Localized fluid flow measurements with an He-Ne laser spectrometer

Ignition of turbulent non-premixed flames

The axial and radial velocity components w and u, and the concentration c of a Rhodamine 6G dye were measured simultaneously in a turbulent buoyant jet, using laser-Doppler anemometry combined with a recently developed laser-induced-fluorescence concentration measurement technique. These non-intrusive techniques enable measurements in a region of plume motion where conventional probe-based techniques have had difficulties. The results of the study show that the asymptotic decay laws for velocity and concentration of a tracer transported by the flow are verified experimentally in both jets and plumes. The momentum and volume fluxes and the mean dilution factor are determined in dimensionless form as a function of the normalized distance from the flow source. Contradictory results from earlier experimental plume investigations concerning the decay laws of w and c and the plume width ratio b_c/b_w are discussed. The turbulence properties and the transition from momentum-driven jets to buoyancy-driven plumes are presented. The turbulence is found to scale with the mean flow as predicted by dimensional analysis and self-similarity. Buoyancy-produced turbulence is found to transport twice as much tracer as jet turbulence. Although velocity statistics in jets and plumes are found to be highly self-similar there is a strong disparity in the distribution of tracer concentration in the two flows. This occurs in the time-average mean flows as well as the r.m.s. turbulent quantities. Instantaneous concentration fluctuations are found to exceed time averages by as much as a factor of 3. The experimental results should provide a reasonable basis for validation of computer models of axisymmetric plumes.

/pdf/investigations-of-round-vertical-turbulent-buoyant-jets-fts3lg6qh5.pdf

Investigations of round vertical turbulent buoyant jets

The lift-off heights and visible-flame lengths of jet diffusion flames in still air have been determined for hydrogen, propane, methane and ethylene. The flame lift-off height varies linearly with the jet exit velocity and is independent of the burner diameter for a given gas. The results support the assumption that if the burner exit flow is choked the burner can be approximated by an equivalent convergent-divergent nozzle at whose exit the flow has expanded to ambient pressure. The data for different gases can be collapsed onto a single curve if they are plotted in terms of the appropriate non-dimensional groupings. These results and previous results for blow-out stability suggest that diffusion flames blow out when the base is lifted to between 0.65 and 0.75 times the height at which stoichiometric concentration is reached at the jet axis. It can be deduced from the experimental results that, at the base of the flame. the ratio of turbulent burning velocity to laminar burning velocity varies a...

Lift-Off Heights and Visible Lengths of Vertical Turbulent Jet Diffusion Flames in Still Air

Mixing, transport and combustion in sprays

Detailed measurements of turbulent concentration parameters in a round free methane jet up to 70 diameters downstream from the jet source are described. The concentration changes were obtained using a Raman spectrometer designed for the measurement of a methane vibrational transition function, and processed using a photon correlator. Mean and fluctuating concentration levels are given, together with the concentration probability density distribution.A theory for interpreting the statistical aspects of the signal is presented and results are compared with other data from the literature on jets of uniform and non-uniform density. It is shown that the intensity of concentration fluctuations has an asymptotic value of 28·5% in the far field region of the jet. One interesting feature of the data is the deviation from Gaussian statistics along the jet centre-line.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112078002190

The turbulent concentration field of a methane jet

The concentration field in compressible sonic jets of natural gas over the pressure range from 2 to 70 bar has been studied using gas chromatography. At these pressures the jet expands immediately downstream of the nozzle in order to equilibriate with ambient conditions. The behaviour of such underexpanded jets is shown to be similar to classical free jets provided that an appropriate scaling factor is employed to describe the effective size of the jet source. This notional source is referred to as the pseudo-diameter and a theroretical analysis for its derivation is presented. The experimental data support this approach, and the decay constants for the jet concentration field agree with previous measurements in incompressible jets. Comparison with results obtained from a high pressure jet of ethylene at 8 bar indicate that the effect of jet fluid density in concentration decay is the same as in classical free jets.

The Structure and Concentration Decay of High Pressure Jets of Natural Gas

The paper describes experiments on ignitability carried out in a turbulent jet of natural gas. The turbulence properties of this jet, in terms of its mean and fluctuating velocity and concentration parameters have been extensively measured using laser Raman spectroscopy, and in the 17th Combustion Symposium the authors presented a theory in which the turbulent concentration parameters were used to derive a flammability factor and predict ignition probability. By using a single spark source this theory has been tested and results are presented which demonstrate the validity of the theory and define the flammable boundary of the jet. The results confirm that in a turbulent flow it is not meaningful to consider just the mean concentration in order to define the flammability or probability of ignition. However, in the absence of intermittency, this probability will approach zero when the mean falls below about half the lower flammable limit.

Ignition probabilities in turbulent mixing flows

This paper describes the measurement of turbulent concentration parameters using a laser Raman spectrometer. Autocorrelation and probability analysis are applied to investigate the concentration field of a turbulent free jet of natural gas having a Reynolds number of 16000. Results of mean and rms concentration are presented along the jet axis; the asymptotic value of volume fraction fluctuation intensity was found to be ∼30%. A method for obtaining the probability density function is described; this was found to be approximately gaussian on the centre line but highly non-gaussian in off-axis intermittent regions. This information is combined with simplified chemical kinetics to calculate factors relating to ignition probability in such turbulent mixing flows.

Studies of flammability in turbulent flows using laser Raman spectroscopy

The paper gives the results of aerodynamic studies of highly turbulent flows produced in the downstream region of an abrupt change in flow area. Information has been deduced from the autocorrelation function of light scattered in laser velocimeter experiments, and is compared with numerical solutions of the Navier Stokes equations incorporating a two-equation model for turbulence.

http://iopscience.iop.org/article/10.1088/0022-3727/8/4/017/pdf

A D Birch

Papers

The turbulent concentration field of a methane jet

The Structure and Concentration Decay of High Pressure Jets of Natural Gas

Ignition probabilities in turbulent mixing flows

Studies of flammability in turbulent flows using laser Raman spectroscopy

Photon correlation spectroscopy and its application to the measurement of turbulence parameters in fluid flows