Stefan Schlechtriem

Optical Investigation of the Combustion Process in Paraffin-based Hybrid Rocket Fuels

This paper summarizes the investigations on the combustion behavior of paraffin-based hybrid rocket fuels with gaseous oxygen (GOX) as oxidizer in a 2D slab burner with windows on two sides. High speed video data and Schlieren measurements have been evaluated to give an insight into the combustion phenomena of paraffin-based hybrid rocket fuels. The combustion flame shows a periodic, wave like structure. This behavior is noticed for different types of paraffin fuels. The values of the liquid layer viscosities are the main difference between the fuels. This affects the droplet entrainment process during combustion and also the regression rates of the fuels. Entrainment and regression rate increase for decreasing fuel liquid layer viscosity.

TMF test based validation of numerical methods for the analysis of heat-loaded walls

** †† ‡‡ , The creation of validation data for CFD, thermal, structural and life time analyses of actively cooled thermally loaded wall structures of rocket engines by means of ThermoMechanical Fatigue (TMF) - tests is discussed in this paper. During a TMF test, only a small section of the hot gas wall of the real engine (the so called TMF panel) is tested. For such a TMF panel, realistic cooling conditions similar to a full scale rocket engine are chosen. The 2d measurement of the thermal field of the heat loaded structure provides (together with the measurement of the temperature, pressure and mass flow rate of the coolant of the TMF panel) data for the combined validation of the CFD analysis of the coolant flow and the thermal analysis of the wall structure. The measurement of the deformation of the thermally loaded structure provides (together with the already determined temperature distribution and the above mentioned pressure measurements of the cooling channels) data for the validation of the structural analysis of the thermally loaded structure. Counting the number of laser loading cycles (laser on-off) until the TMF panel fails (by cracks appearing on the laser loaded side of the cooling channels) provides data for the validation of (either post processing or damage parameter based) life time analyses of thermally loaded structures.

Viscosity and Regression Rate of Liquefying Hybrid Rocket Fuels

The combustion behavior of paraffin-based hybrid rocket fuels with gaseous oxygen as an oxidizer has been analyzed in detail. Regression rate tests have been done in a two-dimensional radial microburner at the DLR, German Aerospace Center and at the Space Propulsion Laboratory. Fuel samples have been characterized by viscosity measurements, tensile tests, and a differential scanning calorimeter. Tensile tests showed significant improvement in maximum stress and elongation when polymers in low concentration were added to the paraffin samples. The values of the liquid fuel viscosities differed significantly between the selected fuels. This affected the droplet entrainment process during combustion and the regression rates of the fuels. The entrainment and regression rate increased for the decreasing fuel liquid layer viscosity. An exponential relation has been found between the liquid fuel layer viscosity and the regression rate, which can be used to predict the regression rate of new liquefying fuels by measuring their viscosity.

Evaluation of paraffin-based fuels for hybrid rocket engines

This paper summarizes the investigations on the combustion behavior of paraffin-based hybrid rocket fuels with gaseous oxygen (GOX) as oxidizer. Combined experimental activities have been done at the DLR Institute of Space Propulsion in Lampoldshausen and at the Space Propulsion Laboratory (SPLab) of Politecnico di Milano. Regression rate tests have been done in a 2D radial micro burner at the DLR and at the SPLab. Fuel samples have been characterized by viscosity measurements, tensile tests and differential scanning calorimeter (DSC). Tensile tests shows significant improvement in maximum stress and elongation when polymers in low concentration are added to the paraffin samples. The values of the liquid fuel viscosities differ signifcantly between the fuels. This affects the droplet entrainment process during combustion and also the regression rates of the fuels. Entrainment and regression rate increase for decreasing fuel liquid layer viscosity. An exponential relation has been found between the liquid fuel layer viscosity and the Regression rate, which can be used to predict the regression rate of new liquefying fuels by measuring their viscosity.

