Won Soon Chang

Effective thermal conductivity of sintered metal fibers

For the effective thermal conductivity of dry and fluid-saturated sintered metal fibers, existing correlations are compared to experimental data, and a new empirical expression that compensates for the effect of the aspect ratio of the fibers is presented. It is shown that the present equation predicts the data with 10% deviation, while the Acton equation, which has been recommended in some references, has approximately 200% error.

Transient Behavior of Axially Grooved Heat Pipes with Thermal Energy Storage

A novel design of a high-temperature axially grooved heat pipe (HP) incorporated with thermal energy storage (TES) to mitigate pulse heat loads are presented. The transient behavior of the HP/TES device was simulated using a three-dimensional numerical model based on finite-difference approximations. A phase-change material (PCM) encapsulated in cylindrical containers was used as thermal energy storage. The transient response of three different HP/TES configurations were compared: 1) heat pipe with one big empty cylinder installed in the vapor core, 2) heat pipe with one big PCM cylinder, and 3) heat pipe with six small PCM cylinders. From the numerical results, it was found that the PCM is very effective in mitigating the adverse effect of pulse heat loads on normal heat pipe operation.

Comparison of high heat flux cooling applications

The advent of LSI/VLSI systems has made possible the development of advanced electronic systems operating in the multi-GHz regime. such high speed systems will be of multichip construction to increase miniaturization, packing, and heat dissipation density. Similar advances in high laser-power optics have resulted in significant increases in heat flux density. The stringent temperature uniformity specifications on these systems demand innovative means of applying state-of-the-art technology in enhancing heat removal. Promising cooling techniques that will meet the future thermal control requirements for these electronic and optics packages are presented. These concepts involve the use of microchannel, droplet impingement, jet impingement, and flow boiling in straight or curved channels.

Active Cooling of Metal Oxide Semiconductor Controlled Thyristor Using Venturi Flow

A metal oxide semiconductor controlled thyristor (MCT) is a solid-state high-current switching device. Because of its high-current and high-heat dissipation, this device requires an advanced cooling arrangement. A sample MCT device was successfully tested in a conduction mode up to 95 A using a new technique called venturi cooling. Steady-state operational tests were performed under various coolant temperatures and flow rates. The highest device temperature was 168.5°C, whereas the power dissipation and heat flux were 170 W and 257 W/ cm2, respectively. Comparison with a commercial liquid-cooled cold plate showed that the cooling effectiveness is nearly double for the venturi flow. Measured junction-to-case thermal resistance of the MCT was 0.213°C/ W for venturi flow compared to 0.421°C/W for the commercial cold plate. Venturi flow cooling is highly recommended for MCT applications.

Finite element eigenvalue method for solving phase-change problems

An eigenvalue method has been developed for solving multidimensional phase-change problems with the initial temperature at noncritical temperature. This method gives a closed-form analytical expression for the temperature field in terms of the eigenvalues and eigenfunctions of a characteristic equation derived from the generalized coordinate Lagrangian form of the heat conduction equation with phase change. The method yields reasonably accurate results with a coarse finite element mesh. It also has no critical time-step restrictions for stability. When long-time solutions are needed, excessive numerical computations are required by conventional finite difference or finite element methods due to the small time steps needed for time marching. With the present method, large time steps can be chosen to approximate the phase-change rate. In addition, only a few dominant eigenvalues and eigenfunctions are needed to achieve the same results obtained by using the complete set. These features result in very significant savings in computing time. For both the examples of one-dimensional solidification and solidification within a square, solutions can be obtained within a few iterations if appropriate relaxation factors are used. The results using the present method compare well with the exact solution for the one-dimensional problem and with a semianalytical similarity solution for the square.

Finite difference simulation of transient heat pipe operation

An existing heat pipe numerical model has been modified in this study to better predict transient characteristics. Variable spacing in the axial direction has been incorporated into the nodal system to account for different evaporator, adiabatic section, and condenser lengths. Good agreement has been observed up to 350 K when compared to experimental data obtained from a refrigerant-11 heat pipe. The study shows that a finer grid in the evaporator (condenser) overpredicts (underpredicts) the operating temperature. The number of nodes in the adiabatic section appears not to affect the prediction significantly. Two different expressions for the effective thermal conductivity are also compared using the numerical model. 14 references.