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The heat transfer performance of a closed-loop pulsating heat pipe (CLPHP) having 2.2 mm inner diameter is experimentally studied at different filling ratios (40%, 50%, 60% and 70%) in a heat load range of 50–300 W using distilled water as the PHP fluid in vertical orientation with evaporator at the bottom. This experimental study validate that the optimum charging ratio is 50% for CLPHP while 40% filling ratio is good at lower heat inputs. Optimum filling ratio gives an average reduction of 26.0% and 14.8% in thermal resistance compared to 70% and 60% fill ratios. The heat transfer performance of the CLPHP is also studied at various volume fractions of graphene–water nanofluid using 50% filling ratio. This study shows that the optimum volume fraction for this CLPHP is 0.045%. The volume fraction of 0.045% gives an average reduction of 27.3% in thermal resistance for all heat inputs and a maximum temperature reduction of 24 °C for the evaporator wall temperature at 275 W heat input. When optimum values of filling ratio and volume fraction are used, CLPHP gives the best heat transfer performance. A correlation is also developed for CLPHP by considering the influential parameters. [ABSTRACT FROM AUTHOR]

The thermal management system of batteries plays a significant role in the operation of electric vehicles (EVs). The purpose of this study is to survey various parameters enhancing the performance of a heat pipe-based battery thermal management system (HP-BTMS) for cooling the lithium-ion batteries (LIBs), including the ambient temperature, coolant temperature, coolant flow rate, heat generation rate, start-up time, inclination angle of the heat pipe, and length of the condenser/evaporator section. This review provides knowledge on the HP-BTMS that can guarantee achievement of the optimum performance of an EV LIB at a high charge/discharge rate. [ABSTRACT FROM AUTHOR]

The novelty of this work is a new type of stack, namely, the parallel wave stack (PWS). In this paper, we study the performance of an air-based standing wave thermoacoustic refrigerator using the PWS. We use small heat pipes with a heat transfer rate of approximately 30 W as heat exchangers for the system. We investigate the effect of the ratio of stack length (Ls) to the distance between stack center and reflector (Xs) on the performance of the refrigerator parallel wave stacks with a blockage ratio (BR) of 67 %, 71 %, and 74 %, and with Ls/Xs ratios of 0.74, 0.96, and 1.20. We tested and compared these under a cooling load (Qc) of 0.5 W to 2 W and a frequency of 150 Hz. The results show that a BR of 71% with an Ls/Xs ratio of 0.96 provides a minimum cold-side temperature (Tc), and a maximum temperature difference (ΔTm), a maximum coefficient of performance, and a maximum relative coefficient of performance. This new type of stack can provide a higher performance. The structure of the stack is also stronger than others. This stack is an alternative and can be used in a thermoacoustic refrigerator. The results obtained from the present study will be very useful for improving the performance of the thermoacoustic refrigerator in the future. [ABSTRACT FROM AUTHOR]

We investigate, in this analysis, the entropy generation characteristics associated with the transport of nanofluid in a microfluidic channel under the combined influences of applied pressure gradient and electrical forcing. In our study, the nanofluid is subjected to an asymmetric cooling at the channel walls, while the conductive transport of heat through the channel walls is also taken into account. We show that the underlying thermo-electro-hydrodynamics of nanofluids in the channel lead to entropy generation, attributed to the irreversibilities associated with heat transfer, viscous dissipation, and Joule heating effects. We establish that a non-trivial interplay among these irreversibilities gives rise to an optimum value of the geometrical parameter, viz. the channel wall thickness (δ ), and the thermophysical parameters, viz. the thermal conductivity of the wall (γ) , Biot number (Bi), and the modified Peclet number ( Pe ¯ ), leading to a minimum entropy generation rate of the system. Also, we unveil through this study that changes in the electroosmotic parameter κ ¯ (representative of the EDL thickness) or the composition of the fluid (ϕ , the volume fraction of nanoparticles agglomerates) non trivially alter the optimum values of these parameters. Inferences drawn from this analysis may have consequences in the optimum design of thermal systems/devices, typically used for thermal management in micro-heat exchangers, micro-reactors, and micro heat pipes. [ABSTRACT FROM AUTHOR]

