Transient Bubble Characteristics in Saturated Flow Boiling

To elucidate the transient saturated flow boiling heat transfer characteristics, the data for the bubble characteristics of FC-72 obtained from the present flow visualization are examined in the following. The top views of the boiling flow in a small region around the geometric center of the heated surface for various coolant mass fluxes and imposed heat fluxes are shown in Figs. 4.43 - 4.67 for the transient saturated flow boiling. At first, the bubble characteristics for the limiting cases of constant mass fluxes are illustrated by the photos in Figure 4.43. It is noted in the flow visualization and the results in Figure 4.43 that the vapor bubbles begin to appear as the heated surface temperature exceeds the boiling incipient superheat. In the beginning, tiny bubbles are observed in the active nucleation sites. The bubbles grow and then detach from the heated surface with certain mean bubble departure diameters.

As the imposed heat flux increases, more bubbles are generated on more active nucleation sites and more bubbles detach from the heated surface. Besides, the detached bubbles tend to merge into larger bubbles. Note that the large bubbles become distorted and elongated as they slide on the heating surface. Moreover, at a higher mass flux the bubbles are smaller and the bubble coalescence is less significant.

Next, the bubble characteristics in the transient flow boiling are illustrated by presenting the photos of the boiling flow at eight selected time instants in a typical periodic cycle in Figures 4.44 – 4.67. In these figures the symbol “ t=to ” signifies the time instant at which the instantaneous mass flux is at the mean level and starts to increase with time. The results indicate that for a given imposed heat flux and fixed mean level, amplitude and period of the mass flux oscillation the bubbles get smaller and become more dispersed in the period the instantaneous mass flux increases. The opposite processes take place when the instantaneous mass flux decreases with time.

These changes of the bubble characteristics with the instantaneous mass flux become more significant for an increase in the amplitude of the mass flux oscillation (Figures 4.44 and 4.56 and Figures 4.46 and 4.58). It is of interest to note that the mean level of the mass flux oscillation exhibits significant influences on the bubble characteristics in the transient flow boiling, as evident from comparing the results in Figures 4.44 and 4.46. Besides, the bubble characteristics are only affected slightly by the period of the mass flux oscillation for the small mass flux oscillation amplitude of 5%. But for the large amplitude of the mass flux oscillation of 10% the bubbles are larger for the cases with longer period of the mass flux oscillation (Figures 4.56 and 4.60).

To quantify the bubble characteristics, the measured data for the time variations

site density in a typical periodic cycle are given in Figs. 4.68 – 4.94 for various mean coolant mass fluxes, amplitudes and periods of the coolant mass flux oscillations, and imposed heat fluxes.

The results in Fig. 4.68(a) indicate that the mean size of the bubbles departing from the copper plate is somewhat smaller for the mass flux raised from 300 to 400 kg/m²s in the stable flow boiling. It reflects the fact that the coolant at a higher mass flux and hence at a higher speed tends to sweep the bubbles more quickly away from the heating surface. Now as the coolant mass flux oscillates, the bubble departure diameter varies significantly with time (Figures 4.68(b) - (c)). More specifically, the size of the departing bubbles decreases in the first and fourth quarters of the periodic cycle in which the instantaneous mass flux increases with time. While in the second and third quarters of the cycle an opposite process is noted since the instantaneous mass flux decreases with time. Besides, at a higher imposed heat flux the departing bubbles are larger. Similar trend is noted in Figures 4.69 – 4.71. Comparing the results in Figure 4.68 with Figure 4.69 and Figure 4.70 with Figure 4.71 indicates that at the larger amplitude of the mass flux oscillation the effects of the imposed heat flux on the bubble departure diameter are somewhat smaller. The results in Figures 4.72 – 4.75 indicate that the bubble departure diameters are only affected slightly by the period of the mass flux oscillation.

Next, the data for the variations of the space-average bubble departure frequency with time for various cases are shown in Figures 4.77 – 4.80. In the stable flow boiling the increase in the bubble departure frequency with the imposed heat flux and coolant mass flux is clearly seen. The increase of f with G is ascribed again to the higher drag on the bubbles still attaching to the heated surface by the liquid coolant moving at a higher speed for a higher G. This, in turn, causes an earlier departure of the bubbles from the surface, resulting in a higher departure frequency. For an

oscillation mass flux the bubbles depart from the heated surface at an increasing rate in the first and fourth quarters of the periodic cycle in which the coolant mass flux rises with time. Apparently, in the second and third quarters of the cycle in which G decreases the bubble departing rate reduces. It should be pointed out that the time variations of the bubble departure frequency are somewhat milder when compared with the bubble departure diameter. Figures 4.81 – 4.84 indicate that the bubble departure frequencies are only affected slightly by the period of the mass flux oscillation. The results shown in Figure 4.85 manifest that the mean level of the mass flux oscillation noticeably affects the bubble departure frequency.

Finally, the space-average active nucleation site density on the heated surface affected by the coolant mass flux oscillation is illustrated in Figures 4.86 – 4.89. The results in Figure 4.86(a) indicate that in stable flow boiling the active nucleation site density increases substantially with the imposed heat flux. But the increase is rather mild for the mass flux raised from 300 to 400 kg/m²s. Note that in transient flow boiling the active nucleation site density decreases with time in the first and fourth quarters of the periodic cycle in which G increases. The reverse process appears in the second and third quarters of the cycle in which the coolant flow slows down. At the higher amplitude of the mass flux oscillation and at higher imposed heat flux the temporal variations of Nac is stronger (Figures 4.88 and 4.89). The effects of the period of the mass flux oscillation on the time variations of Nac are rather weak (Figures 4.90 – 4.93). The amplitude of the Nac oscillation is slighter higher for a larger amplitude of the mass flux oscillation (Figure 4.94).