fluent modelling of combustion in a ducted rocket

FLUENT MODELLING OF COMBUSTION IN A DUCTED ROCKET

R.A. StoweDefence R&D Canada – Valcartier

A. deChamplain, Department of Mechanical Engineering, Université Laval, Canada

科技论坛：www.tech-domain.com第1/25页

Defence R&D Canada – Valcartier

OUTLINE• Introduction• Combustor configurations• Direct-connect combustion tests• CFD modelling• Boundary conditions• Two phase flow• Results

– Effect of twostream model on temperatures– Comparison of modelling and experiments– Time-dependency

• Conclusions



INTRODUCTION• solid propellant rocket motors around for hundreds of years,

evolved into highly reliable and powerful systems• simpler than liquid fuel rockets

– performance improvement limited since oxidizer onboard• up to 88% by mass of the propellant

• how do we drastically improve the performance but maintain the simplicity?

• use an airbreathing system: solid fuel ducted rocket– also called a ramrocket or integral rocket ramjet– increased range, higher average speed than solid rocket



SOLID FUEL DUCTED ROCKET• not as widely used as other airbreathing propulsion systems

– gas turbines, liquid fuel ramjets– but less complex (no pumps, compressors, etc.)– uses a gas generator to provide “fuel” for the ramjet

combustor



OPERATION OF A DUCTED ROCKET

• Ducted rocket launched




• Ducted rocket launched• Booster ignites, accelerates to Mach 1.2-1.4




• Ducted rocket launched• Booster ignites, accelerates to Mach 1.2-1.4• Booster burns out, port covers open, gas generator fuel ignites




• Ducted rocket launched• Booster ignites, accelerates to Mach 1.2-1.4• Booster burns out, port covers open, gas generator fuel ignites• Gas generator exhaust injected into the combustor,

mixes with the air, and reacts



DUCTED ROCKET FUEL• gas generator exhaust is “fuel” for ramjet phase

– lots of solid carbon soot, combustible gases



COMBUSTOR CONFIGURATIONS• wanted to look at a wide variety of combustor geometries

– better validate a single CFD methodology • variety of air and fuel injectors, dome heights, nozzles• same configurations for non-reacting flow CFD and water

tunnel (previously presented) and direct-connect combustion experiments

57-100mm (Dome height)

100mmfuel port

air injector diameterair inlet angle

fuel injector

Combustor



DIRECT-CONNECT COMBUSTION TESTS

• to provide validation for CFD modelling predictions• to simplify experiments, used a rich mixture of reacted

ethylene/air to simulate the solid fuel gas generator exhaust– gives similar

exhaust composition, simplified experiments



CFD MODELLING• used CFD package FLUENT V5 (Full Navier-Stokes)• structured grid of approximately 50K hexahedral cells

• RNG turbulence model (implies quasi-steady flow)– flow fully turbulent

(Re = 106)– superior to k-ε

for

recirculating, swirling flow



CFD MODELLING (2)• PDF combustion model

– have separate fuel and oxidizer streams• turbulent diffusion flame, mixing-controlled• accounts for the turbulence/chemistry interaction

– only two extra equations per fuel stream, relatively easy to converge (compared to finite-rate model)

• solve for mixture fraction and its variance which determines density, composition, and temperature

– FLUENT Version 5 requires the incompressible solver• neglected nozzle, M <

0.3 in most of combustor



BOUNDARY CONDITIONS• inlets

– uniform velocity profiles– 10% turbulence intensity– characteristic length 1/4 diameter

• air: vitiated air at ≈

600K• fuel: mixture of reacted ethylene/air

– calculated equilibrium compositions at fuel injector exit– same exhaust composition as GAP/Carbon solid fuel– mole fraction carbon (solid) ≈

40%, hydrogen ≈

25%



TWO PHASE FLOW MODEL• with the PDF combustion model, mixture of fuel and

oxidizer (up to a rich limit) are in equilibrium– assumes infinitely fast reactions– do solids react instantaneously?

• previously used a onestream model– gases and solids treated as single, homogeneous stream– tendency to overpredict temperatures in the combustor

• could model as two fuel streams, one of gases, other as solids with particle model– no reference to this approach in literature cited

• only two phase modelling was kerosene injection



TWO PHASE FLOW MODEL (2)• soot from typical HC flames made up of agglomerates

(several microns) of carbon spheres < 60 nm in diameter• collected soot from

ethylene/air gas generator

• used Malvern instrument– assumes that

particles round– 0.1 to > 200 µm



TWO PHASE FLOW MODEL (3)• SEM of collected ethylene/air soot shows agglomerates of

approximately 75 nm diameter carbon spheres • decomposition of

soot related to exposed surface area– number and size

of 75 nm spheres, not overall size of agglomerates

• trajectories related to mass and overall size of agglomerates



TWO PHASE FLOW MODEL (4)

• coloured by particle density

• trajectories change little with size below 10 micron

• based on this, decided to model particles as 75 nm– surface area for

decomposition model correct

1 nm particles

10 nm particles

75 nm particles

1 µm particles

10 µm particles



TWO PHASE FLOW MODEL (5)• particles decompose gradually into CO gas through

combustor controlled by slower of:• rate of diffusion of oxidizer to particle surface, or• rate of surface reaction kinetics

• diffusion rate: from an expression for pulverized coal over a wide range of temperatures:

rate increases as particle diameter decreases

( )[ ]p

p

dTT

CR75.0

112/∞+

=



TWO PHASE FLOW MODEL (6)• surface kinetics rate: approximation to Nagel-Strickland-

Constable formula for soot oxidation below 2000K:

for 75 nm particles, decomposition is controlled by the surface reaction kinetics

• 500 particles are injected individually from the fuel inlet– Lagrangian reference frame, affected randomly by

turbulence (stochastic particle tracking)• continuous and dispersed (solid) phase calculations coupled• particle density decreases as it decomposes (dp constant)

( )pRTECR /exp22 −=



RESULTS: EFFECT ON TEMPERATURE FIELDS

Twostream model more diffuse, carries heat release further



RESULTS: COMPARISON WITH EXPERIMENTS

twostream difference (16%) almost same as experimental uncertainty (13%), onestream difference 27%

2,4

2exp,4

ttheot

ttT TT

TT−

−=Δη



TIME DEPENDENCY

Water tunnel visualization (unsteady)



TIME DEPENDENCY (2)• graph of

magnitude of oscillations

• ramjet ignition at 3 seconds

• pressure oscillations:– 100 kPa

peak-to-peak– mean pressure

420 kPa



CONCLUSIONS• a twostream PDF model, with the gases and solids treated

separately, was implemented– changed the temperature distribution in the combustor

over the onestream PDF model– improved combustion efficiency predictions

significantly over the onestream PDF model• unsteady phenomena may explain some of the differences

between predictions and experimental results– unsteady flow seen in water tunnel– oscillations of 25% of the mean pressure were

measured in the combustor• might be coupled to combustion processes


fluent modelling of combustion in a ducted rocket

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