Experimental Study of the Effect of Swirl Number and Bluff Body Size on the Stability Map of Premixed LPG Flames in a Tangential Swirl Burner

Abstract

This article presents an experimental analysis investigating the impact of swirl number (S) and annular passage area (B) on the stability limits of the premixed flames in a 20-kW tangential swirl burner. Three nozzles with different diameters (𝐷 𝑁 = 20, 25, and 30 mm) were used to achieve variable geometric swirl numbers (𝑆 𝑔 = 0.918, 1.148, and 1.377).Three central fuel injectors (as a bluff-body) with different diameters (𝐷 𝑖𝑛𝑗 = 6, 8, and 12 mm) were used to obtain different annular passage areas.In the first part of the study, the results revealed that as swirl number (S) increased, flashback limits (FB) decreased (i.e., worsened), and the flame resistance to blowoff (BLO) improved. Thus, the best flame stability map was obtained with increasing S. In the second part of the study, changing the central fuel injector diameter (bluff-body) led to a change in the annular passage area (B) and had an important role in resisting and preventing flashback from combustion-induced vortex breakdown (CIVB). When B was increased (increasing fuel injector diameter), a significant improvement in flashback resistance was observed due to an increase in the momentum of the unburned mixture, while with a decrease in B (reducing fuel injector diameter), the limits of the flashback decreased and moved towards lean regions. However, it was found that the burner operation map increases with increasing B; the best operation map was found at S = 1.377 and B = 0.4, and it was (𝜑 𝐹𝐵 = 1.18 -𝜑 𝐵𝐿𝑂 = 0.42) in the inlet tangential velocity range (𝑊 𝑖𝑛𝑙𝑒𝑡 = 0.58 to 2.84 m/s).Also, tests were carried out without the use of a bluff-body (central fuel injector), and it was found that there is a great impact on the flow field and the location and shape of the central recirculation zone (CRZ); thus, the stability map was narrower than that using the bluffbody. Consequently, adding bluff-bodies to swirling flows improves the mixing properties, increases the intensity of the CRZ, thus enhances the stable operation map. In addition, the bluff-body's presence greatly helps in eliminating CIVB flashback.