Thermal performance evaluation of TiO2-coated copper mesh wick in a cylindrical heat pipe with graphite nanofluid is experimentally analyzed at various inclinations, nanoparticle concentrations and power levels. Boiling heat transfer from the evaporator of a heat pipe depends on both thermal conductivity of the nanoparticle and suspension stability of the nanofluid. The lower the density of the nanoparticle, the better the suspension stability. Spherical graphite nanoparticles having lower density and good thermal conductivity quicken the heat transfer rate and hence the vaporization of base fluid. A hydrophilic coating of TiO2 is done on the copper wick structure to reduce the contact angle of graphite nanofluid. Results showed that the heat pipe worked well at 60° inclination compared to the other tested orientations. For this optimum condition, a reduction in 168.75% of thermal resistance is obtained compared with DI water with uncoated wick at horizontal position and also an improvement in thermal efficiency of 94.07% for 1.0 mass% particle loading. The enhancement in equivalent thermal conductivity is 90.87% for 1.0 mass% compared with DI water. Results from the repeatability test also confirm that the hydrophilic coating over the wick is stable, and results are repeatable. [ABSTRACT FROM AUTHOR]

The heat exchanger pipe diameter has a significant effect on the flow characteristics as well as on the initial investment, operation and overall cost. Increasing fin dimensions increases the annulus hydraulic diameters. Even though the total volume of the heat exchanger remains unchanged between the finned and bare designs, the heat duty increases with increased heat transfer area for the finned design. The fins should be designed, and the dimensions should be calculated with special attention for different flow rates and heat exchanger dimensions. In this study, number, geometry and dimensions of the fins are determined using the algorithms available in the literature. The operational condition optimization is carried out accompanied with the cost analysis. In addition, the effects of the types of working fluids and fouled and clean cases are investigated for the total heat transfer enhancement in parallel with performance, lifetime and cost issues. A detailed analysis is presented for finned and unfinned double-pipe heat exchanger models for pure engine oil and its nanofluid mixtures with Ti, TiO2, Cu, CuO, Al and Al2O3 nanoparticles, multi-wall carbon nanotubes and graphene nanosheet having a constant particle concentration in the liquid phase. The nanofluid is flowing in annulus side, whereas the seawater is flowing in the tube side. It is observed that both the pressure drop and the pumping power increase with the increasing fin number and decrease with the cleanliness factor, whereas the total tube number decreases with increasing fin number. It is found that different types of nanofluids affect the cost and optimum annulus side velocity significantly. The results are summarized in several figures that consider the increasing Reynolds number with the cleanliness factor, the heat transfer coefficient and the pressure drop, the friction factor with changing mass flow rate and the cost values with corresponding annulus side velocities. Finally, the overall characteristics of the trend lines are provided in the figures. [ABSTRACT FROM AUTHOR]

The feasibility of using multiport minichannel (MPMC) as thermosyphon for cooling miniaturized electronic products is experimentally investigated with acetone as the working fluid. A detailed analysis on thermal performance and entropy generation due to heat transfer and pressure drop with the effects of heat load (10-50 W), filling ratio (FR; 40%, 50%, and 60%), and inclination angle (45°, 60°, and 90°) has been carried out. The results showed a reduction of 22.2% and 9.31% in thermal resistance and evaporator wall temperature at optimum filling ratio (OFR) of 50%. Reduction in entropy generation due to heat transfer and pressure drop of 16.6% and 12.3%, respectively, was observed at OFR. Internal fins in MPMC increase the surface area and evaporation rate by enhancing heat transfer leading to a decrease in the rate of entropy generation. Multiport increases surface tension of condensate at right angles to the flow direction along with the effects of gravity and enhancing rate of condensation. A new correlation is developed to predict evaporator wall temperature as a function of heat load and FR. The proposed correlation agrees well with a deviation of ±20% with present experimental results and also with the published literature. Thus, the obtained results will be useful in cooling miniaturized electronic devices. [ABSTRACT FROM AUTHOR]

