Cylindrical Heat Pipe Research Articles

The heat transfer capability prediction of heat pipes based on capillary rise test of wicks

In this study, the capillary limit of heat pipe is predicted on the basis of capillary rise test. Firstly, porous wicks and cylindrical heat pipes are fabricated utilizing copper powders with identical diameters but different shapes. The pore structures and capillary performances of wicks are characterized, and thermal performances of heat pipes are also measured. Results show that high temperature sintering causes wick shrinkage towards the pipe wall, and the shrinkage degree increases with increase of powder irregularity, which results in decrease of wick cross-sectional area and thickness. A curve fitting equation describing capillary rise test is obtained and the capillary performance of the wick also increases with increase of powder irregularity. Fitting results show that the capillary pressure of irregular powders decreases nearly 30% comparing with that of spherical powders, but its permeability increases 533%. The experimental capillary limits of heat pipes sintered with spherical, sphere-like, dendritic and irregular copper powders are 18 W, 20 W, 44 W and 42 W, respectively. Furthermore, the predicted capillary limits at working temperatures are about 7∼11 W smaller than experimental values. Finally, the experimental capillary limit is employed to correct the prediction equation of capillary limit at different working temperatures.

등방성 윅 내 원주 방향 작동 유체 채움 특성이 실린더형 히트파이프의 열성능에 미치는 영향

등방성 윅 내 원주 방향 작동 유체 채움 특성이 실린더형 히트파이프의 열성능에 미치는 영향

INVESTIGATION OF INTRINSIC DYNAMIC CHARACTERISTICS IN AN OSCILLATING HEAT PIPE

Conjugate heat transfer devices based on oscillating phenomenon allow thermal instability to play a crucial role in thermal management. This study proposed and tested a hollow cylindrical oscillating heat pipe (OHP) for managing waste heat sources. Deionized water was filled as a working medium at ratios of 10%-90%. Experiments were conducted with heat inputs 10-120 W; the performance was dependent on the features of self-sustained oscillations. Dry-out was found only under low filling conditions, and this operating fault can be avoided by increasing the working medium. A medium level of approximately 50% is optimal. The intrinsic thermal instability related to the performance of an oscillating thermal design has attracted considerable academic interest. Simplifications and assumptions of traditional modeling approaches render them unsuitable for probing the dynamic characteristics of such thermal designs. A nonlinear autoregressive exogenous (NARX) modeling based on machine learning techniques was introduced to identify the causality of the heat transfer mechanism of the OHP. The discrete models estimated using the NARX modeling accurately simulated OHP dynamics, which were further revealed through spectrum analysis. Energy excitation, shown as frequency resonances, was distinctly related to OHP performance.

Effect of coated mesh wick on the performance of cylindrical heat pipe using graphite nanofluids

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.

Temperature uniformity improvement of a proton exchange membrane fuel cell stack with ultra-thin vapor chambers

Achieving uniform temperature distribution can provide a significant contribution to proton exchange membrane fuel cell performance and durability. Using heat pipes for temperature uniformity can get a simple system and reduce parasitic power. However, current researches on the effect of heat pipes on the temperature distribution are based on the most common cylindrical tube configuration and heat pipes are not integrated into the actual fuel cell during the experiment. They mostly use a heater to simulate the heat generation of the fuel cell. In this study, 6 ultra-thin vapor chambers with a thickness of only 1.5 mm are designed and used for a 5-cell fuel cell stack to reduce the temperature difference in the plane of each layer. Ultra-thin vapor chamber is a type of heat pipes with the advantages of light weight, geometric flexibility and extremely high thermal conductivity. It can provide an effective measure to make the temperature more uniform. The stack is tested under different placement states and cooling conditions. Test results prove that using ultra-thin vapor chamber can obtain a very uniform temperature distribution. Especially under the forced convection condition, the minimum temperature difference on the cathode gas diffusion layer surface can be reduced to 0.3 K. Note that it is better to avoid placing ultra-thin vapor chambers in the unfavorable placement for heat transfer to improve uniformity. The method of thermal management using ultra-thin vapor chambers offers opportunities for uniform distribution of temperature across a fuel cell and compactness.

