Development and testing of a novel horizontal loop thermosyphon as a kW-class heat transfer device

Abstract

• New kW-class horizontal loop thermosyphon is developed to cool electronic components. • Annular mini channel with porous coating in the evaporator ensures efficient two phase fluid flow. • Thermal resistance of thermosyphon is near 0.03 K/W for the heat load of 100–1750 W. • Efficient device to cool electronic and electric transport components, solar receivers. This article describes the design, fabrication, and heat transfer characteristics of a novel type of loop thermosyphon with a horizontal porous evaporator (LTHPE) of the kW-class heat transfer performance. The main goal of this investigation is to improve the heat transfer intensity by increasing the fluid circulation in the evaporator annular channel. The Bond number Bo ≤ 1–2 is typical of this new device. Such thermosyphon with the evaporator annular thickness equal to, or larger than, the capillary limit exhibits some new properties. An additional (to the gravity field) mechanism of liquid flow enhancement with self-exciting oscillation due to the bubble generation in the annular gap is used for intense and efficient fluid circulation through the thermosyphon evaporator in a preferential direction. Experimental data on heat transfer coefficients in boiling and evaporation inside the porous wick were obtained in the evaporator annular horizontal gap flooded and partially saturated with liquid. Due to the two-phase fluid flow along the annular tube, stabilization of heat transfer has been achieved including the subcooled and saturated liquid boiling with evaporation inside the wick. A thin porous coating on the evaporator wall ensures a 2–2.5-fold increase of heat transfer in comparison with the flow boiling heat transfer in a smooth tube. The thermal performance of the LTHPE was investigated at different filling ratios and LTHPE inclination angles to the horizon. The evaporator envelope was made from a copper tube of length 200 mm. A copper sintered powder wick was installed inside the evaporator, and pure water was used as the working fluid. The wick thickness was less than 1 mm. The thermal resistance of the condenser was equal to 0.01 °C/W . The LTHPE evaporator and condenser are connected with each other by flexible minipipes for transferring vapor and liquid. The thermal resistance of LTHPE is relatively insensitive to any changes in inclination, when the angle of the latter to the horizon exceeds 18°. The total thermal resistance of LTHPE does not exceed 0.03 K/W (thermal resistance of evaporator is near 0.02 K/W) with the heat load of 100–1750 W. This device guaranties a shortened start-up time, has a decreased evaporator wall temperature, has a small temperature hysteresis on increasing/decreasing the heat load, and suppresses the temperature pulsations inside the evaporator. The determination of the thermal resistance of evaporator and condenser and the analysis of the temperature field along the LTHPE were the main goals of this research.