Air flow and heat transfer in fan cooled electronic systems

In this paper measurements of surface temperature and air velocity are presented for the case of a printed circuit board with heated metal elements in a fan-induced flow. Surface temperature measurements were performed using infra red thermography, while air flow measurements were performed using Particle Image Velocimetry (PIV). The aim is firstly to provide a physical explanation of the heat transfer process for the board situated in the inlet and exit airflow from the fan, and secondly to provide a set of data to test predictive methods. The results are presented in two parts: first, surface temperature distribution and velocity field data are presented for the case of the fan drawing air from the system. The flow was found to be parallel and uniform. There were symmetrical temperature gradients across the board that were readily explained in terms of board conduction and boundary layer growth. This data set is in strong contrast to that presented in the second part of the paper which deals with the board mounted in the fan exit flow. In this case, both the flow and the heat transfer changed dramatically. The flow here is unsteady and swirling, giving rise to significantly higher Nusselt numbers over the whole of the board. A simple method of understanding these flows is presented to help the system designer to use them to advantage. The implications for increased system reliability, due to both the measured low component temperatures and the lower operational temperature of the fan are significant.