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Commentaries are presented on Ye et al. [1]'s paper, where the authors investigated the performance of heat transfer in evaporator under the case that the parameters of the heat source and temperature of pinch point were identified. They utilized the graphical method of temperature-heat (T-Q) diagram analysis. Also, they utilized the entransy principle in their analysis. Commentaries show the reality that the graphical method of T-Q diagram analysis belongs to Professor Adrian Bejan, who first proposed it in 1977. In addition, many instances in the literature are given to indicate disputes for the entransy principle by oppositionists of "entransy" from various countries. [ABSTRACT FROM AUTHOR]

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

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

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]

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

Organic Rankine cycle (ORC) is suitable to converting the normally hard to utilize low temperature thermal energies, such as geothermal energy, solar energy, and industrial waste heat, to electricity through utilizing low boiling organic working fluids. The performance of ORC system is dramatically affected by the selections of working fluid and working conditions. As a key component of waste heat recovery, the irreversible loss of evaporator also has great influence on the performance of ORC system. In this paper, we study the heat transfer performance in evaporator under the condition that the heat source parameters and pinch point temperature difference are identified. It is found that the heat transfer performance is affected by Cr, the ratio of heat capacity flow rates between the working fluid and the heat source fluid. The equivalent thermal resistance, deducing from the concept of entransy, to measure the irreversability during the heat transfer process is used. Then, the parameter κr, the ratio between latent heat and sensible heat of working fluid is defined. With the parameters Cr and κr, we investigate the relationship between the heat transfer and irreversible loss, and deduce the condition that maximum heat transfer and minimum equivalent thermal resistance occurs. Finally, a calculation method is established to choose the optimum working fluid and the evaporation condition. [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 represents an experimental study about the effect of curves related to thermosyphons and heat pipes with different active fluids and inclination angle at the thermal efficiency. The nanofluid utilized in this work is an aqueous soluble of Al2O3 nanoparticles with 35 nm diameter in pure water. The test saturation level of nanoparticles is 0%, 1%, and 3%wt. All the experiments were conducted and repeated at inclination angle of 30°, 60°, and 90° (vertical). The article presents the gravity impacts on the heat transfer characteristics in different angles and the effects of working fluids and tilt angle of heat pipe tube by the addition of nanoparticles and weight fractions on the thermal efficiency of heat pipe at different inclination. According to the experimental results, the heat pipe at the tilt angle of 60° generates the superior results. At a particle volume concentration of 1%, the use of Al2O3/water nanofluid gives significantly higher heat transfer. [ABSTRACT FROM AUTHOR]

The helical oscillating heat pipe (HOHP) is a high heat transfer heat exchanger with high flexibility in its installation and can therefore be used in a wide variety of applications. In this study, the effect of various parameters on the heat transfer rates of HOHP were used to establish a correlation equation for use in the heat flux prediction, the dimensionless parameters studied were r v/r l, Bo, Nu, We, Ja, Pr, Fr, Co, Ga, Bi, Wo, Oh, and Ku. Experiments were conducted to find out their effects on the heat transfer rates of copper HOHP with internal diameters were 2.03, 3.5, and 4.5 mm. The lengths of evaporator and condenser sections were equal at 1500, 2000, and 2500 mm. The pitch coils were 10, 15, and 20 mm. The working fluids used were R134a, ethanol, and water with the filling ratios were 30%, 50%, and 80% of the total internal volume. The temperature of evaporator section were varied between 60, 70, and 80°C within normal operating conditions in a vertical position. The results of the experiment showed that the internal diameter, lengths of evaporator/condenser sections, pitch coil, type of working fluid, filling ratio and temperature of evaporator section affected the heat transfer rates of the HOHP. The results of dimensionless parameters can establish the correlation equation to predict the heat flux for the HOHP as shown in this paper. In addition, the results of this research can be applied in the designing and construction of HOHP heat exchangers. [ABSTRACT FROM AUTHOR]

In this paper, the empirical correlations for heat transfer characterizations of a small closed end heat pipe with special vapor chamber (SCEHP/SVC) are carried out. The heat transfer performance was introduced by a correlation function of dimensionless parameters namely Kutateladze number (Ku), Bond number (Bo), Weber number (We), Froude number (Fr), Prandtl number (Pr), Jacob number (Ja), Density ratio (DR) and Aspect ratio (AR). SCEHP/SVCs were consists of two main parts such as a small closed end and special vapor chamber (where located at the bottom). Small closed end was made of small copper tubes with 2, 3 and 4.5 mm ID. Number of turns was 10. Special vapor chamber was an ID of 14.5, 17.5 and 20.5 mm. The evaporator, adiabatic and condenser sections were of equal length 50, 100 and 150 mm, with an inclination angle 60o and 90o. Water, ethanol and R-134a were selected as working fluids with filling ratios 30, 40 and 50% of the volume of a special vapor chamber. The operating temperatures were 60, 70 and 80°C that heated by hot water at the evaporator section, while the condenser section was cooled by cold water to 20°C. Experiments were recorded when the system reached a steady state, so as to calculate the heat flux. All experimental results of the heat flux of SCEHP/SVCs at inclined 90o orientation could be correlated in terms of modified Kutateladze number (Ku*) as follows; Ku*90 = 2.25[Bo-1.2 We-2.309 Pr1.503 Ja-4.318 AR0.877 DR1.232] That correlation was used to predict the heat flux with standard deviations (STD) of ±22.12%. [ABSTRACT FROM AUTHOR]

