•The world’s first 6.2-kW heat transport loop heat pipe (LHP) was developed.•The system’s thermal resistance at 6.2 kW was 0.004 °C/W.•The LHP did not reach the operating (capillary) limit, even at ...6.2 kW.•The proposed model indicates that the LHP has a heat transport capability of 10 kW.•Natural air cooling exhibited the lowest system thermal resistance of 0.002 °C/W.
This paper describes the design, fabrication, and heat transfer characteristics of a loop heat pipe (LHP) with a kW-class heat transfer performance. A one-dimensional numerical model incorporating a two-phase flow pattern of the LHP condenser was developed during the design process. Based on the numerical model, a kW-class LHP comprising a single evaporator was constructed. At the evaporator, a stainless steel 316 (SUS316) box-type wick was installed, and pure water was used as the working fluid. Additionally, two types of condensers with diameters of 1/2 and 3/4 in. were manufactured based on the devised numerical model. An experimental investigation of the heat transport performance of the kW-class LHP was conducted under a cooling temperature of 30 °C using four conditions of heat dissipation at the condenser: natural air, forced air, natural water, and forced water convection. The world’s highest heat transport of 6.2 kW was achieved using the 1/2-in. diameter condenser under natural water convection. The thermal resistance between the evaporator and condenser was 0.004 °C/W, and annular flow dominated the condenser’s flow pattern. Under the other cooling conditions, maximum heat transport capabilities of 1.5, 2.5, and 4.5 kW for natural air, forced air, and forced water convection, respectively, were achieved. Conversely, due to vapor penetration from the condenser to the compensation chamber caused by stratified flow at the condenser, lower LHP performances were demonstrated by the 3/4-in. condenser under all cooling conditions. The findings confirmed that to improve heat transfer performance, the appropriate control of the two-phase flow pattern at the condenser is critical, and it is essential for the development of kW-class LHPs.
•The thermofluid behavior of a multiple evaporators LHP was visually investigated.•The positive gravitational head’s effect was investigated.•The vapor-liquid distribution in both evaporator cores ...and CCs was visually confirmed.
In this work, the thermofluid behavior of a loop heat pipe (LHP) with two evaporators and one condenser was experimentally investigated. Acetone was used as the working fluid. To directly investigate the flow patterns and vapor–liquid distribution, visualization tests were conducted in the condenser, evaporator cores, and compensation chambers (CCs) of the MLHP with a variety of heat loads. The experiments were conducted under a gravity-assisted condition (i.e., the condenser was positioned 360 mm higher than the evaporators). Under the normal operating pattern, both evaporators were heated; for the heat load sharing pattern, only one of the evaporators was heated. To conduct visualization tests, the condensing tube of the MLHP was fabricated using a transparent plastic pipe. In each CC of the MLHP, a glass tube was installed. These tests investigated a combination of temperature variations, pressure drops, vapor–liquid distribution and two-phase flow regions. In the normal operating pattern and heat load sharing pattern, the relation between heat leak and two-phase length in the condenser was confirmed. The effect of gravitational head on flow patterns in the condensing tube and heat leak were also investigated. In the heat load sharing pattern, visualization experiments were used to study the vapor–liquid distribution in each evaporator core and CC. In the gravity-driven mode, a lack of heat load sharing was discovered and investigated. In the gravity-capillary codriven mode, nucleate boiling was observed in the unpowered evaporator core.
•Cylindrical evaporator-CC coupling structure was visually analyzed at three tilt angles.•The performances of the vapour-liquid interface in the coupling structure was studied.•The bubble generation ...and its movement in the coupling structure were observed and studied.•Temperature fluctuations of the LHP were related to the oscillations of vapour-liquid interface.•The conclusions are helpful for the heat transfer mechanism of the LHP in gravity applications.
To visually analyze the flow and heat transfer characteristics of the working fluid in a loop heat pipe (LHP), the evaporator-compensation chamber (CC) was designed as a half-sectioned cylindrical structure sealed with a glass window. Using acetone as the working fluid, visualization experiments were performed under different heat loads and tilt angles. The vapour-liquid distribution in the evaporator-CC coupling structure and the temperature variation characteristics of the LHP system were investigated at three tilt angles of the evaporator-CC coupling structure. According to the experimental results including the startup and steady-state operation, some important conclusions have been drawn as summarized below: (1) with the subcooled liquid outflow from the bayonet tube, the vapour-liquid interface in the evaporator-CC coupling structure rises when the heat load is applied to the evaporator; (2) the bubble generation inside the porous wick becomes more intense with an increased heat load, and there are various bubble movement patterns at different tilt angles; (3) the temperature fluctuation of the LHP system is closely related to the oscillation of the vapour-liquid interface in the evaporator core.
