Electronics cooling by extended surface: Refractive index changes flow visualization of the natural convection heat transfer

Abstract

Natural convection heat transfer occurs when the fluid buoyancy motion is induced by density differences themselves caused by the heating. A temperature gradient causes a density variation in a cooling fluid with a related local change in the refractive index. The gradient of refractive index has the effect of bending the light. The thermal load of the device causes an optical deflection in the cooling fluid that an opportune light probe can reveal. Analyzing the deflection of the light probe it is possible to go back to the related temperature gradient. The experimental work in this paper represents a very simple method for the visualization of refractive index non homogeneities in a phase object: the temperature gradients in a cooling fluid for buoyancy-induced convective flow can be visualized in an electronic system during its operation. The developed experimental set-up allows to reveal local refractive index changes in a phase objects. A fringe pattern is acquired, through the cooling fluid under analysis, with a digital camera two times: the first one with the fluid at rest, the second one with the thermal load due to the electronic device normal operation. By the means of the MATLAB processing of the acquired images it's possible to reveal the shape and the directions of the thermal flow lines for the cooling fluid. In this way we can obtain a deeper understanding of the optimal convection working volume or information for the optimization of the relative spatial positioning of the several electronic components in a complex electronic system, like a printed circuit board (PCB). The experimental set-up was optically implemented: the analysis is absolutely no-contact and carried out without distortion for the thermal flow and without alteration for the temperature gradients in the fluid under test. The proposed technique has been applied on two typical heat extraction situations recurrent in the electronic devices: are presented the experimental results of the visualization of the natural convection buoyancy driven air flow for an heat sink and a power resistor. In both the cases it was possible to visualize the bouyancy induced flow generated, in air, by the heated sample and understand the shape of the isogradients lines in the test field and the involved working volume in the cooling fluid. The results presented show that is possible to monitore the onset and the development of the natural convection thermal flow and the perturbation in the thermal gradient map caused by externally added air flow with a simple and cheap noninvasive optical setup.