Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Principles of Thermodynamics System The system is whatever we want to study. It may be as simple as a free body or as complex as an entire chemical refinery. We may want to study a quantity of matter contained within a closed, rigid-walled tank, or we may want to consider something such as a pipeline through which natural gas flows. The composition of the matter inside the system may be fixed or may be changing through chemical or nuclear reactions. The shape or volume of the system being analyzed is not necessarily constant, as when a gas in a cylinder is compressed by a piston or a balloon is inflated. Boundary The system is distinguished from its surroundings by a specified boundary, which may be at rest or in motion. Closed System A closed system is defined when a particular quantity of matter is under study. A closed system always contains the same matter. There can be no transfer of mass across its boundary. A special type of closed system that does not interact in any way with its surroundings is called an isolated system. The figure shows a gas in a piston–cylinder assembly. When the valves are closed, we can consider the gas to be a closed system. Fig.: piston cylinder assembly Control Volume In most cases it is simpler to think instead in terms of a given region of space through which mass flows. With this approach, a region within a prescribed boundary is studied. The region is called a control volume. Mass may cross the boundary of a control volume. Fig.: control volume Property A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior of the system. In the physical sciences, an intensive property is a physical property of a system that does not depend on the system size or the amount of material in the system. By contrast, an extensive property of a system does depend on the system size or the amount of material in the system. For example, density is an intensive quantity while mass and volume are extensive quantities. State The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties. All other properties can be determined in terms of these few. Process When any of the properties of a system change, the state changes and the system is said to have undergone a process. A process is a transformation from one state to another. However, if a system exhibits the same values of its properties at two different times, it is in the same state at these times. A system is said to be at steady state if none of its properties changes with time. © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Cycle A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning First Law of Thermodynamics When any closed system is taken through a cycle, the net work delivered to the surrounding is proportional to the net heat taken from the surroundings and the converse is also true. These statements may be expressed in mathematical form by – ΣdQ ∝ ΣdW dQ = dU + dW Internal Energy Internal energy of a system may be defined as the amount of energy summing all the energies of all the atoms, ions and molecules of that system. The value of internal energy depends on temperature and pressure. Measurement of total internal energy is not possible. But change in internal energy is equal to amount energy transferred by heat and work together. A change in the internal energy between two states is independent of the path between them. Internal energy of an isolated system is constant. Second Law of Thermodynamics It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir, and do an equivalent amount of work on the surroundings. In other words, it is impossible to construct a system which will operate in a cycle and transfer heat from a cooler to a hotter body without work being done on the system by the surrounding. Enthalpy Enthalpy is a thermodynamic property of a system that can be defined as the summation of internal energy and the product of pressure and volume of that system. A change in enthalpy under constant pressure condition is equal to the change in internal energy of the system and the work done by the system on its surroundings. ∆H = ∆U + ∆(PV) Entropy In thermodynamics, entropy is a measure of how much of the energy of a system is potentially available to do work and how much of it is potentially manifest as heat. In classical thermodynamics, the entropy is defined only for a system in thermodynamic equilibrium. Statistical mechanics explains entropy as the amount of uncertainty which remains about a system, after its observable macroscopic properties have been taken into account. For a given set of macroscopic variables, like temperature and volume, the entropy measures the degree to which the probability of the system is spread out over different possible quantum states. ∆S = © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Steady Flow Energy Equation Q W m h V z = = = = = = rate of heat transfer between the control volume and its surroundings rate of work transfer between the control volume and its surroundings mass flow through the control volume specific enthalpy of the working fluid velocity of the working fluid reference height of the system Some Steady Flow Engineering Devices Device Steady Flow Energy Equation Heat Exchanger (Boiler & Condenser) Q = m (h2 – h1) Turbine & Compressor W = m (h1 – h2) Throttling h2 = h1 © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Properties of Fluids Pressure Pressure (often termed as absolute pressure) is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. Absolute Pressure = Atmospheric Pressure + Gauge Pressure Because pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., inches of water). The most common choices are mercury (Hg) and water. The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows: 1 atm = 101.325 kPa = 14.7 psi = 10.3 m of H2O = 760 mm of Hg Temperature In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics. If no net heat flow occurs between two objects, the objects have the same temperature; otherwise heat flows from the hotter object to the colder object. This is the content of the zeroth law of thermodynamics. The basic unit of temperature in the International System of Units (SI) is the kelvin. The kelvin and Celsius scales are defined by two points, absolute zero and the triple point water. Absolute zero is defined as being precisely 0 K and −273.15 °C. Absolute zero is where all kinetic motion in the particles comprising matter ceases and they are at complete rest. At absolute zero, matter contains no thermal energy. Phases of a Pure Substance Fig.: arrangement of atoms in different phases Fig.: illustration of constant pressure change from liquid to vapor for water © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Fig.: T – v diagram for the heating process of water at constant pressure Fig.: T – v diagram of constant pressure phase change process at different pressures A state at which a phase change begins or ends is called a saturation state. The liquid states along the line segment 1–2 are sometimes referred to as subcooled liquid states because the temperature at these states is less than the saturation temperature at the given pressure. These states are also referred to as compressed liquid states because the pressure at each state is higher than the saturation pressure corresponding to the temperature at the state. When a mixture of liquid and vapor exists in equilibrium, the liquid phase is a saturated liquid and the vapor phase is a saturated vapor. If the system is heated further until the last bit of liquid has vaporized, it is brought to point 4, the saturated vapor state. The intervening two phase liquid–vapor mixtures can be distinguished from one another by the quality, an intensive property. For a two phase liquid–vapor mixture, the ratio of the mass of vapor present to the total mass of the mixture is its quality, x. In symbols, © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET The value of the quality ranges from zero to unity: at saturated liquid states, x = 0, and at saturated vapor states, x = 1. When the system is at the saturated vapor state, further heating at fixed pressure results in increases in both temperature and specific volume. A state such as this is often referred to as a superheated vapor state because the system would be at a temperature greater than the saturation temperature corresponding to the given pressure. Triple Point In thermodynamics, the triple point of a substance is the temperature and pressure at which three phases (for example, gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa. The single combination of pressure and temperature at which water, ice, and water vapor can coexist in a stable equilibrium occurs at exactly 273.16 k and a partial vapour pressure of 0.611 kPa. Critical Point In thermodynamics, a critical point, also called a critical state, specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. There are multiple types of critical points such as vapor-liquid critical points and liquid-liquid critical points. The vapor-liquid critical point denotes the conditions above which distinct liquid and gas phases do not exist. In water, the critical point occurs at around 647 k and 22.064 MPa. As the critical temperature is approached, the properties of the gas and liquid phases approach one another, resulting in only one phase at the critical point: a homogeneous supercritical fluid. The heat of vaporization is zero at and beyond this critical point, so there is no distinction between the two phases. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure at the critical temperature. Fig.: T – v diagram of a pure substance © Nusair Mohammed Ibn Hasan ME 267: Fundamentals of Mechanical Engineering Department of Mechanical Engineering, BUET Fig.: P – v diagram of a pure substance ** See Attached Table for Properties of Water © Nusair Mohammed Ibn Hasan