Thermal Performance Analysis of Cylindrical Heat Pipes Induced in a Shell Assisted Heat Exchanger

Thermal Performance Analysis of Cylindrical Heat Pipes Induced in a Shell Assisted Heat Exchanger

Liquid phase enhanced sintering of porous aluminum for cylindrical Al-acetone heat pipe

In this study, porous aluminum is successfully sintered on the inner surface of a slim aluminum pipe utilizing liquid phase enhanced sintering method, and cylindrical heat pipe is successfully fabricated using acetone as the working fluid. The process mainly includes sintering, acetone filling, vacuuming, secondary degassing and argon welding. The sintering experiments indicate that pure aluminum powders are poorly sintered; however apparent sintering necks between aluminum powders are formed with copper particles added. Moreover, aluminum powders agglomerate and average pore size increases obviously when copper mass fraction reaches 4.5 wt%. Thermal performance test results of Al-acetone heat pipe demonstrate that the temperature nonuniformity is small and the maximum heat transfer capability is 7 W in horizontal orientation. The total thermal resistance of the heat pipe is 0.6 °C/W within the heat load range of 3–7 W.

Experimental inquisition of heat pipe: performance evaluation for different fluids

ABSTRACT A comprehensive study for testing of cylindrical heat pipe has been carried out with a number of experiments. Characteristics and performance of heat pipe were evaluated individually with water, methanol, and acetone at different inclination angles (θ). Results obtained show that heat pipe charged with acetone has the smallest thermal resistance and highest heat transfer coefficient compared to other working fluids, hence identified as the best working fluid for the prepared heat pipes. Also, the best angle of inclination for the working heat pipe is 45° as the thermal performance of the heat pipe is optimal at this angle.

Heat transfer performance of cylindrical heat pipes with axially graded wick at anti-gravity orientations

To increase the heat transfer capability at anti-gravity orientations, heat pipes with homogeneous and axially graded wicks were experimentally investigated. The homogeneous wick (0.8 mm thickness) sintered with 125–150 μm (coarse) copper powder operated at 100 W without dryout at horizontal orientation, while that sintered with 75–97 μm (fine) copper powder had lower critical heat load about 80 W. The critical heat load decreased tremendously with increasing anti-gravity angles and it was about 20 W at totally anti-gravity orientation. However, the axially graded wick, sintered with fine and coarse powder at evaporator section and the rest section of the pipe, respectively, exhibited critical heat load of 35 W, nearly 75% increment than homogeneous wick with same thickness. Further study shows that increasing the thickness of axially graded wick significantly improved the critical heat load although the thermal resistance increased simultaneously. At a tilted angle of 90°, the critical heat load increased from 35 W to 45 W when increasing thickness from 0.8 mm to 1.1 mm and it further increased to 96 W when increasing thickness to 1.35 mm. The reconciliation between the capillary force and the permeability of axially graded structure promoted the increase of the critical heat load notably.

Experimental study on thermal performance of copper nanofluids in a miniature heat pipe fabricated by wire electrical discharge machining

A cylindrical miniature heat pipe (MHP) with a diameter of 1.9 mm and a length of 130 mm was designed and fabricated via wire electrical discharge machining in this study. A special elastic fixture was designed for the micro grooves machining process. Water based copper nanofluids with mass concentrations ranged from 0.5 wt% to 1.5 wt% were prepared and tested in the MHP. Both the transient response and heat transfer capability of the MHP with copper nanofluids and DI water were studied experimentally. The MHP with nanofluids had a slightly slower startup response compared to the one with DI water. However, the heat transfer capability of the MHP with nanofluids was greatly improved to 250 mW (q = 8.82 W/cm2), while that of the MHP with the basefluid (DI water) was only 100 mW (q = 3.53 W/cm2). At the heat load of 150 mW, the MHP with 0.5 wt% nanofluid showed a minimum thermal resistance of 0.33 °C/W. The effects of mass concentration, filling ratio and heat load were investigated as well.

