🧊Thermodynamics II Unit 11 – Compressible Fluid Flow

Key Concepts and Definitions

• Compressible fluid flow involves fluids with significant density changes due to variations in pressure and temperature
• Mach number ($M$) represents the ratio of the fluid's velocity to the local speed of sound and characterizes the compressibility of the flow
  • Subsonic flow: $M < 1$
  • Sonic flow: $M = 1$
  • Supersonic flow: $M > 1$
• Stagnation properties refer to the fluid properties (pressure, temperature, and density) that would be achieved if the fluid were brought to rest isentropically
• Critical properties are the fluid properties at which the Mach number equals 1 (sonic conditions)
• Isentropic flow assumes no heat transfer or friction, and the entropy remains constant throughout the flow
• Fanno flow is an adiabatic flow with friction, leading to entropy increase along the flow path
• Rayleigh flow is a frictionless flow with heat transfer, causing entropy to change along the flow path

Fundamental Equations

• Continuity equation expresses the conservation of mass in a compressible flow: $\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \vec{V}) = 0$
• Momentum equation represents the conservation of momentum: $\rho \frac{D\vec{V}}{Dt} = -\nabla p + \nabla \cdot \tau + \rho \vec{g}$
• Energy equation describes the conservation of energy: $\rho \frac{De}{Dt} = -\nabla \cdot \vec{q} - p(\nabla \cdot \vec{V}) + \phi$
• Equation of state relates pressure, density, and temperature for a given fluid (ideal gas law: $p = \rho RT$)
• Speed of sound equation: $a = \sqrt{\frac{\gamma p}{\rho}}$, where $\gamma$ is the specific heat ratio
• Isentropic relations for pressure, temperature, and density ratios as functions of Mach number
• Fanno and Rayleigh flow equations relate fluid properties along the flow path

Types of Compressible Flow

• Internal flow occurs within a confined space, such as pipes, ducts, or nozzles
  • Characterized by the presence of solid boundaries that guide the flow
• External flow occurs around objects immersed in a fluid, such as airfoils, vehicles, or buildings
  • Characterized by the absence of solid boundaries confining the flow
• Steady flow implies that fluid properties at any point do not change with time ($\frac{\partial}{\partial t} = 0$)
• Unsteady flow occurs when fluid properties at a point vary with time ($\frac{\partial}{\partial t} \neq 0$)
• One-dimensional flow assumes that fluid properties vary only in the flow direction, neglecting variations in other directions
• Two-dimensional and three-dimensional flows consider variations in fluid properties in multiple spatial dimensions
• Compressible flow regimes:
  • Subsonic ($M < 1$)
  • Transonic ($M \approx 1$)
  • Supersonic ($M > 1$)
  • Hypersonic ($M >> 1$, typically $M > 5$)

Shock Waves and Expansion Waves

• Shock waves are thin regions of abrupt changes in fluid properties (pressure, density, temperature, and velocity) that occur when a supersonic flow encounters an obstacle or a sudden change in flow direction
  • Normal shock waves are perpendicular to the flow direction
  • Oblique shock waves form at an angle to the flow direction
• Shock wave properties can be calculated using normal and oblique shock relations, which relate the fluid properties across the shock
• Expansion waves occur when a supersonic flow encounters a sudden expansion or change in flow direction, leading to a decrease in pressure, density, and temperature, and an increase in velocity
• Prandtl-Meyer expansion fans are a series of infinitesimal expansion waves that turn the flow gradually, maintaining isentropic conditions
• Shock-expansion theory combines shock wave and expansion wave analysis to predict the flow field around supersonic objects (wedges, cones)
• Shock-boundary layer interaction can lead to flow separation, increased drag, and heat transfer rates

Nozzles and Diffusers

• Nozzles are devices used to accelerate a compressible fluid by converting pressure energy into kinetic energy
  • Converging nozzles accelerate subsonic flow and decelerate supersonic flow
  • Diverging nozzles accelerate supersonic flow and decelerate subsonic flow
  • Converging-diverging (CD) nozzles can accelerate a fluid from subsonic to supersonic velocities
• Nozzle flow regimes depend on the pressure ratio between the inlet and outlet:
  • Subsonic flow throughout the nozzle
  • Choked flow at the throat (sonic conditions)
  • Supersonic flow in the diverging section
  • Over-expanded or under-expanded flow at the exit
• Diffusers are devices used to decelerate a compressible fluid and increase its pressure
  • Subsonic diffusers have a gradually increasing cross-sectional area
  • Supersonic diffusers often employ a combination of shock waves and area changes to decelerate the flow efficiently
• Nozzle and diffuser performance is characterized by parameters such as thrust, mass flow rate, and efficiency

Applications in Engineering

• Jet engines (turbojets, turbofans) rely on compressible flow principles for propulsion
  • Inlet, compressor, combustion chamber, turbine, and nozzle components
• Rocket engines use converging-diverging nozzles to accelerate the exhaust gases to supersonic velocities
• Supersonic wind tunnels employ nozzles and diffusers to generate and control high-speed flow for aerodynamic testing
• Compressible flow analysis is essential in the design of high-speed vehicles (aircraft, spacecraft, missiles)
• Gas pipelines and distribution systems involve compressible flow in the transport of natural gas and other compressible fluids
• Compressed air systems, such as pneumatic tools and air brakes, utilize compressible flow principles
• Refrigeration and air conditioning systems involve the compression and expansion of refrigerants, which are modeled as compressible fluids
• Shock wave phenomena are relevant in explosion and blast wave analysis, as well as in medical applications (lithotripsy)

Problem-Solving Techniques

• One-dimensional isentropic flow analysis using isentropic relations and tables
• Normal and oblique shock wave calculations using shock relations and tables
• Prandtl-Meyer expansion fan calculations using Prandtl-Meyer function and tables
• Fanno flow analysis using Fanno flow equations and tables
• Rayleigh flow analysis using Rayleigh flow equations and tables
• Nozzle and diffuser design calculations, including area ratios, pressure ratios, and mass flow rates
• Shock-expansion theory for analyzing supersonic flow over wedges and cones
• Numerical methods for solving complex compressible flow problems (finite difference, finite volume, finite element)
  • Computational Fluid Dynamics (CFD) simulations
• Experimental techniques for measuring compressible flow properties (pressure probes, temperature sensors, Schlieren imaging, Particle Image Velocimetry)

Real-World Examples

• Supersonic aircraft (Concorde, SR-71 Blackbird) and their aerodynamic design challenges
• Jet engines in commercial and military aviation (turbofans, turbojets, ramjets, scramjets)
• Rocket engines for space launch vehicles (SpaceX Falcon 9, NASA Space Shuttle)
• High-speed wind tunnels for testing aircraft, vehicles, and components (NASA Glenn Research Center, AEDC)
• Supersonic inlets and nozzles in jet engines and rocket engines (Bell X-1, North American X-15)
• Shock waves generated by explosions, blast waves, and supersonic projectiles
• Shock wave phenomena in medical applications (extracorporeal shock wave lithotripsy for kidney stone treatment)
• Natural gas pipelines and compressor stations for long-distance transport
• High-pressure compressed air systems in industrial applications (pneumatic tools, air brakes)
• Supersonic flow in valve and regulator designs for pressure control in various industries