Cylindrical Heat Pipe Research Articles

The Influence of Horizontal Longitudinal Vibrations and the Condensation Section Temperature on the Heat Transfer Performance of a Heat Pipe

This study is primarily focused on the influence of horizontal longitudinal vibrations and the condensation section temperature on the heat transfer performance of a grooved cylindrical copper heat pipe, with a length of 600 mm and an outer diameter of 8 mm. Longitudinal vibrations with frequencies of 3, 4, 5, 6, and 9 Hz and amplitudes of 2.8, 5, 10, 15, 20, and 25 mm, which would give accelerations in the range of 0.1–1.01 g, were experimentally tested. The condensation section temperature was set at 20, 30, or 40°C. A heating jacket and a cooling sleeve were installed at the evaporation and condensation sections of the test cell to mimic a constant heat flux and a constant temperature boundary, respectively. When the heat pipe started to vibrate horizontally in the longitudinal direction, this vibration caused an increase in the heat transfer of the heat pipe that was directly proportional to the input vibration energy below 500 mm2 Hz2. When the value of the vibration energy exceeded this value, the heat transfer enhancement per unit vibration energy decreased rapidly. Along with the decrease in the condensation section temperature, the average temperature of the heating section decreases. The influence of the condensation section temperature on the maximum heat transfer is much greater than that of the vibrations.

Neutron Imaging of Alkali Metal Heat Pipes

Abstract High-temperature heat pipes are two-phase, capillary driven heat transfer devices capable of passively providing high thermal fluxes. Such a device using a liquid-metal coolant can be used as a solution for successful thermal management on hypersonic flight vehicles. Imaging of the liquid-metal coolant inside will provide valuable information in characterizing the detailed heat and mass transport. Neutron imaging possesses an inherent advantage from the fact that neutrons penetrate the heat pipe metal walls with very little attenuation, but are significantly attenuated by the liquid metal contained inside. Using the BT-2 beam line at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, preliminary efforts have been conducted on a nickel-sodium heat pipe. The contrast between the attenuated beam and the background is calculated to be approximately 3%. This low contrast requires sacrifice in spatial or temporal resolution so efforts have since been concentrated on lithium (Li) which has a substantially larger neutron attenuation cross section. Using the CG-1D beam line at the High Flux Isotope Reactor (HFIR) of Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, the first neutron images of high-temperature molybdenum (Mo)-Li heat pipes have been achieved. The relatively high neutron cross section of Li allows for the visualization of the Li working fluid inside the heat pipes. The evaporator region of a gravity assisted cylindrical heat pipe prototype 25 cm long was imaged from start-up to steady state operation up to approximately 900 °C. In each corner of the square bore inside, the capillary action raises the Li meniscus above the bulk Li pool in the evaporator region. As the operational temperature changes, the meniscus shapes and the bulk meniscus height also changes. Furthermore, a three-dimensional tomographic image is also reconstructed from the total of 128 projection images taken 1.4o apart in which the Li had already cooled and solidified.

Comparison between evaporative cooling and a heat pipe assisted thermal loop for a commercial wind tower in hot and dry climatic conditions

Increasing focus on reducing energy consumption has raised public awareness of renewable energy resources, particularly the integration of natural ventilation devices in buildings such as wind towers. The purpose of this paper was to compare the traditional evaporative wind tower technique with a proposed wind tower system consisting of heat pipes. Computational Fluid Dynamics (CFD) was used to develop a numerical model of a wind tower system and simulate the air flow pattern around and through the device. A baseline heat exchanger section containing cylindrical heat pipes was constructed to simulate the multiphase flow behaviour of two-phase heat pipe working fluids including water and ethanol. Heat transfer rate was obtained at 113 and 106W for water and 72 and 116W for ethanol respectively. The second part of the study incorporated the cylindrical heat pipes within the control domain of a roof-mounted wind tower, highlighting the potential to achieve minimal restriction in the external air flow stream while ensuring maximum contact time, thus optimizing the cooling duty of the device. A comparison was established with the conventional evaporative cooling methodology. The proposed cooling system consisting of heat pipes was capable of reducing the air temperatures by 12–15K, depending on the configuration and operating conditions. The technology presented here is subject to IP protection under the QNRF funding guidelines.

Comparative study on heat pipe performance using aqueous solutions of alcohols

This paper deals with the performance characterization of heat pipes using an aqueous solution of long chain alcohols like n-Butanol, n-Pentanol, n-Hexanol and n-Heptanol as working mediums. These solutions are called as self-rewetting fluids, since these fluid mixtures possess a non-linear dependence of the surface tension with temperature. A cylindrical heat pipe made up of copper with two layers of wrapped screen is used as a wick material and partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability like thermal efficiency and thermal resistance at different inclinations and input power levels. A number of tests have been performed with heat pipes, filled with various aqueous solutions of alcohols with a concentration of 2 ml/l in de-ionized water (DI water) on volume basis. The results obtained for heat pipes using self rewetting fluids show improved performances, when compared to DI water heat pipes.