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

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

Highlights • Constructal design of shell-and-tube evaporator is conducted. • Evaporator is used in Kalina cycle with ammonia-water working fluid. • A complex function minimization is taken as optimization objective. • It composed of heat transfer rate and total pumping power. • External diameter of the heat transfer tube and mass flow rates are optimized. Abstract On the basis of constructal theory, structure design of a shell-and-tube (ST) evaporator with ammonia-water working fluid is conducted in this paper. A complex function composed of heat transfer rate (HTR) and total pumping power (TPP) is taken as optimization objective, and the heat transfer area of the tubes is taken as the constraint. The complex function of the ST evaporator is minimized, and the corresponding optimal external diameter (ED) of the heat transfer tube (HTT) and optimal mass flow rates (MFRs) of the ammonia-water and hot water are obtained. Compared with the initial design, the complex function, HTR and TPP are decreased by 17.24%, 22.88% and 64.14% after constructal optimization, respectively. It illustrates that the complex function is a tradeoff between the heat transfer and fluid flow performances of the evaporator. As the ammonia concentration increases, the overall performance of the evaporator is improved. The inlet temperature of hot water has certain influences on the optimal MFRs of the ammonia-water and hot water, but has no influence on the optimal ED of the HTT. The optimal results obtained in this paper can provide some guidelines for the optimal designs of the ST evaporators, and can be further extended to the optimal designs of the ST condensers and even the whole systems of Kalina cycles. [ABSTRACT FROM AUTHOR]

The self-rewetting fluid is the liquid which have an anomalous surface tension increasing with temperature when the temperature exceeds a certain value. This particular property can cause the subcooled liquid to be drawn towards the heated surface if a dry patch appears, thus making it possible for the self-rewetting fluids to become promising working fluids. This paper presents an overview of the recent developments of the research on boiling heat transfer using self-rewetting fluids. Thermophysical properties, pool and flow boiling experiments and heat pipe applications of self-rewetting fluids have been reported in this paper. It can be found that the use of self-rewetting fluids in a wide range of applications appears promising. However, further detailed and valuable theoretical and numerical investigations are necessary for us to better understand the boiling phenomenon of self-rewetting fluids. [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]

This paper presents a novel study on performance of pulsating heat pipe using ammonia as working fluid. Firstly, a new full visualization experiment with high speed camera is conducted, to study the relationship between the flow patterns and thermo-hydrodynamics of the PHP. The tested PHP, consisting of 6 turns, is fully made of quartz glass tubes with 6 mm outer diameter and 2 mm inner diameter. The filling ratio for ammonia fluid is 70%. Wall temperature fluctuations of several key positions are recorded under a series charge of heat transfer rates, which are from 25 W to 520 W. In visualization results, the motion identities, flow pattern variations, breakup and coalescence between the vapor plugs and liquid slugs are illustrated and discussed. In addition, the actual velocity of the vapor is computed from the experimental measurement in this paper, which is never found in other literature but is very important for the mechanism study in theoretical model. Furthermore, a theoretical model including the dynamical characteristics and heat transfer is investigated coupled with the thermal driving force, friction force and capillary force variations as the flow patterns changed at different transport powers, which are derived from the full visualization experiment. The theoretical results are compared with the experimental results analytically, and the operating mechanism of PHP will be discussed in detail finally. [ABSTRACT FROM AUTHOR]

The paper proposes a model of heat transfer in the evaporator of the spacecraft thermal control system. The model allows to calculate the average temperature of the evaporator wall and to build a "boiling curve" in a wide range of thermal loads. Adequacy of the model is confirmed by experimental studies on an aluminum thermal sink with high longitudinal thermal conductivity in the range of parameters typical for the thermal control systems of spacecrafts. Ammonia is used as a working fluid. The model might be recommended for use in zero gravity and normal ground conditions. [ABSTRACT FROM AUTHOR]

