A NEW TECHNIQUE FOR MEASURING CONVECTIVE HEAT TRANSFER COEFFICIENTS IN SOLID MATRICES FOR COMPACT HEAT EXCHANGER DESIGN

摘要

Measuring internal heat transfer coefficients in solid matrices has been a challenge in the design of Compact Heat Exchangers (CHXs). Steady-state methods have proven to be infeasible, and, although numerous transient methods have been explored, it is our opinion that none have distinguished themselves in terms of convenience and accuracy. In the present study, a new and unique combined experimental and computational technique for determining the internal heat transfer coefficient within randomly stacked woven-screen matrices is presented and results are obtained for air as the working fluid. To obtain the local heat transfer coefficient the solid phase matrix is subjected to a uniform step change in heat generation rate via induction heating, while the fluid flows through under steady state flow conditions. The transient fluid phase temperature response between the inlet and outlet of the sample is measured. The heat transfer coefficient is determined by comparing the results of a numerical simulation based on Volume Averaging Theory (VAT) with the experimental results. The local heat transfer coefficient is defined from VAT in terms of several lower-scale integral and differential terms present in the averaged thermal transport equations. Obtaining the heat transfer coefficient experimentally provides closure to the general VAT thermal energy equations. Several matrices were selected for this experimental study and the results are presented in terms of Nusselt number over a Reynolds number range of about 100 to 400. The characteristic length scale in the dimensionless numbers is the porous media hydraulic diameter derived from the VAT-based governing equations. It is proposed that this new method can provide a convenient and accurate tool for CHX designers.