An Innovative Investigation on Fluid-to-Fluid Modeling of Post-Dryout Heat Transfer in Thermal Energy Systems

Dali Yu,Yijiang Fan,Chi Xu,Chongju Hu

doi:10.3389/fenrg.2021.819060

Abstract

Post-dryout heat transfer is an important thermal hydraulics phenomenon happening in the loss-of-coolant accident of water-cooled nuclear reactors and the off-design conditions of water-cooled ceramic breeder blanket in fusion reactors. It is necessary to research the flow and heat transfer mechanisms of post-dryout. To economically and comprehensively study on the post-dryout heat transfer characteristics, this work conducted fluid-to-fluid modeling research on water and R-134a data banks by using Buckingham dimensional analysis methodology. In addition, a new interpolation method was developed and verified to make the scaling between the two data banks possible. Finally, a scaling method of post-dryout heat transfer was proposed and assessed by using experimental data. The results showed that the obtained scaling method is reliable in certain conditions.

Highlights

1.1 BackgroundPost-dryout heat transfer phenomenon widely exists in both the loss-of-coolant accident (LOCA) of a pressurized water reactor (Yoo et al, 2020) and the off-design conditions of water-cooled ceramic breeder blanket in the Chinese Fusion Engineering Test Reactor (Cheng et al, 2020)
A post-dryout heat transfer region can be encountered once the contact between the liquid film and the heated surface cannot be maintained due to continuous liquid film depletion, and the liquid phase is only in the form of dispersed droplets or small fragments, which usually occurs at a void fraction greater than 80% (Groeneveld, 1973)
Based on the experience of theoretical investigations on postdryout heat transfer, the independent variables that determine the wall temperature in a post-dryout flow in a round tube can be described as below: The system describing variables: P, G, q′′w, Δi, LT, DT, and g,where P is the system pressure; G is the inlet mass flux; q′′w is the wall heat flux; Δi is the subcooling of the inlet flow and equals the enthalpy difference of the saturation liquid and the flow at inlet; LT is the heated length of the tube, DT is the inner diameter of the tube; and g is gravitational acceleration

Summary

Introduction

1.1 BackgroundPost-dryout heat transfer phenomenon widely exists in both the loss-of-coolant accident (LOCA) of a pressurized water reactor (Yoo et al, 2020) and the off-design conditions of water-cooled ceramic breeder blanket in the Chinese Fusion Engineering Test Reactor (Cheng et al, 2020). A post-dryout heat transfer region can be encountered once the contact between the liquid film and the heated surface cannot be maintained due to continuous liquid film depletion, and the liquid phase is only in the form of dispersed droplets or small fragments, which usually occurs at a void fraction greater than 80% (Groeneveld, 1973). A post-dryout region consists of a developing region (unstable film boiling) and a fully developed region (stable film boiling) (Yu et al, 2018). While the direct wall–droplets contact becomes less frequent, the vapor temperature and velocity structures and the droplets distribution over the cross-section of the flow are well rearranged, the flow develops into a relatively stable state, which is called fully developed post-dryout region, characterized by a stable mist flow pattern and no wall–droplets wet contact. Heat transfer in the full-range post-dryout regime involves various heat exchange paths among the vapor phase, droplets, and the heated wall, including 1) convective heat transfer from wall to vapor; 2) interfacial heat transfer from vapor to droplets; 3) contact heat

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