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Recently, thermal regulators based on adsorption in a heat pipe have been proposed. The advantage of these so-called "switchpipes" over similar approaches is their low on state thermal resistance. In this paper, we propose a methodology to calculate and adjust the activation temperature of such switchpipes. For this purpose, we use a mass balance-based model that considers both the heat transfer properties of the heat pipe itself, which depend on the amount of working fluid, and the adsorption equilibrium of the adsorbent used. This model can be used not only to describe the activation behavior of a given heat pipe but also to optimize the configuration of a heat pipe for specific operating conditions and to select appropriate adsorbents. In this paper, we also propose definitions for basic indicators of the activation properties of the heat pipe, such as the activation temperature and the activation temperature span. Finally, a simplified calculation method is presented that allows the selection of the correct adsorbent among all adsorbents with Type IV and Type V adsorption isotherms. [ABSTRACT FROM AUTHOR]

Given that two-phase closed thermosiphons (TPCT) are a prevalent and efficacious means of heat transfer. However, non-condensable gas (NCG) may potentially impact the heat transfer performance of the aforementioned devices. Nevertheless, the theoretical analysis of heat transfer in TPCT containing NCG is not fully comprehensive and therefore requires further supplementation by means of relevant experiments. This paper presents the development and experimental investigation of a theoretical heat transfer model for a TPCT containing NCG. The research encompasses the optimal fluid-filling ratio of R22 and R410a working fluid and the impact of NCG on heat transfer in the condenser section of TPCT. Experimental findings indicate that TPCT with R22 and R410a working fluids at a fluid-filling ratio of 60% and 50%, respectively, demonstrate excellent isotherm and heat transfer efficiency. The presence of NCG affects the condenser section heat transfer process of the vapor, leading to a 2 °C decrease in the average temperature of the condenser section of the TPCT (Tca). In comparison with the TPCT without NCG, it was observed that an increase in the mass of NCG from 0.0097 to 0.0197 mol resulted in a reduction in the effective length of the condenser section (La) and effective heat transfer rate (κ) of R22 TPCT. The decrease in La was 75.1 mm, while the decrease in κ was 15.02%. Furthermore, at the same NCG mass, the effective heat transfer rate of R410a TPCT is evidently superior to that of R22 TPCT. The NCG in the TPCT can be removed by using a check valve. Nevertheless, this will result in a reduction in the fluid-filling ratio of the TPCT. The temperature of the R410a TPCT containing 0.0197 mol NCG with a fluid-filling ratio of 50% is comparable to that observed in fluid-filling ratio of 40% after the NCG is exhausted. [ABSTRACT FROM AUTHOR]

Since their invention by Akachi in 1990s, pulsating heat pipes (PHPs) have attracted widespread interest and application in practice, e.g., grinding, chip cooling, the thermal management of batteries, etc., owing to their notable efficiency in heat transfer and their simplicity and flexibility in structure. Key factors influencing the heat transfer efficacy of pulsating heat pipes are mainly attributed to the thermophysical properties of the working fluid, the structural parameters, and the operating conditions. Research on pulsating heat pipes is conducted through theoretical investigations, numerical simulations, and visual experiments. In this paper, the research on PHPs in recent decades is reviewed with the consideration of the heat transfer performance mechanism and application of pulsating heat pipes, especially research under operation conditions such as with "status with motion" and with "inconsistent heat flux". [ABSTRACT FROM AUTHOR]

Gravity-assisted separation heat pipes (GSHPs) are extensively utilized in telecommunications base stations and data centers. To ensure year-round cooling, integrating GSHPs directly with a vapor compression refrigeration system is a viable solution. It is unavoidable that the refrigeration system's lubricant will infiltrate the heat pipe loop, thereby affecting its thermal performance. This paper examines the performance of a GSHP, which features a water-cooled plate heat exchanger as the condenser and a finned-tube heat exchanger as the evaporator, when the working fluid (R134a) is contaminated with a lubricant (POE, Emkarate RL-46H). The findings are compared with those from a system free of lubricant. The experimental outcomes indicate that the presence of lubricant degrades the heat transfer efficiency, particularly when the filling ratio is adequate and no significant superheat is observed at the evaporator's outlet. This results in a 3.86% increase in heat transfer resistance. When the charge of the working fluid is suboptimal, the average heat transfer resistance remains relatively constant at a 3% lubricant concentration yet increases to approximately 5.27% at a 4–6% lubricant concentration, and further to 12.32% at a 9% lubricant concentration. Concurrently, as the lubricant concentration fluctuates between 3% and 9%, the oil circulation ratio (OCR) varies from 0.02% to 0.11%. [ABSTRACT FROM AUTHOR]

