Laser Doppler velocimeter measurements on supersonic mixing nozzles that employ gas-trips

Abstract

A cold flow laser Doppler velocimeter study was performed in the mixing region of supersonic mixing nozzles that employed gas-trips for enhanced mixing; the nozzles were operated with room-temperature nitrogen under simulated laser flow conditions. A determination was made of three mean and turbulent velocity components, in addition to two turbulent shear stress elements, under both trip-on and trip-off operation. To preclude velocity biasing, the primary, secondary, and streams were simultaneously seeded with 0.357-/*m-diam latex spheres. The objectives of this activity were to acquire basic cold flow information on nozzles in order to help develop some insight into the tripped mixing phenomenon and to acquire information that could be correlated with sophisticated computer models of the mixing process. The velocimeter results appear to support ob- servations that were made earlier during a related laser-induced fluorescence visualization study. N many combustion devices (e.g., chemical rocket and jet engines, lasers) unmixed reactants are independently fed via adjacent mixing nozzles into a reaction chamber, where mixing and chemical reactions occur. It has been found that the mixing phenomenon has a significant effect on the per- formance of the device. Gas-trips have recently been installed in various mixing nozzles in an attempt to enhance mixing and thereby improve device efficiency. With this approach, gas is injected through small orifices that are located in the nozzles near the exit plane in an attempt to trip the flow; that is, as one theory goes, trigger premature transition from laminar flow. The use of gas-trips in the cavity mixing nozzles of chemical lasers has resulted in more than a doubling of the laser power. Infrared radiometer scans of the mixing region have verified that the power increase was due to enhanced reactant mixing. * While a number of theories have been proposed to explain this enhanced mixing, evaluation of these theories has been frustrated by a lack of extensive experimental in- formation. Because of this, a cold flow experimental project was initiated on chemical laser nozzles that employ gas-trips. The primary objective of this activity was to acquire basic gasdynamic information on supersonic nozzles, under simulated laser flow conditions, to help develop some insight into the trip-enhanced mixing phenomenon. Earlier in this program, testing was conducted on four different supersonic slit nozzle arrays that utilized gas-trips. Included were extensive pitot probing and flow visualization studies using laser-induced fluorescence of selectivity seeded iodine vapor.2 In this phase of the program, the mixing region of a BCL- 10 nozzle array was probed with a laser Doppler velocimeter (LDV). In addition to satisfying the primary program objective, a secondary goal of the LDV phase was to acquire information that could be correlated with sophisticated computer models of the mixing process.3'5 Hence, measurements were made of three mean and turbulent velocity components, as well as two turbulent shear stress elements, under both trip-on and trip-off cold flow operation. A considerable amount of gasdynamic research has been