lecture9a - MECH 6251/498D MECH 7221 Non-Ideal Nozzle Flow...

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1 MECH 7221 Non-Ideal Nozzle Flow Analysis • Non-ideal effects in real nozzle flow Objectives Reading assignment: Sutton and Biblarz Chapter 6 Saturn V engine
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2 Ideal Rocket Nozzle Flow • Working fluid is homogeneous perfect gas • No heat transfer ( q = 0, adiabatic) • No frictional loss, no boundary layer loss • No shocks • Invariant gas composition in nozzle • Steady flow • One-dimensional flow, i.e. flow is axial and properties are constant across any plane normal to flow • Chemical equilibrium in combustion chamber
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3 Real Nozzle Effects Stagnation pressure loss in the chamber ¾ non-isentropic flow, including heat and mass transfer, friction • Two-dimensional flow (divergence, varying properties) Boundary layer and wall friction ¾ lower velocity in BL: effects include pressure gradient, heat transfer, wall roughness, nozzle geometry Multiphase flow ¾ liquid drops and solid particles have higher density (thus lower velocity) ¾ Momentum transfer from gas to large drops also slows gas down • Unsteady flow
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4 Real Nozzle Effects Nozzle flow chemical kinetics ¾ Re-association of relatively unstable (high positive heat of formation) molecules as gas cools in the nozzle • Throat erosion leading to lower expansion ratio • Non-uniform properties ¾ mixing loss can be a major effect • Real gas (not perfect gas) properties • Non-optimal expansion
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5 Flow in ideal rocket nozzles One-dimensional Isentropic Area ratio is only important geometric variable Real Nozzles flow is never truly one-dimensional shape of nozzle walls is important Entire nozzle shape must into account variations in velocity and pressure on surface normal to streamlines Other influences on flow: -friction - heat transfer - composition change - shocks Shape of the supersonic or divergent part of the nozzle will dictate shock formation and performance gain/loss. Rocket Analysis
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6 Performance Definition “Four” types (Sutton & Biblarz) 1) Theoretical performance (based on calculations, loss types specified) at operating conditions 2) Delivered (actually measured) 3) Performance at standard conditions -P o = 1000psia, optimally expanded at SL or stipulated ε in vacuum - propellant combination, not propulsion system, performance 4) Guaranteed minimum performance Associated conditions must be clearly defined: chamber and ambient pressure (SL or vacuum) nozzle geometry ( α , ε , etc.) propellants and propellant conditions (T, composition, O/F) type of thermochemical analysis (equilibrium chemistry or invariant composition during nozzle flow)
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7 Sources of Losses 1. We have already discussed the effect of non-axial flow at the nozzle exit.
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lecture9a - MECH 6251/498D MECH 7221 Non-Ideal Nozzle Flow...

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