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This paper researches the heat transfer equation and thermal balance equation of a shell-and-tube evaporator; constructs an accurate mathematical model for the evaporator; and derives equations including detailed and accurate calculation methods for all heat transfer coefficients, such as the refrigerant side heat transfer coefficient, water side heat transfer coefficient, refrigerant kinematic viscosity, density, and specific enthalpy. Adopting this approach involves fitting the relationships between the density, thermal conductivity, kinematic viscosity, and enthalpy of R134a refrigerants in saturated vapor and liquid states. The relationships between superheated gas enthalpy, density, and temperature were also assessed, and heat transfer coefficients were obtained through calculation methods and microelement heat transfer relationships in both the single-phase and two-phase zones, matching empirical formulas concerning the relationship between superheated enthalpy and temperature. Notably, the research utilizes the Simulink approach without relying on M files and S functions to establish the evaporator's two-phase and superheated zones, as well as an overall simulation model which provides intuitive internal coupling relationships and the coefficient calculation process in the formulas and uses the function "Algebraic Constraint" instead of "memory" or "1/z" to solve algebraic loops, thereby avoiding computation deviations introduced by delays and iterations. Finally, simulation calculations were conducted, and an experimental platform was designed and built for experimental verification which can validate the derived mathematical models. The simulation results, including the evaporator pressure, and chilled water outlet temperature with variation in chilled water mass flow rate, closely matched the experimental outcomes. The simulation model is concise and intuitive. Modifying parameters such as the thermal conductivity of the model material is straightforward, thereby alleviating the workload for researchers. It also facilitates an understanding of model principles for beginners. Moreover, the database generated from the model allows for fault analysis, diagnosis, and decision evaluation. [ABSTRACT FROM AUTHOR]

In many energy-related applications, components with high heat loads, such as power electronics, play an important role. Pulsating heat pipes (PHPs) are an effective solution to deal with the increasing heat load of these components. In many real-life applications, the PHP must work against gravity and still be able to operate efficiently. However, the majority of present flat-plate PHP designs do not perform well under this condition. Therefore, this paper presents a flat-plate PHP with a conventional channel design optimized for gravity-independent operation. The PHP was capable of transmitting a heat output of 754 watts in all orientations, while the testing heater in use never exceeded a temperature of 100 °C. No indications of dryout were observed, implying that the maximum thermal load the PHP can handle is even higher. Additionally, three different condenser zone sizes were tested with the PHP. Previously published results indicated that there is a specific range of suitable condenser zone sizes, and performance problems will occur if the condenser zone size falls outside of this range. The findings from this work point in the same direction. [ABSTRACT FROM AUTHOR]

An air-conditioning system (ACS), which consumes large amounts of high-grade energy, is essential for maintaining the indoor thermal environment of modern buildings. However, an ACS consumes almost half of the total energy of the building. Therefore, it is necessary to reduce the energy consumption of the ACS to promote energy conservation and emission reduction in the building sector. In fact, there is an abundance of waste heat and low-grade energies with the potential to be utilized in ACS in nature, but many of them are not utilized efficiently or cannot be utilized at all due to the low efficiency of thermal energy conversion. Known as a passive thermal transfer device, the application of a heat pipe (HP) in the ACS has shown explosive growth in recent years. HPs have been demonstrated to be an effective method for reducing building cooling and heating demands and energy consumption in ACS with experimental and simulation methods. This paper summarizes the different HP types applied in the ACS and provides brief insight into the performance enhancement of the ACS integrated with HP. Four types of HPs, namely tubular HP (THP), loop HP (LHP), pulsating HP (PHP) and flat HP (FHP), are presented. Their working principles and scope of applications are reviewed. Then, HPs used in natural cooling system, split air conditioner (SAC), centralized ACS (CACS) and cooling terminal devices are comprehensively reviewed. Finally, the heat transfer characteristics and energy savings of the above systems are critically analyzed. The results show that the performance of the HP is greatly affected by its own structure, working fluid and external environmental conditions. The energy saving of ACS coupled with HP is 3–40.9%. The payback period of this system ranges from 1.9–10 years. It demonstrates that the HP plays a significant role in reducing ACS energy consumption and improving indoor thermal comfort. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to develop SRF formulations for LHP performance enhancement. In this paper, the solute of SRF is prepared, and butanol and its isomer, 2-butanol, are selected. In this paper, concentrations of the 2-butanol aqueous solution (10%, 15%, and 20%) plus the butanol 6% aqueous solution were used to measure the surface tension of four different formulations of SRF and water. It was found that the higher the solute concentration became, the stronger the Marangoni effect was, and the more obvious the surface tension change of the 2-butanol 20% aqueous solution was. Water, the butanol 6% aqueous solution and the 2-butanol 20% aqueous solution were filled into LHP respectively, and the heat transfer performance was measured. The 2-Butanol 20% aqueous solution improved LHP performance by about three times compared with water, and the lowest total thermal resistance was only 1/4 that of water. Therefore, the 2-butanol 20% SRF aqueous solution is an ideal formula for improving the LHP heat transfer performance. [ABSTRACT FROM AUTHOR]

In recent years, the development of electronic chips has focused on achieving high integration and lightweight designs. As a result, pulsating heat pipes (PHPs) have gained widespread use as passive cooling devices due to their exceptional heat transfer capacity. Nevertheless, the erratic pulsations observed in slug flow across multiple channels constitute a significant challenge, hindering the advancement of start-up and heat dissipation capabilities in traditional PHP systems. In this paper, we introduce a flat plate pulsating heat pipe (PHP) featuring adaptive structured channels, denoted as ASCPHP. The aim is to enhance the thermal performance of PHP systems. These adaptive structured channels are specifically engineered to dynamically accommodate volume changes during phase transitions, resulting in the formation of a predictable and controllable two-phase flow. This innovation is pivotal in achieving a breakthrough in the thermal performance of PHPs. We experimentally verified the heat transfer performance of the ASCPHP across a range of heating loads from 10 to 75 W and various orientations spanning 0 to 90 degrees, while maintaining a constant filling ratio (FR) of 40%. In comparison to traditional PHP systems, the ASCPHP design, as proposed in this study, offers the advantage of achieving a lower evaporation temperature and a more uniform temperature distribution across the PHP surface. The thermal resistances are reduced by a maximum of 37.5% when FR is 40%. The experimental results for start-up characteristics, conducted at a heating power of 70 W, demonstrate that the ASCPHP exhibits the quickest start-up response and the lowest start-up temperature among the tested configurations. Furthermore, thanks to the guiding influence of adaptive structured channels on two-phase flow, liquid replenishment in the ASCPHP exhibits minimal dependence on gravity. This means that the ASCPHP can initiate the start-up process promptly, even when placed horizontally. [ABSTRACT FROM AUTHOR]

