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Solar energy offers a sustainable solution to address the increasing energy demand and environmental concerns in both industrial and domestic applications. To enhance the efficiency of solar collectors, researchers have integrated heat pipes, which are passive devices for effectively transferring heat to a working fluid. This integration has a wide range of applications for solar thermal collectors, including water and space heating, drying processes, and desalination. Conventional solar collectors have limitations in high-temperature applications. When heat pipes are integrated with these collectors, they have demonstrated a higher temperature range and improved performance. This paper presents a comprehensive review of the various techniques and innovations adopted by researchers over the last few decades to maximize the thermal performance of heat pipe solar collectors (HPSC). The review begins by covering the fundamental concept and working principle of heat pipes. Heat pipes are sealed copper tubes with an inner wick structure and a small amount of working fluid. Researchers have explored several techniques and innovations to enhance the thermal performance of HPSC. These include optimizing geometrical parameters such as length, diameter, and orientation of heat pipes to maximize heat transfer. Improvements in wick structure design and the selection of working fluids have also been investigated to enhance heat transfer efficiency. This review provides an overview of the techniques and innovations employed by researchers to enhance the efficiency of heat pipe solar collectors. By addressing the identified research gaps, further advancements can be achieved in maximizing the thermal performance of HPSC, leading to more efficient utilization of solar energy. [ABSTRACT FROM AUTHOR]

The critical review presented here exclusively covers the studies on battery thermal management systems (BTMSs), which utilize heat pipes of different structural designs and operating parameters as a cooling medium. The review paper is divided into five major parts, and each part addresses the role of heat pipes in BTMS categorically. Experimental studies, numerical analyses, combined experimental and numerical investigations, optimum utilization of a phase-change material (PCM) with a heat pipe (HP), oscillating heat pipe (OHP), and micro heat pipes combined with PCM for Li-ion BTMS using heat pipes are presented. The usage of HP's and PCM can keep the temperature of the battery system in the desirable limit for a longer duration compared to other traditional and passive methods. More emphasis is made on how one can achieve a suitable cooling system design and structure, which may tend to enhance the energy density of the batteries, improve thermal performance at maximum and minimum temperature range. Arrangement of battery cells in a pack or module, type of cooling fluid used, heat pipe configuration, type of PCM used, working fluid in a heat pipe, and surrounding environmental conditions are reviewed. According to the study, the battery's effectiveness is significantly influenced by temperature. The usage of flat HPs and heat sink proves to be the best cooling method for keeping the battery working temperature below 50 °C and reduces the heat sink thermal resistance by 30%. With an intake temperature of 25 °C and a discharge rate of 1 L per minute, an HP that uses water as a coolant is also effective at regulating battery cell temperature and maintaining it below the permissible 55 °C range. Using beeswax as a PCM in HPs reduces the temperature of BTMS by up to 26.62 °C, while the usage of RT44 in HPs reduces the temperature of BTMS by 33.42 °C. The use of fins along with copper spreaders drastically decreases the temperature capability of HPTMS by 11 °C. MHPA shows excellent performance in controlling the battery temperature within 40 °C. The effective thermal management can be done by incorporating heat pipe alone or by coupling with liquid cooling or metal plate. However, extensive and extended research is required to improve thermal management to safely and effectively use the battery for day-to-day applications. [ABSTRACT FROM AUTHOR]

Pulsating heat pipes are widely used in cooling electronic devices, automotive radiators, refrigeration, and air conditioning systems. The addition of nanoparticles is one of the effective ways to improve the heat transfer characteristics of fluids. This paper, based on large eddy simulation (LES) numerical method and field synergy theory, investigates the heat transfer characteristics of Al2O3/Cu mixed nanofluid in a self-excited oscillating heat exchanger tube. Through multi-objective optimization of field synergy angles, field synergy factors, and structural parameters, the enhanced heat transfer mechanism of mixed nanofluids in a self-excited oscillating heat pipe is investigated. It is found that there is a significant negative correlation between the heat transfer coefficient h at the wall surface of the heat exchanger tube and the synergistic angle β. The incorporation of nanoparticles improves the synergistic properties between the velocity and temperature fields. The optimum structural parameters are L/D = 0.469931, α = 128.9904° and d2/d1 = 2.047937, and the optimization increases Nu by 1.92%, decreases f by 10.46%, and decreases the temperature synergy angle β by 2.24°. At the same time, the orthogonality or lack of orthogonality between isotherms and streamlines determines the extent of synergistic performance in the countercurrent vortex. [ABSTRACT FROM AUTHOR]

Nanofluids can effectively enhance the heat transfer performance of different types of heat pipes according to the existing research. This paper provides a comprehensive literature review on the application of nanofluids in thermosyphon heat pipe, pulsating heat pipe and wick heat pipe, respectively. The effect of important variables such as different nanoparticles, concentrations, packing ratios, inclination angles and thermal loads on heat pipe performance is discussed. Studies shows that there is a certain concentration of nanoparticles for highly efficient utilization. High concentration of nanoparticles leads to higher thermal resistance mainly due to the increase in dynamic viscosity and the possibility of particle agglomeration. The enhanced heat transfer performance is caused by the enhanced Brownian motion, improved surface wettability, increased nucleation sites and inherently high thermal conductivity of nanofluids. Finally, the current problems and future trends of heat pipe technology based on nanofluids are discussed. [ABSTRACT FROM AUTHOR]

