Takeo Tomita

Visualization of the formation of separation bubbles on a bell-shaped nozzle surface in relation to serious side-load

Elimination of serious side-loads are essential in the design of first stage-rocket nozzles. Recent studies have indicated that the major origin of side-loads is a change of separation pattern. In the present paper, shear sensitive liquid crystal (SSLC) was applied to the nozzle wall to visualize changes of separation pattern and to determine the mechanism that produces side-loads. The results showed that SSLC applied to the nozzle wall successfully visualized separation, reattachment and re-separation of the boundary layer and a rapid movement of separation points. The CFD code based on the axisymmetric Navier-Stokes equation was firmly anchored by visualized shear stress distributions, wall pressure distributions and shadow graphs. Introduction Elimination of excessive side-loads during start-up and shut-down transients is one of the most difficult issues in the design of first-stage rocket nozzles. This has long been the subject of investigation but detailed studies on such things as proper contours of the nozzle which would minimize side-loads by optimizing nozzle efficiency have not been conducted. Recently, intensive studies have been conducted to determine the origin of side-loads in the Vulcain nozzle during start-up and shut-down transients [l]. M. Frev et al. |2-4l investigated truncated perfect nozzles and thrust optimized nozzles and found * Researcher, Kakuda Space Propulsion Laboratory, NAL, Member AIAA t Senior researcher, Kakuda Space Propulsion Laboratory, NAL, Member AIAA

Control of Transition between Two Working Modes of a Dual-bell Nozzle by Gas Injection

Group leader, Kakuda Space Propulsion Laboratory, NAL, Member AIAA 11 Associate Senior engineer, NASDA Copyright ©2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. that the highest side-loads occur in the thrust-optimized nozzle, when the type of separation changes from free shock separation (FSS) to restricted shock separation (RSS), or vice versa. The side-loads of the truncated perfect nozzle, in which only free shock separation occurs, are significantly lower, namely, only about one-third as high [4]. The authors are investigating methodology for the design of the nozzle contour of compressed truncated perfect (CTP) nozzles, proposed by J. D. Hoffman [5], which would optimize nozzle efficiency without severe side-loads. A CTP nozzle contour is obtained by linearly compressing the truncated perfect nozzle (TP nozzle) contour in the axial direction to obtain the desired nozzle length. A discontinuity in the nozzle slope produced in the above compression procedure is e l iminated by introducing a cubic equation which smoothly connects the linearly compressed curve to the initial circular curve. If strong compression is applied to a TP nozzle with a nozzle length longer than that of the original TP nozzle, the nozzle exit angle will be smaller compared with that of the original TP nozzle, resulting in reduction of divergence loss with a probable consequent Fig. 1 Cold flow test facility

Combustion and Heat Transfer Modeling in Regeneratively Cooled Thrust Chambers (Optimal Solution Procedures for Heat Flux Estimation of a Full-Scale Thrust Chamber)

*† ‡ For a dual-bell nozzle to be employed in a future reusable launch vehicle, active control of working mode transition is essential. To establish a technique for such control, the effectiveness of secondary injection was investigated by conducting cold-flow tests. The secondary injection was introduced from the inflection position of the nozzle contour perpendicular to the thrust axis. As a result, the secondary injection successfully changed the transition condition. Based on the results, a system and a sequence to realize the active control of the transition between two working modes of a dual-bell nozzle are herein discussed.

INFLUENCE OF A GAP FOR FILM COOLING ON TRANSIENT FLOW CHARACTERISTICS OF ROCKET ENGINE NOZZLES

Combustion flowfields in GH2/LOX sub-scale calorimeter chambers with multi-injector elements and full-scale thrust chamber are investigated using Reynolds-Averaged NavierStokes simulation, in which the finite rate chemistry with the H2/O2 detailed reaction mechanism is taken into account. The computed wall heat flux distributions are compared to that of the simplified cases to reduce a computational cost. The considered simplifications are a presence of reaction and a number of injector rows. At first, these simplifications are validated in the simulation of sub-scale chambers. The reaction is essential for the prediction of heat flux because it makes change the species distribution in a thermal boundary layer on a thrust chamber wall. A heat flux using a combustion simulation with only outermost injectors shows a good agreement with that with an original configuration near a face plate. On the other hand, it overestimates the heat flux around nozzle and throat parts. It is clarified that this overestimate comes from the shortage of unburned hydrogen near a chamber wall in the simplified method. Next, the simplification of the number of injector rows are applied to the simulation of full-scale thrust chambers. The effectiveness of this simplification for the prediction of wall heat flux is revealed. The optimal solution by using of the simplification is proven to be effective for the prediction of heat flux in a full-scale thrust chamber.

Aerodynamic Characterization of Linear Aerospike Nozzles in Off-Design Flight Conditions

Axisymmetric computation was applied to the transient flow in a sub-scaled LE-7A nozzle with three different configurations of a joint section between its upper nozzle and nozzle extension sections, such as a backward facing step for film-cooling, to investigate the influences of the flow disturbances produced by such nozzle surface shapes on the transient flow characteristics and to examine the possible cause of side-loads observed in the experiments. Rapid movement of the separation point occurred when it passed through the joint section. One of reasons for the occurrence of a highly asymmetrical flow causing large side-load is considered to be that the location of the separation point becomes very sensitive to the flow condition when the separation point passes through the joint section.

