Development of bubble dynamic parameter models in subcooled flow boiling of saline solution

Abstract

Concentrated saline solutions are inevitably used as coolants in the industry heat exchangers. From the aspect of simulation prediction, the closure models for RPI model, including the models of bubble departure diameter (Dw), departure frequency (f) and active nucleation site density (Nw), are of great importance for the overall success of heat transfer calculation. However, the applicability of the existing closure models for the nucleate flow boiling of saline solutions is yet to be answered. In this paper, the experiments of subcooled flow boiling of the NaCl solutions with concentration range of 0%–6% were conducted. The experimental data of the three bubble parameters, Dw, f and Nw, were obtained under different operating conditions, making up for the shortage of the bubble parameter data of saline solution in literature. The impacts of key influencing parameters for each of the three bubble parameters were discussed. For model development, the prediction performance of several existing representative Dw, f and Nw models on saline solution were studied. The results indicate that the available models for Dw, f and Nw show rather large deviations for saline solutions. Therefore, new models for Dw, f and Nw were developed according to the correlation method and the influencing parameter analyses. The parameters and dimensionless groups, which can be used to characterize the variations of Nw, Dw and f, were determined. The dimensionless parameters including Boiling number (Bo), density ratio (ρ⁎), dimensionless surface tension, Prandtl number and a suggested dimensionless temperature (ΛT), can be used to characterize the variation of Dw. And the dimensionless groups of Bo, ρ⁎, ΛT, and the non-dimensional bubble departure diameter, can be used to correlating with the non-dimensional departure frequency. As for Nw, the wall superheat and solution concentration are the most critical factors for model correlating. The new correlations for predicting the Dw, f and Nw during subcooled flow boiling in both the pure water and saline solutions were built and validated.