As flexible electronic technologies rapidly developed with a requirement for multifunction, miniaturization, and high power density, effective thermal management has become an increasingly important issue. The oscillating heat pipe, as a promising technology, was used to dissipate high heat fluxes and had a wide range of applications. In this paper, we reported the fabrication and heat transfer performance evaluation of a polymer-based flexible oscillating heat pipe (FOHP) prepared using 3D printing technology. The 3D-printed inner surface presented excellent wettability to the working fluid, which was beneficial for the evaporation of the working fluid. Ethanol was selected as the working fluid, and the influence of the filling ratios range of 30–60% on heat transfer performance was analyzed. It was found that a 3D-printed FOHP with a filling ratio of 40% presented the best heat transfer performance with the lowest thermal resistance, and the fabricated heat pipes could be easily bent from 0° to 90°. With the best filling ratio, the thermal resistance of the FOHPs increased with larger bending angles. In addition, the 3D-printed FOHP was successfully applied for the thermal management of flexible printed circuits, and the results showed that the temperature of flexible printed circuits was kept within 72 °C, and its service life was guaranteed. [ABSTRACT FROM AUTHOR]

The application of batteries has become more and more extensive, and the heat dissipation problem cannot be ignored. Oscillating Heat Pipe (OHP) is a good means of heat dissipation. In this paper, the methods to improve the energy conversion and flow thermal performance of micro-channel OHP are studied and summarized. The working principle, heat transfer mechanism, advantages and applications of PHP are also introduced in detail in this study. Proper adjustment of the micro-channel layout can increase the heat transfer limit of PHP by 44%. The thermal resistance of two-diameter channel PHP is 45% lower than that of conventional PHP. The thermal resistance of PHP under uneven heating can be reduced to 50% of the original. PHP pulse heating can alleviate the phenomenon of dryness. Different working fluids have different effects on PHP. The use of graphene nano-fluids as the work medium can reduce the thermal resistance of PHP by 83.6%. The work medium obtained by the mixture of different fluids has the potential to compensate for the defects while inheriting the advantages of a single fluid. [ABSTRACT FROM AUTHOR]

The miniaturisation of systems has widely attracted researchers to contribute to the thermal performance enhancement utilising efficient heat transfer devices. Pulsating heat pipe (PHP) is an inimitable passive two-phase energy conversion/transport device capable of transferring extensive heat energy by oscillating liquid slugs and vapour plugs from hot to cold areas suitable for the management of generation of heat in electronics. Several theoretical, experimental and visualisation studies are made on PHP to understand operational mechanisms and to arrive at optimal conditions. Although PHP has potential application in the field of heat management, its effective utilisation is limited due to complex operational mechanisms.It is observed that PHPs are effective and reliable devices for utilisation in numerous energy systems. It is observed that PHPs are effective and reliable devices for utilisation in numerous energy systems. Advanced research is directed towards numerical simulations on the evaluation of PHP performance with non-conventional working fluids. It is found that the use of ternary hybrid nanofluid in PHP has scope for research work. The objective of this paper is to review the existing methodologies for enhancing the PHP overall performance and its applications. [ABSTRACT FROM AUTHOR]

With the highly integrated development of electronic components, higher requirements are put forward for heat transfer components, there is an urgent need for high-performance heat transfer components. Oscillating heat pipe (OHP) is novel types of heat pipe with excellent heat transfer capability. However, the heat transfer limit hinders their operation and application. This paper proposes an OHP under trans-critical conditions with the working fluid of R218 and explores its operating characteristics and heat transfer capability. Visualization experiments and pressure monitoring were conducted to analyze the state changes within the OHP under trans-critical conditions. After evaporation section temperature surpassing the critical temperature of R218, the pressure increases with the temperature, once the pressure reached near the critical pressure, OHP can transcend the limitation of the critical temperature and realize efficient oscillating operation under trans-critical conditions. With stable operation, the working fluid velocity can reach up to 1.49 m/s, and the OHP under trans-critical conditions presented excellent heat transfer performance with the thermal resistance of 0.22 °C/W. This paper proves the feasibility of the OHP under trans-critical conditions and provides a basis for further research. [ABSTRACT FROM AUTHOR]

