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A double-pipe helical heat exchanger was studied numerically and experimentally for both heat transfer and hydrodynamic characteristics. / Results from the numerical trials show that the inner Nusselt numbers in the heat exchanger were similar to literature data, despite the different boundary conditions. Nusselt numbers in the annulus were correlated to a modified Dean number. It was shown that the thermal resistance in the annulus to be the greatest limiting factor for the heat transfer, and heat transfer rates could be increased by increasing the inner tube diameter. / The Prandtl number was shown to affect the inner Nusselt number; however the effects were much greater at low Dean numbers. These differences were attributed to the difference in the developing thermal and hydrodynamic boundary layers. The studies with the thermally dependent thermal conductivities showed that the Nusselt number correlated well with a modified Graetz number. / Thermally dependent viscosity had little effect on the heat transfer; however it affected the pressure drop. Furthermore, it was shown that by keeping the flow rate in the inner tube or the annulus constant, the pressure drop in that section can be affected by changes in the flow rate in the opposite section, due to the change in the heat transfer rate and hence the average temperature and viscosity of the fluid. Non-Newtonian fluids showed little effect on the heat transfer rates, though they significantly affected the pressure drop relations. / The uniformity of the residence time and the temperature distribution were both increased in the inner tube with increasing flow rates. It was shown that a smaller gap size in the annulus resulted in more uniform residence times. Temperature distributions in the inner tube and the annulus were affected by changes in the flow velocity in the opposite section, with lower flow rates resulting in more uniform temperature distributions. Implications of using parallel flow versus counterflow, heating versus cooling, and flow rate are discussed. / Overall heat transfer coefficients and Nusselt numbers were calculated for the experimental data. The inner and annulus heat transfer coefficients were determined using Wilson plots. The results were compared to the numerical data and literature values and showed reasonable agreement. (Abstract shortened by UMI.)

The performance of membrane heat exchangers for HVAC total energy recovery systems was evaluated through experimentation and detailed system modelling. The operating principle of the membrane heat exchanger (enthalpy heat exchanger) is based on passing ambient hot and humid supply air over one side of a porous membrane heat exchanger surface and cold and less humid room exhaust air on the other side of the transfer surface. Due to the gradient in temperature and vapour pressure, both heat and moisture are transferred across the membrane surface causing a decrease in temperature and humidity of the supply air before it enters the evaporator unit of the conventional air conditioner. Hence both sensible and latent energy are recovered. In this study, both experimental and numerical investigations were undertaken and mathematical models were developed to predict the performance of the latent heat recovery heat exchangers for use with conventional air conditioning systems. The membrane moisture transfer resistance was determined by a laboratory-scale permeability measurements. It was found that the membrane heat exchanger performance is significantly influenced by the heat exchanger flow profile and shape, heat and moisture transfer material characteristics, air velocity and air moisture content. Improvement of membrane heat exchanger performance requires an in depth study on flow, temperature and moisture distribution in the heat exchanger flow paths. Thus, a commercial CFD package FLUENT is used to model the membrane heat exchanger. However, software of this type cannot model moisture diffusion through the porous transfer boundary. Therefore, two user defined function codes have been introduced to model the moisture transfer in latent energy heat exchangers. The annual energy consumption of an air conditioner coupled with a membrane heat exchanger is also studied and compared with a conventional air conditioning cycle using the HPRate software. Energy analysis shows that in hot and humid climates where the latent load is high, an air conditioning system coupled with a membrane heat exchanger consumes less energy than a conventional air conditioning system. The membrane heat exchanger dehumidifies the air before it enters the air conditioning system, resulting in a decrease in energy consumption in comparison with conventional air conditioning system.