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

The objective of this paper is to investigate startup conditions of a closed loop oscillating heat pipe (CLOHP). The CLOHP are made of a long copper capillary tube with an inner diameter of 2.03 mm. The number of turns was 4, 5 and 7. The ends of the tube are connected to complete the loop. The evaporator, adiabatic and condenser sections are all an equal length of 50 mm. R123, acetone and water were used as the working fluids with a 50% filling ratio. The CLOHP was operated horizontally and the condenser temperature was controlled at 25 ∘C. A low-voltage high-current heater was used as the heat source and the cooling medium was a solution of water and ethylene-glycol with a 1:1 mixing volume ratio. The number of turns is 7, changing the working fluid between R123, acetone and water, it was found that the heat transfer rate increases from 5.72 W using R123 as working fluid, to 7.35 W and 14.71 W using acetone and water as working fluids, respectively. The frequency of the working fluid decreased from 0.94 cycle/s using R123 as working fluid, to 0.65 cycle/s and 0.19 cycle/s using acetone and water as working fluids, respectively. The amplitude increased from 0.36 °C using R123, to 0.55 °C and 3.62 °C using acetone and water, respectively. In the case of using water as working fluids, changing the number of turn from 4 to 5 and 7 turns, it was found that the heat transfer rate increased from 11.43 W using 4 turns, to 12.25 W and 14.71 W using 5 and 7 turns, respectively. The frequency did not change switching to 5 turns but did change from 0.18 cycle/s to be 0.19 cycle/s when using 7 turns. The amplitude increased from 2.35 °C using 4 turns, to 3.24 °C and 3.62 °C using 5 and 7 turns, respectively. [ABSTRACT FROM AUTHOR]

In recent times pulsating heat pipes turn into a pleasing field of study for researchers as useful devices to enhance and intensify heat transfer. Experimental data under various operating conditions for a better theoretical understanding are needed. This paper presents an experimental study on a closed loop pulsating heat pipe made of copper with 14 U-turns. The experiments were carried out for different working fluids (acetone, distilled water, ethanol and methanol) and their mixing, heat input, filling ratio and evacuation levels to optimize operational conditions. Thermal resistance at steady state specified for performance evaluation and showed better results for acetone. The mentioned working fluids in filling ratio of 60 % have better operation and acetone has the best. As well as experiments in different evacuation levels showed, more evacuation lead to more performance. [ABSTRACT FROM AUTHOR]

• The effect of continuous and discontinuous TPT on η is investigated. • The thermal performance factor of seven different turbulators is compared. • The effect of perforation on discontinuous twisted tapes turbulator is examined. • Discontinuous turbulator has a better effect on heat transfer and pressure drop. • The highest η was obtained for twisted turbulator with a circular perforation. In this paper, the experimental effect of the continuous and discontinuous twisted tapes turbulator (perforated and non-perforated) on the inner side of the internal tube in the double heat exchangers on heat transfer, the friction coefficient, and thermal performance have been discussed. On discontinuous twisted tapes turbulator, 9 holes with different geometries, such as triangular, square, rectangular, circular, and diamond with the triangular arrangement, have been created on turbulator flat surfaces. The Reynolds range is investigated at 5500–10000, and working fluids are on the side of the inner tube and annular space is hot and cold water, respectively. Experimental results have shown that discontinuous turbulator has a better effect on heat transfer and pressure drop than continuous turbulator. As the discontinuous turbulator without a hole has an increase in heat transfer of 8.2 % and a reduction of 9.8 % in the pressure drop coefficient compared to the continuous turbulator. Then the effect of perforation on discontinuous twisted tapes turbulator was investigated. Experimental results show that perforated discontinuous turbulator with circular, square, rectangular, diamond and triangular perforation with the same hydraulic diameter as for discontinuous turbulator without perforation was 20.8 %, 15 %, 11 %, 8.7 %, 5 % increase in heat transfer and 27.7 %, 22.8 %, 17.3 %, 12.1 %, 5.5 % decrease in the coefficient of pressure drop, respectively. Therefore, according to the results, the best thermal performance was related to the perforated of perforated discontinuous twisted turbulator, discontinuous twisted turbulator, and continuously twisted turbulator were respectively. [ABSTRACT FROM AUTHOR]

Comprehensive research work on heat transfer in heat pipe using traditional working fluids has been carried out over the past decade. Heat transfer in heat pipes using suspensions of nanometer-sized solid particles in base fluids have been experimentally and theoretically investigated in recent years by various researchers across the world. The suspended nanoparticles effectively enhance heat transfer characteristics and the transport properties of base fluids in heat pipes. The objective of this paper is to present an overview of literature dealing with recent developments in the study of heat transfer using nanofluids in heat pipes and some important inferences from the various papers are also highlighted. It also discusses the mechanism of heat transfer enhancement or degradation, the existing problems for various heat pipes utilizing nanofluids, and explores the possible application prospects. [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]

