Browsing by Author "Wolfersdorf, Jens von (Prof. Dr.-Ing.)"

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Open Access
Ein Beitrag zur Beschreibung der Transpirationskühlung an keramischen Verbundwerkstoffen
(2019) Schweikert, Sven; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
Wiederverwertbare Raumtransportsysteme gelten als Schlüssel die Kosten für den Zugang zum Weltraum zu senken. Die Realisierung derartiger Transportsysteme wird dabei eng mit effizienten Thermalschutzsystemen verbunden. Die Transpirationskühlung wird in diesem Bezug als besonders interessante Technik zum Schutz thermisch hochbelasteter Komponenten betrachtet. Der erfolgreiche Einsatz dieser aktiven Kühlmethode korreliert hierbei eng mit der Verfügbarkeit geeigneter Materialien. In diesem Zusammenhang gelten keramische Verbundwerkstoffe, sogenannte CMC-Materialien (ceramic matrix composite), als prädestiniert für den Einsatz in der Luft- und Raumfahrt. Gleichwohl die positiven Eigenschaften der Transpirationskühlung bereits seit den 1950er Jahren bekannt sind, bestehen noch Lücken hinsichtlich der theoretischen Beschreibung dieser Kühltechnik in Kombination mit CMC-Werkstoffen. An diesem Punkt möchte die vorliegende Arbeit ansetzen, um anhand experimenteller Untersuchungen gültige Ansätze zur Charakterisierung von C/C, einer Art Referenzmaterial der CMC-Materialien, bei Transpirationskühlung zu finden. Vor diesem Hintergrund wurden als Untersuchungsschwerpunkte dieser Arbeit der materialinterne Wärmeübergang sowie die Beeinflussung der Grenzschicht bei der Transpirationskühlung von C/C-Wandsegmenten gewählt. Hinsichtlich der Charakterisierung des internen Wärmeübergangs wurde ein Verfahren entwickelt, um den volumetrischen Wärmeübergangskoeffizienten von C/C zu bestimmen. Für die untersuchten Betriebspunkte konnte dadurch das lokale thermische Gleichgewicht innerhalb der C/C-Wandsegmente festgestellt werden. Zur Beschreibung der Grenzschichtbeeinflussung durch die Transpirationskühlung, wurden Geschwindigkeits- und Temperaturgrenzschichten vermessen. Basierend auf diesen Messdaten konnte die Selbstähnlichkeit transpirationsgekühlter Grenzschichten herausgearbeitet werden. Zudem wurde für diesen Datensatz mit Hilfe logarithmischer Wandgesetze und der gemessenen Grenzschichtprofile die zugrundeliegenden Reibungsbeiwerte und Stanton-Zahlen ermittelt. Anhand der Analyse der daraus resultierenden Grenzschichtkennzahlen konnte die Unabhängigkeit vom verwendeten Wandmaterial auf die Grenzschichtbeeinflussung durch die Transpirationskühlung gezeigt werden. Basierend auf dieser Erkenntnis gelang es, die beobachteten Wechselwirkungen zwischen Material und Kühlung durch etablierte Modelle zu beschreiben.
Open Access
A contribution to transpiration cooling for aerospace applications using CMC walls
(2011) Langener, Tobias; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
For faster and more efficient air transportation systems sustained hypersonic flight offers a great potential. One possibility is to use scramjet (supersonic combustion ramjet) propelled airbreathing space planes because this propulsion system can be very efficient at very high flight Mach numbers. In the past several large-scale research programs were carried out in the USA, Europe and Russia to solve the technical difficulties, which are inherent with a novel propulsion system. Currently, several research programs are ongoing investigating the technological foundations in this area such as aerodynamic efficiency, system level design and integration, environmental aspects and thermal efficiency, and protection systems. The last is especially addressed by the European research program ATLLAS and the German national project SFB-TRR40, in which the work of this thesis was embedded in. The work presented here focusses on the transpiration cooling technique applied to porous CMC (ceramic matrix composite) materials, which offer a great potential for the use in aerospace applications. The aim was to identify the cooling mechanisms involved and verify and extend models describing these phenomena, which can be found in literature. For this, an experimental study was carried out using the hot-gas flow facility available at the ITLR (Institute of Aerospace Thermodynamics of the Universität Stuttgart) and several porous carbon/carbon CMC samples provided by the DLR (German Aerospace Center) were investigated with respect to their cooling efficiency. First, the material was characterized with respect to their outflow and through-flow behavior in separate test setups. Then, these samples were exposed to heated supersonic and subsonic flows generating different heat loads. The surface temperature of the porous wall segments were determined using thermocouple measurements and in situ calibrated infrared thermography. As coolants gaseous air, argon, and helium were used. Since the models available in literature were not capable of representing the specific thermal phenomena in our test setup, they had to be extended. This was verified by a number of transpiration cooling experiments at different temperature levels and heat loads. With the help of this model, transpiration cooling prediction in aerospace (testing) application within non-adiabatic environment is possible when knowing the main-stream conditions. Furthermore, the pressure drop over the C/C samples was recorded in the transpiration cooling tests as well as in cold-flow experiments after a detailed characterization of the samples with respect to their through-flow behavior. The influence of the non-isothermal wall, which is common in aerospace application, on the pressure loss was identified and the Darcy-Forchheimer equation was extended for non-isothermal through-flow. This approach was verified with the experimental data for different thermal situations, heat loads and coolant gases whilst only the coolant properties and the hot-gas side wall temperature had to be given to obtain a result for the pressure drop. As a last step, the model for the cooling efficiency was coupled with the extended model for the through-flow behavior to eliminate the need to know the wall temperature. This was also verified using the available experimental data. Now, only the main-stream conditions and the coolant properties need to be known. Then, this model was used to give an estimate of the coolant mass-flow rate and the supply pressure drop for several aerospace application related combustion chambers. It was shown that it is possible to use transpiration cooling with hydrogen as a coolant in high-temperature and high-pressure environments given the availability of a suitable wall material allowing reasonable supply pressure levels at the required coolant mass-flow rates.
