Computational and Experimental Transient Assessment of Supercritical Co2 Based Natural Circulation Loops

Abstract

Natural circulation loop is a geometrically simple heat transporting device in which f luid flow establishes due to density gradient across the loop, which is induced as a result of temperature difference in a loop. It is a fundamentally passive system in which the buoyancy force launches fluid circulation by surpassing all the resistive force in the system. This distinctive competence of NCL makes its operation and maintenance highly economical and also highly reliable in terms of safety of men and machinery. Since the inception of natural circulation loops in the field of engineering applications, water and the brine solutions are the most accredited and extensively used working fluids due to its abundance and also its favorable eco-friendly properties. However, extensive research on CO2 applications revealed that it possesses almost all the properties which are essentially required to meet the qualification criteria of heat transport system. In natural circulation loops, supercritical CO2 is gaining more popularity due to its superior transport properties compared to all natural as well as synthetic fluids. Hence detailed analysis of heat transport capabilities and stability behaviour of CO2 in natural circulation loops is very vital. The loop fluid flow dynamics in NCLs for any impetuous changes in operating parameters is of critical importance and hence finding an elegant solution for these dynamics is very much essential to incorporate it in specific applications. Inability of systems to sustain themselves against small perturbations for which physical system is subjected to is considered as instability. NCL has an inherent problem of instability caused by the combined effect of buoyancy, friction and inertial forces at varying operating conditions. Further, due to non-linearity of natural convection process, NCLs are prone to several kinds of instabilities. This instability in fluid flow creates flow oscillation, chaotic non-linear dynamic behaviour and flow reversal. The primary objective of the present work is to accomplish a comparative study on the dynamic performance among various configurations of NCL. Toexplore the instability phenomenon in sCO2 based NCLs which are configured with various types of heat sources i.e., heater, heat exchanger and isothermal wall at the source with a cold heat exchanger (CHX) at sink have been studied by using 2-D as well as 3-D computational fluid dynamics (CFD) simulations. Further, experimental work has been carried out over a range of supercritical pressures (80 bar to100 bar) and heat inputs (250 W to 2000 W) at source. Transient results for mass flow rate, temperature and velocity variation for different operating pressures and temperatures. Obtained results are also validated with the published experimental and numerical data and found in good agreement. Two-dimensional computational fluid dynamics simulation results show the higher instabilities for hot heat-exchanger loop (HHX-CHX) than an isothermal heater and heat-exchanger loop (ISO-CHX). With an increase in heat input, loops attain stability at a faster rate for a given operating pressure. At a lower heat input both the loops show bidirectional fluctuation, whereas it is unidirectional at high heat input. Nusselt number shows that the HHX-CHX loop’s heat transfer capability is more compared to ISO-CHX loops. 3-D numerical analysis is conducted to get moderately accurate transient and stability behavior of sCO2 based NCLs which are configured with three different types of heat sources i.e., heater, hot heat exchanger (HHX) and isothermal wall (ISO) at the source. Three-dimensional unsteady conservation equations (mass, momentum and energy equations) are solved to assess the transient and stability behavior of sCO2 mass flow rate, temperature and velocity as a function of time. Effect of pressure on sCO2 mass flow rate is also assessed to compare its stability behavior in all the configurations. Performance of the loop fluid has been studied by varying the quantum of heat inputs at source by keeping constant mass flow rate and temperature of cooling media at sink. It is observed that irrespective of the boundary condition at source, the loop experiences some initial disturbances or instabilities before reaching steady state. However, time needed for the attainment of steady state varies with the nature of heat input employed at source. Results show a higher magnitude of instabilities in Heater-CHX loop than HHX-CHX and ISO-CHXloops, and these instabilities mitigate at a faster rate in the ISO-CHX loop at all levels of heat input and operating pressure of the loop. It is also observed that as loop fluid operating pressure increases, the instability of the system decreases and the loop fluid mass flow rate increases. Further, the Nusselt number in the case of Heater-CHX loop is more compared to other loops because of its high turbulent kinetic energy. To assess the actual behaviour of the sCO2 based Natural circulation, experimental setup is designed, fabricated and test is conducted at various pressures and temperatures. The practical behaviour shows that instability is inevitable at lower pressure and temperature and system approaches stable conditions at higher pressure and temperature due to stronger and dominating buoyancy forces. The simulation results also revealed the similar behaviour at lower as well as at higher operating conditions.