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底部加熱之收斂及漸擴管內二次流之引發及熱傳增強現象
Onset of Secondary Flow and Heat Transfer Enhancement for Convergent or Divergent Channel Heated From Below
摘要
本研究主旨乃採用實驗方式探討底部加熱之水平、收斂及漸擴管之二次流發展及其對於管道壁面熱傳之影響現象。實驗測試段之底部加熱面保持水平方向同時均勻加熱。對面之頂壁面則爲絕熱，在管道入口寬度固定不變的條件下，向下（向加熱面）或向上傾斜3°。當向下傾斜3°時，管道變爲收斂管（即出口高度較小）；反之當向上傾斜3°。時，管道變爲漸擴管。管道入口面積之展弦比（寬度除以高度）及管道長度與入口高度比分別爲亂6.67與15，流場之雷諾數範圍介於100與2，000之間，浮力參數(Gr/Re^2)介於0與907之間。本實驗利用煙霧視流法觀察二次流之發展過程並對二次流開始發生之位置加以紀錄，從中找出壁面加熱量、雷諾數與二次流開始發生、熱傳係數開始增加及最大熱傳係數發生位置等相關參數之關係式。比較發現漸擴管由於管壁表面爲逆向壓力梯度使得壁面之熱邊界層變厚造成二次流開始發生之位置提前，此現象乃因流場逐漸減速之流體所誘導之不穩定效應造成的結果，相鄰兩縱排對流捲間相互作用變爲激烈而不穩定，下游流場流況極度地紊亂。對於收斂管由於管壁表面爲順頂向壓力梯度，加速之流體可產生穩定流場之效應，使得壁面熱邊界層變薄造成二次流開始發生之位置延後，抑制相鄰兩縱打謝流捲問相互作用造成二次流所誘導之管壁表面熱傳增加量減少，管壁表面熱傳開始增加之位置較漸擴管延後發生，下游流場達到較爲穩定的狀態。
並列摘要
Experimental studies of secondary air flow structure and heat transfer in horizontal parallel-plate, convergent and divergent channels have been carried out. The bottom wall is horizontal and heated uniformly, which the opposite wall is insulated and inclined with respect to the horizontal plate so as to create a convergence and divergence angle of 3° for the convergent and divergent channel. The aspect ratio (width to height) and the ratio of channel length to height at the entrance of the channel is 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2,000 and the buoyancy parameter, Gr/Re^2, from 0 to 907. Secondary air flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the bottom wall and the locations of the onset of secondary flow are recorded at once. Correlations are suggested for the heat flux and for the Reynolds number, and for the locations of the onset of secondary flow, the onset of heat transfer enhancement, the maximum Nusselt number. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier onset of secondary flow due to the destabilization effect of deceleration. The interaction between neighboring vortices and plumes becomes highly unstable and the flow transfers to turbulent immediately. For the convergent channel, the favorable pressure gradient causes a thinner bottom heated layer which results in much later onset of secondary flow due to the stabilization effect of the acceleration. The interactions between neighboring vortices and plumes are suppressed, and results in less pronounced enhancement of heat transfer, later onset of heat transfer enhancement and a much stable flow.