Wick Heat Pipe Research Articles

Erratum to “Experimental investigation of indirect solar dryer integrated with wicked heat pipe”

Erratum to “Experimental investigation of indirect solar dryer integrated with wicked heat pipe”

Modeling the thermal and hydrodynamic performance of grooved wick flat heat pipes

A compact model is developed to predict the thermal and hydrodynamic performance of a flat heat pipe with a rectangular grooved wick. The present model relies on the analytical solution to the energy equation in the wall and an equivalent heat transfer coefficient predicted using a computationally efficient iterative method. This efficient iterative method can also provide a framework for modeling other grooved or porous wick heat pipes for which analytical or semi-analytical solutions to the wall conduction, fluid flow, and film equations exist. Compared to prior numerical tools, the present modeling approach is computationally efficient, making it compelling for use in parametric and optimization studies. Instead of numerically solving a set of coupled differential equations, the present model considers only analytical and semi-analytical solutions for evaporation and condensation heat transfer rates. The non-discretized nature of the present model allows computations on a typical workstation to be completed within seconds as opposed to the hours required for prevalent numerical tools. The present model accounts for varying liquid fill volumes, geometry, and interfacial properties, such as surface tension and contact angle. The present model closely agrees with published numerical and experimental results for wall temperatures and maximum heat transfer rates. Parametric studies, which vary wall thermal conductivity, water contact angle, and groove dimensions are conducted on a previously experimentally investigated heat pipe to demonstrate the present model’s capabilities. The present model found that the maximum heat transfer rate of the heat pipe can be enhanced by about 15 and 20% by varying its wetting angle and groove dimensions, respectively.

Numerical analysis of tapered wick configurations in cylindrical heat pipes

Heat pipes are essential devices in thermal engineering due to their ability to transport heat with remarkable efficiency. They are crucial for heat distribution and control in a variety of applications, such as aircraft systems and electronics cooling. This paper presents a numerical analysis on the performance characteristics of a cylindrical heat pipe featuring a novel tapered wick geometry. A two-dimensional numerical model incorporating non-Darcian liquid transport is formulated and solved at steady state. To investigate the effect of the tapered wick geometry, major characteristics such as axial distribution of vapor pressure, liquid pressure and wall temperature are evaluated and analyzed. Based on the direction of taper, three configurations viz., THPE (Tapering wick heat pipe from evaporator end), THPC (Tapering wick heat pipe from condenser end) and THPA (Tapering wick heat pipe at adiabatic center) are studied. A novel non-dimensional parameter termed as Temperature Pressure Index (TPI) is proposed to signify the extent to which an HP can reduce its evaporator temperature relative to increase in pressure drop. Initially a parametric study is performed. Results showed a superior performance of THPE and THPC designs over THPA in terms of output parameters. The numerical findings demonstrate that when compared to a uniform wick heat pipe, both the THPE and THPC configurations exhibit a notable enhancement of 58 % in effective thermal conductivity under a maximum load of 455 W. Further, by conducting a variance-based (Sobol) sensitivity analysis, the THPC design is found to be marginally better as compared to THPE. Finally, the BOBYQA optimization algorithm is employed to find optimum taper angle for THPC design that maximizes the output parameters. The optimum taper angle of the wick is found to be 0.0372° for minimum average evaporator temperature in the present case. For this optimized THPC design, a reduction in average evaporator temperature of 6.51 K is obtained compared to uniform wick heat pipe. Additionally, the obtained TPI value is about 1.86 times than that of a uniform wick heat pipe.

