In this study an investigation of heat transfer in air cooled Axial Flux Permanent Magnet (AFPM) electrical machines is carried out. Efficiency of electrical machines is strongly influenced by an effective cooling which is provided by forced convection: air enters the generator through the opening in the machine enclosure and leaves it radially, as it is forced out by the rotating discs. The main goal is the enhancement of the heat transfer from the stator where heat is generated in the copper windings. On the other hand the heat transfer to the rotor needs to be minimised in order to keep the magnets' temperature as low as possible. The cooling can be improved by acting on design parameters, such as the distance between the stator and the rotors (commonly named running clearance), the magnet depth, and by acting on operational parameters, such as the rotational speed. The investigation is carried out by using Computational Fluid Dynamics (CFD) software to model air flow and heat transfer inside the AFPM machines. The experimental validation of the numerical models confirmed the capability of the CFD software in predicting the air mass flow rate and the heat transfer in the AFPM machines. The thesis describes the effects of the above mentioned parameters on target quantities, such as the heat transfer coefficients on the generator surfaces, the air mass flow rate through the machine, and the resistive torque. General correlations in non-dimensional form are obtained for the calculation of the heat transfer on the generator surfaces in the AFPM machines as a function of the above parameters. General correlations have also been obtained for the calculations of the non-dimensional air mass flow rate through the machine and for the non-dimensional resistive torque. It was found that the corresponding relationships between the peripheral Reynolds number and local Nusselt numbers on the generator surfaces and the non-dimensional mass flow rate are linear. However, the dependency of Nusselt number on the non dimensional clearance and the magnet depth is non-linear. For the investigated range of the parameters the following was established: an increase in the peripheral Reynolds number results in higher Nusselt number on both the rotating and stationary surfaces of machines; an increase in the running clearance results in the reduction of the Nusselt number on the machine surfaces; the magnetic segments installed on the surface of the flat rotor act as blades of a radial compressor increasing the air mass flow rate and the corresponding Nusselt number on the stator surface. The combination of the non-dimensional running clearance equal to 9 10-3 and the non-dimensional magnet depth equal to 7.3 10-2 was found to be the one which provides the maximum heat transfer from the stator. This piece of information can be used in the design stage for improving the cooling of AFPM electrical machines without increasing the windage losses and contribute in this way to the enhancement of the overall efficiency of this type of machines.

The theoretical analysis of the present work was based on a thermodynamic approach, using the method of inference. An analysis was made to find the macroscopic turbulence transport properties from the description of microscopic behaviour of entities of varying shape, size and velocity. Momentum and energy trans port in a turbulent fluid were investigated and expressions for the eddy diffusivities of momentum and heat proposed. Communication theory has been success: fully used as a means for the interpretation of turbulence parameters. Velocity profiles in simple shear flows, obtained with the present analysis, were compared with those found by others, both experimentally and theoretically. An overall heat transfer similarity parameter was derived with the assumption of a constant turbulent Prandtl number. Measurement of mean velocity, microscale, turbulence intensity and eddy diffusivities of momentum and heat were obtained in a water tunnel Results were obtained for Reynolds numbers from 2 x 10 5 to 9 x 10 5. Pipes were roughened internally with paint mixed with fine particles and roughness ratio r/k ranging between 7.2 x 10 3 and 2.5 x l0 5 with absolute roughness height between 4um and 10um. In the light of the present analysis, it is concluded that the new and more realistic approach to turbulence phenomenon is a useful concept for predicting turbulence transport properties, as well as heat transfer characteristics of a simple shear flow.

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The merits of various numerical methods for the solution of the one and two dimensional heat conduction equation with a radiation boundary condition have been examined from a practical standpoint in order to determine accuracies and efficiencies. It is found that the use of five increments to approximate the space derivatives gives sufficiently accurate results provided the time step is not too large; further, the implicit backward difference method of Liebmann (27) is found to be the most accurate method. On this basis, a new implicit method is proposed for the solution of the three-dimensional heat conduction equation with radiation boundary conditions. The accuracies of the integral and analogue computer methods are also investigated.

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Refrigerants containing chlorine are believed to deplete the ozone layer. They have been phased out and replaced by new ozone-safe refrigerants. This thesis concerns the heat transfer and pressure drop for chlorine-free refrigerant, HFC134a and with two Ester oil mixtures. A test facility was designed, constructed and operated to simulate the evaporator in a heat pump system. The ranges of heat flux, mass flux and evaporating temperature were between 3-40 kW / m2 , 40-375 kg / m2s and 5- 15'C respectively. The overall concentration of oil was varied from 0% to approximately 5% by mass. The results indicated that the nucleate boiling heat transfer in annular flow is dominant when the vapour quality is less than 30% to 50%; heat transfer coefficients were strongly dependent upon heat flux. However, at higher vapour quality values, the heat flux dependence disappeared and the convective heat transfer dominates in this region in which nucleate boiling is suppressed. Dry-out region was observed after the vapour quality was above 90%. The measured pressure drop for pure refrigerant increased with rising exit vapour quality and mass flux but decreased with evaporating temperature. Local heat transfer coefficients for refrigerant-oil mixtures were lower than for the pure refrigerant In most cases The pressure Drops Were always Higher. Data from the present study were compared with existing correlations. A new correlation for pure refrigerant was developed and the results yielded a good agreement with a mean deviation of only 10.55%.

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