Performance Evaluation of Second-Throat Diffuser for High-Altitude-Test Facility
01 Mar 2010-Journal of Propulsion and Power (American Institute of Aeronautics and Astronautics (AIAA))-Vol. 26, Iss: 2, pp 248-258
TL;DR: In this paper, the performance of a second-throat ejector diffuser system employed in high-altitude testing of large-area-ratio rocket motors is considered under various steady and transient operating conditions.
Abstract: The performance of a second-throat ejector―diffuser system employed in high-altitude testing of large-area-ratio rocket motors is considered under various steady and transient operating conditions. When the diffuser attains started condition, supersonic flow fills the entire inlet section and a series of oblique shock cells occurring in the diffuser duct seal the vacuum environment of the test chamber against backflow. The most sensitive parameter that influences the stagnation pressure needed for diffuser starting is the second-throat diameter. Between the throat and exit diameters of the nozzle, there exists a second-throat diameter value that corresponds to the lowest stagnation pressure for starting. When large radial/axial gaps exist between the nozzle exit and diffuser duct, significant reverse flow occurs for the unstarted cases, which spoils the vacuum in the test chamber. However, the starting stagnation- pressure value remains unaffected by the axial/radial gap. Numerical simulations establish that it is possible to arrive at an optimum diffuser geometry that facilitates early functioning of the high-altitude-test facility during motor ignition phase. The predicted axial variations of static pressure and temperature along the diffuser for the testing of a cryogenic upper-stage motor agree well with available experimental data.
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TL;DR: In this article, 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.
Abstract: DOI: 10.2514/1.39219 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 parametersduring thenonpumpingandpumpingmodesof operation.Inthenonpumpingmode,the lowestvacuum chamberpressureisattainedwhentheprimaryjetjustexpandsuptothemixerthroatandtheresultingsingleshock cellsealsthethroatagainstanybackflow.Thestudyillustrateshoweachgeometricandoperationalparameterofthe ejector can be optimized to meet the test requirements in a high-altitude testing facility byensuring 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.
23 citations
TL;DR: In this article, the effect of the initial expansion angle of a parabolic (TOP) nozzles on flow separation pattern and shock structure was investigated numerically, and it was shown that the presence of a restricted shock separation pattern leads to a considerable increment of the critical cross sectional area of the flow inside the diffuser, and the minimum starting pressure of the STED is increased up to 30% after the resizing of the second throat area to eliminate the flow choking inside it.
Abstract: Free or restricted shock separation phenomena can occur inside a thrust optimized parabolic (TOP) nozzle during over-expanded operations. In the case of restricted shock separation, a cap shock pattern forms in the nozzle which leads to a substantial total pressure drop. This induces further related issues in the process of ground testing of such nozzles using a second throat exhaust diffuser (STED). In the present study, the flow physics in several TOP nozzles operating at over-expanded conditions is investigated numerically. At first, the strong effect of the initial expansion angle of a TOP nozzle on flow separation pattern and shock structure is demonstrated. Results reveal that for high initial expansion angles, restricted shock separation occurs even at low nozzle pressure ratios, while free shock separation takes place for small initial expansion angles at even high nozzle pressure ratios. Subsequently, the effect of separation patterns in TOP nozzles on the starting pressure of a STED is studied. Current results show that the presence of a restricted shock separation pattern leads to a considerable increment of the critical cross sectional area of the flow inside the diffuser. Therefore, the minimum starting pressure of the STED is increased up to 30% after the resizing of the second throat area to eliminate the flow choking inside it.
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01 Nov 2019
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References
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TL;DR: In this paper, a review of the fundamental characteristics of the shock train and pseudo-shock is presented, and some simple predictions are made to simulate these very complicated phenomena, and control methods of the pseudo-shocks are also described.
Abstract: The interaction between a normal shock wave and a boundary layer along a wall surface in internal compressible flows causes a very complicated flow. When the shock is strong enough to separate the boundary layer, the shock is bifurcated and one or more shocks appear downstream of the bifurcated shock. A series of shocks thus formed, called “shock train”, is followed by an adverse pressure gradient region, if the duct is long enough. Thus the effect of the interaction extends over a great distance. The flow is decelerated from supersonic to subsonic through the whole interaction region. In this sense, the interaction region including the shock train in it is referred to as “pseudo-shock” in the present paper, as Crocco called it. The shock train and pseudo-shock strongly affect the performance and efficiency of various flow devices. In the present review some fundamental characteristics of the shock train and pseudo-shock are first described. Some simple predictions are made to simulate these very complicated phenomena. Pseudo-shocks appearing in various flow devices are explained. Control methods of the pseudo-shocks are also described. Finally, the current understanding of self-excited oscillation of pseudo-shock is reviewed.
478 citations
"Performance Evaluation of Second-Th..." refers background in this paper
...The challenging task in designing such a diffuser [8–12] is to keep the facility length and starting pressure ratio as low as possible....
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TL;DR: In this paper, the performance of six well-known turbulence models for the study of supersonic ejectors was evaluated and the results showed that the k-omega-sst model agrees best with experiments.
