834707672

IV3-3

The concept of a flamc front is quite elear for the case of a laminar diffusion flame. In turbulent flames reaction zones occur where mixtures are within the limits of inflammability and temperatures are sufficiently high for reaction to take place. A flame front can be considered to exist in the region of the temperaturę maximum where the mixture ratio is generally close to stoichiometric. On the basis of measurements madę by Kent and Bilger⁴ of a hydrogen turbulent diffusion flame in a co-flowing stream of air, flame contours are shown in Figurę 2. The H₂ limit is for a mole fraction of 0.1% and the 0₂ limit for \%. The reaction taking place in the flame would then need to be confined between the fuel and oxidant limit surfaces. The analyses of Barrere⁵ and Bray⁶ show the importance of measuring correlations between velocity, temperaturę and concentration fluctuating components but, as yet, little progress has been madę in the measurement of such ąuantities. The majority of measurements which have been reported in the literaturę have been time-mean average ąuantities.

PHYSICAL PROBES

A large number of measurements reported in the literaturę have been madę with physical probes. These probes need to be sufficiently robust to withstand the high temperaturę conditions, with the presence of dust and liąuid particles, found in flames. The rangę of water-cooled pitot tubes for velocity measurement, suction pyrometers for temperaturę measurement and water-cooled suction probes for gas and solid sampling have recently been reviewed by Beer and Chigier⁷. Probes for laboratory scalę flames are discussed comprehensively by Fristrom and Westenberg⁸. At the time that these probes were used no altemative probes were known or considered feasible for use by the experimenters. Though attempts were madę to minimize the interference effects it was generally not easy to assess the degrees of inaccuracy introduced due to the flow, cooling and catalytic reaction disturbing effects of the probes.

One of the most comprehensive experimental studies of turbulent diffusion flames has recently been madę by Kent and Bilger^3,4’⁹. They examined a hydrogen turbulent diffusion flame in a co-flowing stream of air. There were considerable density gradients in the system, due both to temperaturę variations and differences in the mole-cular weight of species. Water-cooled pitot and static pressure probes were used for velocity measurements. Temperaturę was measured with a Pt/Rh thermocouple covered with a non-catalytic coating and gas samples were withdrawn from the flow isokinetically, using a hot water-cooled probe. Examples of the measurements they obtained are shown in Figures 3, 4 and 5. It is interesting to notę in Figurę 4 that there is an overlap in the hydrogen and oxygen concentrations, in a region of the flame where temperatures were at their maximum, in the vicinity of 2000°K. The hydrogen and oxygen could not co-exist at these high temperatures without a reaction taking place. Because of the turbulent conditions the probe, situated in the flame, withdrew samples from the fuel-rich and fuel-lean sides of the flame front as the flame front fluctuated across the probe. The ąuantities of hydrogen and oxygen measured in the reaction zonę can, therefore, be used in order to get an indication of the local turbulence level. Hawthorne et al² used this “unmixedness” in order to explain the fact that the observed time-mean average flame lengths were approximately 25% greater than the length along the axis to the stoichiometric mixture line-based on mean concentration measurements.

Peak concentrations of nitric oxide occurred for both axial and radial profiles on the rich side of stoichiometry (Fig.5). The ratę of production of NO was derived from the experimental data through the use of the species balance eąuation with the turbulent mass diffusivity derived from the hydrogen element species balance. Production rates for NO were also computed theoretically on the basis of the Zeldovich eąuations, assuming steady State, main species eąuilibrium and adiabatic temperaturę conditions. With these assumptions the production ratę is a function of equivalence ratio alone with a peak at Iow fractions of eąuilibrium close to the stoichiometric concentration.

TURBULENCE MEASUREMENTS

A number of attempts have been madę at measuring turbulence characteristics in flames. The hot wire anemometer cannot, in generał, withstand the high temperaturę conditions of a flame. Rao and Brzustowski¹⁰ have madę turbulence measurements in a plume using a platinum-iridium wire which can be operated with a mean wire temperaturę up to 900°C. Parker¹¹ has madę measurements of turbulence intensities in a flame using an iridium wire but this could only be used in regions of the flame where there was a reducing atmosphere. The probe could not withstand the high temperatures in the oxidizing atmosphere regions of the flame. Hypodermic glass tubes with internal flows of high pressure cooling water and surrounded by a thin film of platinum have been manufactured for turbulence measurements in flames but have generally been found to be too fragile for use in turbulent diffusion flames.

Considerable progress in the measurement of turbulence in flames has been madę by Gunther and co-workers^{12 ,l3}. Gunther reported on turbulence measurements madę by Ebrahimi¹⁴ using a water-cooled condenser microphone probe and Eickhoff¹⁵ using a water-cooled disc static probe. The results are presented in Figurę 6 for measurements of time-mean fluctuating axial, and normal components of velocity measured in flames compared with isothermal jet measurements. These results showed, surprisingly, that turbulence intensities in flames are lower than corresponding measurements in isothermal jets, and thus no evidence was found of flame generated turbulence.