Abas Abdoli

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACT This article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm−2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm−2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.

Multi-Winglets: Multi-Objective Optimization of Aerodynamic Shapes

Various configurations of aircraft wing tip devices have been investigated by performing 3D aerodynamics analysis. The wing tip device in this study was derived from the wing tips of a soaring bird, featuring three smoothly blended elements. Each multi-winglet configuration was integrated into a complete wing-tail-body aircraft configuration. Geometry of each of the three elements in the multi-winglet was defined using 11 parameters, totaling 33 parameters defining a complete multi-winglet geometry. The current design methodology utilized a second order, 3D geometry generation algorithm based on locally analytical smoothly connected surface patches. This algorithm allows for creation of vastly diverse 3D geometries with minimal number of specified design parameters. A 3D, compressible, turbulent flow, steady state analysis was performed using a Navier-Stokes solver on each configuration to obtain the objective function values. Each configuration was analyzed at a free stream Mach number of 0.25 and at an angle of attack of 11 degrees to mimic the takeoff conditions of a passenger aircraft. Multi-objective optimization was carried out using modeFRONTIER utilizing a radial basis function response surface approximation coupled with a genetic algorithm. Maximizing coefficients of lift and lift-todrag ratio, while minimizing coefficients of drag and the magnitude of the coefficient of moment were the four simultaneous objectives. The multi-winglet concept was shown to have superior performance at subsonic and transonic speeds.

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics With a Hot Spot

The ability of various arrays of micro pin-fins to reduce maximum temperature of an integrated circuit with a 4 × 3 mm footprint and a 0.5 × 0.5 mm hot spot was investigated numerically. Micro pin-fins having circular, symmetric airfoil and symmetric convex lens cross sections were optimized to handle a background uniform heat flux of 500 W cm−2 and a hot spot uniform heat flux of 2000 W cm−2. A fully three-dimensional conjugate heat transfer analysis was performed and a multi-objective, constrained optimization was carried out to find a design for each pin-fin shape capable of cooling such high heat fluxes. The two simultaneous objectives were to minimize maximum temperature and minimize pumping power, while keeping the maximum temperature below 85 °C. The design variables were the inlet average velocity and shape, size and height of the pin-fins. A response surface was generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Stress–deformation analysis incorporating hydrodynamic and thermal loads was performed on the three Pareto optimized configurations. Von-Mises stress for each configuration was found to be significantly below the yield strength of silicon.Copyright

Multi-Objective Design Optimization of Multi-Floor, Counterflow Micro Heat Exchangers

Heat removal capacity, coolant pumping pressure drop and surface temperature non-uniformity are three major challenges facing single-phase flow microchannel compact heat exchangers. In this paper multi-objective optimization has been performed to increase heat removal capacity, and decrease pressure drop and temperature non-uniformity in single-flow microchannels. Three-dimensional (3D) 4-floor branching networks have been applied to increase heat removal capacity of a microchannel from silicon substrate (15×15×2 mm). Each floor has four different branching sub-networks with opposite flow direction with respect to the next one. Each branching network has four inlets and one outlet. However, branching patterns of each of these sub-networks could be different from the others. Conjugate heat transfer analysis has been performed by developing a software package which uses quasi-1D thermo-fluid analysis and a 3D steady heat conduction analysis. These two solvers are coupled through their common boundaries representing surfaces of the cooling microchannels. Using quasi-1D solver significantly decreases computing time and its results are in good agreement with 3D Navier-Stokes equations solver for these types of application. The analysis package is capable of generating 3D branching networks with random topologies. 1341 random cooling networks were simulated using this analysis package. Multi-objective optimization using modeFrontier software was performed using response surface approximation and genetic algorithm. Diameters and branching pattern of each sub-network and coolant flow direction on each floor were design variables of multi-objective optimization. Maximizing heat removal capacity, minimizing pressure drop and temperature non-uniformity on the hot surface were three simultaneous objectives of the optimization. Pareto-optimal solutions demonstrate that thermal loads of up to 500 W/cm2 can be managed with 3D 4-floor microchannel cooling networks.Copyright

Conjugate Analysis of Thin Film Heat Spreaders to Reduce Temperature at Hot Spots

The effects of thin film coating on maximum temperature of integrated electric circuits are investigated. A fully three-dimensional conjugate heat transfer analysis was performed to investigate the effects of thin film material and thickness on the temperature of a hot spot. Two different materials, diamond and graphene nano-platelets were simulated as materials for thin films. The thin film heat spreaders were applied to the top wall of the three optimized arrays of micro pin-fins having circular, airfoil and convex cross sections. The electronic chip with a 4 × 3 mm footprint featured a 0.5 × 0.5 mm hot spot located on the top wall which was exposed to a uniform high-level background heat flux. The effective area of coverage of the thin films was also investigated computationally. It was found that thin film heat spreaders significantly reduce the hot spot temperature, allowing for increased thermal loads and therefore increased performance. Furthermore, it was found that thickness of the thin film heat spreader does not have to be greater than a few tens of microns.Copyright

Thermo-Fluid-Stress-Deformation Analysis of Two-Layer Microchannels for Cooling Chips With Hot Spots

