Syed Ashfaq* et al. ISSN: 2250-3676 [IJESAT] [International Journal of Engineering Science & Advanced Technology] Volume-4, Issue-1, 049-060 IJESAT | Jan-Feb 2014 49 Available online @ http://www.ijesat.org STUDIES ON FLOW FROM CONVERGING NOZZLE AND THE EFFECT OF NOZZLE PRESSURE RATIO FOR AREA RATIO OF 6.25 Syed Ashfaq 1 and S. A. Khan 2 1 Research Scholar, Department of Mechanical Engineering, JJT University, Jhunjhunu, Rajasthan, India, & Associate Professo r, Department of Mechanical Engineering, AAEMF’s COE & MS, Pune, Maharashtra, India, ashfaq.scholar@gmail.com 2 Principal, Department of Mechanical Engineering, Bearys Institute of Technology, Mangalore, Karnataka, India, sakhan06@gmail.com Abstract Micro jets are used in the present experimental study as an active control. Flow from converging nozzle to suddenly expanded circular duct of larger cross-sectional area than that of nozzle exit area were studied experimentally, focusing attention on the base pressure and the flow development in the duct. Mach number and nozzle pressure ratio are considered as the flow parameters. The geometrical parameters considered are the area ratio of the sudden expansion duct cross-section area to the nozzle exit area and the length to diameter ratio of the duct. To find the effect of micro jets as an active control on base pressure as well as on the flow field developed in the duct, the micro jets of 1 mm orifice diameter are used. They are located at 90 0 intervals along a pitch circle diameter 1.3 times the nozzle exit diameter. In this study, the area ratio considered is 6.25. The flow parameter, nozzle pressure ratio (NPR) used were from 1.5 to 3, in steps of 0.5 and experiments were conducted for NPR 1.5, 2.0, 2.5 and 3.0. The geometrical parameter, the length to diameter ratio of the enlarged duct was varied from 10 to 1, and tests were conducted for L/D 10, 8, 6, 5, 4, 3, 2 and 1. When the micro jets were activated they are found to influence the base region, taking the base suction to considerably higher values compared to that for without control case for most of the cases. From the present study it is found that as the NPR increases, the effect on base pressure is marginal for NPRs up to 2.5; however, at NPR 3 there is a sudden decrease in the base pressure. Further, it is found that the micro jets are very effective in controlling the base pressure and the active control in the form of micro jets does not disturb the flow field in the duct, however, wall pressure becomes oscillatory at NPR 3. Index Terms: Nozzle pressure ratio, Micro jets, Control Pressure, Base pressure, Sudden expansion --------------------------------------------------------------------- *** ------------------------------------------------------------------------ 1. INTRODUCTION Base pressure at the blunt base of projectile, missiles, and jet has long been one of the important issues from both the viewpoints of fluid dynamics as well as practical engineering applications. The base pressure characteristics of incompressible flows have been well known to date. However, the base pressure at transonic or supersonic speeds would be different due to the compressibility effects and due to the presence of the shock waves. In the present paper, an experimental study has been performed to understand the base pressure characteristics at sub-sonic, sonic and transonic speeds. An emphasis is placed on the control of the base pressure using a simple orifice in the form of micro jets. Many researchers have explored a variety of jet plumes to investigate the flow variables influencing the base pressure. The results obtained were validated with existing experimental and numerical data and discussed in terms of the base pressure and discharge coefficient of the orifice. Base pressure problems of internal flows are also important in practical applications. The base pressure will affect the performance of an ejector nozzle when operated without the secondary flow. Perhaps it may be argued that the present problem can be handled by large-scale numerical computations. Because the viscous effects through the convergent nozzle immediately upstream of a sudden expansion are very small and can be ignored. The objective of this research is the investigation on the base pressure ratio P b /P atm associated with a suddenly expanded internal flow issuing out from a convergent nozzle. The problem of sudden expansion of external compressible flow over the rear of projectiles and its relationship with the base pressure, since the base drag, which is a considerable