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A numerical study is conducted to investigate the impact of a film-cooled dual-bell nozzle extension on its operation mode transition behavior. Therefore, unsteady Reynolds-averaged Navier–Stokes s...

The design of a film cooled dual-bell nozzle is presented. The nozzle is part of a thrust chamber assembly that adopts an existing LOX/GH2 thrust chamber. The dual-bell base nozzle, including the gaseous hydrogen cooling film injection, is a downscaled redesign of an already tested film cooled TIC nozzle. Future hot flow tests at the test facility P8 will study the impact of a ROF variation and a cooling film mass flow variation on the operation mode transition of the dual-bell. For this reason, a homogeneous hot flow and cooling film distribution are mandatory. To meet those demands, extensive numerical studies were performed and design optimizations were derived. The test specimen will be operated under sea level conditions.

International audience; The main objective of a dual bell nozzle is the enhancement of performances based on the principle of auto-adaptation in accordance with the altitude. Indeed, this system has as advantage the auto-adaptation of the flow for two operating modes (at low and high altitude) without mechanical activation. The principle is theoretically simple but structural forces involved can be significant. In this study, a numerical method for the design of this type of nozzle is developed. On the one hand, it is based on a transonic flow approaches to define the starting line on which the supersonic calculations will be initiated. On the other hand, the method of characteristics is used to draw the base nozzle profile. Knowing that the latter is assimilated as a polynomial of the second degree, its constants are calculated from initial conditions. In order to minimize the weight of this nozzle, its truncation proves necessary; this is performed at a point where the best compromise (weight / performances) was respected. The profile of the second curve is calculated to give a constant wall pressure. This is achieved by using the direct method of characteristics applied for a centered expansion wave that the intensity is P2/P1 at the junction. Once the profile is generated, an analysis of the thermodynamic-parameters evolution (such as: pressure, Mach number) and aerodynamic performances is conducted. For more consistency, our results are compared with numerical databases of ONERA nozzle. Simulations of flow in the nozzle with Ansys 13.0 environment for different types of meshes are presented. Also, to offset the effects of the boundary layer, the simulations were performed by using the k-ω SST turbulence model. The obtained results by the method of characteristics and numerical simulation are compared to the computed results of the literature and it was found good agreement and similarity.

A sub-scale test campaign has been conducted in the framework of cooperation between the German Aerospace Center (DLR) and the Japan Aerospace Exploration Agency (JAXA). Both partners had great experience in cold flow dual bell nozzle investigation. The present campaign took place at P6.1 test facility at DLR Lampoldshausen with a common combustion chamber. Two test specimens were tested with similar conditions and measurement systems. The conditions provided at the bench were the combustion of gaseous methane with liquid oxygen at pressure up to 60 bar. The combustion chamber pressure was varied up and down with constant ramps while keeping the mixture ratio constant. Objective of the study was, a part from demonstrating the precision of the design tool, to investigate the sensitivity of the transition to flow property variations (like the one due to the mixture ratio). The transition and retransition conditions, the range of hysteresis and the transition velocity were of particular interest for DLR side. The work of JAXA concentrates on heat flux and combustion instabilities and is presented in the second part of this work.

The dual-bell nozzle is an altitude-adaptive nozzle concept. It combines the advantages of a nozzle with small area ratio under sea-level conditions and a large area ratio nozzle under high-altitude conditions. Reynolds-averaged Navier–Stokes and unsteady Reynolds-averaged Navier–Stokes simulations on two-dimensional axisymmetric grids were conducted at DLR, German Aerospace Center in Lampoldshausen to investigate the transition from one mode to the other of a dual-bell nozzle model with positive pressure gradient extension. A cold flow test campaign conducted at DLR’s cold flow test facility P6.2 provided validation data for the numerical approach. The present study investigates the influence of different turbulence models and feeding pressure gradients on the dual-bell flow transition behavior. Better results were achieved for the Spalart–Allmaras and Reynolds stress turbulence model. A clear impact of the feeding pressure ramp on the dual-bell transition pressure ratio and the flow separation position ...

