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Bubble lift-off diameter is characterized as the size of a bubble rising from a wall, which is vital in the boundary condition of heat transfer model and interfacial area transport equation. In this paper, mechanistic force analysis was conducted to explore a predictive model for bubble lift-off diameter in a vertical channel of subcooled boiling flow. Specifically, the component of growth force normal to the wall and the shear lift force lead to the lift-off of a bubble on the vertical surface. Through force analysis, we found that bubble lift-off diameter is arranged to be related to wall superheat, latent heat, liquid velocity, fluid properties, bulk liquid subcooling, etc. To account for the contribution of the influencing factors, the dimensionless bubble lift-off diameter was correlated with dimensionless parameters, including the Prandtl number, the Reynolds number, the Jacob number, and dimensionless subcooling. The proposed correlation was assessed according to experimental data and the predictions showed good agreement with the data. [ABSTRACT FROM AUTHOR]

Highlights • The force balance approach and correlation approach are conducted to predict bubble departure diameter. • A new semi-empirical correlation for bubble departure diameter is proposed in this paper. • The effects of dimensionless parameters on bubble departure diameters are analyzed. Abstract According to the force balance analysis on a bubble, the bubble departure diameter in horizontal subcooled flow boiling is mainly influenced by quasi-steady drag force, surface tension force and bubble growth force while an additional force, buoyancy force, plays an important role in vertical flow boiling. In this paper, the effects of these forces can be concluded by a series of dimensionless parameters including density ratio of vapor and liquid, Prandtl number, Jacob number and bubble Reynolds number. Based on the different forces, two different characteristic lengths are adopted to non-dimensionlize bubble departure diameters in horizontal flow boiling and vertical flow boiling, respectively. Finally, the semi-empirical correlations for bubble departure diameters in both horizontal and vertical subcooled flow boiling are proposed in this paper based on the force balance analysis and available experimental data from literature. The predicted results using the present correlations agree fairly well with the experimentally measured values with a mean relative error of 19.72%. [ABSTRACT FROM AUTHOR]

Subcooled boiling flow is widely used for heat and mass transfer in industries such as nuclear reactor, electronic equipment, and spacecraft, where the bubble dynamics is essential for understanding the gas-liquid flow behavior. Current studies focused on the gas void fraction or bubble size distribution at a global scale, but the dynamics of single bubbles are still obscure, which inhibits the fundamental understanding of gas-liquid flow and the precise prediction of heat transfer. Here, we develop a numerical method that captures the motion and condensation of bubbles in subcooled boiling flow using Euler-Lagrange method, with a proposed model for spatiotemporal lift-off bubble distribution on the heated wall as boundary condition. Our numerical method is validated by the agreement between the calculated gas void fraction distributions with the existing experimental data in vertical subcooled boiling flow. We further investigate the distribution of bubble position and size in the tube and find that the bubble size distribution shows different modes at different radial positions. Analysis of the single bubble dynamics shows that small and large bubbles suffer from centrifugal and centripetal lift forces, and thus tend to accumulate at a radial position near the wall and near the tube center, respectively, which reveals the mechanism of bubble size distribution. Besides, we find that the centripetal wall lubrication force is essential for the path instability of small bubbles near the wall. These findings promote the fundamental understanding of the gas-liquid phase distribution in the subcooled boiling flow and provide a numerical strategy for the precise prediction of mass and heat transfer in subcooled boiling flow. [ABSTRACT FROM AUTHOR]

• The interfacial parameters in boiling flow were measured by four-sensor conductivity probe under static and vibration conditions. • The time-averaged local parameters under vibration conditions were analyzed. • The instaneous change of local void fraction was analyzed by adopting the phase partitioning method. • The additional forces induced by mechanical vibration were calculated and compared with each other. • The effect of vibration frequency on local void fraction was studied. In this paper, the subcooled boiling flow experiments in a vertical annulus were conducted to study the effect of vibration on local interfacial characteristics. The local interfacial parameters including void fraction, bubble velocity, interfacial area concentration and Sauter mean diameter were measured by four-sensor conductivity probe. The time-averaged local parameters under vibration conditions were firstly measured and analyzed. It is found that the effects of vibration on time-averaged void fraction and interfacial area concentration are very limited. Therefore, we have developed the phase partitioning method to obtain the instantaneous variation of void fraction for further investigation. The instantaneous void fraction along the radial direction varies periodically with the vibration phase. The additional forces induced by vibration are analyzed and compared with each other. Among them, the tangential force is the dominant, which can account for the change tendency of void fraction under vibration. Besides, the variations of void fraction depend on different working conditions. The change tendencies of void fraction near the inner wall and outer wall are opposite in relatively low averaged void fraction regime while those in relatively high averaged void fraction regime are consistent. [ABSTRACT FROM AUTHOR]