An experimental and numerical invetigation of laminar and turbulent natural convection in vertical parallel-plate channels
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.