With continuously increasing number of chips and smaller and smaller CPU sizes, heat fluxes that need to be dissipated from computers are on a rapid increase. CPU cooling being critical to the performance of electronic devices, this field demands considerable research focus. Researchers have been pushing existing computer cooling technologies to their limits and also developing new cooling techniques. Forced convection, spray jet cooling, boiling heat transfer are a few to name. Different technologies have their respective merits and limits in terms of their cooling capability, reliability, ease of manufacturing and durability. Forced convection using air has always been preferred due to its cost effectiveness and reliability. The traditional way of employing this technique has been by using a blower fan that cools the heat sink that dissipates heat from the chips. However, the current heat removal demands better performance than that can be provided by a blower fan alone. Agitation is a strong mixing mechanism that can disturb the near-wall flow, thin the thermal boundary layer and enhance the convective heat transfer. This thesis study carries a detailed study of agitation alone through a Large Scale Mock Up (LSMU) unit which is dynamically similar to a single channel of a heat sink. The LSMU has a translationally oscillating plate (agitator) inside the channel cavity. Time averaged heat transfer coefficients and time resolved velocity measurements have been made along different regions of the channel to characterize the convective cooling performance of the agitator. The ensemble-averaged mean velocity variations show periods of acceleration, deceleration and flow reversal during a cycle as a result of agitator movement. Turbulence is found to increase toward the end of the acceleration phase and persist through the deceleration phase. A parametric study has been done to explore the effects of agitator frequency (f), amplitude (A) and agitation velocity (2πAf) on heat transfer and flow mechanism. The heat transfer coefficient increases with the increase in frequency and amplitude. At a fixed agitation velocity, heat transfer coefficient is mainly governed by the agitation velocity irrespective of the value of amplitude or frequency.
University of Minnesota Ph.D. dissertation. January 2014. Major: Mechanical Engineering. Advisors: Terrence W. Simon, Tianhong Cui. 1 computer file (PDF); xiii, 161 pages.
The Effects of Agitation on convective heat transfer with applications to electronics cooling.
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