Rudolf Günther

Measurements of burning velocity in a flat flame front

The burning velocity of a fuel-oxidant system varies with mixture composition, temperature, and pressure, but the experimental data can be affected by curvature of the flame front, heat losses to the burner and parallel to the flame front, or intermixing with the surrounding atmosphere. A method of avoiding these effects is to measure the burning velocity in the central stream-tube of a button-shaped flame with the particle track method. Here the burning velocity is equal to the velocity at the point where the first sensible temperature rise occurs. Experimental data for CH 4 -air, H 2 -air, CO-air mixtures at an initial temperature or 293°K and atmospheric pressure are presented. A comparison with data from cone-shaped flame shows differences, which are attributed to the influences mentioned above.

Experimental investigation of thermal structure of turbulent premixed flames

Abstract This paper describes an experimental investigation into the thermal structure of turbulent premixed burner flames, placing emphasis on the examination of the influences of equivalence ratio and turbulence intensity. Detailed measurements were carried out of mean, rms, probability density functions, and power spectral density of fluctuating temperature using fine bare wire thermocouple. It was found that the use of the time constant determined from an attentive consideration of PDF leads to plausible results which are consistent with the wrinkled laminar flame model. The PDF profiles have well-defined bimodal shapes; on the unburnt side of the flame zone, the probability of low temperature is higher than that of high temperature, and on the burnt side, the latter is higher than the former. Although, with increasing equivalence ratio, the high temperature peak appears at a higher temperature, essential features are not changed. The PDF profiles are not influenced by turbulence intensity of the unburnt mixture. Mean and fluctuating temperatures were found to increase with equivalence ratio. However, influence of turbulence intensity could not be observed. From the power spectral densities, it was found that there is no influence of the combustion process on the power spectrum. In addition, neither equivalence ratio nor turbulence intensity of the unburnt mixture cause selective changes of the discrete parts of the power spectrum.

Measurements of fluctuating temperature in a free-jet diffusion flame

Abstract Temperature measurements were made in a turbulent free-jet diffusion flame, that include average and fluctuating values as well as probability spectral densities. The measuring system consisted of a thermocouple whose frequency response was determined at each measurement point and compensated with an electrical network. The results of the fluctuation measurements show that the maxima of the root-mean·square (rms) values are situated just outside the main reaction zone. Root-mean-square values of up to 600 K were measured at these locations, whereas they were only about 150 K on the flame axis upstream of the main reaction zone. In some regions of the flame the spectra reveal temperature fluctuations with a frequency of more than 10 kHz. The statistical quantities of mean value, variance, skewness, and flatness of the temperature distribution were determined from the measured probability density function (PDF).

An Experimental Study of Structure and Reaction Rate in Turbulent Premixed Flames

Abstract To close the turbulent combustion equations, the time mean chemical source terms must be estimated. This paper reports the measurements of saturation ion current in the turbulent premixed flame to obtain the mean turbulent reaction rate. Detailed measurements were carried out of mean, rms and probability density functions of saturation ion current using a water-cooled electrostatic probe. For the wrinkled laminar flame model, the saturation ion current is shown to be proportional to the mean turbulent reaction rate. Roughly linear relationship is found between the mean saturation ion current and mean temperature, which results in an important fact that mean turbulent reaction rate is proportional to mean temperature. This Fact supports the Eddy-Break-Up model 3-peak PDF of saturation ion current is observed on the burnt side of the turbulent flame zone, each peak corresponding to unburnt gas, burnt gas and laminar flame zone, and confirms that the present turbulent premixed flame zone is composed...

Ionization measurements in free-jet diffusion flames

Abstract The existence of ions in flames has been known for a long time, and there are many current investigations on ions, mostly in premixed flames that are especially related to kinetics of combustion. There are only a few measurements of ions in turbulent diffusion flames. Considering that ionization is strongly related to the combustion reaction, the measurements of ionization in diffusion flames reported here should give a good idea of combustion progress in these flames. The studies were made in a free-jet flame emerging from an 8-mm nozzle and in the flame of a parallel-stream burner with an 8-mm gas tube and an annular air-stream. Both flames were produced with natural gas and were stabilized by an annular oxygen stream surrounding the gas exit. The measurements include the time mean and the root-mean-square (rms) values of ionization as well as the probability density function (PDF). Frequency analyses and frequency counts were also made to determine characteristic frequencies and scales.

Exchange coefficients and mathematical modelsof jet-diffusion flames

Complete mathematical models for the calculation of the flow and temperature fields of turbulent flames and their combustion chambers are still not available. They could be obtained from calculations of heat exchange, for instance, by nonluminiscent radiation, and a description of the turbulent flow field. The principle lack, at this time, is a method for computing turbulent exchange of momentum,mass, and heat. In the present paper, exchange coefficients for momentum, matter, and heat have been obtained by calculations and by measurements. Attempts have also been made to derive unmixedness values from turbulence-measurements data, since unmixedness, as well as exchange of matter, determines the reaction rate. Transport coefficients for momentum are derived both from time mean values of velocity and from the fluctuating velocity and turbulent microscale. Results of these calculations are in good agreement, the order of magnitude of the turbulent exchange coefficient for momentum was 10–30×10−3 m2/sec. The coefficients increase in enclosed jets with the onset of the recirculation zone. They depend on nozzle diameter and flow velocity. In order to derive unmixedness from turbulence data, a relation between the rates of mixingand combustion is given that is based on the assumption that the combustion rate increases with turbulence intensity and decreases with turbulence scale. An empirical formula is given for the local unmixedness on the flame axis which covers the unmixedness data derived from mean concentrations, excepted in the first part of the jet where very special conditions exist which this formula cannot be expected to cover.