Combustion Visualization and Characterization of Liquefying Hybrid Rocket Fuels

Recent results of data evaluation techniques are summarized in this paper about optical investigations of the combustion behavior of different hybrid rocket fuels. They are analyzed by optical techniques in detail. Tests are performed in a 2D slab burner configuration with windows on two sides. Mainly liquefying Paraffin-based fuels including additives are tested in combination with gaseous oxygen (GOX). High speed video imaging enables the analysis of combustion phenomena at great detail. But at the same time a huge amount of data is created, that has to be evaluated carefully. Clearly, a manual analysis is rather time consuming and probably more susceptible to errors. Therefore, two different techniques are presented in this work, which are used within an automated video evaluation routine. First of all, the Proper Orthogonal Decomposition (POD) technique is applied. Its results deliver linearly uncorrelated variables which are the principal components of the flow field. This method enables a decomposition of the data set into mean, coherent and incoherent parts, thus recognizing the main structures of the flow field and the combustion flame appearing in the video data. Secondly, the Independent Component Analysis (ICA) technique is applied to the same data. It is able to search for statistically independent, or as independent as possible, structures hidden in the data, thus increasing the independence to higher statistical orders with respect to POD. The basis functions found with the ICA are expected to describe the essential structure of the data and to resemble some physical processes involved in the combustion. With both methods it is possible to compute spatial and temporal coefficients, which can be later analyzed by applying a Power Spectral Density (PSD) in order to obtain the excited frequencies and wavelengths during the combustion. Finally, the results of the two methods are compared in order to better understand and interpret them. A comparison between spatial and temporal ICA is also performed. The results collected so far and the comparison of both techniques show that their application is consistent and useful for the automated evaluation of combustion data.

Combustion efficiency sensitivity studies of the API injector concept

Since a few years DLR Lampoldshausen has been engaged in the development of a new injection concept for cryogenic liquid propellant rocket engines. This concept bases on the injection of the liquid oxygen through a large number of simple tubes while the entire hydrogen is injected through the porous face plate.3, 4 The key feature of the API concept is the independency of the atomization, vaporization, mixing and combustion performance of initial injection conditions such as velocity ratio, momentum fluxratio, Weber number or other non-dimensional parameters which are generally deemed important. This statement will be supported by data optained with a 50mm subscale combustion chamber. The influence of the velocity ratio at injection will be shown through a variation of the propellant mixture ratio and/or the chamber pressure.

Optical heating, thermography and deformation measurement of nozzle wall structures

Optical measurement possibilities for actively cooled thermally loaded wall structures of rocket engines by means of Thermo-Mechanical Fatigue (TMF) – tests aiming at the creation of validation data for CFD, thermal, structural and life time analyses are discussed in this paper. During a TMF test, only a small section of the hot gas wall of the real engine (the so called TMF panel) is tested. For such a TMF panel, realistic cooling conditions similar to a full scale rocket engine are chosen. The 2d measurement of the thermal field of the heat loaded structure provides (together with the measurement of the temperature, pressure and mass flow rate of the coolant of the TMF panel) data for the combined validation of the CFD analysis of the coolant flow and the thermal analysis of the wall structure. The measurement of the deformation of the thermally loaded structure provides (together with the already determined temperature distribution and the above mentioned pressure measurements of the cooling channels) data for the validation of the structural analysis of the thermally loaded structure. Counting the number of laser loading cycles (laser on-off) until the TMF panel fails (by cracks appearing on the laser loaded side of the cooling channels) provides data for the validation of (either post processing or damage parameter based) life time analyses of thermally loaded structures.

Design and testing of a porous injector head for transpiration cooled combustion chambers

The present publication describes the design and testing of a porous injector head which was specifically designed towards an application in a transpiration cooled combustion chamber as a part of the German Research Network ”Propulsion 2010”. In order to determine comparable performance data, this injector head was tested with a conventional water cooled 80 mm diameter combustion chamber. This assembly was tested at the test bench P8 at the DLR Institute of Space Propulsion in Lampoldshausen. The resulting data was compared to test data which was obtained by using a conventional 42-element coaxial injector head and an identical combustion chamber configuration. During the operation of the new porous injector head serious instability problems were encountered.

Experiments on Shock-Boundary Layer Interaction and Cooling Efficiency in a Transpiration Cooled Model Scramjet

Transpiration cooling systems are a promising approach to handle the high wall temperatures and heat loads generated in supersonic combustion ramjets (scramjets). The German Aerospace Center DLR has conducted experiments to investigate the applicability of transpiration cooling systems in scramjets and the phenomena resulting out of the interaction between a wedge/ flame holder and coolant secondary flow. Results with nitrogen as coolant for different sintered porous materials, different blowing ratios and different boundary conditions with and without wedge are shown. A detailed analysis using i.a. optical Schlieren investigations is presented. Phenomena like choking and shock-coolant plenum feedback are addressed. Further research is discussed and concretized.

PRELIMINARY EXPERIMENTS ON TRANSPIRATION COOLING IN RAMJETS AND SCRAMJETS

Transpiration cooling systems are a promising approach to handle the high wall temperatures and heat loads generated in supersonic combustion ramjets (scramjets). The German Aerospace Center DLR has developed and set up a versatile test bench to investigate the applicability of transpiration cooling systems in scramjets and the phenomena resulting out of the interaction between such a cooling system, optional shock generators like wedge shaped struts and hot gas main flow. In the publication at hand, the developed test bench and its components are presented in detail, as well as a performance analysis for it based on preliminary experiments using gas sampling and optical Schlieren investigations. Further research and planned experiments are discussed and concretized.