The aspect ratio effect on immersion cooling of discrete flush-heaters in an enclosure cooled from the top

The width effect on natural convection heat transfer due to discrete flush-heated sections of equal height in an enclosure cooled from the top was experimentally investigated. Five heated sections were uniformly distributed along a vertical side wall, where the height of the unheated sections was equal to that of the heated sections. All other vertical surfaces and the bottom plate were insulated. The experiments were conducted for six values of cavity width resulting in a variation in the cavity height-to-width ratio ( aspect ratio ) of 3.67 to 12.22. Ethylene glycol and FC-75 ( a dielectric fluid ) were used as the convective media. The flow visualization results with ethylene glycol revealed a fairly inactive core flow at low power inputs and small cavity width. Higher values of the power input and cavity width transformed a well structured core flow into a time dependent one with higher horizontal velocities toward the "hot" wail. For all the cavity widths, it was found that the heat transfer results for all the heated sections could be unified and presented by a single correlation through use of a local height as the length scale. This correlation includes an explicit relation for the aspect ratio effect on local Nusselt number. The local height of a given heated section was measured from the bottom of the cavity to the mid-height of that section. The data indicated that an increase in the cavity width results in an increase in the heat transfer coefficients of the heated sections. A comparison between ethylene glycol and FC-75 revealed no appreciable Prandtl number effect which is in agreement with a previously reported prediction. A heat transfer coefficient for the top surface was defined based on the total convective heat flux at this surface and an average temperature difference between the heated sections and the top plate. The result shows that for a given heat flux at the top, the highest heat transfer coefficient on this surface can be obtained with the lowest cavity width.