Youhi Morii

Updated Kinetic Mechanism for High-Pressure Hydrogen Combustion

A chemical kinetic model for high-pressure combustion ofH2=O2 mixtures has been developed by updating some of the rate constants important under high-pressure conditions without any diluent. The revised mechanism is validated against experimental shock-tube ignition delay times and laminar flame speeds. Predictions of the present modelarealsocomparedwiththosebyseveralotherkineticmodelsproposedrecently.Althoughpredictionsofthose models (including the present model) agree quite well with each other and with the experimental data of ignition delay times and flame speeds at pressures lower than 10 atm, substantial differences are observed between recent experimental data of high-pressure mass burning rates and model predictions, as well as among the model predictions themselves. Different pressure dependencies of mass burning rates above 10 atm in different kinetic models result from using different rate constants in these models for HO2 reactions, especially for H HO2 and OH HO2 reactions.TherateconstantsforthereactionH HO2 involvingdifferentproductchannelswerefound to be very important for the prediction of high-pressure combustion characteristics. An updated choice of rate constants for those reactions is presented on the basis of recent experimental and theoretical studies. The role of O 1 D, which can be produced by the H HO2 reaction, in the high-pressure combustion of H2 is discussed.

ERENA: A fast and robust Jacobian-free integration method for ordinary differential equations of chemical kinetics

Abstract We herein propose a fast and robust Jacobian-free time integration method named as the extended robustness-enhanced numerical algorithm (ERENA) to treat the stiff ordinary differential equations (ODEs) of chemical kinetics. The formulation of ERENA is based on an exact solution of a quasi-steady-state approximation that is optimized to preserve the mass conservation law through use of a Lagrange multiplier method. ERENA exhibits higher accuracy and faster performance in homogeneous ignition simulations compared to existing popular explicit and implicit methods for stiff ODEs such as VODE, MTS, and CHEMEQ2. We investigate the effects of user-specified threshold values in ERENA, to provide trade-off information between the accuracy and the computational cost.

Numerical Analyses on Ethylene/Oxygen Detonation with Multistep Chemical Reaction Mechanisms: Grid Resolution and Chemical Reaction Model

ABSTRACT The numerical simulations of one- and two-dimensional inviscid detonations for a stoichiometric ethylene/oxygen gas mixture are performed using the reduced chemical reaction model. VW model 1 and VW model 2 are accurate predictions for the ignition delay time compared with the UC San Diego model. Therefore, VW model 2 with 21 species is selected to simulate the ethylene-fueled detonation. The grid resolution study was validated, and it was found that the Zel’dovich-von Neumann-Doering (ZND) structure of the ethylene detonations contains H2O2 in a very short region. Comparing the species mole fraction profiles of one-dimensional analyses with those of the ZND structure, at least more than 10 points in the ΔH2O2 are required to estimate the chemical process accurately. This means that the grid width of three microns is suitable to simulate the detonations under the initial pressure of 0.01 MPa. The grid resolution of two-dimensional detonation simulations affects the detonation cell size as well as hydrogen-fueled detonations. In the case of a channel width d = 1 mm, the single-head detonations are adequately resolved for Δ = 3 μm. However, the detonation cell width becomes irregular for d = 2 mm and Δ = 3 μm.

Numerical Study on Direct Initiation of Cylindrical Detonation in H2/O2 Mixtures: Effect of Higher-Order Schemes on Detonation Propagation

ABSTRACT This aim of this study is to investigate the effect of grid resolution and higher-order accuracy schemes. The size of the reaction induction zone at the time of the local explosion affects the generation of transverse cells. When the reaction zone is large, transverse cells appear. As a result, for calculations in which the grid width is small or for a higher-order scheme, such as a weighted compact nonlinear scheme (WCNS), is used, the cell size increases; an irregular cellular structure is obtained, such as that observed in experiments; and unburned gas pockets and circulation structures caused by Kelvin–Helmholtz instability appear behind the detonation front. In terms of the computational cost, the computational time of 7th-order WCNS is approximately three times that of the 2nd-order monotone upstream-centered schemes for conservation laws (MUSCL) scheme.

Effects of Chemical Reaction Model on H2/O2 Detonation at High Pressures

The relation between the detailed chemical reaction model and detonation simulations is studied by performing the zero-, one-, and two-dimensional numerical analyses using several chemical reaction medels. The comparison of these reaction models at high pressure is performed to get the combustion characteristics such as the ignition delay time, laminar flame velocity, and their sensitivity analysis. The Petersen and Hanson model and Koshi model are chosen for the reaction models in the detonation simulation. Other models are used for the comparison of combustion characteristics only. The numerical results are compared with experimental results and are discussed about the better detailed chemical reaction model in the numerical simulations at high pressure conditions.