Heat pipes transfer heat via phase transformation of the working fluid, where the working fluid will keep the temperature constant and absorb or release a large amount of latent heat during phase transformation. With the development of heat pipe technology, the isothermal performance of heat pipes has been gradually emphasized in many application fields. Most studies focused on the average temperature characteristics of one heat pipe or several heat pipes with the same type, and lacked a comprehensive analysis on the isothermal performance of different heat pipes. In this paper, previous studies on the application fields of the isothermal performance of heat pipes, the isothermal level of heat pipes used in different fields, and the methods to improve the isothermal performance of heat pipes are summarized. The parameters of the wick have little effect on the temperature uniformity of the heat pipe, while the arrangement of the wick has more influence on the uniformity of the heat pipe. The most suitable charge rate is 15% to 30% of the total inner volume, and the best start-up performance and isothermal performance is at approximately 45°. [ABSTRACT FROM AUTHOR]

• The effect of filling pressures on a gravity-assisted cryogenic loop heat pipe was experimentally studied. • Filling pressures were determined based on the presumption of vapor-liquid distribution. • Heat transfer behavior and hysteresis phenomena under different filling pressures were investigated. • Test results show a good agreement with the presumptions regarding vapor-liquid distribution. This paper investigated the effect of filling pressure on the operating characteristics of a gravity-assisted cryogenic loop heat pipe(CLHP) for use in gravity environments such as terrestrial and lunar environments. The CLHP wick is made of sintered stainless-steel fibers with a pore radius of 1.56 μm and designed with a heat transport distance of 2.05 m. The experiments were conducted under gravity-assisted conditions (the condenser was placed 0.1 m higher than the evaporator). Notably, the filling pressure originated from the assumed vapor-liquid distribution in the CLHP under steady-state conditions. The filling pressure was varied from 2.9 MPa to 3.4 MPa in 0.1 MPa increments for six different conditions. Specifically, (1) 2.9 MPa and (2) 3.0 MPa are conditions where the heat leakage due to the vapor phase in the evaporator core is large, while (3) 3.1 MPa and (4) 3.2 MPa are conditions where there is no vapor phase in the evaporator core and the surplus vapor phase escapes to the CC. In general, this condition is considered to be the optimum amount of working fluid for room-temperature LHPs when designing. (5) 3.3 MPa and (6) 3.4 MPa are overfilling conditions that cause the CC to be filled with liquid. The results revealed that the higher the filling pressure, the more obvious the variation in operating temperature caused by the transition of drive modes. The maximum heat transfer capability reached 25 W in cases (1)-(4). In cases (5) and (6), the heat transfer capabilities increased to 30 W, although the operating temperature was higher. Furthermore, the hysteresis effect under different filling pressure conditions was newly confirmed. The power cycling experiments demonstrated that hysteresis in the operating temperature occurred at high heat loads and showed a similar trend to the room-temperature LHP. [ABSTRACT FROM AUTHOR]

Alkali metal heat pipes are the vital components within the core of heat pipe-cooled reactors. The mechanism characteristics of alkali metal heat pipes need to be further analyzed. CFD simulation with the traditional Volume of Fluid method provide an essential means to analyze the flow and heat transfer mechanism in alkali metal heat pipes. For Lee heat and mass transfer equation within the VOF method, the evaporation and condensation coefficients have significant effects on the simulation results, and their values are typically determined empirically, which results in inaccurate or even unreasonable simulation results. To establish a reasonable numerical relationship for the evaporation and condensation coefficients of the working fluid, this paper employs a multi-zone modeling approach for heat pipes and proposes an improved VOF method. During the iterative process, the temperature and pressure values in the corresponding regions are updated based on the iteration results. The mass changes caused by evaporation and condensation processes in each wick and vapor chamber region are calculated and compared with the theoretical value. To validate the proposed method, a high-temperature experimental test platform was constructed, and an 820 mm sodium heat pipe was fabricated. Furthermore, experimental research was carried out at different heat pipe inclination angles and under various heat transfer powers, with the experimental results being compared to those obtained from the model simulations. The simulated temperature values at different points of the model agree well with the experimental values at different heat transfer levels. This research provides insights into the multiphase distribution and pressure change within the heat pipe, offering important references for the optimization design of alkali metal heat pipes. [ABSTRACT FROM AUTHOR]

