HEAT TRANSFER IN FORCED BOILINGINSmE HORIZONTAL TUBES

Before the start of the lab lesson ~ou should be able to exRlain / answer thefollowing Roints or Questions regardin2 boiling inside horizontal tubes:

1. Describe different forms of boiling with varying heat tIux or temperaturedifference.

2. What is meant with complete versus incomplete evaporation?

3. What driving temperature difference is here used?

4. What non-dimensional numbers that affect the heat transfer are used inPierre's equations (2) and (3)1

5. How have Pierre's non-dimensional equations (2) and (3) been derived?

HEAT TRANSFER

REA T TRANSFER IN FORCED BOILINGINSmE HORIZONTAL TUBES

OBJECT:

The purpose of the lab lesson is to examine the heat transfer in forced boiling(evaporation) inside horizontal tubes at varying heat flux. This includes measuring,calculating and then representing the results in various diagrams.

THEORY:

"Heat transfer in forced boiling inside tubes" is summarized on p.32 in the "Heattransfer, Collection offormulas and Tables and Thermal Properties" (1997) by EricGranryd [1].

In forced boiling inside horizontal tubes the following equation can be used:

q/A = h.At = (lnR/A).Ahev (1)

where q / A is the heat flux over the evaporator [W 1m2]; A the heat transfer area [m2];h is the coefficient of heat transfer in boiling [W/(m2.K)];

L1t = mean temperature difference between the tube wall and the evaporation

temperature (corresponding to the evaporation pressure) [K];lnR = refrigerant mass flow through the tube [kg/S];L1hev = enthalpy change over the evaporator [J/kg].

The following two equations established by Pierre have been derived fromexperimental data.

The equation for incomQlete evaQoration (vapor quality at exit of tube < I) inhorizontal tubes is:

= C}'ReoKfo,5Nu

The equation for comQlete eva~orationhorizontal tubes is:

Nu = C2.ReO.8.KjO.4

where Nu = h 'd I k is the Nusselt number [ - ];Re = 4' mRI(Ji'd',u) is the Reynolds number [

LAB LESSON NO.4

(2)

(with a vapor superheat of 5 Ii 7 C) in

C 20 (Re. Kf 0.5)°08 (3)

- ];

2

Kf = Ahev l(g'L) is the "boiling number" ( - );d; L = inner diameter and length of tube (m);k= thennal conductivity of liquid at evaporation temperature (W/(m'K));jJ = dynamic viscosity of liquid at evaporation temperature (Nm/s2);g = acceleration due to gravity (= 9.81 m/s).

The constant values Pierre used were C1 = 1,1'10.3 and C2 = 1,0'10-2. However,thennal property values have been revised since Pierre established his equations and ithas been suggested that these constants should be reduced to R: 85% of above values.

EXPERThIfENT AL APP ARA TUS :

The test rigg consists of a refrigerating set-up of evaporation type (See Figure 1).

Evaporation takes place in a concentric heat exchanger, the evaporator, where theboiling refrigerant, R134a, flows in the inner tube while a liquid secondary refrigerant(a water solution) is flowing in the space between the inner and outer tubes. The innertube is a 5/8" copper tube (dy = 16; di = 12 rom) with the effective tube length 6.0 m.The concentric heat exchanger is covered with insulation to minimize influence of theambient. It is possible to observe the flow of the refrigerant at the glass sections. Theinsulation can easily be removed at these sections.

The evaporated refrigerant gas is sucked away from the heat exchanger by thecompressor that also compresses and pumps the gas on to the condenser, that issituated under the board. The condenser used is a shell and tube condenser. Therefrigerant is condensed on the tubes and the heat flow released at the condensation istransferred to cold water flowing in the tubes. The compressed refrigerant gas is inthis way first cooled and then condensed to liquid.

The refrigerant returns to the evaporator through a thermostatic expansion valve (TEin Figure 1) where the refrigerant is throttled (down) to the evaporation pressure. Thevalve automatically controls the flow to the evaporator so that it gives a ratherconstant super heat at the exit of the tube.

is circulated with a pump and an electrical heater providesThe heat can be adjusted with a knob connected to a

The second~ refrigerantheat for the evaporation process.variable voltage transformer.

Three Rrecision manometers (pressure gauges) are used for measuring the pressure inthe condenser, the evaporator and at the compressor inlet.

Measurements of temperatures are made with thermocouples that are attached to alogger and through an HP- VEE program (on RUN) in the computer transferred to anExcel sheet. Note that the HP- VEE program must D.Q.t be on RUN when values aresaved in Excel and when writing in the Excel sheet.

3

The laboratory assistant describes how the personal computer can be used to get agraphic picture of the different temperatures along the evaporator as well as to make

calculations and further representations of the results.

TEST PROCEDURE:

The assistant has normally started the lab lesson and adjusted the equipment for thefirst point of measuring. This means that the following steps have been taken:

- Connect the equipment electrically and check that the water faucet to the condenseris open. Put the voltage transformer in position 0 (%).

