Heat Transfer Processes for Hydrogen and Methane in Cooling Channels of Regeneratively Cooled Thrust Chambers of Cryogenic Rocket Engines

Die enormen Temperaturen und Wärmeströme in einer Raketenbrennkammer machen eine aktive Kühlung der Brennkammerstruktur unabdingbar. Die Regenerativkühlung, bei der der Treibstoff vor der Verbrennung durch die Struktur geleitet wird, ist eine sehr effiziente und weitverbreitete Methode der Kühlung. Kühlkanäle mit einem hohen Aspektverhältnis (Höhe zu Breite Verhältnis) können zu einer besseren Kühlung bei gleichzeitig geringerem Druckverlust führen. Die thermische Schichtung, die bei dieser Art von Kühlkanälen auftreten kann, wirkt dem positiven Effekt entgegen und limitiert das Aspektverhältnis. In der vorliegenden Arbeit werden experimentelle Untersuchungen zur regenerativen Kühlung mit Wasserstoff und Methan bei für Raketenbrennkammern repräsentativen Bedingungen vorgestellt und ausgewertet. Das verwendete Brennkammersegment ist in Umfangsrichtung in vier Quadranten unterteilt, wobei in jeden Quadrant Kühlkanäle mit einem anderen Aspektverhältnis eingebracht worden sind. Für die Auswertung der experimentellen Daten wurde eine inverse Methode verwendet, die es ermöglicht, anhand der gemessenen Strukturtemperaturen den lokalen Wärmestrom und Wärmeübergangskoeffizienten zu bestimmen. Die thermische Schichtung aufgrund von mangelnder Durchmischung in Kühlkanälen mit hohem Aspektverhältnis tritt sowohl bei Wasserstoff als auch bei Methan auf. Für Wasserstoff ist die Ausprägung allerdings deutlich größer. Der Einfluss auf die Heißgaswandtemperatur ist allerdings für beide Kühlmedien vergleichsweise gering. Bei Methan kann es aufgrund der Nähe zum kritischen Punkt zur Trennung einer heißen gasartigen Schicht an der Wand und der kalten flüssigartigen Kernströmung kommen. Dieser heat transfer deterioration (HTD) genannte Effekt führt zu einem lokalen Abfall des Wärmeübergangs und einem drastischen Anstieg der Heißgaswandtemperatur. Die wesentlichen Einflussfaktoren auf diesen Effekt sind der Druck bzw. die Nähe zum kritischen Punkt, das Verhältnis von Wärmestromdichte und Massenstrom pro Fläche qw/G sowie das Aspektverhältnis. Basierend auf den Experimenten wurden numerische Simulationen durchgeführt, die die Ergebnisse stützen und erweitern. Die Auswertung dieser Simulationen zeigt eine weitestgehend gute Übereinstimmung mit den experimentellen Ergebnissen. In der Nähe des kritischen Punktes und insbesondere wenn es zu HTD kommt, zeigen die Simulationen eine systematische Abweichung und ein deutliches Überschätzen der Heißgaswandtemperatur

Reducing Hydrogen Boil-Off Losses during Fuelling by Pre-Cooling Cryogenic Tank

Boil-off losses occur when gaseous hydrogen has to be released from a cryogenic tank due to liquid hydrogen evaporating. These are a substantial drawback for all areas in which liquid hydrogen is discussed as a potential fuel to limit the climate impact. Especially boil-off losses during fuelling are one of the most significant source of losses along the liquid hydrogen pathway. To analyse and minimize such losses, simulations of the filling process are performed with the simulation tool EcoSimPro. The simulations are validated with an analytical solution. The results show that boil-off losses can be significantly reduced by pre-cooling the cryogenic tank with liquid nitrogen. This method is most effective for relatively small tanks that could be used e.g., in small air crafts or air taxis

Cryogenic Liquid Hydrogen (LH2) system for Refueling an Airplane, and Use for other Mobile or Stationary Applications

Liquid hydrogen (LH2) is used for energy supply for various stationary and mobile applications. Current political and economic developments show the need of storable, decentralized produced, green energy to ensure supply of fuel, heat and power for an adequate price. Hydrogen can be part of the solution for those global problems. Especially LH2 is promising as fuel for mobile applications. It is advantageous because of the high energy density, and therefore is used for space applications since decades. Currently, there are many activities, to transfer the LH2 technology to other applications, e.g. aerospace and trucks

Modulation of aggression in male mice: influence of group size and cage size

Aggression in group-housed male mice is known to be influenced by both cage size and group size. However, the interdependency of these two parameters has not been studied yet. In this study, the level of aggression in groups of three, five, or eight male BALB/c mice housed in cages with a floor size of either 80 or 125 cm2/animal was estimated weekly after cage cleaning for a period of 14 weeks. Furthermore, urine corticosterone levels, food and water intake, body weight, and number of wounds were measured weekly. At the end of the experiment, tyrosine hydroxylase (TH) activity, testosterone levels, and weight of spleen, thymus, testes, and seminal vesicles were determined. Results indicate a moderate increase of intermale aggression in larger cages when compared to the smaller cages. Aggression in groups of eight animals was considerably higher than in groups of three animals. The increase of agonistic behavior was observed both in dominant and subordinate animals. Physiological parameters indicate differences in stress levels between dominant and subordinate animals. It is concluded that aggressive behavior in group-housed male BALB/c mice is best prevented by housing the animals in small groups of three to five animals, while decreasing floor size per animal may be used as a temporary solution to decrease high levels of aggression in an existing social group.

