Miniature Heat Pipe Research Articles

Micro and miniature heat pipes – Electronic component coolers

The time of beginning of heat pipe science was near 40 years ago with first heat pipe definition and prediction of most simple cases. Micro and miniature heat pipes have received considerable attention in the past decade. The interest stems from the possibility of achieving the extremely high heat fluxes near 1000 W/cm 2, needed for future generation electronics cooling application. Now at the computer age some changes of basic equations are performed, more powerful predicting methods are available with increasing awareness of the complexity of heat pipes and new heat pipe generations. But even today heat pipes are still not completely understood and solution strategies still contain significant simplifications. Micro and miniature heat pipes have some additional complications due to its small size. A short review on the micro and miniature heat pipes is presented.

Design and performance optimization of miniature heat pipes in LTCC

In this paper, performance evaluation of miniature heat pipes in LTCC was made by numerical analysis, and the optimum miniature heat pipe design was defined. The effect of the groove depth, width and vapor space on the heat transfer capacity of miniature heat pipes was analyzed. Prototypes of the miniature heat pipes in LTCC were fabricated. Preliminary charging, sealing and thermal measurement of the miniature heat pipes were performed, and the challenges were discussed.

Conjugate Heat Transfer of a Disk-Shaped Miniature Heat Pipe

ABSTRACT This study employs a computational fluid dynamics (CFD) software package to obtain a two-phase three-dimensional flow field and conjugate heat transfer results for a disk-shape miniature heat pipe (DMHP) that is used to dissipate heat from a laser diode. A mixture model is introduced into the governing equations for calculating the quality of mixture in the two-phase flow field. Predicted thermal resistances are presented at three different fluid charge volumes of 18%, 55%, and 92% at different heating powers of the laser diode. The predicted thermal resistance of the DMHP agrees well with measured results given in our previous study [1].

Double-sided IPEM cooling using miniature heat pipes

Integrated power electronic module (IPEM) planar interconnect technologies offer opportunities for improved thermal management by allowing thermal access to the upper side of the power devices. In this paper, the feasibility of using miniature heat pipes to achieve effective double-sided cooling is investigated by analyzing the complete thermal circuit associated with the power device. A nominal case was modeled using the ANSYS(tm) finite element software in a single-sided and double-sided configuration. The numerical model predicted that the double-sided configuration would result in a 13/spl deg/C reduction in the maximum temperature compared to the single-sided case, for the same 100 W/cm/sup 2/ power dissipation in the semiconductor die. This corresponds to a 15% decrease in the maximum temperature rise relative to ambient or a similar increase in allowable power dissipation. Twenty-eight percent of the heat was removed from the upper side of the IPEM in the double-sided case. An additional benefit associated with double-sided cooling was a significant reduction in the spatial temperature gradients along the surface of the IPEM which would translate to lower thermally induced stress and higher reliability. The sensitivity of the numerical predictions to important parameters; including the dielectric conductivity, contact conductance, and heat sink characteristics are numerically investigated. An experimental fixture was fabricated and used to measure a miniature rectangular heat pipe's performance characteristics and the solder joint resistance at its evaporator and condenser interfaces in order to validate the numerical model inputs and demonstrate the required heat pipe capacity. The tested heat pipe was limited to approximately 80 W/cm/sup 2/ heat flux in a vertical, evaporator-over-condenser orientation. This limit was not observed in a vertical, gravity-assisted orientation for applied heat flux up to 125 W/cm/sup 2/. Equivalent heat pipe resistances of approximately 0.12 and 0.08 K/W were measured in these orientations, respectively. The contact resistance of the indium solder joint was measured and found to be approximately 0.1 cm/sup 2//spl middot/K/W.

An investigation of oscillating motions in a miniature pulsating heat pipe

A mathematical model for predicting the oscillating motion in a pulsating heat pipe is presented. The model considers the thermal energy from the temperature difference between the evaporator and condenser as the driving force for the oscillating motion, which will overcome both the frictional force and the force due to the deformation of compressible bubbles. The results show that the oscillating motion depends on the temperature difference between the condensing section and evaporating section, the working fluid, the operating temperature, the dimensions, and the filled liquid ratio. The results of this investigation will assist in the development of miniature pulsating heat pipes capable of operating at increased power levels.

