Yoshio Nunome

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

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.

High-Frequency Flame Oscillation Observed at a Coaxial LOX/LH2 Injector Element

To study the mechanism of the initiation of combustion instability as hydrogen injection temperature decreases, a hydrogen temperature ramping test was conducted with a single coaxial injection element with LOX/LH2 at a chamber pressure of 8.0 MPa. Two types of injectors were used in the tests. One employed a straight bore LOX post and the other employed a taper-reamed LOX post for better atomization of LOX. The combustion flame was visualized with a high-speed video camera at a rate of 6,000 frames per second. Results showed that unstable combustion was initiated when the hydrogen injection temperature decreased to less than a certain cryogenic temperature. By observing the movement of the prominent pattern of OH emission on the flame, the flame was found to propagate downstream at a constant speed with the flame angle remaining constant during stable combustion. On the other hand, injection pressure peaks appeared during unstable combustion. In this case, a block of flame with strong OH emission was occasionally observed. A block of flame caught up with an anterior block and coalesced into a large block with strong OH emission. This coalesced block of the flame is herein termed “flame burst”.

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

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.

Stability Index for Injection-Coupled Instability in Full-Scale Firing Tests

To construct a stability estimation tool for injection-coupled instability in rocket engines, a linear stability analysis was applied. The amplifying factor related to injector-element acoustics and the damping factor related to chamber acoustics were defined. A complex vector, which is a function of these factors, was introduced. The stability for gain and that for phase were defined by the vector, and the bulk stability was defined by these stabilities. Because of time-lag findings obtained from subscale firing tests, the instability index, which was defined as inverse of the bulk stability, was defined only by the stability of gain. To validate the tool, the stability of cryogenic oxygen/hydrogen firing tests with a full-scale chamber was rated. The average acceleration was applied as an instability index for the firing tests. By setting the instability index by the tool at 1×103 as the threshold that divides “unstable” and “stable,” the stability obtained by the tool and that of the firing test showed...

Experimental and Numerical Study on Characteristics of Fuel Mixers for a Liquid Rocket Engine

Some rocket engines have a fuel mixer upstream of the injector to mix two hydrogen flows of different temperatures. In the mixing process, this fuel mixer may generate large fluctuations of flow properties, which in turn may increase combustion pressure fluctuations. Therefore, fuel mixers must be designed carefully to prevent such large fluctuations. In addition, fuel mixers must have good mixing characteristics and be free of large flow property fluctuations even at off-design points when rocket engines require deep-throttling capability. In this study, we experimentally and numerically investigated the effects of fuel mixer configuration in a rocket engine on the downstream flow properties. In the experiments, we used three different mixer models with different cryogenic hydrogen injection hole configurations (small holes, large holes, and a mixture of both size holes), and conducted experiments using cryogenic hydrogen and gaseous hydrogen under different flow conditions corresponding to engine throttling. The mixers with large injection holes showed better mixing characteristics than the mixer with smaller holes even under conditions of throttling.