Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Kamran Siddiqui

Abstract

The effect of mixed convection on low Reynolds numbers flow inside a horizontal square channel heated from below have been investigated experimentally. The channel flow rate ranged from 0.0210 kg/s to 0.0525 kg/s which correspond to Reynolds numbers between 300 and 750 in the absence of heating. The channel bottom surface temperature was controlled and varied from 30 ºC to 55 ºC (Grashof number ranged between 6.37×106 and 3.86×107). Planer Particle Image Velocimetry (PIV) technique was used to measure two-dimensional velocity fields in the channel mid-vertical plane and two horizontal planes close to the bottom heated wall. Stereo-PIV technique was used to measure velocity fields in the channel cross plane. Natural convection was dominant over forced convection at all measurement conditions and Gr/Re2 ranged between 9 and 206. Mean and turbulent velocity fields were computed and analyzed. It was found that buoyancy-induced secondary flow alters the mean flow behaviour. The mean streamwise velocity in the vertical plane was asymmetric and skewed towards the bottom heated surface. Back flow was observed near the top unheated wall when Gr/Re2 > 55 and its magnitude increased with the increase of bottom surface temperature and/or decrease in the flow rate. Turbulence was induced mainly due to buoyancy-induced secondary flow. The streamwise and spanwise turbulent velocity magnitudes were largest close to the bottom heated surface. The underlying physical processes associated with the low Reynolds number mixed convection were investigated by applying the POD technique on the turbulent velocity fields and through the characterization of turbulent coherent structures formed in the flow. POD analysis revealed intriguing features within the vertical turbulent velocity fields. Local convective cells at different mode levels were observed near the bottom heated wall. Coherent structures were formed extensively in all planes due to the interactions between rising plumes, falling parcels and shear flow. The results showed that the bottom wall temperature had a direct effect on the number of coherent structures generated and their associated characteristics. The flow development was also investigated at five different locations along the channel in the vertical mid-plane. It was found that the flow development length increased with the increase of bottom wall temperature and/or decrease of the flow rate (i.e. increasing Gr/Re2).

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