Thermal performance of heat pipe with suspended nano-particles

Nanofluids are employed as the working medium for a conventional cylindrical heat pipe. A cylindrical copper heat pipe of 19.5 mm outer diameter and 400 mm length was fabricated and tested with two different working fluids. The working fluids used in this study are DI-water and Nano-particles suspension (mixture of copper nano particle and DI-water). The overall heat transfer coefficient of the heat pipe was calculated based on the lumped thermal resistance network and compared with the heat transfer coefficient of base fluid filled heat pipe. There is a quantitative improvement in the heat transfer coefficient using nano-particles suspension as the working medium. A heat transfer correlation was also developed based on multiple regression least square method and the results were compared with that obtained by the experiment.

EXPERIMENTAL ANALYSIS OF CYLINDRICAL HEAT PIPE USING COPPER NANOFLUID WITH AN AQUEOUS SOLUTION OF n-HEXANOL

This study analyzes the thermal performance of heat pipe using copper nanofluid in n-Hexanol. The cyli ndrical heat pipe is filled with de-ionized water, copper nanofluid, an aqueous solution of n-H exanol and copper nanoparticle in an aqueous soluti on of n- Hexanol separately and tested for its performance. The nanofluids kept in the suspension of conventional fluids have the potential of superi or heat transfer capability than the conventional fluids due to their improved thermal conductivity. The experiments are conducted for various heat inpu ts and inclinations angles of the heat pipe. The experimental results are evaluated in terms of its performance metrics like thermal efficiency and the rmal resistance and compared with DI water.

Experimental study of nanofluid effects on the thermal performance with response time of heat pipe

Nanofluids, which are suspensions of nanoparticles in base fluids, have been introduced as new enhanced media for heat transfer, especially during the last decade. An experimental investigation has been carried out on nanofluid effects on the thermal performance of a medium-sized cylindrical meshed heat pipe, in both transient and steady states. Thermal resistance and response time of the heat pipe are the characteristics of transient and steady states, respectively. In the present study, the response time is defined based on the variation of the heat pipe surface temperature. Suspensions of silver nanoparticles in DI-water were utilized in various concentrations of 50, 200 and 600 ppm. The experiments have been performed under heat rates in the range 300–500 W. The least utilized concentration of nanofluid resulted in the best performance. Some discussions are presented for reverse and negative effects of high concentration nanofluids. Whereas the heat pipe operation with DI-water was more stable, by using nanofluids, the thermal resistance and response time of the heat pipe decreased up to 30% and 20%, respectively, compared to DI-water.

Modeling of heat and mass transfer in the liquid film of rotating heat pipes

Axially rotating heat pipes are devices used to dissipate heat flux through the latent heat of phase change of a fluid at saturation state. In these systems, centrifugal forces due to heat pipe rotation allow the liquid to flow. The present study focuses on cylindrical heat pipes due to the simplicity and cost of the manufacturing of theses devices. This paper presents a steady-state model of heat and mass transfers along the rotating heat pipe. An optimization procedure, which permits to work at a given saturation temperature and so allows to study the parameters influence for given thermophysical properties, has been developed to perform the calculations. The hypotheses of the model are also discussed and examined. Finally, a parametric study has been carried out, using water as a working fluid, to highlight the influence of parameters such as rotational speed, temperature difference between the evaporator wall and the vapour, or the mass of fluid, on the heat transfers. Finally, the operating conditions conducive to maximal performance have been described.

Effect of droplets generation in the boiling limit conditions: Heat pipe visualization

Heat pipe devices, for their typical working mechanisms, are particularly suitable for zero gravity applications, and have also been considered for applications in space satellites with nuclear propulsors, thanks to the absence of mobile systems for the coolant fluid circulation. The present work reports the results of the experimental tests carried out on a conventional cylindrical heat pipe designed to investigate on possible thermal-hydraulic performance improvements and to evidence the importance of the droplets generation that determines the boiling limit and consequently the heat transport capability. Some of the tests showed that the droplets generation, due to the nucleate boiling regime, can cause a liquid recirculation and a consequent reduction of liquid evaporation with a heat transport capability lower (about 250%) than that estimated from capillary (wicking) limit conditions. For this reason, generally, to predict more realistically the capability for given wick size, it is not sufficient to know the tilt angle, the total length and the diameter of the tube. Some short grooves (cuts of 15 mm in length) made in the screens at the end of the evaporating zone demonstrate their efficiency to increase the heat flux values more than 2 or 3 W/cm 2. The same heat flux increasing, concerning the wick-wall contact improvement, is obtained thickening the coils of the spring used to tight the wick against the internal tube wall.

