Assessment of Thermal Performance of Electronic Enclosures with Rectangular Fins: a Passive Thermal Solution

Passive heat transfer from enclosures with rectangular fins is studied both experimentally and theoretically. Several sample enclosures with various lengths are prepared and tested. To calibrate the thermal measurements and the analyses, enclosures without fins (“bare” enclosures) are also prepared and tested. Surface temperature distribution is determined for various enclosure lengths and heat generation rates. Existing relationships for natural convection and radiation heat transfer are used to calculate the heat transfer rate of the tested samples. The theoretical results successfully predict the trends observed in the experimental data. It is observed that the contribution of the radiation heat transfer is on the order of 50% of the total heat transfer for the tested enclosures. As such, a new correlation is reported for calculating optimum fin spacing in uniformly finned surfaces, with rectangular straight fins, that takes into account both natural convection and radiation. INTRODUCTION Passive cooling, technologies or design features without power consumption, has been used in thermal solutions for various applications such as power electronics, telecommunications, microelectronics, and HVAC systems. Two significant heat transfer modes involved in passive cooling systems are natural convection and radiation. Convection heat transfer from an object, determined using Newton’s cooling law ( ) T T ( hA Q s conv ∞ − = ), can be enhanced by increasing the heat transfer coefficient, h , and/or by increasing the surface area, A , if the temperature difference is fixed [1]. Extended surfaces, which are popularly known as fins, are extensively used in air-cooled heat exchangers and electronic cooling. Although fins increase the surface area, they lead to an increase in frictional resistance resulting in a lower air flow rate in the vicinity of the finned surface, and at the same time, add more thermal resistance to the conduction path from the base to the ambient. As such, the heat transfer coefficient for noninterrupted finned surfaces is lower than the value for the base plate without fins. Therefore, the overall heat transfer rate of a finned plate could either increase or decrease in comparison with the base plate without fins. These competing trends clearly indicate the need for an optimization study to establish an ‘optimum’ fin design for air cooled enclosures. The effect of fin-spacing on natural convection has been investigated by several researchers. Design information of natural convection for fin arrays can be extracted from experimental studies of Starner and McManus [2], Welling and Wooldridge [3], Harahap and McManus [4], Jones and Smith [5], Donvan and