Thermal Resistance Of Heat Pipe Research Articles

Effect of thin porous copper coating on the performance of wickless heat pipe with R134a as working fluid

The heat transfer characteristics of a thin porous copper-coated wickless heat pipe using R134a as a working fluid is investigated and is compared for its performance with uncoated wickless heat pipe using the same working fluid. An electroplating process was utilised to form a porous structure of copper over the inner surface of the wickless heat pipe. The experiments were carried out in the heat input range between 50 and 250 W. The thermal resistance of heat pipe at three different inclination angles such as 0°, 45° and 90° with horizontal are investigated. The results showed that 45° inclination has the lowest resistance with significant improvement in heat transfer characteristics. The coated wickless heat pipe exhibited a low thermal resistance when compared to uncoated wickless heat pipe. The condenser and evaporator heat transfer coefficients of a coated wickless heat pipe were found to be higher by about 11% and 25%, respectively, when compared to uncoated heat pipe for a heat flux of 10 kW m−2 and inclination of 45°. The magnitudes of dimensionless numbers (such as, Bo, We, Ku and Co) on coated and uncoated wickless heat pipes are also found.

A review on using nanofluids in heat pipes

The thermophysical specifications of working fluid play a key role in thermal performance of various types of heat pipes. Fluids with high thermal conductivity, low viscosity and surface tension are more favorable to be applied in heat pipes. In order to have fluids with higher thermal conductivity, adding nanoparticles can be an acceptable idea. In the present study, the effects of using nanofluids in several types of heat pipes are reviewed. The nanofluids are categorized based on the types of particles (as carbonic, metallic, etc.). Based on the results of the literature review, applying nanostructures in the base fluid can significantly reduce the thermal resistance of heat pipes compared with utilizing pure as operating fluid. For instance, it is observed that using graphene oxide/water nanofluid in pulsating heat pipe reduces the thermal resistance up to 42% in comparison with the water-filled heat pipe. In addition, reviewed studies revealed that the type of nanoparticle, concentration and their stability are among the most important parameters affecting thermal performance. The enhancement in thermal performance of heat pipes by using nanofluid is mainly attributed to higher thermal conductivity of the nanofluids and increase in nucleation sites.

Numerical study of CeO2/H2O nanofluid application on thermal performance of heat pipe

Abstract In this work, a two-dimensional analysis is used to investigate the thermal performance of heat pipe using water and CeO2 /H2O nanofluid as the working fluid. CFD simulation for the thermal performance of heat pipe using CeO2 /H2O nanofluid (0.5, 1.0, &1.5 vol. % concentration) and different heat flux i.e. 10, 15 and 20 kW/m2 has been done. The numerical results show that CeO2 /H2O nanofluid application reduces the surface wall temperatures and thermal resistance of heat pipe as compared to that of water. Experimental and numerical results are in good agreements. Authors concluded that heat pipe using CeO2 /H2O nanofluid as working fluid has better heat transfer characteristics as compared to water. So there is a great future prospect of nanofluids application in heat pipes.

High performance copper-water heat pipes with nanoengineered evaporator sections

This experimental investigation aims to enhance the performance of heat pipes through nanoengineering the evaporator section, by integrating hydrophilic copper oxide (CuO) nanowires on the inner surface. Two types of CuO nanowires have been employed. Copper pipes measuring 440 mm in length with 12.7 mm O.D. and 0.8 mm wall thickness with inner grooves were used to manufacture heat pipes. All heat pipes were charged with ultra-filtered deionized (DI) water as a working fluid. By employing the hydrophilic CuO nanowires coating in the evaporator section of a heat pipe, its performance is substantially enhanced compared to a heat pipe with identical dimensions without the coating. Specifically, thermal resistance is reduced by 81.2% when using Type I CuO and 72% using Type II CuO nanowires compared to a heat pipe without coatings. The effects of the working load and orientation on the heat pipe thermal resistance have been systematically examined.

