boiling heat transfer exp

8/3/2019 Boiling Heat Transfer Exp

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TWO PHASE HEAT TRANSFER


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TWO PHA SE HEAT TRANSFER

Objectives

1. Determine the heat flux and surface heat transfer coefficient as functions of the

temperature excess at constant pressure; i.e., construct a boiling curve.

2. Determine the maximum heat flux (critical heat flux) as a function of pressure; compare

the value with the theoretical data generated using the Zuber and Tribus correlation.

3. Determine and explain the effect of system pressure on the maximum heat flux4. Determine and explain the effect of system pressure on the heat transfer coefficient.

5. Perform a heat balance on the apparatus and compare to theory. Can you apply

the heat balance to the film boiling region to get a more accurate estimate of max heat

flux?

For extra credit, model a similar system in Polymath or Excel to show how pressure can

affect the heat transfer coefficient. Hint: you may have to assume a simplified

distribution of discreet surface cavity sizes.

Theory

Boiling Heat TransferWhen a liquid at the saturation temperature is in contact with the surface of a

solid (usually a metal) at a higher temperature, heat is transferred to the liquid,and a phase change (evaporation) of some of the liquid occurs. The nature and

rate of this heat transfer changes considerably as the temperature difference

between the metal surface and the liquid is increased.

Convective BoilingWhen the metal surface is slightly hotter than the liquid, convective currents

carry the warmed liquid to the surface, and evaporation is largely at the surface

with little ebullition (turbulence).

Nucleate BoilingAs the metal surface temperature is increased, small bubbles of vapor appear on

the heating element surface. At first, some of these bubbles may collapse,

giving up their latent heat to the liquid. The rest of the bubbles rise to thesurface where they burst and release the vapor. As the temperature difference

increases further, almost all of the bubbles make it to the surface without

condensing.

Surface tension in the liquid offers great resistance to the birth of a bubble.

Initially the bubbles form at nucleation sites on the surface where minute local

cavities or gas pockets exist. The ability of these vapor bubbles to form,

expand, and rise to the surface depends on the size of the surface cavity (note

that for a given surface there is a unique range or distribution of cavity sizes),

the system pressure, the liquid vapor pressure and surface tension, and the


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temperature difference between the heating element and the liquid. The

dependence of bubble formation on these variables can be seen by combining a

force balance on the gas-liquid interface of the bubble with the Clausius-

Clapeyron equation which you should recall from physical chemistry and

thermodynamics classes.

In full nucleate boiling, bubbles form vigorously with considerable turbulence.

This action leads to very high heat transfer rates. From an industrial point of

view, this is by far the most important and most used regime of boiling.Film Boiling

Above a critical surface-liquid temperature difference, the surface becomes

vapor-locked and the liquid is unable to wet the surface. When this happens,

there is a considerable reduction in the heat transfer rate. If the heat input to the

metal is not reduced to match the lower ability of the surface to transfer heat, the

metal temperature will rise until radiation heat transfer plus the limited film

boiling heat transfer from the surface is equal to the energy input. This

condition can result in a failure or burn-out. In this experiment, film boiling is

reached and sustained for a limited period of time.

Heat flux

The heat transferred and the heat flux are calculated using the equations below.

Remember that Q is calculated in Watts. The exact dimensions of the heating element will be required to

calculate the areaA. These dimensions are indicted on the experiment casing.

The heat transfer coefficient (h) is then given by

Zuber and Tribus Correlation

Brock and Bird Method for Computing Surface Tension ()

A

Q=FluxHeat

IV=Q

TA

Q=h

+1

)-(gh

24=

A

q

l

v

1/2

2v

vl

1/4

vgf

max


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where,

Pcr = Critical pressure (atm)

Tcr = Critical temperature K

hfg =Latent heat of

boiling (fluid-gas)

B = Brock and Bird parameter (dimensionless)Tbr = Tb /Tcr(dimensionless)

= Surface tension (dynes/cm = mN/m)

OPERATING INSTRUCTIONS

During Use

Control the saturation pressure to the desired level by:

a. Varying the cooling water flow rate. Increasing the water flow rate resultsin a decrease in the vapor pressure.

b. Varying the power supplied to the heater. Increasing the power supplied to

the heater results in an increase in the vapor pressure.

High Temperature Cut-Off

Under no circumstances should the setting on this control be above 300C. For normaloperation, it is advisable that the control be set to shut the system off at 250C.

After Use

a. Switch off the electrical supply.

b. Circulate cooling water until the pressure has dropped to atmospheric.

NoteInitial readings from the voltmeter and ammeter should be taken from the lowest scales

(low power inputs). When the power input is sufficient enough to exceed this scale, the

two pole switch should be adjusted to allow the larger voltmeter and ammeter scales to be

utilized. The switch is labeled to indicate which scale relates to each switch position.

0.281-T-1

PT+10.1207=B

)T-(1BTP=

b

crb

11/9

r1/3cr

2/3cr

r

r

ln


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EXPERIMENTAL APPARATUS

VISUAL DEMONSTRATION OF THE THREE MODES OF

BOILING

EXPERIMENT 1

Objective:


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To visually demonstrate the three modes of boiling: convective, nucleate, and film boiling using

R-113 refrigerant.

Procedure:

1. Turn on the electric and water supply. Adjust the power output to 100-150 Watts and

maintain the pressure at 5 psig by switching buttons 1 and 3 (button 1 is the power button; it

should always be turned on). Carefully watch the liquid surrounding the copper heatingelement. Convection currents will be observed and at the same time, liquid will be seen to

collect on the condenser coils, indicating that evaporation is proceeding at a low rate. When

the pressure reaches 5 psig, purge air until the pressure reaches 2.5 psig.