The microchannel heat sink (MCHS) has become the most relevant micro-heat exchanger for a small area in need of an effective high heat removal system. However, heat dissipation from the microchips where the MCHS is utilized—the microprocessor and microcontroller—is getting higher with the sizes getting smaller. A more effective coolant is needed to address the increasing heat load from the microchip and nanofluid, nanosized particles dispersed in a base fluid, is among those explored. This paper reports a new potential use of non-metallic nanofluid, boron nitride nanotube (BNN), that is capable of improving the overall performance of a rectangular MCHS. A heuristic method, multi-objective genetic algorithm (MOGA), is employed to simultaneously minimize the thermal resistance and pressured drop of a boron nitride nanotubes (BNN) nanofluid-cooled MCHS to obtain optimized dimensions at various mass concentrations. The method is capable in achieving conflicting objectives, minimization of the thermal resistance and the pressure drop; an increase in the former decreases the latter and vice versa. In addition, experimental thermophysical properties of the BNN nanofluid are used to provide reliability to the optimization outcomes in identifying the best BNN concentration for cooling of a MCHS at 50 °C. The optimization results showed that as the thermal resistance decreases, the pressure drop decreases. For mass concentrations of 0.001, 0.003, 0.005, 0.01 and 0.03% the thermal resistance is the lowest, 0.0711 K/W at 0.01 mass.%. at 0.0115 W. The thermal resistance is lowered by 5.34% compared to water for the same operating conditions. These results indicate the great potential of BNN nanofluid as a coolant in the electronics cooling system. Optimization with the MOGA provides a fast analysis into the potentials of any coolants for MCHS applications as shown here, reliability being provided by experimentally obtained thermophysical properties. [ABSTRACT FROM AUTHOR]

Managing the heat generated in equipment such as microprocessors requires optimal thermal design. As a type of superconductor, heat pipes (HPs) are a suitable choice for this purpose. The performance of micro-HPs can be affected by the body material and working fluid, among other parameters. The present paper examines the use of a nanofluid inside a flat plate micro-HP under various working conditions. Platinum (Pt), copper (Cu), and aluminum (Al) were used as body materials and argon (Ar), ethanol (Eth), and water (H2O) were used as the working fluid. The effect of adding 5, 10, and 15 nanoparticles (NPs) of the same substance as the body was also studied. Given the scale of the problem (maximum length of 1 μm), a molecular dynamic (MD) simulation was performed. First, the results were validated, and then the performance of the HP was examined under various conditions. According to the results, increasing the number of nanoparticles helped improve thermal performance. The highest heat flux corresponded to Cu–Eth and was equal to 2092 W/cm2. On the other hand, the highest heat flux for Pt was obtained using Eth and equaled 1992 W/cm2. The highest and lowest mass transfer rates are achieved using Ar and H2O at 40 % and 26 %, respectively. Increasing the number of nanoparticles from 5 to 15 np in Pt–Eth led to a 23.5 % rise in the heat flux. Cu has the highest evaporation and condensation rate among the metals under study. The findings of this study can help for a better understanding of the working condition of micro-HPs on a molecular scale, which helps better design such devices in the electronic industry. It should be noted that the results are validated using different methods. [ABSTRACT FROM AUTHOR]

The heat pipes are two-phase flow passive and reliable devices that transfer heat effectively and are vastly utilized in thermal systems. A summary of experimental and numerical studies related to advanced technologies of applications of heat pipes and thermosiphons is offered in this review. This paper focused mainly on the hybrid combinations of heat pipes and phase change materials, heat pipes with nanofluids and heat pipes, and modern electronic devices. Also, the influence of operating variables, such as working fluids, heat inputs, filling ratios, and inclination angles, has been involved. The hybrid systems that combine phase change materials with heat pipes to improve their thermal performance are discussed. Low thermal conductivity of phase change materials and heat pipe overheating has been addressed using these hybrid systems. The impact of nanofluids on the heat pipes' thermal behaviour is also discussed. Nanofluids enhance the thermal properties of heat pipes by increasing their thermal performance. Heat pipe technology is being used in the thermal management of electronics to enhance their cooling systems. Addressing overheating issues of electronic devices will improve their performance and helps to achieve their robust, small, and flexible design. [ABSTRACT FROM AUTHOR]

Most of the energy consumed worldwide is wasted as heat. The usage of this waste heat both increases the efficiency of the system and reduces environmental pollution. The Organic Rankine Cycle (ORC) has been recognized as a dependable technology for turning low-temperature heat sources into electricity, and it is also a promising solution for recovering wasted heat. This paper investigated the possibility of generating power from the low-temperature waste heat using a Two-stage Compression Refrigeration Cycle (TCRC) integrated with the ORC. The energy, exergy, exergoeconomic, and environmental low-temperature ORC for six working fluids (Ammonia, R161, R410a, R152a, R407C, and R32) were thoroughly investigated. The temperature of the heat source required by the ORC is restricted to the experimental temperature of the TCRC, which is the basis for finding the appropriate working fluids. The study demonstrated that R152a, among the other six working fluids, has a maximum power generation of 44.79 kW, an energy efficiency of 5.37%, and an exergy efficiency of 52.48%, respectively. In addition, R161 and R407C, with a value of the product of 41.36 $ GJ−1, are also economical. Ammonia and R410a have the least exergy destruction in the system, with 104 kW and 109 kW, respectively. From an economic aspect, Ammonia and R152a are the most economical fluid for the suggested system, with a payback period of approximately 3 years. Also, R152a performs a significant role in keeping the environment clean by reducing the emission of CO2 and NOx, 187.3 tons/year and 30.96 tons/year, respectively. [ABSTRACT FROM AUTHOR]

