Heat transfer characteristics of natural gas/air swirling flame impinging on a flat surface

An experimental study has been conducted to investigate the heat transfer characteristics of compressed natural gas (CNG)/air swirling flames impinging on a flat surface. Qualitative flame structures have been studied by taking direct photographs of impinging flames. The radial temperature distribution of swirling free and impinging flames is presented at different axial heights under fixed operating conditions. Effects of dimensionless separation distance (1–6), Reynolds number (3500–6000), equivalence ratio (1–1.5) and helical vane swirler angle (0–60°) on heat transfer characteristics have been investigated. The heat transfer characteristics of swirling and non-swirling flames have been compared under similar operating conditions. A dip in the heat flux at and around stagnation point was observed in almost all cases which could be the main cause of non-uniformity even in case of heating with swirling impinging flames. The dip in heat flux becomes more pronounced at higher Reynolds number. The heat flux distribution on the impingement plate was more uniform with swirl as compared to without swirl. Also, heat flux distribution was observed to be more uniform at moderate separation distance and at larger helical vane swirl angle. Stagnation point heat flux variation remains almost constant at higher separation distances for different values of Reynolds number and equivalence ratios. There was significant decrease in average heat flux with increase in separation distance. It was further observed that average heat flux increases sharply with Reynolds number whereas the variation was not that steep with change in equivalence ratio.

► Qualitative flame structures of swirling impinging flames studied.
► Heat transfer distribution found more uniform at moderate separation distance and larger helical vane angle.
► Radial distribution of heat transfer for swirling flame and non-swirling flame compared.