangers have performance characteristics that are superior to state of the art macro-scale heat exchanger designs.

27

28

Advantages over Macro-Scale Heat Exchangers

• Substantially better performance

Improves heat transfer coefficient with large number of

smaller channels

• Size

Smaller size allows for an increase in mobility and uses

• Light Weight

Lower weight reduces the structural and support

requirements

• Cost

Lower costs due to less material being used in fabrication

29

Disadvantages of Microscale Heat Exchangers

One of the main disadvantages of microchannel heat exchangers is the high

pressure loss that is associated with a small hydraulic diameter.

This prevents the uniform flow of the cooling material along the channel.

Microchannels are sometimes fairly long and absorb most of the heat along

the first section of the channel.

This makes them less able to absorb heat along later sections.

In order to get the maximum performance out of a microchannel heat

exchanger, there needs to be a balance between the desirable high heat

transfer coefficient and the undesiarable pressure loss.

Due to the small scale of microchannel passages, wall roughness can be very

important in determining how high the heat transfer coefficient is.

30

Applications of Microscale Heat Exchangers

• Microscale heat exchangers are being used to help along the

development of fuel cells.

• The compact microchannel fuel vaporizer (CMFV), which is a

microscale heat exchanger, is a main component of a microchannel

fuel processor that will hopefully enable fuel cell powered vehicles.

• Conventional heat exchangers are too large to be used in this

application, nor can they deliver the kind of performance needed in

this application.

• The microscale heat exchanger is also making possible a portable fuel

cell power supply.

• This power supply could make batteries obsolete.

31

• It will have a longer run time than a battery of comparable

weight.

• It could also be used in place of portable generators that operate

with an internal combustion engine.

• These fuel cells would operate more quietly and with a greater

efficiency than an engine driven generator.

• Problems with refueling a generator in a remote location could

also be solved be this new portable fuel cell.

32

Currently used in these Industries

• Automotive vehicles

• Commercial and Residential Heating/Cooling systems

• Aircrafts

• Manufacturing industries

• Cooling Electronic devices

Fundamentals of

Liquid Cooling

Thermal Management of Electronics

San José State University

Mechanical Engineering Department

Air as a Coolant

PROS:

• Simplicity

• Low Cost

• Easy to Maintain

• Reliable

CONS:

• Inefficient at heat removal

(low k and Pr)

• Low thermal capacitance

(low ρ and Cp)

• Large thermal resistance

Using Alternate Coolants

• As electronic components get smaller and heat transfer

requirements increase air becomes a less efficient coolant

• Liquid cooling provides a means in which thermal resistance can

be reduced dramatically

Types of Liquid Cooling

• Indirect –

The coolant does not come into contact with the electronics.

• Direct (Immersion) –

The coolant is in direct contact with the electronics.

Fluid Selection

• Is the fluid in direct contact with the electronics?

No.

Water will normally be used due to the fact that it is cheap and has superior thermal properties.

Yes.

A dielectric must be used.

Consideration must be given to the thermal properties of different dielectric fluids.

Microchannels

• Microchannels are most commonly used for indirect liquid

cooling of IC‟s and may be:

Machined into the chip itself.

Machined into a substrate or a heat sink and then attached

to a chip or array of chips.

Microchannels

• Example: Thermal

Conduction Module used

on IBM 3080X/3090

series

• Heat is transmitted

through an intermediate

structure to a cold plate

through which a coolant

is pumped

Incropera, pg. 3

Microchannels

• Rth,h – Conduction Resistance through the chip

• Rth,c – Contact Resistance at the Chip/Substrate Interface

• Rth,sub – 3-D Conduction Resistance in the substrate (spreading resistance)

• Rth,cnv – Convection Resistance from the substrate to the coolant

Incropera, pg. 155

Note that this network ends with the mean fluid temperature.

If we use the inlet fluid temperature, we also need to include Rcaloric

Contact Resistance

• Contact resistance is proportional to roughness, and inversely proportional to

pressure.

• Contact resistance depends on several factors

Surface roughness

Pressure holding the two surfaces together

Type of the fluid in the void space between two surfaces

The interface temperature

• Keeping the viscous liquid like “glycerin” in the interface, reduces the contact

resistance between two aluminum surfaces by a “factor of 10” at a given pressure.

• As a thumb rule, the “typical contact resistance” is approximately equal to “5 mm

of additional thickness”.

• Contact resistance may be reduced by use of special bonding greases or insertion

of a soft metal foil between the two surfaces.41

essurePr

RoughnesscetanresisContact

Motivating Example

Laminar flow through a rectangular channel:

Kandlikar and Grande, pg. 7 Kandlikar and Grande, pg. 8

Pressure Drop in Microchannels

• The pressure drop due to forcing a fluid through a small channel may produce design limitations.

V is the mean flow velocity

L is the flow length

ρ is the fluid density

f is the friction factor, depends on the

aspect ratio.

Limitations may include:

1) Pumping Power

2) Mechanical Stress Limitation of

the Chip Materialh

2

D

VLf2p

Pressure Drop Example

• If chip power increases

mass flow rate must

increase

• If mass flow rate

increases pressure drop

increases Kandlikar and Grande, pg. 9

)TT(c

Qm

i,wo,wp

.

Optimization of Microchannels

• How should the channels in the silicon substrate be designed for optimal

heat transfer?

• Should the channel be deep or shallow?

• Make sure to give a valid reason.

