Behaviour of free gas jet

First we will see the behaviour gas when it exits a single Laval nozzle in the surrounding which consists of air. A gas on exiting through a nozzle spreads in the surrounding and is called “free gas jet”, because spreading is not confined. Figure 13.2 show a free jet in the surrounding. It is characterized by the

potential (L_P) and supersonic (L_C) core length as shown in the figure. In the potential core no entrainment of the surrounding occurs and hence velocity of gas in both axial and radial direction is that at the exit value. Beyond the potential core both radial and axial velocity begins to decrease due to entertainment of the surrounding. Radial spreading of the jet can be seen in the figure. However a point Z₁ is reached in the free gas jet at which the gas velocity attains a sonic value (M=1). Within the length L_C gas velocity is above the supersonic value in both radial and axial direction. Beyond the supersonic core length the gas velocity is subsonic. Thus radial spreading and axial velocity decay beyond the potential core are the main characteristics of a free gas jet

Due to spreading mass of the jet increases which means that concentration of the gas at plane Z=0 decreases due to entrainment of the surrounding. If oxygen is flowing through the nozzle, concentration of oxygen at plane Z₂ is lower than at Z₁ and at Z=0. But mass of jet (jet consists of main fluid + surrounding) at  mass at . Axial velocity of the jet is a function of axial distance measured from the nozzle exit.