摘要:

Results are presented of direct numerical simulations (DNS) of an unsteady, three-dimensional, temporally developing, compressible mixing layer under both non-reacting and reacting non-premixed conditions. In the reacting case, a simple chemistry model of the type A + rB -»(1 + r)Products is considered. Based on simulated results, it is shown that at sufficiently large Reynolds numbers the global and statistical features of mixing transitions are similar to those observed experimentally. At sufficiently large Mach numbers, it is shown that eddy shockletsdo indeed exist in three-dimensional (3D) flow. However, the strength of these shocks is less than that in two-dimensional (2D) layers of the same compressibility level. Aided by the analysis of the DNS data, the extent of validity of the “Steady Laminar, Diffusion Flamelet Model” (SLDFM) and the “Conditional Moment Method“ (CMM) are assessed. In the evaluation of the SLDFM, DNS results for different stoichiometric coefficients and reaction types are analyzed. It is shown that DNS results compare well with model predictions as the magnitude of the Damkohler number is increased. The agreement is improved as the value of r is increased and also as the effects of exothermicity become more pronounced. In the assessment of the CMM, it is shown that the conditional reaction rate can be reasonably approximated in terms of the conditional averages of the scalar variables. Also, the cross-stream variation of the conditional scalar mean values is negligible. However, this is not the case for the variation of higher order moments of the scalar variables.

ISSN:0010-2202    

DOI:10.1080/00102209408935423

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209408935422

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

An experimental study is made of the structure of turbulent nonpremixed flames in the vicinity of the exit of hydrocarbon jets. The jets are issued upward from a circular tube which is located inside a coflowing low-velocity air stream. Schlieren—direct and TiOz scattering-direct synchronized photography is used to visualize the flame zone and various boundaries. A stable flame zone is seen to exist at the exterior of the boundary of fuel jet. The location of the flame zone is found to be influenced by the diffusion velocity of fuel in addition to the stoichiometric volume ratio and entrainmenl of reactants. The presence of a stable laminar flame region near the burner tip is experimentally confirmed. However, in the core region near the tip, the flow is turbulent from the entrance and eddies grow to large sizes in the downstream direction. A schematic characterization is proposed which illustrates the basic structure of the turbulent nonpremixed flame in the near field.

ISSN:0010-2202    

DOI:10.1080/00102209408935424

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A structural model of turbulence, composed of vortex tubes which are based on direct simulations of turbulence, is used to model the flame area enhancement found in direct simulations of passive flame propagation. The resulting model produces a turbulent premixed flame speed ST within a chamber which has the experimentally observed integral length scale and turbulence intensity behavior. The excess flame area is proportional to where R, is the flame radius,. is the Taylor length scale and the diameter of the vortex tube, u’ is the turbulence intensity and SL is the burning velocity. This first model yields a linear dependence between and and additionally, gives as the steady-state planar propagation, where A is the integral scale. This planar relation can also be expressed as s/t/ReJ\5SL, and this relation has been shown to correlate experimental turbulent flame speed results and direct turbulence simulations of passive flamelet propagation. A second propagation model uses a function that exhibits a maximum value of to replace the in the above excess area estimate. This eddy structure model yields departure from the linear u’/SL behavior of the first model, and this departure agrees with the experimental results. The maximum in is caused by vortical structures which are not space filling, and depends upon the ratio of. The two constants in this eddy structure model are determined from experimental results, however, their values are close to those estimated from the spatial structure of vorticity. A model of flame propagation for internal combustion applications is proposed and the effect of the chamber geometry upon the turbulent flame speed is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209408935425

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We study the propagation of combustion waves through porous samples in which two reactions occur. The first is a gasless solid-solid reaction between two solid species in the porous solid matrix to form a solid product, while the second is a solid-gas reaction in which gas delivered to the reaction site through the pores of the sample reacts with one of the solid species to form both solid and gaseous products. We consider the case of coflow filtration, in which the direction of gas flow is the same as the direction of propagation. The gas, consisting of both chemically active and inert components, filters to the reaction zone through the product region thus transferring heat from the high-temperature products to the unburned mixture.

ISSN:0010-2202    

DOI:10.1080/00102209408935426

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Laser Doppler Velocimetry measurements are performed in flat flames, stabilized on a newly developed flat-flame burner. It is shown that the velocity component perpendicular to the main flow direction, induced by expansion in the reaction zone and buoyancy in the burnt gas, is significant. A method is derived with which the temperature profile of flat flames can be determined from two-dimensional Laser Doppler Velocimetry measurements. Good agreement is found when compared to one-dimensional flame calculations and experimental data obtained from the literature. A method is presented to describe the influence of expansion and buoyancy on the burnt gas flow.

ISSN:0010-2202    

DOI:10.1080/00102209408935427

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The motion of LDV seeding particles under the influence of viscous and thermophoretic forces in the rapidly-accelerating, high-temperature-gradient flame environment was studied via the counterfiow premixed twin-flame configuration. Results demonstrate that thermophoretic force can induce significant lag between the fluid and,particle velocities in the active preheat zone of a flame, and suggest that caution should be exercised when interpreting LDV data obtained in this region. A thermophoretic velocity correction to the LDV-determined velocity, with known experimental or computational temperature profile, is proposed. Additional considerations of LDV diagnoslics and its determination of laminar flame speed are also presented.

ISSN:0010-2202    

DOI:10.1080/00102209408935428

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In this paper, we report the results of an experimental investigation of methane-air combustion within a porous medium burner for various equivalence ratios and flow rates. Measurements of emissions and radiant output are presented. The results indicate that CO and NOx, emissions increase with flame speed. For a given equivalence ratio, however, NOx, emissions are fairly constant over the range of flame speed studied. The radiant output increases but the radiant thermal efficiency decreases with flame speed.

ISSN:0010-2202    

DOI:10.1080/00102209408935429

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A simultaneous thermogravimetric/differential thermal analyzer with a range from 20 to 1600°C was used to study the oxidation of Ti powder. The influences of powder morphology, initial oxygen and nitrogen content and the effects of heating rate and sample size were investigated. All the oxidation experiments terminated at the sample ignition point with an associated heating effect that induced sample melting. The ignition temperature for oxidation increased with higher initial oxygen content, and the nature of the ignition process changed: the transition from passivity to combustion via a regime of gradual oxidation collapsed, bypassing the initial oxidation process observed for powders with lower initial oxygen contents. Cracking of the protective oxide scale played an important part in this behavior. The complete history of the powder needs lo be known in order to characterize its reactivity: simple measurable powder characteristics like particle size, surface area and oxygen content alone, are not sufficient for the determination of powder reactivity.

ISSN:0010-2202    

DOI:10.1080/00102209408935430

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A simultaneous thermogravimetnc/differential thermal analyzer with a range from 20 to 1600°C was used to study the nitridation of Ti powder. The influences of powder morphology, initial oxygen and nitrogen content and the effects of heating rate and sample size were investigated. The sample size and heating rate played an important part in the ignition behavior. The Ti powder ignited much less readily in nitrogen than in oxygen: an order of magnitude larger sample size was required. It was found that the α & rarrtl; β transition of Ti at 882°C is a necessary condition for ignition in nitrogen, unlike the oxidation process, where ignition occurs below the transition temperature. The nitridation and oxidation behavior of Ti were compared and major differences were identified.

ISSN:0010-2202    

DOI:10.1080/00102209408935431

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor