•A flat evaporator made of aluminum alloy was designed for ground applications.•R1233zd(E), an eco-friendly refrigerant, was selected as the working fluid.•The LHP could dissipate the heat of 190 W ...with an effective length of 790 mm.•The thermal performance was stable even at heat sink temperature of 30 °C.•The minimum LHP thermal resistance was 0.197 °C/W at 0 °C heat sink temperature.
Flat evaporator loop heat pipes with good thermal performance and compact volume have been widely used in electronic device applications. A flat-disk evaporator loop heat pipe made of aluminum alloy was designed for terrestrial application in this paper. R1233zd(E) was selected as the working fluid for its ultra-low toxicity and environmentally friendly. The pumping force of the system, driving the working fluid circulation, was generated by the sintered capillary wick made from nickel powder. Considering the requirements for electronics and the capillary limit of the wick together, the target temperature was below 75 °C and the effective heat transfer length was set as 790 mm. With a heat sink temperature of −10 °C, it could dissipate the heat of 190 W (11.43 W/cm2). The performance investigation under bad external conditions was also carried out. A relatively broad operating range between the heat load of 10 W (0.60 W/cm2) and 130 W (7.82 W/cm2) was observed with a 30 °C heat sink temperature. However, slight temperature overshoot occurred during the start-up test but the system was able to self-regulate and stabilize quickly. Moreover, the variable heat load test, imitating the heat-dissipating demand for actual electronic devices, demonstrated that this system responded fast and operation failure did not happen. The minimum thermal resistance of the evaporator and the total LHP was 0.134 °C/W and 0.197 °C/W, respectively.
Heat pipes are becoming increasingly popular as passive heat transfer technologies due to their high efficiency. This paper provides a comprehensive review of the state-of-the-art applications, ...materials and performance of current heat pipe devices. The paper is divided into four main parts; low temperature heat pipes, high temperature heat pipes, thermal modelling of heat pipes and discussion. The low and high temperature sections present an extended list with suitable working fluids and operating temperatures, along with their compatibility with casing materials. Furthermore, the sections focus on some of the most widespread industrial applications, such as solar, nanoparticles, Rankine cycles, nuclear, thermoelectric modules and ceramics, in which heat pipe technologies offer many key advantages over conventional practises. The third part of the paper consists of a thorough analysis of the thermal modelling side of heat pipes. Internal and external thermal modelling techniques, theories and methodologies are presented in this section, for various applications such as non-Newtonian fluids, nano-fluids, solar, geothermal, automotive, hybrid storage and nuclear systems. The final part of the paper discusses the limitations of heat pipes and the reasons why they are not implemented in more aspects of our lives. Operational limitations, cost concerns and the lack of detailed theoretical and simulation analysis of heat pipes are some of the point covered in this section. Finally, some of the recent and future developments in the field are discussed.
•A comprehensive review on the heat pipe technology in the literature is included.•The current state of the art for this technology is included and detailed.•The potential for this technology is illustrated with sample real case studies.
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•Investigated the effect of using heat pipe solar collector integrated with pyramid-shaped solar still.•Conducted the effect of using water and ethanol as working fluid.•An optimal ...filling ratio was 40%.•The maximum hourly and accumulated yield were 1235 and 6970 ml/m2, respectively.•The cost of produced fresh water was 0.0137 $/l.
In this paper, a modified pyramid-shaped solar still (MPSS) including a conventional pyramid-shaped solar still (CPSS) integrated with a heat pipe solar collector (HPSC) was fabricated and experimentally assessed in the outdoor conditions of Mashhad, Iran (latitude of 36°18', longitude of 59°34'). Two different working fluids (water and ethanol) were used at three different filling ratios (FRs). Results showed that using the HPSC increased the temperature difference between the glass cover and the basin water. Therefore, using the HPSC augmented the hourly and accumulated yield. The accumulated yield was escalated to 6970 and 3300 ml/m2 for the MPSS (with water as the working fluid at the FR of 40%) and the CPSS, respectively. Moreover, the HPSC increased exergy, although the instantaneous efficiency was rather invariant. Based on the experimental data the values of the total internal heat transfer coefficient have been calculated. Furthermore, the best performance of the MPSS was obtained with water as the working fluid at the FR of 40% whereas ethanol at the FR of 40% also showed a very good performance. Finally, the economic analysis revealed that the MPSS was economically reasonable and an estimated cost for 1 L of distillate was 0.0137 $ for the proposed MPSS.
•Three loop heat pipes with less than 1.6 mm in thickness were developed.•Diffusion bonding reveal to be a promising fabrication technology of thin devices.•The thermal performance of LHPs was ...reduced by decreasing their thickness.•The LHPs were able to transfer low and large heat fluxes with natural convection.•The LHPs are potential solutions for thermal management of thin electronic devices.
