Working regime identification for natural circulation loops by comparative thermalhydraulic analyses with three fluids under identical operating conditions

Computational investigation for comparative thermalhydraulic analyses of rectangular natural circulation loops is performed to propose a guideline for selecting the working fluid and nature of the loop, subcritical or supercritical, under identical levels of operating parameters like pressure, heating power and coolant temperature. A 3-d uniform-diameter loop geometry is developed with horizontal heating and cooling. Heating is provided in constant heat flux mode, whereas cooling is through a constant temperature sink. Due to favourable thermophysical properties and environmental conformity, water, CO2 and R134a are selected as possible working fluids. Operational parameters are set so as to have sub- to supercritical condition for CO2, supercritical for R134a and single-phase liquid for water. Mass flow rate for supercritical fluid rapidly increases with heater power, when the fluid is allowed to cross the pseudocritical point during its passage through the heater, and exhibits a maxima. Drastic fall in mass flow rate can be observed beyond the maxima, accompanied by a jump in maximum fluid temperature and a rapid decline in sink-side heat transfer coefficient. That can be identified as heat transfer deterioration in supercritical natural circulation loops, a highly undesirable situation from loop safety point of view. Allowable working range of heater power can be enhanced by increasing system pressure and decreasing sink temperature. For any specified set of operating conditions, CO2-based supercritical loops are better choice till the appearance of maxima, whereas single-phase water-based loops are preferable beyond that limit due to consistently-higher level of heat transfer coefficient. R134a can substitute water at higher powers owing to the lesser inventory requirement and similar level of maximum fluid temperature, but may suffer from chemical stability issues.