Heat pipes are broadly applied for thermal management of devices with high heat flux. Due to the dependency of their efficient operating range on the boiling point of the working fluids, employing special types of fluids is necessary to have applicable heat pipes at very low temperatures. Hydrogen, neon and nitrogen are among the most attractive working fluids for heat pipes in order to reach efficient heat transfer in cryogenic conditions. According to the reviewed studies on cryogenic heat pipes, similar to the medium or high-temperature heat pipes, their thermal performance depends on different factors such as filling ratio, input power, structure and type of heat pipe and working fluid. Employing appropriate fluids, applicable for the required operating temperature, provides heat pipes with high effective thermal conductivity in cryogenic conditions. The effective thermal conductivity of the cryogenic heat pipes is several orders of magnitudes higher than that of the conductive metals in the similar temperature which makes them as desirable thermal management devices for low-temperature applications such as cooling superconductors. In this paper, a comprehensive review is carried out on the researches focused on the cryogenic heat pipes and the factors influencing their heat transfer features to provide a useful and applicable reference for the future studies with relevant topics. This review study is categorized based on the type of heat pipes: conventional heat pipes, pulsating heat pipes and thermosyphons (gravity-assisted heat pipes). The reviewed studies have demonstrated that the operating fluid and heat load have substantial role in the performance of heat pipes. Finally, some recommendations for future studies are provided to reach further enhancement in the performance of cryogenic heat pipes. [ABSTRACT FROM AUTHOR]

In this paper, the heat transfer rate of CuO nanoparticle suspended cold-water flow in a double pipe heat exchanger is evaluated. The average size of CuO nanoparticles is 35 nm is dissolved in deionized water, flowing in the counter direction of a hot-water pipe. The velocity and the discharge rate were maintained constant and the dispersion rate of CuO nanoparticles was varied (0, 0.1, 0.2, and 0.3 mass%). The system inlet temperature (hot water) was 50 °C and the cold-water inlet temperature was found to vary with respect to the suspended particle proportion. The experimental results showed that the CuO nanoparticle has highly influenced to reduce the cold-water inlet temperature and heat transferred rate found to be drastically increased from 183 to 3488 kW. Subsequently, with appropriate input details and boundary conditions, the experimentation was simulated with valid computation fluid dynamics software. Simulated results confirmed that the lower concentration of CuO nanoparticle had less impact compared to that of higher concentration of suspended particles. It is found that the experimental results and simulated data found to be very similar, and hence it is confirmed that the proposed model can be executed for different heat exchanger designs. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer enhancement in a double pipe heat exchanger with a twisted tape as a swirl generator in both sides is investigated numerically. The three-dimensional numerical simulations have been done by finite volume method using a commercial CFD code. The spatial discretization of mass, momentum, turbulence dissipation rate, and turbulence kinetic energy equations has been achieved by a second-order upwind scheme. A SIMPLE algorithm has been used for velocity–pressure coupling. To calculate gradients, Green–Gauss cell-based method has been utilized. The swirl generator is considered in both channels of the heat exchanger (hot and cold streams). Three types of nanofluids containing Al2O3, CuO, and SiO2 are considered as heat transfer fluid in the inner channel (hot fluid flow). Results showed that the maximum thermal performance belongs to CuO/water nanofluid at Re = 5492 by 7% enhancement in thermal performance in comparison with net water. Also, the SiO2/water nanofluid at Re = 3343 has the minimum enhancement in thermal performance by 2.5% in comparison with the net water. In a lower Reynolds number (Re = 3343), the greatest thermal performance is obtained for a case with ϕ = 4% by 1% improvement in thermal performance compared to pure water. However, the case with ϕ = 5% presents a lower thermal performance by 3.7% compared to pure water. However, in higher Reynolds number (Re = 5492), the highest thermal performance is achieved for the case with ϕ = 4% by 6.4% improvement in thermal performance compared to pure water and the lowest improvement belongs to the case with ϕ = 3% by 1.3% enhancement in thermal performance compared to pure water. [ABSTRACT FROM AUTHOR]

Heat pipes are silent heat transfer devices that work on the motion of boiling and condensation process. They have been used in space crafts, heat recovery and ventilation, power conversion, energy, and electronics cooling applications. Over the past few eras, several necessary upgradations in heat pipe technologies have happened to implement new advanced fluids, design modification, and modified wick structures. Heat transfer enhancement due to these upgradations/implementations has been deliberated in many studies. In this paper, performance studies of heat pipes with deposition of nanoparticles and suitable coating made on the wick structure are reviewed. Various heat transfer mechanisms involved in heat pipes with nanoparticles deposition on the evaporator are summarized. Also, the various heat transfer mechanisms in heat pipes while using a nanofluid and a porous coating in the evaporator are summarized. This review shall offer a superior comprehension of the improvement required in cooling devices and help future research fraternity to develop advanced cooling gadgets. [ABSTRACT FROM AUTHOR]

This paper introduces an experimental study for concentrating solar cell performance cooled by using flat heat pipe. The cell represents the heat pipe evaporator, and the heat pipe condenser is cooled by using a rectangular finned heat sink. This study is investigated at various heat pipe condenser and adiabatic regions lengths and concentration ratios of the radiation intensity incident on the cell, and for forced and free convection airflow cooling through the finned heat sink. The required radiation energy supplied to the cell is provided by solar simulator. The findings illustrate that cell efficiency and output power increase with increasing the heat pipe condenser and decrease its adiabatic regions' lengths. However, cell efficiency reduces with rising the incident radiation intensity. The heat pipe temperature increases with radiation intensity, but its maximum temperature difference does not change greatly with variation solar intensity. Cooling the cell by heat pipe increases its output power by 24.3% compared to free convection without utilizing heat pipe at incident energy 500 W m−2. Using forced convection with double condenser length increases the cell output power by about 9.1% compared to one heat sink for free convection at falling radiation intensity 3000 W m−2. [ABSTRACT FROM AUTHOR]

