Influence of aspect ratio on the mixing behavior of plane jets at low reynolds numbers

Jets of rectangular cross section find application in many instances. However, most of the experimental works are confined to higher Reynolds number turbulent jets. The present work deals with an experimental study, conducted in a two dimensional unventilated plane air jet issued from a converging type nozzle to a quiescent room, which is specially designed for this purpose. Nozzle with variable aspect ratios of 40, 20 and 10 are used for the study. Measurements are done by using a DANTEC hotwire anemometer with a 55P11 platinum coated tungsten probe of 5 μdiameter. Measurements are taken up to an axial distance of x/d = 260. The jet with lower aspect ratio seems to be mixing better, which is evident from the higher half width growth rate and faster mean velocity decay rate. At a Reynolds number of 2000, a linear spread rate is observed roughly up to x/d = 100, followed by a nonlinear spread. But the distance at which linear growth rate terminates is also a function of Re which is revealed from the study at a lower Re of 500. Lower the aspect ratio is, greater is the development of three dimensionality in the far field. Integral length scales are found to increase with increasing axial distance and decreasing aspect ratio as well as Re. In general, the mixing properties of jets are found to be dependent on the aspect ratio due to the combined influence of three dimensionality effect and change in integral length scale. This factor should be considered while selecting the aspect ratio for specific applications. The information obtained here can be applied to jet flows in boiler furnaces, dryers, combustion chambers of jet engines and stationary gas turbine plants of electric power stations, chemical reactors, etc.