Basic Mechanical Engineering (202001202) PDF

Basic Mechanical Engineering (202001202) Chapter – 1: Introduction Prepared by :Prof. Bhavik A Ardeshana Mechatronics Department G H Patel College of Engineering & Technology (A Constituent College of CVM University)  Prime movers and its types...

Basic Mechanical Engineering (202001202) Chapter – 1: Introduction Prepared by :Prof. Bhavik A Ardeshana Mechatronics Department G H Patel College of Engineering & Technology (A Constituent College of CVM University)  Prime movers and its types System, Change of state, Path, Process, Cycle Content Concept of Force, Pressure, Energy, Work, Power Concept of Heat, temperature, Specific heat capacity, Internal Energy, Enthalpy Statements of zeroth law and first law 22-07-2024 Chapter-1: Introduction 2 Prime mover A prime mover is defined as a device which converts energy from natural sources into mechanical energy or useful work (shaft power). Examples of prime movers are: Wind turbine, steam turbine, water turbine, I.C. Engine, etc. 22-07-2024 Chapter-1: Introduction 3  Prime movers use various natural sources of energy like fuel, water energy, atom, biomass, wind etc. 1. Fuel: When fuel is burnt, heat energy is generated. Amount of heat generated by burning of fuel depends upon calorific value of that fuel. By using heat engine, the heat energy is converted into mechanical energy (shaft power). Various types of fuels are coal, petrol, diesel, gas etc. 2. Water Energy: Water stored at high elevation contains potential energy. When water starts flowing, potential energy gets converted to Sources of kinetic energy. Hydraulic turbine is a prime mover which converts kinetic energy of flowing water into mechanical energy. For example energy water stored in dam contains potential energy. 3. Atoms (Nuclear Energy): Heat energy produced by the fission (nucleus is divided into two or more fragments) or fusion (two lighter atomic nuclei fuse to form a heavier nucleus) of atoms may be used to produce heat. This heat is used to produce shaft power by heat engines. 4. Non-conventional Energy Sources: These energy resources replace themselves naturally in a relatively short time and therefore will always be available. Examples of these resources are solar energy, wind energy, tidal energy, bio energy, solid wastes etc. Almost all non- conventional energy resources offer pollution free environment. Types of prime movers The prime mover can be classified according to the sources of energy utilized. The classification of prime movers is shown in figure. 22-07-2024 Chapter-1: Introduction 5  Basic Definitions Mass: It is quantity of matter contained in a body. It does not depend upon gravitational force. The fundamental unit of mass is the Kilogram (kg). Weight: Weight is the force exerted by gravity. Weight of body is dependent upon gravitational force, so it is not constant. Weight = Mass x Gravitational acceleration W=m×g 22-07-2024 Chapter-1: Introduction 6 Basic Definitions Force: It is push or pull acting on a body which changes or tends to change the state of rest or uniform motion of the body. As per Newton's second law of motion Force ∝ acceleration F=mxa In SI unit (International system), unit of mass is kg and unit of acceleration is m/s2 and unit of the force is Newton (N). When m = 1 kg, and a = 1 m/s2 then F = 1 N. 1 N: When unit mass is given unit acceleration then the force produced is 1 N. 22-07-2024 Chapter-1: Introduction 7 Basic Definitions Pressure: 22-07-2024 Chapter-1: Introduction 8 Basic Definitions Atmospheric Pressure: 22-07-2024 Chapter-1: Introduction 9 Basic Definitions Gauge Pressure: The pressure relative to the atmosphere is called gauge pressure. This pressure is measured by pressure gauge. Absolute pressure: It is the pressure measured with reference to absolute zero pressure. It is the pressure related to perfect vacuum. Mathematically, Absolute pressure = Atmospheric pressure + Gauge pressure Vacuum: The pressure below atmospheric pressure is called vacuum. A perfect vacuum is obtained when absolute pressure is zero; at this instant molecular momentum is zero. The relation between different pressures is given in Figure. 22-07-2024 Chapter-1: Introduction 10 Basic Definitions Relation between different pressures 7/22/2024 Chapter-1: Introduction 11 Basic Definitions Work: Work is said to be done when a force moves the object through a distance in direction of force. Hence, Work = Force x Distance moved into direction of force. W=F×d 𝐹 W= × (𝐴 × 𝑑) 𝐴 W =P×V Unit of work is N∙m or Joule (J). 