Achieving the global electricity demand and meeting the United Nations sustainable development target on reliable and sustainable energy supply by 2050 are crucial. Portable energy storage (PES) units, powered by solid-state battery cells, can offer a sustainable and cost-effective solution for regions with limited power-grid access. However, operating in high-dust and high-temperature environments presents challenges that require effective thermal management solutions. This paper is a comprehensive review of thermal management systems for PES units, with a specific focus on addressing the challenge of overheating in airtight designs. The review of various active and passive cooling systems is conducted through extensive study of the relevant literature, which is significant in providing insights into the operation, performance parameters, and design options for different cooling system technologies. The findings from this review show heat pipe (HP) technologies as key cooling-system solutions for airtight PES units. Specifically, loop and oscillating HPs, as well as the vapour chamber, offer desirable features such as compactness, low cost, and high thermal conductivity that make them superior to other alternatives for the cooling systems in PES. The insights and knowledge generated via this review will help facilitate the design and development of innovative, efficient, and reliable PES units, thereby contributing to the advancement of off-grid renewable energy applications and enabling sustainable power solutions worldwide. Furthermore, an appropriate design of PES units can help in reducing capital and maintenance costs. [ABSTRACT FROM AUTHOR]

This paper presents a novel technology to improve the energy efficiency of refrigeration and air-conditioning (RAC) systems by applying a condenser cooling approach. The approach is based on the integration of an innovative radiation-enhanced thermal diode tank (RTDT) with a RAC system. The thermal diode tank (TDT), consisting of heat pipes and an insulated water tank, is a passive device to generate cooling water at a minimum night ambient temperature. When the radiation-enhanced heat pipe (RHP) is equipped with the TDT, it becomes an RTDT, which could theoretically lower the water temperature below the ambient temperature. In this study, a radiation-enhanced thermal diode tank (RTDT) is proposed to supply cooling water to the RAC system. Simulation models for the proposed RTDT-assisted RAC (RTDT-RAC) system are developed in order to investigate the impacts of the tank size to cooling capacity (TS/Qc) ratio, day/night ambient temperature fluctuations on the system's coefficient of performance (COP) and the energy saving percentage (ESP). The results show that a greater day/night ambient temperature difference and a larger TS/Qc value can both enhance the COP and ESP of the RTDT-RAC system. The optimal and threshold TS/Qc values were 1 m3/kW and 0.18 m3/kW, respectively. These findings demonstrate the potential of the RTDT-RAC system to achieve significant energy savings and provide valuable insights for the design and optimization of an RTDT-RAC system. [ABSTRACT FROM AUTHOR]

In this paper, the effect of solar intensity on the heat pipe tip temperature in a heat pipe type—evacuated-tube solar collector (HP-ETSC) was investigated. A simple relation was proposed, relating the solar intensity to the heat pipe tip temperature generated from the experimental data. This simple empirical relation was applied in a set of heat transfer equations derived to predict the heating medium temperature at the manifold outlet of the evacuated-tube solar collector. The calculated results corresponding to two types of heating medium, i.e., palm oil and water, were compared with experimental results from the literature. The results show that the average error was 6.41% for the case of palm oil and 4.66% for the case of water. Based on the case of water as a heating medium fluid, it was found that the flow rate of the heating medium fluid affected the accuracy of prediction, as the percentage error increased with the heating medium flow rate. The maximum percentage error increased from only 1.83% for a water inlet flowing at a Reynolds number of about 2.4 × 103 to 15.23% for a water flow rate at a Reynolds number of about 2.6 × 104. The correction factor was added into the correlation to predict the heat transfer coefficients of heating medium fluids. With this correction factor, the maximum error could be reduced from 11.78% to 7.29% for the palm oil case and from 15.23% to 5.57% for the water case. The average errors corresponding to palm oil and water cases could be reduced to 0.74% and 1.26%, respectively. [ABSTRACT FROM AUTHOR]

An oscillating heat pipe (OHP) is an effective heat transfer device for the thermal management of electronic devices. However, the heat transfer mechanism of the OHP was not fully understood due to its complicated operation characteristics. In this paper, the thermal performance of an OHP was experimentally studied. The condensation and evaporation temperature variations were monitored under different heat inputs and were then used to evaluate the OHP system operating characteristics. Thermal resistance was used as a key parameter to evaluate the thermal performance of the OHP system. The results indicated that as the heat input increased from 25 to 100 W, the average thermal resistance decreased while the stable evaporating and condensing temperatures increased. The equivalent heat transfer coefficient was derived theoretically. It showed that the reciprocal of the radial heat transfer coefficient increased with increasing liquid film thickness. Based on this result, an empirical correlation was proposed to evaluate the thermal resistance of an OHP system. This correlation was validated using both the experimental data provided in this study and the data collected from the open literature. The comparison results indicated that the proposed empirical correlation could reasonably predict the thermal resistance under different filling ratios and heat inputs. [ABSTRACT FROM AUTHOR]

Various types of heat pipes are available to purchase off the shelf, from various manufacturers, but most of them have strictly defined geometry and technical parameters. However, when there is a need to use a heat pipe (HP) with an unusual size and shape or working conditions other than the standard ones, it becomes very costly to order them from manufacturers, especially in small quantities, and only a few producers are willing to fulfill such an order. This paper presents a detailed description and step-by-step method of manufacturing and testing a low-cost HP with specific properties and geometry, cooperating with a modular heat recovery system based on the use of phase change materials (PCM) for electromobility applications. The presented heat pipes were made entirely by hand, primarily with the use of basic workshop tools, without the use of specialized and automated CNC machines. Utensils used during the process were either made by hand or using desktop FDM 3D printers. During the evaluation of heat pipes' performance within PCM (coconut oil), simple statistical functions were used. One-dimensional and two-dimensional histograms were used to visualize data obtained during this research. The presented method allows the manufacturing of heat pipes that are, on average, able to melt about 35% more PCM than an empty copper pipe with the exact same geometry. The HPs' performance in coconut oil was evaluated on the basis of their future applications. [ABSTRACT FROM AUTHOR]

