Measurement of the Local Group Combustion Number of Droplet Clusters in a Premixed Spray Stream

摘要

We have succeeded previously in the experimental observation of the group combustion behavior of spray flames by statistically and spectrally processing the light-emission signals from radicals together with the Mie-scattering signal fromdroplet clusters in a premixed spray flame. It was interesting that even uniformly premixed droplet streams burn in groups due to their eddy motion and preferential flame propagation, hence generating temporal and spatial variation in the scale of droplet clusters. In the present study, time-series signals of the light emission on OH and CH bands, the Mie scattering from droplets, and the size and velocity of droplets with a phase Doppler anemometer (PDA) were monitored simultaneously. This high dat arate and high validation rate of PDA measurement accomplished by the optimizedneously. The high data rate and high vlaidation rate of PDA measurement accomplished by the optimized design of optics enabled us to distinguish instantaneous characteristics of individual droplet clusters in the flame. Thus, we estimated the modified group combustion number G_C for individual drople tclusters using the measured mean separation distance between droplet centers and the total number of droplets for each droplet cluster. Note that Chiu et al. defined their original group combustion number for a quasi-steady laminar combustion of a cloud of equally spaced drops based on the theory of isolated droplet combustion. We examined the transition of group combustion mode of droplet clusters as a function of the distance from the burner port using G_C. It was found that the derived G_C values were concentrated within the range of the original diagram predicted by Chiu et al. The G_C value averaged over all droplet clusters passing a monitoring spot, G_C, fell within the range of external group combustion mode at the initial stage of combustion and moved to the range of internal group combustion mode as the evaporation and combustion of the cluster proceeded, which was in good accordance with the previous experimental observations.