Experimental and numerical analysis of the transient response of a miniature heat pipe

The aim of this research is to analyse the behaviour of a miniature heat pipe during transient events. This type of heat pipe is used for thermal control of electronic equipment in space applications. A one-dimensional model has been developed. It allows to calculate the transient evolution of a series of profiles along the heat pipe: liquid and vapour fluxes; liquid and vapour pressures; and wall, liquid and vapour temperatures. It is based on the numerical solution of the differential equations for mass, momentum and energy conservation in the two phases, the equation of thermodynamic equilibrium at the interface, and the energy equation of the wall. This model uses three hydraulic parameters (wick porosity, pore radius and permeability) and three thermal parameters (condenser and evaporator thermal conductances, saturated wick capacitance) which have to be determined from experiments. The model was validated with tests on a 2.1 × 10 mm 2 rectangular cross section flat copper-water heat pipe (length = 200 mm, wick: metallic mesh). Steady-state tests have first been performed to determine the performance characteristics—temperature difference as a function of the heat rate—up to the capillary limit, for different inclination angles. These tests allow to determine the values of the hydraulic parameters and the thermal conductances of the heat pipe. As a first approximation, constant values of the parameters can be used to cover the whole range of the experimental conditions. The transient behaviour has then been investigated, resulting from a step variation of the evaporator heat rate.

Enhancement of thermal performance in a sintered miniature heat pipe

The performance of a sintered miniature heat pipe is enhanced. With the capillary limitation, porosity takes priority over the wick structure parameters that would affect the heat transfer capacity. Since sintered dendritic copper powder has higher porosity, it is used to mix with pore former (Na2CO3) in experiments for increasing porosity, and hence enhancing the thermal performance. The results show that, for a heat pipe with a 3mm outer diameter and 200 mm effective length, the heat transfer rate is up to 16.5W and the thermal resistance is 0.9°C/W. In comparison with the unmixed case, the performance increases about 40%.

Axial heat conduction model to predict maximum heat remove of miniature heat pipe based on grey model theory

Computer chip is always accompanied by the increase of heat dissipation and miniaturization. The miniature heat pipes are widely used in notebook computer to resolve the heat dissipation problems. Maximum heat removed model of miniature heat pipes building by grey model is presented. In order to know the foundation for modeling, the smooth grade of error examination is inquired and the accuracy of grey relational grade is verified. The model can be used to select a suitable heat pipes to solve electric heat problems in the future. Final results show that the grey model only needs four experiment data and its error value is less than 10%, further, it is better than computational fluid dynamics (CFD) model.

Experimental and Theoretical Analysis on Enhanced Flat Miniature Heat Pipes

A combined experimental and analytical study is realized to verify the miniature heat pipe (MHP) concept for cooling high-power dissipation electronic components and determine the potential advantages of constructing arrays of miniature channels as an integrated part of a heat pipe. A theoretical model is developed to predict the capillary limit and the thermal resistance of the flat MHP (FMHP). The model includes flow analysis and thermal study of the heat transfer modes in the capillary structures. To verify the mixed capillary structure concept and validate the proposed analytical model, an FMHP including a mixed capillary structure, rectangular grooves and screen wick, is manufactured, and a filling apparatus is developed charge such FMHPs. The heat transfer improvement obtained by comparing the heat pipe thermal resistance to the heat conduction thermal resistance of a copper plate having the same dimensions as the tested heat pipe is demonstrated for different heat input flux rates.

Experimental Investigation of Flat Miniature Heat Pipes with Three Kinds of Micro Grooves

The flat miniature heat pipes (FMHPs) with three kinds of micro grooves, including two kinds of deep rectangular grooves and one kind of zigzag groove, are fabricated employing the electric-discharging machining wire cutting method. In order to study systematically the influence of gravitation on the performance of the heat pipes, axial grooves are distributed on only one side of the interior perimeter of the FMHPs. The heat pipes with different charging ratios and working fluids are tested under different tilt angles, so as to study the influence of factors such as the gravitation, charging ratio, groove structure, etc., on the performance of the FMHPs. Experimental results concluded that the influence of the gravitation on the performance of the FMHPs is obvious in the axial direction, while in the radial direction the influence is obvious only if the charging ratio is larger. The optimal charging ratio of the FMHPs with deep grooves is 2.3, and the FMHPs with deep grooves have excellent heat transfer performance.

