Driving and damping mechanisms for transverse combustion instabilities in liquid rocket engines

This work presents the analysis of a transverse combustion instability in a reduced-scale rocket engine. The study is conducted on a time-resolved database of three-dimensional fields obtained via large-eddy simulation. The physical mechanisms involved in the response of the coaxial hydrogen/oxygen flames are discussed through the analysis of the Rayleigh term in the disturbance-energy equation. The interaction between acoustics and vorticity, also explicit in the disturbance-energy balance, is shown to be the main damping mechanism for this instability. The relative contributions of Rayleigh and damping terms, depending on the position of the flame with respect to the acoustic field, are discussed. The results give new insight into the phenomenology of transverse combustion instabilities. Finally, the applicability of spectral analysis on the nonlinear Rayleigh and dissipation terms is discussed

Parametric Analysis of Cooling Properties of Candidate Expander-Cycle Fuels

Flow evolution and heat transfer capability in the cooling system of liquid rocket engines heavily depend on propellant thermophysical properties. Coolant thermophysical property analysis and modeling is therefore important to study the possibility of relying on a regenerative cooling system, whose performance is crucial to determine feasibility and convenience of pump-fed liquid rocket cycles of the expander type. The aim of the present study is to compare the behavior of different liquid fuels for expander-cycle engines. They are light hydrocarbons, binary mixtures of them, and liquefied natural gas, which is a mixture made basically of methane and minor fractions of other light hydrocarbons and nitrogen. A parametric analysis is carried out by a validated numerical solver to compare temperature increase, pressure loss, and heat transfer evolution for the different fuels along the same straight tube and subjected to assigned heat fluxes. Results show that similar engine performance can be obtained by the different candidate expander-cycle fuels, but significant differences can be seen in the flow evolution through the cooling channels

Direct numerical simulation of nucleate boiling in micro-layer regime

The physical mechanisms associated with the evolution of a micro-layer beneath a bubble and the transition between contact line and micro-layer regimes are investigated with fully resolved numerical simulations, in the framework of nucleate pool boiling. Capturing the transition between these two regimes has been possible for the first time using very refined grids and parallel computations. Indeed, grids with a cell size under 1 l m must be used in order to capture thermal and dynamical effects in the micro-layer. Such multiscale computations require advanced code capabilities. The present simulations are used to analyse the physical processes involved in the formation and depletion of a micro-layer. A parametric study is carried out to investigate the impact of the main parameters affecting the presence of the micro-layer. From these results, the limit conditions between nucleate boiling in micro-layer and contact line regimes are deduced. Neglecting the micro-layer would lead to erroneous results because it has a strong influence on the overall bubble growth. Therefore the present results could be of major interest for designing models of nucleate pool boiling in larger scales computations, when the micro-layer cannot be resolved

Direct numerical simulation of nucleate boiling in zero gravity conditions

Understanding and controlling nucleate pool boiling phenomena in zero gravity conditions is fundamental for space applications. An analytical model for the equilibrium radius reached by a bubble nucleated in sub-cooled conditions is established in this work and verified numerically. Indeed, direct numerical simulations of two phase flows conjugated with the heat conduction in the solid wall are carried out in order to verify and correct the analytical model. Fine grids, with cells size of the order of the micron, are manda- tory in order to capture the subtle equilibrium between condensation and evaporation that characterises stationary conditions. This has been possible thanks to the house made solver DIVA, validated for nucleate pool boiling simulations, and that permits to carry out parallel numerical simulations. Results show that the equilibrium radius of the bubble is a function of the thermal gradient, of the Jakob numbers associated with condensation and evaporation and of the apparent contact angle. The analysis of the thermal field is carried out and an interpretation of the physical processes that characterise the equilibrium is given. In addition, useful information on the heat transfer behaviour, reported in terms of Nu numbers, completes the work

Multidisciplinary Design and Architecture Optimization of a Reusable Lunar Lander

With renewed interest in lunar exploration and the upcoming deployment of the lunar space station, the Lunar Orbital Platform-Gateway (LOP-G), a scientific community, is focusing on the design of a lander to bring people back to the lunar surface. This work focuses on optimizing two aspects of the lunar lander concurrently: the mission architecture and the vehicle design, often treated independently in the literature. A methodology is introduced to enumerate and preliminarily rank all possible mission architectures. The best mission architectures are then coupled with a multidisciplinary design optimization process by modeling the various components of the spacecraft and optimizing over a set of design parameters. The need for fast computational models, particularly in trajectory optimization, resulted in an analytical approximation of gravitational losses. This work resulted in a hierarchy of mission architectures that are ranked according to the average mass necessary to perform the mission. This work is intended to help a system engineer designing a lunar lander in choosing the best number of vehicles, the number of reuses, and the mission profile for his/her mission requirements

Exploration of combustion instability triggering using Large Eddy Simulation of a multiple injector Liquid Rocket Engine