Dual bell nozzle is a promising one among the altitude adaptation nozzle concepts, which offer increased nozzle performance in rocket engines. Its advantage is the simplicity it offers due to the absence of any additional mechanical device or movable parts. Hence it offers reliability along with improved nozzle performance as demanded by future launch vehicles. Among other issues, the flow transition to the extension nozzle of a dual bell nozzle is one of the major issues being studied in the development of dual bell nozzle. A parameter named over-expansion factor, which controls the value of the wall inflection angle, has been reported to have substantial influence in this transition process. This paper studies, through CFD and cold flow experiments, the effect of overexpansion factor on flow transition in dual bell nozzles., {"references":["Hagemann, G., Immich, H., Nguyen, T. V., and Dumnov, G. E.,\n\"Advanced Rocket Nozzles,\" Journal of Propulsion and Power, Vol. 14,\nNo. 5, September-October 1998 pp. 620-634","Genin, C., and Stark, R., \"Experimental Study on Flow Transition in\nDual Bell Nozzles,\" Proceedings of the 45th AIAA/ASME/SAE/ASEE\nJPC and Exhibit, August 2009, Denver, Colorado. AIAA 2009-4855.","Bijukumar K. S., Mathew George, Baiju A. P., Balachandran P.,\nRajamanohar and N. K. Gupta, \"Development of Dual Bell Nozzle for\nFuture Launch Vehicles,\" Proceedings of IISc Centenary Conference &\nExhibition on Aerospace Engineering 2009,May 2009, Bangalore, India.","Tomita, T., Takahashi, M., and Sasaki, M., \"Investigation on\nCharacteristics of Conventional-Nozzle-Based Altitude Compensating\nNozzles by Cold-flow Tests (II)-Side-Load Characteristics During\nTransition,\" Proceedings of the 43rd AIAA/ASME/SAE/ASEE JPC and\nExhibit, July 2007, Cincinnati, OH. AIAA 2007-5472.","Tomita, T., Takahashi, M., Sasaki, M., and Tamura, H., \"Investigation\non Characteristics of Conventional-Nozzle-based Altitude Compensating\nNozzles by Cold-Flow Tests,\" Proceedings of the 42nd\nAIAA/ASME/SAE/ASEE JPC and Exhibit, July 2006, Sacramento,\nCalifornia. AIAA 2006-4375.","Kimura, T., Niu, K., Yonezawa, K., Tsujimoto, Y., and Ishizaka, K.,\n\"Experimental and Analytical Study for Design of Dual Bell Nozzles,\"\nProceedings of the 45th AIAA/ASME/SAE/ASEE JPC and Exhibit,\nAugust 2009, Denver, Colarado. AIAA 2009-5149.","Miyazawa, M., Takeuchi, S., Takahashi, M., \"Flight Performance of\nDual-Bell Nozzles,\" AIAA 2002-0686","Otsu, H., Miyazawa, M., Nagata, Y., \"Design Criterion of the Dual-Bell\nNozzle Contour,\" IAC-05-C4.2.08.","Nasuti, F., Onofri, M., and Martelli, E., \"Role of wall shape on the\nTransition in Dual-Bell Nozzles,\" 39th AIAA/ASME/SAE/ASEE JPC\nand Exhibit, Jul. 2003, Huntsville, Alabama. AIAA 2003-4911"]}

