The paper contains an analytical review of developments, results of tests and simulation of loop heat pipes (LHPs) with disk-shaped, rectangular and flat-oval evaporators. Two main directions have ...been noted in the development of flat evaporators, which may be arbitrarily separated into evaporators with opposite replenishment (EORs) and evaporators with longitudinal replenishment (ELRs). The bodies of such evaporators are made of stainless steel, copper, aluminum. For making wicks use is made of sintered powders and mesh of stainless steel, nickel, titanium, copper, polytetrafluoroethylene (PTFE) and ceramics. Monoporous and biporous capillary structures are considered. Water, ammonia, methanol, ethanol, and acetone have been tested as working fluids. The best results were shown by the combination “copper–copper–water” at temperatures above 70 °C, when on trials an evaporator thermal resistance of less than 0.01 °C/W and a heat flux close to 1000 W/cm2 were achieved. For temperatures below 70 °C the most efficient combination is “stainless steel–nickel–ammonia”.
•A review of development of loop heat pipes with flat evaporators is presented.•Evaporators with opposite replenishment and longitudinal replenishment are defined.•Stainless steel–nickel–ammonia is the most efficient combination for 40–70 °C.•Copper–copper–water combination is the most efficient for 70–100 °C.
•A novel LHP with a flat evaporator has been proposed and tested.•SEM, porosity, and capillary rise of different sintered powder size are measured.•The heat transfer performance of the novel LHP is ...studied at heat loads from 50 to 750 W.•The performance comparison of the novel LHP is presented.
A novel loop heat pipe (LHP) with a flat evaporator has been proposed to meet the heat dissipation requirement of high heat flux server CPU. The sintered porous copper wick was provided with a double arrayed vapor channels to increase the evaporation area. And a 1 mm thick smaller particle size copper powder embedded with two 0.5 mm thick stainless steel rings was sintered on the inner surface of the compensation chamber to enhance the strength, and increase the working fluid into the porous wick such that the liquid saturation was enhanced and the dry zone was reduced at high heat flux. Additionally, a dual layer structure condenser was presented to increase the cooling area of the condenser. In this study, the systematic experimental studies were carried out mainly focusing on the optimization of powder size, heat transfer performance of the novel LHP and performance comparison of the novel LHP. The results demonstrated that the sintered copper powder of the 106–150 μm particle size shown the best capillary performance in comparison with other powder sizes. More importantly, in the heat load range of 500–750 W, the novel LHP of filling ratio of 46.1% exhibited better operating thermal characteristics. The minimum values of the LHP thermal resistance and the total system thermal resistance at the heat load of 750 W were 0.040 °C/W and 0.090 °C/W, respectively. Compared with the conventional LHP and the simple water block, the novel flat evaporator LHP was a more promising option when the heat load was higher than 250 W.
With the rapid development of microelectronics and the telecommunication industry, a variety of high performance, portable and slim electronic devices have become available. Miniaturization of ...devices and increased packing density of electronics can generate “hot spots” i.e. a high heat flux on a small area. Thus, in such devices the heat management requirements go beyond the limits of typical approaches and the development of miniaturized, high-performance thermal management concepts to cool high-performance, compact electronic devices is urgently required. To this direction, micro and nanofabrication methods can provide solutions in both miniaturizing existing concepts of passive cooling as well as in improving their performance. In this review, we start by introducing the most commonly used metrics used to evaluate the performance of passive cooling devices (i.e. vapor chambers and flattened heat pipes) together with the most prominent performance limitations. Then, in the main part, we present state of the art examples of microfabricated, thin vapor chambers and flattened heat pipes on rigid substrates (i.e. using metals and silicon), but also vapor chambers on thin and flexible polymeric or composite materials. Finally, the main conclusions and the steps which should be followed to further enhance the performance of such devices are summarized in the conclusions and future perspectives section.
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•Microfabrication methods can provide ultra-thin vapor chambers.•Ultra-thin (< 500 μm) vapor chambers are rare in the literature.•Metals, Silicon and polymers are used for heat pipes and vapor chambers.•Flexibility and low thickness are the two basic requirements.
•The 3D printing technology is applied to design and manufacture the wicks.•Vapor trap in the wick with small pore radius resulted in severe heat leak.•Vapor permeation through the wick at high heat ...loads were observed.•The maximum heat transfer coefficient was up to 44,379 W/m2K.
