Functional mathematical model of a hydrogen-driven combustion chamber for a scramjet

A functional mathematical model of a hydrogen-driven combustion chamber for a scramjet is described. The model is constructed with the use of one-dimensional steady gas-dynamic equations and parametrization of the channel configuration and the governing parameters (fuel injection into the flow, fuel burnout along the channel, dissipation of kinetic energy, removal of some part of energy generated by gases for modeling cooling of channel walls by the fuel) with allowance for real thermophysical properties of the gases. Through parametric calculations, it is found that fuel injection in three cross sections of the channel consisting of segments with weak and strong expansion ensures a supersonic velocity of combustion products in the range of free-stream Mach numbers M_∞ = 6–12. It is demonstrated that the angle between the velocity vectors of the gaseous hydrogen flow and the main gas flow can be fairly large in the case of distributed injection of the fuel. This allows effective control of the mixing process. It is proposed to use the exergy of combustion products as a criterion of the efficiency of heat supply in the combustion chamber. Based on the calculated values of exergy, the critical free-stream Mach number that still allows scramjet operation is estimated.     

微小空间内丙烷/空气火焰传播特性与加氢爆燃实验

在内径150 mm的圆盘狭缝微型燃烧室内,实验探讨了在常温常压下,不同当量比的丙烷/空气预混气以及掺氢的丙烷/空气混合气在电火花点火后向外传播的特性,通过高速摄影方法获得了在狭缝间距为2.0、2.5、3.0、5.0 mm时微燃烧室内的火焰传播形态。实验中观察到火焰传播存在光滑、皱褶和断裂三种火焰锋面形态。当量比的增加和狭缝间距的减小会使火焰更容易发生褶皱。随着火焰的传播,火焰半径逐渐增大,火焰传播速度整体呈下降趋势。火焰传播速度随着间距的减小先增大后减小,在间距3 mm时最大。因为壁面散热的影响,微尺度效应在降低火焰传播速度和增加火焰不稳定性方面具有重要作用。掺入氢气能提高预混气的火焰传播速度,在间距2.5 mm的微燃烧腔中还观察到了爆燃现象。     

One-dimensional numerical model for degradation and combustion of polymethyl methacrylate

A transient one-dimensional model is applied to study degradation and combustion of a poly methyl methacrylate (PMMA) sample. Ignition of PMMA is a complex interaction among different mechanisms, including solid fuel degradation, heat transfer, in-depth absorption of radiation, surface regression, gas-phase advective heat/mass transfer, and combustion. The present task has the significant feature of coupling the solid and gas phases. Besides the mathematical model has been solved numerically by using a fast iterative method and has yielded realistic results.     

Two modes of operation of a model combustion chamber as a thermoacoustic autooscillating system

It is shown that soft and hard modes of operation of a model combustion chamber as an autooscillating system are possible. In the case of oscillations with transverse acoustic waves we: a) determined the ranges of these modes experimentally; b) detected oscillatory hysteresis (persistence) effects and observed the abrupt appearance and disappearance of oscillations during gradual variation of the parameters. We also noted excitation of autooscillations when finite perturbations acted on the gas column in the combustion chamber in the case of the hard mode.     

Comparison of numerical methods and combustion models for LES of a ramjet

Ramjets are very sensitive to instabilities and their numerical predictions can only be addressed adequately by Large Eddy Simulation (LES). With this technique, solvers can be implicit or explicit and handle structured, unstructured or hybrid meshes, etc. Turbulence and combustion models are other sources of differences. The impact of these options is here investigated for the ONERA ramjet burner. To do so, two LES codes developed by ONERA and CERFACS compute one stable operating condition. Preliminary LES results of the two codes underline the overall robustness of LES. Mean flow features at the various critical sections are reasonably well predicted by both codes. Disagreement mainly appear in the chamber where combustion positions differ pointing to the importance of the combustion and subgrid mixing models. The two LES produce different energy containing motions. With CEDRE, a low frequency dominates while AVBP produces different ranges of low frequencies that can be linked with acoustic modes of the configuration. To cite this article: A. Roux et al., C. R. Mecanique 337 (2009).     