Low global warming potential working fluid R1234ze(Z) is anticipated to be the working fluid of the choice for the moderately high temperature heat pump in industrial applications. In this paper, the thermodynamic theoretical studies of the working fluids R114, R134a, R227ea, R236fa, R245fa and R1234ze(Z) were performed, and the theoretical analysis showed that the thermodynamic properties of R1234ze(Z) were better than other working fluids. The experimental investigations of the working fluids R134a, R227ea, R236fa, R245fa and R1234ze(Z) were carried out on a vapor compression heat pump system in the condensation temperature range of 70–85 °C with the cycle temperature lift (difference between condensation temperature and evaporation temperature) of 45 K. In addition, the experimental cycle performance of R1234ze(Z) with the cycle temperature lift of 58 K was also tested and compared with that with the cycle temperature lift of 45 K in the condensation temperature range of 70–85 °C. The coefficient of performance (COP) of R245fa was first measured to test the credibility of the newly developed apparatus. The test showed that the COP of R1234ze(Z) could reach up to 3.55 when the condensation temperature was 85 °C with the temperature lift of 45 K. The above assessments demonstrate that R1234ze(Z) is suitable for moderately high temperature heat pump applications. [ABSTRACT FROM AUTHOR]

Using materials with porous natures to enhance the thermal efficiency of various thermal systems has attracted much attention. This is owing to the low cost, light weight, and great features of these materials to improvement in the thermal efficiency of thermal systems. In addition, using porous materials, as a passive method, does not consume any external energy. Increasing thermal efficiencies of different thermal systems by using these materials is related to many factors including the kind of thermal system, position of insert, porosity, thermal conductivity, thickness, and nature of these materials. This paper aims to perform a comprehensive review on the application of different materials with porous structures in different thermal systems such as heat pipes, thermosyphons, electronic cooling modules, boiling systems, phase change materials, etc. In addition, a brief discussion is presented about the application of porous model to simulate fluid flow and heat transfer in plate-fin heat exchangers. [ABSTRACT FROM AUTHOR]

This paper presents a novel study on the performance of closed-loop oscillating heat pipe (CLOHP) using iron oxide (Fe3O4) as the working fluid for three types of condensers. The tested CLOHP consists of six turns made of copper tubes, 4.5 mm outer diameter and 3 mm inner diameter with heating power input in a range of 0–200 W. The experimental results showed that the thermal performance of the CLOHPs has been improved when the corrugated horizontal condenser was used compare to straight and corrugated vertical condensers. Based on 800 sets of available experimental data, the results show that the CLOHPs with corrugated horizontal condenser had better thermal performance when charged with Fe3O4/water at 2% mass concentration. [ABSTRACT FROM AUTHOR]

A large portion of world's final energy consumption is employed in thermal form. The proper techniques can be used to improve the efficiencies of thermal energy systems and decrease the energy consumption. Phase change material (PCM) storage modules can be coupled with different thermal systems to improve their thermal efficiency. This paper reviews the real potentials of coupling PCM storage modules to heat pipes and heat pumps in terms of energy saving and thermal energy management. The different systems suggested by researchers are described, and the main findings of the studies are provided. The advantages and disadvantages of these combined systems are discussed, and some suggestions for future studies are provided. The results show that heat pipes show great heat transfer ability and have good efficiency in improving heat transfer within PCMs. Moreover, heat pipes can be incorporated into PCMs to decrease the thermal resistance. The purpose of PCM coupled with heat pipe is to incorporate the large thermal conductivity of heat pipe with the large latent heat capacity of PCM. PCMs have ability to balance the unconformity between supply and demand in heat pumps and decrease the size of storage tank used in a heat pump module. [ABSTRACT FROM AUTHOR]

This paper presents the thermodynamic and techno-economic performance calculation of heat pipe evacuated tube collector (HP-ETC) solar water heating system by energy, exergy analysis and different economic parameters for Indian composite climate of Jammu. The HP-ETC solar water heating system of 1.69 m2 was designed and fabricated for a family with an average size of six persons. The experiments were done for six different mass flow rates of working fluid (water) such as 20, 30, 40, 50 and 60 L per hour (LPH). The highest average energy and exergy efficiencies were obtained to be 72% and 5.2%, respectively, for 20 LPH, while the lowest values of respective parameters were 55% and 1.25% at 60 LPH flow rate. The maximum and minimum average outlet temperatures from collector were found to be 76.4 and 45 °C at 20 and 60 LPH, respectively. It has been observed that heat pipe ETC water heating system is better than direct flow evacuated tube collector-based water heating system. Also, the exergy efficiency was found to be much lower than energy efficiency for all the selected mass flow rates. The techno-economic analysis of heat pipe evacuated tube solar water heating system was done by three methods, namely annual cost, life-cycle savings and payback period. The cost of hot water production at desired temperature was found to be INR 0.12 per liter, whereas that of the electric geyser and gas geyser was INR 0.40 and 0.26, respectively. The payback period of heat pipe ETC solar water heater was estimated to be 4 years, which is much less than the life of HP-ETC solar water heating system. [ABSTRACT FROM AUTHOR]

As a new type of condensation-heat-transfer facility, nano-porous ceramic membrane tube is surrounded by nano-porous wall surface, which is hydrophilic to capture the vapor or condensate. In general, water vapor will condense into liquid near the wall surface or inside its nano-sized pores, wherein its latent heat will be released synchronously. Thus, an amount of heat and water can be recovered by using nano-porous ceramic membrane tube. Experiments are performed using a 10-nm ceramic membrane tube and a traditional condensation-heat-transfer tube (a 304 stainless steel tube in the experiment) with the same dimensions. The gas mixture consisting of nitrogen (N2) and water vapor is employed as working fluid, and the low-temperature water is used as the cold source. Great attention is paid to contrastively evaluate the heat-transfer and water-recovery performance of two kinds of tubes at different influential parameters (e.g., inclination angle). Compared with stainless steel tube, ceramic membrane tube has a more outstanding heat-transfer characteristic; it can enhance the cooling water temperature by 125% maximally. In addition, condensation-heat-transfer process of 10-nm ceramic membrane tube is influenced by inclination angle more significantly. In the 10-nm ceramic membrane tube, it has highest condensation-heat-transfer coefficient at horizontal flow (θ = 0°) and lowest at vertical down flow (θ = − 90°). Finally, water-recovery mass flux in the 10-nm ceramic membrane tube is higher than that in stainless steel tube, especially for the transmembrane pressure difference is higher than 3.83 kPa. This paper is useful for ceramic membrane condenser arrangement and design. [ABSTRACT FROM AUTHOR]