22-07-2024 Chapter-1: Introduction 12 Basic Definitions Power: Power is defined as the rate of doing work OR the power is work done per unit time. Workdone Mathematically, Power= and unit of power is Joule/second. time In SI unit Joule/second is called Watt (W) Watt is very small unit, so another larger units are megawatt (MW), Kilowatt (kW). 22-07-2024 Chapter-1: Introduction 13 Basic Definitions Energy: Energy means capacity for doing work. The unit of energy is the unit of work i.e. Joule. In our daily life unit of energy use is Kilowatt hour (kWh). Forms of energy: The different forms of energy are; 1. Mechanical energy 2. Thermal or heat energy 3. Chemical energy 4. Electrical energy 5. Nuclear energy 22-07-2024 Chapter-1: Introduction 14 Basic Definitions “Energy can neither be created nor be destroyed but the total amount of energy remains constant. It is possible to convert one form of energy into another form of energy.” This is called the law of conservation of energy. High and Low Grade energy: The second law of thermodynamics prohibits the complete conversion of heat into work. Sources of energy may be divided into two groups viz. a) High grade energy: Energy that can be completely converted (neglecting loss) into the work. Examples: Mechanical work, Electrical energy, Water power, Wind and tidal power, Kinetic energy of jet. b) Low Grade energy: Only a certain portion of energy that can be converted into mechanical work (shaft power), that energy is called low grade energy. Examples: Thermal or heat energy, Heat derived from combustion of fuels, Heat of nuclear fission. 22-07-2024 Chapter-1: Introduction 15 Basic Definitions Types of energy: Energy may be classified as: (1) Stored energy (2) Energy in transition (1) Stored energy: The stored energy of a substance may be in the form of mechanical energy, internal energy, nuclear energy etc. (2) Energy in transition: Energy in transition is the energy transferred as a result of potential difference. This energy may be in the forms of heat energy, work energy, electrical energy. 22-07-2024 Chapter-1: Introduction 16 Basic Definitions Types of Mechanical Energy: There are two types of mechanical energy (1) Potential energy (2) Kinetic energy 1. Potential energy: The energy which a body possesses by virtue of its elevation or position is known as its potential energy. Example: Water stored at higher level in a dam Potential energy, P.E.= m× g ×h Where m = mass of body in kg, g = gravitational acceleration = 9.81 m/s2 h = height in meter, 22-07-2024 Chapter-1: Introduction 17 Basic Definitions Types of Mechanical Energy: 2. Kinetic Energy: The energy which a body possesses by virtue of its motion is known as its kinetic energy. Example: Jet of water coming out from nozzle. 1 Kinetic Energy, K.E.= ×m×𝑣 2 2 Where m = mass of body in kg, v = velocity of body in m/s. 22-07-2024 Chapter-1: Introduction 18 Basic Definitions Temperature: One is well familiar with the qualitative statement of the state of a system such as cold, hot, too cold, too hot etc. based on the day-to-day experience. The degree of hotness or coldness is relative to the state of observer. The temperature of a body is proportional to the stored molecular energy i.e. the average molecular kinetic energy of the molecules in a system. Unit of temperature In the International system (SI) of unit, the unit of thermodynamic temperature is Kelvin. It is denoted by the symbol K. However, for practical purposes the Celsius scale is used for measuring temperature. It is denoted by degree Celsius, ℃. 22-07-2024 Chapter-1: Introduction 19 Basic Definitions Absolute zero temperature: It is the temperature at which the volume occupied by the gas becomes zero. This is the lowest temperature that can be measured by a gas thermometer. 22-07-2024 Chapter-1: Introduction 20 Basic Definitions Temperature Scale: A look at the history shows that for quantitative estimation of temperature a German instrument maker Mr. Gabriel Daniel Fahrenheit (1686-1736) came up with idea of instrument like thermometer and developed mercury in glass thermometer. In the year 1742, a Swedish astronomer Mr. Anders Celsius described a scale for temperature measurement. This scale Later on became very popular and is known as Centigrade Scale or Celsius scale. Some standard reference points used for international practical Temperature Scale are given in Table. 22-07-2024 Chapter-1: Introduction 21 Basic Definitions Heat: When two bodies at different temperatures are brought into contact there are observable changes in some of their properties and changes continue till the two don't attain the same temperature if contact is maintained. Thus, there is some kind of energy interaction between two bodies which causes change in temperatures. This form of energy interaction is called heat. Heat may be defined as the energy interaction at the system boundary which occurs due to temperature difference only. Heat flows in the direction of decreasing temperature 22-07-2024 Chapter-1: Introduction 22 Basic Definitions Interchange of heat: Let us consider two bodies, hot body and cold body. When hot body comes in contact with cold body, the heat will flow from hot body to cold body. This interchange of heat is due to temperature difference. See figure. There will be no heat flow if both the bodies in contact have equal temperature. Interchange of heat 22-07-2024 Chapter-1: Introduction 23 Basic Definitions Units of heat: Heat is a form of energy. In SI system, unit of heat is taken as joule. Kilojoules (kJ) and Mega joule (MJ) are recommended larger units of heat. Calorie (cal.) is also unit of heat. Generally Kilocalorie (kcal) is quantity of heat required to raise temperature of unit mass of water through one degree Celsius or Kelvin. 1 kcal = 4186.8 joules = 4.1868 kilo joules 22-07-2024 Chapter-1: Introduction 24 Basic Definitions Specific heat: It is defines as the quantity of heat required to raise the temperature of unit mass of the substance by one degree. The unit of specific heat is kJ/kg K or J/kg K depending on the unit of Q. From the definition of specific heat, the heat transfer Q is written as Q = m × C× ΔT 𝑸 C= 𝒎×Δ𝑻 Heat capacity: The product of mass and specific heat is called the heat capacity of the substance. Specific heat is function of temperature; hence it is not constant but varies with temperature. Generally it is assumed that it is constant. 22-07-2024 Chapter-1: Introduction 25 Basic Definitions Specific heats in thermodynamics: The solids and liquids have only one value of specific heat but a gas is considered to have two distinct values of specific heat capacity. (i) A value when the gas is heated at constant volume, 𝐶𝑉 (ii) A value when the gas is heated at constant pressure 𝐶𝑃 The specific heat at constant volume 𝐶𝑉 : It is defined as the heat required to increase the temperature of the unit mass of a gas by one degree as the volume is maintained constant. The specific heat at constant volume 𝐶𝑃 : It is defined as the heat required to increase the temperature of the unit mass of a gas by one degree as the pressure is maintained constant. 22-07-2024 Chapter-1: Introduction 26 Basic Definitions Change of state : The various states of substance (Phases) are Solid, Liquid and Vapour/Gas. When heat is supplied to a substance at the solid phase, its temperature rises until it starts converting into liquid. Phase change terminology 22-07-2024 Chapter-1: Introduction 27  Basic Definitions Change of state : Melting point: It is the temperature at which the solid is converted into liquid when heat is supplied. Boiling point: It is the temperature at which the liquid is converted into vapour when heat is supplied. Critical point: It is the temperature and pressure above which only one phase is existing i.e. vapour. Triple point: Triple point of a substance refers to the state at which substance can coexist in solid, liquid and gaseous phase in equilibrium. For water triple point is 0.010 ℃ i.e. at this temperature ice, water and steam can coexist in equilibrium. 22-07-2024 Chapter-1: Introduction 28 Basic Definitions Thermodynamic systems : A system is defined as a quantity of matter or a region in space chosen for study. Examples: quantity of steam, turbine etc. 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. These terms are shown in 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. The system is identified by a boundary around the system. A system and its surroundings together are called the universe. Universe = system + surroundings 22-07-2024 Chapter-1: Introduction 29 Basic Definitions Types of system: There are three types of system (1) Open system, (2) Closed system and (3) Isolated system 1) Open system: In an open system mass and energy (in form of heat and work) both can transfer across the boundary (figure). Most of the engineering devices are open system. Examples: Boiler, turbine, compressor, pump, I.C. engine etc. 22-07-2024 Chapter-1: Introduction 30 Basic Definitions 2) Closed system: Closed system exchange energy in