Validation of 1D model for methane/air/Pt combustion in stagnation flow

A 2D model is built on the package of FLUENT to study the effects of radial aspect ratio (R/W), length-to-width ratio (L/W), strain rate (S _R), and buoyancy (Ri=Gr/Re ²) on the validation of the simplified 1D model. In the present 2D model, the methane/air homogeneous reaction mechanism of Peters and the methane/air/platinum heterogeneous reaction mechanism of Deutschmann are applied. By comparison between the 1D and 2D numerical results, it is found that the validation of 1D model is highly related with the catalytic stagnation reactor configuration. For length-to-width ratio L/W = 1 configuration, 1D laminar model is applicable when the radial aspect ratio R/W > 0.4. For R/W = 0.6, the reactor exhibited 1D characteristics when L/W < 1. Compared with the temperature and species profiles, the velocity distribution along the axis is more sensitive to the change of radial aspect ratio and length-to-width ratio. With increasing of the strain rate, the flame front goes closer to the catalytic wall surface and the difference between the 1D and 2D results decreases. For a valid 1D simulation, it is recommended that the strain rate should be greater than 20 s^-1. The effects of natural convection can be neglected when Ri < 5.     

Optimizing an oscillation absorber in a combustion chamber

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 5, pp. 79–88, September–October, 1993.     

Numerical analysis of supersonic combustion ramjet with upstream fuel injection

This paper describes possible fuel injection scheme for airbreathing engines that use hydrocarbon fuels. The basic idea is to inject fuel at the spike tip of the supersonic inlet to achieve mixing and combustion efficiency with a limited length combustion chamber. A numerical code, able to solve the full Navier–Stokes equations in turbulent and reacting flows, is employed to obtain numerical simulations of the thermo‐fluidynamic fields at different scramjet flight conditions, at Mach numbers of M=6.5 and 8. The feasibility of the idea of the upstream injection is checked for a simple axisymmetric configuration and relatively small size. The results are discussed in connection with the potential benefits deriving from the use of new ultra high temperature ceramics (UHTC). Copyright © 2003 John Wiley & Sons, Ltd.     

Magnetohydrodynamic interaction in hypersonic air flow past a blunt body

Hypersonic MHD air flow past a blunt body in the presence of an external magnetic field is considered. The MHD effect on the flow consists in a significant increase in the shock wave stand-off from the body surface and a significant reduction in the heat flux to the wall (up to 50%). It is shown that even in the presence of a strong Hall effect the intensity of the magnetohydrodynamic interaction in the plasma behind the shock wave remains at a high level commensurable with the ideal case of the absence of a Hall effect.     

An extension of CVDV model to Zeldovich exchange reactions for hypersonic non-equilibrium air flows

A new preferential vibration-dissociation-exchange reactions coupling model – labelled CVDEV – resulting from an extension of the well-known Treanor and Marrone CVDV model, has been derived to take into account the coupling between the vibrational excitation of the and molecules and the two Zeldovich exchange reactions. Analytical expressions for the exchange reactions coupling factor and for the average vibrational energy lost – or gained – by a molecule through an exchange reaction have been developed. The influence of such a coupling has been shown by means of numerical simulations of hypersonic air flows through normal and bow shock waves. Code-to-code comparisons between our model and other recent approaches have been conducted. The infrared radiation of nitric oxide behind a normal shock wave resulting from computations with the CVDEV model has been compared with other coupling model results and to recent shock tube experimental data. These comparisons have shown a good agreement of our model results with the experimental data. In this context, the results show the prominent influence of vibration coupling on the first Zeldovich reaction, and the absence of vibration coupling effects on the second Zeldovich reaction. Received 30 June 1997 / Accepted 3 December 1997     

How does the rarefaction of the air affect hypersonic vehicles

The scope,physical concept,and mathematical model for describing how the rarefaction of the air affects on hypersonic flows have been discussed.A new method,nam...     

Measurement of the flow field in a diesel engine combustion chamber after combustion by cross-correlation of high-speed photographs

A cellular cross-correlation technique is applied to high-speed photographs of the luminous phase of combustion in a high-speed direct-injection diesel engine. The method enables the velocity and vorticity distributions in the combustion chamber to be evaluated. The results obtained from the basic technique are refined to remove spurious results and to complete the definition of the flow field by applying data validation, interpolation, and smoothing. The velocity and vorticity fields evaluated at two swirl ratios show the way in which the basically solid body swirl motion interacts with the fuel jets in the combustion chamber. A better understanding of the post-combustion fluid motion is obtained, and this should be of help in validating CFD codes and also the design of engines.The combustion photographs which provided the data for this work were obtained by Mr. E. H. Clough, K. K. Rao, P. Gomes and R. Pruce. Funding for the programme was provided by the UK Science and Engineering Research Council. Financial support for one of the authors (J.-H. S.) was provided by the Sino-British Friendship Scholarship Scheme. To all these people and organisations the authors express their grateful thanks.     

