Computational study of laminar flow heat transfer enhancement by dimples and protrusions

Abstract

The significant increase in heat fluxes rejected from electronic equipment and other small devices has continued with the unrelenting demand for technological improvement. There is therefore a need to increase the performance of the heat exchangers operating at these small scales. Consequently, the flow in these heat exchangers is likely to be laminar, resulting in poor thermal performance. Hence, some means of heat transfer enhancement is needed. It appears that protrusion and dimples may sufficiently increase heat transfer rates without a large pressure drop penalty. A comprehensive numerical study was undertaken of the flow and thermal fields along with the heat transfer distribution in the vicinity of isolated, height to print diameter ratio (h/D=0.22 and 0.5) spherical dimples and protrusions in shallow rectangular channels. The purpose of the study was to develop an understanding of flow structures, the resultant thermal fields and heat transfer in the vicinity of a single obstacle. In addition, different combinations of small and large dimples and protrusions have been investigated in order to obtain a comprehension of effect of the interaction between the flow fields of these heat transfer enhancement devices when placed in an array. The numerical approach used in this research was validated by the excellent agreement between the velocity and thermal fields generated by the computer code when modelling the flow and heat transfer in a shallow rectangular channel containing a large protrusion and the experimental data obtained by particle image velocimetry and thermochromic liquid crystal sheets. For the range of Reynolds numbers and channel aspect ratios studied the flow around the small protrusion and dimple is steady. However, these small obstacles generated surprisingly complex flow structures and concomitant thermal fields. When the large protrusion is used the flow becomes unsteady at a critical Reynolds number. A thorough study of the unsteady and time-averaged flow and temperature fields are presented as well as the resultant heat transfer distributions on the channel surfaces at a Reynolds number of 1600. Similar studies were performed for the steady flow in the vicinity of isolated large dimple. The introduction of a single small protrusion in a shallow rectangular channel leads to a larger enhancement in heat transfer than a single small dimple. This enhancement is dependent on Prandtl and Reynolds numbers. Whilst a similar result is found when isolated large protrusions and dimples are employed, the average augmentation achieved when the large protrusion is used is four times that obtained when a small protrusion is employed at the same Reynolds number. This massive heat transfer enhancement wit ha large protrusion is associated with an effectiveness factor of 1.7 in contrast with 1.15 for the small protrusion; making the large protrusion a cost effective method of heat transfer enhancement. As a result, a number of large protrusion/dimple combinations were studied in order to develop an understanding of the interactions between the flow fields developed by individual obstacles so as to be able to design arrays which would maximise the heat transfer without producing undesirably high pressure penalties.