Experimental investigation of indirect solar dryer integrated with wicked heat pipe

Experimental investigation of indirect solar dryer integrated with wicked heat pipe

Performance Analysis of Water Heat Pipe for Application of the Passive Cooling System in Nuclear Power Plants

Incorporating heat pipes into passive cooling systems in nuclear reactors offers the benefits of passive operation without external power, a simple design, and high thermal capacity. Accurate thermal performance prediction of the heat pipe is crucial for ensuring safe reactor design and operation. Prior studies on nuclear reactor systems utilizing heat pipes have focused on thermosyphons, which operate by gravity. However, to expand the range of heat pipe applications in reactor systems, experimental investigations of large-scale heat pipes driven by capillary pumping force are required. In this study, a water heat pipe with a 25.4-mm diameter and 4-m length was manufactured to provide thermal experimental results under extreme conditions, such as system rollover or loss-of-cooling accidents. A three-dimensional (3D) printing technique was used to fabricate the high-performance lattice capillary wick structure by combining cubic and diamond lattice structures. The 3D printed wick structure showed 21 to 165 times higher capillarity and enhanced surface properties compared to the screen mesh wick structure. Compared to wickless thermosyphons, the 3D printed wick heat pipe exhibited higher thermal conductivity, stable operation in both vertical and horizontal orientations, and faster startup under extreme conditions.

A novel coaxial heat pipe with an inner vapor tube for cooling high power electronic devices

Improving heat transfer limit of heat pipe is important for their application in high-power equipment system. Shear stress at the liquid–vapor interface affects the heat transfer capacity of heat pipe. In this study, a novel coaxial heat pipe is designed to eliminate vapor entrainment from interfacial shear stress for the first time. An inner thin-walled tube is sintered with the powder wick in the adiabatic section by one-step sintering process. The effects of filling ratio and powder size of sintered wick on the thermal performance of coaxial heat pipe are investigated experimentally. The results indicate that the optimal powder size range for the sintered wick of coaxial heat pipe is 106–125 μm, while the optimal filling ratio is about 125 %. It means that the sintered wick with 106–125 μm powder can achieve better trade-off between capillary performance and permeability. The coaxial heat pipe can operate at heat loads of 70 ∼ 140 W with the total thermal resistance less than 0.06 °C /W. Moreover, compared with the normal heat pipe, the heat transfer capacity of coaxial heat pipe is increased by 57 %, while the evaporation temperature is decreased by 9.6 ∼ 19.1 °C and the total thermal resistance is decreased by 41 %∼320 %. The inner tube can improve the replenishment efficiency of working liquid by eliminating interfacial shear stress, resulting in the significant heat transfer improvement. The design of coaxial heat pipe is an effective and low-cost solution to improve the thermal performance of heat pipe.

Selective laser melting manufacturing and heat transfer performance study of porous wick in loop heat pipe

Porous wicks, characterized by high capillary pumping force, lightweight, and excellent permeability, have been widely used in heat transfer. Currently, porous wicks are fabricated using powder sintering, porous foaming, and solvent volatilization techniques. However, there is the problem that the shape, size, and pore distribution of the porous structure are difficult to control. This paper proposes a novel 3D printing method to fabricate porous wicks using selective laser melting. The unit diameter and length are optimized to obtain a porous wick with high porosity and capillary pumping force. The effect of laser power on the surface morphology and porosity of the porous wick is investigated by modulating the 3D printing process parameters. The results show that the porosity of the porous wick can be regulated by changing the diameter and length of the pore unit. Decreasing the laser processing power and attaching unmelted solid-phase particles to the pore surface can further increase the porosity. When the laser power is 125 W, the unit diameter is 0.4 mm, and the unit length is 0.6 mm, the porous wick has strong capillary pumping force. The 316L porous material is modified by heat treatment to obtain super-hydrophilic properties. The porous wick is applied to the evaporator of the loop heat pipe, which significantly improves the heat transfer performance. When the heat load is 200 W, the evaporator temperature is 57.09 °C, and the thermal resistance is 0.025 °C/W.

