Numerical investigation of high-pressure combustion in rocket engines using Flamelet/Progress-variable models

The present paper deals with the numerical study of high pressure LOx/H2 or LOx/hydrocarbon combustion for propulsion systems. The present research effort is driven by the continued interest in achieving low cost, reliable access to space and more recently, by the renewed interest in hypersonic transportation systems capable of reducing time-to-destination. Moreover, combustion at high pressure has been assumed as a key issue to achieve better propulsive performance and lower environmental impact, as long as the replacement of hydrogen with a hydrocarbon, to reduce the costs related to ground operations and increase flexibility. The current work provides a model for the numerical simulation of high- pressure turbulent combustion employing detailed chemistry description, embedded in a RANS equations solver with a Low Reynolds number k-omega turbulence model. The model used to study such a combustion phenomenon is an extension of the standard flamelet-progress-variable (FPV) turbulent combustion model combined with a Reynolds Averaged Navier-Stokes equation Solver (RANS). In the FPV model, all of the thermo-chemical quantities are evaluated by evolving the mixture fraction Z and a progress variable C. When using a turbulence model in conjunction with FPV model, a probability density function (PDF) is required to evaluate statistical averages of chemical quantities. The choice of such PDF must be a compromise between computational costs and accuracy level. State- of-the-art FPV models are built presuming the functional shape of the joint PDF of Z and C in order to evaluate Favre-averages of thermodynamic quantities. The model here proposed evaluates the most probable joint distribution of Z and C without any assumption on their behavior.Comment: presented at AIAA Scitech 201

Low-dimensional modelling of flame dynamics in heated microchannels

This paper presents simulations of stoichiometric methane/air premixed flames into a microchannel at atmospheric pressure. These simulations result from numerical resolutions of reduced-order models. Indeed, combustion control into microchannels would be allowed by fast simulations that in turn enable real-time adjustments of the device's parameters. Former experimental studies reported the occurrence of a Flame Repetitive Extinction/Ignition (FREI) phenomenon provided that a temperature gradient is sustained at the channel's walls. Conducting unsteady one-dimensional simulations including complex chemistry, a late numerical study tried to explain the occurrence of this phenomenon. The present study therefore explores low-dimensional models that potentially reproduce the FREI phenomenon. Provided a calibration of some empirical constants, an unsteady two-dimensional model including one-step chemical reaction is shown to decently reproduce the FREI regime all along the range of mixture flow rates investigated by the experimental studies. Complementing the aforementioned numerical study, furthermore, when the channel's diameter is varied, the two-dimensional model unveils an unstable regime that a one-dimensional model cannot capture. As two-dimensional hydrodynamics appears to play a key role into the flame's dynamics, therefore the heat rate released by the microcombustor, one-dimensional models are not believed to deliver an adequate strategy of combustion control into such microchannels.Comment: 37 pages, 12 figure

Oxider to Fuel Ratio Shift Compensation Via Vortex Strength Control in Hybrid Rocket Motors

Hybrid motors have existed as a hypothetical propulsion system for decades in a wide range of upper stage rocket motors due to their simple, robust, non-toxic, and versatile nature. However, inherent to hybrids is Oxidizer to Fuel ratio (O/F) shift over time, which results in performance losses for the majority of the rocket’s lifetime. The purpose of this study is to develop a hybrid rocket motor capable of manipulating O/F at will, resulting in an engine which eliminates the undesirable effects of O/F shift. By developing and refining a numerical simulation, a novel injector system, and an open-loop control scheme, this thesis demonstrates programmable O/F manipulation in an experimental hybrid engine

Development of Improved CFD Tools for the Optimization of a Scramjet Engine

In the present work, a plugin has been developed for use with the DoD HPCMP CREATE-AV Kestrel multi-physics solver that adds volumetric source terms to the energy equation. These source terms model the heat released due to combustion, but are much more computationally efficient than a full chemistry model. A thrust-based optimization study was then carried out under the control of Sandia National Laboratories\u27 Dakota toolkit. Dakota was allowed to control the amount of heat added to three regions of the scramjet combustor. The plugin was then extended to consider ignition delay time. By comparing ignition delay time to dwell time, it is possible to determine whether the fuel in a cell should be combusted. Results from this analysis are compared to results gathered using a 22-species chemistry model. The ignition delay source term is shown to capture relevant flow physics at a reduced computational cost. Additionally, the expression for second-law (exergetic) efficiency for a scramjet engine is derived and optimized using Dakota. Finally, Dakota was extended to control the geometry of the scramjet engine, allowing for the numerical optimization of the scramjet expansion system. The results from these computationally-efficient optimizations can then be used to inform researchers of potentially optimal solutions before higher-fidelity models are used

