Environmental Engineering Reference
In-Depth Information
are believed to in
uence the impingement and accumulation of alumina droplets,
which then affects erosive behavior, it is necessary to accurately predict both the
particle size and trajectory. For design purposes, accurate prediction must allow for
numerical simulation of particle size and trajectory for economic purposes. Recent
work in particle size and trajectory using real time radiography (RTR) and
numerical simulation demonstrated predictive capabilities for low solid-to-gas
ratios (Xiao et al. 2003 ). Another study presented image processing methods to
effectively process RTR images for larger particle sizes (Xiao et al. 2005 ).
Since erosion primarily occurs in the nozzle throat vicinity, the alteration of the
nozzle throat geometry could increase SRM thrust ef
fl
ciency. This improvement is
expected through mitigating impingement, accumulation, and
flow of alumina in the
nozzle. A throat-area increase greater than 5 % in most SRMs is considered
excessive (Wong 1968 ). The working
fl
fl
fluid velocity contour in SRM is affected by
the multiphase
flow in the combustion chamber; therefore, the performance is
affected by the nonequilibrium behavior of exhaust gas limits. The motor combus-
tion chamber and the nozzle geometry are ideal for molten alumina agglomerate
formation, resulting in two-phase
fl
flow losses (Borass 1984 ; Salita 1992 ; Hess et al.
1992 ). Agglomerates lower the propulsive ef
fl
ciency of the exhaust
fl
flow since the
agglomerates cause additional drag force on the
flow by not expanding in the nozzle
(Xiao et al. 2003 , 2005 ). Particular alumina particle-gas mixture ratio and particle
size are known to reduce the specific impulse, with reductions as high as 6 %
(Bandera et al. 2011 ; Holtzmann 1964 ). The severe operating environment of SRMs
combustion chambers has temperatures ranging from 3,000 to 3,500 K with pres-
sures of 2.0
fl
10 7 Pa or greater. Given the alumina melting temperature of 2,327 K at
atmospheric pressure and the evaporation temperature about 3,200 K; alumina exists
in liquid form with possible evaporation. Erosive damage can be caused by the re-
entrainment of the alumina
×
film near the nozzle throat and impingement on the
diverging section of the rocket nozzle. Therefore, it is necessary to obtain a better
understanding of this phenomenon for SRM design.
The breakup mechanism of liquid alumina in SRMs is complicated. The attach-
ment and accumulation of liquid alumina on and to the wall of the de Laval nozzle
(Salita 1995 ) produces unstable shear-driven interactions with the surrounding air.
Shear instability causes the development of a liquid
lmwave which leads to the onset
of breakup on the wave crest. The breakup level grows with the increasing magnitude
of surrounding gas velocity; as a result, the observance of liquid alumina droplet size
reduction is seen. This results in decreased resistance to droplet discharge by the gas
fl
flow rather than the adherence of the liquid alumina to the nozzle wall.
A method of reducing the negative effects of aluminum-based propellants through
geometric modi
cation was employed. This method seeks to induce alumina
breakup through SRM nozzle geometry change, a ramp is placed into the com-
bustion
flow to encourage adequate mixing to prevent nozzle damage. The ramp is
used to induce turbulent
fl
flow, enhancing breakup and two-phase mixing. The alu-
mina particles are expected to be largely carried past the nozzle in the exhaust
fl
fl
ow.
Experimental and numerical investigation of geometric modi
cation as given in
Amano et al. ( 2014a , b ) is discussed for SRM nozzle design.
Search WWH ::




Custom Search