Abstract:
An experimental investigation into the effects of shock/wake and
shock/flame interaction on the base pressure of axisymmetric bodies at
Mach 2 has been carried out. This investigation has determined the
effects of various forms of shock generator (axisymmetric cowls, twodimensional
wedges and 'delta' wings) on the base pressure. Shock
waves generated by over-expanding the airflow in an open-jet wind tunnel
have been used to determine the effect of shock strength on the base
pressure of an axisymmetric fuel injector. Both peripheral bleed and
axial bleed of hydrogen fuel have been examined and the effect of shock
compression on the resulting flame has been determined. In the axial
bleed case nitrogen and hydrogen bleed without combustion has also been
examined. The effect of varying the airflow stagnation temperature has
also beeninvestigated.
It is demonstrated herein that there is a distinct shock/wake
interaction position that maximises the base pressure, that with
interaction at this optimal position the static pressure rise across
the shock wave can be communicated in full to the base of the centrebody,
and that favourable aerodynamic interference between the wake and a
cowl of 50 convergent-divergent internal section can give rise to a net
drag reduction. The shock/wake and shock/flame experiments demonstrate
that a significant base thrust can be generated, however, the fuel
efficiency decreases with increasing shock strength. It is shown that
the fuel specific impulse is a function of shock strength, interaction
position and bleed mode (peripheral or axial). The onset of boundary
layer separation due to the adverse pressure gradient encountered when
the base pressure is high appears to limit the useful addition of wake
combustion. Finally, it is demonstrated that the base pressure, with
and without combustion, is only a weak function of airflow stagnation
temperature.