An integrated heat pipe coupling the vapor chamber and two cylindrical heat pipes with high anti-gravity thermal performance

An integrated heat pipe (IHP) coupling the vapor chamber and two cylindrical heat pipes is proposed in this study. Cylindrical heat pipes were connected with the top surface of the vapor chamber to extend the area of the condenser. The copper powder rings and graded pore-size wick are used to shorten the circulation path of working fluid and strengthen the capillary pressure and wick permeability of the wick structure. The effect of various filling ratios (30%, 40%, 60%, and 80%) and different inclination angles (0°, 90°, and 180°) have been investigated experimentally. The results showed that the filling ratio has a significant impact on the heat transfer performance of IHP, and the high filling ratio IHP shows better thermal properties. Specifically, the maximum temperature is lower than 83 °C at the heat load of 150 W when the filling ratio is 60%. Moreover, the integrated heat pipe shows better anti-gravity thermal performance than a conventional heat pipe. The lowest anti-gravity thermal resistance is 0.157 K/W with a heat load of 80 W. As compared with other heat pipes reported in this literature, the IHP has a good thermal performance and can adapt to many inclination angles.

Development of a thermionic diode detector for laser spectroscopy

This paper presents work on the development of a thermionic diode detector for laser spectroscopy of atoms. Basically, this is a positive ion-detector that exploits the negative space charge accumulated around the cathode of a diode. In this device, a thin and straight hot cathode wire is stretched axially into a long cylindrical heat-pipe that constitutes a device similar to an electronic vacuum diode. The ions in the diode space-charge region are produced as a result of photoionization or collisional ionization of lithium atoms. The design aspects and measurement of its electrical characteristics are described. A new method based on the resonant photoionization of lithium is used to measure the gain of the ion-detector. An application is also demonstrated to measure the photoionization cross-section (σ = 7.8 ± 1.6 Mb) of lithium near its first ionization threshold for two-photon excitation at 639.145 nm. Important features of the thermionic diode are its easy, versatile and sturdy design with high gain (104) and sensitivity that make it a valuable and cost-effective tool for spectroscopists.

Investigation of Thermal Performance of Cylindrical Heat Pipe Using Silver Nanofluid. Part I: Experimental Evaluation

Cylindrical heat pipes are widely used in various applications to transfer the heat effectively and efficiently. Besides parameters of heat pipe, its thermal performance mainly depends on the characteristics of working fluid also. Recently, various nanofluids have been considered as the most promising working fluid in different heat transfer applications. In the present paper thermal performance of cylindrical heat pipe using silver nanofluid as working fluid has been experimentally evaluated. Extensive experiments have been performed on cylindrical copper heat pipe having two-layer screen mesh filled with silver nanofluid in filling ratio of 40% with a vacuum of 10−3 torr. The thermal performance of heat pipe has been evaluated in terms of thermal resistance, thermal conductivity and overall heat transfer coefficient by varying parameters such as size of nanoparticle, concentration of nanoparticle, angle of inclination of heat pipe, heat load on heat pipe, average evaporator temperature and average condenser temperature to propose the best combination of parameters. The results indicate that the cylindrical heat pipe kept at 45° inclination angle and filled with silver nanofluid of 35 nm size of silver nanoparticle in 0.3% volume concentration yields higher thermal performance when it is operated at 120–140 W heat input. Thus, the cylindrical heat pipe under study is suitable for applications which require operation of heat pipe at some angles, such as cooling of high-wattage LED street lights.

Investigation of Thermal Performance of Cylindrical Heat Pipe Using Silver Nanofluid: Part-II: Prediction Using Artificial Neural Network

