Numerical analysis of flow structure and heat transfer characteristics in square channels with different internal-protruded dimple geometrics

Flow characteristics and heat transfer performance for a rectangular channel surfaces with internal-protruded dimples are investigated numerically. The special dimpled surface is a conventional dimple cavity with a protrusion structure mounted within it. The object is to enhance the flow mixing and the corresponding heat transfer, especially in the flow-recirculating region where heat transfer is very low in a conventional dimple cavity. All turbulent fluid flow and surface heat transfer results are obtained using computation fluid dynamics with a verified k-ε RNG turbulence model. The locations of protrusion mounted in the dimple cavity along streamwise direction are the main design parameters. The inlet Reynolds number ranges from 7500 to 27,500 conducted in four rectangular channels with different internal-protruded dimple geometrics. From this study, the internal-protruded dimple structures suppress recirculation flows in the upstream parts of the dimples. Small-scale recirculation flows are mainly distributed around the internal protrusions. In addition, flow separations and reattachment structures are also changed by the added internal protrusions, and the ejection from dimples is weakened compared with that from the conventional spherical dimples (Case A). Internal-protruded dimples greatly improve the averaged local heat transfer of dimple curved surface by suppressing the recirculation flows and enhancing heat transfer in the upstream parts of dimples. Internal-protruded dimples also have the advantage of reducing pressure loss penalty compared to the conventional spherical dimples (Case A) by reducing the form drag in the upstream and the friction drag in the downstream. Case D (the internal protrusion is rightly in the center of the correspond dimple) obtains the best overall thermal performance in terms of Nu/Nu0/(f/f0)1/3 parameter for all the considered Reynolds numbers. Case B (horizontal distance between the center of the internal protrusion and the dimple is 16 mm) provides the highest heat transfer with the largest pressure loss penalty at relatively high Reynolds numbers due to the special geometry structure, which makes the flow impingement on the downstream stronger. The overall thermal performance is greatly improved by the internal-protruded dimples compared with the conventional spherical dimples without the addition of added heated surface areas.