A good representation for the change in the mean bubble diameter with respect to the experimental parameters as illustrated in Table 3.6 for the high and low temperature saturated and supersaturated tests, HPT_FR II, III, V & VI, and their relevant experimental uncertainties, are illustrated in Fig. 4.44 The resultant bubble ratios measured at the five focal planes across the pipe depth are given in Fig. 4.45.
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Fig. 4.44: Mean bubble size ratios Rx/Ro for saturated and supersaturated tests
(Experiment: HPT_FR II & V (Top) and supersaturated tests, HPT_FR III & VI (Bottom) in Table 3.6).
Through the consideration of the resultant experimental errors, the bubble size ratios illustrated through the charts as in Fig. 4.44, suggest that at bulk fluid saturation and super saturation conditions, bubble size ratios close to unity are expected. Hence, minimal bubble dissolution or growth was measured through the bulk fluid system flow line saturation range of 1 to 1.1. Furthermore, in line with the observations done in the bubble dissolution investigation, no measurable trends are evident with a change in the bulk fluid temperature. The measured bubble size ratios at saturation conditions (ratio of 1) resulted to be consistently marginally less than unity, hence suggesting a minimal degree of bubble
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dissolution. Such an effect can be attributed to a limited bubble breakage hence resulting in a reduction in the mean diameter between sight glasses HSG1&2. Furthermore, this phenomenon could be attributed to the tolerance range of the
TGM system. A direct comparison of these results cannot be made with existing
literature sources due to the minimal consideration given by literature to similar physical scenarios. However, the findings of the present study tend to be in agreement with those reported by Epstein and Plesset (1950), who stated that at saturation conditions, an isolated stationary bubble is expected to be stable against diffusion, but will still result in dissolution over time due to the effects of surface tension.
Bubble growth at bulk fluid super saturation conditions of 1.1, is insignificant, particularly when compared to the bubble dissolution measured in under saturated bulk fluid conditions. Hence, the results of the present study suggest that bubble growth due to gas diffusion from the bulk fluid into free bubbles in turbulent bubbly flow is minimal at low super saturation levels. The observed phenomena could be attributed to the slower bubble growth process particularly when considering the limited time range for bubbles to flow between sight glasses HSG1&2, with a maximum of 6.9 seconds. In their numerical modelling for bubble growth, Sun and Beckermann (2010) reported an increase in radius in a square root of time fashion for bulk fluid super saturation ratios of 1.1 and 1.2.
A reference to the theory of bubble nucleation and growth for isolated stationary bubbles in supersaturated conditions as discussed in Section 4.4 suggests that bubble growth is diffusion controlled. Such views were reported by Epstein and Plesset (1950), Liebermann (1957) and Cable and Frade (1988). A number of studies have been reported on the numerical simulation of bubble growth in two- phase bubbly flows. However, the open literature gives little consideration to related experimental studies. In their numerical investigation into bubble growth in liquids and melts with super saturation conditions, Arefmanesh et al. (1992) and Sun and Beckermann (2010) stated that the bubble growth process is in general complicated, involving simultaneous mass, momentum and energy transfer between the expanding bubble and the fluid surrounding it. Arefmanesh et al. (1992) reported that due to these complexities, there is no known analytical solution to predict bubble growth under general conditions. Similar conclusions
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were made by Payvar (1987) and Shafi and Flumerfelt (1997). The latter reported that bubble growth numerical solutions are arbitrary as bubble growth dynamics could be dependent on a combination of complex physical conditions, particularly in turbulent flow conditions as is the case with the present study. Hence, our results suggest that even though the bubble nucleation and growth at the heat exchanger wall is significant at similar saturation ratios, the same cannot be said for the free bubbles in bubbly turbulent flow as typical in the system flow line. Therefore, the growth of bubbles at the primary heat exchanger wall can be attributed to the presence of nucleation cavities which are not present in the bulk fluid. This confirms the phenomenon of heterogeneous nucleation at the primary heat exchanger wall as defined by Jones at al. (1999a). Hence, bubble growth due to mass diffusion for free bubbles in a turbulent flow can be considered to be negligible over a horizontal pipe distance of 2.3 m at the system flow line.
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Fig. 4.45: Bubble size ratios Rx/Ro measured at intervals along the pipe depth for saturated tests (Experiments: HPT_FR II (Top) and HPT_FR V (Bottom) in Table 3.6).
Fig. 4.46: Bubble size ratios Rx/Ro measured at intervals along the pipe depth for super saturated tests (Experiments: HPT_FR III (Top) and HPT_FR VI (Bottom) in Table 3.6).
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Cable (1967) reported that it is far easier to obtain a reasonable agreement between theory and experiments for dissolving bubbles in under saturated solutions when compared to bubbles growing in super saturated solutions. Similarly, through the investigation of air bubble growth by rectified diffusion, Crum (1977) reported that as the surface tension was reduced as is the case at elevated bulk fluid temperatures, the observed and predicted bubble growth rates differ significantly. This further emphasizes the complexity of the physical scenario characterizing the present study and the inherent difficulties in fully understanding the mechanism leading to bubble behaviour. Figs. 4.45 and 4.46 illustrate the measured bubble ratios across the pipe depth together with the relevant experimental uncertainties. Through a consideration of the experimental uncertainties in the measured data, the charts do not suggest particular trends, hence implying a uniform bubble behaviour across the pipe section at saturated and super saturated conditions.