Thermodynamics: Systems, Cycles, and Applications
Thermodynamic Systems
Types of Systems
Closed System: Exchanges energy but not mass with its surroundings.
Open System: Exchanges both energy and mass with its surroundings.
Isolated System: Exchanges neither energy nor mass with its surroundings.
Thermodynamic Properties
Properties define the state of a system and can be:
- Intensive: Independent of mass (e.g., pressure, density).
- Extensive: Dependent on mass (e.g., volume, energy).
- Specific Properties: Extensive properties per unit mass.
Thermodynamic Equilibrium and State Changes
Equilibrium occurs when properties are uniform and constant over time. A change of state occurs when properties vary.
Static Changes: Occur through non-equilibrium states.
Quasi-static Changes: Occur through states close to equilibrium and can be:
- Reversible: Can be reversed without affecting the surroundings.
- Irreversible: Involve dissipative effects like friction.
First Law of Thermodynamics
Closed Systems
Energy is conserved: ΔE = ΔU + ΔKE + ΔPE. For systems with negligible kinetic and potential energy changes, ΔE = ΔU, where ΔU is the change in internal energy.
Open Systems
Energy and mass are exchanged. Steady flow occurs when properties are constant at a fixed point but may vary spatially.
Second Law of Thermodynamics and Entropy
Entropy: A state function related to the system’s disorder or randomness.
Carnot Cycle: A reversible cycle with maximum efficiency, operating between two heat reservoirs at constant temperatures.
Available Energy
The maximum work obtainable from a system as it reaches equilibrium with its surroundings.
Thermodynamic Reservoirs
- Work Reservoir: Exchanges work with the system.
- Heat Reservoir: Exchanges heat with the system at a constant temperature.
- Mass Reservoir: Exchanges mass with the system.
Properties of a Pure Substance
- Saturated Liquid: At the boiling point, ready to vaporize.
- Subcooled Liquid: Below the boiling point.
- Saturated Vapor: At the condensation point, ready to condense.
- Superheated Vapor: Above the condensation point.
- Wet Vapor: A mixture of saturated liquid and saturated vapor.
Power Plants and Cycles
Rankine Cycle
A steam power cycle used in thermal power plants. Efficiency is improved by increasing the average temperature of heat absorption and decreasing the average temperature of heat rejection.
Rankine Cycle Variations
- Reheat: Steam is partially expanded, reheated, and then further expanded to increase efficiency.
- Regeneration: Steam is extracted from the turbine to preheat the feedwater, increasing efficiency.
Gas Turbines
Rotary engines used for power generation, propulsion, and other applications. Advantages include compactness, ease of regulation, and a good power-to-weight ratio.
Humid Air
Atmospheric air containing water vapor. Humidity indices include:
- Absolute Humidity: Mass of water vapor per unit mass of dry air.
- Relative Humidity: Ratio of actual water vapor content to the maximum possible at a given temperature.
- Dew Point: Temperature at which condensation begins.