Asymmetric annular flow in horizontal circular macro-channels: Basic modeling of liquid film distribution and heat transfer around the tube perimeter in convective boiling
This paper presents the modeling of the liquid film distribution and heat transfer during convective boiling in horizontal, annular flows to be applied in such applications where non-uniform heat flux occurs. In general, prediction methods in the literature totally ignore the influence of the non-uniformity in the annular film (thin at top while thick at bottom) on the heat transfer process whereas local measurements around the perimeter of horizontal tubes show a significant variation, up to a factor of four times or more in thickness and up to 25-30% or more in heat transfer from top to bottom. Therefore, starting with the original suite for symmetrical annular flow models for convective boiling, condensation, entrainment, void fraction and two-phase pressure drops (Cioncolini and Thome (2009, 2011, 2012, 2012) [8-111) and their recent paper (Cioncolini and Thome (2013) ) for predicting the threshold between symmetric and asymmetric annular flow, the new features added here are the predictions of the asymmetric annular film thickness and perimeter-wise heat transfer coefficients around the internal perimeter of horizontal tubes. To do this, a new set of 24 algebraic equations is proposed to provide the void fraction, liquid entrainment, pressure drop, liquid film distribution and heat transfer around the perimeter with a simple calculation procedure. Predictions of the new model have been compared against experimental databases with a satisfactory agreement. (C) 2014 Elsevier Ltd. All rights reserved.