Forming Process and Mechanism of Microcapillary Grooves of Miniature Cylindrical Heat Pipe

Miniature heat pipe is one of promising means of removing high heat flux for electronics cooling in limited available space. How to produce narrow and deep microcapillary grooves is the critical issue in promoting performance of axially grooved miniature heat pipe. In this work, the microcapillary grooves for miniature cylindrical heat pipe was fabricated by using oil-filled high speed spinning method. The forming process of microcapillary grooves was investigated. The influence of technical parameters on characteristics of microcapillary grooves was analyzed and the optimal technical parameters were found out. The experimental results show that drawing speed ranging from 48cm/min to 64cm/min, spinning ball number of 4 and big distance between the end of multi-tooth mandrel and the center of spinning balls within 3.7 mm should be chosen to obtain narrow and deep microcapillary grooves. In addition, the forming mechanism was investigated by observing microstructure of cross-section of microcapillary grooved tubes.

Performance characteristics of cylindrical heat pipes with multiple heat sources

Abstract A steady-state analytical model for cylindrical heat pipes subject to either constant heat flux or convective cooling is presented. The proposed model couples two-dimensional heat conduction in the heat pipe’s wall with the liquid flow in the wick and the vapor hydrodynamics, and is capable of modeling multiple uniform heat sources (evaporators). A parametric study on the effect of axial heat conduction in the heat pipe wall was carried out. It was found that the exclusion of the effects of axial heat conduction in wall can result in an overestimate of the pressure drops in heat pipe by about 10%, depending on the heat pipe specifications. A simple method was developed to predict and quantify the criteria for the importance of the axial heat conduction in the heat pipe’s wall. A significant saving in computational time is achieved by use of the proposed analytical model compared to full numerical simulations. The developed model provides a useful tool to evaluate the capillary limit of the cylindrical heat pipes and can be used for optimization and design applications.

Thermal Performance of cylindrical Heat Pipe Using Nanofluids

A cylindrical copper heat pipe with a 19.5 mm outer diameter and a 400.0 mm length was filled with three different working fluids and tested for different heat inputs in the range of 100-250 W. The working fluids tested were de-ionized water, silver-water colloid, and copper water. Experimental results showed that the wall temperature reduction obtained was 3-27° C. The efficiency of the heat pipe was enhanced by 14 % as compared with the heat pipe filled with the base fluid. Furthermore, it was found that an increase in the metal fraction in copper-water nanofluids lead to enhancement in thermal efficiency of the heat pipe. Thermal conductivities of copper-water nanofluids were measured, showing a 30% enhancement with a 0.1 wt % of copper nanoparticles.

DEVELOPMENT OF ADVANCED MINIATURE COPPER HEAT PIPES FOR A COOLING SYSTEM OF A MOBILE PC PLATFORM

At present, miniature heat pipes are widely used for cooling systems of mobile PCs. This paper presents an overview of the development of miniature heat pipes (MHP) with a sintered copper wick with emphasis to different ways of heat pipe wick optimization. Several designs of cylindrical heat pipes with 4-, 5-, and 6-mm outer diameter and 200-mm length, which are the most attractive for the CPU cooling system, have been tailored, tested, and compared to theoretical values in this study to evaluate its thermal performances. A new design of 4and 5-mm-outer diameter miniature copper heat pipes with innovative wicks is suggested as a promising candidate for present and future cooling systems of high-power mobile PC platforms.

Effect of working fluid on the performance of a miniature heat pipe system for cooling desktop processor

The heat transfer performance of a miniature heat pipe system (MHPS) used for cooling a desktop computer processor is presented in this paper. The MHPS consists of 6 parallel cylindrical miniature heat pipes (MHPs) which are connected to a copper block at the evaporator section and which are provided with 15 parallel perpendicular copper sheets at the condenser section, used as external cooling fins. Acetone and ethanol are used as working fluids. As heat source a processor is employed which is attached to the copper block. Heat transfer characteristics of the individual MHPs and the complete MHPS using the two working fluids are experimentally determined. The results show that the maximum and steady state temperature of the processor has been significantly reduced by using MHPs with acetone, more than with ethanol, instead of a conventional finned aluminum heat sink with cooling fan. Additional use of a fan results in a much lower processor temperature for both working fluids.

An investigation of the thermal performance of cylindrical heat pipes using nanofluids

In this work, a two-dimensional analysis is used to study the thermal performance of a cylindrical heat pipe utilizing nanofluids. Three of the most common nanoparticles, namely Al 2O 3, CuO, and TiO 2 are considered as the working fluid. A substantial change in the heat pipe thermal resistance, temperature distribution, and maximum capillary heat transfer of the heat pipe is observed when using a nanofluid. The nanoparticles within the liquid enhance the thermal performance of the heat pipe by reducing the thermal resistance while enhancing the maximum heat load it can carry. The existence of an optimum mass concentration for nanoparticles in maximizing the heat transfer limit is established. The effect of particle size on the thermal performance of the heat pipe is also investigated. It is found that smaller particles have a more pronounced effect on the temperature gradient along the heat pipe.

