Mohammed T. Ababneh

On the charging and thermal characterization of a micro/nano structured thermal ground plane

As power densities in electronic devices have increased dramatically over the last decade, advanced thermal management solutions are required. A significant part of the thermal resistance budget is commonly taken up by the heat spreader, which serves to reduce the input heat flux and connect to an increased area for heat removal. Thermal ground planes are devices that address this issue by utilizing two-phase heat transfer achieving higher effective thermal conductivities than conventional solid heat spreaders. This study describes the need for and design of a charging station to accurately dispense the working fluid and a thermal characterization experiment to characterize performance. The design study includes detailed analysis of accuracy and validation of the setup.

Charging Station of a Planar Miniature Heat Pipe Thermal Ground Plane

Thermal ground planes (TGPs) are planar, thin (thickness of 3 mm or less) heat pipes which use two-phase heat transfer. The objective is to utilize TGPs as thermal spreaders in several microelectronic cooling applications. TGPs are innovative high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. Moreover, being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and can operate in high adverse gravitational fields due to nanoporous wicks. A key factor in the design of the TGP is evacuation prior to filling and introduction of the proper amount of working fluid (water) into the device. The major challenge of this work is to fill heat pipes with a total liquid volume of less than 1 ml, without being able to see into the device. The current filling station is an improvement over the current state of the art as it allows for accurate filling of microliter sized volumes. Tests were performed to validate performance of the system and to verify that little to no noncondensable gasses were introduced to the system. Careful calibration of the amount of liquid introduced is important. Therefore, calibration of the burettes utilized for a liquidfill range of 0.01 ml to 100 ml was important. The magnitude of the pressure inside the TGP device is also an important factor. Charging station validation demonstrated the capability of charging TGPs with accuracy of ±1.64 μl. Calibration curves for the burettes and error characterization curves for a range of liquid charging volumes will be presented and discussed in this paper.

Thermo-Fluid Model for High Thermal Conductivity Thermal Ground Planes

The thermal ground plane (TGP) is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the TGP, including the effects of the presence of non-condensable gases (NCGs). Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the TGP decreases when the substrate and/or wick are thicker and/or with the presence of NCGs. Moreover, it was demonstrated that the thermo-fluid model may be utilized to optimize the performance of the TGP by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the TGP. The wick porosity plays an important effect on maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the TGP. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the TGP before reaching the capillary limit is inversely proportional to input power.© 2012 ASME

Thermal-Fluid Modeling For High Thermal Conductivity Heat Pipe Thermal Ground Planes

The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat ...

Charging Station of a Planar Miniature Thermal Ground Plane

Thermal ground planes (TGPs) are flat thin (less than 1 mm thick) heat pipes that can be used as a thermal spreader in a variety of microelectronic cooling applications. Like conventional heat pipes, TGP’s utilize two-phase cooling. Major advantages, include the ability to integrate directly with the microelectronic substrate for a wide range of applications; and the ability to operate in an adverse gravity environment of up to 20g. Other advantages include a very high thermal conductivity, reliability, no moving parts, electrodes, or need for external power. A key factor in the design of the TGP is evacuation prior to filling and introduction of the proper amount of working fluid (water) into the device. The major challenge of this work is to fill heat pipes with a total liquid volume of less than 1 ml, without being able to see into the device. The current filling station is an improvement over the current state of the art as it allows for accurate filling of micro liter sized volumes. Tests were performed to validate performance of the system and to verify that little to no non-condensable gasses were introduced to the system. Careful calibration of the amount of liquid introduced is essential. Therefore, calibration of the burettes utilized for a liquid fill range of 0.1 ml to 100ml was important. The magnitude of the pressure inside the TGP envelope is also an important factor. Calibration curves for the burettes and error characterization curves for a range of liquid charging volumes will be presented and discussed.Copyright

Thermal Modeling and Experimental Validation for High Thermal Conductivity Heat Pipe Thermal Ground Planes

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. In addition to TGPs and flat heat pipes, some investigators refer to similar devices as vapor chambers. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nanoporous wicks) can operate in high adverse gravitational fields. A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor–liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor–liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGPs, and compare with experimental results.

Thermal Modeling for High Thermal Conductivity Thermal Ground Planes

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nano-porous wicks) can operate in high adverse gravitational fields.A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor-liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGP’s, and compare with experimental results.Copyright

Fluid Flow and Capillary Limit in Heat Pipe Thermal Ground Planes

This work shows the solution of the fluid flow and the capillary limit in heat pipe thermal ground planes after solving the temperature field. In addition, the effect of wall shear stress and the interfacial shear stress in the liquid pressure of the TGP is studied. In order to obtain more accurate results it is necessary to solve the velocity and thermal fields in both the liquid saturated wick and the vapor. It is also important to account for the mass, momentum and energy balances at the interface between the vapor and liquid. Previous work demonstrated that for the TGP’s which utilize water as the working fluid, the Jacob number is very small. A consequence of this is that convection of liquid with the wick is much smaller than conduction and the temperature may be solved independently of the velocity field. These solutions were presented in previous work. A key feature of the thermal model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. One important result uses a solution for the evaporation and condensation rates and hence normal velocities at the interface. The results show that for all of the TGP’s lengths, the ratio between the pressure drop in the vapor and the pressure drop in the liquid is close to zero. Therefore, the pressure drop in the liquid will determine the capillary limit in the TGP.Copyright

Non-Dimensional Analysis for the Fluid Flow in the Flat Heat Pipes

Flat heat pipes (heat spreaders) are similar to cylindrical heat pipes. But they have received significant attention recently because of their advantages over conventional cylindrical heat pipes with regard to their large surface area, isothermal heat delivery, geometry fit. For example, Thermal Ground Planes (TGPs) are flat, thin (less than 3 mm thick) heat pipes which utilize phase change cooling. The goal is to use TGP’s as universal heat spreaders in microelectronic cooling applications. These TGPs will act as a new generation of high-performance, integrated systems to work at a high power density without difficulties from temperature gradients, increased weight, or extra complexity. In addition to being able to dissipate high thermal powers, they have very high effective thermal conductivities and can operate in high adverse gravitational fields due to nano-porous wicks.This work shows the effect of vapor pressure, wall shear stress and the interfacial shear stress in the liquid pressure of the flat heat pipes and a comparison with CFD results. Also, this paper offers a design for flat heat pipe charts that avoids the effects of vapor pressure, wall shear stress and the wick-vapor interface to the liquid pressure for most well-known working fluids.Copyright