Abstract

Heat transfer enhancement using the rib turbulators of various shapes and sizes has been the topic of research for past several decades. But high heat transfer also comes with a higher pressure drop penalty in case of rib turbulators. On the other hand, dimples have been known for comparable heat transfer enhancement with much lower pressure drop penalty as compared to rib turbulators. In dimples, a pair of counter rotating secondary vortices forms and sheds in diagonal directions and that increases turbulence and transport of heat in wall normal direction which results in improved heat transfer from that surface.

It has been studied that having protrusions in addition to dimples improve heat transfer further but the mechanisms of heat transfer in these cases is not fully understood. Furthermore, the presence of dimple and protrusions on the same surface give rise to negative poisson ratio (NPR) characteristic which makes these geometries even more interesting due to the improved material strength. The vortical structures which form in the wake of protrusions, alter the flow structures in dimples and these interactions are non-linear and unsteady. High fidelity CFD simulations are needed to predict these interactions correctly.

In the present work, a Large Eddy Simulation (LES) is performed for flow in rectangular channel with one of the walls having dimples and protrusions on it. This simulation includes entry length as well as the fully developed reason of the flow. The periodic sides are used to reduce the computational time.  There are seven and five rows of features in streamwise and spanwise directions, respectively. The shape of dimples and protrusions is spherical with print diameter of 0.0254 m and height/depth to diameter ratio of 0.25. The channel height, width, and length are 0.0254 m, 0.2032 m, and 0.508 m, respectively. The Reynolds number based on the dimple/protrusion print diameter is 10,000. Inline configuration with alternation dimples and protrusions are used in streamwise and spanwise direction.

Mean flow field results are compared with PIV data at few spanwise planes in streamwise direction as the validation case. Turbulence statistics is analyzed to understand the flow and heat transfer in more details. Effort is made to find the dominant structures in flow and the associated frequencies by performing FFT of on transient data. Transient snapshots of the flow are utilized to understand the shape of vortical structure and to track their path in the flow. It is also be analyzed how separation behind the protrusions interacts with the shear layer over the dimples and affects the flow features inside and downstream of the dimples.  Finally, heat transfer enhancement mechanism is explained in the light of above analysis. The difference in flow and heat transfer behavior in entry region and fully developed regions are also highlighted.