An experimental and numerical study of mantle heat exchangers for solar water heaters

摘要

A systematic study of mantle heat exchangers (MHEs) for solar water heaters hasbeen carried out using a combination of experiments, numerical simulations and theory.A MHE consists of a narrow annular passage (the mantle) wrapped around a storagetank in which fluid flows, transferring its heat to the storage tank contents. It was shownthat the exergy of the stored water should be used to determine the efficiency of heatexchangers for solar water heaters. A new parameter that isolates the stratificationcomponent of the exergy was derived, and it was named the stratification efficiency.The effect of curvature on mixed convective flow and heat transfer in narrow annulihas been shown to be negligible. Thus, heat transfer experiments and particle-tracking-velocimetrywere performed in rectangular cavities. Using results of the experimentsand numerical simulations, the flow in the mantle was shown to be predominatelylaminar mixed convection.The effect of a non-isothermal heat-transfer-wall on the mean Nusselt number forconvective heat transfer in a cavity has been quantified using a stratification correctionparameter. When the stratification correction parameter is taken into account, the meanNusselt number is independent of the Grashof number because, due to their currentdesign, MHEs operate with a heat exchanger effectiveness close to 1. A new meanNusselt number correlation has been developed for a range of heat exchanger sizes andboundary conditions, and it includes a correction for a non-isothermal heat-transfer-wall.While not affecting the mean Nusselt number significantly, buoyancy forces doaffect the local heat flux distribution. A comparison of the heat flux distribution fordifferent rectangular cavity configurations has shown that any one configuration may bemore effective as a heat exchanger for solar water heaters, depending on the boundaryconditions. This has been confirmed using an exergy analysis with data from transientheating experiments in a half-scale model horizontal MHE with either a bottom inlet ora top inlet.The new Nusselt number correlation has been incorporated into a model of acomplete thermosyphon solar water heater with a horizontal MHE. The predicted netenergy savings agreed with experimental data to within 4%, over a 10-monthmeasurement period.