The ultra-long gravity heat pipe has a long heat transfer distance and narrow working fluid flow channel within its tube. Due to these unique design features, the vapor–liquid counter-flow and heat transfer characteristics of these heat pipes are more complex than those found in conventional-size heat pipes. This paper innovatively proposes the design of a segmented visualization window structure of an ultra-long gravity heat pipe, which successfully overcomes the challenge of visualizing the internal flow during operations. A visualization experimental platform, measuring 40 m in height with an inner diameter of 7 mm and the aspect ratio up to 5714, was built to investigate the evolving characteristics of two-phase flows with an increasing heat input and the impact of the phase change flow characteristics on the thermal performance of ultra-long gravity heat pipes. The results obtained can provide guidance for the development of the internal structure of ultra-long gravity heat pipes that are being applied in exploiting geothermal energy. The results show that, at low heat input (200 W, 250 W), there are separate flow paths between the condensate return and the steam, but the high hydrostatic pressure due to the height of the liquid injection results in the presence of an unsaturated working fluid with a higher temperature in the liquid pool area, which has a lower evaporation rate, limiting the heat transfer through the heat pipe. It is found that if increasing the heat input up to 300 W, the evaporative phase change in the heating section becomes intense and stable. At the same time, despite the intermittent formation of liquid columns in the adiabatic section due to the vapor–liquid rolls, which increases the resistance to the vapor–liquid counter-flow, the liquid columns are blocked for a short period of time, and the path of steam rises and the condensate return is smooth, which does not seriously affect the steam condensation and liquid return evaporation. At this point, the overall temperature of the heat pipe is evenly distributed along the tube and the heat transfer performance is optimal. When the heat input further increases (350 W, 400 W), a large amount of condensate is trapped in the upper part of the adiabatic section and the condensing section by long liquid plugs for a long time. At this point, the condensate flow back to the heating section is significantly reduced, and the steam is seriously prevented from entering the condensation section, resulting in a significant increase in the temperature gradient between the lower part of the evaporating section and the upper part of the adiabatic section and deterioration of the heat transfer performance. [ABSTRACT FROM AUTHOR]

The cooling system is the auxiliary equipment that consumes the most energy in a data center, accounting for about 30 to 50% of the total energy consumption. In order to effectively reduce the energy consumption of a data center, it is very important to improve the heat exchange efficiency at the chip level. Compared with air cooling, single-phase cold plate liquid cooling, and immersion liquid cooling, the flat loop heat pipe (FLHP) is considered to be a better chip-level cooling solution for data centers. It has extremely high heat transfer efficiency and heat flux variability, and it can avoid the operation risk caused by liquid entering the server. In this paper, a FLHP with an evaporator designed with a "Tesla valve" flow channel configuration is developed. Experiments on the FLHP are carried out, focusing on the installation angles and cooling condition factors. The results show that an inclination angle of 20° is the critical point of the influence of gravity on the performance of the FLHP; to ensure good operation of the FLHP, the installation angle should be greater than 20°. The equivalent heat transfer coefficients of the FLHP condenser under different cooling conditions are calculated. It is found that water cooling can provide higher cooling heat transfer coefficients with lower energy consumption and operating noise. Additionally, the heat transfer limit, operating temperature uniformity, and start-up stability of the FLHP are significantly improved under water cooling conditions. The maximum heat load of the FLHP is up to 230 W, and the temperature difference of the evaporator surface can be controlled within 0.5 °C, under 20 °C water cooling. Finally, using the FLHP for thermal management of the chip, its heat transfer efficiency is 166 and 41% higher than that of air cooling and water cooling, respectively. [ABSTRACT FROM AUTHOR]

This study concerns the numerical simulation and the experimental implementation of a low-scale Phase Change Material-based (PCM-based) condenser, to be included in a PCM-based portable cooling systems. In this category of cooling systems, the PCM can be integrated either in the condenser or in the evaporator. In the present study, the PCM is integrated in the condenser of the vapor compression cycle to absorb the heat power released from the refrigerant fluid (R134a) during condensation, thus eliminating the need to transfer heat to the external environment. The main objective of the present study is to realize and validate a numerical model capable of simulating both the refrigerant fluid and the PCM thermofluid dynamics. For this purpose, a commercial solver was used for the implementation of the developed numerical model, and experimental tests were performed to validate the numerical simulations results. The paper reports the details and test results of both the numerical model and the experimental apparatus. The simulation results indicate a good accordance between the numerical and experimental data. [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]

It is of practical significance to study the multi-physical processes of solid state nuclear systems for device design, safety analysis, and operation guidance. This system generally includes three multi-physical processes: neutronics, heat transfer, and thermoelasticity. In order to analyze the multi-physical field behavior of solid state nuclear system, it is necessary to analyze the laws of neutron flux, temperature, stress, and other physical fields in the system. Aiming at this scientific goal, this paper has carried out three aspects of work: (1) Based on Galerkin's finite element theory, the governing equations of neutronics, heat transfer, and thermoelasticity have been established; (2) a neutronics-thermal-mechanical multi-physical finite element analysis code was developed and verified based on benchmark examples and third-party software for multi-physical processes; (3) for a solid state nuclear system with a typical heat pipe cooled reactor configuration, based on the analysis code developed in this work, the neutronics-thermal-mechanical coupling analysis was carried out, and the physical field laws such as neutron flux, temperature, stress, etc., of the device under the steady-state operating conditions were obtained; and (4) finally, the calculation results are discussed and analyzed, and the focus and direction of the next work are clarified. [ABSTRACT FROM AUTHOR]

The mine return air flow has the characteristics of basically constant temperature and humidity all year round and is a high-quality waste heat resource. Its direct discharge not only wastes energy but also causes environment pollution. It has important economic value and application prospect to solve the problem of shaft antifreeze using new technology to recover the waste heat of mine return air. Gravity heat pipe is widely used in the heat recovery of mine return air. Its heat transfer process is a complex process with multiple parameters. The current research focuses on the influence of a single factor on heat transfer, which has many limitations. To analyze the effects of different parameters on the heat recovery effect of gravity heat pipe in mine return air and to optimize heat pipe heat exchanger parameters in the heat exchange system, mathematical models of gas–water countercurrent heat and mass transfer, entransy dissipation and exergy efficiency were established in this paper, based on the entransy dissipation theory. Under the condition of the given initial parameters, the effects of different parameters on the dimensionless factor, β, of heat transfer, total heat transfer, and entransy dissipation thermal resistance were analyzed. The experimental and calculation results show the entransy dissipation theory can be used to evaluate the heat transfer performance of the gravity heat pipe. When the entransy dissipation thermal resistance was minimum, the heat transfer performance was optimal. During the heat transfer process between the mine return air and the gravity heat pipe with high humidity under a given working condition, increasing the Reynolds number was beneficial to increase the heat transfer dimensionless factor, β. [ABSTRACT FROM AUTHOR]