Experimental study on effect of wick structures on thermal performance enhancement of cylindrical heat pipes

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

Effect of graphenenano-fluid on heat pipe thermal performance for passive heat removal in nuclear spent fuel storage pool

After Fukushima Dai-ichi nuclear power reactor accident, spent fuel in spent fuel storage pool (SFSP) became an important thing to pay attention to, due to its decay heat release. If station blackout occurred, the active system for SFSP cooling system will experience malfunction to remove the product of decay heat. To keep spent fuel safe, heat pipe as passive cooling system device can be used to remove spent fuel decay heat even if the active cooling system failed. Heat pipe with 6 m on length will be proposed as apassive cooling system in SFSP. For that, it is necessary to analyze the effect of Graphene nano-fluid as heat pipe working fluid. The objective of this research is to know the effect on Graphene nano-fluid to enhance the heat pipe thermal performance and to know the heat transfer phenomena inside the heat pipe based on Graphene nano-fluid. Graphene nano-fluid with 1% weight concentration was used as working fluid with filling ratio of 80%. The experimental investigation is conducted with varying the evaporator heat load of 1000, 1500, 2000, and 2500 W. Water as coolant flows in thecondenser with aconstant volumetric flow rate of 8 L/min. The experiment results show that there was anovershoot, zigzag and stable phenomena inside the heat pipe. The thermal resistance of heat pipe is obtained at 0.015 °C/W. The use of Graphene nano-fluid as working fluid can enhance the heat pipe thermal performance significantly and can be used as an alternative working fluid in heat pipe for thepassive cooling system in SFSP of nuclear power plant.

Experimental and Numerical investigation on enhancement in thermal characteristics of Sintered Copper wick Heat pipe using deionized water as fluid

The enhancement in heat transfer characteristics of a copper sintered wick heat pipe with DI water is experimentally and numerically studied, for 15° tilt angle, having 15.88 mm external diameter, 14 mm internal diameter and 565 mm long. The effect of heat input and flow rate on the heat pipe thermal resistance, heat transfer coefficient in evaporator and condenser sections and thermal efficiency are investigated. The experimental results are evaluated for the surface temperatures of evaporator and condenser sections, flow rate conditions also measured at the condenser of heat pipe. Interestingly, a temperature difference of 4.2 C is observed between the heat pipe condenser and the evaporator section. The results showed maximum temperature obtained at the evaporator and minimum temperature obtained at the condenser section because heat input is applied at the evaporator section and cooling is provided at the condenser section. The absolute temperature and thermal resistances of both evaporator and condenser sections are observed that experimental values greater than the numerical, because in experimentation the convection losses occurs. The obtained thermal resistances are very close to zero showing that low thermal resistance, the higher will be the heat transfer. It is observed that the variation of efficiency with power input is decreases. As the heat input is increases the efficiency values are decreasing because of convection losses. Heat pipes are common in many application fields like Aerospace, Electrical and Electronic equipment, Medicine and Human body temperature control and Transport systems and deicing.

Numerical case study of packed sphere wicked heat pipe using Al2O3 and CuO based water nanofluid

Nanofluids have been receiving increased attention worldwide and various research groups have tried to engage it within heat pipes systems to study the subsequent thermal enhancement. The observations based on the reviewed literature showed that the theoretical investigations on nanofluids in heat pipes are very few and hence validating the experimental findings is difficult.In this paper, a two-dimensional numerical model is developed to study the thermal performance of a packed sphere heat pipe utilizing nanofluids due to the recent development of nanofluids and porous media structures. Two of the most common nanoparticles, namely Al2O3 and CuO based water are considered. The substantial change in the heat pipe thermal resistance, liquid pressure, axial and radial velocities profiles in presence of suspended nanoparticles within the base fluid are discussed. The effect of particle size on the thermal performance of the heat pipe is also investigated. Results showed that Al2O3 nanofluid slightly outperformed CuO nanofluid heat pipes especially with decreasing nanofluid particle diameter. It was found that reduction of 68% on thermal resistance can be obtained for 9% CuO nanofluid concentration level which trend to reach 20% without significant shear stress effect. Nanofluid based heat pipe is able to dissipate up to 26% more heat without experiencing an increase in the wall temperature and an optimal nanofluid concentration level and diameter are found to better heat pipe performance.

Thermal characteristics studies on sintered wick heat pipe using CuO and Al2O3 nanofluids

In the present experimental work, the thermal characteristics of cylindrical sintered wick heat pipe are investigated using CuO and Al2O3 nanofluids. The size and morphology of the nanoparticles are maintained as constant to analyze the distinctive performances of nanoparticles on the thermal enhancement of heat pipe. The effect of inclination angle and heat input on the thermal performance of heat pipe is also studied. The addition of nanoparticles has a notable influence in surface temperature of heat pipe and it is gradually reduced with increasing concentration. The reduction for 0.5wt.%, 1.0wt.% and 1.5wt.% of CuO nanofluids are 2.1°C, 5.9°C and 4.7°C respectively, whereas for the same concentrations Al2O3 nanofluids obtain only 0.9°C, 3.6°C and 5.3°C respectively compared with DI water at horizontal position. Thermal resistance of heat pipe is dramatically reduced with increasing heat flux at low heat input and the reduction is diminished for peak loads. The optimum performance is attained for both CuO and Al2O3 nanofluids at 45° inclination angle. In contrast, the optimum concentration is varied i.e., 1.0wt.% for CuO and 1.5wt.% for Al2O3 nanofluids. The evaporation and condensation HTC is increases about 32.99% and 24.59% respectively for CuO and Al2O3 nanofluids at 45°.

