Turbulent burning velocity of hydrogen–air premixed propagating flames at elevated pressures

The paper reviews the practical problems in measuring a turbulent burning velocity that gives the mass rate of burning. These largely centre on identifying an appropriate flame surface to associate with the turbulent burning velocity, u t , and the density of the unburned mixture. Such a flame surface has been identified, in terms of the mean reaction progress variable,c, for explosive flame propagation in a fan-stirred bomb. Measurement ofc makes possible an estimation of the flame surface density, Σ, from the relationship Σ = kc (1 −c). It is shown that in such explosions, mass rates of burning derived from the measured total flame surface area agreed well with those found from the measured turbulent burning velocity. Flamelet considerations identify appropriate dimensionless correlating parameters for u t . As a result, correlations of turbulent burning velocity divided by the effective rms turbulent velocity, are plotted against the turbulent Karlovitz stretch factor, K, for different values of the Markstein number for flame strain rate, Ma sr . These plots cover a wide range of variables, including pressure and fuels, and are indicative of different regimes of turbulent combustion. At the lower values of K, there is some evidence of increases in u t and k due to high-frequency flame surface wrinkling arising from flame instabilities. These increase as Ma sr becomes more negative. It is found from the developed value of the mean flame surface density throughout the flame brush that, to a first approximation, an increase in u t for a given mixture is accompanied by a proportional increase in the volume of the brush. The analysis shows that the volume fraction of the turbulent flame brush that is reacting is quite small.

show abstract

“…Explosively, they have been measured in fan-stirred bombs [24][25][26]. Values of Ma sr tend to decrease with increasing pressure.…”

Section: Turbulent Burning Velocity Correlation and Regimes Of Combusmentioning

confidence: 99%

The Problems of the Turbulent Burning Velocity

Bradley

Lawes

Mansour

2011

Flow Turbulence Combust

View full text Add to dashboard Cite

show abstract

“…There is a good agreement between the τ values measured in the present work and the calculation curve of τ d = τ d (T). The slight excess of the calculated data can be explained by the use of the laminar burning velocity u l to calculate τ d which is somewhat lower than the turbulent burning velocity [29].…”

mentioning

confidence: 88%

Measurement of Ignition Limits and Induction Times of Hydrogen–Air Mixtures Behind the Incident Shock Wave Front at Low Temperatures

Павлов

Gerasimov

2014

J Eng Phys Thermophy

View full text Add to dashboard Cite

We have made an experimental study of the ignition of a stoichiometric hydrogen-air mixture behind the incident shock wave front in a shock tube. The ignition limits and the induction times of the mixture behind the wave front in the range of temperatures T = 550-960 K and pressures P = 0.01-1.1 MPa have been determined. Much lower values of the ignition temperature compared to the experimental results obtained in refl ected shock waves, as well as to the classical Lewis and Elbe, have been registered. The measured induction times in the above temperature and pressure ranges are in good agreement with theoretical estimates.Introduction. The ignition limits of combustible gaseous mixtures are an important characteristic of combustion and detonation processes. Most of the classical experimental works on determining the above limits in hydrogen-air mixtures were carried out under the conditions when the gaseous mixture was practically in the state of rest or moving with a low velocity [1][2][3][4]. The corresponding theoretical calculations were oriented to these experiments [5,6]. At present, in connection with the works aimed at developing hypersonic passenger airplanes burning hydrogen fuel [7], the investigation of the ignition characteristics in supersonic hydrogen-air fl ows is of interest. Therefore, the supersonic character of the gaseous mixture fl ow behind the incident shock wave in the shock tube determines the attractiveness of using this tool in pertinent experiments connected with the practical realization of the combustion process in hypersonic fl ights.Most investigations of the process of ignition of hydrogen-air mixtures in shock tubes were conducted behind refl ected waves where the gas is stationary with respect to the walls of the shock tube [8][9][10][11][12][13]. The measurand thereby was the ignition induction time of the mixture. The aim of the present work is to determine the ignition limits and the induction times of hydrogen-air mixtures in incident shock waves and compare the obtained data with the results of experiments performed behind refl ected shock waves. As the results of the investigations have shown, the value of the induction time at low-temperature ignition of the mixture is determined by the time of propagation of the combustion zone from the boundary layer heated by the supersonic fl ow into the entire volume of the shock tube.Experimental. Experiments were performed on a shock tube with an internal radius r = 28.5 mm which consisted of a high-pressure chamber of length 1 m and a low-pressure chamber of length 4.5 mm separated by a diaphragm. When the diaphragm breaks, in the low-pressure chamber the shock wave compressing and heating the investigated gas propagates. In experiments, we used devices for recording emission spectra, measuring the shock wave velocity, and recording pressure profi les in the shock wave (Fig. 1). The gas-dynamic parameters in the shock wave front were calculated for each particular experiment with the use of the "Gaseq" program from the initial p...

show abstract

“…Solid lines refer to results obtained by extrapolating the measured flame speeds to zero stretch then dividing the flame speed by the density ratio [12]. Dotted lines correspond to u l values computed using u l = u n,min , where, u n is the stretched entrainment burning velocity.…”

Section: Laminar Burning Velocitymentioning

confidence: 99%