The effects of nozzle taper angle on in-nozzle flow and nozzle tip-wetting under flash boiling conditions

Abstract

As an important structural design, the taper angle of an injector has been proved to have a significant influence on spray behavior under both subcooled and flash boiling conditions. However, present studies on taper angle mostly focus on the morphology of the far-field spray, the detailed in-nozzle process as well as the influencing mechanism have not been discussed. To illustrate how the taper angles influence the spray characteristics, backlit imaging method has been adopted on two-dimensional optical nozzles with different taper angle designs. Analyses have been made from in-nozzle bubble development to the unique tip-wetting phenomenon at the counterbore of the injector has been discussed in detail. A novel bubble index is proposed in this paper to describe the in-nozzle flow development. It is found that the design of the taper angle would have a significant impact on the in-nozzle pressure distribution, which would then result in different bubbling characteristics even under the same superheat index, and the contact mechanism between phase boundary and counterbore surface would change with different taper angle designs and tend to result in reverse tip-wetting tendencies under flash boiling conditions compared with subcooled conditions.