Flow Instability and Its Mitigation In Supercritical Co2 Based Natural Circulation Loops: Numerical and Experimental Study

Abstract

A natural circulation loop (NCL) is a thermal energy transport system in which circu- lation solely occurs due to density gradient from a high-temperature source to a low- temperature sink without using any prime mover. Due to thermal imbalance in NCL, fluid flow oftenly subjects to instability in the form of oscillatory behaviour and flow reversal which may lead to catastrophic incidences. The underlying physics of instabil- ity for supercritical CO2 based NCL is complex. Hence, numerical and experimental investigations are carried out for supercritical CO2 based NCLs focus on the flow insta- bility and its mitigation. Three-dimensional computational fluid dynamics (CFD) simulation for super- critical CO2 based NCL is carried out to explore the effects of pressure and heat inputs on instability and determine the possible cause of its occurrence. Investigation shows that for supercritical CO2, there is a threshold point that decides the nature of flow. A heat input lower than a threshold value causes repetitive-reversal flow, while at higher heat input, the flow changes to stable or unidirectional flow. With an increase in heat input, the system attains stability for a given operating pressure. In addition, a possible mechanism for continuous flow oscillation and measurement of instability with differ- ent pressure in unstable loops is also proposed. The novelty of this investigation emphasizes the design of a modified Tesla type valve and its integration in the loop to assist the unidirectional flow of loop fluid, and in turn, reduces the instability. Results show that the use of a single modified Tesla valve leads to better stabilization for all supercritical pressures and heat inputs. It is also found that a loop with a single Tesla mitigates the temperature and velocity oscillations with a marginal reduction (8%) in the heat transfer performance. However, the use of a single modified Tesla type valve in NCL is not capable of mitigating the instability in the case of low heat inputs with operating pressures far away from the pseudocritical point. NCL integrated with two modified Tesla type valves is used to promote the uni- directional circulatory movement of loop fluid and to decrease the magnitude of insta- bility. Results are obtained with supercritical CO2 based twin Tesla-NCL and compared with regular-NCL and single Tesla NCL at different heat inputs and operating pressures. It is found that an increase in the number of Tesla valves, mitigates the instabilities in the NCL operated away from pseudo-critical region at lower heat inputs. However, the use of twin Tesla type valves in NCL drops the heat transfer capability by 15% com- pared to regular NCL. To validate the simulation results and check the practical feasibility, an experi- mental setup of NCL integrated with a modified Tesla valve is designed and developed. Experiments are carried out to comprehend the instability in supercritical CO2 based NCL. Experimental results show that the unidirectional fluid flow circulation can be achieved in the loop with the Tesla valve, which makes it an efficient technique to com- bat instability.