With the rapid development of information science and technology, the demand for computer data processing is increasing, resulting in the rapid growth of the demand for high-power and high-performance solid-state drives (SSDs). The stable operation of SSDs plays an important role in ensuring the reliable working conditions and appropriate temperature of information technology equipment, rack servers, and related facilities. However, SSDs usually have significant heat emissions, putting forward higher requirements for temperature and humidity control, and consequently the heat sink system for cooling is essential to maintain the proper working state of SSDs. In this paper, a new type of thin heat pipe (THP) heat sink is proposed, and the heat transfer performance and cooling effect are experimentally and numerically studied. The numerical results are compared with experimental results, which showed an error within 5%. Single and double heat pipes were investigated under different input powers (from 5 W to 50 W) and different placement angles between 0° and 90°. The heat transfer performance of the new heat sink is analyzed by the startup performance, the evaporator temperature, and the total thermal resistance. The results show that the new double THPs with a 90° angle have a great advantage in the heat transfer performance of SSDs. The research is of great significance for the design and optimization of the SSDs' cooling system in practical applications. [ABSTRACT FROM AUTHOR]

This paper aims to deeply explore the influence of different inclination conditions on the heat transfer characteristics and broaden the application scene of a pulsating heat pipe. A test device for the heat transfer performance of a pulsating heat pipe under different inclination angles is designed and built. Under the condition of 70% liquid filling rate, ethanol and HFE-7100 are selected to carry out the experimental test with heating power of 40–140 W and dimensionless thermal difference of 0–0.56. The heat transfer performance, the temperature in the evaporation section and the internal pressure fluctuation of the pulsating heat pipe were experimentally studied. The results show that under the condition of uniform heat flux, for ethanol working medium, when the pulsating heat pipe is heated at 40 W, the operating thermal resistance varies significantly with different installation angles. At this time, the operating thermal resistance of a pulsating heat pipe with installation angles of 45°, 70°, 90° and D90° is 1.38 °C/W, 1.60 °C/W, 1.73 °C/W and 2.07 °C/W, respectively. With the increase in installation angle, the operating thermal resistance also increases gradually, reaching the maximum at 90°. At low heating power, the effect of the installation angle on the ethanol working medium is significantly greater than that of HFE-7100 working medium. The HFE-7100 working medium showed lower operating thermal resistance at low heating power, but with the increase in heating power, the operating thermal resistance of the two working medium gradually approached a 70% filling rate. Under non-uniform heating conditions, when HFE-7100 is used as a working fluid, the operating thermal resistance of a pulsating heat pipe under different heating power was lower than that of the ethanol working medium. The operating thermal resistance is less affected by the installation angle, and the overall heat transfer performance is better. The phenomenon in which the ethanol working medium is obviously affected by the installation angle can be improved by non-uniform heating conditions. For ethanol working medium, when the dimensionless heat difference reaches 0.33 under the condition of a 45° installation angle, the average temperature fluctuation in the evaporation section appears gentle. At this installation angle, the internal working medium of the four elbow pulsating heat pipe devices used in this research more easily forms a cycle in the pipe than the 90° installation angle. [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]

This paper reviews the manufacturing process of ultra-thin heat pipes and the latest process technologies in detail, focusing on the progress of the shape, structure, and heat transfer mechanism of the wick. The effects of the filling rate and tilt angle on the heat transfer performance of the ultra-thin heat pipe, as well as the material selection of ultra-thin heat pipes, is sorted out, and the surface modification technology is analyzed. Besides, the optimal design based on heat pipes is discussed. Spiral woven mesh wick and multi-size composite wick have significant advantages in the field of ultra-thin heat pipe heat transfer, and comprehensive surface modification technology has huge potential. Finally, an outlook on future scientific research in the field of ultra-thin heat pipes is proposed. [ABSTRACT FROM AUTHOR]

Pipe jackets are widely used in engineering as a component for heating or cooling reactors or other equipment. In this paper, fluid flow and heat transfer performances in straight or helical pipes having bow cross-sections with the central angle α in the range of 90–180° were numerically simulated using water as the medium under turbulent flow conditions. The results show that, under the same volume flow rate, the bow cross-sectional pipe with α less than 180° can enhance heat transfer and shows better comprehensive heat transfer performance compared with the half-pipe with α being 180°. As a result, less heat exchange surface (or the weight) of the bow cross-sectional pipe is needed for transferring the same amount of heat. Specifically, for the helical pipe having the bow cross-section with α being 90°, the weight of the pipe can be reduced by about 80%. In order to facilitate the engineering design of bow cross-sectional pipe jackets, correlation formulas for Nu and f of the whole straight pipe and the helical pipe were modified to include the influence of the central angle α of the bow cross-sections. [ABSTRACT FROM AUTHOR]

This paper presents a new type of split natural cooling system that maximizes the use of natural cold energy to significantly reduce the power consumption of the air conditioning system in data centers. A split natural cooling system module, which consisted of indoor and outdoor heat exchanger based on micro heat pipe arrays connected by liquid circulation system, was selected for experimental research. The heat transfer process and flow characteristics were analyzed under different outdoor environment temperatures, air and water flow rates, and different ratios of heat transfer components (N) of indoor and outdoor heat exchangers. To improve the utilization of natural cold energy, two kinds of heat dissipation conditions, namely room and heat channel-based, were proposed. The indoor temperature of two conditions at 28 °C and 38 °C were simulated in the laboratory at constant temperature-humidity, respectively. Results indicated that the air flow rate had a greater influence on the heat transfer performance than the water flow rate. The pressure drop of the air and water sides was at a lower level, and the fitting curve of the pressure drop was obtained to provide a reference for the heat exchanger design and equipment selection. When the ratio of heat transfer components (N) of the indoor and outdoor heat exchanger was approximately 0.75, the split natural cooling system showed optimal comprehensive performance. Under heat channel-based conditions, the maximum heat transfer rate reached 12.4 kW, and the maximum energy efficiency ratio was 17.15; the maximum heat transfer rate and the maximum energy efficiency ratio increased by 42.5% and 22.64% compared with the room-based condition, respectively. The fitting curve of the energy efficiency ratio was calculated under different outdoor temperatures at two heat dissipation conditions. [ABSTRACT FROM AUTHOR]

Thermal switches contribute to efficient and safe thermal management of components and overall systems in various technical applications by actively controlling heat transfer in response to varying thermal loads and ambient conditions. Heat pipes are passive heat transfer devices constituting an integral part of various thermal management systems such as in spacecraft or consumer electronics thermal control. Heat pipes also form a promising approach for thermal switches due to their high effective thermal conductivity. In this paper, a wickless copper-water heat pipe based thermal switch with an electromagnetic linear actuator is presented. The magnetically actuated motion of a plunger integrated into the heat pipe affects the latent heat transport cycle leading to a switchable heat transfer. Thermal measurements conducted to determine the total thermal resistance of the heat pipe demonstrate the efficacy of the thermal switch. It was found that the thermal resistance of the heat pipe was increased by up to 53% in off state while the heat pipe performance in on state was not significantly affected by the integrated mechanism. [ABSTRACT FROM AUTHOR]

