Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page1 Chapter 1 Introduction 1.1Prime movers “Prime mover is a device which uses natural sources to convert their energy into mechanicalenergy or useful work (shaft power)”. 1.2 Sources of energy Prime movers are using various natural sources of energy like fuel, flow of river water, atom,biomass, wind etc. (i)Fuel:When fuel is burnt, heat energy is liberated. Amount of heat liberated 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). Fuel is the most widely used source of energy. (ii) Flow of river water: This is another useful source of energy. Water stored at high elevation contains potential energy. When water starts flowing, potential energy gets converted to kinetic energy. Hydraulic turbine is a prime mover which converts kinetic energy of flowing water into mechanical energy. (iii) Atoms (Nuclear Energy): Nuclear energy or atomic energy is recent development. Heat energy produced by the fission or fusion of atoms may be used to produce shaft power by heat engines. (iv)Nonconventional Energy Sources: these energy resources replenish themselves naturally in a relatively short time and therefore will always be available. The examples of these resources are solar energy, wind energy, tidal energy, bio energy, solid wastes etc. Almost all nonconventional energy resources offer pollution free environment and also help in maintaining the ecological balance. Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page2 1.3 Types of prime movers The prime mover can be classified according to the sources of energy utilized. 1.4 Force and mass (i) Force:Force is the product of mass and acceleration of the body upon which it is applied. As per Newton's second law of motion Forceα acceleration ... (1.1) where, m is (constant) mass of body in kg. F = m × a In SI unit (International system), unit of mass is kg and unit of acceleration is m/s2. •Weight:Weight is force exerted bygravity Weight=Mass x Gravitational acceleration Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page3 Weight of body is dependent upon gravitational force, so it is not constant. w = m×g (ii) Mass: Mass is the quantity of matter and it is constant. It does not depend upon gravitational force.The fundamental unit of mass is the Kilogram (kg). It is the mass of the platinum iridium lump kept at severs, France. 1.5 Pressure Pressure is the property offluid and it is defined as force per unit area. Pressure =Force/Area or P =F/a N/m2 •The unit of pressure isN/m2, N/m2is known as Pascal (Pa) 1 Pa = 1 N/m2 ... (1.2) • Pressure gauges, Manometers etc are used to measure gauge pressure and Barometer is used to measure atmospheric pressure. Atmospheric pressure is the pressure exerted by atmosphere. It varies with locationon earth. Standard atmospheric pressure is a pressure of atmospheric air at mean sea level. It is defined as the pressure developed by a mercury column of 760 mm. If we take density of mercury equal to 13595.09 kg/m3 and gravitational acceleration equals to 9.80665 mls2 • Standard atmospheric pressure will be = 13595.09x9.80665xO.760 N/m2 = 1.01325 x105N/m2. Or Pa = ℎ • Absolute pressure is measured with reference to absolute zero pressure. It is the pressure related to perfect vacuum. Mathematically, Absolutepressure = Atmospheric pressure + Gauge pressure • Vacuum is defined as the pressure below atmospheric pressure. 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 Fig. 1.1. Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page4 Fig.1.1.Relation between different pressures 1.6 Work Work is said to be done when a force moves through a distance. Ifa part of the boundary of a system undergoes a displacement under the action of a pressure, the work done W is the product of the force (Pressurexarea) and the distance it moves in the direction of the force. Hence, Work=Force x Distance moved into direction of force. • 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 • Ifthe 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 • Unit of work (W)=Unit of force x Unit of displacement = or joule N×m 1.7 Power Power is defined as the rate of doing workORthepower is work done per unit time, Mathematically Power = Work done/ Timeand Joule/second ...(1.3) In SI unit Joule/second is called Watt (W) Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page5 Watt is very small unit, so recommended larger units are (MW), etc.Kilowatt (kW), Megawatt • The power available at the engine shaft is called Brake power (B.P.) and the power developedby the engine is called Indicated power(LP.). 1.8 Energy "Energy", a word derived from the Greek word "Energia", means capacity for doing work. • The unit of energy is the unit of work i.e. Joule. Another important unit of energy is Kilowatt hour (kWh) which is derived from the unit of power kilowatt. • 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 It is possible to convert one form of energy into another form of energy. Heat engine is a device which converts heat energy into mechanical energy. Energy can neither be created nor be destroyed but the total amount of energy remains constant before and after the transformation. 