Two-layer single phase flow microchannels were studied for cooling of electronic chips with a hot spot. A chip with 2.45 2.45 mm footprint and a hot spot of 0.5 0.5 mm in its center was studied in this research. Two different cases were simulated in which heat fluxes of 1500 W cm 2 and 2000 W cm 2 were applied at the hot spot. Heat flux of 1000 W cm 2 was applied on the rest of the chip. Each microchannel layer had 20 channels with an aspect ratio of 4:1. Direction of the second microchannel layer was rotated 90 deg with respect to the first layer. Fully three-dimensional (3D) conjugate heat transfer analysis was performed to study the heat removal capacity of the proposed two-layer microchannel cooling design for high heat flux chips. In the next step, a linear stress analysis was performed to investigate the effects of thermal stresses applied to the microchannel cooling design due to variations of temperature field. Results showed that twolayer microchannel configuration was capable of removing heat from high heat flux chips with a hot spot. [DOI: 10.1115/1.4030005]

Human heart conjugate cooling simulation: Unsteady thermo-fluid-stress analysis

The main objective of this work was to demonstrate computationally that realistic human hearts can be cooled much faster by performing conjugate heat transfer consisting of pumping a cold liquid through the cardiac chambers and major veins while keeping the heart submerged in cold gelatin filling a cooling container. The human heart geometry used for simulations was obtained from three-dimensional, high resolution CT-angio scans. Two fluid flow domains for the right (pulmonic) and left (systemic) heart circulations, and two solid domains for the heart tissue and gelatin solution were defined for multi-domain numerical simulation. Detailed unsteady temperature fields within the heart tissue were calculated during the conjugate cooling process. A linear thermoelasticity analysis was performed to assess the stresses applied on the heart due to the coolant fluid shear and normal forces and to examine the thermal stress caused by temperature variation inside the heart. It was demonstrated that a conjugate cooling effort with coolant temperature at +4°C is capable of reducing the average heart temperature from +37°C to +8°C in 25 minutes for cases in which the coolant was steadily pumped only through major heart inlet veins and cavities.

Optimized Multi-Floor Throughflow Micro Heat Exchangers

Multi-floor networks of straight-through liquid cooled microchannels have been investigated by performing conjugate heat transfer in a silicon substrate of size 15×15×1 mm. Two-floor and three-floor cooling configurations were analyzed with different numbers of microchannels on each floor, different diameters of the channels, and different clustering among the floors. Thickness of substrate was calculated based on number of floors, diameter of floors and vertical clustering. Direction of microchannels on each floor changes by 90 degrees from the previous floor. Direction of flow in each microchannel is opposite of the flow direction in its neighbor channels.Conjugate heat transfer analysis was performed by developing a software package which uses quasi-1D thermo-fluid analysis and a 3D steady heat conduction analysis. These two solvers are coupled through their common boundaries representing surfaces of the cooling microchannels. Using quasi-1D solver significantly decreases overall computing time and its results are in good agreement with 3D Navier-Stokes equations solver for these types of application.Multi-objective optimization with modeFRONTIER software was performed using response surface approximations and genetic algorithm. Maximizing total amount of heat removed, minimizing coolant pressure drop, minimizing maximum temperature on the hot surface, and minimizing non-uniformity of temperature on the hot surface were four simultaneous objectives of the optimization. Pareto-optimal solutions demonstrate that thermal loads of 800 W cm−2 can be effectively managed with such multi-floor microchannel cooling networks. Two-floor microchannel configuration was also simulated with 1,000 W cm−2 uniform thermal load and shown to be feasible.Copyright

Effect of cooling fluids on high frequency electric and magnetic fields in microelectronic systems with integrated TSVs

A fully 3D conjugate numerical analysis was performed to reveal the effects of air, R134a refrigerant and water on electromagnetic fields of electronic cooling designs made of arrays of micro pin-fins with integrated Through-Silicon-Vias (TSVs). The integrated TSV cooling configuration included 8 cylindrical TSVs with 150m diameter each and 200m height. The external dimensions of the silicon substrate were 900700280m. Each TSV encapsulated four equally spaced copper vias each having a diameter of 40m. The impacts of the presence of the stationary cooling fluids without heat transfer on TSVs electric and magnetic fields were examined for five different frequencies; 100MHz, 500MHz, 1GHz, 5GHz and 10GHz. Then, separately, the effects of moving cooling water with temperature-dependent physical properties were studied while exposing the cooled micro pin-fin array to a uniform heat flux of 500Wcm2. For the case of stagnant and moving cooling fluids it was found that water influences the electric field twice as much as either R134a or air and that this influence decreases only negligibly with the increase in frequency of the electric current passing through the TSVs. The influence of the presence of the stagnant and moving cooling fluids on the magnetic field is orders of magnitude smaller and reduces rapidly with the increased frequency. The effects of three different cooling fluids (air, R134a and water) on the magnetic and electric fields of arrays of micro pin-fins having integrated TSVs were studied at five different frequencies using fully 3D conjugate heat transfer/electro-magneto-hydrodynamics analyses.The coolant can have significant impact on the electric field of such micro-electronic packages.

Human heart preservation analyses using convective cooling

Purpose – Currently, human hearts destined for transplantation can be used for 4.5 hours which is often insufficient to test the heart, the purpose of this paper is to find a compatible recipient and transport the heart to larger distances. Cooling systems with simultaneous internal and external liquid cooling were numerically simulated as a method to extend the usable life of human hearts. Design/methodology/approach – Coolant was pumped inside major veins and through the cardiac chambers and also between the heart and cooling container walls. In Case 1, two inlets and two outlets on the container walls steadily circulated the coolant. In the Case 2, an additional inlet was specified on the container wall thus creating a steady jet impinging one of the thickest parts of the heart. Laminar internal flow and turbulent external flow were used in both cases. Unsteady periodic inlet velocities at two frequencies were applied in Case 3 and Case 4 that had four inlets and four outlets on walls with turbulent fl...