• The flow region near the nozzle in double concentric jets.

  • Authors: Horsman H
  • Publication date:

    October 1963


1. Summary

The type of double concentric jets considered in this paper consists of a central round air jet surrounded by an annular air jet issuing into stagnant air surroundings. Detailed measurements of the mean velocity and static pressure distributions have been made in the region close to the exit of the nozzles and the effect of varying the ratio of the velocities in the central and annular jets has

been examined.

It is shown that for a short distance from the nozzle exit both the central and annular jets can be treated as separate jets and that their respective deviation from their separate jet behaviour depends essentially upon the ratio of their velocities and the distance by which they are separated.

The flow patterns are different from those considered experimentally by Forstall and Shapiro and theoretically by Squire and Trouncer who considered the mixing of two parallel streams of fluid with varying velocity ratios. In the double concentric jet the annular jet sets up a region of subatmospheric pressure with an associated closed

ring vortex in the central region. When the central jet is introduced, it has tv overcome the pressure gradient set up by the annular jet. Both the central and annular jets attempt to entrain fluid from their surroundings. The central jet may be completely absorbed by the annular jet or the annular jet may be completely absorbed by the central jet, depending upon the relative momenta of the two jets.

Due to the separation between the jets the effect of the velocity ratio on the decay of the velocities of the central jet does not agree with the prediction and experiments for the case when there is no separation.

The effect of the velocity ratio on the static pressure distribution

is shown for the case of the pure annular jet when the vortex occupies the full space between the inner boundaries of the annular jet, as well as for the cases where the vortex is confined to the space between the jets. The length and strength of the vortex progressively decreases with the velocity r~tio.

The static pressure distributions within the jets show a subbatmospheric minimum in the region of the vortex centre followed by

a super atmospheric pressure at the point of convergence of the streammlines. In the regions of high velocity the turbulence generates pressure defects so that the pressures measured within the jets are

a combination of the pressure forces required to deflect tne streammlines and those generated by the turbulence. These pressure effects

can be clearly distinguished when the flow is varied from the dominant annular jet case to that of the dominant central jet.

The total momentum flux has been calculated at each cross section of the flow and it is shown that corresponding to the changes of pressure, the velocity momentum flux changes along the axis in such a manner that the total momentum flux is conserved.

- 2 -

When the two jets have combined they behave essentially as an equivalent single jet but their effective origin and outside boundaries are dependent upon the velocity ratio as well as upon

the geometry of the nozzles. The origin of the combined jets is shown to move progressively from positive axial values for the pure central jet to negative values well behind the nozzle exit for the annular jet.

With the aid of temperature and C02 tracer techniques the concentrations of central and annular fluids are shown to decay in a similar manner

to the velocities and are also dependent upon the ratio of the velocities in a manner which is essentially different from that found

by Forstall and Shapiro.

  • Research: Technical Reports

  • Document: G 2/a/6