Thermodynamics Chapter One Part I PDF

1 Thermodynamics Chapter one (Part I) Introduction to Thermodynamics Prepared by Dr. Mohammed Abd El Aziz Hassan Automotive and Tractor Engineering Dept., Faculty of Engineering, Minia University. Introduction...

1 Thermodynamics Chapter one (Part I) Introduction to Thermodynamics Prepared by Dr. Mohammed Abd El Aziz Hassan Automotive and Tractor Engineering Dept., Faculty of Engineering, Minia University. Introduction and Basics Concepts Thermodynamics 2  Chapter (1) Objectives ▪ Identify the special terminology related to thermodynamics by accurately defining basic concepts to provide an appropriate structure for understanding the principles of thermodynamics. ▪ Review the metric SI and English unit systems used during study. ▪ Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle. ▪ Discuss properties of a system and define density, specific gravity, and specific weight. ▪ Review concepts of temperature, temperature scales, pressure, and absolute and gage pressure. ▪ Introduce an intuitive systematic problem-solving technique. 3  Introduction to Thermodynamics 1. Thermodynamics And Energy 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. Currently, the term is widely understood to include all aspects of energy, and energy transformations including power generation, refrigeration, and relationships among the properties of matter. One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an 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. 4  Introduction to Thermodynamics A person who has a greater energy input (food) than energy output (exercise) will gain weight (store energy in the form of fat), and a person who has a smaller energy input than output will lose weight. The change in the energy content of a body or any other system is equal to the difference between the energy input and the energy output, and the energy balance is expressed 𝑬𝒊𝒏 − 𝑬𝒐𝒖𝒕 = ∆𝑬 5  Introduction to Thermodynamics Application Areas of Thermodynamics Thermodynamics is commonly encountered in many engineering systems and other aspects of life. An ordinary house is, in some respects, an exhibition hall filled with wonders of thermodynamics. Many ordinary household equipment and appliances are designed, in whole or in part, by using the principles of thermodynamics. Some examples include the electric or gas range, heating and air-conditioning systems, refrigerators, pressure cookers, water heaters, showers, iron, and even computers and the TV. 6  Introduction to Thermodynamics On a larger scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordinary cars to airplanes 7  Introduction to Thermodynamics For example, the pressure of a gas in a container is the result of momentum transfer between the molecules and the walls of the container. However, one does not need to know the behavior of the gas particles to determine the pressure in the container. It would be sufficient to attach a pressure gage to the container. This macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. A more elaborate approach, based on the average behavior of large groups of individual particles, is called statistical thermodynamics. This microscopic approach is rather involved and is used in this text only in the supporting role. 8  Introduction to Thermodynamics 2. Importance of Dimensions and Units o Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units. o Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary, 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. o Several unit systems have been developed over the years. Two sets of units are still in common use today: the English system, which is also known as the United States Customary System (USCS), and the metric SI, which is also known as the International System. o The SI is a simple and logical system based on a decimal relationship between the various units, and it is being used for scientific and engineering work in most of the industrialized nations, including England. 9  Introduction to Thermodynamics In 1960, the General Conference of Weights and Measures (CGPM) produced the SI, which was based on six fundamental quantities, and their units were adopted in 1954 at the Tenth CGPM : meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (°K) for temperature, and candela (cd) for luminous intensity (amount of light). In 1971, the CGPM added a seventh fundamental quantity and unit: mole (mol) for the amount of matter. 10  Introduction to Thermodynamics Based on the notational scheme introduced in 1967, the degree symbol was officially dropped from the absolute temperature unit, all unit names were to be written without capitalization even if they were derived from proper names. However, the abbreviation of a unit was to be capitalized if the unit was derived from a proper name. For example, the SI unit of force, which is named after Sir Isaac Newton (1647– 1723), is newton (not Newton), and it is abbreviated as N. Also, the full name of a unit may be pluralized, but its abbreviation cannot. For example, the length of an object can be 5 m or 5 meters, not 5 ms or 5 meter. Finally, no period is to be used in unit abbreviations unless they appear at the end of a sentence. For example, the proper abbreviation of meter is m (not m.) 11  Introduction to Thermodynamics Standard prefixes in SI units As pointed out, the SI is based on a decimal relationship between units. The prefixes used to express the multiples of the various units are listed in Table. 