Diffusion bonding process was used to manufacture three different thin flat loop heat pipes (LHP), two with 1.52 mm and one with 0.92 mm of thickness. The proposed LHPs were designed to be candidates for thin electronics cooling applications, such as smartphones. The devices attended to the following heat transfer requirements, based on recent electronic technologies: dissipation of both low (2 W/cm2) and high heat fluxes (8 W/cm2) at natural air convection. A workbench, capable of evaluating the LHP thermal performance, was developed to simulate the operating condition of a chip processor with 1 cm2. The LHPs were investigated experimentally using ethanol as the working fluid in three orientations: horizontal, gravity-assisted and against-gravity. As the main problem of operating thin LHPs with low heat fluxes is the heat leakages from the evaporator, a theoretical model was proposed to predict the conduction heat transfer between the evaporator and the liquid line. The minimum thermal resistance of 0.2 °C/W was achieved for the 1.52 mm thick device at 8 W/cm2. Experimental results confirmed that the thickness reduction of heat pipes leads to a decrease in their thermal performance. The heat leakage of the thinner LHP to the liquid line was approximate 17% of the total heat transferred, while the thicker LHP showed a heat leakage of 10%. Although still demands development, two of the three proposed loop heat pipes showed good heat transfer capacity, keeping the evaporator temperature, supposedly in contact with the electronic component in actual applications, below 100 °C. This device is shown to be suitable for the thermal management of thin electronic devices, especially mobile phones.
Experiments have been conducted on a flat rectangular loop heat pipe with water and methanol at heat loads from 40 to 320 W at three orientations of the heat pipe (0°, 45°, 90°) and two filling ...ratios (33% and 50%). Performance parameters such as thermal resistance, effective thermal conductivity and capillary limit are calculated from the experimental data. The results exhibited that the thermal performance of water is better than methanol with least thermal resistance 0.13 K/W, maximum capillary limit 2626.72 W and an effective thermal conductivity 84490.48 W/mK at a heat load 320 W in horizontal position for 33% filling ratio.
An experimental study was conducted to improve the steady-state thermal performance of a loop heat pipe (LHP) under high heat fluxes by employing a bypass line. The LHP had a sintered metal wick and ...a flat evaporator, of which the planar dimensions were 40mm×50mm. The wall and tubing system were made of stainless steel, and distilled water was used as the working fluid. The bypass line was installed between the vapor channel of the evaporator and the liquid reservoir to control the thermal performance of the LHP. A control valve was placed in the bypass line to enable changing between the normal and bypass line operation modes. An experimental investigation was conducted to identify the effect of the bypass line on the LHP performance, from the viewpoints of the temperatures at representative points and the thermal resistance. The steady-state performances of the LHPs with and without the bypass line were analyzed and compared with each other, under thermal loads of 100 W–260 W (21.2 W/cm2). Typical results showed that the evaporator wall temperature was decreased by approximately 45 °C, resulting in reduction of thermal resistance by 28.1 %, for the LHP with the bypass line.
•Steady-state performance of a loop heat pipe can be improved by the bypass line.•Startup time of a loop heat pipe can be significantly reduced by the bypass line.•Under high heat flux, the bypass line can decrease the evaporator wall temperature of a loop heat pipe.•Under high heat flux, the bypass line can lower the thermal resistance of a loop heat pipe.•The bypass line passively controls temperature without additional power consumption.
A loop heat pipe (LHP) with a composite-material evaporator has been proposed to overcome the disadvantages of heat leak through the evaporator sidewall and easy deformation of the flat evaporator. ...The evaporator was composed of two materials: red copper (heating surface) and 316L stainless steel (upper part), and it had features that a reinforced rib structure was designed on the heating surface to improve the strength of the evaporator. The two sintered nickel wicks were incorporated inside the evaporator. In the horizontal orientation, startup performance and response to variable heat load of the proposed LHP were tested with the heating block temperature below 80 °C. The experimental results showed that the LHP with a composite evaporator could operate stably at heat load range from 10 W to 140 W at heat sink temperature of 25 °C and 35 °C, and had better startup performance. Fig. 1 gave the temperature difference between the evaporator outlet and the evaporator back. With the increase in heat load, the LHP with a composite-material evaporator had a increasingly larger temperature difference than that of the LHP with a single-material evaporator. Compared to the LHP with a single-material evaporator, heat leak to the CC through the evaporator sidewall was effectively reduced.
The temperature difference between the evaporator outlet and the evaporator back for the evaporator with the same structure.
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•A composite-material evaporator has been proposed for the flat type loop heat pipes.•The LHP with a composite-material evaporator was studied for reducing heat leak through the evaporator sidewall.•The proposed LHPs could work at heat load range from 10 W to 140 W with the heating surface temperature below 80 °C.•A way to design the flat evaporator has been put forward for further facilitating its development.