• Experimental study on spray cooling with microencapsulated phase change suspension. • Latent heat of suspension improves spray cooling by up to 33.9 % relative to water. • Excessive suspension concentration results in the deterioration of spray cooling. • Optimal spray rate or height exists when liquid-film exactly covers heat surface. • Optimal inlet temperature is 10 °C lower than melting peak temperature of particle. This paper reveals the spray cooling heat transfer mechanism with the microencapsulated phase change suspension as the working fluid based on an experimental investigation. The heat transfer performances between the spray cooling and jet impinging with suspension are compared. A series of key parameters affecting the heat transfer are analyzed, including suspension concentration, nozzle diameter, spray height, inlet temperature, and spray inclined angle. It concludes that the spray cooling of suspension has a stronger heat transfer capability relative to the jet impinging, but the pressure loss is correspondingly larger. Compared with the water, the spray cooling heat transfer of phase change suspension is superior owing to the latent heat ability of particle core, and the maximum improvement in the current tests is 33.9 %. However, excessive suspension concentration can result in the deterioration of heat transfer because of the larger viscosity. There is an optimal volumetric flow rate and an optimal spray height for the spray cooling, where a suitable spray cone angle is present so that the heating surface may be exactly covered by the liquid-film. The latent heat absorption characteristic of phase change core leads to the existence of an optimal spray inlet temperature, which is about 10 °C lower than the melting peak temperature of the particle core. For the cases of incomplete-coverage, the decrease of the spray inclined angle may enlarge the liquid-film covering area and thence improve the heat transfer. A theoretical analysis indicates that the suspension in current tests has a high complete-melting rate (σ ≥ 93.7 %) when using in the spray cooling. The results may provide the beneficial support for the potential application of spray with phase change suspension for the cooling of electronic devices. [ABSTRACT FROM AUTHOR]

Dual compensation chamber loop heat pipe (DCCLHP) is a promising technology for future avionics cooling. In this work, the effect of orientations on the steady state and startup performance of a DCCLHP with kW class heat transfer capacity was investigated experimentally. Experimental results show that for high capacity DCCLHP, the variation of condenser performance at different orientations is a decisive factor in system performance, which could result in up to a 50 % decrease or 10 % increase in the heat transfer limit. In addition, the lack of condenser performance could also have a potential negative impact on the startup performance. By establishing a method to analyze the operating temperature, the authors provided an in-depth analysis of the related mechanisms. The effect of gravity could change the force condition and therefore the thickness of the condensate in the condenser, which in turn affects the condenser capacity and the working fluid temperature flowing into the compensation chamber. As a result, for practical applications, it is necessary to focus on the installation of the condenser to avoid the orientations that tend to cause the increase in the thickness of the condensate film, such as the condensate flow inside the condenser was opposite to gravity. The research content of this paper could deepen the understanding of LHP technology and contribute to the realization of application in acionics cooling in the future. [ABSTRACT FROM AUTHOR]

Thermal management technology based on loop heat pipes (LHPs) has broad application prospects in heat transfer control for aerospace and new energy vehicles. LHPs offer excellent heat transfer performance, reliability, and flexibility, making them suitable for high-heat flux density, high-power heat dissipation, and complex thermal management scenarios. However, due to limitations in heat source temperature and heat transfer power range, LHP-based thermal management systems still face challenges, especially in thermohydraulic modeling, component design, and optimization. Steady-state models improve computational efficiency and accuracy, while transient models capture dynamic behavior under various conditions, aiding performance evaluation during start-up and non-steady-state scenarios. Designs for single/multi-evaporators, compensation chambers, and wick materials are also reviewed. Single-evaporator designs offer compact and efficient start-up, while multi-evaporator designs handle complex thermal environments with multiple heat sources. Innovations in wick materials, such as porous metals, composites, and 3D printing, enhance capillary driving force and heat transfer performance. A comprehensive summary of working fluid selection criteria is conducted, and the effects of selecting organic, inorganic, and nanofluid working fluids on the performance of LHPs are evaluated. The selection process should consider thermodynamic properties, safety, and environmental friendliness to ensure optimal performance. Additionally, the mechanism and optimization methods of the start-up behavior, temperature oscillation, and non-condensable gas on the operating characteristics of LHPs were summarized. Optimizing vapor/liquid distribution, heat load, and sink temperature enhances start-up efficiency and minimizes temperature overshoot. Improved capillary structures and working fluids reduce temperature oscillations. Addressing non-condensable gases with materials like titanium and thermoelectric coolers ensures long-term stability and reliability. This review comprehensively discusses the development trends and prospects of LHP technology, aiming to guide the design and optimization of LHP. [ABSTRACT FROM AUTHOR]