The 3D printed wicks, whose structural parameters could be controlled by changing the distance between neighboring structural cylinders, were manufactured to experimentally investigate the effect of pore size (d) on the heat performance of the loop heat pipe (LHP). With the help of the 3D printing technique, the order 3D printed wicks with all pores interconnected were fabricated to avoid high randomness and appearance of closed pores in the traditional sintered wicks. In addition, high porosity, suitable pore radius, low effective thermal conductivity and high permeability were also realized in the same wick. A flat LHP with a transparent evaporator and vapor line was fabricated to observe the flow motion in the evaporator and the vapor line. It was indicated that the LHP with a 3D printed wick (d = 200 μm) made of stainless steel could start successfully in about 100 s at a low heat load of 20 W (2.83 W/cm2). And a stable operation in a wide heat load range from 20 W to 160 W (22.63 W/cm2) was achieved with the allowable evaporator wall temperature of 100 oC. The minimum evaporator thermal resistance of the 3D printed wick was 0.031 K/W at a heat load of 140 W with a corresponding maximum heat transfer coefficient of 44,379 W/m2K. The minimum LHP thermal resistance was 0.181 K/W at a heat load of 160 W.
•A new concept of LHP as a heating device for EV batteries was proposed.•A new transient calculation model using the Newton–Raphson method was constructed.•The LHP for battery heating was fabricated ...and tested by attaching the simulated battery blocks to the condenser.•The efficacy of the LHP as a battery heating device was demonstrated.
This paper reports design, fabrication, and experimental results of a loop heat pipe (LHP) as a heating device for batteries of electric vehicles. Usually, LHPs are used as a cooling device. In this study, however, the LHP is used as a heating device by attaching battery blocks which are the heating target, to the condenser of the LHP. A new transient analytical model was also constructed using the implicit method, which is a multidimensional extension of the Newton–Raphson method. The analytical results are compared with the test results from the point of the transient behavior of the heating LHP. A cylindrical type evaporator with an outer diameter of 27 mm and a length of 150 mm was used. A flat perforated tube with a length of 1625 mm was used as the condenser. R 134a was selected as the working fluid. The condenser of the LHP is connected to the simulated battery blocks, which were made of 48 aluminum-alloy blocks (100 × 128 × 26.5 mm). First, the operation test was conducted at room temperature, and the basic thermal performance and the concept of the battery heating were evaluated. Next, the experiment was conducted in a thermostatic bath which was set at −20 °C. The experimental results showed the simulated battery blocks reached above 0 °C at 14,600 s. The temperature increase rate was estimated to be 0.06–0.09 °C/min, and it can be considered that this LHP can be used as a heating device under actual low-temperature conditions.
•Novel bio-wick is prepared and characterized for loop heat pipe.•Porosity, capillary rise, pore size, and contact angle are measured.•Significant temperature difference between evaporator and ...condenser is noticed.•Maximum capillary pressure generated in the carbon wick is 20,571 Pascal.•Evaporator temperature decreased in carbon wicked heat pipe about 7 °C.
Compact loop heat pipes are widely used in high-performance compact electronic devices. The wick structure plays a vital role in the design of compact loop heat pipes. The performance of a wick dramatically influences the performance of the heat pipe. There are different kinds of wick structures used to enhance the performance of heat pipes such as screen wick, sintered wick, and grooved wick. In this study, a natural bio-carbon based wick structure is introduced to study the performance of a compact loop heat pipe. The Carbon material for the wick structure is prepared by carbonizing the Karuvelam wood as it possesses high water-absorbing capacity and ability to withstand high temperatures. The wick parameters such as porosity, capillary rise, pore size, and contact angle are investigated in order to study the effect of heat input, the impact of wick structure on temperature distribution and equivalent thermal resistance of the heat pipe. The results reveal that the temperature difference between the evaporator and condenser decreases effectively so that the thermal resistance decreases. This work reveals that the bio-carbon wick structure shall be a cost-effective alternative solution for a loop heat pipe mechanism.
•Ammonia LHP demonstrated efficiency in a wide range of external conditions.•Most profound effect on LHP operation is exerted by heat sink cooling conditions.•Maximum heat load of 500 W was achieved ...at a horizontal position of the device.•Tests of server cooling system with LHPs showed 8% decrease in energy consumption.
A study has been made of thermal characteristics of an ammonia loop heat pipe (LHP) with an effective length of 421 mm equipped with a cylindrical evaporator 10 mm in diameter in a wide range of external conditions. The slope angle was varied from −90° to +90°, the temperature of the liquid cooling the heat sink from 0 to +60 °C, and the liquid flow rate from 0.1 to 4 L/min. It is shown that the most favorable position an LHP is the horizontal one. At a cooling temperature of 0 °C and a flow rate of 4 L/min the maximum heat load reached 500 W. In this case the thermal resistance of the “heat source – cooling liquid” system was 0.147 °C/W. It is also shown that changes in the value of the maximum heat load at all slope angles did not exceed 19% of the maximum value. The greatest effect on the LHP maximum heat load, operating temperature and thermal resistance was exerted by the heat sink temperature and the efficiency of its cooling. LHPs tests in a real cooling system of 1U computer server demonstrated energy savings of 8%.
