Numerical study of heat transfer enhancement in micro-channels using synthetic jets

摘要

High performance electronic devices have been reported to approach 1MW/m2 so that current heat dissipation devices will not be able to cope with increasing heat flux. It has therefore been proposed that in order to manage the ever-increasing heat rejection demands, it will be necessary to have cooling fluid flowing through microchannels equipped with synthetic jets.To account for the deflection of the membrane of the synthetic jet actuator, a novel moving mesh algorithm has been developed. The solution methodology has been validated by comparing the velocity fields generated by the in-house code against experimental data. The conjugate heat transfer problem is solved by determining the temperature distributions in a heated solid and the fluid flowing in the microchannel attached to a silicon wafer microchip. It is shown that 2D studies overestimated the cooling effect, so that 3D effects must be included to properly assess potential of synthetic jets. The use of synthetic jet in cross flow was shown to yield similar results whether constant properties or variable properties were used. However, results obtained with constant properties were slightly more conservative, which coupled with advantages of reduced computational resources and CPU time, meant that constant properties were used in the majority of calculations. A wide range of parametric studies was performed using one jet with varying heat fluxes, actuator diaphragm amplitudes and frequencies. The hot regions in the silicon wafer resulting from the fluid flowing undisturbed in a microchannel are removed when the synthetic jet is switched on thereby significantly lowering the maximum temperature in the wafer. The advantage of higher diaphragm amplitudes is the creation of larger mixing regions and stronger vortices, thereby making cooling operation of the synthetic jet more effective. Similarly, increasing the frequency of the diaphragm leads to an increase of heat transfer enhancement. A parametric study was performed with the two jets being in phase and 180 degree out of phase at various operating frequencies and diaphragm amplitudes in order to create a more uniform heat transfer from the wafer. Out of several frequencies studied, the addition of an extra actuator produced more significant effect at a frequency of 560Hz whether the jets was in or out of phase. It follows that the optimal frequency depends on the jet arrangement.