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. Types of energy: Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page6 Energy may be classified as (1) Stored energy (2) Energy in transition The stored energy of a substance may be in the form of mechanical energy, internal energy, nuclear energy etc. Energy in transition is the energy transferred as a result of potential difference. This energy may be in the forms of heat energy, work energetic. • Types of MechanicalEnergy: There are two types of mechanical energy (a) Potential energy (b) Kinetic energy (a) Potentialenergy: 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, ... (1.4) P.E.= w.h Where w=weight of body in N, h=Height in meter, m=mass of body in kg, P.E.= m×g×h g=Gravitational acceleration = 9.81 m/s2. (b) Kinetic Energy: The energy which a body possesses by virtue of its motion is known as itskinetic energy. Example: Jet of water coming out from nozzle.1 Kinetic Energy N.m Where m = maKs.sE of= bodmy vin kg, v = velocity of body in m/s. 1.9 Heat When twobodiesat 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 at the boundary which causes change in temperatures. This form of energy interaction is called heat. • Definition of Heat: Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page7 Heat may be defined as the energy interaction at the system boundary which occurs due to temperature difference only. ' When heat is removed from a body or supplied to it, there are some changes found to happen suchas (a) change of temperature, (b) change of volume, (c) change of state (solid to liquid, liquid to gas, etc.), (d) change of physical properties, etc. • Positive and negative heat In general, the heat transferred to the system is considered as positive heatwhile the heat transferred from the system is considered asnegative heat. Mass of the substance, specific heat, and temperature difference are the factors on which the heat transfer rate depends. •Comparisonof work and Heat Similarities: (a) Both are path functions (b) Both are boundary phenomenon (c) Both are associated with a process, not a state (d) Systems possess energy, but not work or heat Dissimilarities: (a) In heat transfer, temperature difference is required. (b) In a stablesystem there cannot be work transfer, however, there is no restriction for the transfer of heat. (c) Heat is low grade energy while work is high grade energy. 1.10 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. • Definition: Qualitative indication of the relative hotness can be exactly defined by using thermodynamic property known as temperature. • Unit of temperature In the International system (SI) of unit, the unit of thermodynamic temperature is Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page8 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 (OC) • Absolute zero temperature: It has been found that a gas will not occupy any volume at a certain temperature. This temperature is known as absolute zero temperature. This is the lowest temperature that can be measured by a gas thermometer. • 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 1.1. 1.11 Units of Heat Heat is a form of energy. In SI system, unit of heat is taken as joule.Kilojoules (kJ) andMega 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 kilojoules 1.12 Specific heat capacity Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page9 Specific heat capacity is also known as specific heat. The specific heat capacity of a substance may be defined 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 J/kg °c. This unit is small, so kJ/kg-K or kJ/kg °c is recommended larger units. Mathematically, the heat transfer rate Q is written as Where, C = specific heat in kJ/kg.K, m = mass of substance in kg, Q = m×c×△T Temperaturedifference in K. The product of mass and specific heat is called the heat capacity of the substance. △T • Specific heat is function of temperature; hence it is not constant but varies with temperature. Generally it is assumed that it is constant. • 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 may be defined as the heat required to increase the temperature of the unit mass of a substance by one degree as the volume is maintained constant. Same way one can define the specific heat at constant pressure (Cp), here pressure is p maintained constant. 1.16 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. From the first law of thermodynamics, we can say that the amount of heat transferred to a body is not fully converted t work. When heat (Q) is supplied to a body, some amount of heat is p 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 Darshan Institute of Engineering & Technology Elements of Mechanical engineering (2110006) Chapter-1Introduction Page10