In heat transfer applications, heat pipes are widely- preferred because of some characteristics such as low cost, being able to be produced in any size and low maintenance cost make them superior. Moreover, the working fluid to be employed substantially affects the heat transfer characteristics of a heat pipe. In this paper, effects of nanoparticle addition into the ethylene glycol on heat pipe’s thermal performance were analysed experimentally. Every test was done using two variant working fluids, ethylene glycol and dolomite nanoparticles-doped ethylene glycol, respectively. Dolomite nanoparticles (2% by weight) and Sodium Dodecyl Benzene Sulfonate (0.5% by weight) were doped into the ethylene glycol while preparing the dolomite/ethylene glycol nanofluid. After filling in the heat pipe, experiments were realized under changing working conditions. Using experimental data, efficiency and thermal resistance of the heat pipe were examined. Viscosity of the each working fluid was determined. The contact angle –wettability measurements were also performed to specify the effects of surface active agent addition. The obtained findings revealed that nanoparticle inclusion inside the base fluid, i.e. ethylene glycol, improved the thermal performance (efficiency) and decreased the heat pipe’s thermal resistance substantially. [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]

Highlights • A proof of concept oscillating heat pipe grinding wheel was investigated. • The oscillating heat pipe is potential to augment heat transfer in grinding zone. • The flow patterns determine the thermal transfer performance of the OHP. • The inner diameter and working fluid have coupled effects on heat transfer capacity. Abstract A massive amount of heat is generated during the high-efficiency grinding process, which leads to a serious burnout problem that limits the increase of the material removal rate. The oscillating heat pipe (OHP) can augment the heat transfer in the grinding zone to avoid the burnout. An oscillating heat pipe grinding wheel that is a combination of a grinding wheel and OHPs was proposed in this paper. Aimed to determine geometric dimensions and operating parameters of the OHP, experimental investigations were carried out to study effects of working fluid, inner diameter, and heat flux on the heat transport capability of OHPs. The dynamics of flow change and coupled effects of geometric dimensions and thermophysical properties of working fluids were further analyzed by visualization. The OHP filled with acetone shows an advantage in heat transfer. Within the critical diameter range, a large inner diameter is better for the thermal performance. As heat flux increases, changes of flow pattern and motion modes from bubbly flow to annular flow and from oscillation to circulation enhance the thermal performance. The inner diameter of 3 mm and acetone as the working fluid are preferred for better cooling effects. [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 wide variety of heating and cooling applications use heat exchangers. The increase in energy prices, the requirement for size reduction, and restriction on greenhouse gas emissions has led to the need for finding ways to develop efficient heat exchangers. A cost-efficient way to enhance the model of a heat exchanger by visualizing the effects of the design parameters is using Computational Fluid Dynamics (CFD). The reason for this exploration was to lead an examination of the varieties/changes in the general intensity move process for a Finned-Tube Heat Exchanger (FTHE), also known as Air Coil Heat Exchanger (ACHE) with a variety of plan boundaries like the quantity of tubes, course of action of tubes, and the material utilized for the intensity exchanger. The widely used heat exchanger that uses refrigerant R314a and air as the working fluids was simulated with different design modifications. The simulated results exhibited as to how the number of tubes, arrangement of coils/tubes, material of tubes, and density / spacing of fins, effects the pressure drop, temperature and velocities profiles, and heat exchangers' transfer of a heat. The use of copper coils improved the heat transfer by approximately 61% as compared to aluminium coils. [ABSTRACT FROM AUTHOR]

An extensive development of two-phase thermal control systems based on two- and multi-phase devices (standard heat pipes, loop heat pipes, heat switches, etc.) offers a potential to assume an increasingly important position in the instrumental thermal control for a number of space and earth applications. However, experimental data regarding long-life testing for such systems is insufficient. The first ammonia loop heat pipe (LHP) was developed in 1976 in the Ural Polytechnic Institute under supervision of Prof. Gerasimov, Iu. F. (1921–2001) (Kiseev, 1977; Gerasimov et al., 1981; Maydanik, 2005) [1–3]. This paper features its long-life test, covering 41 years with varying operation times. The life test consists of two parts: permanent and periodic LHP operation (approx. 5000 h) and LHP stand-by storage (approx. 152,600 h). Experimental investigations were performed using the developed loop heat pipe based on composite nickel wick with effective pumping pores of 0.5–10 µm in radius and 1 × 10 −14 m 2 to 4 × 10 −12 m 2 in permeability. We have provided experimental data demonstrating the impact of orientation, varying heat load and temperature on the LHP operation. In addition, a qualitative and quantitative chemical element analysis of the distilled water in a loop heat pipe without capillary structure (loop thermosyphon) was performed before and after its long-term operation (one year) to identify any possible interaction between the working fluid and the pipe materials. [ABSTRACT FROM AUTHOR]

The paper examines the problem of temperature distribution in a flat disk-shaped evaporator of a loop heat pipe (LHP) with counter heat and liquid flows. It analyzes the effect of filtration of a cold liquid through a wick on the evaporator temperature field. Calculations have been made for six working fluids, namely, water, methanol, ammonia, acetone, freon R141b and freon R152a. The heat flux supplied to the evaporator varied from 2.83·10 4 to 4.24·10 5 W/m 2 . The Peclet number Pe varied in the range from 0 to 3.73. An analysis of the results has shown that at Pe number up to 0.73 the contribution of the convective component to the total heat transfer in the evaporator is insignificant. A criterion K Pe has been suggested for evaluation of the cooling capacity of the liquid in an LHP wick. [ABSTRACT FROM AUTHOR]