This paper experimentally investigated exergetic performance analysis of vapor compression refrigeration system using R450a as a replacement for R134a at different evaporator and condenser temperatures within controlled environmental conditions. The exergetic performance analysis of the vapor compression refrigeration system with test parameters including efficiency defects in the components, total irreversibility, and exergy efficiency of the refrigeration system was performed. Findings showed that the total irreversibility and exergy efficiency of the vapor compression refrigeration system using R450A refrigerant were lower and higher than R134a by about 15.25–27.32% and 10.07–130.93%, respectively. However, the efficiency defect in the condenser, compressor, and evaporator of the R450A refrigeration system was lower than R134a by about 16.99–26.08%, 5.03–20.11%, and 1.85–15.85%, respectively. Conversely, efficiency defect in the capillary tube of the R450A refrigeration system was higher than R134a by about 14.66–78.97% under similar operating conditions. Overall, it was found that the most efficient component was the evaporator, and the least efficient component was the compressor for both refrigerants. [ABSTRACT FROM AUTHOR]

The paper presents a thermal performance analysis of a thermosyphon filled with graphene oxide nanofluid. It focuses on factors influencing thermal resistance and geyser boiling processes, such as the presence of surfactant (here: sodium dodecyl sulphate), the time-related deterioration of nanoparticles, and working conditions. Results indicate that GO nanofluids improved heat transfer at low heat loads and that enhancements were limited to the evaporator section. Although thermal resistance of the device was similar for all tested working fluids at high evaporator temperatures, the time-dependent behaviour was different. Both water and GO nanofluid facilitated geyser boiling. Sodium dodecyl sulphate inhibited the phenomenon for pure water, but the same concentration of surfactant in the graphene oxide nanofluid did not show the same effect. SEM analysis showed that substantial amount of surfactant attached to the surface of graphene oxide flakes, which explains the previously described behaviour. [ABSTRACT FROM AUTHOR]

Since the last decades, solar energy has been used worldwide to overcome foreign dependency on crude oil and to control the pollution due to a limited source of non-renewable energy. Evacuated tube solar collectors are the most suitable solar technology for producing useful heat in both low and medium temperature levels. Evacuated tube solar collector is capable of working in hot, mild, cloudy or cold climates where flat plate collector is not an option. The objective of this review paper is the detailed investigation of evacuated tube solar collectors having heat pipe and direct flow are reviewed. All the design parameters which influence the collector performance are investigated and discussed in this work. More specifically, the tracking system, the collector design, the mass flow rate, the optical design and the kind of the working fluid are the main studied parameters. Moreover, this work focuses on the latest developments and advances, providing a review of experimental and numerical studies reported. Lastly, this work presents the future ideas that can be carried out to improve the performance of evacuated tube solar collectors. [ABSTRACT FROM AUTHOR]

High heat generation from electronic devices needs to cool down properly to prevent overheating. Loop heat pipe (LHP) is one of the excellent cooling devices for high heat generation of electronic devices. Nanofluid is a working fluid which has nanoparticle dispersed in based fluid. Nanofluid is proven to have better thermal performance compared with conventional fluids. In this paper, we investigate on the thermal performance of loop heat pipes using different types of nanofluids. The desired nanofluids used in this study are diamond nanofluid, aluminium oxide (Al2O3) nanofluid and silica oxide nanofluid (SiO2) with 0–3% of mass concentrations. The results showed that as the mass concentration of nanofluids increased, the thermal resistance for diamond nanofluid and Al2O3 nanofluid decreased, but SiO2 nanofluid results show the opposite trend of thermal resistance with increasing mass concentration. The lowest thermal resistance is 3.0872 °C W−1, 3.1465 °C W−1 and 3.2816 °C W−1 for diamond, Al2O3 and SiO2, respectively. Moreover, all types of nanofluids show better heat transfer performance compared with water. Diamond nanofluid also had higher heat capacity than Al2O3 nanofluid as it had a lower vapour line temperature reading. Optimization result also shows that diamond nanofluid has better thermal enhancement than Al2O3 with 1.19% of mass concentration. [ABSTRACT FROM AUTHOR]

Thermal analysis has been used in foundry applications to assess the quality of the melt before casting. The high-end thermal analysis techniques such as DSC or DTA are expensive and not suitable for foundry applications. The Computer-Aided Cooling Curve Analysis (CA-CCA) method based on one thermocouple has been widely used as a batch process with poor control over heat extraction and cooling rates during solidification. A heat pipe apparatus has been developed as a thermal analysis tool. The apparatus can assess the melt quality more accurately, as well as, allow for better control of heating and cooling rates. Moreover, the solidification process can be modeled more accurately, and thus the casting parameters affecting the casting quality can be closely simulated and consequently controlled. In this paper the principles of a heat-pipe assisted thermal analysis system are highlighted. The advantages of the new system are described and the possibility of its adoption in melt assessments is discussed. [ABSTRACT FROM AUTHOR]

In this manuscript, the innovative design of a hybrid system is investigated for distilled water and hot water production using the heat pipe-equipped vacuum tube collector system assimilated with solar still (HTP-ETCS-SS). The proposed hybrid system is compared with traditional solar still (SS) in context of energy, exergy and financial analysis with two different depths, i.e., 40%, and 60%, and varying flow rate (4, 8, and 12 LPH). The results show a rise in distilled water productivity by 152.9%, 108%, and 51.9% with 40% depth and 162%, 152.5%, and 92.3% with 60% depth with a varying flow rate of 4, 8, and 12 LPH, respectively, in contrast to traditional solar still. The improvement in energy and exergy efficiency for distilled water output is gained between 40–22.2% and 0.7–4.1%, respectively, with the designed system. However, the system's energy and exergy efficiency lie between 64.8–42.4% and 28.4–16.4%, respectively, for both distilled water and hot water productivity. The 8 LPH flow rate is found optimum in terms of distilled productivity and hot water production. The designed system's distilled water and hot water unit costs are 0.1037$ and 0.0276$, respectively. [ABSTRACT FROM AUTHOR]

