Abstract: Boiling heat exchangers are widely used in petroleum, chemical and other industrial fields. However, the current assumption of the onset entrainment fraction of annular flow in the model for critical heat flux lacks theoretical basis. Based on the relevant experimental results and theoretical understanding of churn flow, the effects of the onset entrainment fraction on the accuracy of critical thermal load prediction is analyzed systematically. The results indicate that the larger droplet size and larger momentum within the churn flow zone have a greater impact on the downstream annular flow field characteristics, especially when the mass flow rate is higher, the selection of the onset entrainment fraction has a drastic effect on the critical heat load prediction accuracy and outlet dryness. Therefore, this paper proposes the improvement idea of advancing the onset calculation of the critical heat load prediction model from the onset of annular flow to that of the churn flow. The improved liquid film flow model not only provides higher prediction accuracy, but also is more in accord with the actual physical process.

Key words: churn flow, liquid film flow model, critical thermal load, droplet entrainment