Non-intrusive optical diagnostics of co- and counter-swirling flames in a dual swirl pulverized coal combustion burner

•Dual swirl-stabilized pulverized coal flames are evaluated with co- and counter-swirl.•Behavior of internal recirculation zone and stagnation point is revealed using the PIV.•Heat release characteristics are investigated using the flame color measurements.

Optical non-intrusive measurements are performed to a 10 kWth laboratory-scale dual swirl pulverized coal combustion burner in order to elucidate the behaviors of internal recirculation zone (IRZ) and heat release region. The pulverized coal flame is operated with four different swirl combinations: co-swirling (low- and high-swirl) and counter-swirling (low- and high-swirl). The flow field is measured using particle image velocimetry (PIV). The IRZ area increases for the high-swirl conditions than that in the low-swirl conditions. With changing from co- to counter-swirl combination, the IRZ appearance changes from a heart- to an elongate-shape IRZ because double stagnation points for the co-swirling flames merges to one stagnation point for the counter-swirling flames. For the co-swirling conditions, a tube-type vortex is detected at the end of the IRZ near the double stagnation points. However, it is not present in the counter-swirling conditions. From two-color pyrometry, the overall temperature in the high-swirl flames is lower than the low-swirl flames. For the counter-swirling flames, higher temperature is observed due to the better mixing, which allows for a more intense combustion reaction of pulverized coal particles. Flame color measurements (OH and CH chemiluminescence band light emission) shows that the devolatilization of coal particles is observed at more upstream region comparing with the heat release reaction for volatile gas combustion. For the high-swirl flames, the CH∗ band intensity for heat release decreases, however its overall dimension increases. For the counter-swirling flames, the higher heat release is observed.