Thermodynamics I__________________________________________________________________ _ AAiT Chapter 1 Introductory Concepts and Definition 1.1 Introduction Thermodynamics may be defined as follows : • Thermodynamics is an axiomatic science which deals with the relations among heat, work and properties of system which are in equilibrium. It describes state and changes in state of physical systems. • Thermodynamics is the science of the regularities governing processes of energy conversion. • Thermodynamics is the science that deals with the interaction between energy and material systems. Thermodynamics can be defined as the science of energy. Although everybody has a feeling of what energy is, it is difficult to give a precise definition for it. Energy can be viewed as the ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power generation, refrigeration, and relationships among the properties of matter. Some Energy Conversion application 1 Steam Power Plant Nuclear Power Plant Internal Combustion Engines Gas Turbines Refrigeration Systems 1 Read the additional hand out. _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 1 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Area of Application Selected Areas of Application of Engineering Thermodynamics • Aircraft and rocket propulsion • Alternative energy systems o Fuel cells o Geothermal systems o Magnetohydrodynamic (MHD) converters o Ocean thermal, wave, and tidal power generation o Solar-activated heating, cooling, and power generation o Thermoelectric and thermionic devices o Wind turbines • Automobile engines • Bioengineering applications • Biomedical applications • Combustion systems • Compressors, pumps • Cooling of electronic equipment • Cryogenic systems, gas separation, and liquefaction • Fossil and nuclear-fueled power stations • Heating, ventilating, and air-conditioning systems o Absorption refrigeration and heat pumps o Vapor-compression refrigeration and heat pumps • Steam and gas turbines o Power production o Propulsion _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 2 of 12 Thermodynamics I__________________________________________________________________ _ 1.2 AAiT Closed, Open, and Isolated System A thermodynamic system, or simply system, is defined as a quantity of matter or a region in space chosen for study (A system is a finite quantity of matter or a prescribed region of space). 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. Figure 1.1 The system. It is not difficult to visualize a real boundary but an example of imaginary boundary would be one drawn around a system consisting of the fresh mixture about to enter the cylinder of an I.C. engine together with the remnants of the last cylinder charge after the exhaust process. Figure 1.2 The real and imaginary boundaries _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 3 of 12 Thermodynamics I__________________________________________________________________ _ AAiT 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. Closed system 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. Figure 1.3 Closed system. Figure 1.4 Closed system with movable boundary. Open system 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. Figure 1.5 Open system _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 4 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Isolated system 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. Figure 1.6 Isolated system Since some of the thermodynamic relations that are applicable to closed and open systems are different, it is extremely important that we recognize the type of system we have before we start analyzing it. 1.3 State, Equilibrium, Process and Properties State Consider a system that is not undergoing any change. The properties can be measured or calculated throughout the entire system. This gives us a set of properties that completely describe the condition or state of the system. At a given state all of the properties are known; changing one property changes the state. 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. _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 5 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Figure 1.7 Thermal Equilibrium Process Any change from one state to another is called a process. During a quasi-equilibrium or quasistatic process the system remains practically in equilibrium at all times. We study quasiequilibrium processes because they are easy to analyze (equations of state apply) and workproducing devices deliver the most work when they operate on the quasi-equilibrium process. Figure 1.8 A process between states 1 and 2 and the process path. 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 _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 6 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1.9 The P-V diagram of a compression process. Cycle A process (or a series of connected processes) with identical end states is called a cycle. Below is a cycle composed of two processes, A and B. Along process A, the pressure and volume change from state 1 to state 2. Then to complete the cycle, the pressure and volume change from state 2 back to the initial state 1 along process B. Keep in mind that all other thermodynamic properties must also change so that the pressure is a function of volume as described by these two processes. Figure 1.10 Cyclic Process _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 7 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Property Any characteristic of a system is called a property. Some familiar properties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation. Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size—or extent—of the system. Total mass, total volume and total momentum are some examples of extensive properties. An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition, as shown in the figure below. Figure 1.11 Intensive and Extensive Properties Steady-Flow Process Consider a fluid flowing through an open system or control volume such as a water heater. The flow is often defined by the terms steady and uniform. The term steady implies that there are no changes with time. The term uniform implies no change with location over a specified region. Engineering flow devices that operate for long periods of time under the same conditions are classified as steady-flow devices. The processes for these devices is called the steady-flow process. The fluid properties can change from point to point with in the control volume, but at any fixed point the properties remain the same during the entire process. State Postulate As noted earlier, the state of a system is described by its properties. But by experience not all properties must be known before the state is specified. Once a sufficient number of properties are _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 8 of 12 Thermodynamics I__________________________________________________________________ _ AAiT known, the state is specified and all other properties are known. The number of properties required to fix the state of a simple, homogeneous system is given by the state postulate: The thermodynamic state of a simple compressible system is completely specified by two independent, intensive properties. 1.4 Dimension and Units Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units. Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fundamental dimensions, while others such as velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. 1.5 Quantity Dimension Units Mass M Kg Length L m Time T s Specific volume, Pressure and Temperature Specific Volume It is the volume occupied by a unit mass of a substance (the reciprocal of density), and it is designated by the letter 𝑣𝑣. = v Volume V = = m3 / kg mass m (1.1) Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative density, and 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, for which rH2O =1000 kg/m3). That is Specific gravity: SG ( ρ s ) = ρ ρH O (1.2) 2 _____________________________________________________________________________________ Compiled by Yidnekachew M. Page 9 of 12 Thermodynamics I__________________________________________________________________ _ AAiT Pressure Pressure is defined as a normal force exerted by a fluid per unit area. We speak of pressure only when we deal with a gas or a liquid. Since pressure is defined as force per unit area, it has the unit of newtons per square meter (N/m2), which is called a pascal (Pa). Force F N = = = Pascal = Pa Area A m 2 P = (1.3) The pressure unit pascal is too small for pressures encountered in practice. Therefore, its multiples kilopascal (1 kPa= 103 Pa) and megapascal (1 MPa= 106 Pa) are commonly used. Three other pressure units commonly used in practice, especially in Europe, are bar, standard atmosphere, and kilogram-force per square centimeter: 5 1bar=10 = Pa 0.1 = MPa 100kPa 1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum (i.e., absolute zero pressure). Most pressure-measuring devices, however, are calibrated to read zero in the atmosphere and so they indicate the difference between the absolute pressure and the local atmospheric pressure. This difference is called the gage pressure. Pressures below atmospheric pressure are called vacuum pressures and are measured by vacuum gages that indicate the difference between the atmospheric pressure and the absolute pressure. Absolute, gage, and vacuum pressures are all positive quantities and are related to each other by Pgage = Pabs − Patm (1.4) P= Patm − Pabs vac (1.5) _____________________________________________________________________________________ Page 10 of 12 Compiled by Yidnekachew M. Thermodynamics I__________________________________________________________________ _ AAiT Figure 1.12 Absolute, gage, and vacuum pressures. A device called manometer, it is commonly used to measure small and moderate pressure differences. A manometer mainly consists of a glass or plastic U-tube containing one or more fluids such as mercury, water, alcohol, or oil. To keep the size of the manometer to a manageable level, heavy fluids such as mercury are used if large pressure differences are anticipated. Consider the manometer shown in Fig. 1-13 that is used to measure the pressure in the tank. Since the gravitational effects of gases are negligible, the pressure anywhere in the tank and at position 1 has the same value. Furthermore, since pressure in a fluid does not vary in the horizontal direction within a fluid, the pressure at point 2 is the same as the pressure at point 1, P 2 = P 1 . The differential fluid column of height h is in static equilibrium, and it is open to the atmosphere. Then the pressure at point 2 is determined directly from Eq. 1.6 to be = P2 Patm + ρ gh (1.6) W W mg ρVg ρ Ahg P2 = Patm + , = == = ρ hg = ∆P A A A A A = P2 Patm + ρ hg ∆P = P2 − Patm = ρ gh This pressure difference is determined from the manometer fluid displaced height as ∆P = ρ gh _____________________________________________________________________________________ Page 11 of 12 Compiled by Yidnekachew M. Thermodynamics I__________________________________________________________________ _ AAiT Temperature Although we are familiar with temperature as a measure of “hotness” or “coldness,” it is not easy to give an exact definition of it. However, temperature is considered as a thermodynamic property that is the measure of the energy content of a mass. When heat energy is transferred to a body, the body's energy content increases and so does its temperature. In fact it is the difference in temperature that causes energy, called heat transfer, to flow from a hot body to a cold body. Two bodies are in thermal equilibrium when they have reached the same temperature. If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This simple fact is known as the zeroth law of thermodynamics. _____________________________________________________________________________________ Page 12 of 12 Compiled by Yidnekachew M.
