S D Houston

Nucleate pool boiling on horizontal tubes : a convection-based correlation

Abstract Pool boiling on horizontal tubes leads to a bubbly flow layer around the lower periphery of the tube in which bubbles slide around the surface. Heat transfer is by enhanced liquid convection at the surface around the bubble and by rapid evaporation of a thin layer under the sliding bubble. Both these mechanisms are a function of the vapour flow rate and a correlation of the following form is developed: Nu = AF(p) Re 0.67 b Pr 0.4 . A is a function of the critical pressure alone, F(p) is a function of the reduced pressure alone and Re b is based on the mean vapour mass flux from the surface and the tube diameter. The correlation applies to water, refrigerants and organics boiling on tubes of 8–50 mm in diameter. Bearing in mind the wide scatter found in pool boiling experiments the correlation fits well with the available world data and is particularly suitable for use in equipment design calculations.

The effect of diameter on boiling heat transfer from the outside of small horizontal tubes

Recent studies have suggested that tube diameter is a relevant parameter which should be included when correlating heat transfer data for pool boiling on horizontal tubes. However, there are few data for tubes of diameter less than 8 mm, and the diameter effect for small tubes has not been determined. This paper reports the results of an experimental investigation using tubes of 1–6 mm diameter. Tests have been carried out with water and refrigerant R141b at heat fluxes ranging from 1 to 90% of the critical heat flux. The results indicate that correlations which show an inverse relationship between heat transfer coefficient and diameter should not be applied for tubes of less than 6 mm diameter. Conventional pool boiling correlations derived from data acquired from boiling on wires and flat plates yielded acceptable results when applied to the test data from this study.