| 摘要: | 本論文係應用數值方法，探討在考慮輻射效應下，超音速高溫衝擊流的流場與熱傳分析。整個幾何模型由於軸對稱，簡化成二維模型。而紊流模式採用k-?模型。輻射熱傳利用離散座標法求解，包含放射、吸收與散射效應。數值計算主分為三大部分，第一部分為改變噴嘴出口條件，第二部分為考慮輻射熱傳效應，調整各項參數，第三部分為探討周圍障礙物之影響。 在高溫超音速噴流時輻射熱傳效應對衝擊板表面之溫度與熱傳分布有很大的影響，且輻射熱傳遠比對流熱傳大，對於流體的速度和溫度場則影響甚小。增加出口速度會造成壁面溫度增加，輻射熱傳量增加，但對流熱傳量減少。改變出口壓力時，在軸心附近的溫度會逐漸下降，輻射熱傳量減少，對流熱傳量增加，遠離軸心時三者變化趨勢則會反轉。 改變氣體吸收係數對衝擊面輻射熱傳量會有極大的影響，氣體吸收係數增加，輻射熱傳量逐漸減少。較高的壁面放射率可令衝擊面溫度降低，但溫度梯度則會增加。探討不同光學厚度與散射比的關係時，發現散射比較小時，光學厚度對衝擊面輻射熱傳量影響較大，散射比越高，光學厚度的影響越小。 加入圍阻體會使流場產生迴流，不同圍阻體高度會改變迴流區的範圍與強度，對衝擊流場與溫度場產生些許的影響，如衝擊面溫度、輻射熱傳量、軸心壓力與溫度等，其變化趨勢為先增加再減少。增加圍阻體與軸心的距離會令圍阻體的溫度與熱傳分布皆逐漸下降，迴流區對衝擊流場的影響亦逐漸減少。 The flow and heat transfer characteristics of a hot supersonic impinging jet are studied. Two-dimensional cylindrical, steady, turbulent flow is simulated using a k-ε model. The discrete-ordinates method is used to solve the radiative transfer equation for radiation. Solutions are presented for the temperature distribution, heat flux, and pressure along the impingement wall. The mach number, pressure, and temperature along the axisymmetric line are also presented. The numerical results can be divided into three parts. For the first part, we change the condition of the jet at nozzle exit. For the second part, the radiation effects are considered and the radiative properties of the fluid are adjusted. For the third part, the influence of the presence of a surrounding object on the impingement wall is studied. The results show, for hot supersonic impinging jet, the radiative transfer has great effects on the temperature and heat flux on the impingement surface. The radiative heat flux is much more than the convective heat flux. For the fluid, radiation effects have only minor influences on velocity and temperature fields. As the jet velocity at the nozzle exit increases, the radiative heat flux increases, but the convective heat flux decreases. When changing the pressure at the nozzle exit, the temperature, radiative heat flux on the impingement wall near the axisymmetic line decreases with pressure increase, and the convective heat flux decreases to zero. On the other hand, far away from the axisymmetic line, the trend reverses. The gas absorption coefficient is very important to the radiative heat flux on the impingement wall. When the gas absorption coefficient increases, the radiative heat flux decreases. An increase in the wall emissivity reduces the temperature of the impingement wall, but increases the temperature gradient. If scattering albedo is increased, the influence of the optical thinkness on the radiative heat flux on the impingement surface is decreased. When a surrounding object is placed on the impingement plate, the fluid that impinges onto the wall circulates back to the jet center. The strength and size of the circulation depend on the location and height of the object. As a result of this interaction between the circulation and the main jet stream, the convective and radiative heat fluxes, and the temperature on the surface of the surrounding object decrease as the object is moved away from the jet center line. |
