Post on 07-Aug-2020
Mechanics of Fluids, SI, Fifth Edition Potter, Wiggert, Ramadan
Chapter 9 Compressible
Flow
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• Not all gas flows are compressible flows, neither are all compressible flows gas flows.
• Examples: airflows around commercial and military aircraft, airflow through jet engines, and the flow of a gas in compressors and turbines.
9.1 Introduction
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• Reminders from previous chapters:
• Continuity equation
• Momentum equation
• Energy equation
9.1 Introduction
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• Reminders from previous chapters:
• Thermodynamic relations
• Ideal gas law
• Isentropic flow
9.1 Introduction
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• Sound wave:• Travels with a velocity c relative to a stationary observer.
9.2 Speed of Sound and the Mach Number
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• Speed of Sound
• Using the ideal gas law
• Mach number (dimensionless velocity)
9.2 Speed of Sound and the Mach Number
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9.2 Speed of Sound and the Mach Number
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If M < 1, the flow is a subsonic flow, and if M > 1, it is a supersonic flow.
Angle α of the Mach cone is given by:
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9.2 Speed of Sound and the Mach Number
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9.2 Speed of Sound and the Mach Number
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9.3 Isentropic Nozzle Flow
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Examples: diffuser near the front of a jet engine, exhaust gases passing through the blades of a turbine, the nozzles on a rocket engine, a broken natural gas line.
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9.3 Isentropic Nozzle Flow
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1. If the area is increasing, dA > 0, and M < 1, we see that dV must be negative, that is, dV < 0. The flow is decelerating for this subsonic flow.
2. If the area is increasing and M > 1, we see that dV > 0; hence the flow is accelerating in the diverging section for this supersonic flow.
3. If the area is decreasing and M < 1, then dV > 0, resulting in an accelerating flow.
4. If the area is decreasing and M > 1, then dV < 0, indicating a decelerating flow.
5. At a throat where dA = 0, either dV = 0 or M = 1, or possibly both.
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9.3 Isentropic Nozzle Flow
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Critical ratios:
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9.3 Isentropic Nozzle Flow
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For air with k = 1.4, the critical values are:
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9.3 Isentropic Nozzle Flow
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If pe > pr , the flow exiting the nozzle is able to turn sharply.
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9.3 Isentropic Nozzle Flow
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Converging–diverging nozzle:
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9.3 Isentropic Nozzle Flow
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The purpose of a nozzle is to convert stored energy into kinetic energy, while the purpose of a diffuser is to recover the pressure.
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9.3 Isentropic Nozzle Flow
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9.3 Isentropic Nozzle Flow
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9.4 Normal Shock Wave
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Shock wave: A large disturbance that propagates through a gas.
The changes that occur across a shock wave take place over an extremely short distance.
Stationary shock wave in a tube:
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9.4 Normal Shock Wave
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The three equations below allow us to determine three unknowns:
Continuity equation
Energy equation
Momentum equation
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9.4 Normal Shock Wave
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A finite wave that converts a subsonic flow into a supersonic flow is an impossibility.
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9.4 Normal Shock Wave
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9.4 Normal Shock Wave
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9.4 Normal Shock Wave
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9.5 Shock Waves in Converging-Diverging Nozzles
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9.5 Shock Waves in Converging-Diverging Nozzles
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9.5 Shock Waves in Converging-Diverging Nozzles
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9.6 Vapor Flow Through a Nozzle
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The critical pressure ratio for steam
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9.6 Vapor Flow Through a Nozzle
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9.6 Vapor Flow Through a Nozzle
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9.7 Oblique Shock Wave
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The oblique shock wave turns the flow so that the velocity vector is parallel to the plane wall.
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9.7 Oblique Shock Wave
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The tangential components of the two velocity vectors do
not enter the equations.
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9.7 Oblique Shock Wave
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9.7 Oblique Shock Wave
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9.8 Isentropic Expansion Waves
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9.8 Isentropic Expansion Waves
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Prandtl-Meyer function: The angle θ through which the supersonic flow turns.
The collection of Mach waves that turn the flow are referred to as an expansion fan.
The supersonic flow remains attached to the wall as it turns the corner, even for large abrupt angles.
Turning angles greater than 90˚ are possible in supersonic flows.
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9.8 Isentropic Expansion Waves
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9.9 Summary
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