Unnikrishnan Vadakkan

Purdue University, West Lafayette, Indiana, United States

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Publications (7)0 Total impact

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    ABSTRACT: A transient, three-dimensional model for thermal transport in heat pipes and vapor chambers is developed. The Navier–Stokes equations along with the energy equation are solved numerically for the liquid and vapor flows. A porous medium formulation is used for the wick region. Evaporation and condensation at the liquid–vapor interface are modeled using kinetic theory. The influence of the wick microstructure on evaporation and condensation mass fluxes at the liquid–vapor interface is accounted for by integrating a microstructure-level evaporation model (micromodel) with the device-level model (macromodel). Meniscus curvature at every location along the wick is calculated as a result of this coupling. The model accounts for the change in interfacial area in the wick pore, thin-film evaporation, and Marangoni convection effects during phase change at the liquid–vapor interface. The coupled model is used to predict the performance of a heat pipe with a screen-mesh wick, and the implications of the coupling employed are discussed.
    International Journal of Heat and Mass Transfer. 01/2010;
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    ABSTRACT: The paper introduces the novel concept of using carbon nano tube (CNTs) based wick structures for high performance heat pipes and vapor chambers. This ongoing research aims to replace the copper wick structures with high conductive CNT wick structures. Individual carbon nanotubes possess extremely high thermal conductivities of the order of 2000-3000 W/m-K. With such a material as the wick in a heat pipe, the effective thermal conductivity of the fluid saturated wick will be significantly higher that a copper-based wick.
    Semiconductor Thermal Measurement and Management Symposium, 2007. SEMI-THERM 2007. Twenty Third Annual IEEE; 04/2007
  • U. Vadakkan, G.M. Chrysler, S. Sane
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    ABSTRACT: A numerical study is performed to characterize the thermal and mechanical performances of silicon/water vapor chambers as heat spreaders for electronics cooling applications and to compare their performance against Cu heat spreaders. 2D flow and energy equations are solved in the vapor and liquid regions, along with conduction in the wall. An equilibrium model for heat transfer and a Brinkman-Forchheimer extended Darcy model for fluid flow are solved in the wick region. In addition to thermal modeling, FEA is also performed to study the impact of the proposed design on die stresses. The study shows that this system can match or thermally perform better than a more standard Cu spreader while also reducing the compressive stress in the Si by as much as 96%. Analysis shows that there are two main factors contributing towards the reduction of stress in the Si die, namely, the better CTE match between the Si die and the Si heat spreader and higher compliance (less stiffness) of the vapor chamber compared to standard heat spreaders. Thus Si vapor chambers provide a good design alternative to a standard Cu heat spreader without compromising on the reliability and performance of the Si.
    Semiconductor Thermal Measurement and Management Symposium, 2005 IEEE Twenty First Annual IEEE; 04/2005
  • Je-Young Chang, Unnikrishnan Vadakkan, Ravi Prasher, Suzana Prstic
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    ABSTRACT: Use of heat pipes (or vapor chambers) is considered as one of the promising technology to extend the capability of air cooling. This paper reports the test results of vapor chambers using two different sets of test heaters (copper post heater and silicon die heater). Experiments were conducted to understand the effects of non-uniform heating conditions on the thermal performance of vapor chambers. In contrast to the copper post heater which provides ideal heating condition, silicon chip package was developed to replicate more realistic heat source boundary conditions of microprocessors. The chip contains three metallic heaters: a 10 × 12 mm heater in order to provide uniform heating, a 10 × 3 mm heater in order to simulate a localized heating, and a 400 × 400 μm heater in order to simulate the hot spots on actual microprocessors. In the experiment, the highest heat flux from the hotspot heater was approximately 690 W/cm2 . Test results indicated that both conduction heat transfer and phase-change phenomena played key roles in the evaporator. The study found that the evaporator resistance was almost insensitive to non-uniform heating conditions, but was clearly dependent on the amount of power applied over the die area. In addition, a simple one-dimensional thermal model was developed to predict the performance of vapor chambers for non-uniform heating conditions and the results were compared against experiments.
    ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems collocated with the ASME 2005 Heat Transfer Summer Conference; 01/2005
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    ABSTRACT: In the present study, the thermal performance of flip chip electronics packages was evaluated by characterizing the amount of voiding present in the Solder Thermal Interface Material (STIM) which is placed between the die and Integrated Heat Spreader (IHS). The study found that the thermal resistance, Rjc (resistance between the Si die and IHS), is dependent upon the amount of voiding present as well as the location of the voiding in the STIM. The study also described the techniques to reduce the STIM voids in flip chip packages and identified the key process parameters to improve the thermal performance. The process parameters varied in this study consisted of STIM thickness, dwell time and temperature, flux weight, and many others. A detailed DOE and statistical analysis were carried out to determine the impact of the parameters mentioned above toward reducing the quantity of voids in the STIM. The analysis showed that for the packages under consideration, the primary process parameters that affect the STIM voiding are cure time, flux weight and TIM thickness. This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
    ASME 2005 Summer Heat Transfer Conference collocated with the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems; 01/2005
  • Unnikrishnan Vadakkan, Suresh V. Garimella, Jayathi Y. Murthy
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    ABSTRACT: A three-dimensional model has been developed to analyze the transient and steady-state performance of flat heat pipes with discrete heat sources. Three-dimensional flow and energy equations are solved in the vapor and liquid regions, along with conduction in the wall. Saturated flow models are used for heat transfer and fluid flow through the wick. In the wick region, the analysis uses an equilibrium model for heat transfer and a Brinkman-Forchheimer extended Darcy model for fluid flow. Averaged properties weighted with the porosity are used for the wick analysis. The state equation is used in the vapor core to relate density change to the operating pressure. The density change due to pressurization of the vapor core is accounted for in the continuity equation. Vapor flow, temperature and hydrodynamic pressure fields are computed at each time step from coupled continuity/momentum and energy equations in the wick and vapor regions. The mass flow rate at the interface is obtained from the application of kinetic theory. Predictions are made for the magnitude of heat flux at which dryout would occur in a flat heat pipe. The input heat flux and the spacing between the discrete heat sources are studied as parameters. The location in the heat pipe at which dryout is initiated is found to be different from that of the maximum temperature. The location where the maximum capillary pressure head is realized also changes during the transient. Axial conduction through the wall and wick are seen to play a significant role in determining the axial temperature variation.
    Journal of Heat Transfer-transactions of The Asme - J HEAT TRANSFER. 01/2004; 126(3).
  • Unnikrishnan Vadakkan, Jayathi Y. Murthy, Suresh V. Garimella
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    ABSTRACT: A stable numerical procedure is developed to analyze the transient performance of flat heat pipes for large input heat fluxes and high wick conductivity. Computation of flow and heat transfer in a heat pipe is complicated by the strong coupling among the velocity, pressure and temperature fields with phase change at the interface between the vapor and wick. A structured collocated finite volume scheme is used in conjunction with the SIMPLE algorithm to solve the continuity, energy and momentum equations. In addition, system pressurization is computed using overall mass balance. The stability of the standard sequential procedure is improved by accounting for the coupling between the evaporator/condenser mass flow rate and the interface temperature and pressure as well as the system pressure. The improved numerical scheme is applied to a flat two-dimensional heat pipe and shown to perform well. Parametric studies are performed by varying the vapor core thickness of the heat pipe and the heat input at the evaporator. The model predictions are validated by comparing the heat pipe wall temperatures against experimental values.
    ASME 2003 Heat Transfer Summer Conference; 01/2003