2. Increase the wattage in increments (around 15 Watts). Observe the surface temperature of

the heating element and the liquid temperature at each increment. Nucleate boiling will be

seen, and as the power input is increased, vigorous boiling will occur. The temperature

difference between the liquid and the heating element surface at this point should still be

below 70C.

3. Increase the power in smaller increments and observe the liquid to heating and 75C, thetemperature of the heating element will rise quickly. This indicates that film boiling is

occurring. At this point watch the temperature CLOSELY. As it approaches 250degrees, reduce the electrical power input to about 100 Watts. When film boiling occurs,

the rate of evaporation falls to a low level. Observation of the heater surface will show that it

is now enveloped in an almost unbroken film of vapor.

4. The power output at which film boiling occurs will be used in Experiment 2 to determine the

size of increments to obtain the proper number of data points.


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DETERMINATION OF THE HEAT FLUX AND SURFACE HEAT

TRANSFER COEFFICIENT

EXPERIMENT 2

Objective:

To determine the heat flux (Q/A) and the surface heat transfer coefficient (h) up to and beyond the

critical temperature difference point at a constant pressure.

Procedure:

1. After reaching steady-state, start taking the data according to the data table. Increase the

power input in increments. Continue the increments until film boiling is reached.

2. When film boiling is reached, reduce the heat input to about 100 Watts. Go to the next

pressure (7.5 psig) by switching off button 3 and switching on

button 4.

DETERMINATION OF THE HEAT FLUX AND SURFACE HEAT

TRANSFER COEFFICIENT

EXPERIMENT 3

Objective:

To determine the maximum heat flux at four pressures.

Procedure:

1. Follow the procedure from Experiment 2 for pressures of 5.0, 7.5, 10.0, and 12.5, psig to

determine the maximum heat flux at these pressures. Remember that at each pressure you

need to reach film boiling and maintain up to 250 F. Choose the appropriate heat input

increments so that you will have enough data to make a graph.

2. After the 12.5 psig run is completed, reduce the wattage (or turn it off). Then set the

pressure down to 10.0 psig by switching button 5. After 2 minutes, set it to 7.5 psig. Then

wait 2 more minutes before setting it to 5.0psig, etc.. This prevents the sudden contraction of

the glass tube. Let the GTA know that the experiment is finished.


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MINIMUM REPORT REQUIREMENTS

1. Plot the log (heat flux) vs. log (temperature difference) at each pressure, and obtain the

maximum heat flux values from these graphs.

2. Compare values determined from (1) above to theoretical values determined using the Zuber

and Tribus correlation. For calculation of the surface tension, use the Brock and Bird

method.

3. Plot log (heat transfer coefficient) on a linear axis vs. the temperature difference on a log axis

at each pressure.

4. Obtain a correlation between the heat transfer coefficient and pressure for nucleate boiling

(see below).

Correlation between heat transfer coefficients and pressure

At some constant pressure, heat transfer coefficient (W/m 2 - C) is a function ofTx (temperature

difference between the heating element and the saturated liquid, in C) as follows:

where, a and b are constants. To obtain these constants, make a plot of ln (h) vs.ln (Tx). The slope of the linear regression is b and they-intercept is ln a. Since we have 4 differentpressure runs, we can take the average of the a and b values (assuming the standard deviation is not toogreat). Use the data analysis- regression routines in Excel to get 95% confidence level estimates of the

parameter values.

The following equation takes into account the variation of pressure in the heat transfer coefficients

where, c is a constant, and href is the heat transfer coefficient at a reference pressurepref.

Combining the above two equations gives

)T(a=hb

x

p

ph=h

ref

c

ref

p

p)T(a=h

ref

c

b

x


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This time, use multiple variable linear regression in Polymath to determine the constants a, b, and c. You

will need to first transform the equation to get a linear form:

Polymath (on PCs in 228 Textile has an example problem of this type that is easily accessed through the

menu system. By tabulating all combinations of ln(Tx ), ln(p/pref), and the dependent variable ln(h), andusing the regression routine, you will get least squares estimates of ln(a), ln(b), and ln(c) along with 95%

confidence levels on these parameters. By taking the antilogs of these values and uncertainties (95%

confidence intervals), you will be able to get estimates of a, b, and c from our original equation. The

confidence interval tells you what range of values are likely to be the actual parameter values with 95%

certainty. Compare the results to experiment. Try graphing as a surface plot in Excel, or (preferably)

Sigma Plot. How do the values for a and b compare to those calculated for a and b?

+ p

pc)T(b+a=h

ref

x lnlnlnln


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SAMPLE DATA SHEET: Experiment 2

PRESSURE: 5 psig

Temperatures (C)

Coolant H 2ORun No. Voltage

(Volts)

Current

(Amps)

Liquid Metal In Out

Flow

Rate

of

H 2O


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SAMPLE DATA SHEET: Experiment 3

PRESSURE: 7.5 psig

Temperatures (C)


(Volts)

Current

(Amps)

Liquid Metal In Out

Flow

Rate

of

H 2O

PRESSURE: 10 psig

Temperatures (C)


(Volts)

Current

(Amps)Liquid Metal

In Out

Flow

Rate

of

H 2O

PRESSURE: 12.5 psig

Temperatures (C)


(Volts)

Current

(Amps)Liquid Metal

In Out

Flow

Rate

of

H 2O


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PRESSURE: 12.5 psig

PRESS URE: 15.0 psig

Temperatures (C)

Coolant H2ORun No. Voltage

(Volts)

Current

(Amps)Liquid Metal

In Out

Flow

Rate

of

H 2O


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