Heat transfer characteristics of dual flame with outer swirling and inner non-swirling flame impinging on a flat surface

摘要

An experimental study has been conducted to evaluate the heat transfer characteristics of CNG/air dual flame (outer swirling and inner non-swirling flame) impinging on a flat surface. Effects of variation of outer swirling flame Reynolds number (Re(o) = 3000-11,000), inner non-swirling flame Reynolds number (Re(i) = 900-1700), dimensionless separation distance (H/D_h = 1-5) and equivalence ratio of outer swirling flame (Φ(o) = 0.8-1.2) on heat transfer characteristics have been investigated at fixed inner flame equivalence ratio (Φ(i) = 1.0) and swirling insert helix angle of 40°. A significant effect on shape and size of the inner non-swirling flame has been observed due to central recirculation zone established by the outer swirling flame and vice versa. Impingement pressure distribution shows that the pressure peaks are the locations of peak heat fluxes on the impingement surface. Effect of presence of inner flame on heating characteristics is considerably less at larger separation distances (H/D_h ≥ 5) due to dominating effect of recirculation zone over the central axial flow. Heating characteristics showed that the heat flux distribution lines corresponding to inner flame were pushed inward towards burner axis with increase in outer swirling flame Re(o) at fixed Re(i). On the other hand, the inner boundary of the outer flame had shifted outwards with increase in Re(i) at fixed Re(o). Average heat flux imparted to the target surface increases with decrease in separation distance and increase in Re(o). There has not been much gain in the average heat transfer to the impingement surface beyond certain value of Re(o) because the rate of increase of average heat flux decreases with increase in Re(o) due to entrainment effect. Relative deviation in heat flux distribution on the impingement surface decreases with increase in separation distance. In most of the cases it again starts increasing for H/D_h ＞ 2. In many cases, with further increase in separation distance beyond H/D_h of 3, the deviation starts continuously decreasing. Thus, it is concluded that for a dual flame impinging on a flat surface, best operating conditions prevail at moderate Re(o) (7000-9000) and H/D_h of 2.