Thermodynamics Introduction and Basic Concepts by Assit Prof

Thermodynamics Introduction and Basic Concepts by Assit. Prof. Channarong Asavatesanupap Mechanical Engineering Department Faculty of Engineering Thammasat University

2 INTRODUCTION Thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure. One of the most fundamental laws of nature is the conservation of energy principle It simply states that during an energy interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed.

3 Closed, Open, and Isolated Systems A thermodynamic system, or simply system, is defined as a quantity of matter or a region in space chosen for study. The region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary. The boundary of a system may be fixed or movable. Surroundings are physical space outside the system boundary. Surroundings System Boundary Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study.

4 A closed system consists of a fixed amount of mass and no mass may cross the system boundary. The closed system boundary may move. Examples of closed systems are sealed tanks and piston cylinder devices (note the volume does not have to be fixed). However, energy in the form of heat and work may cross the boundaries of a closed system.

5 An open system, or control volume, has mass as well as energy crossing the boundary, called a control surface. Examples of open systems are pumps, compressors, turbines, valves, and heat exchangers. An isolated system is a general system of fixed mass where no heat or work may cross the boundaries. An isolated system is a closed system with no energy crossing the boundaries and is normally a collection of a main system and its surroundings that are exchanging mass and energy among themselves and no other system.

6 Properties of a system Any characteristic of a system is called a property. Some familiar properties are volume V, mass m, density r, pressure P, temperature T and etc.

Density is defined as mass per unit volume Water@ 20 C , 1 atm r = 998 kg/m 3 The reciprocal of density is the specific volume, which is defined as Specific Gravity SG is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4 C).

Temperature (T) is a measure of the average energy of motion, or kinetic energy, of particles in matter. (or a measure of hotness and coldness) Temperature scales common scale: Celsius scale °C SI unit Fahrenheit scale °F English unit The Celsius scale is related to the Fahrenheit scale by Thermodynamic scale(Absolute scale): Kelvin K SI unit Rankine R English unit

Temperature scales The common scales are related to the absolute scale by SI unit English unit Example: Water boils at 100 C at one atmosphere pressure. At what temperature does water boil in F, K and R. T(°F) = 100 x 1. 8 + 32 = 212 °F T(K) = 100 + 273. 15 = 373. 15 k T(R) = 212 + 459. 67 = 671. 67 F

10 Pressure(P) is the force per unit area applied in a direction perpendicular to the surface of an object P For English system, = F A N (Pa) m 2

11 Pressure scales Absolute scale: Absolute pressure is the pressure that is measured relative to absolute zero pressure (absolute vacuum). Gage scale: Gage pressure is the pressure that is indicated on a pressure-measuring device (called a pressure gage). Generally, the device is calibrated to read zero in the atmosphere.

12 Vacuum pressure Pressures below atmospheric pressures are called vacuum pressures. A device that is used to measure vacuum pressure is called a vacuum gage. Pressure Symbol Absolute Pa Gage Pg Pressure Eng. unit Absolute psia Gage psig

Example A pressure gage connected to a valve stem of a truck tire reads 24013 k. Pa at a location where the atmospheric pressure is 100 k. Pa. What is the absolute pressure in the tire, in k. Pa and in psia? The pressure in psia is What is the gage pressure of the air in the tire, in psig?

14 Intensive and Extensive properties • Intensive properties are those that are independent of the mass of a system. • Extensive properties are those whose values depend on the size—or extent—of the system.

15 Internal energy (U) is defined as the sum of all the microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and can be viewed as the sum of the kinetic and potential energies of the molecules. Properties Extensive Intensive Gas: Symbol U u=U/m Unit J [Joule] J/kg U = CV T = m c. V T [k. J/kg] where CV = heat capacity [k. J/K] c. V = specific heat Capacity [k. J/kg-K]

16 State, Equilibrium, Process, and Properties State A set of properties that completely describe the condition of the system is called “state of the system” Water @ P=Patm and T=20 C Liquid Equilibrium A system is said to be in thermodynamic equilibrium if it maintains thermal (uniform temperature), mechanical (uniform pressure), phase (the mass of two phases, e. g. , ice and liquid water, in equilibrium) and chemical equilibrium.

17 Process Any change from one state to another is called a process. During a quasiequilibrium or quasi-static process the system remains practically in equilibrium at all times. We study quasi-equilibrium processes because they are easy to analyze (equations of state apply) and work-producing devices deliver the most work when they operate on the quasi-equilibrium process. Liquid water State 1 Heating process Steam State 2 In most of the processes that we will study, one thermodynamic property is held constant. Some of these processes are Process Property held constant isobaric pressure isothermal temperature isochoric volume isentropic entropy (see Chapter 7) System Boundary F Water Constant Pressure Process

18 We can understand the concept of a constant pressure process by considering the above figure. The force exerted by the water on the face of the piston has to equal the force due to the combined weight of the piston and the bricks. If the combined weight of the piston and bricks is constant, then F is constant and the pressure is constant even when the water is heated. We often show the process on a P-V diagram as shown below.

19 First Law of Thermodynamics is an expression of the conservation of energy principle. Ein – Eout = DEsys where DEsys = Efinal – Einit

20 Closed System First Law Energy can cross the boundaries of a closed system in the form of heat or work. According to classical thermodynamics, we consider the energy added to be net heat transfer to the closed system and the energy leaving the closed system to be net work done by the closed system. So where Sign Convention Heat transfer (Q) into a system is defined as positive Heat transfer (Q) out of a system is defined as negative Work (W) done by a system is defined as positive Work (W) done by surrounding is defined as negative

21 Normally the stored energy, or total energy, of a system is expressed as the sum of three separate energies. The total energy of the system, Esystem, is given as The change in stored energy for the system is Now the conservation of energy principle, or the first law of thermodynamics for closed systems, systems is written as

22 If the system does not move with a velocity and has no change in elevation, the conservation of energy equation reduces to No motion (DKE=0) + No elevation change (DPE=0)

23 Example Gas in a piston Heat transfer into a system Tsys Tsurr > Tsys Q Positive

24 Tsys Tsurr < Tsys Q Negative

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26 Heat and work transfer into a system

27 Example A container of 3 kg air is heated by a candle as shown in the figure. Determine the amount of heat if the temperature of air rises by 20 C Air in a container is considered as a closed system First law for a closed system Assumption: 1. No motion of the system. 2 No change of elevation. 3 No work transfer. 4 Air behaves like ideal gas

28 0 DU = CV DT (Ideal gas) Q Q net = Cv. DT = m cv DT = 3 kg x 1. 012 k. J/kg-K x 20 K = 60. 7 k. J