In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2).

Nowadays, due to the tremendous development of data centers (DCs), studying the effective cooling methods that can face to the challenges such as the high power or heat flux dissipation and the efficient electricity consumption in DCs has never been unnecessary. Loop heat pipe (LHP), a two-phase heat transfer device, is being considered as one of the potential solutions for the above problems. This paper introduces the experimental study on the thermal performances of LHP functioning under gravity assisted condition with different working fluids that are water and ethanol (C2H5OH). This LHP has the flat-rectangular evaporator with the stainless-steel (SS) sintering wick installed inside. The results demonstrate that under the same condenser cooling condition, water LHP performed better than ethanol LHP. In the case of water LHP, when heating power was increased from 33 to 535 W, the temperature at the top surface of the heating block raised from 38oC to 110oC. With the ethanol LHP, this temperature reached 133oC at the heating power of 395 W. If temperature limitation of microprocessors functioning inside the DCs is recognized at 85oC, the cooling capabilities of LHP are 220 W and 350 W corresponding to the working fluid are ethanol and water respectively. In addition, the discussions about the difference in boiling heating transfer characteristics as well as condenser performances between water LHP and ethanol LHP are also presented in this study.

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Nowadays, extreme growing of telecommunication and information technology causes the significant changes in the electronics belonging to the high-performance computer, data centers that are miniaturizing in their size and increasing the heat power dissipation. As a result, it is important to study on new cooling methods rather than conventional air cooling to warrant electronic device operate durably and stably. Besides, saving electricity power consumed by cooling systems is another considerable concern. Loop heat pipe, a passive heat transfer device which functions based on phase change processes and natural forces like gravity or capillary force, has been being one of potential solutions for the above challenges. In this paper, a loop heat pipe with flat evaporator was fabricated to investigate its cooling characteristics during start-up, stable operating period at different heat loads under gravity–assisted condition. This loop heat pipe was charged with ethanol whereas sintered stainless steel wick was the capillary structure. Time for successful start-up shortened from 13 minutes to 4 minutes as heat load increased from 30 W to 225 W. When heating power supplied to loop heat pipe’s evaporator was adjusted from 30 W to 400 W (14.8 W/cm2), the heating block surface’s temperature increased from 33oC to 133oC. This temperature could be maintained below 85oC, electronics limitation temperature in industry, if heat power released from the heating block is smaller than 220 W (8.14 W/cm2). Besides, the change of different types of thermal resistance such as total thermal resistance, evaporator and condenser thermal resistances with heating power is discussed detail in this paper.

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