Heat pipes are one of the heat transfer devices adopted in various applications due to its high heat transfer capability. Recently, heat pipes are charged with different nanofluids to increases its performance. Thermal performance of heat pipe charged with nanofluids depends of various factors related to operating conditions, working fluids, heat pipe, etc. Thus, extensive experimentation work is required to recommend appropriate parameters for different industrial applications. In such cases, in order to reduce experiments and to propose the appropriate combination of parameters, experimentation results need to be modeled using fundamentals of simulation. In the Part I of the paper, extensive experiments have been carried out on cylindrical heat pipe with silver nanofluid as working medium. In the present paper, total of 96 artificial neural network models have been created and trained on the basis of the total 700 data sets presented in Part I of the paper. Out of 700 data sets, eight data sets have been randomly separated for post-training simulation of the model, while 692 data sets have been employed for formulating the ANN model using the ANN tool of MATLAB student version available at the college. The models have been prepared to predict the thermal performance of cylindrical heat pipe under study when one, two and three parameters have been considered as output neurons one by one and six parameters as input neurons. Out of six input parameters, heat load, average evaporator and average condenser temperature are related to operating condition, size and concentration of silver nanoparticles are related to working fluid, and inclination angle is related to position of heat pipe. Trained 96 ANN models have been simulated using eight data sets, and the best ANN model has been proposed on the basis of minimum value of error in prediction in terms of MSE, NMSE, MAE, MARD and MRE. The best ANN model identified for one, two and three output neurons is single-layer feed-forward backpropagation network with six neurons of hidden layer, cascade feed-forward backpropagation network having three neurons at hidden layer and single-layer feed-forward backpropagation network with 12 neurons of hidden layer, respectively. It is also concluded that for single output neuron, any one type of error in prediction can be considered to select the best ANN model. But for more number of output neurons, comparison of various types of error in prediction is essential to judge the best ANN model.

Transient Modelling of Rotating and Stationary Cylindrical Heat Pipes: An Engineering Model

Rotating wickless and stationary capillary cylindrical heat pipes are widely used heat transfer devices. Transient behavior of such heat pipes has been investigated numerically with computational fluid dynamics and lumped parameter models. In this paper, the advantages of both methods are combined into a novel engineering model that is low in computational cost but still accurate and rich in the details it provides. The model describes the interior dynamics of the heat pipe with a 2D representation of a cylindrical heat pipe. Liquid and vapor volumes are coarsely meshed in the axial direction. The cells are allowed to change in size in the radial direction during simulation. This allows for tracking the liquid/vapor interface without having to implement fine meshing. The model includes the equations for mass, momentum and energy and is applicable to both rotating and stationary heat pipes. The predictions of the model are validated with other experimental, numerical, and analytical works having an average deviation of less than 4%. The effects of various parameters on the system are explored. The presented model is suitable for the simulation of heat pipe systems in which both the level of detail and the computational cost are crucial factors.

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.

Combined effect of bending and flattening on heat transfer performance of cryogenic sintered-wick heat pipe

Heat transfer characteristics of a cryogenic sintered-wick miniature bent and flattened heat pipe has been experimentally investigated in this paper. The heat pipes are made of oxygen free copper for the container and nitrogen is used as the working fluid. The wick was composite wick made from sintered copper powder and grooved copper pipe. The original cylindrical copper heat pipe was 6 mm in outer diameter and 200 mm in length. The heat pipes with the bending radius of 21 mm and the bending angle of 0°, 30°, 60° and 90° and the final thickness of 2.5, 3.0, 4.0 mm were constructed and tested. The operating temperature of heat pipe was at approximately 78.25 ± 0.10 K. It was found that the performance of the heat pipe in terms of thermal resistances depends on both bending angle and final thickness. Both bending and flattening affect the thermal resistance by increasing it. The bending increased the thermal resistance from 0.88 to 1.07 K/W (6 mm, 0° to 6 mm, 90°) and the flattening increased the thermal resistance from 0.88 to 2.24 K/W (6 mm, 0° to 2.5 mm, 0°). And the combined effect of bending and flattening increased the thermal resistance from 0.88 to 1.56 K/W (6 mm, 0° to 3 mm, 90°). The bending affects heat pipe by wick deformation that obstructs liquid flow. The flattening affects the thermal resistance by the collapse of wick structure which causes a broken liquid path of returned working fluid from the condenser to the evaporator and by liquid clogging in the condenser.