• A reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) was fabricated. • Micro-grooved structures allowed faster startup and more stable operation of the MGTPLT. • The convection heat transfer ratio was proposed to evaluate the heat transfer enhancement. • The MGTPLT reduced evaporator temperatures and achieved thermal performance enhancement. The two-phase loop thermosyphon (TPLT) offers significant advantages in high-efficiency and long-distance heat transport for various thermal and energy-related applications without external power consumption. In this paper, the startup and heat transfer characteristics of a reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) were experimentally investigated and compared with a loop thermosyphon having a smooth-tube evaporator (STPLT). R245fa was used as a working fluid at volumetric filling ratios of 50–80 %. The results demonstrated that the additional microgrooves allowed a shorter startup time characterized by a lower maximum temperature with respect to the temperature overshoot. Compared to the STPLT, the MGTPLT exhibited lower evaporator temperatures and better temperature uniformities; the micro-grooved structures could alleviate local intermittent dry-out at moderate and high levels of power input for filling ratios of 50 and 60 %. The addition of micro-grooved structures enhanced the heat transfer performance of the TPLT characterized by lower thermal resistances. The convection heat transfer ratio (CHTR) was proposed to evaluate the enhancement of the convection heat transfer within the evaporator region after using microgrooves. The CHTR showed a decreasing tend with increasing the power input. At moderate filling ratios, the value of the CHRT was greater than 1.1, and an approximately 18 % increase in the convection heat transfer was achieved under the combined condition of 60 % filling ratio and 250 W power input. The fundamental mechanism of heat transfer performance enhancement with the aid of micro-grooved structures was analyzed and elucidated. [ABSTRACT FROM AUTHOR]

· Different change rate of cross-sections along the flow field is investigated. · Microchannel condenser is used in loop heat pipe. · Effects of operating height situation and microchannel structure are studied. · Smooth operation and anti-gravity performance are effectively improved with the help of tapered channel. The microchannel condenser has the advantages of high condensation efficiency and low flow resistance. The vapor can liquefy and return to the evaporator rapidly, and the heat transfer performance and the smooth operation of the loop heat pipe can be improved. Suitable change rate of the cross-section along the microchannel flow field can adjust the pressure drop and flow rate of working fluid, and improve the anti-gravity performance of the loop heat pipe. In this paper, microchannels with different change rates of cross-section along the flow field, including 0.5 mm straight channel, 1 mm straight channel, divergent channel, and tapered channel, are designed and applied as condensers in loop heat pipes. The entrance size of the gradient microchannel is 0.5 mm or 1 mm, which uniformly expands to 1 mm or shrinks to 0.5 mm along the flow field. The vapor and cooling water form a countercurrent direction of motion in the microchannel, and the heat exchange is carried out through the bottom of the channel. The influence of the microchannel condenser structure and operating height situation between the evaporator and condenser on the heat transfer performance of the loop heat pipe are experimentally investigated. The results show that the loop heat pipe achieves an extremely low thermal resistance of 0.004℃/W with gravity assist. Under the gravity-free operation situation, the maximum evaporator temperature of the loop heat pipe with 0.5 mm straight channel and 1000 W heat load is 128.1 °C, and the vapor pressure drop fluctuated is in the range of -1000 ∼ 2500 Pa. Under the anti-gravity operation, the loop heat pipe with the tapered channel has effective transfer performance at 500 W, and the influence factor is only 28.1 %. [ABSTRACT FROM AUTHOR]