the form of heat and work with its surroundings but there is no mass transfer across the system boundary. The mass within the system remains constant though its volume can change against a flexible boundary. Example: cylinder bounded by a piston with certain quantity of fluid, pressure cooker etc. 22-07-2024 Chapter-1: Introduction 31 Basic Definitions 3) Isolated system: There is no interaction between system and surroundings. It is of fixed mass and energy, and hence there is no mass and energy transfer across the system boundary. Example: The universe and perfectly insulated closed vessel (Thermo flask) 22-07-2024 Chapter-1: Introduction 32 Basic Definitions Sign convention for heat and work: Work If the work is done by the system on surrounding, e.g. when a fluid expands pushing a piston outwards, the work is said to be positive. Work output of the system = + W If the work is done on the system by surrounding e.g. when a force is applied to a piston to compress a fluid, the work is said to be negative. Work output of the system = - W Heat In general, the heat transferred to the system is considered as positive heat (+Q) while the heat transferred from the system is considered as negative heat (-Q). 22-07-2024 Chapter-1: Introduction 33 Basic Definitions Comparison between heat and work Similarities 1) Both are energy interactions. 2) Both are transient phenomena. 3) Both are boundary phenomena. 4) Both represents energy crossing the boundary of the system. 5) They are not the property of the system. 6) Both are path functions. Dissimilarities 1) Heat transfer is the energy interaction due to temperature difference only while work is not. 2) Heat is low grade energy while work is high grade energy. 3) Heat is thermal energy transfer while work is mechanical energy transfer across the system boundary. 22-07-2024 Chapter-1: Introduction 34 Basic Definitions Internal energy: In non-flow processes, fluid does not flow and has no kinetic energy. There is very small amount of change in potential energy because change in centre of gravity is negligible. When heat is supplied to a body the amount of heat transferred to a body is not fully converted to work. When heat (Q) is supplied to a body, some amount of heat is converted into external work (W) due to expansion of fluid volume and remaining amount of heat causes either to increase its temperature or to change its state. Internal Energy is one type of energy which is neither heat nor work; hence it is stored form of energy. It is denoted by U. Mathematically, Q =W +U where Q is amount of heat, W is work and U is internal energy. The internal energy per unit mass is called specific internal energy. Below equation is referred as non-flow energy equation. In other words, for non-flow process 22-07-2024 Chapter-1: Introduction 35 Basic Definitions Enthalpy: Enthalpy is a thermodynamic property of fluid, denoted by H. It can be defined as the summation of internal energy and flow energy. Mathematically, H= U + PV Internal energy Flow work On unit mass basis, the specific enthalpy could be given as h = u + pv From expression of enthalpy it is clear that as we cannot have absolute value of internal energy, the absolute value of enthalpy cannot be obtained. Therefore only change in enthalpy of substance is considered. 22-07-2024 Chapter-1: Introduction 36 Basic Definitions Thermodynamic property: “A thermodynamic property refers to the characteristics which can be used to describe the physical condition or state of a system.” Examples of thermodynamic properties are: Temperature, Pressure, Volume, Energy, Mass, Velocity, etc. Types of thermodynamic property: 1) Intensive Property 2) Extensive Property 3) Specific Property 22-07-2024 Chapter-1: Introduction 37 Basic Definitions Types of thermodynamic property: 1) Intensive Property: Intensive property is independent of the mass of the system. Its value remains same whether one considers the whole system or only a part of it. Example: Pressure, Temperature , Density, etc. 2) Extensive Property: Extensive property depends on mass of the system. Examples: Mass, Volume, Total energy, Enthalpy etc. 3) Specific Property: Extensive properties per unit mass are called specific properties. 𝑉 𝐻 Example: Specific volume 𝑣 = and Specific enthalpy ℎ = 𝑚 𝑚 22-07-2024 Chapter-1: Introduction 38 Basic Definitions State: State refers to the condition of a system as described by its properties. It gives complete description 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 (show in figure). 