Investigations of the thermal performance of a cylindrical wicked heat pipe

The effect of wick structure on the performance of capillary-driven heat pipes is analyzed numerically. Three types of wick structure are investigated: copper screen, sintered steel, and sintered copper. Using lattice Boltzmann method, the entire heat pipe is modeled including heat transfer through the wall and heat and mass transfer in the liquid-wick and vapor regions. Comparison between the present model and analytical results available in the literature shows excellent agreement. The liquid velocity profiles and temperature distribution in the wall are also presented for different working fluids and discussed. Comparison is made between performance of heat pipes with different wick structures and working fluids. Results indicate better performance, with a factor up to 1.7, of heat pipes using water as working fluid with sintered copper structure than with other wick structures. In addition, it is found that the larger the liquid's radial velocity, the less the thermal resistance at given heat input power. Both a simple equivalent resistance circuit and numerical data suggest strongly that the most impactive fluid properties on the heat pipe performance seem to be the latent heat of vaporization, the molecular mass, the change rate of the saturation pressure with respect to temperature, and the conductivity of its liquid phase.

Experimental study of the thermal characteristics of a heat pipe

The performance of a cylindrical heat pipe under various operating conditions was experimentally analyzed. The heat pipe consists of a copper tube with a diameter of 12.7 mm and 4 layers of copper screen mesh act as the wick. The heat pipe was equipped with a vacuum ball valve in order to charge the heat pipe with different amounts of working fluid. The effect of the working fluid fill volume, inclination angle, and heat input on the equivalent thermal resistance of the heat pipe were investigated. The results showed that in gravity-assisted orientations (condenser located above evaporator), the inclination angle has a negligible effect on the performance of the heat pipe. However, for gravity-opposed orientations (evaporator above condenser), as the inclination angle increases, the temperature difference between the evaporator and condensation increases which results in a higher thermal resistance. It was also found that if the working fluid in the heat pipe is under-filled, the capillary limit of the heat pipe decreases dramatically. However, overfilling of the heat pipe degrades the heat pipe performance due to excess liquid interfering with the evaporation-condensation mechanism. The results obtained from this experiment were also used to verify a previously developed numerical model. Very good agreement between the numerically predicted results and those from the experiment were obtained, thus validating the numerical technique developed and reported in authors’ previous papers.

Heat and mass transfer in a cylindrical heat pipe with a circular-capillary wick under small imposed temperature differences

Heat pipes are efficient in transferring heat and have been applied in various thermal systems. Previous models of heat pipes use the heat rate through the pipe as an input parameter, and therefore lack predictive capabilities. Here, we demonstrate, using a simple heat pipe, that if the evaporation and condensing kinetics are properly modeled, then the heat rate is predicted. We consider a cylindrical heat pipe with the inner wall lined with a circular-capillary wick. The capillaries are filled with a partially wetting liquid, and the center of the pipe is filled with its vapor. Initially, the heat pipe is at temperature T0 and the system is under thermodynamic equilibrium. Then, one end of the pipe is heated to T0 + ΔT, while the other end cooled to T0 − ΔT, and the system reaches a steady state. The equilibrium vapor pressure at the hot end is higher than that at the cold end, and this pressure difference drives a vapor flow. As the vapor moves, the vapor pressure at the hot end drops below the equilibrium vapor pressure which induces continuous evaporation from circular pores on the wick surface. At the cold end, the vapor pressure exceeds the equilibrium vapor pressure so that the vapor condenses and releases the latent heat. The condensate moves back to the hot end through the capillaries in the wick to complete a cycle. We assume that the pore size is infinitesimal compared with the pipe dimensions. Thus, pore-level events can be treated separately from pipe-level events. The evaporation rate in each pore is solved in the limit the evaporation number E→∞, and an analytic leading-order solution is obtained, assuming ΔT/T0 ≪ 1. The evaporation rate is incorporated into vapor-flow and energy-balance equations along the pipe. Two dimensionless numbers emerge from these equations: the heat pipe number, H, which is the ratio of heat transfer by vapor flow to conductive heat transfer in the liquid and wall, and the evaporation exponent, S, which controls the evaporation gradient along the pipe. We find that vapor-flow heat transfer dominates in heat pipes and H ≫ S ≫ 1. Under these conditions, the non-dimensionalized heat rate through the insulated pipe is found to be simply S. Analytic solutions are also obtained for the pipe temperature and all the other variables. For maximum evaporative heat transfer, we find an optimal pipe length for fixed pipe cross-sectional dimensions, and an optimal wick thickness for a fixed pipe length. These optimal pipe length and wick thickness can help to improve the design of heat pipes and are found for the first time.