Micro-channel evaporator for space applications – 2. Assessment of predictive tools

Abstract

This study is the second part of a two-part study addressing the effectiveness of micro-channel evaporators for space applications. The first part provided pressure drop and heat transfer data for FC-72 that were acquired with a test module containing 80 of 231μm wide×1000μm deep micro-channels. The tests were performed in three flow orientations: horizontal, vertical upflow and vertical downflow over broad ranges of mass velocity and heat flux. The present part uses these experimental results to assess the accuracy of published predictive tools. The two-phase heat transfer coefficient data are compared to predictions of 15 popular correlations, and pressure drop data to 7 mixture viscosity relations used in conjunction of the Homogeneous Equilibrium Model (HEM), and 18 correlations based on the Separated Flow Model (SFM). These models and correlations are carefully assessed in pursuit of identifying the most accurate tools. In addition, three important criteria for implementing micro-channel flow boiling in space systems are proposed: avoiding large pressure drop, avoiding critical heat flux (CHF), and negating the influence of body force. It is shown that micro-channels require significantly smaller mass velocities to negate body force effects than macro-channels, and are therefore very effective for space applications.