The thermal management of electronic devices such as high-end central processing units, graphic processing units, insulated gate bipolar transistor, and circuit breaker in low-voltage switchboard operating between 20 and 380 W is investigated using a miniature loop heat pipe (mLHP) having nanoparticle-coated evaporator with graphene–water nanofluid. The thermal evaporation method is used to deposit copper nanoparticles on the evaporator surface for a coating thickness of 400 nm. The experimental results show that the combination of nanoparticle coating and nanofluid gives the highest heat transfer compared to the mLHP having uncoated and coated evaporator with distilled water as a working fluid. The use of nanofluid enhances the heat transfer performance of mLHP with an average reduction of 45.2% in thermal resistance and an average enhancement of 113.4% in evaporator heat transfer coefficient for the optimum nanofluid volume concentration of 0.006%. Similarly, the lowest evaporator temperatures are also obtained with the same concentration of nanofluid for various heat loads. The experimental results of mLHP having nanoparticle coating and nanofluid are also found to be repeatable from the repeated tests and suitable for a long-term operation. [ABSTRACT FROM AUTHOR]

• 20.2% reduction in evaporator temperature noted with the use of internal condenser. • Rate of condensation enhanced by 4.15% with the use of internal condenser. • Lowest thermal resistance is noted with the use of internal condenser at 90° angle. In conventional heat pipe designs, cooling in condenser region is typically achieved externally through forced or natural convection. This approach limits surface area and often leads to condensation inefficiency. This inefficiency prolongs vapour residence time, hinders liquid reflux and causing problems like increasing operating temperatures, and pressures. To tackle this problem, an innovative flat heat pipe (FHP) design with internally cooled condenser is proposed. The design includes a tube-in-tube assembly where the coolant is circulated inside the condenser region to enhance the condensation rate. Test are conducted at different heat loads (75 W, 195 W, 315 W), coolant flow rates (ṁ=10–25LPH), and inclination angles (IA=0°-90°). Results demonstrated a superior performance of the proposed FHP design compared to conventional heat pipes. At IA-0° and ṁ-25LPH, it is observed a reduction of 20.2% and 48.1% in the evaporator-wall temperature and evaporator pressure. In addition, at IA-90°, the lowest thermal resistance of 0.0237 °C/W is noted at 315 W. An enhancement of 4.15% in the rate of condensation is noted, when the inclination angle changes from 0° to 90°. Notably, at 315 W heat load with inclination angle of 90°, a minimum evaporator pressure of 0.25 bar and a maximum vapour velocity of 1.7 m/s is observed. [ABSTRACT FROM AUTHOR]

• Multi-channel heat pipe is experimentally studied with acetone as working fluid. • Optimum liquid reservoir L res = 5 mm and fluid volume of 2.5 ml dissipated 90 W. • 35.4 % enhancement in heat dissipation rate noted for axial face (α a) condition. • 24.5 % reduction in temperature fluctuation noted for lateral side inclination (α l). • Antigravity heat transport capability noted up to 15 W at α a = −15o. The heat transfer characteristics of a miniatured flat heat pipe (MFHP) with multi-channels, featuring a port diameter of 1.18 mm, is investigated experimentally. Various operating parameters are considered, including the working fluid volume (V f = 1.5, 2.5, and 3.5 ml), length of the liquid reservoir (L res = No reservoir, 5, and 10 mm), orientation such as axial face (α a) or lateral side (α l), inclination angles (α = −15 to 90o), and cooling water flow rates (ṁ i = 10, 15, and 20 LPH). Based on the experiments, the optimal values for the working fluid volume, reservoir length, and flow rate are determined as V f = 2.5 ml, L res = 5 mm, and ṁ i = 20 LPH, respectively. Further analysis reveals that, the heat dissipation rate for the axial face is significantly higher than that of the lateral side, with an average percentage increase of 35.4 %. However, the lateral side outperforms the axial face in terms of stabilizing the evaporator wall temperature, reducing fluctuations by an average of 24.5 %. Moreover, the presence of multi-channels allows the MFHP in axial face orientation to dissipate a maximum heat load of 15 W against gravity at an inclination angle of α a = −15o. Finally, the variations in MFHP operation based on the orientation and its underlying physical mechanisms that contribute to enhancing heat transfer are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This study presents the improvement in heat transfer performance of a heat pipe using silver nanoparticles dispersed in DI (De-Ionized) water. The nanoparticles suspended in conventional fluids have superior heat transfer capability due to improved thermal conductivity. The heat pipes are tested for heat inputs ranging from 20W to 100W in five steps, which is suitable for removing heat from power transistors in electronics and processors in computers. The effect of various operational limits and test parameters such as heat inputs, volume fraction, vapour temperature on the thermal resistance, evaporation and condensation heat transfer coefficients, are experimentally investigated. The tested silver nanoparticles volume concentration ranged from 0.003% to 0.009% with average nanoparticle diameter of 58.35nm. The experimental results are evaluated in terms of performance metrics by direct measurement of vapour temperatures in the centre core of heat pipe. A substantial reduction in thermal resistance of 76.2% is observed for 0.009vol.% concentration of silver nanoparticles. Further an enhancement in the evaporation heat transfer coefficient of 52.7% is observed for the same concentration. The use of nanoparticles enhances the operating range of heat pipe by 21% compared with that of DI water. [Copyright &y& Elsevier]