Disk-shaped miniature heat pipe (DMHP) with radiating micro grooves for a to can laser diode package

A mounting base integrated with disk-shaped miniature heat pipe (DMHP) is designed for laser diode TO can package in the present study. The heat spreading performance of the disk-shaped miniature heat pipe is also presented. The present mounting base is made of aluminum (6061 T6) other than the conventional TO can package with oxygen free copper. The mounting base shows different thermal resistance with different working fluid charge volume. By optimizing the working fluid charge volume, the thermal resistance of the present mounting base will become lower than the conventional base with an oxygen free copper disk for TO can package. Moreover, this novel design can be manufactured on a massive scale and the fabrication cost can thus be effectively reduced.

Silicon heat pipes used as thermal spreaders

An increase in power densities in electronic devices is a direct consequence of their miniaturization and performance improvements. We propose the use of flat miniature heat pipes with micro capillary grooves to spread heat flux across a heat sink. Models of the structure were developed to calculate heat transfer limitations and temperature drops. A brass/water prototype was fabricated to demonstrate the feasibility of heat spreading using this type of heat pipe. Simulation and experimental results obtained with the prototype are described. The dissipated power reached 110 W/cm/sup 2/ without heat transfer limitations. The results are then extended to the design of this type of heat pipe in silicon. Thermal performance was calculated. Simulation, experimental results and the fabrication process are presented.

Analytical and experimental investigation on the operational characteristics and the thermal optimization of a miniature heat pipe with a grooved wick structure

A mathematical model for heat and mass transfer in a miniature heat pipe with a grooved wick structure is developed and solved analytically to yield the maximum heat transport rate and the overall thermal resistance under steady-state conditions. The effects of the liquid–vapor interfacial shear stress, the contact angle, and the amount of initial liquid charge have been considered in the proposed model. In particular, a novel method called a modified Shah method is suggested and validated; this method is an essential feature of the proposed model and accounts for the effect of the liquid–vapor interfacial shear stress. In order to verify the model, experiments for measuring the maximum heat transport rate and the overall thermal resistance are conducted. The analytical results for the maximum heat transport rate and the total thermal resistance based on the proposed model are shown to be in close agreement with the experimental results. From the proposed model, numerical optimization is performed to enhance the thermal performance of the miniature heat pipe. It is estimated that the maximum heat transport rate of outer diameter 3 and 4 mm heat pipes can be enhanced up to 48% and 73%, respectively, when the groove wick structure is optimized from the existing configurations. Similarly, the total thermal resistance of these heat pipes can be reduced by 7% and 11%, respectively, as a result of optimization.

High performance flat miniature heat pipes fabricated by UD-LIGA process

The objective of this work is to process a high performance flat miniature heat pipes (FMHP). The FMHP is fabricated by ultra-deep LIGA (UD-LIGA) process developed by SRRC. Moreover, the performance of the FMHP is theoretically estimated. The dimensions of microgrooves on the FMHP are 1 mm in depth, 100 μm in width, and 60 mm in length. The theoretically predicted results show that the dissipated heat flux can reach to 391 W at a working temperature of 100 °C.

High performance miniature heat pipe

High performance miniature heat pipes are developed for the cooling of high heat flux electronics using new capillary structures made of a folded copper sheet fin. Using the folded sheet fin, capillary flow channels with fully and partially opened grooves are made by electric-discharge-machining technique. It is easy to form the capillary grooves as dense as desired through the present fabrication techniques. Heat pipes with two different capillary structures and different fill amounts are tested in the horizontal orientation. Three heating modes of the evaporator are simulated by activating different numbers of chip resistors. The heat pipe with partially opened groove wick performs better than that with fully opened groove wick. The condenser heat transfer coefficient is higher by 120% or greater in the case of the former wick type compared to the latter at an operating temperature of 110 °C. Heat fluxes higher than 140 W/ cm 2 are achieved using concentrated heating modes.

Novel Groove-Shaped Screen-Wick Miniature Heat Pipe

Miniature heat pipes (MHP) are passive heat transport devices mainly considered in electronics packaging for high heat flux acquisition and transport. Their applications could include thermal management of a variety of electronic devices such as computer processors and laser diodes. A novel screen-wick design suitable for MHP is described. The new design promises improved performance and ease of fabrication and is recommended in place of the familiar forms of rectangular groove design MHP. Design and fabrication details, along with steady-stale horizontal orientation performance test results of a proof-of-concept rectangular copper-water heat pipe are presented. Heat flux, temperature difference, and heat transfer coefficient data are compared with the literature data of a comparable flat MHP with a machined groove wick. The performance of the new design matches the comparable groove design. The highest applied evaporator heat flux at the heater surface was 115 W/cm 2 at an operating temperature of 90°C and evaporator-to-adiabatic temperature difference of 37°C. Results on the heat transfer coefficients are also presented.

