Hole extension effect on transpiration flow efficiency

The present analysis of the hole extension effect on the transpiration flow efficiency is a part of the research [1] which aims at defining a physical transpiration model of the flow through perforated plates. Perforated walls find a wide use as a method of flow control and effusive cooling. Some data on the L/D (hole length to diameter ratio) effect on the flow structure and mass flow rate may be found in the literature [2, 3], but all those works concern holes of a diameter at least one order of magnitude larger than those used in the simulations presented in this paper. Due to the size of the analyzed holes and their cylindrical shape, the only method of analysing the flow through such holes is the numerical method. In the conducted simulations, the holes were D = 0.6 mm, 0.3 mm and 0.125 mm in diameter and the perforation values were equal to 4%, 5%, 8% and 10%. The L/D ratio was changed between 0.25 and 8. The data bank of the flow through the cylindrical holes was produced. The hole extension has a significant influence on the obtained mass flow rate and, consequently, on the transpiration flow efficiency. In addition, entrance effects appear to be important.