Advanced Hybrid Loop Technology for High Heat Flux Laser Cooling

Abstract

High energy laser diode system dissipates a very high heat flux heat, on the order of 100W/cm≤. While the pumped two-phase loops using spray and jet impingement have been considered as the laser cooling solutions, the complexity associated with the two-phase system by flow reconditioning and high pressure nozzle-related problems are major shortcomings. This paper discusses the advanced hybrid cooling loop technology integrating active liquid pumping with passive capillary liquid management. The hybrid loop evaporators use the sintered porous wick structure for a passive vapor-liquid phase separation. The result is a simple yet high performance cooling system that can remove high heat fluxes over large heat input areas found in such as solid state laser systems. In the paper, the hybrid loop system and the prototype test results are discussed. The hybrid loop system was demonstrated to operate effectively at various earth gravitational orientations with multiple evaporators in 3-dimensional arrangement under varying and asymmetrical heat inputs. The prototype hybrid loop was also demonstrated to remove high heat fluxes in excess of 188W/cm≤ from heat input areas over 5cm≤ with evaporator thermal resistances as low as 0.16°C-cm≤/W. The results represent significant improvements over state-of-the-art heat pipes, loop heat pipes and spray cooling devices in terms of performance, robustness and simplicity. I. Introduction High-power solid-state laser systems generate the high heat flux waste heats from various laser components. The laser diode array and laser gain media dissipate the high heat flux heats of 500W/cm≤ and 100W/cm≤ respectively over large surface areas on the order of 100cm≤. In order to enhance lifetime and optical conversion efficiency of the laser systems, the laser component temperatures need to be maintained below the maximum allowable temperatures. Two-phase cooling systems using phase change process (i.e., evaporation) have been considered to be more effective way to meet such high heat flux cooling requirements than the conventional air or liquid cooling systems. One of the fundamental advantages using the two-phase systems is the large heat transfer coefficient in the evaporator (or cold plate) due to thin-film boiling. The boiling heat transfer coefficient ranges from 10,000 to 100,000W/m≤-°C which is one-order-of-magnitude greater than that of the conventional liquid cooled cold plates. The larger heat transfer coefficients can be directly translated into smaller thermal resistances and consequently higher heat flux capabilities. Another advantage is the large latent heat capacity of the two-phase systems which could be typically two-orders-of-magnitude larger than the sensible heat of the similarly-sized liquid cooling systems. Therefore, the two-phase systems require much less coolant flow and therefore more compact hydraulic systems. Furthermore, isothermal cooling through the evaporation process help the solid state laser systems reduce the temperature non-uniformity and thus ease the thermal stresses in the laser components such as laser diode array and laser gain media.