Improvements in using geothermal sources can be attained through the installation of power plants taking advantage of low and medium enthalpy available in poorly exploited geothermal sites. Geothermal fluids at medium and low temperature could be considered to feed binary cycle power plants using organic fluids for electricity "production" or in cogeneration configuration. The improvement in the use of geothermal aquifers at low-medium enthalpy in small deep sites favours the reduction of drilling well costs, and in addition, it allows the exploitation of local resources in the energy districts. The heat exchanger evaporator enables the thermal heat exchange between the working fluid (which is commonly an organic fluid for an Organic Rankine Cycle) and the geothermal fluid (supplied by the aquifer). Thus, it has to be realised taking into account the thermodynamic proprieties and chemical composition of the geothermal field. The geothermal fluid is typically very aggressive, and it leads to the corrosion of steel traditionally used in the heat exchangers. This paper analyses the possibility of using plastic material in the constructions of the evaporator installed in an Organic Rankine Cycle plant in order to overcome the problems of corrosion and the increase of heat exchanger thermal resistance due to the fouling effect. A comparison among heat exchangers made of commonly used materials, such as carbon, steel, and titanium, with alternative polymeric materials has been carried out. This analysis has been built in a mathematical approach using the correlation referred to in the literature about heat transfer in single-phase and two-phase fluids in a tube and/or in the shell side. The outcomes provide the heat transfer area for the shell and tube heat exchanger with a fixed thermal power size. The results have demonstrated that the plastic evaporator shows an increase of 47.0% of the heat transfer area but an economic installation cost saving of 48.0% over the titanium evaporator. [ABSTRACT FROM AUTHOR]

• A new way of supercritical startup of a cryogenic loop heat pipe without additional power consumption was proposed. • The CLHP could achieve supercritical startup successfully from the ambient temperature, even in the harsh conditions. • The CLHP with nitrogen as working fluid performed stably and efficiently during the normal operation. • Experimental studies on the temperature hysteresis and power cycling of the new CLHP were conducted. Space applications require low power consumption, light weight and no gravity-assistant heat transfer devices. Cryogenic loop heat pipe (CLHP) is an effective heat transport device, which is able to fulfil supercritical startup with the help of a secondary evaporator or a secondary loop. However, both of the secondary evaporator and the secondary loop require additional power consumption on the CLHPs, which increase the burden of a cryocooler. This paper presents a new way of supercritical startup of a cryogenic loop heat pipe with simple structure and reliable operation performance. A flexible porous wick was introduced into the liquid line, connecting the condenser and the evaporator, to provide capillary force as the motive power for the liquid in the condenser flowing into the evaporator, which dispensed with the gravity assistance and additional power consumption. The CLHP operated at liquid-state nitrogen temperature range with nitrogen as working fluid. Investigations on the supercritical startup process and the operation process were presented in the paper. The CLHP could be cooled down from 299 K and realize supercritical startup without additional power consumption. 20 W heat loads were transported across a 0.51 m distance, and dryout was not happened all through the experiments. Furthermore, the supercritical startup process with 0.5 W heat load on the evaporator from the initial ambient temperature, and the operation process of 15 W-1 W-15 W-1 W-15 W heat load on the evaporator were also investigated. The CLHP with new way of supercritical startup performed well during the tests. The experimental results were presented and analyzed in this paper. [ABSTRACT FROM AUTHOR]

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

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

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

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

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

Summary: Large separate heat pipe is a potential selection to establish the passive cooling system to remove decay heat from the Spent Fuel Pool. In this system, heat pipes are expected to work properly and effectively for a long time without human intervention. Due to the physical dimensions and structures, flow patterns, and fluid distribution in large‐scale heat pipes differ from those in conventional ones. Based on the condensation flow patterns, a flow condensation model is presented. Then, a separate heat pipe model considering filling ratio effects is developed. Using the new model, the mean absolute errors of simulation are 5.5% (water), 5.1% (ammonia), and 4.3% (R134a). While in the conventional condensation model, these errors are 29.4%, 9.7%, and 10%. It is found that the influence of filling ratio varies with working fluids. For water heat pipe, the optimal filling ratio is about 17%. Higher or lower filling ratios lead to degradation of heat transfer capacity. For R134a or ammonia, the optimal filling ratio is a range, about 20% to 70%. The boundary of the range relates to both evaporator outlet vapor quality and heat pipe downcomer liquid column height. [ABSTRACT FROM AUTHOR]