Current Developments in Heat Pipes: An Overview

Heat transfer is one of the most difficult tasks in thermal management of electronic components and directly influences cost, reliability and performances. Heat pipes are efficient heat transfer devices used in many applications, e.g. electronic systems, spacecrafts thermal control, energy collectors, power generation, chemical engineering, air conditioning, engine cooling and others. At normal operating temperatures all heat pipes have heat transfer limits imposed by the capillary transport capabilities in presence of high heat fluxes. Typical wick or capillary structures and common working fluids show advantages in some aspects and disadvantages in others. After a general introduction and a short description of the history of heat pipes developments, an overview of the current intellectual property environment and market is presented to identify trends for future developments. Then an overview of recent patents related to heat pipes technology is presented, with particular reference to: flat heat pipes, flexible heat pipes, innovative wick structures, phase change materials, innovative working fluids. Concerning flat and flexible heat pipes, a number of patents are shown that are characterized by different geometries and materials and, differently from usual cylindrical heat pipes, are particularly suited for use in microelectronics applications. Patents related to innovative wick structures are shown that are aimed to overcome some limitations associated with usual wick structures (woven mesh or sintered powder). Patented heat pipes architectures are also described that are designed to enlarge the operating temperature range by using appropriate phase change materials. In addition, recent patents are shown that exploit the use of binary mixtures as working fluids to improve the heat pipes efficiency compared to conventional pure working fluids.

VAPOR FLOW ANALYSIS IN PARTIALLY-HEATED CONCENTRIC ANNULAR HEAT PIPES

The steady-state laminar and incompressible vapor flow in four partially-heated concentric annular heat pipe (CAHP) is studied. The governing equations are solved numerically, using finite volume approach based on collocated grids. The first order upwind scheme and the QUICK scheme are used in the numerical solution. The vapor pressure distributions and velocity profiles along the annular vapor space are predicted for a number of test cases in the range of low to moderate radial Reynolds numbers. The results show that in a partially-heated annular heat pipe, as the radial Reynolds number increases, a number of recirculation zones may be created at both ends of the evaporator and condenser sections. The size and location of the recirculation zones are predicted and their effects on the performance of an annular heat pipe are discussed qualitatively.

OPERATION LIMITS FOR ROTATING CYLINDRICAL HEAT PIPES

The rotation in cylindrical heat pipes becomes important at high angular velocity, when recirculation cells appear, grow and scatfter in the vapor phase from the evaporator section, reducing the efficiency of the heat pipe and obstructing the phase change. The 1-D formulations are not yet able to provide such estimation. In this work, a steady rotating cylindrical heat pipe is simulated, coupling the vapor zone and porous medium. Several values for angular and linear Reynolds number are shown, for several boundary conditions in the condenser section. A limiting curve for heat pipe operation is presented for each boundary condition.

Analysis of cylindrical heat pipes incorporating the effects of liquid–vapor coupling and non-Darcian transport—a closed form solution

This paper presents a two-dimensional analytical model for low-temperature cylindrical heat pipes. A closed-form solution which incorporates liquid–vapor interfacial hydrodynamic coupling and non-Darcian transport through the porous wick for the first time, is obtained for predicting the vapor and liquid velocity and pressure distributions. In addition, the steady-state vapor and wall temperatures for a given input heat load in the evaporator region and a convective boundary condition in the condenser region, are obtained. The effects of liquid–vapor interfacial hydrodynamic coupling and non-Darcian transport through the porous wick on the vapor and liquid velocity and pressure distributions as well as the heat pipe capillary limit are discussed and assessed. The analytical solutions of the axial vapor and wall temperature distributions, the vapor and liquid pressure distributions, and the centerline vapor velocities compare very well with both experimental and numerical results. This work constitutes for the first time a comprehensive analytical solution which provides closed form solutions for the vapor and liquid flow as well as the operating temperature and the maximum heat removal capability of the heat pipe.

Entropy generation in a heat pipe system

A thermodynamic model of conventional cylindrical heat pipes is developed based on the second law of thermodynamics. Entropy generation, an important parameter of the heat pipe performance, is caused by the temperature difference between the hot and cold reservoirs, the frictional losses in the working fluid flows, and the vapor temperature/pressure drop along the heat pipe. The ambient temperature in the condenser section and the convection heat transfer coefficient in the transport section can be adjusted to minimize entropy generation in the heat pipe system. A detailed parametric analysis is presented in which the effects of various heat pipe parameters on entropy generation are examined.