Numerical Analysis of Critical Heat Flux for Vertically Upward and Downward Flows in Circular Pipe Sections

Abstract

Two-phase flows are encountered commonly in the heat transfer equipment used in power plants, chemical and nuclear industry. To improve the efficiency and to reduce the size of heat transfer equipment, it is necessary to have multiple passes where the two-phase flow travels in upward and downward directions. However, this flow pattern would bring in additional risks of premature tube burnout and flow instabilities, especially when the flow travels in downward direction. The liquid momentum and buoyancy forces oppose each other, resulting in a more complex flow. Testing of the heat transfer equipment is expensive and time consuming. An alternative approach to optimize the design and to reduce the number of experiments is to use validated numerical tools. From the literature review, it was found that there were several investigations carried out numerically and validated with experiments for both upward and downward flows at high pressures. Only a few investigations listed the difficulties associated with the convergence of the numerical models at low pressures and low mass fluxes. But, none of the papers reported the definition of low pressure and low mass flux, where numerical convergence would be a challenge. In the present investigation, a total of 112 numerical simulations were carried out for fixed geometry and process conditions for both upward and downward flows using commercially available numerical tool ANSYS FLUENT to understand the numerical challenges for different pressures. The numerical models were validated with the available experimental data in open literature before extending to current simulations. The pressures were varied between atmospheric pressure to 7.01 MPa, and the degree of inlet sub-cooling was varied between 10 and 40 °C lower than the saturation temperature at corresponding pressures. The current results showed that the convergence was difficult with pressures up to 1.5 MPa or less and for both upward and downward flows.