Investigation of flow instability using axially decreased power shape in parallel channels with water at supercritical pressure

Realizing the economic viability of SCWR, a GEN IV reactor, several research activities have been carried to address challenges associated with a system operated at supercritical pressures as a result of drastic changes in fluid properties at these supercritical pressures. These challenges include enhanced heat transfer EHT, deteriorated heat transfer HTD and flow instability among many others. The research activities mostly focused on CFD and experimental studies involving single tube due to the complexity of parallel channel flow and other non-circular flow geometries. Research in parallel channels is needed to address related supercritical heat transfer challenges and to provide more realistic information to the SCWR core design. This study investigated flow instability in parallel channels with water at supercritical pressures adopting axially decreased power shape ADPS. The effects of pressure, mass flow rate, and gravity on flow instability were investigated. Sensitivity analysis of some selected turbulence models and time steps were first carried out with the aim of selecting suitable turbulence model and time step for the numerical simulations. For the system operated at system pressure of 23 MPa, inlet temperatures from 180 degrees C to 360 degrees C, and system mass flow rates of 125 kg/h and 145 kg/h, the system stability decreases with inlet temperature at the high mass flow rate with only lower threshold as instability boundary, but there is a threshold power corresponding to a particular inlet temperature below which stability decreases and above which stability increases with inlet temperature for the low mass flow rate. The system stability increases with increase of system mass flow rate at low inlet temperatures, but decreases with increase of system mass flow rate at high inlet temperatures. With the increase of system pressure at 125 kg/h to 25 MPa, there is different threshold power with particular inlet temperature below which stability decreases and above which stability increases with inlet temperature. The system operated at high pressure is more stable than that operated at low pressure. The effect of stability of a system operated with or without gravity influence is similar to that of the system operated at low pressure or at high pressure respectively. The system operated without gravity influence is more stable than that operated with gravity influence. For the system operated at system pressure of 23 MPa, inlet temperatures from 180 degrees C to 260 degrees C, and system mass flow rates of 125 kg/h and 145 kg/h, the trends of the numerical results are in agreement with the trends of the experimental results. The obtained numerical instability boundary finding that the system is more stable at larger mass flow rate is the opposite of the corresponding experimental instability boundary finding. The numerical dynamics characteristics finding that the system is more stable at low mass flow rate contradicts the corresponding experimental dynamics characteristics finding that mass flow rate has less effect on flow instability. The numerical tool predicted quite close to the experimental results at larger mass flow rate. The numerical tool adopted largely under-predicted experimental amplitude and quite well predicted experimental period of the inlet mass flow oscillations. The adopted 3D numerical tool, STAR-CCM+ code could capture dynamics characteristics of the flow quite well and also predict flow instability in the parallel channels. However, there is evidence that the presence of heating structures in the geometrical model adopted may change the predicted behavior, as shown in previous works. More relevant experiments at supercritical pressures should be carried out for validation of numerical tools adopted for similar studies. (C) 2018 Elsevier Ltd. All rights reserved.