Research on Thermal Resistance of Micro Heat Pipe with Trapezium-Grooved Wick

As an efficient heat conducting unit, micro heat pipe is widely used in high heat flux microelectronic chips, and thermal resistance is one of the factors that are crucial to its heat transfer capacity. Based on heat transfer theory, this paper established a theoretical model of total thermal resistance through analyzing the structure and heat transfer performance of circular heat pipe with trapezium-grooved wick, simplified the model and tested the micro heat pipe for its total thermal resistance performance by setting up a testing platform. The testing results show that when the micro heat pipe is in the optimal heat transfer state, its total thermal resistance well coincides with that from the established theoretical model. As for a micro heat pipe with trapezium-grooved wick, its total thermal resistance first decreases, then increases with heat transfer capability increment, and reaches the minimum when it is in the optimal state of heat transfer performance. That too much working fluid accumulates in evaporation section and the vapor velocity is rather low is the main cause for the greater thermal resistance when the pipe is in low heat transfer quantity, yet the greater total thermal resistance when the pipe is in high heat transfer quantity is mainly caused by the working fluid drying up in condensation section. The total thermal resistance is related to many factors, such as the thermal conductivity of tube-shell material, wall thickness, wick thickness, the number of the grooves, the lengths of condensation and evaporation sections, the diameter of vapor cavity etc.. Therefore, the structure parameters of a micro heat pipe with trapezium-grooved wick should be rationally designed according to specific conditions to ensure its heat transfer capacity and total thermal resistance to meet the requirements and be in the optimal state.

A SIMPLE MATHEMATICAL MODEL TO PREDICT HEAT PIPE MAXIMUM HEAT TRANSFER, EQUIVALENT THERMAL CONDUCTIVITY, AND THERMAL RESISTANCE

The objective of this study is to provide a simple mathematical model to predict the performance of a heat pipe such as the maximum heat transfer rate, equivalent thermal conductivity, and the thermal resistance based on the experiences of industrial products. The maximum heat transfer calculation is based on the maximum capillary provided by the wick against the total pressure drop in the heat pipe. The calculation of the capillary structure of the wick and pressure drop in the heat pipe is based on the traditional pressure drop formulas for vapor and liquid flow. For heat pipe thermal resistance calculation, there is a new proposal of an additional parameter function f (Leff) that accounts for the vapor and fluid flow pressure drop dependent on the heat pipe length. This function dictates that the thermal resistance of the heat pipe increases with the heat pipe length. Without this additional function, the heat pipe thermal resistance would remain constant regardless of the heat pipe length, and this assumption may be incorrect. Analysis results showed that the maximum heat transfer and thermal conductance (inverse of thermal resistance) of heat pipe decreases as the heat pipe length increases. The correspondent equivalent thermal conductivity increases with the heat pipe length in the maximum heat transfer limit range. Theoretical calculation is validated by experimental results and is discussed in this paper.

Effect of receiver diameter ratio on thermal performance of natural circulation steam generation system as applied to parabolic trough collector using water as working fluid

Flow boiling heat transfer of water in a thermosyphon loop based natural circulation solar steam generation system with a horizontally-arranged receiver towards parabolic trough collector (PTC) applications was studied experimentally. The diameter ratio (Rd) of the receiver was varied over a range from 1.0 to 2.0 to explore its effect on flow boiling heat transfer, especially the stratified flow boiling characteristics in the receiver tube, and thermal performance of the thermosyphon system. The results show that flow boiling heat transfer in the receiver was improved since Rd was increased to 1.2 due to a flow pattern transition which eliminated the effect of gas–liquid stratification on surface wettability deterioration in the receiver. An optimal tube diameter ratio was found to be 1.4 with a heat transfer coefficient enhancement by 12.2% in comparison to the baseline case. Simultaneously, the rise of Rd resulted in an enhancement of backflow in the inlet of receiver. The effect of backflow on isothermal improvement in the falling tube of thermosyphon, however, was found to be irrelevant to the total heat pipe thermal resistance (RHP).