燃烧轻气炮多级渐扩型燃烧室流场特性数值研究

为了分析多级渐扩型燃烧室结构对燃烧轻气炮氢氧燃烧特性的影响, 通过计算流体力学方法, 分别对采用传统圆柱型燃烧室和多级渐扩型燃烧室的燃烧轻气炮氢氧燃烧发射过程进行数值模拟。对比结果表明, 多级渐扩型燃烧室结构能够明显地减小燃烧室压力波动幅度, 提高氢氧燃烧稳定性; 多级渐扩型燃烧室内形成回流区, 可以减小气流轴向运动速度; 火焰扩展形态与渐扩型结构相吻合, 燃烧反应区表面变化平稳; 多级渐扩型燃烧室结构对氢氧火焰传播过程和压力波动现象有着重要影响。     

The necessary conditions for stable combustion of a powder in a half-closed chamber

The low-frequency stability of the steady-state combustion cycle is investigated in linear approximation for a powder in a half-closed chamber, taking account of incompleteness of combustion, heat losses at the walls, and dynamic erosion. The necessary conditions for stable combustion are obtained. Qualitative conclusions are drawn of the incompleteness of the chemical reactions taking place and of the destabilizing effect of thermal losses and erosion on the combustion process. Only two out of three possible steady-state mechanisms are stable. The existence of limits of combustion with respect to pressure is shown.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 90–98, November–December, 1972.     

Radiation of a shock layer during hypersonic flow of air about a spherical segment

The problem on the flow of radiating air about a spherical segment is solved. A comparison of results obtained with investigations of the flow of a radiating gas about a sphere and the flow of gas about a spherical segment, with radiation left out of account, is made. The influence of radiation in the neighborhood of the rim of the segment on the flow in the shock layer is considered, and it is shown that it does not exert a significant influence on the fields of the gas-dynamical parameters because the latter are determined by processes occurring near the axis of symmetry, due to the phenomenon of radiation freezing.Moscow. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 101–106, January–February, 1972.     

Study of detonation initiation in hydrogen/air flow

While extensive studies have been conducted concerning the formation of detonation waves in various combustible gaseous mixtures under static conditions since the 1950s, there is very little experimental work on simple flowing systems. In this study, experiments on the deflagration to detonation transition (DDT) of a hydrogen–air flow system were carried out to see the effects of tube diameter, equivalence ratio, and flow types in a premixed and non-premixed flow. Tube diameters used were 25, 50, and 100 mm. The premixed experiments show that the larger tube diameter provides a wider range in run-up distance, reduction of L _DDT/D (ratio of the run-up distance, L _DDT to tube diameter), and expansion of the detonable concentration limit by spreading the cell width. The result of the non-premixed experiments show that similar values of the run-up distance to the premixed experiments are obtained at an equivalence ratio of about 1.0, however, fluctuations of DDT occur near the DDT concentration limit. Under laminar flow conditions at a Reynolds number of less than 2,300, the difference between the two systems could not be observed. However, when the Reynolds number increases towards turbulent conditions, the DDT run-up distance decreases compared to that of static flow conditions. This paper is based on work that was presented at the 21st International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

Evolution of detonation propagation through an annular gap into a combustion chamber

Some results of theoretical studies of detonation processes in combustible gaseous mixtures are discussed for a model geometry of large combustion chambers of detonation engines in the case of mixtures of hydrogen and oxygen-enriched air. The effect of geometric characteristics on the operation of pulse detonation engines is analyzed. In particular, the propagation of detonation waves in tubes of small diameter to larger volumes and the evolution of detonation under the action of converging shock waves are considered.     

Behavior of shock trains in a hypersonic inlet/isolator model with complex background waves

The background flow field of a scramjet isolator that accommodates a shock train contains complex compression and expansion waves, referred as ??background waves,?? causing large streamwise and transverse parameter gradients upstream of the shock train. Therefore, the available results of shock train research obtained by direct-connect methods might be not applicable for real scramjet isolators. Special tests are therefore performed for an inlet/isolator model. Close coupling is found between the shock train and the background shocks. The pointing direction of the leading shock switches upwards and downwards repeatedly during the upstream propagation of the shock train. Three unstable stages with substantial oscillations are also observed, interlaced with four stable stages. In addition, the interference of the background shock waves increases the sustainable back-pressure ratio and decreases the length of the shock train. However, this does not mean that the background waves in the isolator should be intensified intentionally.