Ensuring the structural integrity of the monolithic core, which houses critical components such as heat pipes and fuel rods, is crucial for the safety of a microreactor. This research introduces a hybrid heat pipe shutdown rod as a novel passive safety system to address potential temperature rises in the monolithic core during accidents. It is designed to perform a dual function under accident conditions. It simultaneously absorbs neutrons and removes heat through structural modifications to the existing shutdown rod. Specifically, this system provides a heat transfer path within the monolithic core without increasing the size of the microreactor. By selecting cesium as the working fluid, we aimed to achieve rapid operation of the heat pipe to quickly reduce the temperature gradient in the monolithic core under accident conditions. The hybrid heat pipe was fabricated and evaluated and found to develop continuous flow even at temperatures around 205.1°C. However, its unique structure causes a pronounced converging–diverging effect, resulting in a temperature drop from about 70–170°C in the evaporator region, followed by a slight recovery to below 50°C in the condenser. This effect arises from changes in the cesium vapor mass flow rate due to phase changes and variations in the flow area between the evaporator and condenser. Despite these effects, the use of liquid cesium as the working fluid enables the hybrid heat pipe to operate under natural convection, avoiding startup problems that could cause flow blockage. [ABSTRACT FROM AUTHOR]

A sintered wick was formed in a heat pipe through the process of sintering a mixture of copper powder with particle sizes of 100 µm and 200 µm, mixed with a pore-forming agent. The heat pipe's thermal resistance, which affects its heat transfer efficiency, is determined during manufacturing according to the powder type, thickness of the sintered wick, and filling rate of the working fluid. Heat transfer efficiency was then tested at various inclination angles (0°, 45°, and 90°) to evaluate the performance of heat pipes. Regardless of the filling amount and test angle, the 200 μm copper powder type exhibited superior heat transfer efficiency compared to the 100 μm type. After analyzing heat transfer performance at various filling rates between 20% and 50%, it was determined that the heat pipe's optimal heat transfer capability occurred at a working fluid filling rate of 30%. The width of the wick was directly related to the heat transfer performance. [ABSTRACT FROM AUTHOR]

• Thermal behavior of overfilled sodium heat pipe was experimentally investigated to broaden the understanding of the filling ratio effect on heat pipe performance. • Excess liquid propagation occurs at high heat flux conditions, leading to the formation of a liquid pool at the condenser end. • The temperature gradually increases as the liquid propagates to the condenser end, instead of experiencing abrupt rise. However, sudden dry-out occurred where the propagation of excess liquid was not clearly observed. • The capillary limitation model reasonably predicts the limit power of overfilled sodium heat pipe under horizontal operating conditions. Operating limitations model of heat pipes are difficult to evaluate the effect of the charging amount of working fluid, which is one of the crucial factors in heat pipe performance of startup and maximum heat transfer. To reduce the knowledge gap on the effect of filling amount, the purpose of this paper is an investigation of the thermal behavior of excess liquid. Overfilled sodium heat pipe having about 260% filling ratio was fabricated and tested with the change of boundary and initial condition. Sodium heat pipe showed good isothermal characteristics that the temperature difference between evaporator and condenser reduced as the development of continuum flow until reaching operating limit condition. The characteristics of an overfilled heat pipe appear at power increased after the continuum flow fully developed to the condenser that sudden temperature rise about 100 °C in the overall heat pipe except the end region of the condenser where a continuous temperature about 50 °C drop occurs. Excess liquid moves toward the condenser end and forms the liquid pool and lessening the active condenser length which reduces the maximum heat transfer of heat pipe and induced the operating limitation. The results showed that this phenomenon only occurs at high heat flux conditions as sufficient vapor flow is required to allow excess liquid to propagate to the condenser and determined the operating limitation of overfilled sodium heat pipe that continuous temperature rises as the liquid propagates to the end of the condenser rather than the abrupt rising such as dry out. However, the operating limitations occur sudden dry-out of the evaporator similar to the conventional results of liquid metal heat pipe when the liquid propagation did not occur. Compared to the entertainment limit model, there was a significant difference in the dry-out point of the experimental results with excessive liquid propagation. It was confirmed that the horizontal operation of overfilled heat pipe was in good agreement with the prediction of capillary limit model within 22.6% of the maximum error. [ABSTRACT FROM AUTHOR]

Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux. [ABSTRACT FROM AUTHOR]

A wide variety of heating and cooling applications use heat exchangers. The increase in energy prices, the requirement for size reduction, and restriction on greenhouse gas emissions has led to the need for finding ways to develop efficient heat exchangers. A cost-efficient way to enhance the model of a heat exchanger by visualizing the effects of the design parameters is using Computational Fluid Dynamics (CFD). The reason for this exploration was to lead an examination of the varieties/changes in the general intensity move process for a Finned-Tube Heat Exchanger (FTHE), also known as Air Coil Heat Exchanger (ACHE) with a variety of plan boundaries like the quantity of tubes, course of action of tubes, and the material utilized for the intensity exchanger. The widely used heat exchanger that uses refrigerant R314a and air as the working fluids was simulated with different design modifications. The simulated results exhibited as to how the number of tubes, arrangement of coils/tubes, material of tubes, and density / spacing of fins, effects the pressure drop, temperature and velocities profiles, and heat exchangers' transfer of a heat. The use of copper coils improved the heat transfer by approximately 61% as compared to aluminium coils. [ABSTRACT FROM AUTHOR]