Effects of Plume Impingements of Clustered Nozzles on the Surface Skin Friction

Spike surface flowfields and base wake behaviors of clustered linear aerospike nozzles under the presence of external flow were characterized using surface pressure measurements, flow visualizations using the background-oriented schlieren method, and theoretical modeling. The test model consisted of three clustered cell nozzle modules or a nonclustered two-dimensional nozzle with an exit Mach number of 3.5, followed by a straight section and a contoured spike. The model was exposed to an external flow of Mach 2.0 to simulate off-design transonic flight conditions. The measured surface pressure distributions indicated that the entire flowfield on the spike surface became more two-dimensional under the presence of external flow. Periodic compressions and expansions of the cell nozzle jet were eliminated, but the pressure continued to increase downstream. The results of the flow visualizations found that this was mainly governed by the external flow that controls the jet expansion by forming a slip surface. ...

Flow Visualizations of A Simplified Linear Aerospike Model Using Background Oriented Schlieren

Skin-friction values on the surfaces of clustered nozzles were measured using a pointwise local skin-friction sensor. This sensor has a floating head that can detect the local shear force using the flow passing over the head surface. Cold-flow wind-tunnel experiments were carried out with clustered nozzle models. The test models had three or four clustered cell nozzles, or a nonclustered two-dimensional nozzle with a designed Mach number of 3.5, followed by a straight planar section. The experimental conditions were chosen according to the cell nozzle jet expansions. Hence, some conditions involved an oblique shock wave impinging on the bottom surface and others did not. The measured local skin-friction coefficients were compared to those predicted by using an existing prediction model. The results showed that the local skin-friction value significantly increased when the cell nozzle expansion was under the overexpansion condition, whereas the measured values were identical to those predicted by the exist...

Experimental Study on the Aerodynamic Performance of Clustered Linear Aerospike Nozzles

Flow interaction between external flow and cell jet flows of a simplified clustered linear aerospike model was experimentally investigated based on the surface pressure measurements and a flow visualization using background oriented Schlieren (BOS) technique. The test model has four clustered cell nozzles with a designed Mach number of 3.47 and is connected to a straight section following a 12-degree-inclined straight ramp. It is set up in a Mach 2.0 supersonic wind tunnel to simulate conditions of supersonic external flow impinging on the cell nozzle jets and ramp surface. Under conditions with or without external flow, pressure distributions on the ramp surface were measured and compared to flow visualization results. Results of measured pressure distributions on the test model surface for the with-external flow case were obviously different due to the external flow: ramp surface pressures were maintained at a certain level above the environmental pressure level according to the cell jet expansion conditions, and the pressure distribution for conditions without-external flow can be nearly approximated by the method of characteristics. Furthermore, the external flow makes the entire flowfield on the ramp more uniform compared to that for the conditions without-external flow by eliminating periodic compression/expansion feature of jet. Flow visualization found that those tendencies are mainly governed by the fact that the external flow controls jet expansion by pressing it against the ramp wall surface. While flowfield was investigated, BOS technique itself was assessed to have better sensitivity and accuracy for a given setup.

Combustion Instability Phenomena Observed During Cryogenic Hydrogen Injection Temperature Ramping Tests for Single Coaxial Injector Elements

Aerodynamic performance of the simplified test model of clustered linear aerospike nozzles was experimentally investigated and results were compared with those of a theoretical model. The model is focused on predicting the ramp pressure distribution in the case of external-flow-exposed conditions. The test model has three or four clustered cell nozzles with a designed Mach number of 3.47 and is connected to a straight section and a 12degree-inclined straight ramp. It is set up in a Mach 2.0 supersonic wind tunnel to simulate conditions of supersonic external flow impinging on the cell nozzle jets and ramp surface. Under conditions with or without external flow, pressure distributions on the ramp surface and cell base pressures were measured. Results of measured pressure distributions on the test model surface were obviously different due to the external flow: ramp surface pressures were maintained at a certain level above the environmental pressure level according to the cell flow conditions, and the pressure distribution for conditions without-external flow can be nearly approximated by the method of characteristics. This pressure behavior was successfully predicted by the theoretical model established in this study. Furthermore, the external flow makes the entire flowfield more two-dimensional compared to that for the conditions without-external flow. The influence of external flow on the cell base pressure behavior was found as reduced cell base pressure distribution due to virtually and locally accelerated external flow at the lip on the top wall of the cell nozzle exit.

A Combustion Instability Phenomenon on a LOX/Methane Subscale combustor

For LOX/LH2 shear coaxial injectors, it is well-known that high-frequency combustion instabilities may occur when the injection temperature of hydrogen decreases below a certain value, but the mechanism of the initiation of combustion instability with a coaxial injector is still not clear. In the present study, firing tests were conducted with five types of single shear coaxial injector elements by using LOX and LH2 as propellants to further investigate the mechanism of the initiation of combustion instability during temperature-ramping changes during hydrogen injection. Results showed that unstable combustion was initiated when the hydrogen injection temperature decreased to less than a certain cryogenic temperature. The combustion instabilities observed in the present firing tests are discussed and classified into three different types.