Effective cooling technology is intensely demanded to cool the electronic devices within small space under high heat flux. Mini-grooved flat heat pipe (FHP) with simple structure, close contact with heat source, uniform temperature and high thermal conductivity can satisfy the demand of transferring the great heat rapidly and weakening the hot spot of electronic devices. As fine wick feature and distribution played a critical part in improving the performance of mini-grooved FHP, a thermal experiment was established to explore the heat transfer characterization of mini-grooved FHP with different wick structures and distributions. And a mathematical model in one dimension was set up to investigate the fluid flow and heat transmission performance of FHP with V-type or rectangle-type grooves, expecting to assist the experiment in understanding the operating mechanisms of mini-grooved FHP further. Effects of input heat, inclined angle, working temperature and wick structure were examined. It is found that among the V-type, rectangle-type and block-type mini-grooved FHPs, V-type FHP presents better heat transfer rate, while block-type FHP displays larger maximum heat transfer amount. V2 sloped convex gradient mini-grooved FHP owns the optimum overall performance. It possesses the merits of narrower grooves at the evaporation section, wider grooves at the condensation section and larger vapor chamber space, which can optimize the liquid and vapor circulation processes inside the FHP. Its thermal resistance and maximum temperature reduce by almost 9.7% and 3.4% separately in comparison with those of V1 straight mini-grooved FHP, which is beneficial to guarantee the reliability and stability of electronic devices. [ABSTRACT FROM AUTHOR]

Condensation is a main process in the compression refrigeration cycles. The low performance of split air conditioners prompted researchers to find different methods to improve their performance. One of the methods that can be used to enhance the performance of split air conditioners is the use of porous inserts in the tubes of the condenser. An experimental test rig, which consists of a compressor, condenser, expansion valve, and evaporator, is used for this issue. Stainless steel balls with porosities of 49%, 38%, 30%, and 100% were used. The condenser temperature was varied from 30 to 44 °C, while the air ambient temperature varied from 27.5 to 37.5 °C. The tested refrigerant is R454C. Evaporation capacity, coefficient of performance, power consumption, and volumetric evaporation capacity are experimentally investigated for different operating parameters. Heat rejection rate, mass flow rate, pressure drop, and compressor discharge temperature are also investigated. Percentages of increase of 53.3% and 25.7% are recorded for cycle coefficient of performance and power consumption, respectively, by changing porosity from 100 to 30%. Also, average percentages of increase of about 50.5% and 35.5% are recorded for heat rejection rate and pressure drop, respectively. The coefficient of performance and evaporation capacity are increased by 58.4% and 63.9%, respectively, when decreasing air ambient temperature from 37.5 to 27.5 °C. Power consumption increases by 26% when increasing air ambient temperature from 27.5 to 37.5 °C. Compressor discharge temperature considerably increases at high condensing temperatures and low porosities. [ABSTRACT FROM AUTHOR]

One of the most commonly used techniques in heat transfer enhancement studies is loose-fit twisted tape with variety of configurations. Optimizing the configurations prevents additional effort, time and cost losses. In this regard, the current research examined the impacts of design parameters identified as twist ratio, clearance ratio, and Reynolds number of the loose-fit twisted tape with trapezoidal wings used in a heat exchanger pipe on pressure drop and heat transfer. For the optimization study, the impacts of three various twist and clearance ratios, and six various Reynolds numbers on the friction factor, Nusselt number, and thermohydraulic performance factor identified as performance parameters were evaluated. The optimum conditions were revealed by conducting the signal-to-noise ratio analysis of the Taguchi method for the purpose of maximizing/minimizing the response variables on an individual basis. As methodology, multi-criteria decision-making problem was applied because of the optimum conditions depending on many variables and various response variables. The solution of the multi-objective optimization problem was provided by employing a pair of methods of the AHP/ TOPSIS. The analysis of variance was performed to determine the extent to which every parameter contributed to the response variables. According to the ANOVA findings, the most effective parameter was the Reynolds number. The Reynolds number of 18000, the twist ratio of 2, and the clearance ratio of 0.0178 were chosen as the multi-objective optimum conditions. In relation the results, an approach regarding the optimum design parameters, which has not been revealed before in the use of loose-fit twisted tape, has been put forward. [ABSTRACT FROM AUTHOR]

In this study, we have employed a strategy for the optimization of the concentrations of iron oxide ( Fe 3 O 4 )-multi-walled carbon nanotubes (MWCNTs)/deionized (DI) water hybrid nanofluid based on non-dominated sorting genetic algorithm-II (NSGA-II) before examining it inside the pulsating heat pipe (PHP), taking into account the basic criteria for PHP performance optimization. The thermophysical properties of the hybrid fluid were optimized to obtain concentrations with the highest thermal conductivity and specific heat capacity and the lowest dynamic viscosity and density. Also, the thermal performance of the PHP charged with the optimized concentrations of the hybrid nanofluid was tested. Results showed that the hybrid nanofluid containing 0.597 vol. % of Fe 3 O 4 and 0.100 vol % of MWCNTs provides the best thermal performance for the PHP. Further, the thermal performance of the PHP with base fluid (DI water) was tested. The observations indicated a better start-up and a lower thermal resistance of the PHP with DI water. The thermal resistance of the PHP charged with mentioned hybrid nanofluid and DI water at a heat input of 70 W is 0.185 ∘ C W - 1 , which is almost 49.45 % lower than that of PHP charged with other hybrid nanofluids. Therefore, it can be concluded that although the Fe 3 O 4 -MWCNTs/DI water hybrid nanofluid has a relatively high thermal conductivity, its high viscosity, low specific heat capacity, and sedimentation of the nanoparticles deteriorate the thermal performance of the PHP compared to the base fluid due to increased concentration. [ABSTRACT FROM AUTHOR]