•An ammonia LHP 21 m long has been developed and tested.•Tests were conducted at a horizontal orientation and heat sink temperatures of 4 °C and 20 °C.•The LHP demonstrated serviceability in the ...range of heat loads from 200 to 1700 W.•The minimum value of the “heat source – heat sink” thermal resistance was at a level of 0.034 °C/W.
Loop heat pipes (LHPs) are passive heat-transfer devices which may be used in energy-efficient systems of recovery of low-potential heat, for heat transfer from renewable energy sources, and also in systems of thermal regulation of different equipment with remote heat sinks or sources. The paper shows the results of development and thermal tests of an LHP 21 m long made of stainless steel with ammonia as a working fluid. The device had a cylindrical evaporator 24 mm in diameter with an active zone length of 188 mm equipped with a nickel wick with an effective pore radius of 1.05 µm and a porosity of 70%. The LHP condenser made in the form of a pipe-in-pipe heat exchanger 310 mm long was cooled by running water with a temperature of 4 °C and 20 °C. Tests were conducted at a horizontal orientation of the device. At a cooling temperature of 20 °C a maximum heat load of 1700 W (12 W/cm2) was achieved at a vapor temperature of 62 °C, which corresponded to a heat source temperature of 89 °C. In this case the thermal resistance of the “heat source – cooling liquid” system was equal to 0.034 °C/W.
•A novel loop heat pipe with a vapor-driven jet injector is proposed and tested.•A directional motion of liquid is formed in the CC to take away the heat leakage.•The coupling relationship of the ...injector and the LHP is studied.•The injector improves the heat transfer performance of LHP substantially.
Loop heat pipe (LHP) is an efficient and passive heat transfer device. However, heat leakage may lead to several problems such as temperature oscillation and even startup failure, which hinders the practical application. To weaken the negative effects of heat leakage and improve the performance of LHP, a novel loop heat pipe with a vapor-driven jet injector (LHPI) is proposed. The injector, which is driven by the vapor from the evaporator, can suck the hot liquid and form a directional motion of liquid in the compensation chamber to remove the heat leakage. The results indicate that the LHPI can operate with heat load range of 50–550 W and heat leakage higher than 20% using deionized water as working fluid. The maximum heat flux of LHPI is 48.6 W/cm2, corresponding to a total thermal resistance of 0.27 °C/W. While maintaining the heating wall temperature within 85 °C, the maximum heat flux reaches 22.1 W/cm2, corresponding to a thermal resistance of 0.33 °C/W. The heat transfer performance of LHPI is greatly improved compared with most of the conventional LHPs.
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The implementation of high power density coupled with limited space available in the cooling of electronics demands a highly efficient miniature loop heat pipe (mLHP). This study experimentally ...investigates the influence of a nanofluid on the thermal characteristics of a specially designed mLHP and explores the mechanism of heat transfer enhancement of the nanofluid in the mLHP. The nanofluid is composed of deionized water and Cu nanoparticles and has an average diameter of 50 nm. Reductions of 12.8% and 21.7% are achieved in the evaporator wall temperature and total thermal resistance, respectively, while the heat transfer coefficient (HTC) of the evaporator increases 19.5% when substituting the nanofluid with 1.0 wt% of deionized water at a heat load of 100 W. There is an optimal mass concentration for the nanofluids, which corresponds to the maximum heat transfer enhancement. The optimal mass concentration is 1.5 wt%. The thermal performance improvement of the mLHP using the nanofluid results from the reduction of the contact angle, the enhancement of boiling heat transfer, and a deposited nanoparticle coat on the boiling surface.
•A miniature loop heat pipe (mLHP) is designed for electronics cooling system.•Contact angle of Cu–water nanofluid is studied.•Water recirculation rate of nanofluid is more than that of pure water during boiling.•The effects of nanofluid on thermal performance of mLHP are investigated.
•A novel loop heat pipe with a vapor-driven jet injector and a boiling pool is proposed.•The injector removes the heat leakage and provides liquid for the boiling pool.•The boiling pool is the main ...heat dissipation component with high heat flux.•The coupling relationship of the evaporator and the boiling pool is studied.•Visualization study is carried out to reveal the operation mechanism of the novel loop heat pipe.
To remove the heat leakage in the compensation chamber (CC) and increase the maximum heat flux of the loop heat pipe, a novel loop heat pipe with a vapor-driven jet injector and a boiling pool (LHPIB) was proposed. The injector driven by the vapor from an evaporator was designed to remove the heat leakage and provide liquid for the boiling pool. The boiling pool was the main heat dissipation component where high heat flux was achieved. Comprehensive experimental research was conducted to evaluate the heat transfer performance of the LHPIB, and visualization study was carried out to reveal its operation mechanism. The results indicated that a timely liquid supply from the injector to the boiling pool was essential for a smooth startup. The LHPIB worked well under variable operating conditions and a new steady state was established quickly when the heat load changed. Maintaining the evaporator heat load at about 238 W, the maximum boiling pool heat load reached 507 W, with the corresponding heat flux up to 105 W·cm−2, which was higher than most of the conventional loop heat pipes. In general, the LHPIB is a promising candidate for heat dissipation of high heat flux and multi-heat sources.
