Introduction to Fluid Properties

Introduction to Fluid Properties

• Any characteristic of a system is a property, categorized as intensive or extensive.
• Intensive properties are independent of system mass (e.g., temperature, pressure, density).
• Extensive properties depend on system size (e.g., volume, mass).
• Specific properties are extensive properties per unit mass.
• Specifying enough intensive properties defines a system's state.
• The state postulate says two independent intensive properties completely describe a simple compressible system's state.

Continuum Hypothesis

• Treating a fluid as continuous, homogeneous matter, ignoring molecular gaps is convenient.
• This continuum model is valid if the system's length scale is significantly greater than the mean free path (Knudsen number is small, typically less than 0.01).
• The model disregards the discontinuous nature of atoms.
• Otherwise, gas flow and individual molecule consideration are required.

Density and Specific Gravity

• Density (ρ) is mass per unit volume (kg/m³).
• Specific volume (v) is the reciprocal of density (m³/kg).
• Specific gravity (SG) is the ratio of a substance's density to the density of water at 4°C.
• Weight density (γ) is weight per unit volume (N/m³).

Vapor Pressure and Cavitation

• Saturation temperature (T_sat) is the temperature at which a pure substance changes phase at a given pressure.
• Saturation pressure (P_sat) is the pressure at which a pure substance changes phase at a given temperature.
• Vapor pressure (P_v) is the pressure exerted by a substance's vapor in equilibrium with its liquid at a given temperature; equal to saturation pressure.
• Partial pressure is the pressure of a gas in a mixture; must be less than or equal to the vapor pressure in the absence of liquid.
• Cavitation is vaporization due to pressure dropping below vapor pressure; resulting in destructive high pressure waves; a concern in hydraulic turbines and pumps.
• Supercavitating torpedoes use cavitation to their advantage.

Energy and Specific Heats

• Energy (E) exists in various forms: thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, nuclear).
• Total energy (E) is the sum of all forms of energy.
• Internal energy (U) is the sum of all microscopic energy forms related to a substance's molecular structure.
• Kinetic energy is energy due to motion.
• Potential energy is energy due to elevation in a gravitational field.
• Enthalpy (h) is the total energy per unit mass of a flowing fluid and includes internal energy and flow energy (PV).
• For incompressible substances (liquids), constant pressure and temperature conditions yield specific enthalpy equations.

Compressibility and Speed of Sound

• Compressibility (κ) is a measure of a fluid's resistance to compression.
• Bulk modulus of elasticity (κ) describes a fluid's resistance to compression.
• Isothermal compressibility (α) is the inverse of the coefficient of compressibility.
• Coefficient of volume expansion (β) expresses how density changes with temperature at constant pressure.
• The speed of sound (c) is the speed at which an infinitesimally small pressure wave travels through a medium.
• Mach number (Ma) is the ratio of fluid velocity to the speed of sound in the fluid—high Mach numbers indicate compressible effects.

Viscosity

• Viscosity (µ) is the resistance of a fluid to flow, dependent on temperature and practically independent of pressure for liquids.
• Fluids where stress is linearly proportional to rate of deformation are called Newtonian fluids.
• Dynamic viscosity (µ) measures absolute viscosity.
• Kinematic viscosity (ν) is dynamic viscosity divided by density.
• Viscosity of liquids decreases with increasing temperature, while the viscosity of most gases increases with increasing temperature.
• Viscosity is independent of deformation rate for Newtonian fluids.

Surface Tension and Capillary Effect

• Surface tension (σ) is the force acting on liquids at the surface.
• Contact angle (φ) is the angle between the tangent to the liquid surface and the solid surface.
• Liquids "wet" surfaces if φ < 90° and do not wet surfaces if φ > 90°.
• The capillary effect is the rise or fall of a liquid in a narrow tube due to surface tension.
• The height of liquid rise/fall is related to the liquid's surface tension, density, tube radius, and contact angle.