Browsing by Subject "convection heat transfer"
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Item A Computational Study on the Thermal-Hydraulic Behavior of Supercritical Carbon Dioxide in Various Printed Circuit Heat Exchanger Designs(2013-02-04) Matsuo, BryceThere has been an ever-increasing demand for power generation, which is predicted to grow as society becomes more advanced. However, tradition fossil fuels are beginning to deplete, and there is a great necessity for alternative fuel sources that will bridge the gap between energy production and consumption. To decrease the high demand alternative fuel sources are gaining in popularity. The supercritical carbon dioxide Brayton power cycle has been proposed as a possible cycle for nuclear and concentrated solar power generation. Two main advantages of having supercritical carbon dioxide are the large property variations and component size associated with power cycle. Forced convection heat transfer of supercritical carbon dioxide in printed circuit heat exchanger geometries were investigated in the following study using a finite volume framework and the FLUENT 12.1 code. The geometries of interest were: non- chamfered zig-zag, chamfered zig-zag, and air foil. Flow through the three geometries was in the horizontal orientation and subject to a heating mode operation. A range of testing conditions were explored, including operating pressures between 7.5 to 10.2 MPa with the mass flux ranging from 326 to 762 kg/m2-s. Due to the turbulent nature of this problem, the k?E with enhanced wall treatment and shear stress transport k?? turbulence models were considered. With this addition a total of 54 simulations were performed. Results indicated that there was an increase in the heat transfer coefficient as the supercritical carbon dioxide reached the pseudocritical temperature, conversely as there was an increase in operating pressure, the heat transfer coefficient decreased. Nevertheless, this increase near the pseudocritical temperature was due to a sharp increase in the specific heat. Mass flux effects indicated that there was an increase in heat transfer as the mass flux was increased. This was due to the increase in Reynolds number near the pseudocritical temperature. Next, pressure losses were investigated for the three geometries. The non-chamfered zig-zag channel had the greatest pressure loss associated with it, while the air foil channel had the least. Based on the results, the ratio of the friction factor to heat transfer for the non-chamfered and chamfered zig-zag geometries were approximately 2.65 and 1.57 times higher than for the air foil, thus leading to the idea that the air foil channel may be best suited for practical applications. Finally, the simulation results were compared to experimental data and existing correlations. Many existing correlations failed to accurately predict the magnitude of heat transfer, although they exhibited a similar trend. A new correlation was developed for the zig-zag geometries based on the numerical data obtained during this investigation and published experimental data. The new correlation is able to predict the maximum heat transfer coefficient within 12.4%.Item Development of a fuel-powered compact SMA (Shape Memory Alloy) actuator system(Texas A&M University, 2005-02-17) Jun, Hyoung YollThe work presents investigations into the development of a fuel-powered compact SMA actuator system. For the final SMA actuator, the K-alloy SMA strip (0.9 mm x 2.5 mm), actuated by a forced convection heat transfer mechanism, was embedded in a rectangular channel. In this channel, a rectangular piston, with a slot to accommodate the SMA strip, ran along the strip and was utilized to prevent mixing between the hot and the cold fluid in order to increase the energy density of the system. The fuel, such as propane, was utilized as main energy source in order to achieve high energy and power densities of the SMA actuator system. Numerical analysis was carried out to determine optimal channel geometry and to estimate maximum available force, strain and actuation frequency. Multi-channel combustor/heat exchanger and micro-tube heat exchanger were designed and tested to achieve high heat transfer rate and high compactness. The final SMA actuator system was composed of pumps, valves, bellows, multi-channel combustor/heat exchanger, micro-tube heat exchanger and control unit. The experimental tests of the final system resulted in 250 N force with 2 mm displacement and 1.0 Hz actuation frequency in closed-loop operation, in which the hot and the cold fluid were re-circulated by pumps.