Two-phase Heat Spreader Research Articles

Elliptical pillars for the thermal performance enhancement of micro-structured evaporators

In this study, we propose and investigate the employment of elliptical micropillars as a novel evaporator wick structure to boost the performance of capillarity-driven two-phase heat spreaders through high-fidelity numerical modeling that accounts for all pertinent transport mechanisms, including thermocapillary convection. Results reveal that transforming cylindrical micropillars to elliptical ones significantly enhances the overall evaporator performance by improving the hydraulic performance associated with decreased flow resistance and boosted capillary pumping and by improving the thermal performance associated with increased contact (triple) line length. Among the scenarios tested and elliptical aspect ratios swept, it is demonstrated that the heat capacity and heat transfer coefficient can be increased by up to 350% and 52%, respectively. Overall, this study paves the way for the utilization of stretched geometries in the capillary flow direction of pillar-type evaporators by using elliptical micropillars as a demonstrator showcase.

New thermal packaging with a boiling-driven heat spreader for thermal management of the power electronics

• A boiling-driven heat spreader for the power conversion system. • Thermal management performance was compared among 3 heat spreader designs. • Universal support design is a key design factor for multiple heat source problems. • A numerical methodology was proposed for the boiling-driven heat spreader. A new thermal management device with a two-phase flat heat spreader was designed, fabricated, and characterized. The boiling-driven heat spreader (280 mm × 65 mm × 4 mm) is embedded at the thermal packaging air-cooling heat sink for the power electronics. A sintered microporous structure was coated on the surface in the heat source's direction to promote a boiling heat transfer inside the heat spreader. The thermal management devices are tested with the potential applications consisting of the six heat sources placed vertically. As a result, boiling-driven heat spreaders have proven to be an equivalent thermal management method in applications with multiple heat sources and high heat flux targets. In particular, the heat spreader equipped with a dot-type supporter can make a temperature difference between junctions within 7.6℃ at 1500 W in case of air cooling. Compared to the result (Δ T = 28℃ @ 1500 W) of the solid copper plate of the same size, this thermal packaging demonstrated improved thermal management performance. In addition, we propose a numerical analysis for a boiling-driven heat spreader. The numerical analysis results can predict multiple heat sources' temperature distribution more accurately rather than assume a two-phase heat spreader as a solid with high thermal conductivity. This study provides valuable insight into the thermal management and thermal packaging design of power electronic devices. It represents a breakthrough in resolving the reliability problem of thermal packaging technology and temperature non-uniformity.

Thermal performance of a boiling-driven heat spreader using passive flow control

This study investigates a wickless two-phase heat spreader with boiling heat transfer (boiling-driven heat spreaders) for the thermal management of hot spots with a high heat flux of more than 200 W/cm2. Thermal management studies are performed using two different approaches. First, a boiling-driven heat spreader is fabricated to confirm the thermal performance according to the heat flux, orientation, and internal support design. Second, the working mechanism of the heat spreader is established for a visualization test sample (90 × 90 mm2, inner gap of 1 mm), along with the high-speed in-situ visualization of the boiling/condensation phenomena. It is essential to understand the working mechanism of the heat spreader for improving its thermal performance. Consequently, we propose a new advanced boiling-driven heat spreader with improved thermal performance using a passive flow control design. The internal supports provide a bubble escape passage with different pressure drop characteristics. As a result, the thermal performance of the advanced boiling-driven heat spreader is independent of the orientation up to a heat flux of 200 W/cm2, and the thermal resistance is less than 0.17 K/W. Hence, the advanced heat spreader is suitable for dissipating highly concentrated heat in various thermal management applications.

Liquid film boiling enabled ultra-high conductance and high flux heat spreaders

Many of the advanced technologies, e.g., microprocessors, laser and radar, and power transmitters and amplifiers, are becoming increasingly dependent on the ability to dissipate the enormous amount of waste heat in extremely small areas. Two-phase heat spreaders utilizing the liquid-vapor phase change are appealing due to the high performance and potentially low cost. However, thin film evaporation is limited by the capillary dry-out of wicking structures. We demonstrate here an ultra-high thermal conductance and high flux heat spreader enabled by a capillary-driven liquid film boiling process on a hierarchical mesh wicking structure. By manipulating liquid wicking flow and vapor bubble escape, liquid film boiling is demonstrated to have a heat removal capability of 552 W/cm2 from an area of 25 mm2 with a temperature increase of less than 8 K, corresponding to a heat transfer coefficient of 732.5 kW/m2·K. The effective thermal conductivity of the 1-mm-thick two-phase heat spreader is larger than 104 W/m·K.