Open Access
Heat transfer measurements in rotating turbine blade cooling channel configurations using the transient thermochromic liquid crystal technique
(2020) Waidmann, Christian; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
The turbine blades of modern gas turbine engines are equipped with sophisticated cooling schemes. Cooling air is bled from the compressor and passed through internal convective cooling channels with the aim to keep the blade metal temperatures within safe limits. In an effort to increase overall engine performance and efficiency, turbine inlet temperatures are being steadily increased while simultaneously the cooling air consumption is reduced. A reduction in cooling air mass flow however increases the effects of rotation on the internal flow structures and thus the heat transfer distribution of a rotating cooling channel. These effects are Coriolis forces and rotational buoyancy forces that may increase heat transfer significantly at one surface compared to the non-rotating case, while simultaneously the heat transfer may drastically be reduced at the opposite surface. For future cooling scheme developments it is vital to understand the influence of these rotational effects on the heat transfer distribution in order to derive resulting temperature distributions inside the turbine blade. A well established measurement technique is the transient thermochromic liquid crystal (TLC) technique that is applied in various investigations of non-rotating cooling channels to evaluate locally resolved heat transfer data. The aim of this thesis is to apply this measurement method for the investigation of rotating cooling channels in order to assess the influence of the rotational effects on the heat transfer distribution. A test rig has been developed that was specifically designed to apply the transient TLC measurement method to rotating cooling channels. Several challenges had to be mastered in the process. First of all, the test model and all rotating components had to be designed or selected to withstand the specified centrifugal forces. Second, a new co-rotating camera unit had to be developed to allow the TLC signal to be captured during rotation. Exemplary results of a test campaign with a total of 32 experiments are discussed. The results are presented as locally resolved distributions of Nusselt number ratios as well as line and area averaged data. Furthermore, for each rotating experiment also a corresponding stationary experiment was conducted. This allowed a direct visualization of the rotational effect on the heat transfer distribution by calculating the local normalized Nusselt number ratio between the rotating and the non-rotating experiment.
Open Access
Investigation of thermal loads onto a cooled strut injector inside a scramjet combustion chamber
(2016) Dröske, Nils Christoph; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
For future aviation or space transportation systems, scramjets could provide a complement or even an alternative to conventional propulsion systems. However, due to the high-enthalpy flow environment, scramjet development still implies considerable technical challenges. One of the most relevant issues is the need for an efficient fuel injection and mixing system. It has to guarantee a stable and reliable combustion process, as the flow residence time inside the engine is only in the order of several milliseconds. Strut-based injection systems have proven to be a suitable choice due to their ability to provide fuel directly into the center of the flow. In contrast to wall-based injection systems, however, struts are exposed to the complete aerodynamic heat loads of the flow, which necessitates active cooling to avoid structural damages. As experimental facilities are hardly able to reproduce flight conditions over a long period of time, a numerical approach is inevitable to assess the heat loads onto a strut and to evaluate the internal cooling mechanism. Within the present thesis, a numerical solver for the conjugate simulation of heat transfer in supersonic flows was developed and integrated into the OpenFOAM software package. A thorough validation for a variety of data from both literature and in-house studies was conducted. The accurate prediction of different phenomena relevant for supersonic flows could be verified. The solver was then applied to the evaluation of an internally cooled strut injector. In a first step, the injector was investigated at moderate flow temperatures. Experimental data for different flow temperatures and coolants was obtained using infrared thermography of the injector surface. A comparison to numerical simulations led to the identification of characteristic well and poorly cooled zones along the injector surface, which could be explained by features of either the external or the internal flow field. Finally, the lobed strut injector was studied numerically at hot gas conditions representative for the ITLR model combustor, where no experimental data of the surface is available. Besides the leading edge, a second hot zone was identified towards the trailing edge of the strut, which was attributed to the impact of the reflected leading edge shock wave onto the surface. Activation of internal air cooling was found to lower the general temperature level, but to have only a small effect on the leading edge. Instead, heat conduction towards the cooled combustor side walls provided a considerable part of the cooling in this area. Switching to hydrogen as coolant led to a further reduction of the injector temperature at a considerably lower coolant mass flux, without changing the overall characteristics of the cooled injector. Changing to more realistic, hotter combustor side walls for a hydrogen-cooled strut caused a generally higher injector surface temperature. While the hottest injector regions were found to be near the side walls, the leading edge could still be partially cooled by the internal hydrogen flow.