Improving thermal performance of miniature heat pipe for notebook PC cooling

A miniature heat pipe (MHP) with woven wire wick was used for cooling a notebook PC. The cross-sectional area of the pipe is reduced by about 30% of the original, when the diameter of the MHP is pressed from 4 to 2 mm for packaging in a notebook PC. In the present study, a test of the MHP has been performed in order to review the thermal performance by varying pressed thickness, total length of MHP, wall thickness, heat flux and inclination angle. New wick types were considered for overcoming low heat transfer limits, which occur when the MHP is pressed to a thin plate. Through a performance test, the limiting thickness of pressing is shown to be within the range of 2–2.5 mm. When the wall thickness of 0.4 mm is reduced to 0.25 mm for minimizing conductive thermal resistance through the wall of heat pipe, the heat transfer limit and thermal resistance of the MHP were improved about 10%. While the thermal resistance of the MHP with central wick type is lower than that of the MHP with circular oven wire wick, the thermal resistance of the MHP with composite wick of woven/straight wire is higher than that of the MHP with circular woven wire wick. From the performance test conducted on the MHP cooling modules with woven wicks, it is seen that the T jc (junction temperature of the processor) satisfies a demand condition of being between 0 and 100 ° C. This shows the stability of the MHP as a cooling system of notebook PCs.

Thermal performance of sintered miniature heat pipes

Investigation has been carried out on the thermal performance of sintered miniature heat pipes with 3 mm outer diameter. In the theoretical analysis, the influence of wick structure parameters is determined by using the theory of capillary limitation. As a result, the degree of importance is found to be as follows: porosity, powder diameter and thickness of wick structure. In the experiments, heat pipes with sintered dendritic copper powder wicks were fabricated and tested. The maximum heat transfer rate is about 13 W with an effective heat pipe length of 20 cm. By adopting the formulae developed for both sintered spherical powder and fiber and adjusting their proportion, the agreement between experimental results and prediction is found to be quite good in the tested operation temperature range.

Application of Miniature Heat Pipe for Notebook PC Cooling

Miniature heat pipe(MHP) with woven-wired wick was used to cool the CPU of a notebook PC. The pipe with circular cross-section was pressed and bent for packaging the MHP into a notebook PC with very limited compact packaging space. A cross-sectional area of the pipe is reduced about 30% as the MHP with 4mm diameter is pressed to 2mm thickness. In the present study a performance test has been performed in order to review varying of operating performance according to pressed thickness variation and heat dissipation capacity of MHP cooling module that is packaged on a notebook PC. New wick type was considered for overcoming low heat transfer limit when MHP is pressed to thin-plate. The limiting thickness or pressing is shown to be within the range of 2mm∼2.5mm through the performance test with varying the pressing thickness. When the wall thickness of 0.4mm is reduced to 0.25mm for minimizing conductive thermal resistance through the wall of heat pipe, heat transfer limit and thermal resistance of MHP were improved about 10%. In the meantime, it is shown that the thermal resistance and heat transfer limit for the MHP with central wick type are higher than those of MHP with existing wick types. The results of performance test for MHP cooling modules with woven-wired wick to cool a notebook PC shows the stability as cooling system since T(sub)j(Temperature of Processor Junction) satisfy a demand condition of 0∼100 under 11.5W of CPU heat.

Experimental study on the performance of miniature heat pipes with woven-wire wick

The thermal density of electronic system has been increased continuously because high speed and high density are required for them. The heat dissipation of CPU for a notebook PC has been recently increased to be more than 10 W, but, on the other hand, the available packaging space has been decreased. Therefore, it has become inevitable to perform cooling by using miniature heat pipes (MHPs). In the present study, a new woven-wire-type wick for the MHP is developed, which has a large capillary limit and a high productivity. These MHPs with diameters of 3 mm or 4 mm are applicable to small-sized electronic parts such as CPU of a notebook PC. Because the operational characteristics of MHPs with the diameters of 3 mm or 4 mm are different from those of general medium-size heat pipes, performance tests have been conducted in order to review heat-transfer characteristics and effects of various factors on the performance of MHPs. The design factors under consideration are fill ratio of working fluid, length of heat pipe, length of evaporator and condenser, inclination angle of installation, number of wick strand and thermal load.