The effect of void fraction correlation and heat flux assumption on refrigerant charge inventory predictions

Ten void fraction correlations and four heat flux assumptions are evaluated for their effect on refrigerant charge inventory predictions. Comparisons between mass inventory predictions are made for condensers and evaporators over representative heat pump operating ranges of saturation temperature, mass quality, and mass flux. The choice of void fraction model is found to have a major effect on refrigerant inventory prediction. The maximum variation of predictions ranges from a factor of 10 for low-ambient, heating-mode evaporators to 4.2 for cooling-mode evaporators and 1.7 for high-ambient cooling-mode condensers assuming no subcooling. The correlations of Hughmark, Premoli, Tandon, and Baroczy are found to give the highest predictions and closest agreement to measured total system charge. The choice of heat flux assumption is shown to be insignificant for forced-flow evaporators and of secondary to possibly equal importance to choice of void fraction model for condensers. Implications for charge balancing, off-design and transient performance prediction, and unit reliability are discussed.