A Review of State of the Art in Modeling of Air-to-Refrigerant Heat Exchangers for HVAC&R Applications

Air-to-refrigerant heat exchangers are a key component in all air-conditioning, heat pump and refrigeration systems. The most common of types of air-to-refrigerant heat exchangers are tube-fin and microchannel heat exchangers. There has always been a great emphasis on understating the underlying physics and improving the performance of these heat exchangers. More recently, researchers have been investigating the use of small hydraulic diameter flow channels as well as novel heat transfer surfaces for use in such heat exchangers. The novel designs not only include shape optimized tubes, but also tube bundles with varying tube and fin geometries. In order to design optimum heat exchangers for a given application, it is crucial to use a reliable thermal-hydraulic model to evaluate the performance of air-to-refrigerant heat exchangers. In the last two decades, significant strides have been made in modeling of tubefin and microchannel heat exchangers. The different modeling techniques include the use of performance maps, LMTD and epsilon-NTU based methods and fully discretized finite volume approaches. In terms of accuracy, the finite volume models are by far the preferred ones. The goal of this paper is to present the state of the art in finite volume modeling of air-to-refrigerant heat exchangers and to highlight research that stretches the boundaries of conventional heat exchanger modeling methods. The review starts out with a comprehensive survey of finite volume models in the literature and their capabilities to account for the various underlying physical phenomenon. High level modeling paradigms are derived and the best practices are highlighted. The various methods of geometry and circuitry representation and solution methodologies are summarized. Majority of these models rely on empirical correlations for local heat transfer and pressure drop evaluations. The use of such correlations has its own challenges and the lessons learned from the literature in this context are highlighted. Air and refrigerant flow maldistribution, especially in microchannel heat exchangers, is a critical phenomenon that needs to be accounted for in such models. Refrigerant flow maldistribution models in the literature range from user-specified quality and mass flow distribution profiles to more sophisticated methods that use CFD-based co-simulation techniques. The different techniques for handling dehumidifying conditions, such as those in an evaporator, are summarized. Lastly, recent developments in the field of optimization of air-to-refrigerant heat exchangers is presented. The review concludes with some thoughts on what the future of air-to-refrigerant heat exchanger design and optimization might be.