Flat plate two-phase heat spreader on the thermal management of high-power electronics: a review

Flat plate two-phase heat spreader on the thermal management of high-power electronics: a review

On the effect of structural forces on a condensing film profile near a fin-groove corner

Estimation of condenser performance of two-phase passive heat spreaders with grooved wick structures is crucial in the prediction of the overall performance of the heat spreader. Whilst the evaporation problem in micro-grooves has been widely studied, studies focusing on the condensation on fin-groove systems have been scarce. Condensation on fin-groove systems is actually a multi-scale problem. Thickness of the film near the fin-groove corner can decrease to nanoscale dimensions, which requires the inclusion of nanoscale effects into the modeling. While a few previous studies investigated the effect of dispersion forces, the effect of structural forces has never been considered in the thin film condensation modeling on fin-groove systems. The present study utilizes a disjoining pressure model which considers both dispersion and structural forces. The results reveal that structural forces are able to dominate dispersion forces in certain configurations. Consequently, by intensifying the disjoining pressure, structural forces lead to a sudden change of the film profile (slope break) for subcooling values which are relevant to engineering applications.

Design, fabrication, and performance evaluation of a novel orientation independent and wickless heat spreader

Concentrated heat loads from electronic components present significant thermal management challenges in defense, space, and commercial applications. Liquid-vapor phase-change promises efficient heat dissipation due to the high latent heat of vaporization of the working fluid. Here we report a novel wickless two-phase heat spreader wherein an aqueous solution of surface-active ionic liquid (SAIL) is completely filled within a narrow gap. Unlike typical two-phase passive devices in literature, our device does not rely on gravity or wicks for fluid recirculation. The force of repulsion due to the interaction of SAILs adsorbed at the liquid-vapor interfaces of thin-liquid films contained between neighboring bubbles nucleating at the hotspots is strong enough to cause bubble departure at all orientations. High-speed bubble visualization and infrared thermography suggest that these non-coalescing bubbles quickly spread over a larger area and condense to release the heat to a sink placed at a gap of 2.5 mm. This passive mechanism of heat dissipation demonstrates orientation independent and near-isothermal performance from 0.5−1.1 MW/m2 in the two-phase regime. We believe that the orientation independent performance of this easy to fabricate passive device provides a platform to design next generation thermal management strategies for earth and microgravity applications.

Experimental investigation of a confined flat two-phase thermosyphon for electronics cooling

A novel type of two-phase heat spreader based on a flat confined thermosyphon is proposed for electronics cooling applications. Two wickless flat copper-water heat pipes with an inner thickness of 3 mm were experimentally investigated for two-phase flow visualizations and characterization of thermal performance. The effects of heat input, filling ratio, inclination, and saturation temperature were studied. Experimental results show that the confinement of the fluid inside the heat spreader induces confined boiling phenomenon with a strong coupling between condensation and boiling mechanisms. They also highlight an enhancement of heat transfer and interesting performance such as high heat transfer capability (tested up to 10 W/cm2 with a corresponding thermal resistance around 0.07 K/W at an optimum filling ratio), low sensitivity to inclination and higher degree of freedom on heat sources location compared to a classical thermosyphon.

Effect of charging ratio on thermal performance of a miniaturized two-phase super-heat-spreader

The effects of charging ratio on the thermal performance of a thin novel flat heat pipe (FHP) heat spreader under natural air convection were investigated systematically. Temperature difference and temperature distribution along the central axis of the flat heat pipe were measured carefully. Results show that there exists an optimal charging range for the FHP and a very small temperature difference was achieved, demonstrating superior heat spreading capability. Furthermore, a temperature inverse increase was observed in the condensation section. In addition, within the range of optimal charging ratios the flat heat pipe was insensitive to the gravity force. All the results indicate this novel type of two-phase heat spreader is very suitable for mobile electronics cooling in view of its high-performance as well as its easy processing.

Thermal Spreading Analysis of Rectangular Heat Spreader

A unique nondimensional scheme that employs a source-to-substrate “area ratio” (e.g., footprint), has been utilized for analytically determining the steady-state temperature field within a centrally-heated, cuboidal heat spreader with square cross-section. A modified Laplace equation was solved using a Fourier expansion method providing for an infinite cosine series solution. This solution can be used to analyze the effects of Biot number, heat spreader thickness, and area ratio on the heat spreader's nondimensional maximum temperature and nondimensional thermal spreading resistance. The solution is accurate only for low Biot numbers (Bi < 0.001); representative of highly-conductive, two-phase heat spreaders. Based on the solution, a unique method for estimating the effective thermal conductivity of a two-phase heat spreader is also presented.

