Complex chemical kinetics in supersonic nozzle flow

An approximate solution is obtained for the heat transfer to an incompressible turbulent boundary layer with arbitrary freestream-velocity and surface-temperature distributions. The analysis is based on the simultaneous solution of the boundarylayer momentum and energy integral equations and employs several simplifying assumptions, the most important of which is the approximation that the boundary-layer velocity and totaltemperature profiles and the skin-friction coefficient are independent of pressure gradient. Solutions are given for twodimensional and axisymmetric flows. Although the flow in supersonic nozzles is not incompressible, an adaptation of the aforementioned result, termed the quasiincompressible solution, is suggested for calculating the heat transfer to such nozzles. In order to estimate rapidly the maximum rate of heat transfer which occurs near the throat of a supersonic nozzle, a throat approximation is developed which involves only the nozzle supply conditions, throat opening, and throat radius of curvature. The application of these solutions to a sample nozzle gave the following results using the quasi-incompressible analysis: (1) The heat-transfer coefficients based on solution of both the momentum and energy equations were between 0 and 15 per cent lower than the values based on a solution of the momentum equation alone, and (2) the heat-transfer coefficients that included the effects of variable wall temperature were between 0 and 15 per cent lower than the values obtained assuming constant wall temperature. The throat-approximation, heat-transfer coefficients were within 10 per cent of the corresponding quasiincompressible values.

for saving energy and protecting the environment, natural gas has been widely used in internal combustion engines, which makes the study on the ignition characteristics of natural gas/diesel mixtures important. In this work, the effects of trace methane addition on the ignition delay of n-heptane/air mixtures are numerically studied using a detailed n-heptane mechanism under marine engine-like conditions. The simulations are carried out based on the software CHEMKIN-PRO 18.0 with a closed homogeneous reactor. Results show that the prolonged ignition delay times (IDs) of n-heptane/air mixtures are observed over the whole initial temperature range after methane is added, and the increment of IDs in the negative temperature coefficient (NTC) region is significantly higher than that in high temperature region. The sensitivity analysis indicates that both inhibition and promotion effects of important elementary reactions on n-heptane oxidation are weakened because of methane addition. However, the weakening influence on the promoting effect is more prominent. In addition, the inhibition effect of some elementary reactions that are related to the methane oxidation is enhanced. Thus, the IDs of n-heptane/air mixture are prolonged. The analyses of reaction rate of production (ROP) show that the both the production and consumption rates of key radicals decrease significantly in NTC region after methane is added, but it is negligible in the high temperature region. The study can extend the theoretical basis of ignition characteristics of methane/n-heptane blends under elevated temperatures and pressures.

We intend to close the gap between the numerical description of hydrocarbon flames using a large number of elementary reactions and the asymptotic description using very few reaction steps. To proceed in this direction we first reduce the hydrocarbon chemistry to the smallest number of steps that still provides a realistic flame structure. For the example of methane oxidation these are the four reaction steps $$CH_4 + 2H + H_2 O = CO + 4H_2$$ (I) $$CO + H_2 O = CO_2 + H_2$$ (II) $$2H + M = H_2 + M$$ (III) $$O_2 + 3H_2 = 2H + 2H_2 O$$ (IV) Their rates are algebraicly complicated expressions that contain kinetic data from 9 elementary reactions. If standard values for these data are used, the flame velocity of a stoichiometric methane-air flame is calculated as 45.6 cm/sec . The reaction rates maybe simplified to include only data from the five most important reactions $$\begin{gathered}CH_4 + H \to CH_3 + H_2 \hfill \\CH_4 + OH \to CH_3 + H_2 O \hfill \\CO + OH \rightleftarrows CO_2 + H \hfill \\O_2 + H + M \to HO_2 + M \hfill \\O_2 + H \rightleftarrows OH + O \hfill \\\end{gathered}$$ where the rate expression of the fourth of these reactions is modified. This model is proposed for the asymptotic analysis of the flame structure.

Velocity gradient tensor (VGT) analysis of high-order accurate direct numerical simulation data of supersonic pipe, nozzle, and diffuser flows at computationally moderate Reynolds numbers is performed. Joint probability density functions of second and third invariants of the VGT conditioned on positive and negative dilatation levels are presented in the near-wall viscous layer, buffer layer, log layer, and core region of these flows. For flow regions with positive dilatation, there is a preference for unstable flow topologies, while regions with negative dilatation show a preference for stable flow topologies.

