AIRSIDE PASSIVE HEAT TRANSFER ENHANCEMENT, USING MULTI-SCALE ANALYSIS AND SHAPE OPTIMIZATION, FOR COMPACT HEAT EXCHANGERS WITH SMALL CHARACTERISTIC LENGTHS

Abstract

The study of compact heat exchangers (HX) is a very common, although broad topic that draws interest from many engineering applications. Most technologies contain at least one HX serving as a fundamental component for the proper system functioning. The rapid worldwide population growth, increasing demand for energy resources, widespread environmental concerns, space exploration efforts and economy are all good reasons for developing smaller, lighter and more efficient HX’s. This research sheds the light on the next generation of heat exchangers, with a focus on air-to-fluid applications. For incompressible flows and low-pressure applications, the HX’s airside thermal resistance is the major limitation to overall thermal conductance. On conventional surfaces fins are required, but bring many drawbacks. Among these include being prone to fouling/frosting, reduced heat transfer coefficient, higher friction resistance, and more material consumption. Tubes by nature provide more valuable heat transfer than do fins; there is little focus on tubes in the literature. The first objective of this work is to discuss the fundamental aspects of primary (tubes) and secondary (fins) surfaces, with the aid of numerical analyses. The latter demonstrates how the reduction of characteristic length and novel shapes impact surface performance and compactness of finless and finned tubes. A further discussion is presented arguing that conventional fin concepts are not always beneficial. The second objective of this work entails developing a comprehensive multi-scale analysis with topology and shape optimization methodology leveraging automated CFD simulations and approximation assisted optimization. Novel finless air-to-fluid HX concepts were developed, for single-phase and two-phase applications, and achieved more than 20% reduction in size, 20% better performance and 20% less material than state-of-the-art HX’s including microchannel HX’s. Two prototypes (one manufactured in metal 3D printing) were tested in an in-house wind tunnel. The numerical predictions agree with the experimental results in less than 5% deviation for total capacity, 10% for airside heat transfer coefficient and 20% for air pressure drop. Finally, the last objective is to present the development of robust and computationally inexpensive tools that can accurately predict CFD simulation responses for conventional tube and fin surfaces using small diameter tubes (<5.0mm).