狹窄水平雙套管中R-410A冷媒流量震盪與可變功率之週期性流動沸騰研究

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。流道之間隙為2.0 mm。我們探討了冷媒質通量或熱功率振盪、周期、飽和溫度、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，冷媒平均質通量從300到500 kg/m2s，振幅為10,20 和30%，熱震盪振幅為10,30和50%，周期都分別為20、60、120s，平均冷媒飽和溫度為5, 10和15℃。 在第一部份探討流量震盪的影響。流量在振盪過程中，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。在流量震盪的時候，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。 在第二部份探討熱功率震盪的影響。熱量在震盪過程中，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，但是會有相當明顯的壁溫落後其熱量變化的時間，也就是熱量最高時，壁溫會在過一小段時間才到達最高。另外當熱量振盪的振幅和週期經過時間平均化之後的飽和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。在高振盪振幅、長週期的情況下對其壁溫氣泡脫離直徑、脫離頻率和成核址密度會造成更強烈的振盪情況發生。

An experiment is carried out here to investigate the heat transfer and associated bubble characteristics in time periodic flow boiling of refrigerant R-410A in a horizontal narrow annular duct subject to a time periodic mass flux or heat flux oscillation. Both the imposed mass flux and heat flux oscillations are in the form of triangular waves. Effects of the refrigerant mass flux oscillation, heat flux oscillation, and refrigerant saturated temperature 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 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% and the amplitude of the heat flux oscillation is fixed at 10, 30 and 50% of their respective mean levels and , and the period of the G or q oscillation is fixed at 20, 60 and 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃. The gap of the duct is fixed at 2.0 mm. The measured boiling heat transfer data are expressed in terms of the boiling curves and boiling heat transfer coefficients along with the time variations of the heated wall temperature. In the first part of the present study the measured heat transfer data for the R-410A flow boiling for a constant coolant mass flux are first compared with the time-average data for the flow subject to a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average boiling heat transfer coefficients. Then, we present the data to elucidate the effects 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 exhibits much weaker effects on the Tw oscillation and the mean refrigerant mass flux exerts 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, the oscillations of dp, f and nac are somewhat like triangular waves. In the first half of the cycle in which the mass flux decreases linear increases in dp and nac and linear decrease in f are found. The effect of on nac oscillation is much stronger than on dp and f oscillation 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. Besides, a small time lag in the Tw oscillation is also noted. In the second part of the present study the measured heat transfer data for the R-410A flow boiling for a constant heat flux are also first compared with the time-average data for a time periodic heat flux oscillation. This comparison shows that the time-average heat transfer coefficients are not affected by the time periodic heat flux oscillation to a significant degree. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over a wide range of the mean imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation gets stronger for a higher amplitude and a longer period of the imposed heat flux oscillation and for a higher mean imposed heat flux. Moreover, 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. The effects of the 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. When the mean imposed heat flux is close to the heat flux corresponding to that for the onset of stable flow boiling, intermittent flow boiling appears. A flow regime map and an empirical correlation are given to delineate the boundaries separating different boiling flow regimes in the annular duct subject to imposed heat flux oscillation. Furthermore, the bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time and at the same frequency as the heat flux oscillation. The results also show that the oscillations in dp, f and nac get larger for a long period and a larger amplitude of the impose heat flux oscillation and for a higher mean imposed heat flux. Furthermore, the bubbles become smaller and more dispersed after the time lag when the imposed heat flux decreases with time. The opposite processes take place at increasing heat flux.