Sarod Fullpaper PDF
Sarod Fullpaper PDF
Sarod Fullpaper PDF
ABSTRACT
The main challenge involved in design of a scramjet engine is to achieve steady combustion in supersonic flow. One of the
methods for flameholding is to create turbulence with the help of struts and injecting fuel into the turbulent wake regions,
and this is called “strut-based injection”. A parametric study is conducted varying the axial location of fuel-injecting
struts in a kerosene-fueled scramjet engine at Mach 6 and angle of attack of 2˚, the x-location of strut-base being our
parameter. The isolator length decreases with each configuration (struts move upstream) keeping the rest of the engine
geometry intact. The flow is simulated as a 3D compressible turbulent reacting flow on PARAS-3D software. Non-
reacting flows are also simulated to observe the effect of combustion. The variation of cumulative axial and normal force
coefficients, pitching moment, load distribution, pressure distribution on the wall and in the flow-field, temperature,
Mach number and the mass flow rates of different species involved are observed. The pressure is observed to be the
maximum at the strut-base for a particular configuration, which increases with decreasing isolator length. Drag and inlet
adiabatic efficiency do not change significantly. Also, with more length available for combustion, more products are
formed and combustion efficiency increases. A desirable configuration would maximize lift, thrust and efficiency;
minimize pitching moment and non-uniformity of reaction in the flow-field. The accuracy of the results can be confirmed
after comparing these results with those from other CFD codes.
Key Words: scramjet, hypersonic, inlet ramp, isolator, combustion chamber, nozzle, strut, turbulence, mixing
efficiency, combustion efficiency, pressure ratio, pressure loss, separation bubble, surrogate fuel, Cartesian mesh.
initial Mach speed from which it can accelerate, just
like a ramjet. Scramjets are suited for hypersonic flight
NOMENCLATURE (M>5), as in conventional ramjets there will be
significant loss of stagnation pressure and severe
L = Length of the vehicle increment of temperature (exceeding material limit) at
𝑥̅ = non-dimensionalised x coordinate (x/L) those speeds, but in scramjets air is still supersonic in
𝑦̅ = non-dimensionalised y coordinate (y/L) the combustion chamber, so we get better efficiency
and lesser issues with material science.
𝑧̅ = non-dimensionalised z coordinate (z/L)
P = Static pressure Scramjet stands for Supersonic Combustion
P0 = Stagnation pressure Ramjet, i.e., a ramjet (external combustion engine
T = Static temperature with no moving parts to generate thrust) where
T0 = Stagnation temperature combustion happens in supersonic stream of air.
M = Mach number Scramjets are suited for hypersonic flight (M>5), as in
φ = Equivalence ratio conventional ramjets there will be significant loss of
f = fuel-air mass flow rate ratio stagnation pressure (to make the hypersonic stream
subsonic before combustion, typically though a series
of oblique shocks ending with a normal shock) and
1. INTRODUCTION severe increment of temperature (due to extreme
When it comes to moving fast through pressure ratio across the inlet, which exceeds material
atmosphere, air-breathing vehicles have a significant limit) at those speeds. In a scramjet, we do not
advantage over rockets, that is, there is no need to encounter a normal shock in decelerating a hypersonic
carry separate oxidizer within the vehicle. That is the stream to lower supersonic speed, thus pressure ratio,
application of air-breathing propulsion is being looked total pressure recovery and temperature increment are
upon in every field of aerospace. To cruise at within practically acceptable limits.
hypersonic Mach numbers, the typical air-breathing
A typical scramjet engine consists of four
solution is a supersonic-combustion ramjet aka
major parts, viz – inlet, isolator, combustion chamber
scramjet. Scramjet has no moving parts, and
and nozzle. Inlet (usually consists of one or more inlet
compression and expansion happen only because of
ramps) and isolator compress the air flow through a
variable area. So, they cannot start from rest and need
series of oblique shocks externally and internally.
a mechanism (usually a rocket booster) to take it to an
Combustion increases the pressure and temperature of
Symposium on Applied Aerodynamics and Design of Aerospace Vehicle (SAROD 2022)
December 15 -17, 2022, Hyderabad, India
the flow and creates high pressure differential across lengths, viz – 0.01913L, 0.03826L, 0.05739L and
the nozzle. The energy released in combustion is 0.07652L (case 2, 3, 4 and 5 respectively). Thus, the
extracted in the nozzle in the form of kinetic energy isolator length goes from 0.1175L to 0.1116L from
and thus thrust is obtained. There are two major case 1 to case 5 (decreasing in steps of 0.01913L).
challenges when it comes to scramjet operation, viz,
creating enough turbulence for fuel-air mixing and
flameholding. This is done by injecting fuel into 3. MODELLING
subsonic recirculation pockets created by different
types of obstructions in the flow, like struts or cavities
etc. Profile of the scramjet walls plays a big role in the The 3D models of these 5 configurations are
lift and net thrust obtained. Extensive optimization created using Solidworks® and imported into ISRO's
study is required to find a balance between in-house PARAS-3D (Parallel Aerodynamic
performance and efficiency, and accordingly the Simulator) software. This tool has the following
geometry takes shape[5,7]. features:
USSR flew the first working scramjet GLL 1. It can generate 3D mesh or grid automatically
Kholod in 1991[6]. But research on scramjets had based on some grid parameters as user inputs.
