4.4 Thermodynamics and heat transfer

Thermodynamics and heat transfer are crucial aspects of engineering physics. They explain how energy moves and changes in systems, from engines to buildings. Understanding these concepts helps engineers design efficient machines and solve real-world energy problems.

Laws of thermodynamics govern energy behavior, while heat transfer mechanisms show how it moves between objects. These principles apply to everything from power plants to refrigerators, making them essential for creating better, more sustainable technologies.

Thermodynamics Concepts

Temperature and Heat

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• Thermodynamics deals with heat, work, temperature, and their relation to energy, radiation, and physical properties of matter
• Temperature measures average kinetic energy of particles in a substance (Kelvin, Celsius, Fahrenheit)
• Heat transfers thermal energy between systems due to temperature difference (joules, calories)
• Internal energy encompasses total energy within a system (kinetic and potential energy of particles)
• Specific heat capacity quantifies heat required to raise temperature of unit mass by one degree
  • Crucial for understanding thermal behavior of materials
  • Examples: Water (4.18 J/g°C), Aluminum (0.897 J/g°C)

Entropy and Equilibrium

• Entropy measures disorder or randomness in a system
  • Related to second law of thermodynamics and irreversibility of natural processes
  • Examples: Ice melting, gas expanding
• Zeroth law of thermodynamics establishes concept of thermal equilibrium
  • Provides basis for temperature measurement
  • Example: Two objects in contact reaching same temperature over time
• Thermal equilibrium occurs when systems have equal temperatures
  • No net heat transfer between systems
  • Example: Coffee in a mug eventually reaching room temperature

Laws of Thermodynamics

First Law of Thermodynamics

• Also known as law of conservation of energy
• States energy cannot be created or destroyed, only converted between forms
• Mathematical expression: $ΔU = Q - W$
  • ΔU: change in internal energy
  • Q: heat added to system
  • W: work done by system
• Examples:
  • Burning fuel in an engine (chemical energy to mechanical energy)
  • Hydroelectric dam (gravitational potential energy to electrical energy)

Second Law of Thermodynamics

• Introduces concept of entropy
• States total entropy of an isolated system always increases over time
• Carnot efficiency represents maximum theoretical efficiency of heat engine
  • Operates between two temperatures
  • Derived from second law
• Examples of increasing entropy:
  • Spontaneous mixing of hot and cold water
  • Rusting of iron

Thermodynamic Cycles and Processes

• Thermodynamic cycles analyze performance of heat engines and refrigeration systems
  • Carnot cycle: ideal reversible cycle
  • Otto cycle: internal combustion engines
  • Rankine cycle: steam power plants
• Thermodynamic processes describe system changes under specific conditions
  • Isothermal: constant temperature
  • Isobaric: constant pressure
  • Isochoric: constant volume
  • Adiabatic: no heat transfer
• Exergy quantifies maximum useful work extractable from a system in given environment
  • Example: Determining efficiency of power plant based on available energy

Heat Transfer Mechanisms

Conduction

• Transfers thermal energy through direct contact between particles of matter
• Governed by Fourier's law of heat conduction
• Thermal conductivity (k) quantifies substance's ability to conduct heat (W/m·K)
  • High k: good conductors (metals)
  • Low k: good insulators (wood, foam)
• Examples:
  • Heat transfer through a metal pot on a stove
  • Conduction through walls of a building

Convection

• Involves heat transfer by movement of fluids or gases
• Categorized as natural (free) or forced convection
• Heat transfer coefficient (h) quantifies convective heat transfer (W/m²·K)
• Examples:
  • Natural convection: hot air rising in a room
  • Forced convection: fan blowing air over a hot surface

Radiation

• Transfers heat through electromagnetic waves
• Governed by Stefan-Boltzmann law and surface emissivity
• View factor accounts for geometric relationship between radiating surfaces
• Examples:
  • Heat from the sun warming Earth
  • Infrared heat lamps in food service

Thermal Systems Analysis

Heat Exchangers

• Transfer heat between two or more fluids at different temperatures
• Classified by flow arrangement: parallel, counter, or cross-flow
• Effectiveness-NTU method analyzes performance when outlet temperatures unknown
• Log mean temperature difference (LMTD) method determines heat transfer rates with known inlet/outlet temperatures
• Examples:
  • Car radiator cooling engine coolant
  • Shell and tube heat exchanger in chemical plants

Thermal Insulation

• Characterized by R-value or thermal resistance
• Overall heat transfer coefficient (U) combines effects of conduction, convection, and radiation
• Thermal bridges increase heat transfer in insulation systems
  • Require special consideration in building design and energy efficiency calculations
• Transient heat transfer analysis essential for time-dependent behavior
• Examples:
  • Fiberglass insulation in home attics
  • Vacuum-insulated panels in refrigerators

Refrigeration and Air Conditioning

Vapor Compression Cycle

• Most common refrigeration cycle
• Four main components: compressor, condenser, expansion valve, evaporator
• Coefficient of Performance (COP) measures efficiency
  • Ratio of cooling or heating effect to work input
• Refrigerants chosen based on thermodynamic properties, environmental impact, safety
• Examples:
  • Household refrigerators
  • Automotive air conditioning systems

Psychrometrics and HVAC Systems

• Psychrometrics studies air-water vapor mixtures
• Psychrometric chart determines properties of moist air
  • Relative humidity, dew point, enthalpy
• HVAC systems maintain desired indoor environmental conditions
• Energy efficiency influenced by insulation, compressor efficiency, heat exchanger design
• Examples:
  • Central air conditioning in office buildings
  • Dehumidifiers in basements