Experimental and theoretical study of combustion jet ignition

A combustion jet ignition system was developed to generate turbulent jets of combustion products containing free radicals and to discharge them as ignition sources into a combustible medium. In order to understand the ignition and the inflammation processes caused by combustion jets, the studies of the fluid mechanical properties of turbulent jets with and without combustion were conducted theoretically and experimentally. Experiments using a specially designed igniter, with a prechamber to build up and control the stagnation pressure upstream of the orifice, were conducted to investigate the formation processes of turbulent jets of combustion products. The penetration speed of combustion jets has been found to be constant initially and then decreases monotonically as turbulent jets of combustion products travel closer to the wall. This initial penetration speed to combustion jets is proportional to the initial stagnation pressure upstream of the orifice for the same stoichiometric mixture. Computer simulations by Chorin's Random Vortex Method implemented with the flame propagation algorithm for the theoretical model of turbulent jets with and without combustion were performed to study the turbulent jet flow field. In the formation processes of the turbulent jets, the large-scale eddy structure of turbulence, the so-called coherent structure, dominates the entrainment and mixing processes. The large-scale eddy structure of turbulent jets in this study is constructed by a series of vortex pairs, which are organized in the form of a staggered array of vortex clouds generating local recirculation flow patterns

Numerical and Experimental Investigation of Hybrid Rocket Motors Transient Behavior