A loop heat pipe (LHP) is one of the most efficient two-phase heat transfer devices. During its operation, a portion of the heat load applied to the evaporator is transferred to its compensation chamber (CC) through the evaporator sidewall, which is known as a heat leak, decreasing the performance of the LHP startup to a certain degree. To reduce a heat leak through the evaporator sidewall, an LHP with a composite-material evaporator was proposed. The evaporator is composed of two types of material, namely, red copper (heating surface) and 316L stainless steel (upper part), and has a reinforced rib structure on the heating surface to improve the evaporator strength. Two sintered nickel wicks are incorporated inside the evaporator. The experimental results demonstrate that the LHP with a composite-material evaporator can operate successfully within a heat load range of 10–140 W while heating the surface to below 80 °C at a heat sink temperature of 25 °C and 35 °C. Compared with an LHP with the same evaporator structure (but with a different in material) 17, the temperature difference between the evaporator outlet and the CC left (right) is smaller under the same heat load, indicating that the heat leak through the evaporator sidewall is reduced.
The LHP with a strengthened ribbed plate has provided an effective way to solve the deformative nature of the flat evaporator to the vapor pressure, and the area of the active zone for the evaporator ...is also greatly improved. The experimental results showed that this system possessed good operating performance at the given operating conditions, especially having good temperature uniformity on the surface of heating block with heat load varying, as shown in Fig. 1. The proposed LHP system has high possibility to address thermal issues with large footprint.
Temperature uniformity of heating block.
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•A new flat evaporator structure was proposed to overcome its deformative nature.•The evaporator was implemented in LHP and R245fa was chosen as the working fluid.•The LHP system possessed good startup performance and thermal sensitivity.•The returning liquid could be equally distributed in two parts of the CC.•The heating block surface with 54.76 cm2 possessed good temperature uniformity.
The flat evaporator loop heat pipes (FELHP) possess many advantages because of the presence of the flat thermo-contact surface, but at the same time, one of the noteworthy drawbacks is unable to withstand high vapor pressure inside the evaporator when the evaporator active zone becomes very large, which hinder them further development. In this paper, a new evaporator structure with a strengthened ribbed plate on the side of the heating surface was proposed to overcome deformative nature of the flat surface. This evaporator structure with an active zone of 74 mm × 74 mm was applied in the field of the LHPs for positive pressure system, and can be used for cooling the chips with large thermal footprint. The copper was chosen as the evaporator material, and R245fa as the working fluid. In this investigation, operating performances of the LHP with a strengthened ribbed plate, including startup behavior, transient response to variable heat load, and thermal characteristics, were experimentally tested and theoretically analyzed when the heat sink temperature was controlled at 25 °C and 35 °C in the horizontal orientation. The experimental results demonstrated that the new system operated normally and achieved good temperature uniformity at the given heat load range from 10 W to 160 W with the heating block temperature below 80 °C.
•A loop heat pipe (LHP) with flat evaporator for visualization study was designed.•Two-phase flow characteristics inside the LHP under gravity were investigated.•The evaporator temperatures at the ...boiling incipience were identified.•The flow patterns under different heat loads were systemically investigated.•A minimum LHP thermal resistance of 0.068 °C/W was achieved at 500 W.
Aiming to fully investigate the two-phase flow characteristics inside a loop heat pipe (LHP), a visualization study for a compact copper-water loop heat pipe with flat evaporator was carried out under gravity favorable mode in this work. Under forced water cooling condition with the cooling water temperature of 25 °C, the results indicated that the compact LHP could efficiently dissipate a maximum heat load of 550 W with the evaporator temperature of 91.2 °C. A minimum LHP thermal resistance was 0.068 °C/W, which was obtained at 500 W. In addition, the evaporator temperatures at boiling incipience under different heat loads were evaluated, which showed slight dependence on the input heat load. Finally, the flow patterns during the startup stage and the steady stage associating with different heating loads were recorded and analyzed to investigate the internal operation mechanism of the LHP. As a result, various two-phase flow patterns were found to be related to the heat load, and the occurrence of partial dry-out was observed in the evaporator at the maximum heat load of 550 W, corresponding to a heat flux of 88 W/cm2.
Application of nanofluids as a working fluid can increase the thermal efficiency of heat pipes. The objective of this comprehensive review is to study the previous works to understand the dominant ...mechanisms of heat transfer when nanofluids are used in thermosyphon heat pipe (THP). The influence of heat input, type and concentration of nanoparticles on the thermal behavior of THPs are investigated. In general, nanofluids significantly improve the thermal characteristics of THP by decreasing the thermal resistance and increase the thermal efficiency of THP. The thermal performance of THP strongly depends on operating variables, especially the type of nanoparticles, their concentration and addition of surfactant and heat inputs. Further, the thermal performance of THP at various heat input is remarkably related to the type of nanofluid. This study also presents a review of the applications of the nanofluid-based heat pipes in energy systems. Finally, some challenges are identified and suggestions are made to fertilize the future investigations.
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