Unit 61: Engineering Thermodynamics Lesson 1: Setting the Scene for the Course

Objective The purpose of this lesson is to introduce some of the basic concepts relating to thermodynamics

What is Thermodynamics? So what exactly is Thermodynamics? Thermodynamics is a science in which the storage, the transformation and the transfer of energy are studied.

How is Energy Stored? Energy is stored as… – internal energy (associated with temperature), – kinetic energy (due to motion), – potential (due to elevation) and – chemical (due to chemical composition)

How is Energy Transformed? Energy is transformed from one form (potential to kinetic; chemical to internal energy; etc) to another.

How is Energy Transferred? Energy is transferred across a boundary as either heat or work.

Relating Energy to Macroscopic (large scale) properties Thermodynamics uses mathematical equations to describe the transformations and transfer of energy to material properties such as temperature, density, pressure, volume or enthalpy. – Note: these macroscopic properties of matter are capable of being measured and are often capable of being perceived by our senses. – Note: macroscopic properties of matter contrasts markedly with the microscopic properties of matter such as masses, speeds, energies, etc of the constituent atoms / molecules that cannot be perceived directly by our senses.

Macroscopic properties v Microscopic properties If the atomic model of matter is valid then it should be possible to explain the macroscopic properties of matter in terms of the microscopic and visa versa, since the same thing is being considered but from two different perspectives

Thermodynamic Laws Much of what we know about thermodynamics has been based upon experimental observations, the results of which have been organised into mathematical statements which we term laws (zeroth law, first law, second law and third law)

An Engineer’s Perspective on Thermodynamics An engineer’s objective in studying thermodynamics is most often the analysis or design of a large scale system – anything from an air-conditioning unit to a nuclear power station. A system may be regarded as a continuum in which the activity of the constituent molecules is averaged into measureable quantities such as pressure, temperature, velocity etc.

Thermodynamic Systems A thermodynamic system is a definite quantity of matter contained within some closed surface. This might be an imagined boundary like the deforming boundary of a certain amount of mass as it flows through a pump or it could be an obvious one like that enclosing the gas in a cylinder. All matter and space external to a system is collectively called its surroundings.

Thermodynamic Systems System boundary or wall System under consideration Weight Piston Cylinder Compressed gas (the system) System boundary or wall

Thermodynamic Systems Thermodynamics is concerned with with the interactions of a system and its surroundings or with one system interacting with another. – A system interacts with its surroundings by transferring energy across its boundary. – NO material crosses the boundary of a given system – If the system does not exchange energy with the surroundings, it is called an isolated system

Control Volume In many cases, an analysis is simplified if attention is focused on a volume in space into which or from which a substance flows. Such a volume is called a control volume. – A pump, a turbine, an inflating balloon are examples of control volumes. – The surface that completely surrounds the control volume is called a control surface

Control Volume Pump Liquid inLiquid out Energy in Control surface Control Volume

Thermodynamic System v Control Volume In trying to resolve a particular problem, there is a need to deal with it by considering using a control volume or using a systems approach. If there is a mass flux across a boundary of a region the a control volume is required otherwise a systems approach should be used.

Properties and State of a System The matter in a system may exist in several phases: a solid, liquid or a gas. A phase is a quantity of matter that has the same chemical composition throughout i.e. it is homogeneous (the same throughout in all directions). Phase boundaries separate the phases in what when taken as a whole is called a mixture.

Properties and State of a System A property is any quantity which serves to describe a system (mass, volume, pressure, position, velocity, temperature, etc). Sometimes shape is important when surface effects are considered or colour when radiation heat transfer is being investigated The state of a system is its condition as described by giving values to its properties at a particular instant in time.

Properties and State of a System The essential feature of a property is that it has a unique value when a system is in a particular state and this value does not depend on the previous states that the system passed through that is it is not a path function. Since a property is not dependent on the path any change depends only on the initial and final states of the system.

Properties and State of a System A B Path 1 Path 2 Property 1 Property 2 Note – this is path dependent!

Properties and State of a System φ2φ2 φ1φ1 dφ = φ 2 – φ 1 Thus using the symbol φ to represent a property… This requires that dφ be an exact differential; φ 2 – φ 1 represents the change in the property as the system changes from state 1 to state 2.

Properties and State of a System A relatively small number of independent properties suffice to fix all other properties and thus the state of the system. If the system is composed of a single phase, free from magnetic, electrical and surface effects, the state is fixed when any two properties are fixed!

Properties and State of a System Thermodynamic properties are divided into two general types, intensive and extensive. An intensive property is one which does not depend on the mass (or size) of the system i.e. temperature, pressure, density, etc. They are the same for the entire system. For example if you have 1-g of water or 1-kg; the density is the same! If we bring two systems together, intensive properties are not summed.

Properties and State of a System An extensive property is one that depends upon the mass (or size) of the system i.e. volume, momentum, kinetic energy. If two systems are brought together the extensive property of the new system is the sum of the extensive properties of the original two systems.

Properties and State of a System If we divide an extensive property by the mass a specific property results. Thus specific volume is defined as… v = V m

Thermodynamic Equilibrium When the temperature or the pressure of a system is referred to it is assumed that all points of the system have the same or essentially the same temperature or pressure. When properties are assumed constant from point to point and when there is no tendency for change with time, a condition of thermodynamic equilibrium exists.

Thermodynamic Equilibrium If the temperature say is suddenly increased at some part of the system boundary spontaneous redistribution is assumed to occur until all parts of the system are at the same temperature. If a system undergoes a large change in its properties when subjected to some small disturbance it is said to be in metastable equilibrium

Process When a system changes from one equilibrium state to another, the path of successive states through which the system passes is called a process. If when passing from one state to the next, the deviation from equilibrium is infinitesimal a quasi-equilibrium process occurs and each state in the process may be idealised as an equilibrium state.

Process Many processes such as the compression and expansion of gases in an internal combustion engine can be approximated by quasi- equilibrium processes with no significant loss of accuracy. If a system undergoes a quasi- equilibrium process (such as the thermodynamically slow compression of air in a cylinder) it may be sketched on appropriate coordinates by using a solid line.

Process A B P1P1 P2P2 V1V1 V2V2

If a system however goes from one equilibrium state to another through a series of non-equilibrium states (as in combustion) a non-equilibrium process occurs. A dashed line represents such a process: between (P 1, V 1 ) and (P 2, V 2 ) properties are not uniform throughout the system and thus the state of the system cannot be well defined!

Process A B P1P1 P2P2 V1V1 V2V2 A

Weight Piston Compressed gas (the system) Whether a particular process may be considered quasi-equilibrium or non- equilibrium depends on how the process is carried out. Consider a weight W added to the piston. How would you add W in an non- equilibrium manner and a equilibrium manner?

Process Cycle When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. At the end of the cycle the properties of the system have the same values they had at the beginning.

Process Cycle P V

Iso.. Is derived from a Greek work meaning ‘the same’. – Thus isothermal process is one in which the temperature is constant i.e. the same temperature – An isobaric process is one in which the pressure remains constant – An isometric process is one in which the volume remains constant.

Process Cycle P V isometric isobaric

Units Handout 1 – pg 6 Schaum, ‘Thermodynamics for Engineers’.