Heat spreaders are used to dissipate heat from a localized source (such as an el

Question

Heat spreaders are used to dissipate heat from a localized source (such as an electronic chip). Consider the square heat spreader shown, which has a thickness of w = 3 mm, and an edge dimension of L = 5 cm. The thermal conductivity of the heat spreader is k = 177 W/m/K. The energy released from an electronic chip mounted to the spreader board is transferred away by conduction in board and is lost by convection from the top surface of the board to the surrounding area. The chip dissipates heat at a rate of Q elec =5 W. The convection coefficient for heat transfer to the air is h = 20 W/m2/K, and the surrounding air temperature is T infinity = 25 degree C. It is important to be able to predict the effectiveness of the heal spreader in terms of lowering the chip's operating temperature for a given power dissipation. For simplicity of analysis, suppose the heat spreader is divided into 9 equal size "zones", with the center square being occupied by the electronic chip. A node is placed at the center of each zone, and assigned an average temperature for the zone. Note: Bi = h w / k

Explanation / Answer

A single jet or an array of jets, impinging normally on a surface, is widely used in many engineering applications because very high rates of heating, cooling, or drying can be achieved. Major industrial applications of the impinging jet include cooling of turbine blades and electronic components, annealing of metal sheets, drying of textile and paper products, and deicing of aircraft systems. This talk is intended to give an overview of the fundamentals of impinging jets together with recent advances achieved at the Applied Heat Transfer Lab of KAIST. Specifically, some interesting results on microscopic and macroscopic jets impinging onto a flat surface or a heat sink will be presented. The hydraulic jump, a phenomenon that includes the sudden deceleration of liquid along the wall and affects heat transfer, will be discussed along with the results of hydraulic jump location prediction. Next, a comparison between single-phase and two-phase impinging jets will be shown together with the benefits of using a two-phase impinging jet. Finally, a simple imaging technique based on the hydrodynamics of an impinging jet, the Scanning Flow Impedance Microscope (SFIM), will be presented. The SFIM can be used to image the surface topography of a specimen, and has advantages over conventional forms of microscopy, such as the electron microscope.

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