Open Access
Numerical and experimental investigations on transpiration cooling
(2021) Prokein, Daniel; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
Future challenges in aviation and space travel are closely linked to the development of propulsion engines. Enhancing the performance and reusability while reducing emissions increases the demands on the applied materials and cooling technology. In this context, transpiration cooling is of great interest, particularly if applied to lightweight composite materials. Within this thesis, a combined numerical and experimental approach has been followed to study transpiration cooling for CMC materials and more complex conditions. The research work focuses on experiments and the development of a numerical solver which was integrated into the OpenFOAM software package. The solver allows the simulation of complex transpiration-cooled structures in non-uniform sub- and supersonic hot gas flows. It is validated by means of comparisons to experiments which demonstrate good agreement for all test cases. First, the through-flow behaviour of C/C structures is investigated. The data suggests that the permeability coefficients are independent of the coolant gas used as well as temperature and pressure levels. Secondly, a subsonic turbulent channel flow with porous-wall injection has been simulated. The agreement of numerical boundary-layer profiles to measured data from literature validates the applied injection model for various coolant gases. The main part of the thesis then explores transpiration-cooled C/C structures in supersonic hot-gas flows. For this purpose, experiments in a wind tunnel have been conducted using flat and non-flat porous samples, a shock generator, and various coolants. Transpiration cooling significantly reduces the temperatures of the porous sample and the wake region. The effect depends on the blowing ratio as well as the coolant properties. Variations of the sample wall thickness yield locally increased coolant mass fluxes and intensified cooling. A similar effect was found for more complex main-flow conditions featuring shock waves and expansion fans.
Open Access
Numerical investigation of internal two-pass gas turbine cooling channels under the influence of rotation
(2020) Göhring, Michael; Wolfersdorf, Jens von (Prof. Dr.-Ing.)
Numerical steady-state and transient conjugated simulations are applied to investigate a machine-like cooling channel configuration under the influence of rotation. Among other aspects, the comparison with the experimental data is given special consideration. With the transient thermochromic liquid crystal (TLC) measurement technique, local heat transfer coefficients can be determined. However, the evaluation method assumes that the values are constant over time. Together with the changing thermal conditions and the rotational motion this represents a contradiction, which is investigated in this thesis. As a basis, steady-state simulations are used. For validation, a test case from the literature has been selected to assess fundamental aspects such as turbulence modeling, rotational effects, and the influence of different inlet boundary conditions. This test case is further investigated to analyze the temporal development of heat transfer as it would occur in a transient TLC experiment. The isothermal walls are therefore replaced with a perspex solid body and transient conjugated simulations are performed. In the end, the dependency of heat transfer on the rotational buoyancy can be determined from the temporal development. Based on the preceding findings, the work concentrates on the ribbed channel configuration which has been experimentally investigated on the rotating test rig at ITLR. For an improved comparability with the transient TLC experiment, the color play of the TLCs is imitated from the numerical results. This enables the numerical data to be analyzed with the same evaluation software as the experiments. Although the heat transfer structures are remarkably similar, the heat transfer level of the experimental results is much higher than that of the numerical results. Of particular interest is the fact that the choice of the fluid reference temperature has a considerable influence on the local heat transfer distribution. Finally, the Nusselt numbers are illustrated as the ratio of the rotating case to the non-rotating case. Systematic influencing factors are thereby eliminated, and great similarities can be achieved between numerical simulation and experiment. The simulations not only provide a detailed analysis of the heat transfer characteristics of a machine-like cooling channel configuration, but also make a major contribution to a better understanding of the time-dependent processes in transient TLC experiments.