Optimization of Second Throat Ejectors for High-Altitude Test Facility

Abstract

In the present work, a second throat ejector using nitrogen as the primary fluid is considered for the creation of a low vacuum in a high-altitude testing facility for large-area-ratio rocket motors. Detailed numerical investigations have been carried out to evaluate the performance of the ejector for various operational conditions and geometric parameters during the nonpumping and pumping modes of operation. In the nonpumping mode, the lowest vacuum chamber pressure is attained when the primary jet just expands up to the mixer throat and the resulting single shock cell seals the throat against any backflow. The study illustrates how each geometric and operational parameter of the ejector can be optimized to meet the test requirements in a high-altitude testing facility by ensuring that the primary jet completely expands without a strong impact on the duct wall. When the rocket motor is fully started, due to the self-pumping action, the required vacuum is almost maintained by the exhaust flow itself and the external nitrogen ejector plays only a supplementary role. Numerical predictions for both nonpumping and pumping modes of operation have been validated with experimental data obtained from a scaled-down model of a high-altitude testing facility.Nomenclature d 0 = ejector inlet diameter d 1 = primary nozzle throat diameter d 2 = primary nozzle exit diameter d 3 = mixer throat diameter L 1 = length of the entry duct L 2 = length of the mixing cone L 3 = length of the mixer throat L 4 = length of the divergent duct L 5 = length from primary nozzle exit to mixer throat entrance _ m 1 = flux of secondary flow (rocket exhaust water spray) _ m 2 = flux of primary flow (gaseous nitrogen) P = pressure q = heat transfer per unit area v = velocity = mixing cone convergent angle = entrainment ratio (ratio of secondary mass flux to primary mass flux) = density rr = normal stress in radial direction zr , rz = tangential stress zz = normal stress in axial direction = hoop stress = viscous dissipation Subscripts c = chamber p = primary r = radial s = suction (secondary) v = vacuum z = axial

Performance Evaluation of Second-Throat Diffuser for High-Altitude-Test Facility

“…A prescribed backpressure value below p atm implies the usage of a suitable external ejector to pump the exhaust gas to the atmosphere; on the other hand, the cases of p e > p atm correspond to the situation with no external ejector. More details regarding the external ejector geometry requirements and operating conditions for the testing of large-area-ratio rocket motors are reported elsewhere [26]. The types of steady and unsteady flow simulations carried out in this study are listed in Table 3.…”

Performance Analysis of an Ejector-Diffuser for Vapor Jet Refrigeration

“…Under certain conditions, if the back pressure at primary nozzle exit is higher than the corresponding second critical pressure, shocks will be formed in the divergent portion which give rise to subsonic flow and flow separation. The nozzle flow is said to be "unstarted" ( [8], [9]) under such a condition. However, it is possible to drive the shocks out of the nozzle, either by lowering the back pressure or by increasing the stagnation pressure at nozzle inlet and the nozzle flow will be in "started" condition at this juncture.…”

Investigation on the pressure matching performance of the constant area supersonic-supersonic ejector

“…Its simplicity makes supersonic ejectors found many applications in engineering, such as highaltitude test facilities [1], ejector ramjets [2], and ejector based refrigeration cycle [3], etc. One fluid with higher total energy is the primary flow, and the other fluid with lower total energy is the secondary flow.…”