•A pump-assisted loop heat pipe with ammonia is presented and tested.•A centrifugal pump with a small size and light weight is used.•The maximum heat flux is 36.35 W/cm2 at heat transfer distance of ...2320 mm.•The different performance between ammonia and methanol is demonstrated.
This paper presents a loop heat pipe (LHP) with a pump to overcome the drawbacks of traditional loop heat pipes, such as limited heat transfer distance and temperature oscillation. The mechanical pump used in the loop is a centrifugal micropump which is light-weighted, small-sized and high-speed. In order to avoid the occurrence of cavitation and get a better character of the loop, ammonia is chosen as the working fluid. In consideration of the compatibility between working fluid and material, stainless steel is used to fabricate the whole loop. Meanwhile a biporous nickel wick is sintered to provide the capillary force. The effective heat transfer distance of the loop is 2320 mm. The experiment is conducted under different power of the pump and various temperature of the heat sink. Test results figure out that the system quickly responds to varible heat load and no temperature oscillation occurs. If the heated surface temperature is limited below 80 °C, the loop could handle a maximum heat dissipation of 370 W (heat flux = 36.35 W/cm2) when the power consumption of the pump is 4 W and the temperature of heat sink is −10 °C. Under this circumstance the thermal resistance of the evaporator is 0.176 K/W.
•Current status of loop heat pipe-based solar façade water heating system is summed up.•Evaluation indexes for transient and steady behaviors of loop heat pipe are reviewed.•Three suggestions are ...overviewed for the start-up enhancement of loop heat pipe.•Strategies for improving the performance of solar water heating system are outlined.•Prospects of loop heat pipe-based solar façade water heating system are pointed out.
To meet the demand for a high-efficiency, anti-freezing, and building-integrated solar water heating system, a versatile loop heat pipe-based solar thermal façade water heating system has been developed in recent years. To date, a comprehensive summary of evaluation indexes has not yet been reported for assessing the transient and steady-state performance of the loop heat pipe, which deserves to be determined because the overall thermal efficiency of the system is highly sensitive to the operational performance of the loop heat pipe. In addition, the heat transfer process within the system is analyzed along the three stages of the heat transfer path, namely, solar absorption, outdoor to indoor heat transportation, and water heating, to reveal the working mechanism of the system and clarify the factors influencing its behavior. In line with these three stages and practical requirements, the refined improvement strategies for system performance are elaborately summarized from three corresponding aspects, including the solar absorber structure, heat transfer fluid, and energy storage, to provide a reference for achieving a superior heat transfer effect. The particularity of the system lies in the involved heat and mass transfer phenomena and the latent heat thermal energy storage technology, which are the core issues worthy of in-depth exploration. Therefore, the system’s potential for further development is outlined to promote market penetration. Through the current comprehensive analysis, the evaluation criteria of the operational characteristics of loop heat pipe and loop heat pipe-based solar thermal façade water heating systems as well as performance enhancement techniques are methodically expounded. This review is expected to lay a solid foundation for improving and extending the loop heat pipe-based solar thermal façade water heating system, along with achieving the targets for significant energy-saving and environmental benefits.
•Visualization was carried out to reveal the mechanism of temperature oscillations.•High-amplitude temperature oscillations are caused by intermittent liquid supply.•The oscillation characteristics ...under various operating conditions are presented.•Temperature oscillations are prone to occur under low heat loads.•Low filling ratio and high sink temperature can suppress temperature oscillations.
A novel loop heat pipe with a vapor-driven jet injector (LHPI) shows excellent heat transfer performance. However, high-amplitude temperature oscillations are observed at low heat loads. In this study, a LHPI with transparent compensation chamber is designed and tested to reveal the mechanism of temperature oscillations. The relations between the internal flow and temperature oscillations are investigated by visualization study, and the results show that high-amplitude temperature oscillations are caused by the intermittent liquid supply of the compensation chamber. Meanwhile, the temperature oscillation characteristics of the LHPI under a variety of operating conditions are also studied. Five operation modes of the LHPI are classified as start-up failure mode, high-frequency/low-amplitude oscillating mode, low-frequency/high-amplitude oscillating mode, overshoot mode and smooth mode. Furthermore, a low filling ratio can eliminate the low-frequency/high-amplitude oscillating mode and is therefore conducive to the stable operation of the LHPI, but it elevates the operating temperature under high heat loads. In addition, increasing the sink temperature can also suppress temperature oscillations.
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