Abstract:
Free jet flow is a basic flow configuration found in wide range of applications,
majority of them being mixing, heating or cooling. The present study is therefore focused
to improve the performance of the free turbulent jets within its ‘developing zone’ using
two different mechanisms viz. (a) use of non-circular cross section jet and (b)
introduction of swirl to jet by means of peripheral twist. The present research considers
experimental and computational study of circular and non-circular turbulent jets (having
Reynolds number 7.2 ×104 based on jet exit diameter). A ‘jet modifier pipe’, is used for
the creation of circular and non-circular jets. Essentially, jet modifier pipe is small pipe
through which circular jet is gradually changed to non-circular jet. Axial center line
velocity, jet spread rate, potential core length and jet velocity profiles at various
downstream locations are measured both experimentally and computationally.
Turbulence intensity and tangential velocities at different downstream locations of the
jet are determined numerically.
The comparison of local jet velocity along jet axis indicates that the deceleration
of axial velocity for triangular jet is maximum while that for circular jet is minimum
which is indicating maximum potential core length for circular jet. A consistent
observation is also showcased by jets wherein, the potential core length for circular jet is
maximum, while non-circular jets show lower core length. The peak turbulence intensity
location is found closer to jet exit for triangular and rectangular jets which is an indicator
of efficient mixing. Rectangular jets show saddle shape profile at downstream of the pipe
exit. The rectangular jet possesses an upper hand in terms of jet performance
characteristics from the point of mixing characteristics.
In order to improve the mixing characteristics of non-circular jets, a peripheral
twist has been provided to jet modifier pipe. The parametric analysis included study of
twist rate and length of jet modifier pipe. The study indicates that, increase in twist rate
of jet modifier pipe increases tangential velocity of jet, with rectangular jet outperforming
square and triangular jets. The twisted jet modifier pipe length has an opposite effect
wherein, the smaller the length of pipe, the higher is the tangential velocity. In nut shell
the computational and experimental studies indicate that the peripheral twist introduced
to non-circular free jets are far superior to the circular free jet in the mixing process.