The nozzle are currently presenting the most promising optimisation possibilities to supply the increasing needs of the aerospace industry. The dual bell nozzles allow the altitude adaptation of the flow. The inflection forces the flow to a stable and symmetrical separation, gencrating an optimised thrust for sca leve! operation. Duting the flight, when the ambient pressure decreases under a certain value, the flow suddenly attaches the nozzle wall down to the nozzle end, generating a higher expansion for higher performance.The study presentcd in this document was made at the DLR (German Aerospace Center) as part of a PhD. This fundamental study has for objective the understanding of the flow behaviour in a dual bell nozzle, in order to define the various parameters of influence. A cold flow test series has been conducted on various nozzle geometries. Pressure measurementshave been made and the flow has been observed with schlieren optics in order to determine the influence of the geometry on the evolution of the flow transition from one operating mode to the other, its duration and stability toward pressure variations. Temperature and pressure measurements were also made during a test series under warm gas conditions to investigate the thermal load in the vicinity of the wall inflection.; Les tuyères sont actuellement le sous-système présentant les possibilités d'optimisation les plus prometteuses pour répondre aux besoins toujours croissants de l'industrie aérospatiale.Les tuyères dual bell permettent une adaptation à l'altitude. L'écoulement est forcé à décoller de façon stable et symétrique à l'inflexion, générant ainsi une poussée optimisée pour le fonctionnement à basse altitude. Au cours du vol, lorsque la pression atmosphérique chute sous une certaine valeur, l'écoulement attache soudainement dans toute la tuyère, permettant une détente plus importante pour une poussée plus élevée.L'étude présentée dans ce document a été réalisée au DLR (centre aérospatial Allemand) dans le cadre d'un travail de thèse. Il s'agit d'une étude fondamentale visant à comprendre le comportement de l'écoulement dans une tuyère dual bell, afin de mettre en évidence les paramètres d'influence. Une série de tests en gaz froid a été réalisée sur différentes géométries de tuyères. Des mesures de pression ainsi qu'une observation de l'écoulement à l'aide d'une méthode stéréoscopique de schlieren a permis de conclure sur l'influence de la géométrie sur le déroulement de la transition d 'un mode de fonctionnement à l'autre, sa durée et sa stabilité face aux variations de pression. Une série de tests en gaz chaud a permis de réaliser des relevés de température et pression afin d 'étudier les charges thermiques dans la paroi au voisinage de l'inflexion.

nozzle flow regimes. The most important are strong short time peaks induced by transition and retransition from sea level to high altitude mode and vice versa. In sea level and high altitude mode the dual bell creates less side loads than a comparable conventional nozzle. An additional flow regime could be confirmed, addressed as sneak transition. Contrary to the accepted doctrine, the flow separation in sea level mode will not be continuously fixed at the contour inflection. Before the actual transition to high altitude mode takes place the flow separates within the inflection region. As a result a distinct phase of side loads comparable to separated conventional nozzles is given. A procedure to reduce the regime of sneak transition is proposed.

The contour inflection of the dual bell nozzle forces the flow to a symmetrical and controlled separation under sea level conditions. At a certain altitude, the transition to high altitude mode takes place: the flow attaches rapidly the nozzle extension wall down to the exit plane. During this transition, the separation point moves in the extension generating potential high side load peaks due to its asymmetrical position. A campaign of cold flow tests has been conducted to evaluate the generation of side load. The phenomenology during each phase of the flow has been presented in the first part of this work. The influence of the various geometrical parameters on flow behavior and side load generation is investigated in this study. The extension length is shown to be the critical parameter for flow stability, transition duration and side load generation. A trade o has to be found for the optimization of the dual bell concept to rocket applications.

The dual bell nozzle is a concept of altitude adaptive nozzles. The flow adapts to the altitude by separation at the wall inflection at low altitude, and full flowing at high altitude. To understand the phenomenology of the flow by the transition from sea level to high altitude mode, a series of tests have been made at the cold flow test bench P6.2 at DLR Lampoldshausen. Three nozzles with different geometries have been tested. Two of them were successively shortened and driven unter the same conditions for each extension length. This study yields the influence of the geometric parameters of the basis and the extension on the transition conditions. Furthermore a transition prediction is given.

The dual-bell nozzle concept has been investigated by means of analytical and experimental work. Based on earlier analytical and numerical work published by the authors, different dual-bell nozzles were designed and experimentally tested, in order to explore this concept regarding its aerodynamic characteristics. This experimental work included cold gas subscale tests and hot gas subscale tests, which were performed within a joint German/Russian Research Programme TEHORA, and complementary in the German National Technology Programme TEKAN. It is shown that depending on the type of nozzle contour used for the dual-bell nozzle extension, a sudden transition from sea-level to altitude mode operation can be achieved. Furthermore, important information from the hot gas tests regarding the wall heat transfer were gained.

The flow field of a cold flow dual bell has been numerically simulated with the DLR TAU code. The two operation modes and the transition from one mode to the other are investigated by varying the nozzle pressure ratio. A cold flow test campaign on a sub-scale nozzle model yields experimental data for validation of the simulation. Pressure measurements along the nozzle wall and schlieren optics provides information on the flow field in the nozzle. In addition, the models have been successively shortened to allow the observation of the shock system in the vicinity of the contour inflection.