Prediction of the maximum heat transfer capability of two-phase heat spreaders – Experimental validation

The present paper is devoted to an experimental study to determine the thermal behaviour of a two-phase heat spreader (TPHS) with micro-grooves. The proposed application is the cooling of fuel cell systems. This TPHS aims at reducing the volume of actual cooling systems and to homogenize the temperature in the hearth of fuel cells. The TPHS is flat with a wide evaporating area (190 × 90 mm 2) compared to the condenser area (30 × 90 mm 2). It has been tested with three working fluids: water, methanol and n-pentane. Experimental results obtained with methanol show a temperature difference lower than 1.6 K on the entire evaporator area for a heat transfer rate equal to 85 W and a working temperature equal to 70 °C. The TPHS has been tested in both horizontal and vertical favourable orientation (thermosyphon orientation). The temperature field is similar in both cases for heat transfer rates lower than 155 W. In horizontal orientation, a confocal microscope is used to measure the meniscus curvature radius along the grooves. A two-phase flow model allowing the calculation of the meniscus radius, the liquid and vapour pressures and the liquid and vapour velocities along the TPHS is developed. The comparison between experimental and model results shows the good ability of the numerical model to predict the meniscus curvature radii from which the maximum heat transfer capability of the TPHS is depending.

Enhanced boiling heat transfer simulation from structured surfaces: Semi-analytical model

A semi-analytical model of the bubble dynamics is proposed based on the experimental results reported in the literature on boiling from porous enhanced surfaces. The model considers the ‘flooded mode’ regime of enhancement boiling and is validated for data covering a range of tunnel and pore dimensions. The dynamic model accounts for the temporal evaporation rate variation inside tunnels to arrive at the latent heat flux due to internal evaporation and frequency of bubble formation. The population density is predicted using an empirical formulation, and in turn used to estimate the total heat flux from the porous enhanced surface. The model predicts the heat flux for pool boiling from structured surfaces within ±30% of the experimental data. The model is subsequently used in the prediction of the thermal performance of a novel two-phase heat spreader that employs porous structured surfaces for enhancing boiling heat transfer.

Two-Phase Heat Spreaders Utilizing Microfabricated Boiling Enhancement Structures

Results from various enhancement techniques to improve the performance of a recently developed two-phase heat spreader are reported. The spreader has a central evaporator section, with an integrated condenser along the edges. A micro-fabricated three-dimensional enhancement structure is employed to improve the heat transfer performance of the spreader. This study considers the performance of several liquid coolants to be used with the device and evaluates the effect of initial pressures on the thermal performance of the spreader plate. Liquids with lower boiling points were found to result in lower wall temperatures of the spreader plate due to earlier onset of boiling. Studies also showed an improvement in heat transfer performance with increase in stack height of the enhancement structures used in the evaporator section.

Orientation independent two-phase heat spreaders for space constrained applications

This paper presents a novel orientation independent two-phase heat spreader for constrained space applications.11Patent pending. The two-phase spreader plate has a central evaporator section with an integrated condenser along the edges. A micro-fabricated three-dimensional enhancement structure is employed to improve the heat transfer performance of the spreader plate. The thermal performance of the system is characterized at different power levels and also for various inclination angles. At the maximum heat flux of 6.3W/cm2 under natural convection conditions, the junction temperature of the two-phase spreader was found to be 47.6 °C less than the junction temperature predicted for an equivalent solid metal plate subjected to similar boundary conditions. The performance of the spreader plate is found to be orientation independent as long as the enhancement structure in the evaporator remains flooded with the coolant liquid.

Single chamber compact two-phase heat spreaders with microfabricated boiling enhancement structures

Presents the thermal performance evaluation of a compact single-chamber two-phase heat spreader. The heat spreader setup has a central evaporator section with integrated fins for cooling along the edges. The evaporator employs a micro-fabricated three-dimensional (3-D) copper structure for enhancing boiling heat transfer. The thermal performance of the system was characterized at various power levels and condenser cooling conditions. The size of the boiling enhancement structure and effects of liquid fill volumes on performance were also investigated. Incorporation of the enhancement structure resulted in an improvement in the spreader thermal performance by decreasing the wall temperature at the evaporator by 8/spl deg/C, for a power dissipation of 36 W/cm/sup 2/ at an air speed of 1 m/s. The maximum heat flux obtained based on a maximum evaporator temperature of 75/spl deg/C for an air speed of 1 m/s was 42.5 W/cm/sup 2/. Variation in the liquid fill volume showed negligible effect on the maximum temperature at the evaporator, as long as the enhanced structure was fully flooded.