The time varying saturated flow boiling heat transfer characteristics of R-410A in the annular duct resulting from the imposed temporal refrigerant mass flux oscillation are illustrated by presenting the time variations of the space-average heated surface temperature Tw and boiling heat transfer coefficient hr
ΔG/G
at the middle axial location in the statistical state. For the limiting case of stable flow boiling ( =0) the measured heated surface temperature at z=80 mm for various q are essentially steady (Fig. 4.8). Data for variousTsat, ΔG/G, tp are shown in Figs. 4.9-4.17 for various imposed heat fluxes and mean mass fluxes. The results presented here and from the flow visualization indicate that at low imposed heat flux the single-phase liquid flow prevails in the duct and the heated
surface temperature oscillates significantly with time for a large ΔG/G. At a high imposed heat flux persistent boiling dominates. But, the heated surface temperature oscillation does not become stronger at increasing heat flux. For the in-between intermediate imposed heat flux we observe boiling activity only in a certain fraction of the periodic cycle. Otherwise, the flow is in single-phase state. Thus at this intermediate heat flux we have intermittent boiling in the flow. A close inspection of these data reveals that in the single-phase flow in the first half of the periodic cycle the heated surface temperature increases with time as the imposed mass flux decreases with time. But an opposite trend is noted in the flow dominated by the persistent boiling in which the heated surface temperature decreases as the imposed mass flux is lowered. This phenomenon is somewhat unusual and will be examined in detail later when the quantitative data for the bubble characteristics are analyzed. Note that the temporal oscillation of the heated surface temperature is also periodic in time and is at the same frequency as the mass flux oscillation. The saturation temperature of the refrigerant exhibits relatively weak effects on the Tw oscillation.
Moveover, the Tw
The appearance of the intermittent boiling in the flow needs to be examined closely.
Based on the detailed flow observation, we note that at the imposed heat flux around that for the onset of stable flow boiling bubbles start to nucleate from the heated surface when the refrigerant mass flux is lowered to a certain level in the first half of the periodic cycle and more bubbles nucleate from the surface for a further reduction in the refrigerant mass flux. As a result of that, the heated surface temperature becomes lower. This trend is reversed when the mass flux increases in the second half of the periodic cycle. Specifically, at a certain high level of the mass flux the bubble nucleation stops and the flow is in single-phase state. Obviously, the single-phase flow prevails for a continuing rise in the mass flux. The above processes repeatedly occur forming the intermittent boiling. For the imposed heat flux well below and well above the onset heat flux of stable flow boiling, apparently the duct is dominated respectively by the single-phase liquid flow and persistent boiling. The time instants for the start and stop of the bubble nucleation are marked on the Tw curves for the cases with the intermittent boiling.
oscillation lags slightly behind the mass flux oscillation due to the thermal inertia of the heated pipe wall.
The corresponding time variations of the space-average flow boiling heat transfer coefficient at the middle axial location affected by the refrigerant mass flux oscillation are
shown in Figs. 4.18 – 4.26. The results manifest that the flow boiling heat transfer coefficients also oscillate periodically in time and at the same frequency as the G oscillation. For a longer period and larger amplitude of the mass flux oscillation, the boiling heat transfer coefficients oscillate stronger. For some increase in the imposed heat flux nucleate boiling persists over the entire period of the cycle and we have persistent flow boiling in the duct. It is of interest to note that at this higher q in the first half of the cycle in which G decrease with time, Tw also decrease with time, suggesting that the flow boiling heat transfer over the heated surface is better at a lower G. This trend is opposite to that for the single-phase convection. The results also show that the amplitude of hr
oscillation is not affected by the imposed heat flux in the persistent boiling to a noticeable degree. Moreover, the oscillation in hralso lags slightly behind the mass flux oscillation. It is further noted that in the persistent boiling the amplitude of the hr oscillation is only slightly affected by the heat flux for givenG,ΔG/G tp andTsat.
Finally, we move further to present the data in Fig. 4.27 to elucidate the effects of the experimental parameters on the amplitude of the Tw oscillation over a wide range of the imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results in Figs. 4.27(a) and (b) indicate that for all boiling flow regimes the Tw oscillation is stronger for a higher amplitude and a longer period of the mass flux oscillation. Particularly, the Tw oscillation becomes much stronger for tp raised from 20 seconds to 60 seconds. At the low tp of 20 seconds, which is only slightly longer than the time constant of the present system, the Tw oscillation is rather weak (Fig. 4.27(b)), reflecting that the heated wall is unable to respond instantly to the fast mass flux oscillation for a tp of 20 seconds. But the mean refrigerant mass flux exhibits a nonmonotonic effect on the amplitude of the Tw oscillation, as evident from the data in Fig. 4.27(c). The data given in Fig. 4.27(c) also indicate that in the single-phase flow the oscillation amplitude of Tw increases with the imposed heat flux for a given . Note that when the intermittent boiling appears the Tw oscillation stars to weaken substantially with the increase in the imposed heat flux. At a certain higher q but still in the intermittent boiling regime the Tw oscillation decays to a minimum point and then a further increase in q causes Tw to oscillate in a larger amplitude. We note that as the bubble nucleation starts to appear in the single-phase flow the Tw oscillation is weakened, and this trend continues until the effect of the boiling flow dominates over the single-phase flow for a rise in q to a certain level.
ΔG/G
Then, the Tw oscillation gets stronger for a higher q. For a certain higher q the Tw oscillation decays at increasing imposed heat flux for lower mean mass fluxes and higher saturated temperatures (Figs. 4.27(c) and (d)). Specifically, at intermediate imposed heat flux for q ranging from 12 to 45 kW/m2 in the intermittent and persistent boiling Tw
oscillates in a smaller amplitude at a lower . However, the saturated temperature of the refrigerant exhibits weaker effects on the Tw oscillation shown in Fig. 4.27(d).