Angle of Spread of Free Jets

Abstract

THE value of the angle of spread of free turbulent jets has been of great interest to theoretical workers, and its practical application has become important to engineers in the field of combustion engineering. When such a jet of fluid is injected into quiescent surroundings the original kinetic energy of the jet is gradually dispersed in the turbulent mixing region of the jet. The pressure in the jet is very nearly the same as in the surroundings; but a surface of discontinuity separates the stream and the neighbouring fluid. The boundary is well defined apart from slight local instability. Surrounding fluid is entrained at the boundary so that the velocity of the jet decreases while the mass of fluid increases, the momentum remaining constant. In this way the jet spreads in the form of a cone having a well-defined angle. Measured values of the jet half-angle have been variously reported from 7° to 20°. Using Prandtl's hypothesis, Tollmien¹ calculated the jet half-angle to be 12°. Recently, a new measurement of this angle was made at Sheffield, and the influence of jet fluid density investigated.