Abstract
Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux.
Highlights
In the latest decades, refrigeration science has been an extensive field of research of new technologies to increase the efficiency and cooling capacity of new systems
To increase the cooling performance and find a higher heat removal potential than that offered by air cooling or single-phase liquid cooling, two-phase liquid cooling appears as a very interesting solution
The boiling effect produces a remarkable increase in the convection coefficients as the heat flux increases in the fully developed boiling regime, up to reaching the critical heat flux (CHF) point, which is decisive for the design of a cooling system
Summary
Refrigeration science has been an extensive field of research of new technologies to increase the efficiency and cooling capacity of new systems. The basic one is called pool boiling in which the cooled element is submerged in a boiling fluid. This simple fact has been studied through multiple designs and working fluids, since the coolant ability to transport and remove heat from the pool can be very diverse [1,2]. A more concentrated heat removal is achieved by combining two-phase cooling and forced fluid convection, which is commonly called flow boiling. The flow drag produced by the pumping action increases convection coefficients and displace the CHF point to higher heat fluxes. The most remarkable techniques are the two-phase circulation through microchannel heatsinks [6,7], flow boiling on enhanced surfaces [8,9], and flow boiling jet impingement [10,11]
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