The use of Ground Source Heat Pumps (GSHPs) has grown exponentially around the world over recent decades. The GSHP represents an alternative device to electric heating systems and oil boilers. Additionally, it requires a lower power consumption and less maintenance than combustion-based heating systems. Moreover, the CO 2 emissions produced by a GSHP are lower than other systems based on burning oil, gas, or biomass. However, the main obstacle for the widespread use of GSHPs is the high cost of Ground Heat Exchanger (GHE) installation, a technology that exhibits low thermodynamic efficiencies. Over the past decade, some studies have been conducted to improve heat transfer in GHE pipes using traditional working fluids, creating new pipe materials or designing new heat exchanger configurations. The main contribution of this paper is a summarization of the outcomes of theoretical, numerical and experimental studies to improve heat transfer in GHEs using nanotechnology. Additionally, the development of new fluids (nanofluids) and new materials (nanoparticles and nanocomposites) applied to heat exchanger pipes and the designs and configurations of GHEs are highlighted. As a result, the present review provides a perspective for future research regarding the use of nanotechnology to reduce the costs involved in GHE for GSHP improvement. [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]

This paper deals with the experimental research and verification of a passive cooling system operating on the principle of a loop gravity heat pipe designed for cooling electrical cabinets. This type of cooling works automatically by changing the state of the working substance and thus saves energy consumption. Since the designed cooling system ensures heat transfer from the interior cabinet to the outdoor space, where the heat can naturally dissipate to the surroundings, it is dustproof. The heat pipe consists of an innovative evaporator concept designed to minimize liquid and vapour phase interference in the refrigeration circuit. The aim of the research was to experimentally determine the limit performance parameters of the refrigeration system for different volumes of working medium in the evaporator and decrease heat loss in the cabinet interior. The designed device was verified experimentally and by mathematical calculations as well. The greatest benefit of the work is that the cooling device was able to ensure temperature conditions inside the electrical enclosure at a heat load of 2000 W under 60 °C, 1500 W under 55 °C, 1000 W under 50 °C, 750 W under 45 °C and 500 W under 40 °C. [ABSTRACT FROM AUTHOR]

Associating with thermal management technology, some advanced heat transfer enhancement methods are proposed by combining different kinds of convection heat transfer methods or using new thermal transportation mediums, such as nanofluids. New thermal management technology is widely developed in new energy industries, such as electric vehicles, IoT sensors, photothermal energy conversion, ground thermal energy utilization and propulsion systems. In this editorial paper, some new developments of heat transfer enhancement and thermal management technology within the applications of energy system and propulsion systems are provided. [Extracted from the article]

This paper mainly studies the heat transfer performance of backplane micro-channel heat pipes by establishing a steady-state numerical model. Compared with the experimental data, the heat transfer characteristics under different structure parameters and operating parameters were studied, and the change of heat transfer coefficient inside the system, the air outlet temperature of the back plate and the influence of different environmental factors on the heat transfer performance of the system were analyzed. The results show that the overall error between simulation results and experimental data is less than 10%. In the range of the optimal filling rate (FR = 64.40%–73.60%), the outlet temperature at the lowest point and the highest point of the evaporation section is 22.46 °C and 19.60 °C, the temperature difference does not exceed 3 °C, and the distribution gradient in vertical height is small and the air outlet temperature is uniform. The heat transfer coefficient between the evaporator and the condenser is larger than the heat transfer coefficient under the conditions of low and high liquid charge rate. It increases gradually along the flow direction, and decreases gradually with the flow rate of the condenser. When the width of the flat tube of the evaporator increases from 20 mm to 28 mm, the internal pressure drop of the evaporator decreases by 45.83% and the heat exchange increases by 18.34%. When the number of evaporator slices increases from 16 to 24, the heat transfer increases first and then decreases, with an overall decrease of 2.86% and an increase of 87.67% in the internal pressure drop of the evaporator. The inclination angle of the corrugation changes from 30° to 60°, and the heat transfer capacity and pressure drop increase. After the inclination angle is greater than 60°, the heat transfer capacity and resistance decrease. The results are of great significance to system optimization design and engineering practical application. [ABSTRACT FROM AUTHOR]

Shallow geothermal or ground source heat pump (GSHP) energy systems offer efficient space heating and cooling, reducing greenhouse gas emissions and electrical consumption. Incorporating ground heat exchangers (GHEs) within pile foundations, as part of these GSHP systems, has gained significant attention as it can reduce capital costs. The design and optimisation of GHEs connected in parallel within energy piles have been researched widely, considering symmetrical placement, while the potential misplacement due to construction errors and the optimal placement remain mostly unexplored. This study utilises 3D finite element numerical methods, analysing energy piles with diameters from 0.5 m to 1.4 m, equipped with parallelly connected U-tube and W-tube GHEs. The impact of GHE loop placement is analysed, considering the influence of the ground and concrete thermal conductivities, pile length, fluid flow rate, GHE pipe diameter, and pile spacing. Results indicate a marginal impact, less than 3%, on the overall heat transfer when loops deviate from symmetry and less than 5% on the total heat transfer shared by each loop, except for highly non-symmetric configurations. Symmetrical and evenly spaced loop placement generally maintains favourable thermal performance and ease of installation. This study underscores the flexibility in GHE design and construction with a low risk of thermal yield variations due to uncertainties, particularly with a separation-to-shank distance ratio between 0.5 and 1.5 in a symmetrical distribution. [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]

The potential use of carbon steel in CO2-saturated brine is studied for its potential use in heat exchangers in geothermal applications. A dedicated setup, including a double-pipe heat exchanger, is developed to study the relation between corrosion and the thermohydraulic behavior inside heat exchangers. Hot brine flows inside the inner carbon steel tube, thus corroding the inner surface of this tube. The thermohydraulic behavior of the heat exchanger, i.e., the pressure drop over the pipe and the heat transfer rate through the pipe, are continuously monitored. On the other hand, weight-loss experiments and microscopic analyses are performed on samples that are periodically removed from the setup. The corrosion rate is studied as a function of temperature, i.e., the entrance vs. the exit of the heat-exchanging section, and flow. Therefore, an experiment with static brine and a uniform temperature is used as a reference. The corrosion rate is generally higher in dynamic compared to static conditions. Furthermore, the corrosion rate increases with increasing temperature in dynamic conditions, whereas it decreases with increasing temperature in static conditions. These observations might be explained by the different corrosion products that formed. The corrosion products have no significant effect on the pressure drop over the pipe, but clear fluctuations in the heat transfer coefficient are observed. The origin of these fluctuations should be further studied before the observed heat transfer coefficient can be used as a measure for corrosion. [ABSTRACT FROM AUTHOR]

