heat transfer predictions for forced convective boiling

7/27/2019 Heat Transfer Predictions for Forced Convective Boiling

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Heat Transfer Predictions for

Forced Convective Boiling

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The different flow regimes have significant effects on the heat transfer

characteristics of convective boiling in a tube. While heat transfer for

subcooled liquid and superheated vapor can be easily handled by

correlations for single-phase heat transfer, the intermediate heat transfer

mechanism is complicated by phase change from liquid to vapor. After

boiling is initiated in the tube, vapor bubbles are generated at certain

nucleate sites while the rest of the inner surface of the tube remains in

contact with the liquid. Under these conditions, the heat transfer mechanism

is a combination of two parallel processes: single-phase convection in the

liquid and nucleate boiling.

The overall heat transfer coefficient for convective boiling in an upward

vertical tube can be written as(Chen, 1963)

where and hb are heat transfer coefficients for single-phase convection of

the liquid and nucleate boiling, respectively. Fand S in eq. (1) are dynamic

factors that modify the contributions of single-phase liquid convection andnucleate boiling, respectively

The single-phase heat transfer coefficient for liquid alone can be obtained by

using the Dittus-Boelter/McAdams equation:

The contribution of nucleate boiling is determined by using the correlation

proposed byForster and Zuber (1955)for pool boiling:

The convective boiling factor Fcan be obtained by a regression of

experimental data(Chen, 1963):
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whereXtt is the Lockhart-Martinelli parameter obtained by (seeHeat

Transfer Predictions for Forced Convective Condensation)

The nucleate boiling suppression factor S is

where

is the local two-phase Reynolds number.

Based on more than 10,000 experimental data points for different fluids,

including water, refrigerants, and cryogents,Kandlikar (1990, 1991)

proposed the following generalized heat transfer correlation for convectiveboiling in both vertical and horizontal tubes:

where hNBD and hCBD are the nucleate boiling dominant and convective boiling

dominant heat transfer coefficients, and they are obtained by

where Co is the convective number, Bo is the boiling number, and is the

Froude number.
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The Froude number multiplier, , is

For liquids with a Prandtl number between 0.5 and 2000, a range that covers

most fluids except liquid metal, the single-phase all-liquid heat transfer

coefficient in eqs. (8) and (9) is

where the friction factor fin eq. (14) is given by

Table 1: for copper tube

Fluid Fluid

Water 1.00 R-114 1.24

R-11 1.30 R-134a 1.63

R-12 1.50 R-152a 1.10

R-13B1 1.31 Nitrogen 4.70

R-22 2.20 Neon 3.50

R-113 1.30

The fluid-surface parameter depends on the combination of the liquid and

tube material used. For stainless steel tubing, is taken as 1 regardless of

the type of fluid. For copper tubing, can be obtained from Table 1

Kandlikar (1990; 1991). The above correlation is valid for

Under constant heat flux conditions, the wall temperature will sharply

increase when the heat transfer mechanism inside the tube suddenly
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changes from two-phase to single vapor phase heat transfer. Since the heat

transfer inside the tube always begins with the single liquid phase, then

changes to convective boiling, and finally to single phase vapor, the critical

heat flux (CHF) phenomenon always occurs at a point near the exit of the

tube.

For this reason, CHF for convective boiling inside the tube is a local

phenomenon. There are numerous empirical correlations available in the

literature. The most generalized model to predict CHF for forced convective

boiling is recommended byKatto and Ohno (1984).

For

For

where
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The critical heat flux predicted using eq. (16) agreed reasonably well with avariety of fluids, including water, ammonia, benzene, ethanol, helium,

hydrogen, nitrogen, R-12, R-21, R-22, R-113, and potassium.

Heat transfer coefficients for boiling in both vertical and horizontal tubes are

often measured from experiments using electrical heating that result in

axially and circumferentially uniform heat flux. While this approach can give

reasonable boundary conditions for boiling in a vertical tube,Thome (2004)

pointed out that electrical heating for boiling in a horizontal tube is not

preferred, because circumferential conduction in the tube wall from the hot,

dry-wall condition at the top to the colder, wet-wall condition at the bottom

yields unknown boundary conditions. Therefore, countercurrent hot water

heating that can provide reasonable boundary conditions is preferred.

References

Chen, I.C., 1963, A Correlation for Boiling Heat Transfer to Saturated Fluids

in Convective Flow,ASME preprint 63-HT-34, Presented at 6th National

Heat Transfer Conference, Boston, MA.

Forster, H.K., and Zuber, N., 1955, Dynamics of Vapor Bubbles and Boiling

Heat Transfer,AIChE Journal, Vol. 1, pp. 531-535.

Kandlikar, S.G., 1990, A General Correlation for Two-Phase Flow Boiling

Heat Transfer Coefficient Inside Horizontal and Vertical Tubes, ASME

Journal of Heat Transfer, Vol. 112, pp. 219-228.

Kandlikar, S.G., 1991, Development of a Flow Boiling Map for Subcooled

and Saturated Flow Boiling of Different Fluids in Circular Tubes, ASME

Journal of Heat Transfer, Vol. 113, pp. 190-200.

Katto, Y., and Ohno, H., 1984, An Improved Version of the GeneralizedCorrelation of Critical Heat Flux for the Forced Convection Boiling in

Uniformly Heated Vertical Tubes,International Journal of Heat and Mass

Transfer, Vol. 27, pp. 1641-1648.

Thome, J.R., 2004, Engineering Data Book III, Wolverine Tube, Inc.,

Huntsville, AL.
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heat transfer predictions for forced convective boiling

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