流動液體薄膜受熱破裂之臨界狀態研究

Abstract

本文針對沿垂直平板自由流下液膜的流態，及在平板上因熱點而導致液膜破裂之現象進行實驗。在研究液膜因表面溫度梯度而造成表面張力梯度所產生的熱毛細現象中，所使用的圓形熱點直徑為3mm，是目前已知文獻中最小者。而使用的流體介質有純水及70%甘油水溶液兩種。研究的重點主要放在液體受熱毛細力作用以致拉扯破裂時之臨界條件探討。本文臨界熱通量的範圍為20~1860 kW/m2，液膜流態包含從層流區至穩定波動流區(雷諾數範圍為0.2~55.2)。並且針對熱點尺寸與文獻比較之結果，發現相較於垂直流動方向，沿流線方向之加熱區長度對液膜受熱而破裂之臨界值有較關鍵的影響，而且其長度越長，所需之臨界熱通量越小。本文以純水為工作流體之實驗結果發現，液膜在圓形熱區邊緣破裂之臨界溫度梯度為11.4 ℃/mm，此結果與文獻在矩形熱區邊緣之臨界溫度梯度為10~15 ℃/mm之結果相符，此說明熱點形狀與液膜破裂之臨界條件無直接相關。

A study for liquid free falling thin film rupture by flowing over a vertical plate which includes a hot spot. To investigate the thermocapillary effect due to the surface tension gradient that induces by non-uniform temperature distribution on the liquid thin film surface. A 3-mm diameter circular form hot spot was be used and found the water and 70% glycerine solution in water liquid film breakdown critical conditions. The range of critical heat flux that lead to film breakdown is 20~1860 kW/m2 and the liquid flow regimes are from pure laminar flow to stable wavy flow (Re= 0.2~55.2). According to the results which compared with references can prove that the heater length along the streamwise direction was shown to strongly effect the critical heat flux and the dynamics of liquid film breakdown. More short the heater length needs more large critical heat flux. Kabov et al. (1996) originally proposed that the direct measurements of the temperature field at the film surface using an infrared scanner revealed to that the film surface being great instability even ruptures when surface temperature gradients up to 10~15℃/mm. An experimental result from present data shown that is 11.4 ℃/mm and demonstrated the heater shape isn’t an important issue for film rupturing.