Kinetic modeling of carbon monoxide oxidation and water gas shift reaction in supercritical water

Density functional and microkinetic study of CO2 hydrogenation to methanol on subnanometer Pd cluster doped by transition metal (M= Cu, Ni, Pt, Rh)

For several years we have been studying the chemistry of atomic transition metal ions with simple organic molecules. This research was stimulated by our interest in examining the consequences of oxidation and reduction of transition metals in their gas phase ion molecule chemistry. This is an area of chemistry, redox chemistry, which has received little attention from those of us interested in gas phase chemical dynamics. An initial discovery in our investigations was that atomic transition metal ions are quite subject to oxidative addition; a concerted process in which XY adds to M so that the metal is inserted into the XY bond to form XMY. As Pearson notes [1], it has only been in the last 15 years that oxidative addition has been recognized as an important elementary reaction. In fact our results appear to be the first direct observation of oxidative addition to transition metals in the gas phase. We have been particularly interested in oxidative addition processes that involve formation of metal carbon bonds. We have obtained evidence that such processes occur in alkyl halides and alcohols [2–4], in halobenzenes [5] and in alkanes [6,7].KeywordsOxidative AdditionCyclic KetoneCarbonyl FunctionalityAlkane ReactionSimple Organic MoleculeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

With increased interest in reducing emissions, the staged combustion concept for gas turbine combustors is gaining in popularity. For this work, the effect of CO2 dilution on laminar burning velocities of premixed methane/air flames was investigated at elevated temperature through both experiments and numerical simulations. Validation of the experimental setup and methodology was completed through experimental testing of methane/air mixtures at 1 bar and 298 K. Following validation, high temperature experiments were conducted in an optically accessible constant volume combustion chamber at 1 bar and 473 K. Laminar burning velocities of premixed methane/air flames with 0%, 5%, 10% and 15% CO2 dilution were determined using the constant pressure method enabled via schlieren visualization of the spherically propagating flame front. Results show that laminar burning velocities of methane/air mixtures at 1 bar increase by 106–145% with initial temperature increases from 298 K to 473 K. Additions of 5%, 10% and 15% CO2 dilution at 1 bar and 473 K cause a 30–35%, 51–54% and 66–68% decrease in the laminar burning velocity, respectively. Numerical results were obtained with CHEMKIN [1] using the GRI-Mech 3.0 [2] and the San Diego [3] mechanisms. Excellent agreement was observed between the GRI-Mech 3.0 [2] and experimental data at ϕ ≤ 1.2 for methane/air mixtures at 1 bar and 298 K. However, for mixtures at ϕ < 1.3, 1 bar and 473 K, mixtures at ϕ < 1.2 for 5% and 10% dilutions, and for mixtures at ϕ < 0.9 for 15% dilution, laminar burning velocities predicted by the GRI-Mech 3.0 mechanism [2] were slightly higher than experimental results. The San Diego mechanism [3] showed good agreement with experimental data at ϕ ≤ 0.9 for methane/air mixtures at 1 bar and 298 K. However, for mixtures at ϕ > 0.9, 1 bar and 298 K, mixtures at ϕ > 1.2, 1 bar and 473 K, and mixtures at ϕ > 1.1 with 5%, 10% and 15% dilution, the San Diego mechanism [3] predicted slower laminar flame speeds than the experimental results. On the other hand, the laminar burning velocities predicted by the San Diego mechanism [3] were slightly faster than the experimental results for leaner mixtures. Additionally, the dilution, thermal-diffusion, and chemical effects of CO2 on the laminar burning velocities of methane/air mixtures were investigated numerically by diluting the mixtures with both chemically active and inactive CO2 following the determination of the most important elementary reactions on the burning rate through sensitivity analysis. Lastly, it was shown that CO2 dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length (Lb) with the addition of diluent.

