Review on thermal energy storage with phase change: materials, heat transfer analysis and applications

The boundary layer equations for plane, incompressible, and steady flow are 

$$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Boundary Layer Theory

An introduction to heat transfer

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Numerical Heat Transfer And Fluid Flow

This paper explores the recent research developments in high-heat-flux thermal management. Cooling schemes such as pool boiling, detachable heat sinks, channel flow boiling, microchannel and mini-channel heat sinks, jet-impingement, and sprays, are discussed and compared relative to heat dissipation potential, reliability, and packaging concerns. It is demonstrated that, while different cooling options can be tailored to the specific needs of individual applications, system considerations always play a paramount role in determining the most suitable cooling scheme. It is also shown that extensive fundamental electronic cooling knowledge has been amassed over the past two decades. Yet there is now a growing need for hardware innovations rather than perturbations to those fundamental studies. An example of these innovations is the cooling of military avionics, where research findings from the electronic cooling literature have made possible the development of a new generation of cooling hardware which promise order of magnitude increases in heat dissipation compared to today's cutting edge avionics cooling schemes.

Assessment of high-heat-flux thermal management schemes

Rising thermal dissipation from modern electronics has increased the challenge of cooling using conventional heat sinks. In addition to fans and blowers, focus is turning to active cooling devices for augmenting performance. A piezoelectrically-actuated synthetic jet array is one under consideration. Synthetic jets are zero-net–mass-flow jets realized by a cavity with an oscillating diaphragm on one side and an orifice or multiple orifices on the other side. They generate highly unsteady jetting flows that can impinge upon heated surfaces and enhance cooling. However, the synthetic jet actuation components might interfere with other components of the electronics module, such as the fan, requiring a displacement of the cavity center from the jet array center. Herein, heat transfer enhancement by an inclined piezoelectrically-actuated synthetic jet arrangement in a heat sink for electronics cooling has been experimentally and numerically studied. A wedge-shaped platform is designed to introduce the jets with an inclined configuration into the finned channels of the heat sink. The unit is inclined to avoid interference with other components of the module. The penalty is described in terms of velocities of jets emerging from this wedge-shaped platform, compared to those from an aligned cavity-orifice design. Effects on heat transfer performance for the heat sink are documented. The jets are arranged as wall jets passing over heat sink fins. The experimental study is complemented with a numerical analysis of flow within the synthetic jet cavity. Optimization is done on the number of jets against the penalty on jet velocity for obtaining maximum cooling performance. The jets are driven by piezoelectric actuators operating at resonance frequencies of 700–800 Hz resulting in peak jet velocities of approximately 35m/s from 92, 0.9 mm × 0.9 mm orifices. The results give guidance to those who face a similar interference problem and are considering displacement of the synthetic jet assembly.Copyright © 2013 by ASME

Heat Transfer Enhancement of a Heat Sink by Inclined Synthetic Jets for Electronics Cooling

Measurements of discharge coefficients for several film cooling configurations having hole length-to-diameter ratios of 2.3, 4.6, 6.6, and 7.0 are presented. Recently, it was documented that the velocity distributions over the hole exit plane vary significantly with changes in hole length-to-diameter ratio. This paper documents the effects of such variations on coolant discharge coefficients. Due to the short holes, injection in engines is with a substantial amount of coolant departing the upstream portions of the hole exit plane. This results in a higher rate of momentum exchange with the free stream at that location than for longer holes, which permits more uniform exit flows. Discharge coefficient measurements are discussed in terms of this distribution of velocity. This paper also documents the effects of the hole supply plenum geometry on discharge coefficients. When the coolant flow is delivered to the holes with significant momentum either in the direction of the free stream or opposite to that direction, significant changes in discharge coefficient values are observed.

Measurements of discharge coefficients in film cooling

Neutrally buoyant helium-filled soap bubbles with laser illumination are used to document entry flow separation for oscillating flow in tubes. For a symmetric entry case, the size of the separation zone appears to mildly depend on Reynolds number in the acceleration phase, but is roughly Reynolds number independent in the deceleration phase. For the asymmetric entry case, the separation zone was larger and appeared to grow somewhat during the deceleration phase. The separation zones for both entry geometry cases remain relatively small throughout the cycle. This is different from what would be observed in all-laminar, oscillator flows and is probably due to the high turbulence of the flow, particularly during the deceleration phase of the cycle.

Visualization of entry flow separation for oscillating flow in tubes

Heat transfer performance of air-cooled heat sinks must be improved to meet thermal management requirements of microelectronic devices. The present paper addresses this need by putting actuated plates into channels of a heat sink so that heat transfer is enhanced by the agitation and unsteadiness they generate. A proof-of-concept exercise was computationally conducted in a single channel consisting of one base surface, two fin wall surfaces, and an adiabatic fourth wall, with an actuated plate within the channel. Air flows through the channel, and the actuated plate generates periodic motion in a transverse direction to the air flow and to the fin surface. Turbulence is generated along the tip of the actuated plate due to its periodical motion, resulting in substantial heat transfer enhancement in the channel. Heat transfer is enhanced by 61% by agitating operation for a representative situation. Translational operation of the plate induces 33% more heat transfer than a corresponding flapping operation. Heat transfer on the base surface increases sharply as the gap distance between it and the plate tip decreases, while heat transfer on the fin wall surface is insensitive to the tip gap. Heat transfer in the channel increases linearly with increases of amplitude or frequency. The primary operational parameter to the problem is the product of amplitude and frequency, with amplitude being slightly more influential than frequency. The analysis shows that the proposed method can be used for modern levels of chip heat flux in an air-cooled model forestalling transition to liquid or phase-change cooling.Copyright © 2011 by ASME

A Computational Study of Active Heat Transfer Enhancement of Air-Cooled Heat Sinks by Actuated Plates

Experiments were conducted to determine local heat transfer coefficients on the inside surfaces of a cylindrical cavity that is cooled by a swirling air flow. Temperature-sensitive liquid crystals were used as temperature sensors. Five blowing (cooling) modes were tested: three with swirl numbers of 0.36, 0.84, and 1.73; a fourth with no swirl (axial flow), and a fifth that was similar to the fourth but had the flow direction reversed. Flow visualization and static pressure measurements were performed to improve understanding of the situation. The smoke-wire technique was successfully used to picture the flow patterns. Plots of local Nusselt number along the cavity surfaces were obtained for the five blowing modes and for three different Reynolds numbers. The swirling cases had similar flow fields with higher heat transfer rates near the cavity top and lower rates near the cavity bottom (the opposite of the nonswirling cases). A tornadolike structure on the cavity bottom was observed in the swirling cases. This structure became stronger and more violent as the degree of swirl and the Reynolds number were increased. The Nusselt number curves for the two nonswirling cases were of similar shape, although the flow direction was reversed.

Terrence W. Simon

Papers

Heat Transfer Enhancement of a Heat Sink by Inclined Synthetic Jets for Electronics Cooling

Measurements of discharge coefficients in film cooling

Visualization of entry flow separation for oscillating flow in tubes

A Computational Study of Active Heat Transfer Enhancement of Air-Cooled Heat Sinks by Actuated Plates

Investigation of the effects of flow swirl on heat transfer inside a cylindrical cavity