Evaluation of innovative thermal performance augmentation techniques in heat sinks & heat pipes: Electronics & renewable energy applications

Heat sinks and heat pipes have vast applications including industrial domains requiring high heat flux. Due to numerous uses of these heat transfer devices, there have been great advancement in this area through innovation. This study focuses use of nanofluids and hybrid nanofluids for thermal performance enhancement of heat pipes and heat sinks. There are five different arrangements finless sintered copper wicked heat pipe, water based TiO2 filled heat pipe, TiO2 and SiO2 based hybrid nanofluid filled heat pipe, water cooled cylindrical pin finned heat sink and Fe2O3 and TiO2 hybrid nanofluid cooled heat sink. There are two test rigs one for testing heat pipe configurations and other for heat sinks. All the five elements are tested at constant heat flux ranging from 900 W/m2 to 3600 W/m2, in four equal steps. It’s observed that nanofluids play a very significant role in thermal performance augmentation, the affect is even more significant in heat pipes compared to heat sinks. Interestingly, nanofluids based heat pipes are proved more significant for high heat flux applications, although, nanofluid cooled heat sinks have slight better performance compared to the said but at a higher expense of weight, auxiliaries, space and cost. At 3600 W/m2 nanofluid based heat pipes have 33 % lower thermal resistance compared to water cooled heat sink. Therefore, nanofluid based heat pipes are primarily significant unless use of heat sink deemed dire. Nusselt number of heat sink is calculated at various heat flux and at variable mass flux conditions and found increasing with both parameter.

Preparation of hierarchical microgroove textures on the surface of Al-based wicks by roller pressing and laser scanning irradiation

Fabrication of high-performance heat pipe wicks has become a major challenge due to the increase of heat dissipation requirements for electronic devices. In this paper, we propose a two-step method for the preparation of aluminum liquid-absorbing wicks with hierarchical groove structures by roller pressing and laser irradiation techniques. The effects of laser fluence and velocity on the groove morphology, wettability and capillary effect of the aluminum wicks with hierarchical grooves processed by laser scanning are investigated. The study demonstrates that sub-scale grooves with a period of 20 μm can be created on the surface of the aluminum wick using a laser fluence of 7.166 J/cm2, scanning velocities of 560 mm/s and 630 mm/s, and a pulse distance of 1 μm. The surface roughness of the irradiated aluminum wick was increased and energy dispersive spectroscopy tests showed an increase in the oxygen content of the machined surface, resulting in a surface amplification effect and the formation of a large number of hydrogen bonds in contact with the liquid. Capillary rise experiments demonstrate that hierarchical microgrooves enhance capillary action on the surface of the aluminum wick more effectively than single-scale grooves. This is due to the pumping effect in microgrooves and the diffusion effect in Ω-grooves. The hierarchical microgroove prepared under the above parameters had a capillary rise height of 88.5 mm and a capillary rise speed of 5.5 mm/s. This work proposes a method for producing high-performance wicks that are both efficient and cost-effective.

A Molecular Dynamics Perspective on the Impacts of Random Rough Surface, Film Thickness, and Substrate Temperature on the Adsorbed Film’s Liquid–Vapor Phase Transition Regime

While recent studies have proven an unexpected liquid–vapor phase transition of adsorbed liquid films, a comprehensive description of the mechanisms of different types of phase change regimes over realistic representations of random rough surfaces is absent in the literature. The current comprehensive study investigates the effects of a gold random rough surface, liquid film thickness, and substrate temperature on the liquid–vapor phase change regime of an adsorbed sodium liquid film, considering the evaporator section of a wicked heat pipe (WHP) using a molecular dynamics (MD) simulation. At first, to generate a realistic random rough surface, a new and promising method is proposed that is entirely based on MD simulations. Then, to simulate the evaporator section of a WHP, a unique configuration for eliminating the vapor domain is developed. The simulation results reveal that three distinct regimes, namely, normal evaporation, cluster boiling, and film boiling, could be identified, which are presented on two-dimensional diagrams with the substrate temperature and liquid film thickness as coordinates for the ideally smooth and random rough surfaces. The results also manifest that even though using the random rough surface could lead to different phase transition regimes, the type of regime depends mainly on the substrate temperature and liquid film thickness. Furthermore, this study displays two different modes for normal evaporation. Also, it is shown that the impacts of the liquid film thickness and substrate temperature on the mode of normal evaporation are much more significant than the surface roughness.

