R-410A冷媒在狹窄雙套管中週期性流率或功率振盪之蒸發熱傳研究

Abstract

本研究以實驗方式探討R-410A新冷媒在水平狹窄雙套管中的蒸發熱傳和可看見的蒸發流動特性之實驗研究。流道之間隙固定在2.0mm。我們探討了冷媒質通量振盪、熱通量振盪、週期、振幅、飽和溫度、以及蒸氣乾度對熱傳遞係數及蒸發流動特性的影響。在實驗中，平均冷媒質通量從300到500 kg/m2s，振幅為10,20 和30%，平均熱通量從5到15 kW/m2，振幅為10,30 和50%，周期分別為20、60、120s，蒸氣乾度由0.05到0.95以及冷媒飽和溫度從10到15℃。 第一部份為施加R-410A冷媒流量振盪之蒸發熱傳研究。首先是穩態冷媒質通量與時間平均暫態冷媒質通量振盪之熱傳比較，由實驗數據可發現經過時間平均化後的冷媒質通量振盪蒸發熱傳幾乎與穩態蒸發熱傳一致。在暫態質通量振盪蒸發熱傳實驗時，當質通量振盪的振幅和週期提高，銅管壁溫的振盪振幅也會跟著提高。在實驗中，壁溫的振蕩會稍稍落後於質通量的振盪而有時間落後或稱時滯的現象產生。而我們也發現在中乾度時會開始在管道內產生週期性氣泡流及環形流譜的轉變。另外在前半週期中加熱壁溫會隨著冷媒質通量的遞減而降低，也就是會有蒸發熱傳增強的現象產生，我們將此現象歸因於在測試段入口強烈的質通量振盪導致蒸發流動的冷媒乾度及液膜厚度的改變所造成。 第二部份為施加銅管熱通量振蕩之蒸發熱傳研究。首先是穩態熱通量與時間平均暫態熱通量振盪之熱傳比較，由實驗數據可發現經過時間平均化後的熱通量振盪蒸發熱傳幾乎與穩態蒸發熱傳相似。加熱壁溫和蒸發熱傳流譜也會隨著和熱通量振盪同樣的頻率振盪。在暫態熱通量振盪蒸發熱傳實驗時，當熱通量振盪的振幅加大和週期加長，銅管壁溫的振盪振幅也會跟著增加。值得注意的是由內銅管之熱慣性而導致嚴重的時滯現象也在實驗中被發現。在中乾度時會開始在管道內產生週期性氣泡流及環形流譜的轉變。另外在前半週期中加熱壁溫會隨著熱通量的遞減而降低，且有蒸發熱傳降低的現象產生，我們將此現象歸因於強烈的熱通量振盪導致蒸發流動的冷媒乾度及液膜厚度的改變所造成。 最後，我們將R-410A質通量或熱通量振盪蒸發熱傳遞係數在所有影響參數中的實驗資料做分析，並求出經驗公式。

An experiment is carried out in the present study to investigate the characteristics of time periodic evaporation heat transfer for refrigerant R-410A flowing in a horizontal narrow annular duct subject to an imposed time periodic mass flux oscillation or heat flux oscillation. The mass flux oscillation and heat flux oscillation are both in the form of triangular waves. The gap of the duct is fixed at 2.0 mm. In the study, the effects of the refrigerant mass flux oscillation, heat flux oscillation, R-410A saturation temperature, imposed heat flux and vapor quality of the refrigerant on the temporal evaporating flow heat transfer and the photos of the evaporating flow will be examined in detail. The present experiment is conducted for the mean refrigerant mass flux varied from 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% with the period of the mass flux oscillation tp fixed at 20, 60 and 120 seconds. Besides, the mean imposed heat flux is varied from 0 KW/ m2 to 15 KW/ m2, the amplitude of the heat flux oscillation is chosen to vary from 10% to 50% of the mean heat flux , and the period of the q oscillation tp is also fixed at 20, 60, 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃ for the mean refrigerant vapor quality varied from 0.05 to 0.95. The measured evaporation heat transfer data are expressed in terms of the variations of the heated wall temperature and evaporation heat transfer coefficient with time. In the first part of the study the results for the R-410A evaporation subject to the mass flux oscillation are presented. The measured heat transfer data for the R-410A evaporating flow for a constant coolant mass flux are first compared with the time-average data for a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average evaporation heat transfer. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over wide ranges of the experimental parameters. The results indicate that the Tw oscillation is stronger for higher amplitude and a longer period of the mass flux oscillation. However, a small time lag in the Tw oscillation is also noted. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets better as the mass flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the mass flux increases the opposite processes occur. These unusual changes of the heating surface temperature and heat transfer coefficient with the mass flux oscillation are attributed to the strong effects of the mass flux oscillation on the state of the refrigerant at the duct inlet and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. In the second part of the study results for the R-410A evaporation subject to the heat flux oscillation are also presented. Effects of the mean level and oscillation amplitude and period of the heat flux on the time periodic R-410A evaporation heat transfer have been investigated in detail. We first note that the time-average heat transfer coefficients for the time periodic evaporation of R-410A are not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Then, the heated pipe wall temperature and evaporating flow pattern also oscillate periodically in time and at the same frequency as the heat flux oscillation. Experiment also shows that the resulting oscillation amplitudes of the wall temperature get longer for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. A significant time lag in the heated surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper inner pipe. Moreover, at the intermediate vapor quality changes in the evaporating flow pattern between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets worse as the heat flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the heat flux increases the opposite processes occur. These changes of the heating surface temperature and heat transfer coefficient with the heat flux oscillation are attributed to the strong effects of the heat flux oscillation and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. The effects of heat flux oscillation at extremely short and long periods have been explored. Due to the existence of the thermal inertia of the heated copper duct, the resulting heated surface temperature does not oscillate with time at an extremely short period of the imposed heat flux oscillation. But the oscillation amplitude of the heated surface temperature gets noticeably stronger for an extremely long period of the imposed heat flux oscillation.