Key Thermodynamics and Fluid Mechanics Concepts Defined
Reynolds Number
A dimensionless coefficient related to critical velocity. Above a certain speed, fluid flow transitions from laminar (layered) to turbulent. It can be calculated using fluid properties, velocity, and a characteristic length (e.g., Re = ρvd/μ).
Compressibility Factor (Z)
A measure of the deviation of a real gas from ideal gas behavior. Defined as Z = Pv / RT, where v is the molar volume (V/n), P is pressure, R is the ideal gas constant, and T is temperature.
Triple Line
Represents states on a phase diagram where three phases (e.g., solid, liquid, gas) coexist in equilibrium.
Entropy (S)
A thermodynamic property representing the measure of disorder or randomness in a system. For a reversible process, the change in entropy (ΔS) is defined as the integral of heat transfer (δQ) divided by temperature (T): ΔS = ∫ (δQ / T).
Phase Change Dependency
During a phase change of a pure substance, pressure (P) and temperature (T) are dependent properties; fixing one determines the other.
Dynamic Viscosity (μ)
A measure of a fluid’s resistance to shear stress. The SI unit for the dynamic viscosity coefficient (μ) is N·s/m² (Pascal-seconds, Pa·s) or kg/(m·s).
Diffuser
A device designed to increase the pressure of a fluid by decreasing its velocity.
Buoyancy (Archimedes’ Principle)
A body immersed in a fluid experiences an upward vertical thrust (buoyancy) equal to the weight of the fluid displaced by the body. Buoyant force (FB) = mfluidg = ρfluidVdisplacedg.
Simple System
A system that is large enough for surface effects to be negligible and is not influenced by external electric, magnetic, or gravitational fields.
Specific Heat (c)
The amount of energy required to raise the temperature of a unit mass of a substance by one degree (e.g., one degree Celsius or one Kelvin).
Dew Point Temperature (Tdp)
The temperature to which air must be cooled at constant pressure and water content to reach saturation (100% relative humidity). Below this temperature, water vapor condenses. Example: Fogging on cold glasses occurs when the adjacent air cools to its dew point.
State Postulate
The thermodynamic state of a simple compressible system is completely specified by two independent, intensive properties.
Latent Heat (L)
The heat absorbed or released per unit mass during a phase change (e.g., melting, vaporization) at constant temperature and pressure. Calculated as L = Q / m (Units: J/kg).
Reversible Process
A process is reversible if, after it has occurred, both the system and its surroundings can be returned to their exact initial states without leaving any net change in the universe.
Anomalous Expansion of Water
Water exhibits an anomalous thermodynamic property: it expands upon freezing (its density decreases), unlike most substances. This is why ice floats and lakes freeze from the top down.
Entropy Change (Second Law)
The total entropy of an isolated system (or a system and its surroundings considered together) can never decrease during any process; it increases for irreversible processes and remains constant for reversible processes.
Pressure Head (h)
The height of a column of a specific fluid that would exert a given hydrostatic pressure (P). Calculated as h = P / (ρg), where ρ is the fluid density and g is the acceleration due to gravity.
Closed Heat Exchangers
Devices where two moving fluid streams exchange heat without mixing. Examples include radiators, condensers, evaporators, and boilers.
Magnus Effect
The phenomenon where a spinning object moving through a fluid experiences a sideways force perpendicular to both its direction of motion and its axis of spin.
Quality (x) (Vapor Quality)
Defined only for a saturated liquid-vapor mixture (two-phase or ‘wet’ region). It represents the mass fraction of the vapor phase: x = mvapor / mtotal.
Dry-Bulb and Wet-Bulb Temperatures
The dry-bulb temperature (Tdb) is the ordinary air temperature. The wet-bulb temperature (Twb) is measured by a thermometer with its bulb covered in a water-moistened wick. Tdb and Twb coincide when the relative humidity is 100% (saturated air).
Density of Water Anomaly
The density of liquid water increases as its temperature rises from 0°C to a maximum at approximately 4°C, before decreasing at higher temperatures.
Hydrostatic Paradox
The principle that the hydrostatic pressure exerted by a liquid at a given depth depends only on the depth, fluid density, and gravity, not on the shape or total volume of the container. Consequently, liquid in interconnected vessels of different shapes will reach the same height.
Dynamic Head (Velocity Head)
Represents the kinetic energy per unit weight of a fluid, expressed as an equivalent height. Calculated as v²/2g, where v is the fluid velocity and g is acceleration due to gravity. It’s the height a fluid would need to fall freely from rest to reach velocity v.
Critical Point
The specific state (defined by critical temperature and critical pressure) at which the saturated liquid and saturated vapor phases of a substance become identical and indistinguishable.
Stokes’ Law
Describes the drag force (FD) exerted on a small spherical object moving slowly (low Reynolds number) through a viscous fluid. The formula is FD = 6πμrv, where μ is the fluid’s dynamic viscosity, r is the sphere’s radius, and v is the relative velocity between the sphere and the fluid.
Ideal Gas Model
A simplified theoretical model for gases assuming negligible intermolecular forces and particle volume. It follows the ideal gas law (PV = nRT or Pv = RT). For an ideal gas, internal energy (u) and enthalpy (h) are functions of temperature only: u = u(T), h = h(T) = u(T) + Pv.
Specific Humidity (ω)
The ratio of the mass of water vapor (mv) to the mass of dry air (ma) in a given sample of moist air: ω = mv / ma. It can be related to partial pressures.
Degree of Saturation (μ)
The ratio of the actual specific humidity (ω) of moist air to the specific humidity at saturation (ωs) at the same temperature and pressure: μ = ω / ωs.
Refrigerator and Heat Pump Performance
The performance metrics (Coefficient of Performance, COP) for refrigerators (COPR) and heat pumps (COPHP) depend on their respective objectives (cooling vs. heating). For devices operating between the same temperature reservoirs, COPHP = COPR + 1.
Zeroth Law of Thermodynamics
States that if two thermodynamic systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law establishes temperature as a fundamental property indicating thermal equilibrium.
Kinematic Viscosity (ν)
The ratio of a fluid’s dynamic viscosity (μ) to its density (ρ): ν = μ / ρ. The SI unit is m²/s.