• Gold nanofluid and butanol self-rewetting fluid represent a new and innovative class of heat transfer fluids for advanced cooling. • An experimental analysis is performed on the capillary evaporator section of capillary heat pipe. • Thermal performance is investigated by measuring the thermal resistance of the evaporator. • Gold nanofluid, with 1% Cv, showed a decreasing in the thermal resistance of the system by 13%. • A mixture of self-rewetting gold nanofluid showed the highest thermal performance by decreasing the thermal resistance by 22%. • The phase change is visualized by infrared camera. • This improvement has been explained due to the thermo-physical properties of nanofluid and self-rewetting fluid. This paper presents an experimental analysis of the phenomenon of phase change inside a porous medium using different types of working fluids. It represents the impact of these fluids on improving the characteristics of heat and mass transfer in a Capillary Heat Pipe (CHP). In this study, gold nanoparticles (5 nm in diameter with 1% C v), a self-rewetting binary solution (butanol with 3% C v), and a mixture of self-rewetting butanol and gold nanofluid are considered to be the operating fluids within the CHP. The experiments are carried out after designing and developing the capillary heat pipe section. It consists of a water tank with a pump, an evaporator attached to a copper porous medium on which thermocouples and power supplies are placed. The experimental results showed the positive influence of gold nanoparticles on the thermal system's performance by reducing the thermal resistance by 13% compared to pure water as the base working fluid. In addition, a self-rewetting butanol solution showed improvement in the performance of the capillary evaporator by decreasing its casing temperature. While a mixture of self-rewetting butanol solution (3 % C v) and gold nanofluid (1% C v) exhibited the best performance of heat and mass transfer performance by reducing the thermal resistance of the system by approximately 22 %. To explain the mechanism for improving heat transfer, the phase change phenomenon was visualized by an infrared camera for the three working fluids. It is shown that as the applied power increases, the shape of the vapor pocket developed in the wick also increases, for pure water, until it reaches a stable form. Whereas, with respect to nanofluid and self-rewetting fluid, the shape of the vapor pockets was smaller than that of pure water allowing more efficient mass and heat transfer. The thermophysical properties of these fluids such as thermal conductivity, stability, surface tension, Marangoni, wettability, and capillary forces were presented to ensure and validate the decrease in the vapor pocket as well as the enhancement of the CHP thermal system. [ABSTRACT FROM AUTHOR]

• Experimental analysis on hysteresis phenomenon during flow boiling heat transfer • Dedicated experimental procedure with variation of the imposed heat flux over time • Detection and isolation of parameters affecting the hysteresis phenomenon. • Hysteresis effect related to activation of surface cavities with higher heat fluxes This paper presents an experimental analysis on the hysteresis phenomenon occurring during flow boiling heat transfer in a horizontal stainless-steel tube having an internal diameter of 6.0 mm. Pure R134a is the working fluid employed, working at a saturation temperature and a vapor quality fixed to 40°C and 0.30, respectively, whereas two different mass velocities of 400 and 800 kg/m2s are studied. A dedicated experimental procedure has been carried-out to highlight the variation of the two-phase heat transfer performance due to the hysteresis effect. Specifically, tests performed at the same operating conditions but having a different thermal history (a higher imposed heat flux in the transient phase) were seen to provide lower wall superheat values and therefore higher heat transfer performances. The nucleate boiling contribution is isolated from the overall heat transfer and the parameters affecting the boiling curve behavior are highlighted and discussed, pointing out the differences when increasing and decreasing the imposed heat flux once a peak value is reached. The experimental trends are finally upheld by the boiling theory, considering the hysteresis effect as the activation of the surface cavities undergone a higher given thermal boundary condition. [ABSTRACT FROM AUTHOR]

The research on working fluids of refrigerant/absorbent mixture is important for the development of the absorption refrigeration area. R290/POE-oil mixture is a new potential working fluid, in which R290 (propane) is the refrigerant and POE oil is the absorbent. This refrigerant/oil working fluid has a wide operating range, low generation temperature, and no corrosion to metals. Flow boiling heat transfer performance of the new working fluid in smooth horizontal tubes has been experimentally investigated in this paper. The R290 concentration for the experimental working fluid was selected as 11 wt%. The test sections were horizontal smooth stainless steel annular tubes with outer diameters of 6, 8 and 10 mm. The boiling saturation pressures in the test sections coincided with the actual operation of the absorption refrigeration cycle, which were set as 1.2 MPa, 1.3 MPa, 1.4 MPa, respectively, corresponding to boiling temperatures 55.3 °C, 58.7 °C, 62.5 °C. The mass fluxes were 166 kg m−2s−1, 221 kg m−2s−1, 276 kg m−2s−1, 332 kg m−2s−1 and 387 kg m−2s−1. The effects of heat flux, vapor quality, mass flux, boiling temperature and tube diameter on the flow boiling heat transfer coefficient were displayed and analyzed. Experimental results show that heat flux, mass flux and vapor quality have obvious effects on the coefficients; the nucleate boiling mechanism plays an important role, and the convective boiling mechanism plays a significant role in the boiling process. The present study is of good significance to the application of R290/POE-oil working fluid and the design and optimization of the finned-tube generators in absorption refrigeration systems. • R290/POE-oil working pair has obvious advantages in absorption refrigeration. • The flow boiling heat transfer performance of R290/POE-oil mixture are studied. • The influences of different operating conditions on the coefficient are analyzed. • Both nuclear boiling and convective boiling mechanisms play an important role. [ABSTRACT FROM AUTHOR]