• LMHOHPs with different proportions of sodium-potassium alloy as working fluids were firstly developed and tested. • The influence of NaK proportions and temperature on wettability was studied. • The influence of NaK proportions and working fluid properties on the LMHOHP performance was studied. • The heat transfer performance of LMHOHPs increased with the growth of potassium mass fraction. Liquid metal high-temperature oscillating heat pipes (LMHOHPs) are highly efficient heat transfer devices which can work in high temperature environments above 500 °C. In this paper, LMHOHPs with different proportions of sodium-potassium alloy (hereinafter referred to as NaK) as working fluids were developed, and their start-up and heat transfer performance was investigated experimentally. 310S stainless steel with an inner diameter of 6mm was employed as the pipe material, and the filling ratio was kept as 50%. Under the conditions of input powers 1500∼3500 W and inclination angles 0°∼90°, LMHOHPs with five proportions of NaK (namely, 0, 10, 46, 78 and 100 wt.% of potassium) as working fluids were tested. The results showed that: (1) The wettability between NaK and 310S stainless steel was poor, but it was improved with the increase of temperature and potassium mass fraction. At 350 °C, The contact angles of NaK (K100%) and NaK (K78%) were 72.3° and 81.1°, respectively, which meant that the liquid metal changed from non-wetting state to wetting state. (2) Except that the LMHOHP with NaK (K0%) could not start up at the inclination angle of 0°, the five LMHOHPs could start up successfully and transfer heat through oscillating motion at all inclination angles and input powers. (3) When the heat flux exceeded 4.56 × 106W/m2, the flow patterns of LMHOHPs with NaK (K10%), NaK (K46%), NaK (K78%) and NaK (K100%) changed from oscillating motion to stable unidirectional circulating flow, which significantly improved their heat transfer performance. (4) Due to the influences of saturated vapor pressure gradient, viscosity and wettability, the heat transfer performance of LMHOHPs increased with the growth of potassium mass fraction. The minimum thermal resistance of the LMHOHP with NaK (K100%) was 0.071 °C/W at the input power of 3528 W and the inclination angle of 90°. This study could provide guidance for the working fluids selection of oscillating heat pipes in high temperature environments. [ABSTRACT FROM AUTHOR]

The pulsating heat pipe (PHP) can be used to transfer massive heat to reduce the thermal damage to the cutting tool when machining difficult-to-cut materials. To select a better open PHP, the heat transfer performance was experimentally investigated in this study. The operating characteristics of different types of working fluids under different heat flux were analyzed compared with the closed PHP. Visual experiments were established to verify the experimental characteristics. The effects of heat flux, length, ratio of inner/outer diameter, and inclination angle on equivalent thermal resistance were analyzed. Based on the experimental data and four boosting integrating learning methods, a model for heat transfer performance prediction was proposed. The prediction model based on the CatBoost method had better goodness-of-fit and the best prediction effect. The R2, MAPE, and RMSE of the validation set were the best, which are 0.9258, 7.2564, and 0.1057 respectively. In addition, the contribution of input parameters to the output results was evaluated, while L, Di/Do, and Pr were the top three variables. Sub-tree structures used to explain the prediction model were also presented. This proposed prediction model can be used to select the most suitable open PHP before designing PHP self-cooling tools. [ABSTRACT FROM AUTHOR]