An assessment of rate data for high-temperaturesystems

Supersonic swirling separation technology is an innovative gas conditioning technology to separate heavy hydrocarbon and water vapor from the natural gas. The Laval nozzle, where the condensation occurs, is used to generate supersonic flow and achieved a high degree of supersaturation in natural gas dehydration unit. Therefore, the nozzle shape has a strong impact on the non-equilibrium phase transition and plays a decisive role to the distribution of the nucleation and the growth rate. To optimize the structure of the Laval nozzle and achieve higher separation efficiency, numerical simulation plays an important role in accelerating development cycles and cutting down the cost of experiment. In this paper, to avert the complexity of using the multiphase models and real gas model, a quick and efficiency method is validated and used to determine the location of the nucleation zone and the droplet growth zone. The corrected Internally Consistent Classical Theory (ICCT) model and Gyarmathy model (gya82) were employed to the numerical simulation of a condensing Laval nozzle flow by coupling the N-S equation and condensate mass equation at different nozzle pressure ratios (NPR) and initial supersaturations. The results show that, in a supersonic expansion Laval nozzle flow, high cooling rate results in a high value of supersaturation and nucleation rate. When condensation occurs, the flow is affected by the latent heat released and the total temperature is increased. This method can accurately predict the distribution of the condensing flow parameters, find an optimized flow state to obtain larger droplet, and assure the latent heat released is moderate to maintain a steady flow. Finally, this method is applied to the numerical simulation of a full-scale supersonic swirling separator flow field under different work conditions.

Direct numerical simulation (DNS), based on high-order numerical schemes, is used to study the effects of distributed pressure gradients on the redistribution of fluctuating kinetic energy in supersonic nozzle and diffuser flow with incoming fully developed turbulent pipe flow. Axisymmetric geometries and flow parameters have been selected such that shock waves are avoided and streamline curvature remains unimportant. Although mean extra rates of strain are quite small, strong changes in Reynolds stresses and their production/redistribution mechanisms are observed, in agreement with findings of Bradshaw (J. Fluid Mech., vol. 63, 1974, pp. 449–464). The central role of pressure–strain correlations in changing the Reynolds stress anisotropy is highlighted. A Green’s function-based analysis of pressure–strain correlations is presented, showing remarkable agreement with DNS data.

Mechanism of soot initiation in methane systems

In the present work, a convergent–divergent nozzle with a multi‐inlet combustion chamber of a rocket engine is numerically investigated to precisely understand the flow dynamics and variation of flow properties like pressure, temperature, and velocity distribution. The work also deals with a modified combustion chamber having multiple inlets. According to the entry conditions, when the propellants enter the combustion chamber, they are burning due to high velocity and temperature. Then there is a rapid increase in temperature and pressure in the combustion chamber, and afterward, they decrease over the nozzle's length. In this study, three configurations have been prepared in the design modular of ANSYS, a combined setup of the convergent–divergent nozzle with a combustion chamber with four inlets, six inlets, and one with increased combustion chamber length called a modified combustion chamber configuration. In this analysis, an investigation is performed to see the effect of multi‐inlet and increased length of the combustion chamber on the convergent–divergent nozzle's exit velocity. The results show that the configuration with six inlets modified combustion chamber has a 26.3% increase in velocity than six inlets and 38.6% higher exit velocity than four inlets convergent–divergent nozzle of the rocket engine.

A two-way coupled model between polydisperse particle phases with compressible gases and a density-based coupling implicit solution method, combining the third-order MUSCL with QUICK spatial discretization scheme and the second-order temporal discretization scheme, are constructed based on the discrete-phase model (DPM) and the stochastic wander model (DRWM) in the Eulerian–Lagrangian framework in conjunction with a unitary particulate source (PSIC) approach and the SST k-ω turbulence model. The accuracy of the numerical prediction method is verified using previous supersonic nozzle gas-solid two-phase flow experiments. Numerical simulation of a two-phase jet of dry powder extinguishing agent gas with pilot-type supersonic nozzle was performed to analyze the influence of geometrical parameters, such as the length ratio rL and the area ratio rA of the main nozzle on the two-phase flow field, as well as on the jet performance indexes, such as the particle mean velocity vp,a, velocity inhomogeneity Φvp, particle dispersion Ψp, particle mean acceleration ap,a, etc. By analyzing the parameters, we indicate the requirements for the combination of jet performance metrics for different flame types such as penetrating, spreading, and dispersing.

A method of designing rocket nozzle wall contours to yield optimum thrust is presented. The rocket exhaust gases are treated under the assumption of isentropic, adiabatic and frictionless flow. Nozzle length and ambient pressure appear as governing restrictions under which the thrust is maximized. The solution of this extremal problem yields flow properties required along a suitably chosen control surface, and the nozzle contour is constructed by the method of characteristics to yield this required flow field. A typical numerical example is given. For the propellant combination used in the example, it is inferred from available experimental data that the expanding exhaust gases are in chemical equilibrium. The varying gas properties associated with chemical equilibrium are accounted for in the thrust optimization method. The low altitude performance of such a contoured nozzle is discussed.