- Start the compressor and circulation pump for the secondary refrigerant (check onthe volume flow meter that the pump rotates).

- Adjust the transformer voltage for the first point of measuring.

Different points of measuring are obtained by varying the heat flux over theevaporator. This is done by adjusting the transformer voltage with the knob to thevalue in the following table, unless lab assistant gives other values:

Point 1: 230 V~ Point 2: 210 V~ Point 3: 190 V~ Point 4: 170 V.

When stable temperatures have been obtained the values for a point of measuringmust be saved. (Remember that the HP- VEE program must nQ.t be on RUN whenvalues are saved in Excel). The other measurements of pressure, voltage andamperage are then made and recorded in Table 1. Note that for each pressure youshall also write the corresponding temperature given for R134a on the manometers.

Adjustments are finally made to the next point of measuring!

CALCULATIONS:

Examine first the Excel sheet named Serle 1 Chart with temperature profiles along theevaporator. What observations can be made?

Calculations are then made according to Table 2. Explanation is here given to themain calculations for each point of measuring.

The heat flow q that in the evaporator is transferred to the boiling refrigerant can becalculated from:

(in 'Cp}uc,r,'(t19 - t23) -1.5.(t4 - t,IJq

where the last 11ambient air.

errn

The mean temperature of the secondary refrigerant in the evaporator and the electricalheater is almost the same.

(4)

is subtracted because of heat that leaks into the evaporator from

Therefore, the mass flow times specific heat for the

4

secondary refngeranthe electrical heater giving:

U./ = (m .C~8BC.r..(t24 - t2J) - 0.2.(t4 - tJ)

where the last term is subtracted because the ambient air temperature, . 14, is higher

than the temperature of the electrical heater.t3.

Calculate the coefficient of heat transfer, h, according to Eqn 1. The drivingtemperature difference, At, is the temperature difference between the tube wall andthe boiling refrigerant. To avoid most of the influence from the superheated part of

the tube we can use:

At = (tll+t12+t13+t14+t15+tl~/6 - (t6+t7+t8+t9)/4 (6)

Calculate Nu-, Re- and Kf-numbers. Thermal property values (u and k) for li.gYillR134a at the evaporation temperature can be obtained from Table 8 in Ref. [1]. Theenthalpy change over the evaporator, Ahev, is easiest obtained by drawing the wholeprocess in the attached h -log p diagram (Ap~endix 1).

REPRESENT A TION:

Draw Eqn 3 as a line in Di~am 1. Mark your calculated Nu-values as a function ofRe K~.5. What slope in the diagram would your values give? Adjust with yourcalculated Nu-values a line parallell to Eqn 3. What value for the constant C2 wouldthis line indicate? Compare with comments given to Eqn. 3.

Mark also your calculated Nu-values as a function of Re1 Kfin Diagram 2. How is thegeneral agreement? What other observations can be made?

Mark h as a function of q/A in Diagram 3. How is h affected by the heat flux q/A?(Start with Eqns 1 and 3).

Mark q/A as a function of 111 in Diagram 4. How is 111 affected when the heat flux q/A(with complete evaporation) is changed? (Continue with the method above).

What would the situation be with incomplete evaporation, where Eqn 2 can be used?

The values of h in Diagram 3 are ~ heat transfer coefficients for the whole heattransfer areaA. Would it with this lab lesson rig be possible to estimate ~heattransfer coefficients for the four horizontal parts of the evaporator? If so, how couldthat be done?

t in the evaporator can be determined from a heat balance over

(5)

5

TABLE 1: IvIEASURED V ALVES

6

CALCULATIONS OF MEASURED VALUES

7

FIGUREl: LABORATORY SET OF APPARATUS

~ ~~~~~~= Water

Condenser~Compressor

Section A - A TemReratures along the whole eva~orator

"t 19: 't ., ., ~ . 2 ~ :"":""" I2L 2t-_~ +-" ~ 7 1 ~ J: 1, 1 1:' 1~3 1 ~ ~ 1,5 IV I

,I I I I I ~ ~O

~- Ir j'" /' \

6 7 8 9 I~ -~

Secondary19 refrigerant

Tubewall

Primaryrefrigerant

8

~usselt number. Nu. as a function of of Re Kfo.5.

60

40

30

20

Nugget! number. Nu. as a function of of Re2 Kt:DIAGRAM 2:

1.105 Re. Krill (2.10. S-10.

9

DIAGRAM 3: Heat transfer coefficient h as a function of heat flux. q/A.

h (W/m2.K)

800

600

400

300

200

1-103

Heat flux:.. q/ A as a function of the tem~erature difference. Lit.DIAGRAM 4:

q/ A (W /m1

8.103

6-103

4.103

2.103

1-103

4-103 6.103 8.1032.103 q/ A (W 1m2)

~t (K)

10

~FRIGERANT DIAGRAM FOR R13-!1aMPENDIX L

8~

(Enthalpy - log p diagram)

c-(e Jeq) aJnssaJd

11