Numerical and experimental optimisation of a high performance heat exchanger.

The aim of this research is to numerically and experimentally scrutinise the thermal performance of a typical heat exchanger fitted in a domestic condensing boiler. The optimisation process considered the pins' geometry (circular pins and elliptical pins), pins' spacing, pitch distance, the pressure drop across the heat chamber and the occurrence of thermal hot spots. The first part of the study focused on the effect of altering the circular pins spacing and pins pitch distance of the heat exchanger. Computational Fluid Dynamics (CFD) is used to scrutinise the thermal performance and the air flow properties of each model by changing these two parameters. In total, 13 circular pin models were investigated. Numerical modelling was used to analyse the performance of each model in three-dimensional computational domain. For comparison, all models shared similar boundary conditions and maintained the same pin height of 35 mm and pin diameter of 8 mm. The results showed that at a given flow rate, the total heat transfer rate is more sensitive to a change in the pins spacing than a change of the pins pitch. The results also showed that an optimum spacing of circular pins can increase the heat transfer rate by up to 10%.The second part of the study, focused on investigating the thermal performance of elliptical pins. Four elliptical pin setups were created to study the thermal performance and the air flow properties. In comparison with circular pins, the simulation results showed that the optimum use of eccentricity of elliptical pins could increase the total energy transfer by up to 23% and reduce the pressure drop by 55%. To validate the acquired CFD results, a Thermal Wind Tunnel (TWT) was designed, built and commissioned. The experimental results showed that the numerical simulation under predicted the circular pin models' core temperatures, but over predicted the elliptical models core temperatures. This effect is due to the default values of the standard k - E transport equations model used in the numerical study. Both numerical and experimental results showed that the elliptical models performed better compared to its circular pins counter parts.The study also showed that heat exchanger optimisation can be carried out within a fixed physical geometry with the effective use of CFD.