As the space business is shifting from pure performances to affordability a renewed interest is growing about hybrid rocket propulsion. Hybrid rocket motors are attractive for their inherent advantages like simplicity, reliability, safety and reduced costs. Moreover hybrid motors are easy to throttle and thus they are ideal candidate when soft-landing or energy management capabilities are required. This thesis is mainly involved with a theoretical/numerical study of hybrid transient behavior. The study of transient behavior is a very important aspect in the development of affordable, efficient, stable hybrid motors, particularly when throttling and controllability is concerned. Moreover transient behavior is important also for motors that work at a fixed operating point, not only in the prediction of ignition and shutdown phases but particularly in the analysis of instabilities. The prediction and reduction of instabilities are one of the main challenge in hybrid propulsion (as in general in all rocket motors). The aim of this doctoral thesis is to investigate and simulate hybrid rocket transient behavior through the development of a numerical code. The numerical code is composed by several independent parts coupled together, each one referring to a different subsystem of the hybrid rocket motor. Due to budget and time constraints it has not been possible to perform a dedicated experimental activity for this thesis. However the numerical results have been compared with experimental data obtained from literature, from CISAS partners (like NAMMO), and from other CISAS experimental activities performed both before and during this doctoral period. Each subsystem of the hybrid propulsion unit and its related codes are described in a different chapter. In the first chapter hybrid boundary layer steady combustion is introduced together with a discussion about the effect of steady hybrid regression physics on the shift of motor operating parameters with time. In the second chapter typical necessary or intentional transient events occurring during the operation of a hybrid rocket (ignition, throttling and shutdown) are classified and described. With chapter 3 begins the description of the several sub-models defining hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the solid grain thermal behavior. The main object of this work is to determine the response of the solid fuel to variations of the heat flux on the surface. A 1D numerical model of transient grain thermal response has been developed with this goal. The model is based on the work performed by Karabeyoglu and solves the temperature profile in the direction normal to the surface. In the first paragraph a model suited for classical polymeric fuels is developed. In the second paragraph the grain model is coupled with the boundary layer response in order to investigate typical hybrid low frequency instabilities. In the third paragraph a version of the original grain model suited for liquefying propellants is developed. In fact recently a new class of fast burning fuels has been discovered at Stanford University. These fuels form a liquid layer on the melting surface during combustion, hence the term 'liquefying fuels'. Entrainment of droplets from the liquid-gas interface creates the desired high regression rate by increasing the rate of fuel mass transfer. Several researchers included people at CISAS have experimental confirmed that paraffin-based fuels burn at surface regression rates 3 to 4 times that of conventional hybrid fuels. Others following studies showed with the use of visualization experiments the presences of waves on the liquid surfaces and droplets entrained by the gas flow, confirming original theoretical predictions. The third paragraph is divided in three parts. In the first part the model developed to predict the regression rate and the thermal profile inside a paraffin fuel is presented. The second part deals with the phenomenology of supercritical entrainment. Finally the third part discusses the problem of the closure of the equations to take into account the space-time variability of the entrainment phenomenon. In chapter 4 the attention is focused on the gas dynamic inside the hybrid combustion chamber. For this purpose two time-varying numerical models are developed. The aim of these unsteady codes is to determine the transient behavior of the main parameters of the hybrid rocket motor. The combustion chamber model represents the core of the hybrid rocket motor simulation. In fact the combustion chamber model gives directly the main parameter of a propulsion system, that is, motor thrust. The sub-models presented in the previous and the next chapters define the input parameters for the combustion chamber model. In fact the grain model of chapter 3 determine the fuel mass flow while the tank and feed lines model of chapter 5 gives the oxidizer mass flow. In the first part of this chapter a global 0D time-varying numerical model of the combustion chamber is developed. The code is then coupled with the grain model described in the previous chapter to account for the transient fuel production. It follows a brief discussion about the main hybrid rocket motor characteristic times and their relative values. In the second part a 1D time-varying numerical model of the combustion chamber is developed. The unsteady 1D code is able to simulate all the features of the 0D code. It should add the acoustic response of the system and the spatial variation of the fluid-dynamic unknowns along the flow direction, increasing the accuracy of the results at the expense of an higher computational effort. Chapter 5 end the description of the several sub-models of the hybrid rocket propulsion system. Together with chapter 3 and 4 it composes the code describing hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the oxidizer path. This includes the sub-systems ahead of the combustion chamber like the pressurization system, the main tank and the feed lines. Moreover it considers also the injector elements and some aspects of droplets vaporization and atomization in the combustion chamber. This work is complementary to the one described in chapter 3, defining the input parameters for the core of the code, that is the chamber gas-dynamic model shown in chapter 4. The main object of this work is to determine how the feed system affects the performance parameters of the hybrid motor with time. For this purpose the prediction of several unknowns like the oxidizer mass flow, tank pressure and the amount of residual gases is obtained through the modeling of the principal subsystem behavior. Moreover the full transient coupling between the feed system and the combustion chamber is also investigated. This chapter is divided in three parts. The topic of the first paragraph regards the main tank and the pressurization system. After a brief description of the main alternatives the discussion goes on with the numerical modeling of the typical solutions adopted for hybrid rockets (i.e. pressure-regulated, blowdown and self-press). First of all a numerical model of a pressure fed tank is developed. The code is able to predict several parameters like masses, densities, temperatures and pressures of the gas in the ullage volume and in the pressurant tank, the pressurant mass flow and the filling level of the tank. The model takes into account several aspects like heat losses, liquid oxidizer evaporation, eventual gas phase combustion of the pressurant gas, the use of by-pass and digital valves. Later a numerical model of a self pressurized tank is developed. The code is able to determine the oxidizer mass, temperature, pressure, density and the vapor/liquid volume/mass fractions during the discharge. The numerical results are compared with experimental hot tests performed at CISAS. The second paragraph takes into account the full transient coupling between the feed system and the combustion chamber. The main challenge is to determine the instantaneous liquid mass flow and the relation between the liquid oxidizer and the gaseous oxidizer that takes part in the hybrid motor combustion processes (i.e. droplets vaporization). In this way it is possible to simulate feed system coupled instabilities. The third paragraph deals with the prediction of the mass flow through the injector elements. In particular the behavior of self-pressurized systems is investigated. In this case the chamber pressure is below the vapor pressure of the liquid inside the tank. Consequently cavitation and flashing occur inside the injector elements. This kind of two-phase flow with vaporization involves several important modeling issues. Different models are compared with cold-flow tests performed at CISAS in order to check the accuracy of their predictions. In chapter 6 some advanced techniques developed to increase the regression rate and combustion efficiency of hybrid rockets are investigated with a particular focus on their influence on the transient behavior of the motor, particularly regarding combustion instabilities. The two methods studied in this thesis are the use of a diaphragm in the midst of the grain and the use of a swirling oxidizer injection. The reason for this choice is related to the fact that both solutions have been tested (among others) at CISAS and look very promising with respect to the overcoming of historical hybrid weaknesses. Even if working in very different ways both methods induce a strong increase of the turbulence level and mixing of the reactants in the combustion chamber, promoting a more complete combustion and an higher heat flux on the grain surface. Beside improving significantly hybrid performances this two techniques can affect the stability behavior of an hybrid motor directly (i.e. modifying the flowfield in the chamber) and indirectly (e.g. reducing the chamber length due to increased regression rate). In the final chapter a summary of the activities carried out and the results achieved is given

Analysis of controlled auto-ignition /HCCI combustion in a direct injection gasoline engine with single and split fuel injections

A multi-cycle three-dimensional CFD engine simulation programme has been developed and applied to analyze the Controlled autoignition (CAI) combustion, also known as homogeneous charge compression ignition (HCCI), in a direct injection gasoline engine. CAI operation was achieved through the negative valve overlap method by means of a set of low lift camshafts. In the first part of the paper, the effect of single injection timing on combustion phasing and underlying physical and chemical processes involved was examined through a series of analytical studies using the multi-cycle 3D engine simulation programme. The analyses showed that early injection into the trapped burned gases of a lean-burn mixture during the negative valve overlap period had a large effect on combustion phasing, due to localized heat release and the production of chemically reactive species. As the injection was retarded to the intake stroke, the charge cooling effect tended to slow down the autoignition process. However, further retard of fuel injection to the compression stroke caused the earlier start of main combustion as fuel stratification was produced in the cylinder. In order to optimize the engine performance and engine-out emissions, double injection was investigated by injecting part of the fuel first in the negative valve overlap period and the rest of fuel during the intake or compression strokes. By varying the fueling of each injection, the best engine performance was obtained with the 50/50 fuel injection split ratio, while the lowest total NOx and soot emissions were seen with the optimal split injection ratio of 10/90