Wärmeübergang von Wasserstoff und Methan in Kühlkanälen renerativ gekühlter Schubkammern kryogener Raketentriebwerke

The high temperature and heat flux in a rocket combustion chamber make an active cooling indispensable. In this work tests will be presented that were performed with a cylindrical research combustion chamber segment that is divided into four sectors around the circumference each containing cooling channels with a different aspect ratio. For methane as cooling fluid, due to the vicinity to the critical point, it may come to a separation of a hot gas-like layer at the wall and the cold liquid-like bulk flow. This effect is known as heat transfer deterioration (HTD) and leads to a locally disturbed heat transfer coefficient and a rising hot gas sidewall temperature. The main parameter for this effect are also analysed experimentally. Based on the experiments, numerical simulations were performed. The comparison between simulated and measured values shows satisfactory small deviations. In the vicinity to the critical point, especially when it comes to HTD, the simulations show a systematic deviation and over-predict the hot gas sidewall temperature

Wärmeübergang von Wasserstoff und Methan in Kühlkanälen renerativ gekühlter Schubkammern kryogener Raketentriebwerke

The high temperature and heat flux in a rocket combustion chamber make an active cooling indispensable. In this work tests will be presented that were performed with a cylindrical research combustion chamber segment that is divided into four sectors around the circumference each containing cooling channels with a different aspect ratio. For methane as cooling fluid, due to the vicinity to the critical point, it may come to a separation of a hot gas-like layer at the wall and the cold liquid-like bulk flow. This effect is known as heat transfer deterioration (HTD) and leads to a locally disturbed heat transfer coefficient and a rising hot gas sidewall temperature. The main parameter for this effect are also analysed experimentally. Based on the experiments, numerical simulations were performed. The comparison between simulated and measured values shows satisfactory small deviations. In the vicinity to the critical point, especially when it comes to HTD, the simulations show a systematic deviation and over-predict the hot gas sidewall temperature

Simulation and Optimization of Zero Emission Hydrogen-Based Powertrains Using EcosimPro

The transportation and especially aviation sectors climate impact could be substantially reduced by switching its fuel to liquid hydrogen. However, due to its low boiling point (~20 K), it is a challenging fuel to handle. One of the most critical aspects is the evaporation of cryogenic liquid due to heat input from the environment, the so-called boil-off. To detect and rate all sources of boil-off losses along the liquid hydrogen power train system, simulations were performed with EcosimPro, a software tool that is widely used to simulate propellant lines in space transportation. The model that is simulated consists of a small liquid hydrogen tank an electrical heater and a fuel cell model. The dimension relates to a small aircraft / air taxi or an APU replacement for a medium sized aircraft. The scope of simulations incorporates the fueling of the lH2 tank and various mission scenarios such as long taxiing and waiting times as well as high power demands. The results show that boil-off losses during fueling are one of the most significant sources of losses along the liquid hydrogen pathway. These boil-off losses can substantially be reduced e.g. by subcooling the liquid hydrogen or by pre-cooling the tank with liquid nitrogen before the filling process. The smaller the tank, the higher are potential benefits, therefore mass savings of up to 50% of total fuel mass are possible. An experimental demonstrator that is suitable to validate the numerical simulations is currently under development. First results with liquid nitrogen as a replacement for liquid hydrogen are expected until end of 2022

Development and Design of a LOX/LNG Thrust Chamber by using Computer-Aided Optimization Methods

The regenerative cooling still is one of the most crucial parts in designing a liquid rocket engine. The goal of an effective design is a trade-off between a sufficient cooling of the structure and a low pressure drop in the cooling channels. For an expander type cycle a third requirement arises: A sufficient enthalpy increase of the cooling fluid for the turbopumps to operate effectively. To prevent the combustion chamber from melting, the regenerative cooling has to be designed with a sufficient margin to respect all uncertainties. In this paper, the design of the regenerative cooling system will be described. A numerical method is used to optimize CFD simulations in order to reach all goals

Experimental and Numerical Investigation of Heat Transfer Processes in Rocket Engine Cooling Channels Operated with Cryogenic Hydrogen and Methane at Supercritical Conditions

Hydrogen and Methane are two fluids that are either used or in discussion as propellants for upper and lower stage rocket engines. The conception of a regenerative cooling system is a crucial part in the design of a rocket engine and so is the detailed knowledge of the coolants behavior and the heat transfer capabilities. Hydrogen is a very efficient and well known cooling fluid whereas the properties of methane as a cooling fluid are intensively investigated nowadays. Experiments were performed with a subscale combustion chamber that is divided into four sectors around the circumference each containing rectangular cooling channels with different aspect ratios. Cryogenic hydrogen and liquid methane were used as cooling fluids. These experiments provide a broad data basis that is used for the validation of CFD simulations. The simulations are capable of predicting wall temperatures for high pressure conditions. Thermal stratification effects that are known to limit cooling properties in high aspect ratio cooling channels arise for both fluids, but the effects are much stronger for hydrogen compared to methane. However in the vicinity to the critical point, when it comes to heat transfer deterioration, the simulations show large deviations to the experimental values