This study offers a comprehensive exploration of the cooling strategies and power ratings of electric vehicle motors. Electric vehicles depend on robust electric motors for propulsion. High power and sustained high-speed usage create excessive heat, risking motor failure without effective thermal management. Implementation of effective cooling strategies thus becomes imperative to ensure both the longevity and optimal performance of electric vehicle motors. This article seeks to review range of cooling approaches, encompassing air cooling, liquid cooling, and hybrid cooling, scrutinizing their respective capabilities in efficiently dissipating the heat generated during motor operation. Furthermore, the study investigates the impact of power ratings on the performance of electric vehicle motors in determining its maximum power output with higher ratings. In summary, this article underscores the critical importance of carefully selecting the appropriate cooling strategy and power rating for electric vehicle motors to maximize their efficiency and durability. This insight proves to be invaluable to electric vehicle manufacturers and designers, providing them with essential guidance in the design and selection of electric vehicle motors that meet the demands of modern transportation while contributing to a sustainable future. [Display omitted] • Efficient cooling system design enhances electric motor performance and durability. • Hybrid cooling techniques enhances electric motor performance and torque. • Minimisation of motor energy losses results in higher power output. [ABSTRACT FROM AUTHOR]

Abstract Heat transfer at the liquid-solid interface, especially when the particles are at nanoscale, can dominate the thermal properties of nanofluids. In this study we investigate the liquid-solid interfacial thermal resistance (Kapitza resistance) and particularly analyze the structure of the formed molecular nanolayer around the carbon-based nanoparticles. Employing non-equilibrium molecular dynamics simulation and thermal relaxation method, nanofluids systems with nanoparticles with different diameters and surface wettabilities were investigated. Simulation results reveal that carbon nanotubes (CNTs) with smaller diameters more effectively attract base fluid and thus show lower reduced Kapitza resistances. It was found that the thickness of the nanolayer around the nanoparticle is independent of the carbon/water interaction strength. As expected, it was shown that the value of Kapitza resistance decreases when the interaction strength increases. Based on our acquired results, a correlation was proposed for the interfacial thermal resistance of CNT/water and graphene/water with respect to the intensity of wettability of nanoparticles surface. The insight provided by our atomistic simulations can provide a better understanding of heat transfer in nanofluids systems wherein an accurate local description of heat transfer is crucial. Highlights • CNTs with lower diameter have less interfacial thermal resistance (ITR). • The strength between water and carbon has no effect on the thickness of nanolayer. • The ITR for CNT/water is less than that of graphene/water. • A correlation proposed for the ITR in terms of wettability of the nanoparticle. [ABSTRACT FROM AUTHOR]

Pulsating heat pipes (PHPs) are compact cooling equipment used for various applications. This type of heat pipes can be used in renewable energy systems, cooling electronic devices, heat recovery systems and many other applications. Since PHPs have superior thermal performance, by applying them in energy systems enhance their efficiency. In addition, PHPs are a reliable medium for cooling various devices which have high heat flux. In this study, various works conducted on the applications of PHPs are reviewed and analyzed. It is concluded that PHPs are efficient and reliable devices for utilization in various energy systems. Moreover, at very low temperatures, such as cryogenic applications, they can transfer heat efficiently. For instance, by applying cryogenic PHP it is possible to achieve up to 12,000 W/m K which is much higher than metal thermal conductivity at those temperatures. [ABSTRACT FROM AUTHOR]