The latest developments in solar technologies demonstrated that the solar central receiver configuration is the most promising application among concentrated solar power (CSP) plants. In CSPs solar-heated air can be used as the working fluid in a Brayton thermal cycle and as the heat transfer fluid for a Rankine thermal cycle as an alternative to more traditional working fluids thereby reducing maintenance operations and providing the power section with a higher degree of flexibility To supply thermal needs when the solar source is unavailable, an auxiliary burner is requested. This configuration is adopted in the Julich CSP (J-CSP) plant, operating in Germany and characterized by a nominal power of 1.5 MW, the heat transfer fluid (HTF) is air which is heated in the solar tower and used to produce steam for the bottoming Rankine cycle. In this paper, the J-CSP plant with thermal energy storage has been compared with a hybrid CSP plant (H-CSP) using air as the working fluid. Thermodynamic and economic performances of all the simulated plants have been evaluated by applying both exergy analysis and thermoeconomic analysis (TA) to determine the yearly average operation at nominal conditions. The exergy destructions and structure as well as the exergoeconomic costs of products have been derived for all the components of the plants. Based on the obtained results, the thermoeconomic design evaluation and optimization of the plants has been performed, allowing for improvement of the thermodynamic and economic efficiency of the systems as well as decreasing the exergy and exergoeconomic cost of their products. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of methods to increase the efficiency of heat transfer in heat exchangers. The scope of the research included analysis of efficiency optimization using the example of two tubular heat exchanger structures most often used in industry. The obtained efficiency of heat recovery from the ground of the examined exchangers was over 90%, enabling the reduction of emissions of the heating systems of buildings. The paper presents the results of tests of two types of heat pipes using R134A, R404A, and R407C working agents. The paper also presents the results of experimental tests using the R410A working medium. The results included in the study will also enable the effective use of land as a heat store. [ABSTRACT FROM AUTHOR]

When colloidal particles are deposited in a heat transfer channel, they increase the flow resistance in the channel, resulting in a substantial decrease in heat transfer efficiency. It is critical to have a comprehensive understanding of particle properties in heat transfer channels for practical engineering applications. This study employed the Reynolds stress model (RSM) and the discrete particle model (DPM) to simulate particle deposition in a 3D corrugated rough-walled channel. The turbulent diffusion of particles was modeled with the discrete random walk model (DRW). A user-defined function (UDF) was created for particle–wall contact, and an improved particle bounce deposition model was implemented. The research focused on investigating secondary flow near the corrugated wall, Q-value standards, turbulent kinetic energy distribution, and particle deposition through validation of velocity in the tube and particle deposition modeling. The study analyzed the impact of airflow velocity, particle size, corrugation height, and corrugation period on particle deposition efficiency. The findings suggest that the use of corrugated walls can significantly improve the efficiency of deposition for particles less than 20 μm in size. Specifically, particles with a diameter of 3 μm showed five times higher efficacy of deposition with a corrugation height of 24 mm compared to a smooth surface. [ABSTRACT FROM AUTHOR]

As electronic components progressively downsize and their power intensifies, thermal management has emerged as a paramount challenge. This study presents a novel, high-efficiency finned heat exchanger, termed Flat-Plate Oscillating Heat Pipe Finned Radiator (FOHPFR), which employs arrayed flat-plate oscillating heat pipes (OHP) as heat dissipation fins. Three-dimensional (3D)-printed techniques allow the internal microchannels of the FOHPFR to become rougher, providing excellent surface wettability and capillary forces, which in turn significantly improves the device's ability to dissipate heat. In this study, the 3D-printed FOHPFR is compared with traditional solid finned radiators made of identical materials and designs. The impacts of filling ratio, inclination angle, and cold-end conditions on the heat transfer performance of the 3D-printed FOHPFR are investigated. It is demonstrated by the results that compared to solid finned radiators, the FOHPFR exhibits superior transient heat absorption and steady-state heat transfer capabilities. When the heating power is set at 140 W, a decrease in thermal resistance from 0.32 °C/W in the solid type to 0.11 °C/W is observed in the FOHPFR, marking a reduction of 65.6%. Similarly, a drop in the average temperature of the heat source from 160 °C in the solid version to 125 °C, a decrease of 21.8%, is noted. An optimal filling ratio of 50% was identified for the vertical 3D-printed FOHPFR, with the minimal thermal resistance achieving 0.11 °C/W. Moreover, the thermal resistance of the 3D-printed FOHPFR is effectively reduced compared to that of the solid finned radiator at all inclination angles. This indicates that the FOHPFR possessed notable adaptability to various working angles. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries, crucial in powering Battery Electric Vehicles (BEVs), face critical challenges in maintaining safety and efficiency. The quest for an effective Battery Thermal Management System (BTMS) arises from critical concerns over the safety and efficiency of lithium-ion batteries, particularly in Battery Electric Vehicles (BEVs). This study introduces a pioneering BTMS solution merging a two-phase immersion cooling system with heat pipes. Notably, the integration of NovecTM 649 as the dielectric fluid substantially mitigates thermal runaway-induced fire risks without requiring an additional power source. Comprehensive 1-D modeling, validated against AMESim (Advanced Modeling Environment for Simulation of Engineering Systems) simulations and experiments, investigates diverse design variable impacts on thermal resistance and evaporator temperature. At 10 W, 15 W, and 20 W heat inputs, the BTMS consistently maintained lithium-ion battery temperatures within the optimal range (approximately 27–34 °C). Optimized porosity (60%) and filling ratios (30–40%) minimized thermal resistance to 0.3848–0.4549 °C/W. This innovative system not only enhances safety but also improves energy efficiency by reducing weight, affirming its potential to revolutionize lithium-ion battery performance and address critical challenges in the field. [ABSTRACT FROM AUTHOR]