Abstract: Supersonic ejectors are widely used in a range of applications such as aerospace, propulsion and refrigeration. The primary interest of this study is to set up a reliable hydrodynamics model of a supersonic ejector, which may be extended to refrigeration applications. The first part of this work evaluated the performance of six well-known turbulence models for the study of supersonic ejectors. The validation concentrated on the shock location, shock strength and the average pressure recovery prediction. Axial pressure measurements with a capillary probe performed previously [Int. J. Turbo Jet Engines 19 (2002) 71; Conference Proc., 10th Int. Symp. Flow Visuzlization, Kyoto, Japan, 2002], were compared with numerical simulations while laser tomography pictures were used to evaluate the non-mixing length. The capillary probe has been included in the numerical model and the non-mixing length has been numerically evaluated by including an additional transport equation for a passive scalar, which acted as an ideal colorant in the flow. At this point, the results show that the k–omega–sst model agrees best with experiments. In the second part, the tested model was used to reproduce the different operation modes of a supersonic ejector, ranging from on-design point to off-design. In this respect, CFD turned out to be an efficient diagnosis tool of ejector analysis (mixing, flow separation), for design, and performance optimization (optimum entrainment and recompression ratios).
323 citations
Additional excerpts
...Numerical investigations [19–24] are quite helpful in analyzing the performance of different ejector– diffuser subsystems under design and offdesign modes of operation and also for understanding the flow features, including the structure of shock cells....
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TL;DR: In this paper, the response of choked nozzles and supersonic diffusers to one-dimensional flow perturbations is investigated and a set of boundary conditions is developed that extends the existing work to a nozzle of arbitrary geometry.
Abstract: The response of choked nozzles and supersonic diffusers to one-dimensional flow perturbations is investigated. Following previous arguments in the literature, small flow perturbations in a duct of spatially linear steady velocity distribution are determined by solution of a hyper-geometric differential equation. A set of boundary conditions is then developed that extends the existing work to a nozzle of arbitrary geometry. This analysis accommodates the motion of a plane shock wave and makes no assumption about the nozzle compactness. Numerical simulations of the unsteady, quasi-one-dimensional Euler equations are performed to validate this analysis and also to indicate the conditions under which the perturbations remain approximately linear. The nonlinear response of compact choked nozzles and supersonic diffusers is also investigated. Simple analyses are performed to determine the reflected and transmitted waveforms, as well as conditions for unchoke, 'over-choke' and unstart. This analysis is also supported with results from numerical simulations of the Euler equations. © Cambridge University Press 2007.
104 citations
"Performance Evaluation of Second-Th..." refers background in this paper
...D IFFUSERS find application in many fluid systems such as aircraft, gas turbines, ramjets, wind tunnels, high-altitude-test facilities, and jet pumps [1–3] for converting the flow kinetic energy into static-pressure rise....
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TL;DR: In this article, a straight cylindrical supersonic exhaust diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids were used to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (Ad/At), SED areas to rocket exhaust manifold exit area, SED length to its diameter (L/D), and specific heat ratio of the driving gases (k) on the minimum starting pressure ratio, (Po/Pa)st, of SED.
Abstract: Experiments were carried out on straight cylindrical supersonic exhaust diffusers (SED) using cold nitrogen and hot rocket exhaust gases as driving fluids, in order to evaluate the effects of the ratios of the SED area to rocket nozzle throat area (Ad/At), SED area to rocket nozzle exit area (Ad/Ae), SED length to its diameter (L/D) and specific heat ratio of the driving gases (k) on the minimum starting pressure ratio, (Po/Pa)st, of SED. The rocket nozzle and SED starting transients were also simulated in the models. The study reveals that (Po/Pa)st increases monotonically with increase in (Ad/At) and k. One-dimensional normal shock relations were used in predicting the (Po/Pa)st since the compression in long ducts is basically a normal shock process. Predicted values of (Po/Pa)st were validated with experimental data. SED efficiency factors(ηns) were arrived at based on one-dimensional normal shock relations. ηns goes down at higher values of (Ad/Ae). (Po/Pa)st is lower for lower k values for the same (Ad/At). Cylindrical SEDs exhibit no hysteresis. The results of this investigation were utilised in validating the design of high altitude test (HAT) facility for testing the third stage motor (PS-3) of Polar Satellite Launch Vehicle (PSLV). The simulation of starting transients in the model revealed that the HAT facility shall not be operated in the unstarted phase, because the rocket nozzle may fail due to violent oscillations of the vacuum chamber pressure. These experimental data were also utilised for designing a SED for PS-3 sub-scale motor, the results of which are covered in this paper. The accuracy of measurements are within a range of ±0.4%. Error analysis of the data were carried out and are presented in Appendix A .
64 citations
"Performance Evaluation of Second-Th..." refers background in this paper
...The challenging task in designing such a diffuser [8–12] is to keep the facility length and starting pressure ratio as low as possible....
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TL;DR: Theoretical methods for estimating the required starting pressure ratio and minimum base pressure ratio of axisymmetric ejector-diffuser systems used in rocket altitude simulation are presented in this paper.
Abstract: Theoretical methods are presented for estimating the required starting-pressure ratio and minimum base pressure ratio of axisymmetric ejector-diffuser systems used in rocket altitude simulation. The starting-pressure ratio is determined by applying over-all conservation relations; the theory also predicts the limiting second-throat contraction ratio, which can exceed the normal-shock-starting contraction limit for wind tunnels. Experimental starting data from 79 axisymmetric ejector systems agree with this theory within approximately 10%. The theory for the minimum base pressure ratio for zero secondary flow was developed by modifying Korst's two-dimensional theory to be applicable to axisymmetric systems. The modification consists of a new method for computing the peak recompression static pressure. Measured base pressures of 34 ejector configurations agreed with the latter within 6%.
46 citations
"Performance Evaluation of Second-Th..." refers background in this paper
...Numerical investigations [19–24] are quite helpful in analyzing the performance of different ejector– diffuser subsystems under design and offdesign modes of operation and also for understanding the flow features, including the structure of shock cells....
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...Furthermore, the straight ejector–diffuser requires higher motor stagnation pressure to attain started condition [20]....
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