This study evaluates the effect of using alumina (Al2O3)-water nanofluids stabilized by sodium dodecylbenzene sulfonate (SDBS) on the thermal performance and capillary limit of a screen mesh heat pipe. Nanofluids were prepared using three Al2O3 concentrations (0.1 mass/%, 0.5 mass/%, and 1.0 mass/%) and two SDBS concentrations (0.064 mass/% and 0.32 mass/%). The stability of the nanofluids was studied by means of UV-absorbance measurements prior to the experimental test. The results show that the thermal resistance of the heat pipe decreased by up to 50% with Al2O3 and SDBS at concentrations of 0.5 mass/% and 0.32 mass/%, respectively. Nevertheless, the capillary limit was reduced between 25 and 45 W compared to the operation with water. Finally, the capillary limit was improved by 5 W (compared to water) using an Al2O3 concentration of 0.1 mass/% and no surfactant. [ABSTRACT FROM AUTHOR]

A heat pipe is a passive two-phase heat exchanger technology, as a capillary-driven structure that allows heat transport by maintaining temperature difference. Heat pipe performance can be determined from the value of heat resistance, and nanoparticle can be applied to increase heat pipe performance. This research uses Al2O3 and TiO2 as a coating material for the heat pipe. The methods used in this research were by giving the heat pipe a nano-coating treatment using the electrophoretic deposition process and doing a pool boiling experiment by giving the heat pipe some heat loads. The main data of this research are temperature and bubble growth data. Based on the result of the research, the use of nanoparticles can improve heat pipe performance. The temperature difference between the evaporator and condenser area was calculated 2.38 °C on Al2O3 coating and 3.92 °C on TiO2 coating. Al2O3 nanoparticle coating was able to provide a heat transfer coefficient 480% superior to sample without nanoparticle coating, and 174% better than TiO2 nanoparticle coating. [ABSTRACT FROM AUTHOR]

The loop heat pipe is a passive high-efficiency two-phase transfer device. To study the effects of the liquid and vapor line lengths on the performance of ethane loop heat pipe (ELHP), a performance test system has been set up based on a pulse tube refrigerator (PTR). The lengths of liquid line (LL) and vapor line (VL) of the ELHP are changed in this test system. The LL length is set as 180 mm, 500 mm, 800 mm, 1100 mm and 1500 mm, respectively, when the VL length is 500 mm. In addition, the VL length is set as 800 mm, 1100 mm, and 1500 mm, respectively, as the LL length is 500 mm. The thermal performances of ELHP including startup performance, thermal resistance and temperature difference are investigated in detail under 170 ~ 240 K temperature regions. The experiment results show that the lengths of LL have an important effect on the startup performance when the heat load (HL) of the ELHP is 0 W. The shorter the LL length is, the easier the startup is at 0 W HL. When the LL length is longer, the ELHP is more stable and the temperature oscillation is smaller. The effect of the VL length on the ELHP startup performance is the same as that of the LL length. Due to the thermophysical properties of ethane, the ELHP in the 240 K temperature region has the best performance. [ABSTRACT FROM AUTHOR]

Based on innovative design, a liquid–vapor separation plate condenser with excellent heat transfer performance is invented. It takes less time to discharge condensate from the refrigerant channel and get a thinner liquid film in condensing channel with the new-type condenser than with the conventional one. The evaluation of the heat transfer performance of the liquid–vapor separation plate condenser and the optimization of its structure design is realized with a validated mathematical model. In this numerical simulation, the performance evaluation criterion and penalty factor are used to assess the refrigerant side performance, while the exergy efficiency is used to evaluate the overall performance, which includes the water-side performance. According to the simulation results, the optimal structure of the liquid–vapor separation plate condenser is achieved when altitude ratio is equal to 0.6 and length ratio is equal to 0.5. In this structure, when the condensation temperature of refrigerant R410A is 20 °C, the heat flux is kW m−2, the mass flux is 50 kg m−2 s−1, the performance evaluation criterion is at 1.13, the penalty factor is at 0.89, and the exergy efficiency is at 0.34. All these values outperform those of conventional plate condenser. Compared with conventional one, the heat transfer coefficients of refrigerant of liquid–vapor separation plate condenser increase by 15.98%, and its the pressure drop increases by 16.45% under the setting conditions. [ABSTRACT FROM AUTHOR]

Oscillating heat pipes (OHPs) are applicable in different energy systems such as solar collectors, desalinations and fuel cells as thermal mediums or thermal management devices. Thermal resistance of OHPs is affected by different elements including the architecture of OHP, filling ratio, heat load and operating fluid. Depending on the operating temperature range, appropriate working fluid is used. Acetone is one of the fluids that is applicable for the OHPs working in low or medium temperatures. Performance modeling of OHPs can be conducted by employing numerical simulation and data-driven methods. Utilizing powerful data-driven methods can be preferred in terms of time consumption and computational cost. In the present article, two types of artificial neural network, multilayer perceptron (MLP) and group method of data handling (GMDH), are utilized for thermal resistance modeling of OHPs filled with acetone under various working conditions. To find the models with the highest possible accuracy, different architectures are considered in this work. Models' outputs revealed that the thermal resistance of the aforementioned OHPs can be reliably predicted by using the abovementioned methods. R-squared of the models proposed by using MLP and GMDH are 0.989 and 0.965, respectively. Values of mean squared errors for the mentioned methods are around 0.0045 and 0.0144, respectively. [ABSTRACT FROM AUTHOR]