Experimental and numerical investigation of wicked heat pipe cooling efficacy in oil-immersed transformers

The significance of thermal management in transformers, recognized as a pivotal component within the electricity industry, has prompted numerous studies in this area. Although various methods to decrease hot-spot temperatures in transformers have been introduced, a comprehensive investigation into the effectiveness of wicked heat pipes (WHPs) remains lacking.This study aims to rectify this deficiency by conducting experimental and numerical investigations to assess the impact of WHPs on transformer thermal management. Firstly, the WHPs undergo testing with varied thermal loads to assess their heat transfer efficiency in steady-state operation. This not only monitors the WHP conditions to prevent limitations but also computes the effective thermal conductivity for numerical study input. Next, the thermal management performance of a laboratory transformer with two WHPs was experimentally studied both indoors and outdoors.Additionally, the simultaneous utilization of WHPs and radiator with various configurations is examined experimentally to assess their combined effect. Then, a numerical study investigates how the number of WHPs influences the reduction of the hot-spot temperatures in the transformer. The experimental findings indicate that the use of WHPs led to a 9 % reduction in peak transformer temperature under real conditions. Simultaneous use of the radiator and WHPs, regardless of their arrangement, resulted in an approximate 18 % reduction in the hot-spot temperature. Additionally, numerical simulations suggest that employing eight heat pipes could reduce hot-spot temperatures by up to 9.2 °C, representing a 15 % improvement compared to baseline conditions.

Composite wick heat pipe fabrication and performance study

A composite wick heat pipe is designed and prepared: a copper mesh is fixed on the copper pipe, and copper powder is filled in the gap between the mesh and the mandrel through the mandrel. A test device is assembled to test the maximum thermal conductive power and temperature stabilization time of the heat pipe. The results show that: the composite wick structure and the copper powder structure compared with the ultimate heat transfer performance increased by 20%, when the heat capacity is increased, the overall temperature difference will increase accordingly, and the optimal liquid injection rate of 100%, the ultimate power of up to 100 W, when filling with powder, it has the best thermal conductivity, the ultimate heat transfer power in the state of gravity is three times higher than that of the state of resistance to gravity, and it is 28.6% higher than that in the state of horizontality. The composite wick structure starts 17.6% faster than the copper powder structure.

Numerical simulation-based design of optimized surfaces for condensation heat transfer

Heat transfer enhancement is a well-established research topic due to its industrial relevance. The advent of 3D printing, also known as additive manufacturing, has opened up the pathway for the design of compact heat exchangers with innovative shapes, complex geometries and smaller size. In particular, additive manufacturing technologies speed up the transition from CAD models to physical prototypes and thus numerical simulation tools for two-phase heat transfer are expected in the future to play a key role for the design of heat exchangers. With regard to filmwise condensation, numerical methods capable to account for all the main forces involved (such as gravity, surface tension, vapor shear stress) must be considered for the optimization of the condensing surfaces. The present study is focused on the preliminary design of innovative shapes for enhancing the condensation heat transfer in a grooved wick heat pipe. Most of the condensation heat transfer studies available in the literature deal with vapor condensing inside tubes or over cylindrical/plain surfaces, while the condensation process occurring inside heat pipes has been less investigated. The grooved wick surfaces must be designed with the aim to promote the drainage of the condensate and minimize the thickness of the liquid film forming over the wick structures. In the present work, the behaviour of different surfaces is studied during condensation of refrigerants by performing Volume of Fluid (VOF) 2D numerical simulations with Ansys®Fluent. The numerical results allow to determine the liquid film thickness and the heat flux distribution along the grooves.

