狹窄水平雙套管中R-407C冷媒流量震盪之週期性流動沸騰研究

Abstract

狹窄水平雙套管中R-407C冷媒流量震盪之週期性流動沸騰研究

研究生: 王譯徵 指導教授: 林清發

國立交通大學機械工程學系

摘要 本研究以實驗方式探討R-407C新冷媒在水平狹窄雙套管中流動沸騰熱傳(含次冷及飽和流動沸騰)及相關氣泡特徵之影響。流道之間隙由1.0、2.0至5.0mm。我們探討了冷媒質通量振盪、周期、飽和溫度、入口次冷度、管徑尺寸以及熱通量對熱傳係數及氣泡特徵的影響。氣泡特徵包含氣泡脫離直徑和頻率以及成核密度可由流場觀測得之。在實驗中，冷媒質通量從100到600 kg/m2s，振幅為10,20 和30%，周期分別為20、60、120s，飽和溫度從10到15℃，熱通量從0到45 kW/m2 ，入口次冷度從3到6℃。 由實驗發現當熱通量在振盪過程中，在相對應的瞬時壁溫、熱傳遞係數、氣泡脫離直徑、脫離頻率和成核址密度也會有振盪的現象發生，但是當流量振盪的振幅和週期經過時間平均化之後的飽和態和次冷態流動的暫態流動沸騰熱傳特徵沒有明顯的影響，類似穩態的流動沸騰。然後我們定義單相沸騰、漸歇性沸騰、完全沸騰。除此之外，在高振盪振幅、長週期的情況下對其壁溫會造成更強烈的振盪情況發生。冷媒的飽和溫度和質通量對壁溫的振盪並無比較大的影響。壁溫、氣泡脫離直徑、脫離頻率和成核址密度震盪頻率和流量相同。當入口次冷度和流到間隙增加的情況下對其壁溫和熱傳會造成更強烈的振盪情況發生。在高次冷度的情況下，氣泡脫離直徑、脫離頻率和成核址密度都會跟著遞減。隨流量減少時振幅所造成成核址密度振盪遠大於氣泡脫離直徑和脫離頻率所以使的壁溫降低、熱傳變好。

An experiment is conducted here to investigate the heat transfer and associated bubble characteristics in time periodic flow boiling of refrigerant R-407C in a horizontal narrow annular duct subject to a time periodic mass flux oscillation. The mass flux oscillation is in the form of a triangular wave. Effects of the refrigerant mass flux oscillation, saturation temperature and inlet subcooling, gap size of the duct, and imposed heat flux on the temporal flow boiling heat transfer and bubble characteristics are examined. The bubble characteristics at the middle axial location of the duct are obtained from the flow visualization of the boiling flow, including the time variations of the bubble departure diameter and frequency and active nucleation site density. The present experiment is conducted for the mean refrigerant mass flux varied from 100 to 600 kg/m2s, the amplitude of the mass flux oscillation ΔG/G is fixed at 10, 20 and 30% with the period of the mass flux oscillation tp fixed at 20, 60 and 120 seconds. The mean refrigerant saturation temperature is set at 10 and 15 ℃ for the imposed heat flux varied from 0 to 45 kW/m2 and mean inlet liquid subcooling from 0 to 6 ℃. The gap of the duct is fixed at 1.0, 2.0 and 5.0 mm. The measured boiling heat transfer data are expressed in terms of the boiling curves and boiling heat transfer coefficient along with the variations of the heated wall temperature with time. The measured heat transfer data for the R-407C flow boiling 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 boiling heat transfer. Then, we present the data to elucidate the effect of the experimental parameters on the amplitude of Tw oscillation over a wide range of the imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation is stronger for a higher amplitude and a longer period of the mass flux oscillation. However, the mean saturated temperature of the refrigerant mass flux exhibits relatively weak effects on the Tw oscillation and the mean refrigerant mass flux and duct gap exert nonmonotonic effects on the amplitude of the Tw oscillation. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time and at the same frequency as the mass flux oscillation. Furthermore, in persistent boiling the resulting Tw oscillation is stronger for a longer period and a large amplitude of the refrigerant mass flux oscillation. We also note that increases in the inlet liquid subcooling and duct gap cause stronger oscillations in the boiling heat transfer coefficient and wall temperature oscillation. And a higher inlet liquid subcooling results in causes lower mean levels of nac ,dp and f. The effect of ΔG/G on nac oscillation is much stronger than on dp and f ooscillation causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling opposed, to that in the single-phase flow. But they are only slightly affected by the period of the mass flux oscillation. However, a small time lag in the Tw oscillation is also noted.