高溫衝擊流熱傳特性之研究; Heat Transfer Characteristics of a Hot Impinging Jet.

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题名:	高溫衝擊流熱傳特性之研究;Heat Transfer Characteristics of a Hot Impinging Jet.
作者:	彭仕鈞;Shih-Chun Peng
贡献者:	機械工程研究所
关键词:	高溫衝擊流;超音速衝擊流;熱輻射;離散座標法;supersonic jet;radiation;hot impinging jet;discrete-ordinates method
日期:	2004-02-24
上传时间:	2009-09-21 11:39:22 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本論文係應用數值方法，探討在考慮輻射效應下，高溫衝擊流的熱傳分析。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-e模型。輻射熱傳利用離散座標法求解，包含氣體輻射及固體粉粒的散射效應。數值計算包含兩個部分，在高溫衝擊流主要改變參數為雷諾數大小、衝擊距離、壁面放射率、衝擊板的熱傳導係數以及氣體之光學性質(包括吸收係數和散射係數)，來探討熱傳的特性﹔另一部份為超音速高溫衝擊流，主要探討改變噴嘴出口壓力以及噴嘴出口的馬赫數。在高溫衝擊流部分，結果發現只考慮氣體輻射時，隨著氣體的吸收係數增加，壁面的輻射熱通量和全部熱通量會增加，壁面溫度也會跟著增加﹔當固體粉粒的大小從5µm 增加20µm時，在停滯點處的全部熱通量降低了50% ;衝擊距離從2個噴嘴直徑增加至12個噴嘴直徑時，停滯點的全部熱通量減少了93 %，而且溫度也降低了28%。當雷諾數從15000增加至100000時，停滯點放射出去輻射熱通量增加了93 %;改變衝擊板的熱傳導係數，熱傳導係數愈大能量愈容易從衝擊板散去，所以當熱傳導係數愈大時，衝擊板的溫度分布愈均勻。因此可知在高溫噴流系統下，輻射效應的影響是不容忽略的。在超音速高溫衝擊流發現，在衝擊距離為4個噴嘴直徑長時，噴嘴出口壓力與周圍環境壓力比從1.0到3.0變化時，震波產生的位置會隨著壓力的增加遠離壁面，不過壓力很大時，增加的幅度較不明顯﹔而溫度則是隨著壓力增加而降低的趨勢；當馬赫數從1.0增加至2.7 時，震波的位置會離衝擊面愈近﹔可是再從2.7增加3.05時，震波的位置反而會慢慢分離而遠離壁面。此時受到震波位置不同的影響，溫度及壓力的分布也就明顯的不同，隨著震波靠近壁面，溫度和壓力會上升，但隨著震波的遠離，而有下降的趨勢。因此可知衝擊速度是一個很大的影響參數。 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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