Effect of Copper Content on the Microstructure and Properties of the Sintered Porous Aluminum Wick

Porous aluminum has been widely used as a wick, an electrode, and in other products, due to its advantages of a light weight, pore uniformity, and corrosion resistance. However, the dense alumina layer on the surface of Al powder hinders its densification during sintering. In this paper, porous aluminum was prepared via loose powder sintering with the addition of Cu as a sintering aid. The effects of Cu content on the microstructure and wick properties of the porous aluminum were investigated. The results showed that, with increasing Cu content, the porosity and capillary properties of porous Al decreased, while the compressive strength improved. The optimal Cu content was determined to be 3 wt.% and the prepared porous Al has a porosity of 47.5%, plateau stress of 11.82 MPa, and capillary characteristic parameter of 6.72 × 10−8 N, meeting the requirements for wicks in heat pipes. These findings contribute to the demand for the lightweight design of heat pipes for aerospace applications.

Sintered Wick Heat Pipes with Excellent Heat Transfer Capabilities—Case Study

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.

Permeability measurement of a 3D-printed AlSi10Mg porous medium: comparisons between first results with different experimental and numerical techniques

In this work, the permeability of a 3D-printed, AlSi10Mg porous medium, with porosity ε = 0.3 and an effective pore radius of 48 μm, developed to operate as wick in a sinter-like heat pipe, has been investigated by means of two different experimental approaches, and of two different numerical methods. The two experimental methods are the capillary rise tests, from which permeability was estimated by fitting the theoretical capillary rise curve to the experimental data, and the direct measurement of the the mass flow rate across the porous sample at an imposed pressure difference. The numerical simulations were performed too using two different approaches and software tools, namely, Lattice-Boltzmann with Palabos, and Finite-Volumes with OpenFOAM. In both cases, the simulation domain was reconstructed from a micro-computer aided tomographic scan of a porous medium sample. Preliminary simulations were run on a simple configuration, both to check simulation setup and validate results, and mesh independence was assessed. Then, pressure-driven and velocity-driven simulations of an incompressible fluid flow across the domain were performed, from which the permeability was estimated using Darcy and Darcy-Forchheimer equations. The results show that the methods, while not in complete agreement, provide a useful estimate. The numerical methods also complement the information given by the experimental techniques by highlighting the flow paths, and allow to analyze scenarios of increased and decreased porosity.

The Effect of Wick Permeability and Porous Radius on Capillary and Entrainment Limit in A Heat Pipe Reactor

For heat extraction in nuclear systems, interest in the design of nuclear reactors with heat pipes has increased. The determination of heat limitations is one of the remarkable factors for safety when heat pipes are used for nuclear systems. In this study, capillary and entrainment limit values for the heat pipe were calculated in a heat pipe reactor with potassium working fluid operating at 650 K. Five different effective porous radii (10.1x10-6, 10.225x10-6, 10.35x10-6, 10.425x10-6 and 10.6x10-6 m) and five different wick permeability (4.75x10-12, 5x10-12, 5.25x10-11, 5.5x10-12 and 5.75x10-12 m2) is considered for sintered copper wick heat pipe. While the effects of effective porosity radius, wick permeability, and wick radius on the capillary barrier were studied, only the effects of effective porosity radius were studied. While the effects of effective porosity radius, wick permeability, and wick radius on the capillary barrier are studied, only the effects of effective porosity radius are studied. The highest values of the capillary and entrainment limits are obtained when the porosity radius is 10.1x10-6 m. Besides, maximum capillary limits are achieved when the wick permeability is 5.75x10-12 m2 and the effective porosity radius is 10.1x10-6. This study aims to determine the optimum effective porous radius and wick permeability for this reactor and investigate the effect of effective porous radius and wick permeability on the heat pipe limitations.

