Gravity, radiation, and coflow effects on partially premixed flames
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Our objective is to characterize gravity effects on the structure of laminar methane–air partially premixed flames through detailed simulations. We examine the heat loss due to radiation from similar flames that are established at various gravitational accelerations and coflow velocities. Radiation is modeled using the optically thin assumption that provides a limiting value for the radiation heat transfer. We have validated the simulations with measurements in a representative 1-g flame. The predictions are in good agreement with the measured reaction zone topologies and temperature distributions. The simulations show that when the gravitational acceleration for a representative 1-g partially premixed double flame is instantaneously decreased to zero, it is possible to establish a nearly steady 0-g flame in roughly 2.2 s. The overall effect of radiation on the structure of the 1-g flame is relatively insignificant in contrast to the corresponding 0-g flame. Due to radiation effects, the heights of both the inner premixed and outer nonpremixed reaction zones in the 0-g double flame increase, and the heat release rate intensity near the premixed reaction zone tip decreases. When radiation effects are not included in the simulations, the peak temperatures are nearly the same for the 1-g and 0-g flames. However, with radiation the difference in these temperatures is significant. The decrease in the peak temperature due to radiation for the 0-g flame is nearly five times larger than for the 1-g flame. The value of the radiation fraction for 0-g flames without coflow can be as large as 50%, although it drops significantly in the presence of a coflow. While the flowfields upstream of the inner premixed reaction zone are nearly identical for 1-g and 0-g double flames, they are markedly different in the regions between the two reaction zones as well as downstream of the outer nonpremixed reaction zone. The maximum flame temperatures and local heat-release rates increase as the gravitational acceleration increases, while the radiation fractions and inner flame heights decrease. The flickering frequency also increases from 14.7 Hz at 1-g to 41.4 Hz at 10-g and follows the correlation St∝Fr−0.57 that is in accord with a previous compilation of normal gravity data. The radiation Damköhler number is inversely proportional to the Froude number. The radiation fraction decreases with increasing coflow, and the differences between the maximum flame temperatures and heat-release rates for 1-g and 0-g flames become less pronounced. Results for triple flames are in accord with those for double flames.
