| Summary: | 博士 === 國立交通大學 === 機械工程學系 === 98 === Experiments have been conducted here to investigate how the imposed time periodic refrigerant flow rate or heat flux oscillation affects the saturated and subcooled flow boiling heat transfer and associated bubble characteristics for refrigerant R-134a in a horizontal narrow annular duct. Besides, the evaporation heat transfer of R-134a flow in the same duct are examined. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to measure the boiling heat transfer coefficient and to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In the study the gap of the duct is varied from 1.0 to 5.0 mm with the mean refrigerant mass flux, saturated temperature, imposed heat flux and mean vapor quality respectively ranging from 100 to 600 kg/m2s, 5 to 15℃, 0 to 45 kW/m2 and 0.05 to 0.95. The inlet subcooling is varied from 3 to 6℃. In particular, attention is focused on the time periodic flow boiling characteristics affected by the mean levels, amplitudes and periods of the flow rate or heat flux oscillation.
Some results have been obtained and are reported here. In the first part of the present study, experiments have been carried out to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on the saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics of R-134a in a horizontal narrow annular duct. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial time interval of each periodic cycle. But the intermittent boiling prevails in narrower ranges of the experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation is found to negligibly affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are noted to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting Tw oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in dp, f and nac are noted. Specifically, in the first half of the periodic cycle in which the mass flux decreases with time the departing bubbles are larger and the departure rate is lower but the active nucleation site density is higher. The opposite is the case in the second half of the cycle in which the mass flux increases. The effects of the mass flux oscillation on the departing bubble size and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling, opposing to that in the single-phase flow. But the bubble characteristics are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the Tw oscillation is also noted. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct.
Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film resulting from the refrigerant flow rate oscillation 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 present study, experiments are conducted to investigate how the imposed time periodic heat flux oscillation also in the form of nearly a triangular wave on the refrigerant R-134a saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The results also show that when the mean imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial interval of each periodic cycle. But the intermittent boiling appears in narrower ranges of experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the heat flux oscillation does not noticeably affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the persistent boiling the resulting oscillation amplitudes of the heated surface temperature, heat transfer coefficient and bubble parameters, such as dp, f and nac, get larger 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 Tw oscillation is noted. In the first half of the periodic cycle in which the heat flux decreases with time, after the time lag the heated wall temperature decreases with time, so does the bubble parameters. The opposite processes occur in the second half of the cycle in which q increases with time. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct.
Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and by the liquid film resulting from the heat flux oscillation 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 on the changes of the vapor quality and liquid film thickness in the evaporating flow.
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