Krishna Kota

PARAMETRIC NUMERICAL STUDY OF FLOW AND HEAT TRANSFER IN MICROCHANNELS WITH WAVY WALLS

Wavy channels were investigated in this paper as a passive scheme to improve the heat transfer performance of laminar fluid flow as applied to microchannel heat sinks. Parametric study of three-dimensional laminar fluid flow and heat transfer characteristics in microsized wavy channels was performed by varying the wavy feature amplitude, wavelength, and aspect ratio for different Reynolds numbers between 50 and 150. Two different types of wavy channels were considered and their thermal performance for a constant heat flux of 47 W/cm 2 was compared. Based on the comparison with straight channels, it was found that wavy channels can provide improved overall thermal performance. In addition, it was observed that wavy channels with a configuration in which crests and troughs face each other alternately (serpentine channels) were found to show an edge in thermal performance over the configuration where crests and troughs directly face each other. The best configuration considered in this paper was found to provide an improvement of up to 55% in the overall performance compared to microchannels with straight walls and hence are attractive candidates for cooling of future high heat flux electronics.

Numerical Investigation of Flow and Heat Transfer Performance of Nano-Encapsulated Phase Change Material Slurry in Microchannels

Microchannels are used in applications where large amount of heat is produced. Phase change material (PCM) slurries can be used as a heat transfer fluid in microchannels as they provide increased heat capacity during the melting of phase change material. For the present numerical investigation, performance of a nano-encapsulated phase change material slurry in a manifold microchannel heat sink was analyzed. The slurry was modeled as a bulk fluid with varying specific heat. The temperature field inside the channel wall is solved three dimensionally and is coupled with the three dimensional velocity and temperature fields of the fluid. The model includes the microchannel fin or wall effect, axial conduction along the length of the channel, developing flow of the fluid and not all these features were included in previous numerical investigations. Influence of parameters such as particle concentration, inlet temperature, melting range of the PCM, and heat flux is investigated, and the results are compared with the pure single phase fluid.

Thermal Performance of Microchannels With Wavy Walls for Electronics Cooling

Wavy walls are investigated in this paper as a passive scheme to improve the heat transfer performance of low-Reynolds-number laminar flows in microchannel heat sinks for electronics cooling applications. 3-D laminar fluid flow and heat transfer characteristics in microchannels with wavy walls are numerically studied for a 500-μm hydraulic diameter channel by varying the wavy feature amplitude at different Reynolds numbers (10, 20, 50, and 100). In addition, flow measurements are made using a micrometer-resolution particle image velocimetry technique for understanding the fundamentals of fluid flow in the wavy-walled microchannels for the considered Reynolds numbers. Based on the comparison with straight channels, it was found that wavy channels can provide improved heat transfer performance while keeping the pressure drop within acceptable limits. Accordingly, wavy channels are to found to provide an improvement of up to 26% in the overall performance (which includes the effect of wall waviness on heat transfer, pressure drop, and surface area) compared to microchannels with straight walls for the same pumping power and hence are attractive candidates for cooling of future electronics.

Design of a Thermal Management System for Directed Energy Weapons

A generic model was developed that can provide an estimate of the mass and volume and characterize the usefulness of thermal energy storage (TES) for various thermal management (TM) applications in advanced military aircraft. Conceptual design for low thermal duty cycle electronic heat sink applications was described. The criteria are presented for selecting a design for different end-applications. A thermal resistance model has been developed to analyze and optimize the design. It can be concluded that the heat sink design for the high-energy laser (HEL) system can have a high heat storage ability of 20 MJ, a volume storage density of 86 MJ/m 3 and a mass storage density of 77 kJ/kg. The airborne active denial (AAD) system heat sink design can have a high heat storage ability of 25 MJ, a volume storage density of 76 MJ/m 3 and a mass storage density of 69 kJ/kg. Nomenclature A = surface area of TES chambers (m 2 ) a = vapor chamber width (m) b = vapor chamber length (m) D = width of vapor channel (m) Dh = hydraulic diameter of vapor channel (m) keff = effective thermal conductivity of carbon foam and PCM (W/m⋅K) kfoam = thermal conductivity of the carbon foam (W/m⋅K) kl = thermal conductivity of the working fluid in liquid state (W/m⋅K) kPCM = thermal conductivity of PCM (W/m⋅K) kwall = thermal conductivity of the TES chamber wall (W/m⋅K) kc = thermal conductivity of the vapor chamber plate (W/m⋅K) G = mass flux (kg/m 2 .s) N = number of TES chambers mPCM = total mass of PCM present in the VCTES system (kg) mtot = total mass of VCTES heat sink (kg) Prl = liquid Prandtl number, µl cp,l / kl Pred = reduced pressure, P/Pcritical Psat = normal pressure (gage)/vapor saturation pressure (atm) (=P) Q = total heat absorption capacity of the VCTES heat sink (MJ) Qm = heat storage capacity per unit mass (MJ/kg)

A novel conduction-convection based cooling solution for 3D stacked electronics

The present investigation focuses on the design and thermal parametric study of a unique liquid interface thermal management solution for a 3D chip stack that is embedded within a cavity, in a radial heat sink cooled by an array of synthetic jet actuators. The heat sink module was previously reported by the authors, who achieved an overall heat transfer coefficient of ~70 W/m2.K. The radial heat sink exploits enhanced, small-scale heat transfer that is affected by a central array of synthetic jet actuators. This approach is very effective due to the short radial thermal path of the cooling air along the fins which couples rapid, time-periodic entrainment and ejection of cool and heated air, respectively to increase the local heat transfer coefficient on the air-side. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high power 3D stacked electronics to the heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive and scalable thermal management solution.

