Thermodynamics Quiz: Zeroth Law and Pressure

Course Information

• Course title: 5ENT1129 Thermodynamics for Aerospace
• Lecturer: Dr Burhan Saeed
• University: University of Hertfordshire (UH)

Lecture 1 - Introduction to Thermodynamics

• Thermodynamics is derived from Greek words "therme" (heat) and "dynamis" (power)
• Thermodynamics is encountered in many aspects of life and engineering systems
• Two approaches in thermodynamics:
  • Classical (macroscopic) approach: studies the average behavior of a large group of particles without studying individual particle behavior.
  • Statistical (microscopic) approach: studies the average behavior of a large group of individual particles.
• System: a region in space that encloses a quantity of matter
• Surroundings: everything outside the system
• Boundary: separates the system from its surroundings; has zero thickness and can move.

System Types

• Open system: can exchange energy and mass with the surroundings (e.g. boiler, human body, rocket nozzle)
• Closed system: can exchange energy but not mass with the surroundings (e.g. pressure cooker, home heating system)
• Isolated system: cannot exchange energy or mass with the surroundings (e.g. universe, Dewar flask)

Properties of a System

• Any characteristic of a system is called a property
• Extensive Property: depends on the size or extent of the system (e.g. total mass, total volume)
• Intensive Property: does not depend on the size or extent of the system (e.g. temperature, pressure)
• Specific Property: extensive property per unit mass (e.g. specific volume, specific total energy, specific heat capacity)

Density and Specific Gravity

• Density (ρ): mass per unit volume (kg/m³)
• Specific volume (v): volume per unit mass (m³/kg)
• Specific gravity (SG): ratio of density of a substance to the density of water at a specified temperature (often 0°C or 20°C)
• Specific weight (Ys): weight per unit volume (N/m³)

State and Equilibrium

• State: all the properties have fixed values throughout the system.
• Equilibrium state: no unbalanced potential or driving force can change the system
• Thermal equilibrium: no heat transfer between systems
• Mechanical equilibrium: no pressure change with time
• Phase equilibrium: mass of each phase reaches an equilibrium level and stays there.
• Chemical equilibrium: chemical composition does not change with time

Processes and Cycles

• Quasi-equilibrium process: change in a system undergoes from one equilibrium state to another, the system remains infinitesimally close to an equilibrium state at all times.
• Process diagram: shows the relationship between two properties during a process.
• Isothermal process: occurs at constant temperature (e.g. boiling of water in open air)
• Isobaric process: occurs at constant pressure
• Isochoric/isovolumetric process: occurs at constant volume
• Adiabatic process: occurs with no heat transfer (e.g. compression/expansion in compressors/turbines)

Assessments

• Timed Assessment 1: computer-based test (20%)
• Lab Coursework: Heat Pump experiments (20%)
• Final Timed Assessment: multiple-choice and open questions (60%)

Lecture 2 - Energy Conservation

• Different forms of energy: thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear.
• Total energy of a system on a unit mass basis is denoted by e (kJ/kg).
• Macroscopic energy: related to the motion of a system, e.g., potential energy, kinetic energy
• Microscopic energy/Internal Energy (u or Eint): related to the molecular structure of the system and the degree of molecular activity e.g sensible energy, latent energy, chemical energy, nuclear energy.

Energy Transfer and Heat, Work

• Heat transfer: transfer between two systems (or a system and its surroundings) due to temperature difference.
• Heat transfer is a form of energy
• Work transfer: energy transfer associated with a force acting through a distance
• Work is energy
• Q is heat

Formal Sign Convention on Heat and Work

• Heat transfer to a system is positive (Q>0)
• Heat transfer from a system is negative (Q<0)
• Work done by a system is positive (W>0)
• Work done on a system is negative (W<0)

First Law of Thermodynamics

• Energy can not be created or destroyed, only transformed
• Change in total energy of system = Heat added to system − Work done by system ($\Delta$U = Q − W)
• Adiabatic process: a process where there is no heat transfer (Q = 0)

Adiabatic Process

• Definition: A process during which there is no heat transfer.
• Two ways to be adiabatic: Well insulated, No temperature difference.

Ideal Gas Eqaution Of State

• PV = nRT
  • P = Pressure (measured in Pascal)
  • V = Volume (measured in m³)
  • n = number of moles(measured in mol)
  • R = Universal Gas Constant (8.314 J/mol·K)
  • T = Absolute temperature (measured in K)

Specific Heat

• Specific heat at constant volume (cv): energy required to raise the temperature of a unit mass by one degree at constant volume
• Specific heat at constant pressure (cp): energy required to raise the temperature of a unit mass by one degree at constant pressure
• • For monoatomic gas (e.g., Helium, Neon, Argon): cv=3/2 R and Cp= 5/2 R
  • For diatomic gas (e.g., Hydrogen): cv=5/2 R and Cp = 7/2 R

Adiabatic process of an ideal gas

• PV^k = constant ; k = _cp/_cv
• T.V^k-1 = constant

Different process on the PV (Clapeyron) plane

Energy Analysis Steady Flow Control Volume

• Volume flow rate: the volume of fluid passing a point per unit time (Q = AV)
• Mass flow rate: the mass of fluid passing a point per unit time (m = ρQ = ρAV)
• Steady flow: a process in which the properties at a fixed point in the control volume do not change with time, steady flow processes are often encountered in turbines, compressors, nozzles, diffusers, heat pumps, and refrigerators.

Flow Work or Flow Energy

• Enthalpy [kJ/kg]: the sum of internal energy plus flow energy (h = u + pv)
• Total Enthalpy [kJ]: the sum of total energy plus flow energy (H = U+PV)
• Mass flow rate is often important when dealing with steady flow (m = ρ Q)
• The heat transfer rate and the work transfer rate must be considered

Efficiency and Power

• Mechanical efficiency
• Turbine efficiency
• Compressor efficiency
• Heat pump efficiency
• The power calculations are based on energy transfer

Heat Pump Application, Refrigerators

Entropy, Thermodynamic Processes

• Entropy is a thermodynamic property that quantifies the irreversibility of thermodynamic processes
• In general , $ΣS_{system} + ΣS_{surr} > 0$ (Entropy of the universe always increases)

Reversible Processes

• A process where the system & surroundings are returned completely to their original state at the end of the process.
• Ideally, many processes in thermodynamic systems are reversible.
• The theoretical limit for real processes,

Factors that Contribute to Irreversibility

• Friction (heat generation)
• Heat transfer between bodies with finite thermal capacities.
• Mixing and chemical reactions

The Carnot Cycle

• Series of four reversible processes that constitute a theoretical heat engine cycle.
  • Reversible adiabatic compression
  • Reversible isothermal heating/expansion
  • Reversible adiabatic expansion/compression

Carnot Cycle Thermal Efficiency

• Carnot Heat Engine η =1 - (T_cold/T_hot)
• A Carnot heat engine is a theoretical heat engine that operates on the Carnot cycle and is fully reversible.
• The efficiency of a heat engine must always be less than a Carnot cycle engine when operating between the same two reservoirs.

Reversed Carnot Cycle

• Exactly the same as the Carnot cycle but the heat and work interactions are reversed

Test your knowledge on the Zeroth Law of Thermodynamics and concepts of pressure. This quiz covers key principles, definitions, and phenomena related to thermodynamics. Ideal for students looking to grasp the fundamentals of heat transfer and pressure measurements.