The effect of nanoparticle coating on the performance of a miniature loop heat pipe (mLHP) is experimentally investigated for heat inputs of 20'380 W using distilled water as the working fluid. Applications include the cooling of electronic devices such as circuit breaker in low voltage switch board and insulated gate bipolar transistor. Physical vapor deposition method is used to coat the nanoparticles on the evaporator surface for different coating thicknesses of 100 nm, 200 nm, 300 nm, 400 nm, and 500 nm, respectively. An optimum filling ratio (FR) of 30% is chosen for the analysis. Experimental findings show that the nanoparticle coating gives a remarkable improvement in heat transfer of the heat pipe. An average reduction of 6.7%, 11.9%, 17.2%, and 22.6% in thermal resistance is observed with coating thicknesses of 100 nm, 200 nm, 300 nm, and 400 nm, respectively. Similarly, enhancements in evaporator heat transfer coefficients of 47%, 63.5%, 73.5%, and 86% are noted for the same coating thicknesses, respectively. Evaporator wall temperature decreased by 15.4 °C for 380 W with a coating thickness of 400 nm. The repeatability test ensures the repeatability of experiments and the stability of coatings in the long run. [ABSTRACT FROM AUTHOR]

The paper presents the enhancement in the operational limits (boiling, entrainment, sonic, viscous and capillary limits) of heat pipes using silver nanoparticles dispersed in de-ionized (DI) water. The tested nanoparticles concentration ranged from 0.003 vol. % to 0.009 vol. % with particle diameter of <100 nm. The nanofluid as working fluid enhances the effective thermal conductivity of heat pipe by 40%, 58%, and 70%, respectively, for volume concentrations of 0.003%, 0.006%, and 0.009%. For an input heat load of 60 W, the adiabatic vapor temperatures of nanofluid based heat pipes are reduced by 9 °C, 18 °C, and 20 °C, when compared with DI water. This reduction in the operating temperature enhances the thermophysical properties of working fluid and gives a change in the various operational limits of heat pipes. The use of silver nanoparticles with 0.009 vol. % concentration increases the capillary limit value of heat pipe by 54% when compared with DI water. This in turn improves the performance and operating range of the heat pipe. [ABSTRACT FROM AUTHOR]

A miniature loop heat pipe is theoretically modelled and the predicted results are used to compare the thermal performance and entropy generation with the experimental results. The working fluids used are water and two concentrations of graphene–water nanofluid (0.003 Vol.% and 0.006 Vol.%). The analysis shows that the model is in good agreement with experiments with a maximum variation of 6% in evaporator wall temperature. The entropy generation in mLHP due to heat transfer and pressure drop are also estimated from the model and experimental results. The results show that the use of nanofluid diminishes the entropy generation and increases the second law efficiency. The average reduction in total entropy generation with the use of nanofluid is 23.9% and 34.6% for 0.003 Vol.% and 0.006 Vol.% respectively. Similarly the second law efficiency shows an average increase of 19.4% and 37.5% for the same concentrations. The heat transfer is found to be the dominant factor causing the entropy generation in mLHP and the effect of friction on entropy generation is found to be less at very low volume fractions of nanofluid. [ABSTRACT FROM AUTHOR]

The heat transfer performance of miniature loop heat pipe with graphene–water nanofluid is experimentally analysed. The miniature loop heat pipe used in the study consisted of a square flat evaporator having a size of 20 mm × 20 mm, a compensation chamber placed above the evaporator and transport lines having different diameters. The difference in diameter prevents reverse flow of vapour through liquid line and also increases the flow rate of condensed liquid through liquid line. An optimum filling ratio of 30% of the total volume of the heat pipe is used in all the experiments. The experiments are conducted for a heat load range of 20–380 W using water and graphene–water nanofluid in vertical orientation. The graphene nanosheets having 1–5 nm thickness with very low volume fractions of 0.003%, 0.006% and 0.009% are mixed with distilled water to prepare nanofluid. The experimental results indicate that the nanofluids improve the thermal performance of the miniature loop heat pipe and lower the evaporator interface temperature compared to distilled water. An optimum concentration of 0.006% provides the maximum improvement in heat transfer. The lowest thermal resistance value (0.083 K/W at 380 W) is observed for the optimum concentration and it is 21.6% below the value of distilled water. The evaporator interface temperature reached only 106.3 °C at 380 W which shows a decrease of 10.3 °C compared to distilled water. The experimental results confirm suitability of miniature loop heat pipe filled with graphene–water nanofluid for cooling applications. [ABSTRACT FROM AUTHOR]

