Multiphase Science and Technology A Quarterly

ISSN for PRINT: 0276-1459

For Online Access

Best Paper Award Selection - Editorial Board Site

2000, Volume12

Issue 3&4

302 pages

Issue price - $280.00

M. Mann
Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart, Pfaffenwaldring 9, 70550 Stuttgart, Germany

K. Stephan
Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart, Pfaffenwaldring 9, 70550 Stuttgart, Germany

A detailed micro region model for heat transfer to single vapor bubbles at heated walls is combined with models of bubble population around a horizontal cylinder, and of heat transfer in the thermal boundary layer which is disturbed by the rising bubbles.
The micro region model takes into account the existence of an adsorbed film between wall and vapor and describes heat transfer in the narrow wedge shaped region adjacent to this film. Solution of the micro region model yields the growth rate and the heat transferred to the bubble.
Enhancement of convection by rising vapor bubbles on the lower side of the cylinder is given special attention in a convection model based on bubble population. The area where the thermal boundary layer is disturbed by rising bubbles is estimated. There a portion of the superheated liquid in the thermal boundary layer is removed and replaced by liquid of saturation temperature. The cold liquid warms up again by transient heat conduction, leading to an enhancement of heat transfer.
Nucleate pool boiling heat transfer coefficients from a horizontal cylinder were then modeled by the combination of heat transfer to bubbles adhering at the wall, described by the micro region model, and heat transfer due to convection, described by the convection model. The contribution of convection was found to depend strongly on the heat flux. In fully developed nucleate boiling the contribution of convection undergoes a maximum and decreases again at higher heat fluxes where evaporation becomes the dominating mechanism. The results obtained for i-pentane, propane and R114 at different reduced pressures p_r are in good agreement with experimental data of others obtained with horizontal cylinders.
We can thus state that the model permits prediction of nucleate boiling heat transfer coefficients at flat plates and along horizontal cylinders. The only parameters needed are bubble site densities.

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