Laminar and turbulent natural convection heat transfer and fluid flow processes in asymmetrically heated vertical parallel flat plate channels have been investigated both experimentally and numerically. The experimental part of the investigation was carried out using a fibre optic Laser-Doppler Anemometer (LDA) together with a temperature probe and data acquisition system. The numerical analysis was done using the computational fluid dynamics (CFD) code PHOENICS. The laminar natural convection flow case was examined for a uniform heat flux (UHF) heating mode while the turbulent flow case was examined considering both UHF and uniform wall temperature (UWT) heating modes. Small and large scale channels were constructed to perform laminar and turbulent flow experiments respectively. The channels were formed by a heated wall, an opposing unheated perspex or glass wall and side walls. Velocity profile and time history data along the channel and temperature profiles at channel exit were recorded for the laminar flow case. In the turbulent flow case of UWT heating mode mean and turbulent velocity data and mean temperature data at the channel exit were collected. For the UHF heating mode turbulent flow case, however, both mean and turbulent velocity and temperature data were recorded. The main objective of the experiments was to obtain data by which the numerical analysis could be supported since not enough experimental data is available, especially for the turbulent flow case. The numerical analysis of the laminar flow situation was performed with the standard features of PHOENICS. The turbulent flow case was examined by building into PHOENICS the codes for four different Low Reynolds Number k-Ɛ turbulence models. The grid pattern was optimised for a test case and employed for final computations. Due to the lack of experimental data regarding the location of transition to turbulent flow, computations were done by introducing a level of turbulence at the inlet and solving the governing equations for turbulent flow throughout the channel. The results of measurements justified this approach. The results of numerical and experimental analysis of laminar flow showed good agreement confirming that the numerical method was capable of predicting the flow with good accuracy. Turbulent flow experiments exhibited the characteristics of a developing turbulent channel flow. Low amplitude, high frequency velocity fluctuations were observed close to the channel inlet with higher amplitudes downstream and almost similar fluctuation patterns in the uppermost region of the channel suggesting fully developed turbulence. The temperature data indicated intermittent bursts of high frequency temperature fluctuations near the inlet and continuous high frequency temperature fluctuations of increasing, amplitude downstream. Numerical results of the turbulent flow case have produced good agreement with experiments. The solutions were most sensitive to the level of turbulence introduced at the channel inlet. Reasonable grid independent solutions were obtained for grids greater than 60x60 for most of the turbulence models considered. Velocity and temperature profiles obtained from experiments and numerical analysis are presented for both flow cases. Typical time histories of velocity and temperature at different channel heights and cross-stream locations are presented. From the numerical and experimental results correlations were produced for Nusselt number, and Reynolds number as functions of Rayleigh number, for the UWT heating mode of turbulent flow.

Heat management in high thermal-density systems such as CPU chips, nuclear reactors and compact heat exchangers is confronting rising challenges due to ever more miniaturized and intensified processes. While searching for heat transfer enhancement, micro-channel flow boiling and the usage of high thermal potential fluids such as nanofluids are found to be efficient heat removal approaches. However, the limited understanding of micro-scale multiphase flows impedes wider applications of these techniques. In this thesis work, liquid-vapour phase change and multiphase flow heat transfer in micro-channels were experimentally investigated. Included are studies on the single phase friction, vapour dynamics, liquid meniscus evaporation, two-phase flow instabilities and heat transfer. An experimental system was built. Rectangular microchannels with different hydraulic diameters (571 μm, 762 μm and 1454 μm) and crosssectional aspect ratios were selected. Transparent heating was utilised by coating the micro-channels with a layer of tantalum on the outer surfaces. FC-72, n-pentane, ethanol, and ethanol-based Al2O3 nanofluids were used as working fluids. Pressures and temperatures at micro-channel inlet and outlet were acquired. Simultaneous visualisation and thermographic profiles were monitored. Single phase friction of pure liquids and nanofluids mostly showed good agreement with the conventional theory. The discrepancies were associated with hydrodynamic developing flow and the early transition to turbulent flow, but nanoparticle concentration showed minor impact. After boiling incipient, the single vapour bubble growth and flow regimes were investigated, exploring the influences of flow and thermal conditions as well as the micro-channel geometry on vapour dynamics. In addition, liquid meniscus evaporation as the main heat transfer approach at thin liquid films in micro-channels was studied particularly. Nanoparticles largely enhanced meniscus stability. Besides, flow instabilities were analyzed based on the pressure drop and channel surface temperature fluctuations as well as the synchronous visualization results. Moreover, study on flow boiling heat transfer was undertaken, the corresponding heat transfer characteristics were presented and the heat transfer mechanisms were elucidated. Furthermore, ten existing heat transfer correlations were assessed. A modified heat transfer correlation for high aspect ratio micro-channel flow boiling was proposed. The crucial role of liquid property and microchannel aspect-ratio on flow boiling heat transfer was highlighted.

Thermal analysis is an important topic that can affect the electrical machine performance, reliability, lifetime and efficiency. In order to predict the electrical machine thermal performance accurately, thermal analysis of electrical machines must include fluid flow modelling. One of the technologies which may be used to estimate the flow distribution and pressure losses in throughflow ventilated machines is flow network analysis, but suitable correlations that can be used to estimate the pressure losses in rotor ducts due to fluid shock is not available. The aim of this work is to investigate how the rotation affects the pressure losses in rotor ducts by performing a dimensional analysis. Apart from the additional friction loss due to the effects of rotation, other rotational pressure losses that appear in a rotor-stator system are: duct entrance loss due to fluid shock and combining flow loss at the exit of the rotor-stator gap. These losses are analysed using computational fluid dynamics (CFD) methods. The CFD simulations use the Reynolds-averaged Navier Stokes (RANS) approach. An experimental test rig is built to validate the CFD findings. The investigation showed that the CFD results are consistent with the experimental results and the rotational pressure losses correlate well with the rotation ratio (a dimensionless parameter). It shows that the rotational pressure loss generally increases with the increase in the rotation ratio. At certain operating conditions, the rotational pressure loss can contribute over 50 % of the total system loss. The investigation leads to an original set of correlations for the pressure losses in air ducts in the rotor due to fluid shock which are more suitable to be applied to fluid flow modelling of throughflow ventilated machines. Such correlations provide a significant contribution to the field of thermal modelling of electrical machines. They are incorporated into the air flow modelling tool that has been programmed in Portunus by the present author. The modelling tool can be integrated with the existing thermal modelling method, lumped-parameter thermal network (LPTN) to form a complete analytical thermal-fluid modelling method.