Michael Oschwald

Experimental and Numerical Investigations of Thermal Stratification Effects

In the frame of this work, thermal stratification effects withincooling channel flow of hydrogen cooled combustion chambers are investigated with theoretical approaches. Experimental results with a hydrogen cooled cylindrical combustion chamber segment with four different cooling channel geometries are presented. A new stratification approach that consideres the limited mixing capabilities within the turbulent core flow of the cooling channel is validated with the help of the experimental results.

Atomization and Flames in LOX/H2- and LOx/CH4- Spray Combustion

Hydrogen is a widely used rocket fuel and methane is particularly of interest in Europe as a promising substitute for H2. Experimental investigation of cryogenic reactive coaxial sprays with oxygen as an oxidizer and hydrogen and methane as fuels is conducted to prove whether concepts from LOX/H2 injector design can be transferred to LOX/CH4 injection. The liquid oxygen has been atomized in shear co-axial atomizers, the sprays and the flames have been investigated by visualization methods like shadowgraphy and imaging of the flame emission. LOX-sprays are characterized for both propellants by the intact core lengths and droplet numbers and the combustion is analyzed in terms of the flame anchoring mechanism and the flame spreading angle. The results for LOX/H2- and LOX/CH4-spray combustion are compared, also the influence of the injection conditions of the propellants on atomization and spray flame is discussed. Significant differences of the sprays and flames have been observed for the two propellant combinations at similar injection conditions as defined by Weber number and momentum flux ratio. The flame stabilization process has shown a strong influence on the atomization and flame characteristics.

Ignition of Cryogenic H2/LOX Sprays

First results obtained during the investigation of the transient ignition process in a cryogenic model rocket combustion chamber are reported. Ignition of the propellants LOX and GH2 has been initiated with a pulsed laser and ignition and flame stabilization processes have been analyzed using high-speed visualization methods. Local flame velocities as well as local convection velocities at the point of ignition are determined by image displacement velocimetry methods. Depending on the injection conditions of the propellants, some distinct phases of the transient start-up process could be observed and are discussed in terms of Weber and Reynolds number.

Injector-Driven Combustion Instabilities in a Hydrogen/Oxygen Rocket Combustor

Self-excited combustion instabilities of the first tangential mode have been found in a research combustor operated with the cryogenic propellant combination of hydrogen/oxygen. In a series of consecutive test campaigns, the influence of operating conditions on these self-excited combustion instabilities was examined. This included a variation of the combustion chamber pressure, the mixture ratio, and the propellant temperatures. It has been shown how these operating parameters influence the resonance frequencies of the combustion chamber. The analysis of the influence of operating conditions on the oscillation amplitude of the first tangential mode indicated that the instability occurred when the frequency of the first tangential mode of the combustion chamber was shifted into the frequency of the second longitudinal mode of the liquid oxygen injector. With a variation of the injector length, and therefore its longitudinal resonance frequencies, this hypothesis has been tested. Based on the experimental ...

Resonance Frequencies and Damping of a Combustor Acoustically Coupled to an Absorber

The coupled acoustic system of a cylindrical combustor and an absorber cavity has been investigated experimentally in hot-fire and cold-flow tests. The addition of an absorber volume to the combustor is shown to result in eigenfrequencies of the coupled system systematically different from that of a pure cylindrical resonance volume. The general behavior of the coupled system is explained based on an analytical one-dimensional analysis of the problem. Numerical simulations in two dimensions result in very good agreement with experimental observations. A three-dimensional simulation of a cylinder equipped with 40 absorbers demonstrates the relevance of the reported phenomena for rocket engines equipped with absorber rings. Experimental data on the damping behavior of an absorber are given and the damping of absorbers is discussed based on the numerically obtained acoustic eigenmodes of the coupled system.

LOx Jet Atomization Under Transverse Acoustic Oscillations

Testing has been conducted with the BKH rocket combustor at the European Research and Technology Test Facility P8 for cryogenic rocket engines at DLR Lampoldshausen. BKH has multiple shear coaxial injectors and an exhaust modulation system for forcing excitation of acoustic resonances in the combustion chamber. Optical access windows allow the application of parallel high-speed shadowgraph and flame emission imaging of the near-injector region. This paper reports measurements of the intact liquid oxygen core during forced excitation of the first transverse acoustic mode. High-speed shadowgraph images show that the mechanism of core breakup and atomization differs between off-resonance and first transverse excitation conditions. The core length is found to decrease with increasing amplitude of acoustic pressure, or equivalently with transverse acoustic velocity, with a core length reduction of up to 70% for conditions approaching those of naturally occurring high frequency combustion instabilities. This de...

