Unit
- Closed system: A fixed mass of matter that exchanges energy but not mass with its surroundings. A piston-cylinder assembly is a classic example where work and heat cross the boundary.
- Open system (control volume): Both mass and energy can cross the boundary. Turbines, compressors, and heat exchangers are modeled as control volumes with steady or unsteady flow.
- Intensive properties: Properties independent of system size, such as temperature, pressure, and specific volume. They are uniform at equilibrium and used to define the state.
- Extensive properties: Properties that scale with system mass, like total volume, internal energy, and enthalpy. Specific properties (per unit mass) convert extensive to intensive.
- State postulate: For a simple compressible system, the thermodynamic state is fixed by specifying two independent intensive properties. This is fundamental for property tables and equations of state.
- Phase change and pure substances: Understanding saturation, subcooled liquid, and superheated vapor regions is essential. The T-v diagram illustrates constant-pressure phase change processes like boiling and condensation.
- Quality of a saturated mixture: In the two-phase region, quality (x) indicates the mass fraction of vapor. It is used to determine mixture properties: v = v_f + x (v_g - v_f).
- Equation of state for ideal gases: The ideal gas law (PV = nRT) approximates behavior at low pressures and high temperatures. Compressibility factor (Z) corrects deviations for real gases.
- Property tables and software: Engineers use steam tables, refrigerant tables, and software like
CoolPropto retrieve properties. Accurate property data is critical for performance calculations. - Process paths and cycles: A thermodynamic process connects two states; a cycle returns to the initial state. Examples include isothermal, adiabatic, and polytropic processes.
Code Example
Python
import CoolProp.CoolProp as CP
# Retrieve saturation temperature at given pressure
fluid = 'Water'
P = 101325 # Pa (1 atm)
T_sat = CP.PropsSI('T','P',P,'Q',0,fluid)
print('Saturation temperature at 1 atm: {:.2f} K'.format(T_sat))This code uses CoolProp to obtain the saturation temperature of water at atmospheric pressure. The function PropsSI requires the output property, two independent inputs, and the fluid name. The 'Q',0 indicates saturated liquid. This approach replaces manual table lookup and reduces errors.
Quick Revision
- Closed and open systems define mass and energy interactions.
- Intensive properties are independent of mass; extensive properties depend on mass.
- The state postulate requires two independent properties to fix the state.
- Quality defines vapor fraction in saturated mixtures.
- Ideal gas law approximates behavior; compressibility factor corrects real gas.
- Property databases like
CoolPropenable accurate computations.