Heat Transfer Comparison between Archimedean and Rectangular Spiral Heat Exchangers for Mesoscale Combustors

Heat exchanger optimization is a significant aspect of mesoscale combustor development which utilizes heat recirculation principle. Spiral heat exchangers (SHEs) are usually employed for this purpose. The two commonly used spiral geometries are rectangular spiral and Archimedean spiral. However, there is a non-availability of research data regarding the relative merits and demerits of these two geometries, in particular their heat transfer performance in small scales. Comparison cannot be made using conventional heat exchanger equations and correlations because of the invalidity of the basic assumptions in small scales. In the present work comparison between these two geometries were performed using CFD simulations with the aid of a RANS type software code. The geometries considered are based on a mesoscale combustor developed in our previous study using propane-air combustion. Air was chosen as the working fluid for both hot and cold flows. Comparisons were performed between similar geometries of Archimedean and rectangular spiral heat exchangers (SHE). Similarity between the geometries was achieved by keeping the respective wetted surface areas of hot and cold flows similar for both the SHEs. This was accomplished by performing a simultaneous minimization (with the aid of Matlab Optimization tool box) of the differences between the respective hot and cold fluid wetted surface areas of the spirals. The channel width, depth and channel wall thickness were kept the same for the two geometries. To achieve similar areas the Archimedean spiral had to have higher number of turns. CFD simulations were performed for these two geometries and for two wall material thermal conductivities. Three different inlet Reynolds numbers and five external heat loss coefficients were considered. Results showed that for the adiabatic outer wall conditions, the rectangular SHE had a significant advantage over the Archimedean SHE in terms of heat exchange effectiveness. But as the external heat loss coefficient increases both the geometries provided similar performance. Results showed that the wall thermal conductivity of outer walls of the combustor is a more important parameter than the type of SHE when examining performance under non-adiabatic conditions.