Mini-grooved flat heat pipe (MGFHP) possesses the advantages of high compactness, no mechanical component, super thermal conductivity, and excellent temperature uniformity, which can meet the demand for electronic devices efficiently cooling. In this research, visual and heat transfer experiments were performed to investigate the flow and thermal characteristics inside the MGFHP. Fluid flow and distribution are observed to be quite different in the MGFHP containing different working fluids, which is affected by the physical properties of working fluid, the surface state of the grooved wick, and limited working space. Additionally, the input heat, working fluid type, filling ratio, and wettability obviously affect the thermal conductivity and temperature uniformity of the MGFHP. The deionized water-filled MGFHP possesses lower thermal resistance and higher heat transfer capacity than anhydrous ethanol or hexane filled MGFHP, especially for the copper oxide MGFHP filled with deionized water with a filling ratio of 1.0. Thermal resistance, maximum temperature, and temperature nonuniformity at the condensation section of deionized water-filled copper oxide MGFHP are lower than those of the original copper MGFHP by 31.1%, 3.7 °C, and 0.11 °C for the anhydrous ethanol filled MGFHP and 34.4%, 3.1 °C, and 0.13 °C for the hexane filled MGFHP, respectively. [ABSTRACT FROM AUTHOR]

As obtained geofluids from enhanced geothermal systems usually have lower temperatures and contain chemicals and impurities, a novel power cycle (NPC) with a unit capacity of several hundred kilowatts has been configured and developed in this study, with particular reference to the geofluid temperature (heat source) ranging from 110 °C to 170 °C. Using a suitable CO2-based mixture working fluid, a transcritical power cycle was developed. The novelty of the developed power cycle lies in the fact that an increasing-pressure endothermic process was realized in a few-hundred-meters-long downhole heat exchanger (DHE) by making use of gravitational potential energy, which increases the working fluid's pressure and temperature at the turbine inlet and, hence, increases the cycle's power output. The increasing-pressure endothermic process in the DHE has a better match with the temperature change of the heat source (geofluid), as does the exothermic process in the condenser with the temperature change of the sink (cooling water), which reduces the heat transfer irreversibility and improves the cycle efficiency. Power cycle performance has been analyzed in terms of the effects of mass fraction of the mixture working fluids, the working fluid's flowrate and its DHE inlet pressure, geofluid flowrate, and the length of the DHE. Results show that, for a given geofluid's temperature and mass flowrate, the cycle's net power output is a strong function of the working-fluid's flowrate, as well as of its DHE inlet pressure. Too high or too low of a DHE inlet pressure results in a lower power output. When geofluid temperature is 130 °C, the optimum DHE inlet pressure is found to be 11 MPa, corresponding to an optimum working-fluid flowrate of 6.5 kg/s. The longer the DHE, the greater the corresponding working-fluid flowrate and the higher the net power output. For geofluid temperature ranging from 110 °C to 170 °C, the developed NPC has a better thermodynamic performance than the conventional ORC. The advantage of using the developed NPC becomes obvious when geofluid temperature is low. The maximum net power output difference between the NPC and the ORC happens when the geofluid temperature is 130 °C and NPC's working fluid mass fraction (R32/CO2) is 0.5/0.5. [ABSTRACT FROM AUTHOR]

As the heat dissipation of smart devices increases, cutting-edge cooling solutions are becoming increasingly important. The heat pipe is an efficient device that boosts heat transfer and is recommended to reduce thermal management power. In this study, a concentric annular heat pipe (CAHP) with distilled water as a working fluid is proposed to enhance heat transfer, and experiments and one-dimensional analysis were carried out to predict thermal characteristics and evaluate performance. The CAHP was 90 mm in length, 62 mm in inner diameter, 70 mm in outer diameter, and 0.4 mm in thickness. At the outer surface of the internal CAHP, a two-layer screen mesh wick (500 mesh, Stainless Steel 304) that is 0.34 mm in layer thickness was installed. A ceramic heater (20 mm × 20 mm) was attached to the middle of the outer surface, and the hollow region with 48 fins was cooled by an electric fan. The experiment was carried out with variations in the heat load, the filling ratio of the working fluid, the pitch angle, the roll angle, and the airflow speed, and the one-dimensional analysis was modeled by AMESIM. The experimental results showed that the best thermal resistance of the CAHP was 3.74 °C/W with a supplied heat of 20 W, a pitch angle of −15°, and a Vair of 3 m/s. In addition, the CAHP's 1-D simulation model using AMESIM was verified through the experimental results. However, although the modeling results according to the inclination angle could not be reflected due to the difficulty of implementing multiple orientation structures in the one-dimensional simulation model, the simulation results were found to be almost consistent with the experimental results. Case studies were conducted to understand the various characteristics of the CAHP using the model, and the optimal volume fraction, the porosity, and the number of layers of the wicks were determined to be 10, 0.345, and 2, respectively. [ABSTRACT FROM AUTHOR]