started in both USSR and the West during the cold war 2. Paras generates only cartesian grid.
era. Currently, USA, Russia, China and India are the 3. It solves 3D Navier-Stokes equation using
only nations to have flown a scramjet-powered Finite Volume approach.
vehicle, but only Russia has a fully developed
scramjet-powered vehicle (3M22 Zircon cruise
missile) that has entered production. The turbulence is modelled by means of k-ε
model and u, is the turbulent viscosity. k is the kinetic
Our current study aims at improving, or just energy of turbulence, ε is the dissipation rate and Cμ
to observe the trends in the typical engine performance is a constant = 0.09. The turbulent Prandtl number is
parameters like thrust, drag, lift, combustion taken as 0.92.
efficiency etc. with movement of struts and pick up the
best possible strut location with minimal compromises
The Navier-Stokes equation in the conservative
and maximum performance within practical limits.
form with species conservation equation is solved
2. GEOMETRY using standard finite volume method on an adaptive
Cartesian Mesh. The interface fluxes are computed
The geometry (Figure 1) of the generic using an upwind scheme of the type AUSM. To ensure
vehicle, from nose to tail, includes a multi-ramp intake second order accuracy, linear reconstruction of
for external compression, an isolator, a constant area primitive variables is done from the cell centre to the
duct containing the fuel-injecting struts, a divergent cell interface with a min-mod limiter. In the regions of
section with very small divergence and a nozzle strong gradients, the scheme becomes first order. The
(highly divergent). As we are only focusing on the viscous fluxes are obtained by standard central
performance of the engine, we eliminate the wings and differencing. The solution marches in time for each
tails of the vehicle for the sake of simplicity. cell based on its local time step. Since the species
conservation equations are very stiff due to highly
non-linear production terms, a point implicit scheme is
adopted for the conservation equations alone to
accelerate the flow solution.
In this work, C10H20 has been used as the
surrogate fuel for Kerosene. Surrogate fuels are
mixtures of pure compounds as instead of real fuels for
research, modeling and simulation purposes. They are
blended to mimic specific properties of the real fuel.
A 10 species, 10 step reduced kinetic
Figure 1: Section of the vehicle at the symmetry mechanism with finite rate chemistry has been
plane implemented in PARAS-3D. This is a combustion
One configuration is chosen as baseline (case model rather than the actual combustion mechanism of
1) and 4 other configurations are created by moving kerosene. A combustion model is introduced to reduce
the struts upstream into the isolator by 4 different
Symposium on Applied Aerodynamics and Design of Aerospace Vehicle (SAROD 2022)
December 15 -17, 2022, Hyderabad, India
6. CONCLUSION
1. Higher pressure and temperature are achieved in
the combustion chamber as isolator length is
decreased.
2. Decrease of pitching moment from case 1 to 5 is
a positive development, as it will require less
efforts in the fuselage design.
3. Case 1 has maximum thrust and lift, and it has
decreased slightly thereafter. This is because of
changing only one geometry parameter (strut
location going upstream into the isolator). Better Figure 9: Area-averaged Pressure
thrust and lift can be extracted from cases 2-5 if
we make subsequent changes in the length of the
constant area combustor duct, and the shape of the
divergence and the nozzle.
4. Combustion efficiency decreases from case 1 to 5
and the maximum is for case 1 and 2 where we got
70%. This is because of lower heat
release(confirmed from lower production of CO2)
as we go from case 1 to 5.
5. Pressure recovery of inlet achieved is almost 38%
with respect to the freestream, in all the cases.
6. It is understood that moving the struts upstream
(case 1 to 5), hence decreasing isolator length and
increasing combustor length is not enough if we Figure 10: Mass averaged Temperature
keep the rest of the geometry intact, as we get
lesser thrust, lift and efficiency. Thus, redesign of
the divergence and nozzle is required and
combustor length needs to be decreased to get the
advantage of the higher chamber pressure and
temperature with shorter isolator.
AUTHORS:
1
Hritam Nath:
Hritam Nath is pursuing his
B. Tech. in aerospace
engineering at Indian
Institute of Space Science
and Technology,
Thiruvananthapuram, India.
ACKNOWLEDGEMENT:
This work was possible because of the
recommendation from Dr. Ayyappan G., chairman,
ASRG, IIST. All these simulations were run from the
Vikram Sarabhai Space Centre, Thiruvananthapuram,
India and simulations ran on clusters at NARL,
Tirupati. The process was supported by several
scientists and interns at VSSC.
REFERENCES
[1] Singh, A. K., Das, D., Manokaran, K., G., V.
and Ashok, V. (2018), ‘Propulsion
characteristics of an integrated air breathing
scramjet vehicle concept propulsion
Symposium on Applied Aerodynamics and Design of Aerospace Vehicle (SAROD 2022)
December 15 -17, 2022, Hyderabad, India