In the present research, extremely low pumping speed and special charging method are employed to investigate the effects of pumping speed on the filling ratio and isothermal performance of heat pipe. Results show that the saturation degree of porous wick is about 92% and water loss during vacuuming is significantly decreased when pumping speed is decreased to 0.1 m3 h−1. In addition, excessive extension of evacuation time cannot decrease NCG level further but increase water loss. NCG level after first vacuuming is high, and the temperature difference (ΔT) along heat pipe cooling section is larger than 7 °C. But, ΔT can be significantly decreased when secondary degassing is conducted. Increasing heating temperature and duration both obviously decrease NCG level and increase isothermal performance. The maximum ΔT of the heat pipe can be decreased to 0.5 °C when secondary degassing was conducted at 140 °C for 100 s. Moreover, the decrease in shrunken section length decreases the isothermal performance and 40 mm is enough in the present study. Finally, the water loss of whole fabrication process can be decreased to ~ 0.015 g leading to approximately 1 mass% water loss in heat pipe. [ABSTRACT FROM AUTHOR]

Numerical simulations are performed to analyze the thermal characteristics of a latent heat thermal energy storage system with phase change material embedded in highly conductive porous media. A network of finned heat pipes is also employed to enhance the heat transfer within the system. ANSYS-FLUENT 19.0 is used to create a transient multiphase computational model to simulate the thermal behavior of the storage unit. Copper foam is the porous medium used to enhance the heat transfer and is impregnated with the phase change material, potassium nitrate (KNO3). The effects of the porosity of the metal foam and the quantity of heat pipes on the thermal characteristics of storage unit have been investigated. The results indicated that increasing the quantity of the embedded heat pipes leads to drastic acceleration of both charging and discharging process. Impregnating the copper foam with potassium nitrate phase change material significantly affects the total charging and discharging times of the storage unit. It was shown that the porosity of the metal foam plays a key role in the thermal behavior of the system during the charging and discharging processes. [ABSTRACT FROM AUTHOR]

Generally, there are two main methods to increase the thermal performance of the heat exchanger, which include active and passive methods. Passive mode, unlike active mode, does not need external power which the heat transfer rate can be increased by utilizing changes in the flow regime or direction of the fluid path. In the present survey, numerical analysis of the effect of geometrical and operational parameters on the thermal performance of a convergent–divergent tube is done. Numerical simulations are performed using the commercial CFD code. The investigated geometrical parameters include the large and smaller diameters of the cone's wall, the pitch of the cone and the height of the roughness. Obtained results in the first section indicate that the proposed wavy geometry leads to enhanced heat transfer in the pipe. In the second section of the study, instead of pure water, two types of water-based nanofluids, including water/Al2O3 and water/CuO, are utilized, and the obtained results are compared with pure water. Results indicate that water/Al2O3 nanofluid has better thermal performance than CuO/water and especially pure water. As a result, it can be said that for water/Al2O3 nanofluid at low Reynolds number (Re = 10,000), the case with φ = 4% has 9.29% more thermal performance than pure water which has the highest thermal performance. Also, at high Reynolds number (Re = 20,000), the case with φ = 5% has 7.15% more thermal performance than pure water. The lowest thermal performance improvement in comparison with pure water belongs to the case with φ = 2% at high Reynolds number (Re = 20,000) with about 6%. Also, for water/CuO nanofluid, the case with φ = 3% at low Reynolds number (Re = 10,000) and the case with φ = 5% at high Reynolds number (Re = 20,000) have highest and lowest thermal performance improvement in comparison with pure water with 8.86% and 6.25%, respectively. [ABSTRACT FROM AUTHOR]

In this research, a series of experiments have been performed to study the thermal resistance of an oscillating heat pipe equipped with cooling tower. The effects of filling ratio and input heating power on the thermal resistance of the heat pipe and temperatures of different sections of evaporator and condenser of the heat pipe are investigated and discussed. All tests are taken for input heating power and filling ratio in the ranges of 20–200 W and 10–60%, respectively. A correlation for the thermal resistance is presented, which the effects of input heating power and filling ratio are taken into account in this correlation. The results showed that the heat pipe with filling ratio of 40% and input heating power of 160 W has the minimum value of thermal resistance among all cases considered in this research. Moreover, the thermal resistance decreases about 86% as the input heating power increases in the range of 20–120 W, while this reduction is only 23% by increasing the input heating power in the range of 160–200 W. [ABSTRACT FROM AUTHOR]

The effect of varying wick structures viz. mesh, sintered and composite wick (sintered-mesh) on the thermal enhancement of cylindrical heat pipes is experimentally investigated. In addition, the investigation focused on the effect of inclination angle and heat input of heat pipe. Surfactant free CuO nano-fluid with a mass concentration of 1.0% is used as a working fluid. The energy and exergy analysis of heat pipe was also conducted at various conditions. To analyze the distinctive performance of composite heat pipe, a heat pipe is filled with DI water and the obtained results are compared with nanofluid results. The maximum heat transfer capability of composite heat pipe is improved by 35.71% and 18.75% compared with mesh and sintered wicks. The composite heat pipe with CuO nanofluid as working fluid instead of DI water improves the heat transport capacity by 11.76%. Surface temperature of heat pipe significantly reduces by varying the wick structure viz. mesh, sintered and composite wick. The composite heat pipe with 1.0 mass% of CuO nanofluid obtained 3.7 °C reduction in surface temperature at evaporator section compared with DI water. Thermal resistance of heat pipe is gradually reduces with increasing inclination angle. The maximum reduction is observed for composite wick, sintered and mesh wick heat pipes are 47.50, 43.70 and 24.39% respectively at 45° inclination angle compared with horizontal axis. [ABSTRACT FROM AUTHOR]