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Design, Construction, and Qualification of an Open Loop Wind Tunnel Facility to Quantify Enhanced Heat Transfer/Pumping Power Performance in Isothermal Rectangular-Cross Section Passages
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Hydrocarbon-fueled and nuclear power plants are generally located near rivers, lakes or oceans so that water may be used to condense the turbine cycle working fluid. Since geothermal power plants must be located near their thermal source, air must be used to condense the working fluid when sufficient water is unavailable. Air is less suited for cooling condensers than water. As a result air-cooled condensers require large air-side surface areas and/or large power-consuming blowers, which increase capital costs and reduce plant net output. Experimental and computational research shows that forming grooves in the surfaces of heat transfer passages triggers unsteady flows that, under certain circumstances, increase passage heat transfer for a given fan power compared to flat-walled channels. The overall objective of this research program is to determine if grooved passages can be used to reduce the size and/or fan power consumption of air-cooled condensers used in geothermal power plants. This technology may also be useful for reducing the water consumption of other power plants. The objective of the current thesis work is to develop an experimental method to assess the heat transfer and pumping power performance of rectangular cross-section passages for a variety of wall surface topographies. This includes characterizing the developing region of the passage (near its entrance) for a range of air flow rates (and Reynolds numbers). In this thesis, this method is qualified by comparing flat passage measured data against analytical and simulation results. An open-loop facility was constructed that draws laboratory air (at roughly 22°C) through, sequentially, an inlet nozzle, a 1.02 m long, 0.85 cm tall and 23.2 cm wide test section, a flow straightener/exit plenum, round ducting, a flow rate measurement nozzle, and finally the suction end of a variable speed blower. The upper and lower test section surfaces consist of aluminum blocks that are separated from each other by insulation. Each block has a thermocouple on its inner surface (exposed to the air flow) and a heater bonded to its outer surface. A proportional/integral routine is used to control the fraction of time each heater is powered on, so that the measured block temperature reaches a desired value. The resulting time-averaged heater power (minus losses) is used to determine the heat flux versus location to maintain the heated wall at a desired temperature level. Measurements are performed for wall temperatures between 40 and 70°C, and air flow Reynolds numbers of Re = 800 to 1600. The taps in the passage side walls are connected via a scanning valve to a pressure transducer to measure the pressure versus location in the channel. Two-dimensional steady computational fluid dynamics simulations were performed to predict the pressure drop and heat transfer behavior for the same entrance conditions and air property temperature dependence as the experiment. The pressure measurements were in good agreement with simulations. The time-average energy input to the heaters from the experiment increased linearly with the temperature difference between the walls and inlet air. However, heat losses to the surrounds were between 10 and 30% of the input to the heaters for, respectively, the highest and lowest Reynolds numbers. If this heat loss is assumed to be equally distributed to each heater, then the measured heat flux to the air is in good agreement with the numerical simulations. However, future work experimental designs must better quantify the local variation of heat losses. Once the measurement system is qualified based on comparison to flat passage results, it can be used with confidence to assess the heat transfer performance of grooved or other enhanced passage configurations.