22-07-2024 Chapter-1: Introduction 39 Basic Definitions Equilibrium: The word equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system is in thermal equilibrium if the temperature is same throughout the entire system. Mechanical equilibrium is related to pressure. If there is no change in pressure at any point in the system with time the system is in mechanical equilibrium. Chemical equilibrium is that state when the chemical composition does not change with time and there is no chemical reaction. A system will be in thermodynamic equilibrium only when it satisfies the conditions for all modes of equilibrium. 22-07-2024 Chapter-1: Introduction 40 Basic Definitions Process and path: Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process (show in Figure). 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. There are infinite ways for a system to change from one state A process between states 1 to another. and 2 and the process path 22-07-2024 Chapter-1: Introduction 41 Basic Definitions Cycle: When a system in a given initial state goes through a number of different changes of state or processes and finally returns to its initial state, the system has undergone a cycle. Thus for a cycle the initial and final states are identical. Example: Steam that circulates through a steam power plant Cyclic Process undergoes cycle. 22-07-2024 Chapter-1: Introduction 42 Basic Definitions Zeroth law of Thermodynamics: Zeroth law of thermodynamics states that “the bodies A and B are in thermal equilibrium with a third body C separately then the two bodies A and B shall also be in thermal equilibrium with each other”. This is the principle of temperature measurement. Block diagram shown in figure shows the Zeroth law of thermodynamics. 22-07-2024 Chapter-1: Introduction 43 Basic Definitions First law of thermodynamics: The first law of thermodynamics is the law of conservation of energy and it states that “energy can neither be created nor destroyed, it can be converted from one form to another and total energy remains same”. Let us take water in a container and heat it from the bottom. What will happen? Container and the water inside will get heated up. This heating can be sensed by either touching it or by measuring its initial and final temperatures. What has caused it to happen so? Answer for the above question lies in the energy interactions. First law of thermodynamics provides for studying the relationships between the various forms of energy and energy interactions. First law may be expressed as, Change in total energy = net energy transferred as heat and work ΔE = Q - W 22-07-2024 Chapter-1: Introduction 44 Basic Definitions First law of thermodynamics: Where ΔE is summation of various energies like Internal energy (ΔU), Kinetic energy (ΔKE), Potential energy (ΔPE) etc. ΔE = Q – W= ΔU + ΔKE +ΔPE.... In closed system, mass is fixed and there is no elevation difference and movement. Hence, ΔKE = 0 and ΔPE = 0. ΔU = Q – W For cyclic process ΔE = 0 Hence first law for a cyclic process is Q – W = 0. That is, the net heat transfer and net work done during a cyclic process are equal. 22-07-2024 Chapter-1: Introduction 45 Numerical 1) A steam engine piston having an area of 140cm2 moves a distance of 160 mm inside the cylinder. Find the amount of work done if the pressure exerted upon the head of the piston is (i) 80Kn/m2 and (ii) 650 Kpa. (Ans. W=179.2 J and W=1456 J) 2) Determine the power developed by the water turbine if it receives 800 kg of water per second at a pressure of 2 bar. (Ans. W= 160 kW) 3) Steel having mass of 20 kg and a specific heat of 460J/Kg K is heated from 35°C to 150°C. Determine heat required. (Ans. Q= 1058 kJ) 4) A gas in a cylinder is at a pressure of 9 bar. It is expanded at constant pressure from 0.5 m3 to 1.8 m3. Determine the work. (Ans W= 1170 kJ) 22-07-2024 Chapter-1: Introduction 46 Numerical 5) To pump 200 kg of water in a boiler, 150×103 𝑁 ∙ 𝑚 of work is required. Find the pressure of the boiler. (Ans. P=750000 N/m2) 6) In compressor, the work done on the air is 150kJ. The heat rejected to the surroundings is 60 kJ/kg. Find the change in internal energy. (Ans. ΔU= 90 kJ) 7) A gas enters a system at an initial pressure of 0.5 Mpa and flow rate of 0.15 m3/s and leaves it at a pressure of 0.95 Mpa and flow rate of 0.09 m3/s. During the passage through the system, the increase in internal energy of 22 kJ/s. Calculate the change in enthalpy of the medium. (Ans. ΔH= 32.5 kJ) 22-07-2024 Chapter-1: Introduction 47  Thank you 22-07-2024 Chapter-1: Introduction 48