摘要:

The detection of CH chemiiuminescence is demonstrated as a viable technique to obtain data on the falme sheet topology in turbulent reacting flows. Since the CH chemiluminescence signal originates in the flame zone near the stoichiometric surface, the technique also provides a good visualization of the instantaneous isoconcentration contour corresponding to near stoichiometric mixtures of fuel and air. The technique uses two intensified CCD cameras that are gated to provide images of the instantaneous CH chemiluminescence distribution. Advantages of this technique are that no lasers are needed and that it is straightforward to apply, typically requiring only suitable collection optics and a sufficiently sensitive detector. The primary disadvantage is that it provides a line-of-sight measurement so that, in highly turbulent flows where the flame sheet is significantly convoluted, interpretation of the flame sheet structure is difficult.

ISSN:0010-2202    

DOI:10.1080/00102209708935676

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An experimental study of flame-spreading behavior over thin Mg/PTFE/Mg (magnesium/poly-tetrafluoroethylene/magnesium) films with a thickness of 10/25/10 μm was conducted in both a pure argon and a 10% oxygen/90% argon environment. The objectives of this research was to determine the effect of initial chamber pressure, gap width between two adjacent films, and ambient gas composition on the flame-spreading rate (VFS) of Mg/PTFE/Mg pyrotechnic films. The flame-spreading rate was deduced from the flame-front trajectory measured by an array of fast-response lead-selenide (Pb-Se) infrared photodetectors. The initial chamber pressure was varied from 0.1 to 20.8 MPa. For tests with two films aligned in parallel, the gap width was varied from 50 to 400 um. For single-film tests, VFS was found to decrease as the initial chamber pressure increased; this observed trend is believed to be caused by a greater heat loss to ambient gases in a higher pressure environment. For double-film tests, results showed the existence of an optimal gap width for which VFS is maximized at a given initial chamber pressure and an optimal initial chamber pressure for which VFS is maximized at a given gap width. These results are caused by a combination of several factors, including the required physical space for gas-phase reactions and the heat transfer rates to both unburned films and ambient gases. Addition of oxygen in test environments was found to decrease the flame-spreading rates when compared to those in pure argon. Experimental correlations relating the flame-spreading rate to the initial chamber pressure were developed for both single- and double-film tests.Keywords: Magnesium; polytetrafluoroethylene; flame-spreading rate; photodetector

ISSN:0010-2202    

DOI:10.1080/00102209708935677

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

One-dimensional simultaneous measurements of the species mass fractions of O2, H2H2O, N2and OH radical as well as flame temperature have been carried out in hydrogen jet diffusion flames diluted with Ar using the line-Raman/Rayleigh/LIPF-OH technique. Statistical information about the mixture fraction Z and scalar dissipation rate x can be obtained for evaluation of current finite-chemistry combustion models for the turbulent nonpremixed flames. At an upstream position of x/d = 8, the flamelet approach is found to be qualitatively valid, in agreement with a larger Favre mixture fraction fluctuation than the reaction zone thickness in the Z-space. Transient effect, however, can be important in view of H2and O2superequilibrium. Preferential diffusion effects are negligible due to a relatively large Peclet number. Joint correlations between a reactive scalar T with δ and the conserved scalar Z with γ are found to be different from those reported in nonreacting flows. The observed distinct features can be explained based on the flamelet concept so that statistical independence is argued to be a good approximation. There is evidence showing that large-scale turbulent motion dominates scalar transport in the connected reaction zone regime. Thus, flamelet approach is also favored. Some modeling assumptions used in the flamelet model and the CMC method are discussed. Model refinements based on the present experimental data are suggested as well.

ISSN:0010-2202    

DOI:10.1080/00102209708935678

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Flame spread over the surface of the thick PMMA sheet with heat-source backing in an opposed air flow is investigated experimentally. The flow velocity (U=50–150 cm/s), fuel thickness (δ=8.2–17.4 mm), and fuel backing temperature (Tb=60–150 °C) are varied to examine their effects on the flame spread rate over the thick PMMA. The results show that the rate of flame spread over the fuel with heat-source backing is significantly higher than that without heat-source backing. The local flame spread rate decreases along thedistance of flame propagation due to the decrease in the solid fuel temperature and the temperature of incoming oxidizer ahead of the flame front. In addition, with the same fuel backing temperature, the flame spread rate decreases with increasing the local Biot number of the solid fuel. Moreover, lowering the convective heat transfer of the thermal-backing fuel could approach the flame spread behavior to be that of faster chemical reaction rate. Apower-law correlation for the fiame spread rate over the thick PMMA with heat-source backing is further developed in terms of the fuel backing temperature, and the fuel Biot number, which may be helpful for controlling the flame spread in accidental fires and a useful reference for theoretic aspects.

ISSN:0010-2202    

DOI:10.1080/00102209708935679

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A basic discussion drawn from carbon black studies is presented as applicable to understanding soot primary particle structure. In this context, the relation between bright and dark field transmission electron microscopy (TEM) and selected area electron diffraction (SAED) is discussed. The potential for these techniques to characterize the degree of carbonization or crystallite growth of soot is discussed and then applied to soot thermophoretically sampled at different positions along the axial streamline of an ethylene gas-jet diffusion flame. Bright and dark field TEM image pairs are presented for soot thermophoretically sampled at a series of axial heights above the burner and related to the transformation process of coalesced soot precursor material into solid carbonaceous soot. The potential to quantify the degree of carbonization of the soot is examined via SAED patterns. It is found that higher resolution (via HRTEM or in the SAED patterns) is needed to extend these techniques into a quantifiable diagnostic.

ISSN:0010-2202    

DOI:10.1080/00102209708935680

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Formal analysis of combusion instabilities has been based on expansion of the equations of motion in two small parameters, March numbers characterizing the average and fluctuating flows. The procedure is reviewed and extended in this note explain how new terms involving nonlinear mean flow/acoustic interactions may be incorporated. As a further example of the ordering procedure, the reasoning is given to show why coupling of modes, linear in the average March number, cannot legitimately be ratained when the equations have been expanted only to first order in Mach number

ISSN:0010-2202    

DOI:10.1080/00102209708935681

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Hysteretic behaviour,as found in the dump combustor facility at GALCIT, allows nonlinear active control of the instability, demonstrated recently by Knoop et al.(1996).As in that work,pulses of secondary fuel, based on a simple on/off control law, have been successfully used to drive the transition between the two modes present in the hysteretic region, thereby reducing the amplitude of the pressure oscillations with minimal use of fuel. In order to clarify the origin of the phenomenon, high speed shadowgraph images of the flowfield during the transition between 'unstable' and stabel burning have been taken, showing distinctive features that may help in modeling the observed behaviour. A Preliminary parametric study (type of injector, duration of pulses, type of secondary flow) has also been conducted, showing that the transition can be obtained over a broad range of conditions.

ISSN:0010-2202    

DOI:10.1080/00102209708935682

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The response of laminar premixed fuel-lean methane-air counterflow flames to the combined effects of stretch and cooling by a fine spray of water and NaCl-water as well as surfactant-water solution is studied experimentally. Results show that flame extinction is promoted both by increased flame stretch and by a higher concentration of liquid. An unexpected finding is that at a given concentration an NaCl-water solution is significantly more effective than pure water in causing flame extinction. On the other hand, at the concentration studied, results from a spray of water containing a surfactant ∥Synperonic PE/L62) are indistinguishable from those with pure water.

ISSN:0010-2202    

DOI:10.1080/00102209708935683

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor