Inductive heating of a supersonic hydrogen flow

Die in dieser Arbeit entwickelte, neuartige Plasmaquelle TIHTUS setzt sich aus einem lichtbogenbeheizten Triebwerk mit konvergent-divergenter Düse und einer nachgeschalteten, induktiv beheizten Stufe mit zylindrischem Entladungsrohr zusammen, die den Wasserstoffplasmastrahl der ersten Stufe besonders in den Außenbereichen weiter aufheizt. Mit einer derartigen Nachheizung soll das Leistungsspektrum von Plasmageneratoren und -triebwerken hinsichtlich Schub oder Austrittsgeschwindigkeit und Totaldruck oder spezifischer Enthalpie erweitert werden. Jede der zwei Stufen kann gesondert mit Leistung versorgt und durch jede der zwei Stufen kann Gas zugeführt werden, sodass die Hybridplasmaquelle ausschließlich lichtbogenbeheizt, ausschließlich induktiv oder kombiniert betrieben werden kann. So wird gezeigt, dass nennenswerte Einkopplung von induktiver Leistung in ein reines thermisches Wasserstoffplasma möglich ist. Durch Messung von Plasmaleistung, Schub, Totaldruck, Plasmageschwindigkeit, Wärmestromdichte und Induktionsspulenstrom werden das Betriebsverhalten und die Plasmabedingungen bei zuvor theoretisch und experimentell für Wasserstoff optimierter Betriebsfrequenz von 840 kHz umfassend charakterisiert. Mit einer systematischen Variation der Parameter Leistungs- und Massenstromzufuhr zu den Stufen bei einer Gesamtleistung von 50 kW und 300 mg/s Wasserstoffmassenstrom wird der Einfluss der Nachheizung durch die zweite Stufe auf das Plasma herausgearbeitet. Die Messergebnisse werden mit Hilfe der hergeleiteten theoretischen Modelle für Hybridplasmaquellen und einer numerischen Strömungsberechnung interpretiert. Die Plasmaleistung lässt sich von 13,1 kW bei reiner Lichtbogenheizung mit 25 kW auf 21 kW bei zusätzlicher induktiver Nachheizung mit weiteren 25 kW steigern. Dies geht jedoch mit einer Verringerung des thermischen Wirkungsgrads von 52,4% auf 42,9% einher. Ebenso steigt die Schubstrahlleistung bei Zuschalten der zweiten Stufe bei den gleichen Bedingungen von 4,6 kW bei 18,5% Schubwirkungsgrad auf 5,6 kW bei 11,4% Schubwirkungsgrad an. Dies entspricht Schubwerten von 1,66 N und 1,85 N. Des Weiteren wird beobachtet, dass bei Leistungsaufspaltung zwischen den Stufen gleicher Schub und gleiche effektive Austrittsgeschwindigkeit wie mit der rein lichtbogenbetriebenen Hybridplasmaquelle erreicht werden kann, wenn die Gaszufuhr ebenfalls auf zwei Stufen aufgeteilt wird. Anhand der Totaldruckprofile, die im Maximum 85 Pa betragen, wird bestätigt, dass die induktive Einkopplung am Strahlrand stattfindet. Die Profile verbreitern sich bei zunehmender induktiver Heizung. Dasselbe gilt für die Geschwindigkeitsverteilung, die für steigende Lichtbogenleistung von 3000 m/s auf 9000 m/s ansteigende Werte im Strahlzentrum zeigt. Mit Hilfe der systematischen experimentellen Untersuchung der Nachheizung einer Überschallwasserstoffströmung werden in der vorliegenden Arbeit die theoretischen und experimentellen Grundlagen zur Optimierung des zweistufigen Systems geschaffen. Die entwickelten analytischen Modelle erlauben darüber hinaus, entsprechende Auslegungsrechnungen für zukünftige Entwicklungen durchzuführen.The exhaust velocity of hydrogen that was heated in the stagnation chamber of a rocket engine is limited for various reasons. New thruster concepts are required in order to produce higher exhaust velocity at high thrust. One approach to transfer more power into a plasma flow is to use an afterburner heating mechanism to reheat the edge of the plume without exceeding the material limitations mentioned above. In the framework of the present thesis, a novel thermal-inductive hybrid thruster (TIHTUS) was set up and characterized in a ground test facility. TIHTUS is a two-stage plasma generator where reheating of a hydrogen arcjet plume is realized by means of induction. The novel thruster may be operated in either arc heated (DC), inductively heated (RF), or hybrid mode. Thus, it is shown that a notable amount of inductive power can be coupled into a pure hydrogen plasma flow. Central gas flow through the arc heated stage is expanded into the injection head of the inductive, second stage where a swirl gas flow can be admixed. As a result of the divergent gas jet being redirected through the cylindrical discharge tube, the latter is strongly heated where the plume impinges on the quartz tube. Due to the resulting tube cooling limitations, the investigated operational conditions are limited to a total of 50 kW of power input at a total of 300 mg/s mass flow rate. After the thruste's individual stages are investigated, the combined thruster is characterized by means of several probes. The first investigation on the hybrid thruster is to theoretically determine the optimum operation frequency to 840 kHz [11]. The calorimetric experiments are in accordance to this. The power balance drawn for hybrid operation of the thruster showed that the plasma power measured by the calorimeter rises from 13.1 kW at 25 kW pure arc heating to 21 kW at additional inductive heating with another 25 kW. However, this takes place at a thermal efficiency decrease from 52.4% to 42.9% [3]. The same is the case for the thrust power increasing from 4.6 kW at 18.5% to 5.6 kW at 11.4% efficiency for the same operating conditions [4]. This implies that the power coupled into the plasma downstream of the first stage's nozzle cannot be used efficiently. Varying the power staging at a constant total power of 50 kW shows that increasing power to the second stage leads to a decrease of both plasma power and thrust power. Additionally varying the mass flow staging, however, raises the exhaust velocity and thrust to the same levels as for pure arc heating of the hybrid plasma source. This can ready be reached in hybrid mode although the geometry of the thruster has not been varied, as for example by adding a second nozzle or by minimizing the distance between arc heating and induction heating. Moreover, radially resolved state variables are measured at an axial distance of x=200 mm from the thruster exit. With increasing coupling of RF-power into a constantly arc heated hydrogen plasma, a dilatation of the plume is observed for total pressure with slightly sinking eccentric maxima. This shows that, as expected, mostly the gas close to the wall is heated. In the plume center, total pressure increases with the arc power. Varying the mass flow staging, an increasing swirl gas flow from the second stage adds to the ambient pressure while the plume width remains uninfluenced. The determinion of thrust from total pressure is coincidental to the result of the impact plate measurements that were taken at an axial distance of 860 mm from the thruster exit [4]. The radial distribution of the plasma velocity in the plume is also widened at increasing second stage power. It shows rising values on the plume axis for increasing arc power [7]. At increasing swirl gas flow, the maximum becomes more and more eccentric. At 25 kW input power to each stage and 200 mg/s gas flow to the DC- and 100 mg gas flow to the RF-stage, the highest profile is detected. From velocity and total pressure, under assumption of an equilibrium model, plasma temperature is derived, giving temperatures between 3000 K and 9000 K on the plume axis [7]. With the heat flux measurements and an equilibrium approach, radially resolved mass-specific enthalpy can also be determined. The integral of these values over the plume cross section compares well to the calorimetrically obtained plasma enthalpy. After having interpreted the numerical fluid calculations, it was expected that the inductive heating, coupling mostly into the subsonic edges of the plasma plume, not only contributes to the acceleration but also to a homogenisation of the radial distribution of the state variables in the plasma. By means of the systematic experimental investigation of the present thesis, the fundamental understanding is composed for the layout and optimization of a two-stage hybrid electric thruster. Moreover, the analytical models which are developed permit respective layout calculations for future developments