Adsorption refrigerators are a compelling ecological alternative to compressor refrigerators; global warming forces us to constantly look for alternative sources of energy and cold. Cold production in adsorption chillers is based on the use of heat generated by other processes running in the company. Waste heat from production processes, which has, until now, been irretrievably lost, is a potential source of energy for generating cold via an adsorption unit producing chilled water. Cooling optimizes the use of the heating network in summer and can lead to increased electricity production while reducing heat supply losses. Thus far, attempts to implement adsorption refrigerators for widespread use have not been successful as a result of the low efficiency of these devices; this is directly related to the poor heat and mass transfer conditions in the beds and heat exchangers of adsorption refrigerators. The solutions used so far, such as new working pairs, glued beds or modifications to the structure or cycle length, are still not strong enough for these devices. Therefore, it is necessary to look for new solutions. Using micro- and nanobubbles as media to increase mass and heat transfer in refrigerators is an innovative and pioneering solution. Thus, this document describes the most important features of micro- and nanobubble technology applications in adsorption refrigerators. This article is an introduction and a basis for the implementation of further research, consolidating the existing literature as a review. [ABSTRACT FROM AUTHOR]

With the development of microsatellite technology, the heat generated by onboard components is increasing, leading to a growing demand for improved thermal dissipation in small satellites. Metal powder additive manufacturing technology offers the possibility of customizing and miniaturizing heat pipes to meet the specific requirements of small satellites. This article introduces a small-scale heat pipe designed using selective laser melting (SLM) technology. The heat pipe's material, structure, and internal working fluid were determined based on mission requirements. Subsequently, the SolidWorks 2021 software was used for heat pipe modeling, and the ANSYS 2021R2 finite element analysis software was employed to simulate the heat transfer performance of the designed heat pipe, confirming its feasibility. The heat pipe's structure was optimized using multi-objective regression analysis, considering various structural parameters, such as the channel diameter, vapor chamber height, and narrow gap width. The simulation results demonstrate that the optimized heat pipe achieved a 10.5% reduction in thermal resistance and an 11.6% increase in equivalent thermal conductivity compared to the original heat pipe. Furthermore, compared to conventional metal heat-conducting rods, the optimized heat pipe showed a 38.5% decrease in thermal resistance and a 62.19% increase in equivalent thermal conductivity. The heat pipe was then fabricated using a 3D printer (EOS M280), and a vacuum experimental system was established to investigate its heat transfer characteristics. The experimental results show that the heat pipe operated most efficiently at a heating power of 20 W, reached its maximum heat transfer capacity at 22 W, and had an optimal fill ratio of 30%. These results highlight the excellent performance of the heat pipe and the promising application prospects for SLM technology in the field of small satellites. [ABSTRACT FROM AUTHOR]

This paper reports a numerical study on heat transfer improvement in a double backward-facing expanding channel using different convectional fluids. A finite volume method with the k-ε standard model is used to investigate the effects of step, Reynolds number and type of liquid on heat transfer enhancement. Three types of conventional fluids (water, ammonia liquid and ethylene glycol) with Reynolds numbers varying from 98.5 to 512 and three cases for different step heights at a constant heat flux (q = 2000 W/m2) are examined. The top wall of the passage and the bottom wall of the upstream section are adiabatic, while the walls of both the first and second steps downstream are heated. The results show that the local Nusselt number rises with the augmentation of the Reynolds number, and the critical effects are seen in the entrance area of the first and second steps. The maximum average Nusselt number, which represents the thermal performance, can be seen clearly in case 1 for EG in comparison to water and ammonia. Due to the expanding of the passage, separation flow is generated, which causes a rapid increment in the local skin friction coefficient, especially at the first and second steps of the downstream section for water, ammonia liquid and EG. The maximum skin friction coefficient is detected in case 1 for water with Re = 512. Trends of velocities for positions (X/H1 = 2.01, X/H2 = 2.51) at the first and second steps for all the studied cases with different types of convectional fluids are indicated in this paper. The presented findings also include the contour of velocity, which shows the recirculation zones at the first and second steps to demonstrate the improvement in the thermal performance. [ABSTRACT FROM AUTHOR]

With the continuous application of high-power electronic equipment in aircraft, highly efficient heat transfer technology has been emphasized for airborne applications. In this paper, a thermal management system based on an antifreeze liquid cooling loop and a vapor compression refrigeration loop is presented for high-power airborne equipment in a helicopter. The simulation models of the thermal management system are built in order to study its control strategy for the changing flight conditions. The antifreeze-refrigerant evaporator and air-refrigerant condenser are specially validated with the experimental data. A dual feedforward proportion integration differentiation and expert control algorithm are adopted in the inlet temperature of the cold plate and sub-cooling control of the refrigerant by regulating the compressor speed and the fan speed, respectively. A preheating strategy for antifreeze is set up to decrease its flow resistance in cold day conditions. The control strategy for the thermal management system is finally built based on the above control methods. In this paper, two extreme conditions are discussed, including cold and hot days. Both the simulation results show that the superheated, sub-cooling and antifreeze inlet temperature of the cold plate can be controlled at 3 to8 °C, −10 to −3 °C and 18 to22 °C, respectively. Under the same changing flight envelope, the coefficient of performance of the vapor compression refrigeration loop is relatively stable on the cold day, which is about 6, while it has a range of 2.58–4.9 on the hot day. [ABSTRACT FROM AUTHOR]

This paper deals with the heat transfer performance of a micro-channel backplane heat pipe air conditioning system. The optimal range of the filling rate of a micro-channel backplane heat pipe air conditioning system was determined in the range of 65-75%, almost free from the interference of working conditions. Then, the influence of temperature and air volume flow rate on the heat exchange system were studied. The system maximum heat exchange is 7000-8000 W, and the temperature difference between the inlet and outlet of the evaporator and the condenser is almost 0 °C. Under the optimum refrigerant filling rate, the heat transfer of the micro-channel heat pipe backplane system is approximately linear with the temperature difference between the inlet air temperature of the evaporator and the cooling distribution unit (CDU) inlet water temperature in the range of 18-28 °C. The last part compares the heat transfer characteristics of two refrigerants at different filling rates. The heat transfer, pressure, and refrigerant temperature of R134a and R22 are the same with the change of filling rate, but the heat transfer of R134a is lower than that of R22. The results are of great significance for the operational control and practical application of a backplane heat pipe system. [ABSTRACT FROM AUTHOR]

Therefore, heat transfer during flow with a change of phase in mini-channels turned out to be a necessary solution to achieve high heat fluxes with reasonably miniature geometrical dimensions and a temperature at a not very high level within the heat exchanger device. The authors noted that there was an increase in the heat flux and the heat transfer coefficient for surfaces with microchannels. The transfer of large heat fluxes is one of the most significant issues in modern technology. The authors indicated that the construction of a compact heat exchanger with a group of rectangular mini- and microchannels with enhanced surfaces can improve the heat transfer process. [Extracted from the article]