A Noble Kinetic Model of H2/O2 System Applicable to Liquid Rocket Engine Combustion

A chemical kinetic model for high pressure combustion of H2/O2 mixtures has been developed. Some of the rate constants important in high pressure conditions were updated. Particular attention has been paid to different channels of the H+HO2 reaction, and to the third body efficiency in the H+O2+M and H+OH+M reactions. An analysis of the performance of an updated model is presented by comparing with various experimental data. Although the present model could reproduce most of shock tube data of ignition delay with variety of bath gases, there is still some discrepancy in the pressure dependence of flame speed between model predictions and recent experimental data. These models were validated against a wide range of experimental conditions and, in general, were found to be in good agreement with various experimental data including shock tube ignition delay, flow reactors, and laminar flame speed measurements. Basic characteristics of hydrogen combustion are now well understood because of those excellent modeling works and related experimental studies. Most recent kinetic model of Konnov 3 is probably the most extensively validated. The modeling range covers ignition delay measurements by shock tubes from 950 to 2700 K and from the atmospheric pressure up to 87 atm, and flow tube experiments at around 900 K with pressures from 0.3 to 15.7 atm. The mechanism is also validated against laminar flame burning velocities up to 4 atm. However, as pointed by Konnov, there is still some discrepancy between the model prediction and the measured flame velocities of hydrogen-oxygen-inert gas mixtures at higher pressures. The pressure dependences of mass burning rates for hydrogen mixtures have recently been studied experimentally and numerically by Burke et al. 4 . Flame speeds and mass burning rates were measured for equivalence ratios from 0.85 to 2.5, pressures from 1 to 25 atm. They found that the mass burning rate is increasing with pressure at low pressures, while at grater pressures, the mass burning rate is found to decrease with pressure. At lower pressures, predictions using recently published chemical kinetic models agree well with experimental data. However, the predicted mass burning rates differ significantly from model to model. Burk et al. suggested that

Study on Behavior of Methane/Oxygen Gas Detonation Near Propagation Limit in Small Diameter Tube: Effect of Tube Diameter

ABSTRACT The unstable behavior of the detonation is an important issue in predicting detonation limits and safety problems. The present study aims to obtain information of the unstable behavior near the detonation propagating limit in methane/oxygen gas mixture for various initial pressures and tube diameters. The detonation velocity rapidly decreases near α = 1.0 and deviates from the theoretical value for α < 1.0 conditions (α = πd/λ; d: tube diameter; λ: cell width). The onset of the single-head spinning is α = 1.0. For α < 1.0 conditions, the transition to the single-head spinning from small cells and the failure of cellular patterns were observed. For α > 1.0 conditions, the propagation mode was unstable. The unstable phenomenon means that the cellular pattern repeats multi-head and single-head spinning in turn.

Fast and robust time integration method for sti[|#11#|] chemical kinetic ODEs

A simple yet robust and fast time integration method is proposed for efficiently solving stiff chemical kinetic ordinary differential equations. The proposed method is based on a general formula which preserves the conservation laws for any integration operators constructed using the Lagrange multiplier method. A quasi-steady-state approxixmation is used as the integrator. The time step size is automatically controlled by using a Lagrange multiplier so that the error, which is caused by the Lagrange multiplier method, is small. The results of several ignition problems demonstrate the robustness and accuracy of the proposed method in comparison with other integration methods such as a implicit integration method (VODE), a multi time-scalse method (MTS), and a modified CHEMEQ2. The proposed method, named ERENA, provides the fastest performance for the most of conditions used in this study.

Numerical evaluation of heat exchange mechanism by slit resonators

The characteristic of the heat exchange mechanism by slit resonators is numerically investigated. In general, a rocket engine chamber is filled with hot gas, and slit resonators, which are equipped outside of the chamber wall near faceplate, are filled with cold gas. Therefore, when the resonators work well, the cold gas in the resonator might be exchanged for the hot gas in the rocket engine because of acoustic oscillations or vortex generations and motions. In this paper, the heat exchange mechanism by the slit resonators is numerically evaluated on the effect of intensity of incident wave, slit resonator shapes, and kinematic viscosity. In addition, the new idea of a slit resonator shape for the suppression of the heat exchange and its damping performance are mentioned.

An order estimation of the acoustic losses inside a simulated liquid rocket chamber

Acoustic losses inside a simulated liquid rocket chamber are investigated by numerical simulation coupled with theoretical calculation. In this study, the losses by injector, resonator and chamber are considered and compared. The oscillation amplitude is assumed to be small (within linear range). For injector and chamber, the loss mechanisms, such as the radiation & convection from the inlet or outlet, and viscous & thermal loss at the wall are considered. For a resonator, the viscous & thermal loss would be the major loss factor. A simulated liquid rocket chamber configuration with an injector installed off-center is investigated especially for tangential oscillation modes. It is found that with well-tuned resonator the resonant frequencies and modes would change from those without the resonator. Therefore, the coupled simulation is indispensable for resonator design. Also an order estimation of each acoustic loss factors is conducted. It is found that the viscous & thermal loss of the chamber and resonator dominate the total acoustic loss in the present configuration. However, if several hundreds of injectors is equipped as in actual rocket chamber, the loss related to injectors would become comparable to that related to resonator on the total loss.