Forced convective boiling in liquid nitrogen from discrete heat sources

The quest for high power with electronics operating in a cryogenic environment has given rise to the need for very effective cryogenic cooling of the electronic devices. This dissertation presents an investigation of flow boiling heat transfer with liquid nitrogen from small, discrete heat sources which simulate electronic chips.; The experimental apparatus is designed to allow visual observation of the phase-change phenomena of cryogen (liquid nitrogen) at the heat sources. The effect of the liquid nitrogen flow velocity on the CHF (critical heat flux) has been studied. It has been shown that the thickness of the macrolayers beneath the vapor bubbles plays the most important role in determining the CHF. Experiments with different flow channel heights have been conducted. It is noted that the channel height has little effect on the CHF until the channel height is decreased to about the size of the vapor bubble layer. The experimental setup also allows an array of three in-line heaters. The dimension of the heaters and the distances between heaters can be varied. The upstream heaters have a tremendous effect on heat transfer from the heaters located downstream. The two most important parameters in the multiple-heater situation are the distance between the heaters and the liquid flow velocity.; A model to describe the heat transfer mechanism near CHF was developed using the present results and those of previous investigators. A correlation was established using this model and the experimental data. This correlation provides an excellent fit to a wide range of experimental data.; With the present apparatus, the heat transfer from small, discrete heat arrays using gaseous nitrogen were also studied. A correlation was derived from the experimental data to calculate the heat transfer coefficient.; The results in this dissertation should provide an invaluable guideline for the electronic industry on how compact electronic devices can be packaged without sacrificing phase-change heat transfer benefit.