Heat Transfer Investigation of an Offset Strip Fin Using Liquid Crystals

Abstract

The heat transfer characteristics of an offset strip fin are investigated by the transient liquid crystal method. It is found that there are various flow patterns that may occur, such as separation, reattachment, flow acceleration, expansion, and trailing vortex, and that the heat transfer is directly influenced by them. These flow patterns vary depending on the fin geometry and Reynolds number. For a sparsely stacked fin array (strip thickness-to-fin pitch, t/D < 0.1), the heat transfer pattern is close to a flat plate in a free stream such that the heat transfer is high at the leading edge and decreases along the plate. A distinct pattern change occurs at ReDh ≈ 2300. The heat transfer is high at the leading edge, but decreases rapidly over the first 10–20% of the strip, then increases to a maximum at 25–30%, then decreases again over the rest of the strip. The maximum at 25–30% of the strip indicates flow reattachment there. The same trend is found in a densely stacked fin array (t/D = 0.25) but the pattern change occurs at much lower Reynolds number (ReDh ≈ 800). The influence of flow reattachment is comparable at these Reynolds numbers in both of the fin arrays but it becomes more significant in the densely stacked fin array as Reynolds number increases. It appears that this is due to more dominant flow separation and reattachment, caused by an adverse pressure field near the leading edge of the strip in the densely stacked fin array. The results indicate the importance of the fin pitch in understanding the flow and heat transfer characteristics of offset strip fins.