There are many industrial applications that require improved thermal energy transfer and thereby allow for reduced heat transfer devices resulting in reduced size and weight. Whether free or forced convection is used as a cooling mechanism it is of paramount importance to understand the flow field associated with the cooling strategy, this is especially true for devices where thermal convection occurs at low velocities, or Reynolds Number (Re). It has been shown that small surface protrusions within such a velocity field may enhance or diminish the heat and therefore a thorough understanding of the size and shape of these potential obstacles is necessary if optimum heat transfer is to be achieved.
In order to improve our knowledge of the influence of surface protrusions on heat transfer enhancement, ESI has recently conducted an in-depth analysis of over 20 different geometric shapes, at low Reynolds number, in ESI’s water tunnel facility, Figure 1.
From this analysis, several different geometries were selected for a follow up computational fluid dynamics (CFD) analysis of the protrusions ability to affect the heat transfer as well as to determine its drag component for energy transport considerations; these results were then validated against experimental evidence, Figure 2.
Figure 1: Low Reynolds Number Flow Over a Surface
Protrusion in ESI's Water Tunnel
Figure 2: CFD Analyses of a Single Center-Line Streamline
Approaching a Protrusion on a Flat Surface
From an analysis of these video tests a number of configurations were then subsequently evaluated for heat transfer effectiveness at low-speed air flow (low Re) in ESI’s flow visualization wind tunnel, Figure 4. This was achieved by temperature measurements with and without the protrusions present in the air flow and the Nusselt number determined for each protrusion array. These results were then used to validate the CFD packing density model.
Geometries that produced the largest heat transfer, as represented by the Nusselt Number (Nu), and the lowest drag coefficient (Cd) were then chosen to evaluate the effect of protrusion element packing densities, Figure 3.
Figure 3: Diagrammatic Sketch of the Packing
Densities of Surface Protrusions
From this preliminary analysis, it was determined that the coefficient of friction for a particular array was approximately 50% lower than the clean flat plate condition. Furthermore, the experimental results showed that the heat transfer, in terms of the Stanton number (St), was enhanced by approximately 25%, whereas the computationally determined Stanton number showed an increase of 33% under the same flow conditions.
Figure 4: Flow Visualization Wind Tunnel with a Heat Transfer Plate Model