矽微米線陣列上之池沸騰臨界熱通量

Abstract

本研究以去離子水為工作流體，探討矽微米線高度與加熱器尺寸對池沸騰臨界熱通量(Critical Heat Flux, CHF)的影響，本研究共探討5種不同加熱器尺寸 (0.2 x 0.2, 0.5 x 0.5, 1 x 1, 1.5 x 1.5 和 2 x 2 cm2)，以及4種矽微米線高度(25, 50, 75 和 100 μm)，實驗所得在5種加熱器尺寸的CHF (已扣除熱損失)分別為218, 221, 208 □ 1.1, 159 □ 4.2 和 136 □ 0.8 W/cm2，其熱損失百分比分別為36, 15, 5.8 □ 0.2, 8.1 □ 0.7 和 6 □ 0.4 %，在加熱器尺寸為0.5 x 0.5 cm2時，得到最高之CHF為221 W/cm2，其為目前沸騰熱傳界所得之最高臨界熱通量之一，所得結果顯示當加熱器尺寸大於1 x 1 cm2時，CHF隨著加熱器尺寸減小而增加，且所得之CHF與經由流體不穩定性理論所計算出來的值接近吻合，此驗證了我們之前所提出的假說，但當加熱器尺寸小於0.5 x 0.5 cm2時，CHF不再隨著加熱器尺寸改變，這是由於當加熱器尺寸越小，其表面張力影響越大，造成蒸汽氣泡難以脫離加熱表面，而造成CHF無法隨著加熱器尺寸縮小而更加上升。 此外，在4種不同矽微米線高度所得的CHF (已扣除熱損失)分別為206 □ 0.3, 208 □ 1.1, 184 □ 2.9 和 157.6，其熱損失百分比分別為7.5, 5.8 □ 0.2, 5.7和6.3 %，其結果顯示當矽微米線高度大於50 μm時，所得之CHF與微米線高度是呈反比的關係，隨著矽微米線高度越低，所得CHF則越高，此說明所得之CHF乃受毛細限制(Capillary Limit)所影響;而當矽微米線高度小於50 μm時，所得之CHF不再隨矽微米線高度變化的影響，此說明此時所得之CHF不受毛細限制，其乃由於此時的矽微米線高度較短，其毛細限較大，故所得之CHF並不受毛細限所限制。

In this work, pool boiling of de-ionized (DI) water on the micropillar array-coated surfaces for five different sizes of heater (0.2 x 0.2, 0.5 x 0.5, 1 x 1, 1.5 x 1.5 and 2 x 2 cm2) and four different heights of pillars (25, 50, 75 and 100 μm) were studied. The effects of heater sizes and micropillar heights on Critical Heat Flux (CHF) of pool boiling were discussed. The obtained CHFs after subtracting conduction heat losses for the heater sizes of 0.2 x 0.2, 0.5 x 0.5, 1 x 1, 1.5 x 1.5 and 2 x 2 cm2 are 217.7, 220.5, 208 □ 1.1, 159.4 □ 4.2 and 135.8 □ 0.8 W/cm2 respectively, and the percentages of the heat losses for the five different sizes of heaters are 35.5, 15.3, 5.8 □ 0.2, 8.1 □ 0.7 and 6 □ 0.4 % respectively. The highest CHF of about 220.5 W/cm2 was obtained on the 0.5 x 0.5 cm2 sized heater. This value of CHF is one of the highest CHFs obtained in the field of boiling heat transfer. The CHF increases as heater size reduces when heater size is larger than 1 cm2. The theoretical predications of the modified hydrodynamic theory agree well with the obtained CHFs on these sizes of heaters. This also supports our proposed modified hydrodynamic model. On the other hand, there is no apparently size-dependent-CHF observed when the heater size is smaller than 0.5 x 0.5 cm2. This might be due to that the hydrodynamic theory is not suitable for very small heaters on which the surface tension force is too large for bubble releasing. The obtained CHF (after subtracting conduction heat loss) for the four different heights of micropillars are 205.6 □ 0.3, 208 □ 1.1, 184.3 □ 2.9 and 157.6 respectively, and the percentages of the heat loss for the four different heights of micropillars are 7.5, 5.8 □ 0.2, 5.7 and 6.3 % respectively. The CHF increases as micropillar height reduces when micropillar height is more than 50μm. It suggests that the CHF is limited by the capillary force for these heights of pillars. Nevertheless, there is no micropillar-height-dependent CHF observed when the height of the pillar is shorter than 50μm. This might be due to that the capillary limit it too high to restrict the occurrence of the CHF.