Design of a Dual Latent Heat Sink for Pulsed Electronic Systems

A conceptual design of a dual latent heat sink basically intended for low thermal duty cycle electronic heat sink applications is presented. In addition to the concept, end-application-dependent criteria to select an optimized design for this dual latent heat sink are presented. A thermal resistance model has been developed to analyze and optimize the design, which would also serve as a fast design tool for experiments. The model showed that it is possible to have a dual latent heat sink design capable of handling 7 MJ of thermal load at a heat flux of 500 W/cm 2 (over an area of 100 cm 2 ) with a volume of 0.072 m 3 and a weight of about 57.5 kg. It was also found that, with such high heat flux absorption capability, the proposed conceptual design can have a vapor-to-condenser temperature difference of less than 10°C with a volume storage density of 97 MJ/m 3 and a mass storage density of 0.122 MJ/kg.

Hybrid Liquid Immersion and Synthetic Jet Heat Sink for Cooling 3-D Stacked Electronics

This paper focuses on the design and parametric numerical study of a hybrid heat sink combining a liquid thermal interface with an array of synthetic jet actuators for 3-D chip stack cooling. The air-side heat sink exploits enhanced localized heat transfer achieved via a central array of synthetic jet actuators. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high-power 3-D stacked electronics to the hybrid heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive, and scalable thermal management solution for 3-D stacked die structures. It is shown that this heat sink with a footprint of 76-mm square × 51-mm height can dissipate a total of 41 W of heat/power from the stack for a 44°C average chip temperature rise above ambient (an Rja of ~ 1.06 K/W obtained passively).

Numerical Simulation of Heat Transfer in a Microchannel Heat Sink With Micro Encapsulated Phase Change Material (MEPCM) Slurry

Abstract : High heat flux removal from devices such as Insulated-Gate Bipolar Transistor (IGBT) and Monolithic Microwave Integrated Circuit (MMIC) will be important for future Navy ships. Micro encapsulated phase change material (MEPCM) slurry was used as a heat transfer fluid inside a microchannel instead single phase fluid. Presence of phase change material increases the effective heat capacity of the fluid. The performance of encapsulated phase change material (EPCM) slurry flow in microchannels was investigated using the effective specific heat capacity method. Lattice Boltzmann method was used to simulate the particle paths when the duct shape has different aspect ratios. For higher concentrations, a shear induced migration model was used to simulate the nonhomogeneous particle distribution. Results of the model were used to solve the temperature distribution inside the channels. Parametric study was carried out with water and PAO as base fluids in microchannels of two different widths, 101 urn and 25 urn. Parameters varied include particle concentration, inlet temperature of the fluid, melting range of PCM and base heat flux.

Conceptual Design of a Dual Latent Heat Sink for Thermal Management of Pulse Heat Generating Electronic Systems

Conceptual design of a dual latent heat sink basically intended for low thermal duty cycle electronic heat sink applications is presented. In addition to the concept, end-application dependent criteria to select an optimized design for this dual latent heat sink are presented. A thermal resistance model has been developed to analyze and optimize the design, which would also serve as a fast design tool for experiments. The model showed that it is possible to have a dual latent heat sink design capable of handling 7 MJ of thermal load at a heat flux of 500 W/cm (over an area of 100 cm) with a volume of 0.072 m and weighing about 57.5 kg. It was also found that with such high heat flux absorption capability, the proposed conceptual design can have a vapor-to-condenser temperature difference of less than 10 C with a volume storage density of 97 MJ/m and a mass storage density of 0.122 MJ/kg.

Review of Geothermal and Solar Thermal Power Plants and a Comparative Design Analysis

Thermodynamics indicates that the lower the temperature of a resource, the less energy that could be extracted from it due to lower maximum thermal efficiency. Geothermal resources exist in varying temperatures. The lowest ones (around 120°C), are too small for economic power production. On the other hand, concentrating solar power (CSP) can achieve high temperatures during the day (from 350 to 550°C, based on a Parabolic Trough CSP plant [1]) but once the sun is not shining, that temperature is reduced drastically.Transition to renewable energy systems is an environmentally friendly and potentially rewarding economic decision that society can make nowadays. This paper briefly reviews geothermal and solar thermal based plants in terms of energy growth or decay from one year to another (2012–2013). In addition, an example site location is chosen and the performance of both these types of power plants is analyzed in terms of capacity factor, Thermal Energy Storage (TES) hours, solar multiple, area requirement and Levelized Cost of Energy (LCOE) for a given set of environmental conditions. This analysis is performed using the System Advisor Model (SAM), on which simulation of parabolic trough, power tower, linear Fresnel, dish Stirling and geothermal (binary cycle) energy conversion systems are considered. At the same time, the analysis discussed will take place in a further study which will include economic viability for the two technologies running under the same combined cycle.Copyright