Applied Thermodynamics: Key Concepts

• Applied thermodynamics is the practical application of thermodynamic principles to the analysis and design of real-world systems and processes.
• It focuses on utilizing thermodynamic laws and concepts to solve engineering problems.
• It is crucial in various fields, including mechanical, chemical, aerospace, and environmental engineering.

Key Concepts

• Energy: The capacity to do work, existing in forms like kinetic, potential, thermal, and chemical.
• Work: Energy transfer from a force acting over a distance, often volume changes (gas expansion/compression).
• Heat: Energy transfer due to temperature difference, always flowing from higher to lower temperature.
• System: Defined region of space or matter under analysis.
  • Surroundings: External elements affecting the system's behavior.
  • Boundary: Separates the system from its surroundings.
• State: Condition of a system defined by its properties.
• Property: Macroscopic characteristic of a system (temperature, pressure, volume, density).
• Process: Any change a system undergoes from one state to another.
• Cycle: Series of processes returning a system to its initial state.
• Equilibrium: State where system properties are uniform, with no driving forces for change.
  • Thermal equilibrium: Uniform temperature.
  • Mechanical equilibrium: Uniform pressure.
  • Chemical equilibrium: Uniform chemical potential.

Laws of Thermodynamics

• Zeroth Law: If two systems are in thermal equilibrium with a third, they are in equilibrium with each other, establishing temperature.
• First Law: Energy is conserved, converting from one form to another.
  • Mathematical expression: ΔU = Q - W (ΔU: internal energy change, Q: heat added, W: work done).
• Second Law: Total entropy of an isolated system increases or remains constant (ideal), never decreasing.
  • Entropy (S): Measures system disorder or randomness.
  • Implications: Heat engines cannot be perfectly efficient; processes are irreversible.
• Third Law: As temperature approaches absolute zero, entropy approaches a minimum or zero value, providing an entropy calculation reference.

Thermodynamic Properties

• Temperature (T): Measures average kinetic energy of particles.
  • Units: Kelvin (K) in SI; Rankine (°R) in English units.
• Pressure (P): Force per unit area exerted by a fluid.
  • Units: Pascal (Pa) in SI; pounds per square inch (psi) in English units.
• Volume (V): Amount of space a substance occupies.
  • Units: cubic meters (m³) in SI; cubic feet (ft³) in English units.
• Internal Energy (U): Energy from molecular motion and intermolecular forces.
  • State function, depends on the current state, not the path.
• Enthalpy (H): Defined as H = U + PV (U: internal energy, P: pressure, V: volume).
  • Useful for analyzing constant pressure processes.
• Entropy (S): Measures system disorder or randomness.
  • Entropy increase indicates decreased energy available for work.
• Gibbs Free Energy (G): Defined as G = H - TS (H: enthalpy, T: temperature, S: entropy).
  • Useful for determining the spontaneity at constant temperature and pressure.

Thermodynamic Processes

• Isothermal Process: Constant temperature process.
• Isobaric Process: Constant pressure process.
• Isochoric (Isometric) Process: Constant volume process.
• Adiabatic Process: No heat transfer between system and surroundings (Q = 0).
• Reversible Process: Can be reversed without affecting surroundings; an idealization.
• Irreversible Process: Cannot be reversed without affecting surroundings; all real-world processes.
• Polytropic Process: Follows PV^n = constant (n: polytropic index).

Thermodynamic Cycles

• Power Cycles: Convert heat into work.
  • Examples: Rankine cycle (steam power plants), Otto cycle (gasoline engines), Diesel cycle (diesel engines), Brayton cycle (gas turbine engines).
• Refrigeration Cycles: Transfer heat from cold to hot reservoir, requiring work input.
  • Examples: Vapor-compression, absorption refrigeration cycles.
• Heat Pump Cycles: Transfer heat from cold to hot reservoir, providing heating; similar to refrigeration cycles.

Applications

• Power Generation: Power plant design/analysis using steam, gas turbines, and internal combustion engines.
• Refrigeration and Air Conditioning: Design of cooling systems, air conditioners, and heat pumps.
• Chemical Processes: Analysis of reactions, phase equilibria, and separation processes.
• Aerospace Engineering: Design/optimization of propulsion systems (jet and rocket engines).
• Environmental Engineering: Analysis of environmental systems like air pollution control and waste heat recovery.
• Combustion: Analysis of combustion in engines/furnaces, including energy release and pollutant formation.
• Material Science: Understanding material thermodynamic properties and behavior.

Ideal Gas Model

• Assumptions: Gas particles have negligible volume, no interaction.
• Ideal Gas Law: PV = nRT (P: pressure, V: volume, n: moles, R: gas constant, T: temperature).
• Limitations: Good approximation at low pressures and high temperatures, deviates at high pressures/low temperatures due to intermolecular forces.

Real Gases

• Real gases deviate from ideal behavior.
• Equations of State: Complex equations (van der Waals, Redlich-Kwong, Peng-Robinson) model real gas behavior, accounting for intermolecular forces and volume.
• Compressibility Factor (Z): Quantifies real gas deviation from ideal; Z = PV/nRT.

Mixtures

• Mixtures of ideal gases: Dalton's Law states total pressure equals sum of partial pressures.
• Partial pressure: Pressure each gas exerts if occupying the entire volume alone.
• Mole fraction: Ratio of component moles to total moles in the mixture.
• Psychrometrics: Study of moist air thermodynamic properties (dry air and water vapor), important in air conditioning and meteorology.

Combustion Thermodynamics

• Stoichiometry: Quantitative relationships between reactants/products in reactions.
• Air-Fuel Ratio (AFR): Mass ratio of air to fuel in combustion.
• Theoretical Air: Minimum air for complete combustion.
• Excess Air: Air supplied beyond theoretical, ensuring complete combustion.
• Enthalpy of Formation: Enthalpy change when forming one mole of a substance from its elements in their standard states.
• Heating Value: Heat released during complete combustion of a unit mass/volume of fuel.
  • Higher Heating Value (HHV): Includes water vaporization heat in products.
  • Lower Heating Value (LHV): Excludes water vaporization heat in products.

Chemical Thermodynamics

• Chemical Potential: Gibbs free energy change w.r.t. component mole change.
• Phase Equilibrium: Condition where two or more phases coexist.
• Chemical Equilibrium: Forward/reverse reaction rates are equal, no net change in reactant/product concentrations.
• Equilibrium Constant (K): Measures relative amounts of reactants/products at equilibrium, depends on temperature.

Exergy Analysis

• Exergy (Availability): Maximum useful work from a system reaching equilibrium with surroundings.
• Exergy Destruction: Exergy loss due to irreversible processes, also known as irreversibility.
• Second Law Efficiency: Measures system performance accounting for exergy destruction, always <= 1.