Investigation Into Advanced Architecture and Strategies For Turbocharged Compressed Natural Gas Heavy Duty SI-engine

CNG is at present retaining a growing interest as a factual alternative to traditional fuel for SI engine thanks to its high potentials in reducing the engine-out emissions. Increasing thrust into the exploitation of NG in the transport field is in fact produced by the even more stringent emission regulations which are being introduced into the worldwide scenario. Specific attention is also to be devoted to heavy duty engines given the high impact they retain due to the diesel oil exploitation and to the PM emissions, the latter issue assessing for the need to shift towards alternative fuels such as natural gas. A thorough control of the air-to-fuel ratio appears to be mandatory in spark ignition CNG engines in order to meet the even more stringent thresholds set by the current regulations. The accuracy of the air/fuel mixture highly depends on the injection system dynamic behavior and to its coupling to the engine fluid-dynamic. The amount of injected fuel should in fact be properly targeted by the ECU basing on the estimation of the induced air and accounting for the embedded closed-loop strategies. Still, these latter are normally derived from engine-base routines and totally ignore the injection system dynamics. Thus, a sound investigation into the mixing process can only be achieved provided that a proper analysis of the injection rail and of the injectors is carried out. The first part of the present work carries out a numerical investigation into the fluid dynamic behavior of a commercial CNG injection system by means of a 0D-1D code. The research has been focused on defining the set of parameters to be precisely reproduced in the 0D-1D simulation so as to match the injection system experimental behavior. Specific attention has been paid to the one component which significantly contributes to fully defining its dynamic response, i.e. the pressure reducing valve. The pressure reducer is made up of various elements that retain diverse weights on the valve behavior and should consequently be differently addressed to. A refined model of the pressure reducer has hence been proposed and the model has been calibrated, tested and run under various operating conditions so as to assess for the set-up validity. Comparisons have been carried out on steady state points as well as through out a vehicle driving cycle and the model capability to properly reproduce the real system characteristic has been investigated into. The proposed valve model has proved to consistently replicate the injection system response for different speed and load conditions. A few methodological indications concerning modeling aspects of a pressure regulator can be drawn from the present study. The model has been run in a predictive mode so as to inquiry into the response of the system to fast transient operations, both in terms of speed and load. The model outputs have highlighted mismatches between the ECU target mass and the actually injected one and have hinted at the need for dedicated and refined control strategies capable of preventing anomalies in the mixture formation and hence in the engine functioning. The second part of the present work aims at deeply investigating into the potentials of a heavy duty engine running on CNG and equipped with two different injection systems, an advanced SP one and a prototype MP one. The considered 7.8 liter engine was designed and produced to implement a Sigle-Point (SP) strategy and has hence been modified to run with a dedicated Multi-Point (MP) system so as to take advantage of its flexibility in terms of control strategies. More specifically, a thorough comparison between the experimental performances of the engine equipped with the two injection systems has been carried out at steady state as well as at transient operations. Better performances in terms of cycle-to-cycle variability were proved for the MP system despite poorer mixture homogeneity. Attention has also been paid to the different engine control strategies to be eventually adopted in compliance with the constraints set by the two different layouts. A 0D-1D model has also been built and validated on the experimental data set to be hence exploited for investigating into different strategies both for the SP and for the MP layout. An extensive simulation has been carried out on the effects of the injection phasing on the SP system performance referring to the engine power output and to the air-to-fuel ratio homogeneity amongst the cylinders. Finally, as far as the MP injection system is concerned, the innovative fire-skipping (DSF) or cylinder deactivation has been considered and deployed by means of the numerical model, assessing for an overall decrease in the fuel consumption of 12% at part load operations

Radio astronomy Explorer-B in-flight mission control system development effort

A description is given of the development for the Mission Analysis Evaluation and Space Trajectory Operations (MAESTRO) program to be used for the in-flight decision making process during the translunar and lunar orbit adjustment phases of the flight of the Radio Astronomy Explorer-B. THe program serves two functions: performance and evaluation of preflight mission analysis, and in-flight support for the midcourse and lunar insertion command decisions that must be made by the flight director. The topics discussed include: analysis of program and midcourse guidance capabilities; methods for on-line control; printed displays of the MAESTRO program; and in-flight operational logistics and testing