Spray Behavior in Annular Air Streams

Abstract

Although offering great stability, bluff-body flame holders are seldom used in most of the present practical combustion systems, because in their present design configuration, they achieve less mixing of the air with the fuel than that occurs with annular swirling air and opposed jets. However, by the proper design of their geometric configuration coupled with the operating conditions, the spray shape and the droplet trajectories can be easily controlled aerodynamically. As a result, intimate mixing of the air and the fuel can be obtained. Hence, the bluff-body type of combustor is worth re-examining. In the present study, measurements were made of the spray envelope for a bluff-body combustor having annular air flow under cold flow conditions. The flow rates of the water and air were varied (6.5 × 10³ < Re_w < 1.8 × 10⁴; 2.5 × 10³< Re_a < 10⁵) as well as the size of the annulus (blockage ratio). The spray profile was classified into three different categories. The data is correlated using a ratio of the momentum flux of air that enters the recirculating zone to the momentum flux of the water-(M_a/M_w). With this momentum flux ratio, it should be possible to extrapolate the data from model testing to the prototype. Possible advantages and disadvantages of this type of combustor are presented.