The effect of nanoparticle shape on the thermal resistance of a flat-plate heat pipe using acetone-based Al2O3 nanofluids

In this paper, we experimentally investigate the effect of the shape of nanoparticles in acetone-based Al2O3 nanofluids on the thermal resistance of a flat-plate heat pipe. Acetone-based Al2O3 nanofluids containing sphere-, brick- and cylinder-shaped nanoparticles were prepared, using the two-step method without any surfactants or additives. The flat-plate heat pipes are manufactured with the prepared three nanofluids. The thermal resistances of the flat-plate heat pipes are experimentally obtained. The results show that the thermal resistances of the flat-plate heat pipes with nanofluids containing sphere-, brick- and cylinder-shaped nanoparticles are reduced to 33%, 29%, and 16%, respectively, compared with the thermal resistance of the flat-plate heat pipe with pure acetone. Based on the results, we demonstrate that the shape of nanoparticles in working nanofluids significantly affects the thermal resistance of the heat pipes. Finally, we discuss why nanoparticle shape in nanofluids reduces the thermal resistance of the flat-plate heat pipe using a theoretical approach presented in literature.

Experimental investigation on the effect of nanofluid on the thermal performance of symmetric sintered U shaped heat pipe

In this study, the impact of Entrance Power and Silver nanofluid concentration (with base fluid ethanol and DI-water) on heat pipe thermal performance are considered. In order to reach the aim a U-shaped sintered heat pipe is utilized which causes occupied space to decline. The length of the heat pipe is 135 mm in each branch. On account of recognition the effect of working fluid on heat pipe thermal performance, thermal resistance and overall heat transfer coefficient in base working fluid and nano-colloidal silver are measured in the shape of thermosyphon. The working fluid is with volume percentages of 70 ethanol and 30 distilled water. The average size pertaining to the nanoparticle applied is 40 nm. In addition, the influences of nanofluid concentrations are measured by comparing three concentrations 0.001, 0.005, 0.1 vol%. The range of entrance power is from 10 to 40 W and the temperature of coolant has been changed from 20 to 40 °C. The results of the experiment indicate that by increasing entrance power, the temperatures of the condenser, evaporator and working temperature experience a rise. Furthermore, this causes a decrease of thermal resistance and an increase of overall heat transfer coefficient. A comparison of three concentrations reveals that in concentration of 50 ppm, thermal resistance compared to the base fluid has decreased to 42.26 % and overall heat transfer coefficient has gone up to 1883 (W/m2·°K) . Also, due to unexpected changes in concentration of 1000 ppm, the existence of an optimized concentration for the silver nanofluid in this heat pipe with this geometry has been clear.

Numerical investigation of roll heat pipe type for heat exchangers thermal management

Heat pipes cooling technology is currently spreading out for many applications such as electronic, solar energy, air conditioning systems, space applications, telecommunications, food industries, geothermal systems, heat recovery etc.In this work, two numerical studies have been developed to investigate the thermal performance of a novel type of heat pipe called “Roll heat pipe (RHP)”. Results show that RHP performs better than annular pipes often used in heat exchangers. The effect of boundary conditions on the heat pipe thermal resistance is investigated. Results reveal that the RHP thermal resistance decreases with the heat input and RHP thermal performance is controlled by the outer surface temperature. Heat transfer inside RHP can be enhanced when heat is extracted quickly from the condenser region. Result reveals also that a higher grade of isothermalisation can be obtained compared with conventional tubular heat exchanger for uniform or localized heat inputs in the evaporator region. The objective of this study is to better understand the thermal performance of RHP type for further heat exchanger applications or other designs processes that are transient or steady.

나노유체 특성에 따른 히트파이프 성능해석

In this study, we theoretically investigate the thermal performances of heat pipes that have different nano-fluid properties. Two different types of nano-particles have been used: Al2O3 and CuO. The thermal performances of the heat pipes are observed for varying nano-particle aggregations and volume fractions. Both the viscosity and the conductivity increase as the volume fraction and the aggregation increase, respectively. Increasing the volume fraction helps increase the capillary limit in the well-dispersed condition. Whereas, the capillary limit is decreased under the aggregate condition, when the volume fraction increases. The dependence of the heat pipe thermal resistance on the volume fraction, aggregation, and conductivity of the nano-particles is analyzed. The maximum thermal transfer of the heat pipe is highly dependent on the volume fraction because of the high permeability of the heat pipe . For the proposed heat pipe, the optimum volume fraction of the nano-particle can be seen through 3D graphics.