In this paper, a mathematic model of a closed loop OHP with two branches was established to reveal the motion law of internal working fluid and the mechanism of heat transfer enhancement excited by different heat source modes, that is, uneven pulsing heat source mode (UP-mode), the even pulsing heat source mode (EP-mode) and the alternate heat source mode (A-mode). The simulation result shows that the vapor plug pressure and the liquid slug displacement both present the characteristic of variable amplitude fluctuation under the three heating modes. The amplitude generally changes periodically which is different from that of the continuous external heat source mode, which make the parameters above show additional fluctuation characteristics on the original oscillation law. The specific variation law is affected by the factors such as the heating period and heating interval. The three heat source modes introduced can enhance the transfer heat capability of the OHP under appropriate condition. But if the configuration condition is not suitable, the three modes above will weak the transfer heat ability, even result in that work stability of OHP deteriorates and stops. [ABSTRACT FROM AUTHOR]

• A liquid metal high temperature OHP (LMHOHP) was first developed. • The LMHOHP that can function at a temperature of over 500 °C. • The LMHOHP can transport heat of 3169 W and achieve a thermal resistance of 0.08 °C/W. This paper introduces the development of a liquid metal high-temperature oscillating heat pipe (LMHOHP) that can function under high-temperature conditions (over 500 °C). The LMHOHP has six turns with a total height of 310 mm. The sodium-potassium alloy (potassium 78%) was used as the working fluid, and the 310S stainless steel tube with an inner diameter of 6 mm as the shell tube. The LMHOHP heat transfer performance was investigated experimentally with a filling ratio of 45% at different input powers. Experimental results show that the LMHOHP can effectively generate steady-state oscillating motions at a working temperature of over 500 °C. When the steady-state oscillating motions exist, the temperature difference and thermal resistance between the evaporator and condenser were reduced significantly. At a heat input of 3169 W, the LMHOHP presents a minimum thermal resistance of 0.08 °C/W. This LMHOHP provides a new approach to next-generation high-temperature cooling devices. [ABSTRACT FROM AUTHOR]

• HTC and pressure drop rises with mass velocity and quality in plain and TTI tubes. • Gravity-dominated regimes (stratified-wavy) are more sensitive to vapor quality. • Shear-dominated regimes (intermittent/annular) are more sensitive to mass velocity; • Tapes increase HTC and pressure drop, the less twist ratio, the more increment. • Twist ratio of 9 has the highest thermal performance, usually less than one. This paper deals with condensation flow inside plain and twisted tape inserted (TTI) tubes with diameter and length of 8.7 mm and 700 mm, respectively. R1234yf, identified as a viable alternative for R134a with low global warning potential (GWP), is examined as the working fluid. The heat transfer coefficient and the pressure drop of the flow are measured in different mass velocities and vapor qualities ranging 160–310 kg m−2 s−1 and 0.12–0.84, respectively. A well-equipped test rig consisting of a test condenser and other necessary equipment is employed to precisely adjust the test conditions. Experimental results for the plain tube are compared with some well-known correlations. As a result, Shah and Souza et al. equations are the most reliable equations to predict the flow heat transfer coefficient and its pressure drop, respectively. Different twisted tape inserts with three twist ratios of 6, 9, and 12 are exploited to study the effect of twisted tape geometry. The results indicate that up to 42% and 235% increment in the heat transfer coefficient and the pressure drop, respectively, is associated with implementing twisted tape inserts. In addition, it was shown that, among some correlations proposed previously in the literature, the experimental heat transfer coefficient and pressure drop in TTI tubes can be predicted by Akhavan Behabadi et al. and Hejazi et al. correlations with reasonable accuracy. According to the results, the twist ratio of 6 gives the highest increase in the heat transfer coefficient, while the ratio of 9 is associated with the highest overall enhancement ratio. [ABSTRACT FROM AUTHOR]

This research deals with the effects of working fluid and use of sound waves on the startup and heat transfer characteristics in terms of thermal resistance of a closed loop oscillating heat pipe. The performance of the oscillating heat pipe is checked for different orientations as 90° (vertical position), 60°, and 30°. Initially water is used as working fluid and performance of the oscillating heat pipe is checked with and without sound waves. Then 0.1 wt.% Al2O3-water nanofluid is utilized as working fluid in oscillating heat pipe and its performance is analyzed with and without sound waves. In this work, sound waves of 1 kHz frequency are passed through the evaporator section of closed-loop oscillation heat pipe. Application of sound waves improved the oscillation characteristics of the CLOHP with reduced startup time and enhanced thermal performance at all orientations. In comparison between working fluids, 0.1 wt.% Al2O3-water nanofluid showed better oscillation characteristics at all orientations of CLOHP except at 90° where use of sound waves leads to dry-out condition. [ABSTRACT FROM AUTHOR]

