Visualization study on the mechanisms of net vapor generation in water subcooled flow boiling under moderate pressure conditions
Introduction
Prediction of the axial void fraction profile in subcooled flow boiling is of considerable practical importance in evaluating the two-phase flow instabilities in boiling channels and the neutron moderation and fuel burnup in nuclear reactor cores [1], [2], [3], [4], [5]. In a boiling channel, two-phase flow region is commenced at the point of the onset of nucleate boiling (ONB) where the wall temperature sufficiently exceeds the saturation temperature to permit the first bubbles to appear on the heated surface [1]. It is however known that the void fraction remains low within a certain region and it eventually starts to rise vigorously at the point further downstream from the point of ONB. Since the void faction just downstream from the ONB point is negligibly small in many cases, inception of the vigorous increase of the void fraction is commonly regarded as the onset of significant void (OSV) or the net vapor generation (NVG). It is known that accurate evaluation of the point of NVG is particularly important in predicting the void fraction profile in the subcooled boiling region [6], [7].
In many models for the void fraction profile in subcooled flow boiling, the onset of NVG is associated with the behavior of vapor bubbles [7], [8], [9], [10], [11], [12]. For instance, Levy postulated that the NVG occurs when the sum of the buoyancy and frictional forces attempting to remove the bubble overcomes the surface tension force attempting to hold it on the heated surface [9]. Since the bubble behavior is of importance in evaluating the void fraction as well as the heat transfer [13], a number of visualization studies of subcooled flow boiling were also carried out so far in various configurations and different experimental conditions [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Although the behavior of vapor bubbles produced at the nucleation cavities is also dependent on the surface wettability [15], bubble lift-off from the heated surface followed by the condensation in subcooled bulk liquid has been observed in many experiments of water subcooled upward flow boiling particularly under low pressure conditions [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Whilst, the presence of bubbles sliding along the vertical heated surface for a long distance was also reported by several investigators [25], [26], [27]. The results of these visualization experiments are useful to correlate important parameters in subcooled flow boiling with fundamental values such as the mass flux and heat flux. For instance, Prodanovic et al. [17] and Zeitoun and Shoukri [21] developed empirical correlations for the maximum and detachment bubble diameters and the Sauter mean bubble diameter, respectively. Numerical analyses of the void fraction in the subcooled flow boiling using a drift-flux model [28] and a two-fluid model [29], [3], [30], [31] were also conducted. However, since the mechanisms of NVG have not been clarified sufficiently, fully empirical methods are commonly used to decide the onset of NVG [31].
In view of the present insufficient understanding of the mechanisms governing the bubble dynamics and the onset of NVG in subcooled flow boiling, the present authors also performed the visualization studies using a rather hydrophilic heated surface [32], [33]; in the experiments, filtrated and deionized water was used as the working fluid and the flow direction was vertical upward. Observation of the bubble behavior at ONB revealed that at elevated pressures, the bubbles slide along the heated surface after the departure from the nucleation site under the influence of the shear-induced lift force. Whilst, at low pressures close to the atmospheric pressure, they were lifted off the vertical heated surface immediately after the nucleation to collapse in the subcooled bulk liquid due to condensation. In consequence, bubble life-time at ONB was remarkably shorter in the low pressure experiments. It was discussed that the lift-off limit can be expressed in terms of the Jakob number since the distinct difference in the bubble behavior is mainly caused by the bubble growth rate after the nucleation [32]. The liquid subcooling was then parametrically changed to investigate the mechanisms causing the NVG. It was found that at low pressures, all the bubbles collapsed in the subcooled bulk liquid at the ONB but some bubbles could be reattached to the heated surface when the liquid subcooling was low enough. Since the bubbles slid along the heated surface for a long distance after the reattachment, the bubble life-time was significantly extended and consequently the vaporization rate could noticeably be greater than the condensation rate. It was concluded that the bubble reattachment to the heated surface is a key phenomenon to cause the NVG at low pressures [33]. Under the moderate pressure conditions, however, it was believed that different mechanisms are responsible for the onset of NVG since the bubbles are not lifted off the surface even at ONB. The mechanisms of NVG at elevated pressures are obviously of great importance from the engineering standpoint since most power plants are operated in high pressure conditions. Therefore, in the present work, series of experiments are conducted to explore the important mechanisms in causing the NVG in subcooled flow boiling under elevated pressure conditions.
Section snippets
Experimental description
The experimental apparatus is described only briefly since it was the same as that used in our previous studies [32], [33]. The schematic diagram of the experimental loop is depicted in Fig. 1a. The closed loop mainly consisted of the canned motor pump to circulate the working fluid, two 5 kW preheaters, test section, bypass line that was the auxiliary system to keep system pressure, steam separator, condenser and cooler. The two turbine flow meters measured the flow rates at the main and bypass
Experimental results
Snapshots obtained in each experimental run are displayed in Fig. 3a–d, in which the cross marks indicate the active nucleation sites. Important observation in these photos is that majority of the bubbles are located close to the vertical heated surface since most bubbles slid along the surface after the departure from the nucleation sites even after the condition of OSV was reached. It was also confirmed that bubbles did not stay at their nucleation sites in all the experimental conditions
Conclusion
The process of bubble generation in subcooled flow boiling was experimentally investigated through visualization using a high speed camera to explore the mechanisms of net vapor generation. The experiments were conducted under moderate pressure conditions and a rather hydrophilic surface was used as a heated surface. Important observations made in the present experiments are summarized as follows:
- (1)
At the onset of nucleate boiling, most bubbles departed from the nucleation sites immediately after
Acknowledgement
This work was supported by KAKENHI (No. 20360419).
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