12  Introduction to Thermodynamics Some SI and English Units In SI, the units of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively. The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s). In SI, the force unit is the newton (N), and it is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2. In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 as shown. That is 13  Introduction to Thermodynamics Dimensional Homogeneity In engineering, all equations must be dimensionally homogeneous. That is, every term in an equation must have the same unit. If, at some stage of an analysis, we find ourselves in a position to add two quantities that have different units, it is a clear indication that we have made an error at an earlier stage. So checking dimensions can serve as a valuable tool to spot errors. Example1. Obtaining Formulas from Unit Considerations A tank is filled with oil whose density is ρ = 850 kg/m3. If the volume of the tank is 𝞶 = 2 m3, determine the amount of mass m in the tank ρ = 850 kg/m3 𝞶 = 2 m3 It is obvious that we can eliminate m3 and end up with kg by multiplying these two quantities. Therefore, the formula we are looking for should be 𝒎=ρ𝞶 14  Introduction to Thermodynamics Thus, 𝑚 = (850𝑘𝑔/𝑚3)(2𝑚3) = 𝟏𝟕𝟎𝟎 𝒌𝒈 Discussion Note that this approach may not work for more complicated formulas. Nondimensional constants also may be present in the formulas, and these cannot be derived from unit considerations alone. Caution You should keep in mind that a formula that is not dimensionally homogeneous is definitely wrong, but a dimensionally homogeneous formula is not necessarily right. 15  Introduction to Thermodynamics 3. Systems And Control Volumes A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary (Figure). The boundary of a system can be fixed or movable. Note that : The boundary is the contact surface shared by both the system and the surroundings. Mathematically speaking, the boundary has zero thickness, and thus it can neither contain any mass nor occupy any volume in space. 16  Introduction to Thermodynamics Open and Closed System 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. A closed system (also known as a control mass) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system, as shown in Figure. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system. 17  Introduction to Thermodynamics An open system, or a control volume, as it is often called, is a properly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a control volume. Many engineering problems involve mass flow in and out of a system and, therefore, are modeled as control volumes. A water heater, a car radiator, a turbine, and a compressor all involve mass flow and should be analyzed as control volumes (open systems) instead of as control masses (closed systems). 18  Introduction to Thermodynamics The boundaries of a control volume are called a control surface, and they can be real or imaginary. In the case of a nozzle, the inner surface of the nozzle forms the real part of the boundary, and the entrance and exit areas form the imaginary part, since there are no physical surfaces there. As an example of an open system, consider the water heater shown in Figure. Since hot water will leave the tank and be replaced by cold water, it is not convenient to choose a fixed mass as our system for the analysis. Instead, we can concentrate our attention on the volume formed by the interior surfaces of the tank and consider the hot and cold-water streams as mass leaving and entering the control volume. 19 Introduction to Thermodynamics 20  Fluid Properties 4. Properties Of A System Any characteristic of a system is called a property. Properties are either intensive or extensive. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Total mass, total volume V, 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 Figure 1.1. Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties. Figure 1.1 A criterion to differentiate intensive and extensive properties 21 Introduction to Thermodynamics 22  Fluid Properties Density The density of a fluid, designated by the Greek symbol 𝝆 (rho), is defined as its mass per unit volume. In SI the units are kg/m3. The value of density can vary widely between different fluids, but for liquids, variations in pressure and temperature generally have only a small effect on the value of the small change in the density of water with large variations in temperature is illustrated in Figure 1.2. The density of water at 60 °F is 999 kg/m3. Unlike liquids, the density of a gas is strongly influenced by both pressure and temperature. Figure 1.2 Density of water as a function of temperature 23  Fluid Properties Specific Weight The specific weight of a fluid, designated by the Greek symbol 𝜸 (gamma), is defined as its weight per unit volume. Thus, specific weight is related to density through the equation where g is the local acceleration of gravity. The specific weight is used to characterize the weight of the system. Specific Gravity The specific gravity of a fluid, designated as SG, is defined as the ratio of the density of the fluid to the density of water at some specified temperature. Usually, the specified temperature is taken as 4°C (39.2 °F), and at this temperature the density of water is 1000 kg/m3. In equation form, specific gravity is expressed as 24  Fluid Properties Example 2.1 (Density, Specific Gravity, and Mass of Air in a Room ) Determine the density, specific gravity, and mass of the air in a room whose dimensions are 4 m × 5 m × 6 m at 100 kPa and 25°C (Figure 1-2 ) Assumptions: At specified conditions, air can be treated as an ideal gas. Properties: The gas constant of air is R = 0.287 kPa⋅m3/kg⋅K. Analysis: The density of the air is determined from the ideal-gas relation 𝑃 = 𝜌 𝑅𝑇 𝑃 100 𝝆 = 𝑅𝑇 = 0.287×(273+25) = 1.169 kg/m3 Figure 1-2 Then the specific gravity of the air becomes 𝜌 1.169 𝑆𝐺 = = = 1.169 × 10−3 𝜌𝑤 1000 Finally, mass of the air 𝑚 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 × 𝑣𝑜𝑙𝑢𝑚𝑒 = 𝝆𝞶 = 1.169 × 120 = 140.28 Kg 25  Introduction to Thermodynamics 5. State And Equilibrium Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Figure a system is shown at two different states. Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. A system at two different states. 26  Introduction to Thermodynamics There are many types of equilibrium, and a system is not in thermodynamic equilibrium unless the conditions of all the relevant types of equilibrium are satisfied. For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Figure. Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. However, the pressure may vary within the system with elevation as a result of gravitational effects. The variation of pressure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded. Finally, a system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. A system will not be in equilibrium unless all the relevant equilibrium criteria are satisfied. 27  Introduction to Thermodynamics The State Postulate Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically. That is, specifying a certain number of properties is sufficient to fix a state. State postulate is the number of properties required to fix the state of a system. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific volume, for example, are always independent properties, and together they can fix the state of a simple compressible system. Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. Thus, temperature and pressure are not sufficient to fix the state of a two-phase system. 28  Introduction to Thermodynamics The state of a simple compressible system is completely specified by two independent, intensive properties. A system is called a simple compressible system in the absence of electrical, magnetic, gravitational, motion, and surface tension effects. Otherwise, an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be considered, for example, the elevation z needs to be specified in addition to the two properties necessary to fix the state. The state of nitrogen is fixed by two independent, intensive properties 29  Introduction to Thermodynamics 6. Processes And Cycles A process is a transformation from one state to another state by changing one or more of the system properties is called a process, and the series of states through which a system passes during a process is called the path of the process. To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. A quasi-static or quasi-equilibrium process is a process that occurs while the system stays extremely close to an equilibrium state at all times A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those A process between states 1 and 2 at other parts. and the process path. 30  Introduction to Thermodynamics If the piston is moved slowly, the molecules will have sufficient time to redistribute and there will not be a molecule pileup in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process. A quasi-equilibrium process is an idealized process and is not a true representation of an actual process. Engineers are interested in quasi-equilibrium processes for two reasons. First, they are easy to analyze; second, work-producing devices deliver the most work when they operate on quasi-equilibrium processes. 31  Introduction to Thermodynamics 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). Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasi- equilibrium processes only. The P-V diagram of a compression process. 32 Introduction to Thermodynamics 33