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•Smooth and rough porous copper fiber sintered sheets as wicks are fabricated.•Capillary pumping performance of porous copper fiber sintered sheets are tested and discussed.•Effects of different ...parameters on thermal performance of loop heat pipes are studied.•Better thermal performance with rough porous copper fiber sintered sheets as wick is obtained.•Deionized water with a 30% filling ratio is the optimal combination for designed loop heat pipe.
Smooth and rough porous copper fiber sintered sheets, employed here as wicks for loop heat pipes for the first time, were fabricated using a low-temperature solid-phase sintering method. The capillary performance of these porous copper fiber sintered sheets were analyzed and discussed. The influence of the surface morphology, filling ratio, and working fluid on the thermal resistance, evaporator wall temperature, and start-up time of the loop heat pipes were investigated. The results showed that the capillary pumping amountofworking fluid for both smooth and rough porous copper fiber sintered sheets initially increases rapidly, and then gradually attains a stable state. The curve of the capillary pumping amountofworking fluid can be described as a function that increases exponentially over time. When rough porous copper fiber sintered sheets are used as wicks and deionized water is used as the working fluid, the capillary pumping amount is maximized. Compared to smooth porous copper fiber sintered sheets, loop heat pipes with rough porous copper fiber sintered sheets exhibit a shorter start-up time, lower thermal resistance, and lower evaporator wall temperature. For a filling ratio in the range of 15–45%, loop heat pipes with rough porous copper fiber sintered sheets and a 30% filling ratio show lower thermal resistance and a lower evaporator wall temperature. Ultimately, the use of deionized water as the working fluid with a 30% filling ratio enables loop heat pipes with rough porous copper fiber sintered sheets to be stably operated at a heat load of 200W.
•An ammonia LHP with flat evaporator and long heat transfer distance was designed.•The loop could operate stably in the range of heat loads from 2.5 W to 180 W.•The evaporator inlet temperature ...suffered a staged decline due to vapor expansion.•Two different processes related to vapor phase and heat leak were observed.•A minimum LHP thermal resistance was 0.252 °C/W at heat sink temperature of 10 °C.
Loop heat pipes are passive heat transfer devices which can meet the heat dissipation requirement of high-power electronic devices in aerospace and terrestrial applications. This paper investigates the operating characteristic of a stainless steel-ammonia loop heat pipe with a flat disk evaporator. A biporous wick made from sintered nickel powders was used to produce the capillary force. The heat transfer distance was 1.6 m and the allowable heater surface temperature was below 70 °C. Tests demonstrated that the loop could operate under a heat load ranging from 2.5 W to 180 W (heat flux 0.15–10.8 W/cm2) at heat sink temperature of −10 °C. In addition, variable conductance mode and constant conductance mode existed, and no obvious temperature overshoot or pulsation was observed. The evaporator inlet temperature went through a staged decline due to the effect of initial driving force of vapor expansion during the start-up process. Meanwhile, under the synergy of heat leaks from heater surface and long transport line, whether the vapor phase would form inside the compensation chamber could result in a large discrepancy in temperature trends. The minimum evaporator thermal resistance was 0.096 °C/W at heat sink temperature of −10 °C and the minimum LHP thermal resistance was 0.252 °C/W at heat sink temperature of 10 °C.
•A DCCLHP with an extended bayonet tube was proposed and fabricated.•The DCCLHP could start up at whatever orientations in the terrestrial surroundings.•The DCCLHP exhibited better startup ...performance especially at small heat loads.•Operating instability such as temperature oscillation and overshoot were observed.
Dual compensation chamber loop heat pipe (DCCLHP) is an advanced two-phase heat transfer device, which features an orientation-free operation in the terrestrial surroundings and promises great application potential in the aircraft thermal management. In this work, a DCCLHP with an extended bayonet tube was designed and fabricated, aiming to improve its startup performance especially at a small heat load. The startup characteristics of the DCCLHP in five different orientations was experimentally investigated and theoretically analyzed. According to the experimental results, some important conclusions have been drawn: 1) The DCCLHP with an extended bayonet tube could achieve successful startup under the heat load ranging from 10 to 100 W in the horizontal orientation, and no startup failure occurred. 2) The DCCLHP could start up successfully even at a small heat load of 10 W from -90° to +90° tilt angle in the terrestrial surroundings. 3) Different startup phenomena such as the unique change of the temperature overshoot were observed in the experiments. 4) Operating instability such as the temperature oscillation occurred during the startup process in the favorable directions.
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