Gas-flow measurements in a jet flame using cross-correlation of high-speed-particle images

摘要

Temporal variations of a two-dimensional distribution of velocities in a nitrogen jet and a methane-jet flame are measured by cross-correlation particle-image velocimetry (PIV). Two different approaches to investigating the turbulence characteristics are demonstrated. One gives the distribution of ensemble-averaged velocities and turbulence intensities by means of repetitive PIV measurements using a double-pulse laser for a longer period. The other provides the detailed motion of velocity profiles for a shorter duration, allowing one to analyse the characteristic scale of turbulence using a high-power continuous laser. The accuracy of measurements of the time-averaged velocity and turbulence intensity is quantitatively assessed on the basis of the agreement with the results from hot-wire-anemometry (HWA) measurement. This indicates the feasibility of the PIV measurement, which may supply information about turbulence characteristics. From the measured results for a jet and a jetting flame, it is shown that the velocity gradient in the shear layer in the reacting zone is increased due to the local acceleration caused by buoyancy, resulting in higher turbulence intensities than those in a non-reacting jet. Also, from the change in the distribution of velocity vectors with time, it is clear that the turbulence eddies are carried downstream along the gas motion with little transformation. The time scale of turbulence at each location in the flow is obtained from the autocorrelation function of the velocity fluctuations. Furthermore, this can afford an estimate of the turbulence length scale if one assumes that the Taylor hypothesis is valid and multiplies the time scale and the time-average velocity. It is shown that the characteristic length scales of a flaming jet are about 1.5 times greater than those of a non-flaming jet. The effects of combustion on the turbulence in a flaming jet are discussed in detail on the basis of these experimental results.