This article presents a thermal performance of a small closed end heat pipe with special vapor chamber (SCEHP/ SVC). This study has carried out the effect of parameters (such as pipe size, filling ratio, working fluids and incline angle etc.) on thermal performance. The SCEHP/SVCs consists of 2, 3 and 4.5 mm ID small closed end pipe and special vapor chamber were 14.5, 17.5 and 20.0 mm ID (at evaporator section). The number of meandering turns was 10. The lengths of the evaporator were 50, 100 and 150 mm (The length of evaporator, adiabatic and condenser sections were equal). R-134a, ethanol and water were used as working fluids with filling ratios of 30, 40 and 50%.The evaporator was heated by hot water to 60, 70 and 80°C, whereas the condenser was cooled with water at 20 °C. The inclination angles were 90, 60, 30, 0, -30, -60 and -90 degrees from horizontal plane. The temperatures at significant points (evaporator, adiabatic, condenser, inlet and outlet) were recorded every 10 minutes by data logger. As the result of the experiment, the boiling behaviors of SCEHP/SVCs, which has a small closed end of 2 mm acts like oscillating heat pipe, were like thermosyphon. As inclination angle increases, the heat flux increases. The best working fluid was R134a. The filling ratio, section length and small closed end size increased with decreasing heat flux. In addition, the heat transfer performances were excellent under the condition; the 2 mm of small closed end pipe, 17.5 mm vapor chamber pipe, R134a with filling ratio 30%, and also hot water at 80 °C with inclination angle of 90 degree. [ABSTRACT FROM AUTHOR]

In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2). [ABSTRACT FROM AUTHOR]

Experimental and numerical modeling of a heat pipe included with a phase change heat transfer was developed to assess the effects of three parameters of nanofluid, heat pipe inclination angles, and input heating power. Distilled water (DW) and 1-pyrene carboxylic-acid (PCA)-functionalized graphene nanofluid (with concentrations of 0.06 wt%) were used as working fluids in the heat pipe. A computational fluid dynamic (CFD) model was developed for evaluation of the heat transfer and two-phase flow through the steady-state process of the heat pipe. It was found that inclination significantly affects the heat transfer of the heat pipe. Maximum increment of thermal performance in the heat pipe reached 49.4% by using 0.06 wt% of PCA-functionalized graphene as working fluids. The result associated with this comparison indicates that the highest deviation is less than 6%, consequently confirming that the CFD model was successful in reproducing the heat and mass transfer processes in the DW and nanofluids charged heat pipe. The results of CFD simulation have good agreement between predicted temperature profiles and experimental data. [ABSTRACT FROM AUTHOR]

Based on the theoretical model of a heated ideal working fluid in the cylinder, the optimal motion path of the piston in this system, for the maximum work output, is re-studied by establishing the changed Lagrangian function and applying the elimination method when the initial internal energy, initial volume, finial volume and the process time are given and generalized radiative heat transfer law between the working fluid and heat bath is considered. The analytical solutions of the intermediate Euler-Lagrange arc with square, cubic and radiative heat transfer laws are taken as examples and obtained. The optimal motion path of the piston with cubic heat transfer law, which is obtained by applying the elimination method, is compared with that obtained by applying the Taylor formula expansion method through numerical example. The comparing result shows that the accuracy of the result which is obtained by applying the elimination method is not affected by the length of time of the expansion process of the working fluid, so this result is more universal. [ABSTRACT FROM AUTHOR]

In order to enhance heat transfer in the abrasive-milling processes to reduce thermal damage, the concept of employing oscillating heat pipes (OHPs) in an abrasive-milling tool is proposed. A single-loop OHP (SLOHP) is positioned on the plane parallel to the rotational axis of the tool. In this case, centrifugal accelerations do not segregate the fluid between the evaporator and condenser. The experimental investigation is conducted to study the effects of centrifugal acceleration (0–738 m/s2), heat flux (9100–31,850 W/m2) and working fluids (methanol, acetone and water) on the thermal performance. Results show that the centrifugal acceleration has a positive influence on the thermal performance of the axial-rotating SLOHP when filled with acetone or methanol. As for water, with the increase of centrifugal acceleration, the heat transfer performance first increases and then decreases. The thermal performance enhances for higher heat flux rises for all the fluids. The flow inside the axial-rotating SLOHP is analyzed by a slow-motion visualization supported by the theoretical analysis. Based on the theoretical analysis, the rotation will increase the resistance for the vapor to penetrate through the liquid slugs to form an annular flow, which is verified by the visualization. [ABSTRACT FROM AUTHOR]