Heat transfer characteristics of dual swirling flame impinging on a flat surface

•Inner swirling flame has modified the recirculation zone of outer swirling flame.•Inner swirling flame significantly altered the heating patterns of outer swirling flame.•Better heating uniformity can be achieved by optimizing inner and outer swirl strength.•Selection of radial extent of integration area considerably effects the magnitude and uniformity of heating on the surface.

An experimental investigation has been conducted on the heat transfer characteristics of compressed natural gas (CNG)/air dual flame (with co-swirling inner and outer flames) impinging on a flat surface. Effects of various parameters such as outer vane swirler angle (θo = 30°, 40° and 50°), inner vane swirler angle (θi = 0°, 15°, 22.5° and 30°), outer swirling flame Reynolds number (Reo = 5000–9000), and dimensionless separation distance (H/Dh = 1–4) on heat transfer characteristics have been evaluated. The inner flame Reynolds number (Rei) was kept constant to 1300. A significant difference in the heat transfer characteristics has been observed in the stagnation region for different values of the θo and θi at different H/Dh. High local and average heat fluxes have been observed at smaller separation distances. It has also been observed that the distribution of heat flux on the impingement surface was more uniform for θo of 40° at H/Dh of 2 and 3. Inducing swirl also to the inner flame has made the heat flux distribution further more uniform in the stagnation region and the effect was most favorable at low swirls (θi = 15°). Comparisons were also made for average heat fluxes at different radial extent of integration areas for different operating conditions. It has been found that radial extent of integration area correspond to r/Dh of 5 covers most of the effective heating area of the dual impinging flame. Finally, it has been inferred from the present study that a combination of θo of 40°, θi of 15°, H/Dh of 2 and Reo of 7000 produces the lowest percentage relative deviation and significantly high average heat transfer to the target surface for dual flame impinging on a flat surface.