Experimental investigation of effect of heat load on thermal performance of natural circulation steam generation system as applied to PTC-based solar system

An indoor experimental test rig of parabolic trough collector (PTC)-based natural circulation steam generation system consisting in a thermosyphon loop was presented. A series of five heat loads (0.6–1.2kW) were applied to investigate effect of heat load on thermal performance of the system. Effect of heat load on flow pattern, thermal efficiency and two phase heat transfer coefficient was discussed, respectively. An extended correlation equation was provided for two flow patterns, which is characterized by heat pipe thermal resistance. The critical heat pipe thermal resistance for flow pattern transition was ranged from 34.37K/kW to 33.35K/kW. Simultaneously, thermal efficiency shows a continuous increase as heat load kept rising. The effect of backflow was found to be negligible when heat load increased to 1.1kW. Additionally, the average two-phase heat transfer coefficient in receiver also went up with the rising of heat load for the same flow pattern. Due to the flow pattern transition, which resulted in a dryness fraction drop in receiver, a maximum heat transfer coefficient of 285.86W/m2K was obtained at heat load of 1.0kW under a steam discharging pressure of 0.15MPa.

Effect of Al<sub>2</sub>O<sub>3</sub>-Water Nanofluid on the Heat Performance of Gravity-Assisted Heat Pipe

The aim of this paper is to investigate the effect of nanofluid on the heat transfer performance of heat pipe and to examine the difference of the thermal conductivity between pure water heat pipes and nanofluid heat pipes. In our experiments, Al2O3-water nanofluid and pure water were used as working fluids respectively in gravity-assisted heat pipes. Effects of filling ratio and heating temperature on the thermal performance of heat pipe were investigated. The thermal resistance of heat pipe was analyzed. Our results showed that nanofluid can significantly increase the heat transfer coefficient and enhance the thermal performance of heat pipe.

Thermal Performance Enhancement of L-Shaped Microgrooved Heat Pipe Containing Water-Based Al2O3 Nanofluids

An experimental study was performed to investigate the thermal performance of an L-shaped grooved heat pipe with cylindrical cross section, which contained 0.5 wt% water-based Al2O3 nanofluid as the working fluid. The transient performance of the heat pipe and the effect of cooling water temperature on the heat transfer characteristics of the heat pipe were investigated. The outer diameter and the length of the heat pipe were 6 mm and 220 mm, respectively. Experimental results revealed that the temperature of the cooling water has a significant effect on the thermal resistance of the heat pipe containing nanofluids as its working fluid. By increasing the cooling water temperature from 5°C to 27.5°C, the thermal resistance decreases by approximately 40%. At the same charge volume, test results indicated an average reduction of 30% in thermal resistance of heat pipes with nanofluid as compared with heat pipe containing pure water. For transient conditions, unsteady state time for nanofluids was reduced by approximately 28%, when compared with water as the working fluid.

A model-based approach for analysis of working fluids in heat pipes

Heat pipes (HP) are efficient heat transfer devices, utilizing a working fluid to transfer heat based on evaporation and condensation. To make HPs even more efficient, one viable approach is to modify the utilized working fluid thermophysical properties. In this paper, a simple yet effective method based on dimensional analysis, leading to a reduced-order model, is proposed to analyze and modify working fluid's thermophysical properties and to predict the impact of working fluid modification on thermal resistance of a trapezoidal micro-grooved heat pipe. A validated one dimensional mathematical model based on a semi-analytical hydrodynamic approach is used to estimate the reduced-order model parameters. Alternatively, experimental data can be used for estimation of the reduced-order model parameters. The reduced model is further used to analyze and quantify the contribution of each of the thermophysical properties of working fluid on heat pipe thermal resistance. The simplified reduced-order model yielded a dimensionally reduced form of the numerical model which revealed constant thermal resistance contours. In addition, the reduced-order model successfully predicted the performance of the heat pipe when filled with different working fluids. It is also found that high thermal conductivity, low surface tension, low latent heat of evaporation, high viscosity, and low liquid density are the most favourable thermophysical properties of the working fluid leading to improvement of heat pipe thermal resistance, respectively. The proposed method for working fluid modification is generic and thus applicable to a range of situations associated with the selection and/or design of heat pipe working fluids and mixtures thereof.