There is a growing demand from the industrial sector and the population to cover the need for water temperature increases that can be covered with systems such as heat pumps. The present research aims to increase the heat transfer to the working fluid in a collector/evaporator, part of a solar-assisted direct expansion heat pump. This research was developed using a numerical analysis and by applying computational fluid dynamics; different simulations were performed to compare the performances of collector/evaporators with models exhibiting variations in the cross-section profile under similar conditions. An average incident solar radiation of 464.1 W·m−2 was considered during the analysis. For the comparison, profiles with hexagon-, four-leaf clover-, and circular-shaped sections with floral shapes, among others, were analysed, resulting in a temperature increase at the outlet of the working fluid of 1.3 °C. In comparison, the collector/evaporator surface temperature varied between 4 and 13.8 °C, while the internal temperature of the fluid reached 11.21 °C. Finally, it is indicated that the best results were presented by analysing the profile corresponding to the circular section with the flower shape. [ABSTRACT FROM AUTHOR]

Global optimization of industrial plant configurations using organic Rankine cycles (ORC) to recover heat is becoming attractive nowadays. This kind of optimization requires structural and parametric decisions to be made; the number of variables is usually high, and some of them generate disruptive responses. Surrogate models can be developed to replace the main components of the complex models reducing the computational requirements. This paper aims to create, evaluate, and compare surrogates built to replace a complex thermodynamic-economic code used to indicate the specific cost (US$/kWe) and efficiency of optimized ORCs. The ORCs are optimized under different heat sources conditions in respect to their operational state, configuration, working fluid and thermal fluid, aiming at a minimal specific cost. The costs of 1449.05, 1045.24, and 638.80 US$/kWe and energy efficiencies of 11.1%, 10.9%, and 10.4% were found for 100, 1000, and 50,000 kWt of heat transfer rate at average temperature of 345 °C. The R-square varied from 0.96 to 0.99 while the number of results with error lower than 5% varied from 88% to 75% depending on the surrogate model (random forest or polynomial regression) and output (specific cost or efficiency). The computational time was reduced in more than 99.9% for all surrogates indicated. [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]

Vibrations have attracted much attention as an effective method for enhancing heat transfer in pulsating heat pipes (PHPs). This study mainly investigates and explores the effects of local low-frequency vibrations on the starting-up and heat transfer characteristics of a PHP. The starting-up temperature and average temperatures along the evaporation section of the pulsating heat pipe were experimentally scrutinized, along with thermal performance, under local vibrations on evaporation, condensation and adiabatic sections, respectively. The following important conclusions can be derived by the experimental study: (1) The effect of vibrations at the evaporation section and at the adiabatic section during the starting-up time of the PHP were more significant than that at the condensation section; (2) vibrations at different positions could reduce the starting-up temperature of the PHP—the effect of the vibrations at the evaporation section was the best when heat power was lower, while the effect of vibrations on the adiabatic section was the best when heat power was higher; (3) vibrations at the evaporation and adiabatic sections could reduce the thermal resistance of the PHP, but vibrations at the condensation section had little effect on the thermal resistance of the PHP; (4) vibrations at the evaporation and adiabatic sections could effectively reduce the temperature at the evaporation section of the PHP, but the vibrations at the condensation section had no effect on the temperature at the evaporation section of the PHP. This paper shows that local low-frequency vibrations have positive effects on the heat transfer performances of PHPs. [ABSTRACT FROM AUTHOR]

As a type of micro flat loop heat pipe, s-UTLHP (silicon-based ultra-thin loop heat pipe) is of great significance in the field of micro-scale heat dissipation. To prove the feasibility of s-UTLHP with high heat flux in a narrow space, it is necessary to study its heat transfer mechanism visually. In this paper, a structural design of s-UTLHP was proposed, and then, to realize the working fluid charging and visual experiment, an experimental system including a holding module, heating module, cooling module, data acquisition module, and vacuum chamber was proposed. Deionized water was selected as a working fluid in the experiment. The overall and micro phenomena of s-UTLHP during startup, as well as the evaporation and condensation phenomena of s-UTLHP during stable operation, were observed and analyzed. Finally, the failure phenomenon of s-UTLHP was analyzed, and several solutions were proposed. The observed phenomena and experimental conclusions can provide references for further related experimental research. [ABSTRACT FROM AUTHOR]

A marine condenser with exhausted steam as the working fluid is researched in this paper. Constructal designs of the condenser are numerically conducted based on single and multi-objective optimizations, respectively. In the single objective optimization, there is an optimal dimensionless tube diameter leading to the minimum total pumping power required by the condenser. After constructal optimization, the total pumping power is decreased by 42.3%. In addition, with the increase in mass flow rate of the steam and heat transfer area and the decrease in total heat transfer rate, the minimum total pumping power required by the condenser decreases. In the multi-objective optimization, the Pareto optimal set of the entropy generation rate and total pumping power is gained. The optimal results gained by three decision methods in the Pareto optimal set and single objective optimizations are compared by the deviation index. The optimal construct gained by the TOPSIS decision method corresponding to the smallest deviation index is recommended in the optimal design of the condenser. These research ideas can also be used to design other heat transfer devices. [ABSTRACT FROM AUTHOR]

This article describes an innovative method for the determination of heat flow (specific heat loss; linear heat flow density) from a one-metre length of a twin pipe directly-buried heat network. Such heat losses are currently described by applying analytical procedures based on the heat transfer theory. It is rather complicated to accurately express the heat loss using such procedures, due to the necessity to determine the individual values of thermal resistance. A simpler method to express heat loss is the balance method, as it requires measuring a temperature gradient Δt between the starting point of the heat network and the end point of the heat collection. A suitable measuring device must provide high-accuracy measurements of the temperature. In the case of very well-insulated distribution pipelines and short pipes, the temperature measurements must be accurate to the hundredths of a degree Celsius. It is impossible to install such devices as fixed equipment on every heat distribution network, due to such networks measuring many kilometres and the cost of the appropriate measuring technology. For the aforesaid reasons, the authors created a mathematical model for specific heat losses based on dimensional analysis. This method facilitates the identification of dimensionless criteria based on the relevant dimensional quantities. Functional correlations between the identified criteria may be identified on the basis of the results of physical or numerical experiments. In this study, a database of the results obtained from physical experiments conducted on two heat networks was used. The output of the similarity model was a function describing the heat flow from a one-meter pipe length that was applicable to various alternatives in relation to the geometrical, physical and boundary conditions. The standard deviation of a difference in the heat losses identified by applying the balance method and using the proposed criterial equation for a twin pipe directly-buried pre-insulated heat network was 0.515 W·m−1. [ABSTRACT FROM AUTHOR]