• The channels should be deep so that the hydraulic diameter is small but the channel surface area is large.

• Caution: Making the channels too small may result in unreasonable pressure drop.

Kandlikar and Grande, pg. 9

Microchannel - Issues

• Liquids + Electronics

Self-explanatory

• Fouling Leading to Clogging

Clogging prevents flow of liquids through a channel

Local areas where heat is not pulled away from components at a high enough rate are developed

Microchannel - Issues

• Mini-Pumps

Able to move liquid through the channel at a required rate

Able to produce large pressure heads to overcome the large pressure drop associated with the small channels

Tradition rotary pumps can not be used due to their large size and power consumption

For information on some current solutions refer to

http://www.electronics-cooling.com/html/2006_may_a3.html

Current Research for Single Phase Convection in

Microchannels

• Surface Area

Adding protrusions to the channels to increase surface area.

Adding and arranging fins in a manner that is similar to a

compact heat exchanger.

Microstructures

Silicon

Substrate

Examples of different

geometries:

• Staggered Fins

•Posts

•T-Shaped Fins

Kandlikar and Grande, pg. 10

Current Research for Single Phase Convection

in Microchannels

• Manufacturing Technology

Reducing cost of manufacturing

Producing enhanced geometries

For further information refer to article by Kandlikar and Grange

• Justifying deviation from classical theory for friction and heat transfer coefficients when microchannel diameters become small

Lack of a good analytical model

Surface Roughness

Accurate measurements of system parameters

Ect.

***If you are interested in this take a look at:

Palm, B. “Heat Transfer in Microchannels”. Microscale Thermophysical Engineering 5:155-175, 2001. Taylor Francis, 2001.

Current Research for Single Phase Convection in

Microchannels

Jet Impingement

• Benefits of using a jet in

thermal management of a

surface:

A thin hydrodynamic

boundary layer is formed

A thin thermal boundary

layer is formed

Incropera, pg. 56

Classifying Impinging Jets

• Jets can be:

Free-Surface – discharged

into an ambient gas

Submerged – discharged

into a liquid of the same

type

• Cross Sections:

Circular

Rectangular

• Confinement:

Confined – Flow is

confined to a region after

impingement

Unconfined – Flow is

unconfined after

impingement

Classify the Following Jets

• Liquid jet released into

ambient gas

• Liquid release into

liquid of the same type

Incropera, pg. 56

Incropera, pg. 65

Classify the Following Jets

Unconfined, circular, free-

surface jet

Unconfined, circular,

submerged jet

Incropera, pg. 56

Incropera, pg. 65

Nozzle Design

• Nozzles are designed to create different jet

characteristics

Example: Sufficiently long nozzles will produce both fully

developed laminar or turbulent jets (Shown in b)

Incropera, pg. 58

Flow Regions

• Stagnation Region –

Jet flow is decelerated normal to the impingement surface and accelerated parallel to it.

Hydrodynamic and thermal boundary layers are uniform.

• Wall Jet Region –

Boundary layers begin to grow

Incropera, pg. 62

Degradation of Heat Transfer During Jet

Impingement

• Splattering –

Droplets are eject from the wall jet region due to

the distance the nozzle is from the heat source, and

the surface tension of the jet fluid

• Hydraulic Jump –

An abrupt increase in film thickness and reduction in film

velocity occurring in the wall jet region

Confining Fluid Flow

• Adding a confining wall:

Adds low and high pressure regions

Sometimes adds secondary stagnation regions

Degrades convection heat transfer

Decreases space needed to use jet impingement

Incropera, pg. 69

Two-Phase Boiling in Microchannels

• Fluid entering microchannels is heated to the point where it

boils

• Flow in microchannels is highly unpredictable and can

produce large voids and multiple flow regimes inside of tubes

• No accurate analytical models currently exist; many analytical

models have errors ranging from 10% to well over 100%

Flow Regimes in Two-Phase Applications

Garimella, pg. 107

Immersion (Direct) Cooling

• In direct cooling electronics are immersed into a dielectric liquid

• Closed loop systems (Transformer cooling with oils) are

normally used due to

both the cost of the liquids used and

the environmental issues associated with the liquids escaping

into the atmosphere

Typical Liquids Used in Immersion

Cengel, pg. 920

Boiling Used in Immersion(Reuse of the cooling liquids)

• Electronics expel heat into the liquid

• Vapor bubbles are formed in the liquid

• The vapor is collected at the top of the enclosure where it

comes in contact with some sort of heat exchanger

• The vapor condenses and returns to the liquid portion of the

reservoir

Cengel, pg. 918

Boiling Used in Immersion

• Electronics dissipate heat through the liquid

• Vapor bubbles are generated

• As vapor bubbles rise they come in contact with the cooler liquid produced by an immersed heat exchange and they implode

*The prior example is more efficient due to the heat transfer coefficient associated with condensation

Cengel, pg. 919

Cray-2 Supercomputer

• Cold fluid enters between the

circuit modules

• Convection occurs, pulling

heat from the electronics to the

liquid

• The heated fluid is pumped to

a heat exchanger

• Heat is transfer from the

immersion liquid to chilled

water in the heat exchanger Incropera, pg. 6

Concerns with Immersion

• Introduction of incompressible gasses into a vapor space

This will limit the amount of condensation that is allowed

to occur and degrade heat transfer

• Leakage

Environmental Concerns

Reliability