| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 97 === 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.
|
|---|