This article explores the possibility of analyzing combustion instabilities in liq- uid rocket engines by making use of Large Eddy Simulations (LES). Calculations are carried out for a complete small-scale rocket engine, including the injection manifold thrust chamber and nozzle outlet. The engine comprises 42 coaxial injectors feeding the combustion chamber with gaseous hydrogen and liquid oxy- gen and it operates at supercritical pressures with a maximum thermal power of 80 MW. The objective of the study is to predict the occurrence of transverse high-frequency combustion instabilities by comparing two operating points fea- turing different levels of acoustic activity. The LES compares favorably with the experiment for the stable load point and exhibits a nonlinearly unstable trans- verse mode for the experimentally unstable operating condition. A detailed analysis of the instability retrieves the experimental data in terms of spectral features. It is also found that modifications of the flame structures and of the global combustion region configuration have similarities with those observed in recent model scale experiments. It is shown that the overall acoustic activity mainly results from the combination of one transverse and one radial mode of the chamber, which are also strongly coupled with the oxidizer injectors

Study of flame response to transverse acoustic modes from the LES of a 42-injector rocket engine

The Large-Eddy Simulation of a reduced-scale rocket engine operated by DLR has been conducted. This configuration features 42 coaxial injectors fed with liquid oxygen and gaseous hydrogen. For a given set of injection conditions the combustor exhibits strong transverse thermo-acoustic oscillations that are retrieved by the numerical simulation. The spatial structure of the two main modes observed in the LES is investigated through 3D Fourier analysis during the limit cycle. They are respectively associated with the first transverse and first radial resonant acoustic modes of the combustion chamber. The contributions of each individual flame to the unsteady heat release rate and the Rayleigh index are reconstructed for each mode. These contributions are in both cases low in the vicinity of velocity anti-nodes and high near pressure anti-nodes. Moreover it is noticed that these pressure fluctuations lead to large velocity oscillations in the hydrogen stream. From these observations, a driving mechanism for the flame response is proposed and values for the gain and phase of the associated flame transfer function are evaluated from the LES

Multidisciplinary Design Optimization of a Reusable Lunar Vehicle

The Lunar Orbital Platform-Gateway will be the successor to the ISS and will be placed around the Moon. To bring crew onto the lunar surface, a lunar lander must be designed and used. This work will present a system design tool for lunar landers which utilizes OpenMDAO, a multidisciplinary design optimization library. Moreover, different mission architectures will be compared independently. As a benchmark, a design for a one-stage LH2/LOX will be produced and compared to an existing design

Analysis of Heat Transfer Characteristics of Supercritical Fuels in Rocket Cooling Systems by a Space Marching Numerical Technique

Methane and Liquefied Natural Gas (LNG) have been recently considered both for launch and for in-space applications because of several advantages they present if compared with other commonly used fuels. In particular, several studies are dedicated at the use of methane in liquid rocket engines with turbopumb fed systems. In this framework, the present study focuses on the use of methane or LNG as coolant in regenerative cooling systems. The study has two main purposes. The first is to understand what are the differences between using pure methane or LNG in a cooling system. The second purpose is to investigate on the heat transfer deterioration which is a thermodynamic phenomenon that could affect methane or LNG in cooling channels. The idea is to fulfill these objectives by numerical studies. The test cases that have to be analyzed are straight channels with circular cross section, a length of the order of the meter and a diameter of the order of the millimeter. The Reynolds number is of the order of 10^5 − 10^6 , which implies that the flow is turbulent. The coolant enters the channels with a supercritical pressure (∼10 MPa) and a subcritical temperature (∼ 110 K), which correspond to a very low compressibility. As a consequence, the inlet Mach number are very low (∼ 0.01). High heat fluxes up to 10 MW/m^2 are enforced along the channel. The temperature variations along the channel cause a change in all the thermophysical properties that strongly nfluence the coolant behavior. Thermophysical properties of real fluids and mixtures of real fluids are used to carry out the present investigations. An equation of state based on the Helmholtz free energy is used for the thermodynamic properties. Transport property models are based on the extended corresponding states approach used in combination with accurate models for the transport properties of each considered species. A numerical code is developed pecifically to deal with the test cases of interest. It is based on parabolized Navier Stokes equations which can be solved with a space marching approach. The numerical model, used together with the selected thermophysical models, is validated against experimental data. Finally the developed code is used to obtain the desired results. First a comparison between LNG and pure methane behavior is carried out which permits to emphasize their different properties. In particular, the influence of the LNG composition on the coolant flow is analyzed. Subsequently, study of the deterioration of the heat transfer is addressed both with methane and LNG. Parametric studies permit to understand what are the main parameters involved in the phenomenon and how it can be handled

Conditions for the occurrence of heat transfer deterioration in light hydrocarbons flows

Heat transfer deterioration could affect heated channel flows of supercritical pressure fluids in a number of technological applications. Aim of the present study is to analyze the phenomenon onset and to investigate its dependence on the pressure, for three light hydrocarbons: methane, ethane and propane. To this goal a parametric numerical analysis is carried out on uniformly heated straight channels varying, for each different species, the inlet reduced pressure and the enforced heat flux. A parabolized Navier-Stokes solver is used to carry out the simulations, together with Helmholtz energy equation of state and accurate models for the transport properties. Results lead to an unambiguous definition of the threshold value for the ratio between heat flux and specific mass flow rate which identifies the boundary for heat transfer deterioration onset. On the basis of this definition, for assigned inlet reduced temperature, a correlation for the threshold parameter in terms of reduced pressure is presented for the considered species. (C) 2013 Elsevier Ltd. All rights reserved