| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 92 === The heat transfer characteristics of a hot impinging jet with radiation effects are studied. Two-dimensional cylindrical, steady, turbulent flow is simulated using the k-e model. The discrete-ordinates method is used to solve the equation of radiative transfer for radiation. Solutions are presented for the temperature distribution, heat flux, Nusselt number, and pressure along the impingement wall. The effects of important parameters, such as optical properties (absorption and scattering coefficient of the gas), the nozzle-to-plate distance, the Reynolds number, the surface emissivity of the wall, the thermal conductivity of the plate, the nozzle exit pressure to the ambient pressure ratio (PR), and Mach number of the nozzle exit are examined.
Results show that the radiative heat flux and the total heat flux at the stagnation point are reduced by 94 percent and 77 percent respectively, when the absorption coefficient is decreased from 0.2 cm-1 to 0.005 cm-1. The total heat flux at the stagnation point is reduced by 50 percent approximately and the Nusselt number is reduced by 51 percent as the particle size is increased from 5µm to 20µm. As the emissivity is decreased from 0.9 to 0.1, the radiative heat flux from the impingement plate is decreased by 58 percent, and the total heat flux to the impingement plate is increased by 3 percent at the stagnation point. The temperature slightly decreases as the emissivity er is increased. As the nozzle-to-plate distance (z) is increased from 2 to 12 nozzle diameter (D), the total heat flux of the stagnation point is reduced by 93 percent approximately. The stagnation temperature drops by 28 percent and the Nusselt number is reduced by 93.5 percent when z/D is increased from 2 to 12. The net radiative heat flux increase as the Rc decreases. The temperature of the stagnation point is higher 34 percent as Rc increase from 565 to 15206. The radiative heat flux from the stagnation point is increased by 93 percent when the Reynolds number is increased from 15000 to 100000. When the plate temperature is high, the radiative effect becomes more important. As the Reynolds number increases from 15000 to 100000, the temperature at the stagnation point increases by 52 percent approximately.
The position of the shock wave has a profound effect on the temperature field, flow field, and pressure field distributions. When PR is increased the shock wave moves away from the impingement plate and hence increases the circulation zone between the shock wave and the impingement plate. The shock wave moves closer to the plate as the value is increased from 1.0 to 2.7. Again, the shock wave moves away from the plate and increases the circulation zone when the value is increased from 2.7 to 3.05. The velocity increases rapidly before the shock wave and decreases after the shock wave. As the value becomes larger, the temperature is decreased.
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