• Multiports act as internal fins and enhances evaporation rate of working fluid. • Reservoirs at the ends supply required quantity of fluid & reduce fluid entrainment. • Reduction in heat transfer and pressure drop irreversibilities of 17 & 48% observed. • Multiport design showed good stability by withstanding 2.7 bar pressure at 90 W. Thermal performance of an air cooled minichannel thermosyphon having multiports (10 ports) with hydraulic diameter of 1.18 mm is investigated. Experiments conducted at different heat loads (10 to 90 W), filling ratios (40, 50 and 60 %), inclination angles (30, 45, 60 and 90°), confluence lengths (0, 5 and 10 mm) and cooling air velocities (0.8, 1, 1.2 and 1.5 m/s) using acetone as working fluid. Results showed a maximum heat dissipation of 90 W with evaporator wall temperature of 72.2 °C at 5 mm confluence length. Multiports act as internal fins and enhance the surface area of the thermosyphon. This improves the boiling and evaporation rate, leading to reduced thermal resistance and wall temperature. An average reduction of 17.1 and 48.2 % are respectively noted in the irreversibilities of heat transfer and pressure drop at optimum condition. Liquid reservoirs at both ends of the thermosyphon overcomes the entrainment problem and supply the required quantity of working fluid and prevents it from reaching dry out condition. Analysis revealed that, the internal flow pattern transition is between Slug and Geyser flow for the tested heat loads. The present experimental findings can be a useful guide for engineering design to satisfy the requirement of cooling in miniaturized electronic devices. [ABSTRACT FROM AUTHOR]

An experimental investigation has been carried out to compare the enhancement in the thermal performance of sintered and mesh wick heat pipes by varying the working fluid, inclination angle and heat input. Similar geometrical specifications of 12, 330 and 1 mm respectively are selected for the outer diameter, length and wick thickness and kept constant for both sintered and mesh wick heat pipes. The study focuses on changes in surface temperature distribution, thermal resistance and effective thermal conductivity of heat pipes. The results showed that the maximum reduction in surface temperature is obtained for sintered wick heat pipe at 45° tilt angle and 60° for mesh wick heat pipe with CuO/DI water nanofluid concentration at 1.0 wt.% for both the cases. The reduction in thermal resistance of sintered wick heat pipe is 13.92% higher compared with mesh wick heat pipe for the same heat input, mass concentration and inclination angle. Presence of CuO nanoparticles in DI water and increasing heat input tremendously increases the thermal conductivity of heat pipes. An important observation from this study is the sole effect of sintered wick in heat pipe not only reduces the thermal resistance but also increases the heat transport capacity up to 20 W compared with that of mesh wick. [ABSTRACT FROM AUTHOR]

The heat transfer performance of various thermal devices may be augmented by active and passive techniques. One of the passive techniques is the addition of ultrafine particles (called nanoparticles) to the common heat transfer fluids so that the thermal transport properties of the prepared suspension (called nanofluid) will be enhanced as compared to the base fluid. Nanorefrigerants are a special type of nanofluids which are mixtures of nanoparticles and refrigerants and have a broad range of applications in diverse fields for instance refrigeration, air conditioning systems, and heat pumps. In this paper, a review is performed in order to clarify effect of nanorefrigerant properties (such as nanoparticle type, size and concentration) on heat transfer and pressure drop compared to pure refrigerant. Moreover, studies related to the thermophysical properties, flow and pool boiling, and applications of nanorefrigerants to some specific areas such as domestic refrigerators, heat pipes and air conditioners are also summarized. [ABSTRACT FROM AUTHOR]