The Character Of Bubble Growth And Bubble Detachment And The Effect Of Bubble Behavior On Pressure Drop In Narrow Channel

The narrow channel is characterised by compact structure and enhancing heat transfer, etc. It is wildly used in power industry, chemical industry, petroleum industry, etc. Especially in the thermal-hydraulic analysis of nuclear reactor, it is significant to investigate the bubble growth, the bubble detachment, and the effect of bubble behavior on pressure drop in the narrow channels within the reactor core. The effect of channel dimension on bubble dynamics is considerable. It is very necessary to find out the nature of bubble dynamics within a narrow channel in order to understand the heat transfer and pressure drop in narrow channel profoundly. In this study, visual investigation on flow boiling in a vertical rectangular narrow channel (2 mm) was carried out under different working conditions. The purpose was to probe the bubble growth, the bubble detachment and the effect of bubble behavior on flow boiling pressure drop, and to propose models for predictions of the bubble growth, the bubble detachment diameter and the pressure drop based on experimental and theoretical analysis. The results will be valuable in engineering practice.According to this study, it was found that the effect of the heat flux and the mass flux on the bubble growth and the bubble detachment is very considerable, and it was found that the variation of bubble growth rate and bubble size with heat flux and mass flux is not monotonous. The character of bubble dynamics is affected by system pressure significantly. With increasing system pressure, the bubble growth rate slows down, and the bubble size minishes, and it was found that the bubble detachment time shortens significantly. For the mode of bubble growing, the bubble is at the nucleation site when bubble is growing under lower system pressure, and the growth time is less than 2 ms. While under higher system pressure, the time for bubble growing at the nucleation site is less than 0.3 ms, and the major bubble growth is accompanied with bubble sliding along the heating wall.According to the analysis on dimensionless bubble growth curves obtained under lower system pressure, it was found that the bubble growth curves are linear when dimensionless bubble growth time t+ < 0.3, while they are power law when t+ > 0.3. In this study, a dual curves model was proposed for the prediction of bubble growth. The slop of linear curve, kl, is with relative great fluctuation, while the coefficients of power law, kp and n, keep constants as 1 and 0.25, respectively. The dual curves model can predict the experimental data with an uncertainty less than±15%.After non-dimensionalizing the bubble radius and the bubble growth time by Laplace number and time factor,ξ, respectively, it was found that the dimensionless bubble growth in higher system pressure can be predicted by power law model. The predicted result of power law model is with an uncertainty of±16% when comparing with experimental result.In this study, a model for prediction of bubble contact diameter was proposed. It was found that the bubble contact diameter is dependent on dimensionless bubble growth rate, dR+/dt+, and dimensionless bubble radius, R+, strongly. The coefficients of this model, aw and nw, are determined according to experimental data, and the uncertainty of predicted result is less than±20%.In this study, a bubble detachment diameter predicting model was built up based on a comprehensive force analysis carried out on a growing bubble. This model takes all forces acting on a bubble into account. A correlation relative to the bubble contact diameter was proposed to evaluate the asymmetrical bubble growth force. It was found that the surface tension force has substantial contribution to the bubble detachment. This model is credible for upward flow, downward flow and horizontal flow. It can achieve an uncertainty less than±10% when comparing with the authors'experiment data. The predicted result also is with an uncertainty less than±15% when comparing with the experimental result from literature.A pressure drop model considering the effect of bubble behavior was proposed in this study. Based on the visual investigation on bubble behavior in the vertical rectangular narrow channel, the friction pressure drop was separated as the single phase flow friction pressure drop, the full developed subcooling boiling flow friction pressure drop and the saturated boiling flow friction pressure drop. According to the bubble behavior in the narrow channel, the Saha-Zuber model for real vapor quality and the Zuber-Findlay model for void fraction were calibrated for application in a narrow channel. The pressure drop model can reach an uncertainty less than±10% when comparing the predicted result with the experimental result obtained in this study.According to the investigation on the effect of bubble behavior on pressure drop in the narrow channel, the relationship between the average frictional multipliers,φa2, lo,sub andφa2, lo,sat, for full developed subcooling flow boiling and saturated flow boiling respectively, gravitational pressure drops dimensionless factor,φΔpg, and accelerational pressure drop dimensionless factor,φΔpa, and the outlet void fraction, ?out, were obtained in this study. It was found thatφΔpg decreases linearly with increasing ?out; 2φa, lo,sub,φa2, lo,sat andφΔpa increase with increasing ?out and increase dramatically when ?out is greater than 0.7.