Experimental Investigations of Heat Transfer in a Channel with Ribs and Obstacle
Abstract: Popular Abstract in English This thesis concerns experiments of heat transfer in a rectangular channel with ribs and an obstacle. Liquid crystal thermography (LCT) is used to record the temperature fields in the experiments. LCT is based on measurement of the color response of a heated surface. When illuminated with a white light, the liquid crystals react to temperature changes by changing color, i.e., from red at the low end to blue at the high end of the active temperature range. The liquid crystal used has a bandwidth of 1°C. The research work consists of three cases, namely several periodic ribs, a single obstacle, and interaction of ribs with an obstacle, respectively. The results with periodic ribs placed on one wall expand the current knowledge frontier for two geometrical parameters, i.e., the rib spacing to rib height ratio and the rib height-to-channel hydraulic diameter ratio, for cases where the thermal field is not periodically fully developed. Three different flow velocities were considered. Particular emphasis is on large ratios of the rib spacing to rib height in the first inter-rib regions. It is conjectured that the main flow for all cases separates at the edge of the first rib and reattaches in the first inter-rib region at a distance of about ten times the rib height from the upstream rib edge, where a local maximum of the heat transfer coefficient is found. The effect of the rib height-to-channel hydraulic diameter ratio is investigated by two rib heights in combination with two rib spacings. The significant effect of the small rib height on the local heat transfer in the first inter-rib region is found remarkable compared to the bigger rib height. This effect is explained by the small thickness of the boundary layer in the developing region and it is conjectured that the core flow is strongly disturbed by the presence of high ribs. The second part presents local end-wall heat transfer distributions in the upstream as well as downstream regions of an obstacle. The obstacle has a rectangular cross-section and blocks the whole height of the channel. Three flow velocities are considered. A double peak of locally high heat transfer occurs in the upstream wall-obstacle junction and indicates the existence of a complex vortex system. A primary peak upstream of the junction is explained by a vortex formed at the front corner. The last part aims to control the heat transfer effect of the obstacle by using ribs. A rib is positioned upstream or downstream of the obstacle. Two flow velocities are considered. The spacing between the ribs and the obstacle is a primary parameter. In the upstream region, two spacings and two rib heights were considered. For the small spacing the result is contradictory to that of the large spacing. It is found that the local heat transfer especially in the upstream region is strongly affected by the spacing and rib height. In the downstream region, the local heat transfer is more affected by the flow velocity than by the rib height and spacing. As the rib is in the downstream region, a single spacing prevailed but two rib heights were tested. No impact on the local heat transfer in the upstream region was found. In the downstream region of the obstacle, the local heat transfer was significantly affected by the presence of the rib and its height. The results of this thesis will be of significance for understanding and handling heat transfer issues in gas turbines and heat exchangers.
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