Boiling in small diameter tubes

Abstract

Boiling in small cross section channels has not received as much attention as in large cross section channels. It is noted in the literature that higher heat transfer coefficients can be expected in the small cross section channels. This fact, if correct, could be utilized in the design of compact heat exchangers. Unfortunately, not much has been done to understand the physics of boiling in small cross section channels. It is hoped that this study will generate renewed interest in small cross section channels. The primary objectives of this study were as follows: 1) To design and fabricate a basic apparatus for the study of forced convection boiling in small cross section channels in the Heat Transfer Laboratory of the Mechanical Engineering Department of Texas A&M University. 2) To study the effect of the primary variables on the average heat transfer, the frictional pressure drop and the critical heat flux. 3) To express the independent and dependent variables in proper dimensionless quantities and to develop functional relationships between dimensionless quantities. Three fluids, methanol, 2-propanol, and water, were tested. Six different mass fluxes and three different subcoolings were used. An average heat transfer coefficient was defined and was expressed as a function of bubble number, Reynolds number, Prandtl number and boiling number. The bubble number is the ratio of tube diameter to the critical wave length of Taylor instability. It expresses the significance of bubble diameter when the tube diameter decreases. The average heat transfer coefficient failed to show a dependence on bubble number. The average heat transfer coefficient is a strong function of Re number, and Pr number..