Performance Evaluation of a Rice Drying System Utilizing Wick Heat Pipe as a Heat Exchanger

Indonesia predominantly consists of a population engaged in rice farming as their primary source of livelihood. The rice farmers in Indonesia continue to employ traditional methods in the rice drying process, which poses a significant challenge as it substantially impacts the efficiency of rice processing. Consequently, to address this issue, a rice drying apparatus employing wick heat pipes as heat transfer devices has been developed. In the course of this research, the drying apparatus underwent testing at multiple points, with variations in voltage and airspeed. This was undertaken to ascertain the optimal voltage level at which the wick heat pipes, and the drying chamber would operate effectively. Voltage settings of 180V, 200V, and 220V were used, coupled with airspeeds of 1.5 m/s and 2 m/s. For the 180V setting and an airspeed of 2 m/s, a heat transfer rate (Q) of 1.0552 J/s was achieved, with an efficiency of 0.18%. Conversely, at an airspeed of 1.5 m/s, the heat transfer rate amounted to 1.1306 J/s, and the efficiency reached 0.19%. At 200V and an airspeed of 2 m/s, the heat transfer rate was 1.5075 J/s, and the efficiency was 0.21%, while at an airspeed of 1.5 m/s, the heat transfer rate was 1.1306 J/s, with an efficiency of 0.16%. At 220V and an airspeed of 2 m/s, the heat transfer rate was 1.8009 J/s, and the efficiency was 0.21%, while at an airspeed of 1.5 m/s, the heat transfer rate remained at 1.8009 J/s, and the efficiency also stood at 0.21%. In conclusion, it can be inferred that the wick heat pipes, and the drying chamber demonstrated optimal performance at the 220V voltage setting.

The effect of layer thickness on the geometry and capillary performance of strut-based heat pipe wicks manufactured by laser powder bed fusion

Additive manufacturing (AM) can be used to fabricate heat pipes and two-phase heat sinks which have integrated wicking structures. These devices can be customized and have superior performance when compared with devices with conventionally fabricated wicks. The current work investigates the impact of build rate on the capillary performance of AM wicks fabricated by laser powder bed fusion (LPBF). The build rate was examined by varying the layer thickness from 30 µm to 80 µm for four different strut-based wick geometries: (i) body-centered cubic (BCC), (ii) face-centered cubic (FCC), (iii) simple cubic (SC), and (iv) fluorite. The mass rate-of-rise method was used to quantify the wick hydraulic parameters (namely wick permeability, K, and effective pore radius, reff). Layer thickness has a significant influence on wick permeability; layer thickness of 40 µm result in the highest value for all configurations but have a small effect on the effective pore radius. Layer thickness of 40 µm and 60 µm reached the highest K/reff ratio, primarily because of the permeability increase. We attribute this to poor build quality such as missing struts and less defined edges which facilitate low resistance to fluid flow. The SC 60 and BCC 40 configurations achieved the maximum capillary performance with K/reff of 1.34 µm and 1.42 µm, respectively. Overall, it was found that while increasing the build rate by varying layer thickness may affect the part build quality, it promotes hydraulic performance for some configurations by creating random rough surface morphologies and arteries, which help increase permeability.

Ultrafast Axial Freezing in a Liquid-Filled Capillary Tube.

Liquid-filled capillary tubes are a kind of standard component in life science (e.g., blood vessels, interstitial pores, and plant vessels) and engineering (e.g., MEMS microchannel resonators, heat pipe wicks, and water-saturated soils). Under sufficiently low temperatures, the liquid in a capillary tube undergoes phase transition, forming an ice nucleus randomly on its inner wall. However, how an ice layer forms from the nucleus and then expands, either axially or radially to the tube inner wall, remains obscure. We demonstrated, both experimentally and theoretically, that axial freezing along the inner wall of a water-filled capillary tube occurs way ahead of radial freezing, at a nearly constant velocity 3 orders in magnitude faster than the latter. Rapid release of latent heat during axial freezing was identified as the determining factor for the short duration of recalescence, resulting in an exponential rise of the supercooling temperature from ice nucleation via axial freezing to radial freezing. The profile of the ice-water interface is strongly dependent upon the length-to-radius ratio of the capillary tube and the supercooling degree at ice nucleation. The results obtained in this study bridge the knowledge gap between the classical nucleation theory and the Stefan solution of phase transition.