A Furnace for Imaging of Charring in Materials under High Temperature

This paper presents a new in-situ material diagnosis instrument based on a furnace setup inside an X-ray tomograph. This allows the direct observation of the material decomposition. Data of a representative cork sample are presented here. The paper describes sample and furnace setup and gives direct images of the degradation in exemplary cork samples. The data are correlated to temperature data within the samples and the results include assessment of pyrolysis Progress inside the sample and deduction of pyrolysis temperature

Ablation: Modelling and Evaluation Experiments

High heat flux is encountered in space applications in re-entry flight at the thermal protection system, especially at sharp leading edges such as engine intakes, and in combustion chamber liners. Therefore, apart from the passive, radiatively cooled heat shield of C/C-SiC, active cooling systems are of special interest for the use in severe thermal environments where the passive systems are inadequate. Active cooling includes film cooling with slit, showerhead, effusion, and transpiration configuration, the latter of which will be discussed here. Another approach to thermal protection is the ablative heat shield in which the material transfers the energy into chemical degradation. It is used for high speed and high enthalpy re-entries. The degradation process is a complex combination of physical processes and chemical reactions such as evaporation of moisture, swelling, and pyrolysis within the material and recession due to oxidation and nitridation at the material surface and within pores. The material thermal response is governed by heat transfer to and within the component, the reaction kinetics and blowing of the pyrolysis gases. The contribution presents the analysis of heat transfer between coolant and structure in transpiration cooling and wall temperature response for both, chemically transforming ablation- and transpiration-cooled re-entry thermal protection systems and compares calculatory results to measurement data

Transpiration-Cooled Hypersonic Flight Experiment: Setup, Flight Measurement, and Reconstruction

The present paper presents the in-flight measurement of the transpiration cooling experiment Aktive Kühlung durch Transpiration im Versuch (often referred to as AKTiV) flown on the suborbital reentry configuration Sharp Edge Flight Experiment II (often referred to as SHEFEX II). Thermal response of the structure is measured just up and downstream of a cooled sample with a noncooled reference setup on the opposite side of the vehicle. The measurement shows that the heat flux is reduced upon coolant exhaust. The maximum temperature reduction with 87 K is observed on the porous sample, while downstream the sample, the temperature is reduced by 75 K by film cooling. This corresponds to cooling efficiencies of 58 and 42% on the sample and downstream, respectively. The evaluation is supported by the semi-analytical tool HEATS, based on a transient heat balance at the surface. Comparison of the results with HEATS shows that the heat flux predicted with HEATS is in good concurrence with the measured temperatures on the cooled sample, as well as upstream and downstream with a maximum deviation by 14%. The analysis suggests that the flow condition around the sharp-edged, faceted vehicle remained laminar longer than expected

Secure tightening of a CMC fastener for the heat shield of re-entry vehicles

The results of static and hot tests of ceramic matrix composite fasteners for heat shield attachment to a re-entry vehicle are presented. Herein, the effect of the high temperature cycle occurring during atmospheric re-entry on the reduction of the tightening torque of the CMC fasteners is investigated. From cold testing, it is found that the Young’s modulus of the material decreases with first-time loading and, therefore, previous loading of the fasteners before their application is recommended. In hot testing of a fully integrated fastener connection, the material temperatures within the connection are experimentally simulated. It is shown that with previous straining, a significant improvement of the fastener is achieved. The loosening torque is no longer reduced by thermally induced stretching of the fastener

Prozesseignung und Schubversuche an einem keramischen Faltkern

Im vorliegenden Bericht wird ein neuartiger Faltkern für eine C/C-SiC Sandwichbauweise untersucht. Die Faltstruktur zeigte gute Prozesseigenschaften und wurde in einem Schubversuch durch Einkleben zwischen Aluminium-Deckplatten getestet. Die ermittelten Kennwerte ergeben für den Faltkern mit 81,25 kg/m³ geometrischer Dichte aus C/C-SiC eine Schubfestigkeit von 0,58 MPa bei einem Schubmodul von 16,968 MPa. Durch die vorliegende Untersuchung wird die grundsätzliche Eignung der Faltwabentechnologie für das LSI-Verfahren gezeigt. Für die Herstellung einer kompletten Sandwichstruktur muss im nächsten Schritt die Fügbarkeit der Wabe mit den Deckschichten demonstriert werden. Weiterführende Versuche umfassen außerdem Biegetests und Druckversuche