Thermo- and Fluidmechanical Analysis of High Aspect Ratio Cooling Channels

This study compares the results of three-dimensional and one-dimensional thermal and fluid flow analysis methods, applied to electrically heated tube flow experiments and also to cooling passages of regeneratively cooled combustion chamber well structures. It will be shown, that both methods can be applied very well to the simple case of the tube flow, but the more complex geometry and the boundary conditions in the case of a regeneratively cooled combustion chamber wall structure brings along a great deviation between both methods. This lies in the fact that one dimensional analysis tools neglect thermal stratification. The solution of the Navier Stokes and the energy equation is necessary to include this effect, but this method is to computationally costly to the part of an optimization of regeneratively cooled wall structures. Therefore a new stratification approach based on the usage of empirical correlations for heat transfer and pressure loss is developed. On base of the results of the analysis an experiment is planned to validate the theoretical models.

Theoretical and Experimental Identification of Acoustic Spinning Mode in a Cylindrical Combustor

The analytical solution for the eigenmodes of a cylindrical combustor allows for several realizations of the first tangential mode. The limiting cases of standing and spinning first tangential modes are well known. The general solution for a first tangential mode, however, allows for a large variety of realizations. In this study the analytical solution of the acoustic problem and a numerical time-resolved simulation of the acoustic pressure field are compared with experimental data from hot-fire tests. The experimentally observed pressure data are in excellent agreement with the simulation for all possible theoretical solutions. The approach enables detailed insight into the dynamics of the pressure field and thus constitutes a tool for the characterization of acoustic pressure waves in a cylindrical combustor even when the amplitude of the investigated eigenmode is small as compared with the mean pressure fluctuation.

Experimental Investigation on LOX/CH4 Ignition

Investigation of ignition phenomena is a first step towards obtaining a better understanding of combusting LOx/CH4 propellants. Safe and reliable ignition is essential for a long-life engine design. Atomization and vaporization of a liquid oxygen jet are controlling processes during the ignition transient. The aerodynamic forces between the liquid phase and the coaxial gaseous stream mainly control primary atomization. Non-dimensional numbers are generally used to characterize the atomization process, in particular, the momentum flux ratio J and the We number. To characterize the ignition phase of LOx/CH4, a novel propellant combination being investigated in EU at the moment, an experimental investigation has been performed in the M3 test facility at DLR-Lampoldshausen, Germany. The test bench is equipped with a single-injector combustion chamber with optical access for visualizing the flame development. The mixture ratio O/F was fixed to 3.4. Propellants mass flow rates were chosen to achieve 1.5 bar combustion chamber pressure once a steady state condition is reached. The coaxial injector geometry can be modified to achieve different injection conditions. A pilot flame provides an ignition source after stable cold-flow injection conditions are achieved. This configuration has been chosen to simulate real rocket engine start-up conditions. Injection conditions included a combination of J (from 0.1 to 2.0) and We number (from 2000 to 15000). Flame development was observed by means of an intensified, high-speed CCD camera. The camera is fitted with a UV lens and a narrow band filter to record the OH radical emission created during the combustion process. In this paper, a systematic description of the LOx/CH4 flame development is provided. Due to high image acquisition frequency, the different evolution phases of the flame have been recorded and used to estimate flame propagation velocities. Results are correlated with the J, We and Re numbers to assess the sensitivity of the system to these crucial parameters.

Investigation Of Sub- And Supercritical LOX/Methane Injection Using Optical Diagnostics

The DLR Institute of Space Propulsion at Lampoldshausen has been working on dierent aspects of oxygen/methane combustion for a couple of years. Within this framework, the European High Pressure Research and Technology Test Facility P8 was equipped with a methane fluid system as a first step to enable single injector investigations of high pressure methane injection and combustion processes. The fluid system provides ambient temperature methane at flow rates up to 1 kg/s and at combustion chamber pressures up to 10 MPa. The measurement system has been adapted to the new fluid system to provide a precise determination of injection conditions. Hence, the P8 now allows investigations of LOX/CH4 combustion chamber processes under realistic rocket engine conditions. For the first operational tests, a single injector windowed rocket combustion chamber was operated at various steady-state conditions in the sub-, near- and supercritical regime with respect to the critical pressure of oxygen. The propellants were injected through a single shear coaxial injector element at temperatures of about 120 K and 275 K, respectively. High speed optical diagnostic techniques such as flame emission spectroscopy and shadowgraphy have been applied in the near injector region to assess injection and atomization behavior as well as flame anchoring and stabilization. The paper describes the modifications of fluid and MCC systems at P8 and presents preliminary results of the first test campaign.