IE 211 INTRODUCTION TO ENGINEERING THERMODYNAMICS

Transcription

1 IE 211 INTRODUCTION TO ENGINEERING THERMODYNAMICS Chapter1 Introduction and Basic Concepts

2 INDUSTRIAL REVOLUTION A period in 18th and early 19th centuries Major changes in agriculture, mining, manufacturing, and transportation Emerged in United Kingdom, then spread throughout Europe, North America and eventually the world The First Industrial Revolution, which began in the 18th century, merged into the Second Industrial Revolution around 1850, when technological and economic progress

3 The introduction of steam power resulted in enourmous increase in production capacity. It started with the mechanisation of the textile industries, the development of ironmaking techniques Trade expansion was enabled by the introduction of canals, improved roads and railways.

4 Thermodynamics emerged as a science during the construction of the first successful atmospheric steam engines in England ( ) Thermodynamics term was firstly used in a publication of Lord Kelvin in The first thermodynamic textbook was written in 1859 by William Rankine THERMODYNAMICS deals with energy and can be defined as the science of energy. It gives the relation between energy, heat and work. Therme (heat) Stems from the Greek words dynamis (power)

5 The first and second law of thermodynamics emerged simultaneously in 1850s, primarily out of the work of William Rankine, Rudolph Clausius and Lord Kelvin German physicist, created the science of thermodynamics. The first law of thermodynamics: (conservation of energy) Rudolph Clausius He developed the idea of absolute zero and independently recognized its first and second laws of thermodynamics. With James Prescott Joule, Kelvin discovered that gases cool when allowed to expand, the Joule-Thomson effect. (William Thomson )Lord Kelvin Proposed another thermodynamic temperature scale which also assigned 0 to thermodynamic absolute zero, but used the degree Fahrenheit as its base unit. William Rankine

6 The first law of thermodynamics: (conservation of energy) Energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another. i.e. Falling of a rock Conversion of potential energy to kinetic energy i.e. Human body

7 The second law of thermodynamics: (energy has quality as well as quantity) Heat always flows from a warm body to a cooler one, never the other way. - Actual processes occur in the direction of decreasing quality of energy i.e. Cooling of a cup of coffee

8 THERMODYNAMICS Classical Thermodynamics Analyses the macroscopic properties of the system using classical laws of thermodynamics Statistical Thermodynamics Analyzes thermodynamic properties by relating them to molecular-level models of microscopic behaviour Household Appliances -Heating and air conditioning system -Refrigerator -Water heater -The shower Application Areas

9 In the design of -Automotive engines -Rockets -Jet engines -Power Plants -Solar collectors

10 DIMENSIONS AND UNITS Physical quantity is characterized by dimensions. The magnitudes of the dimensions are specified by units. 1) Fundamentals dimensions; mass(m), length(l), time(t), temperature(t) 2) Secondary (drived) dimensions; velocity(v), energy(e), volume ( ) Two sets of units English System United States Customary System (USCS) Metric, SI, International system Based on decimal relationship between units (milli,m), 10-2 (centi, c), 10-1 (deci,d), 10 1 (deka, da)... Example: Dimension:Length; Unit: METER(m)...mm, cm, m,..,km... Dimension:Mass; Unit: GRAM(g)...mg, cg, g,..., kg

11 SOME SI and ENGLISH UNITS Dimension Mass Length Time Force Energy... SI unit Kilogram (kg) Meter (m) Second(s) Newton(N)= 1 kg.m/s 2 Joules(J) = N.m... English unit Pound-mass (lbm) Foot(ft) Second(s) 1 lbf= lbm.ft/s 2 British thermal unit(btu)... 1) MASS and WEIGHT Mass is an intrinsic property of matter Weight is a force that results from the action of gravity on matter (It s magnitude is determined from Newton s second law) W = m.g (N) g decreases with altitude, so that a body weighs less on top of a mountain On the surface of moon, an astronaut weighs about one-sixth of what he or she normally weighs on earth

12 2) DENSITY = m/v ; (kg/m 3 or lbm/ft 3 ) = f(p,t) For most gases P 1/T For liquids and solids variation of density with pressure is negligable Effect of temperature; water= 998 kg/m 3 20 o C at 1 atm water = 975 kg/m 3 75 o C at 1 atm Specific gravity: density of a substance relative to density of a well-known substance, e.g. water S.G. = / water(at 4 o C) Specific weight: The weight of a unit volume of a substance s =.g

13 3) TEMPERATURE We can measure temperature; by our sensations; freezing cold, warm, hot, red-hot Mercury in glass thermometer (based on the expansion of mercury with temperature) Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Phase Changes; Ice (freezing) point: A mixture of ice and liquid water that is in equilibrium with air saturated with vapor at 1 atm. pressure is said to be at the ice point. Steam (boiling) point: Mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm. pressure is said to be at the steam point.

14 TEMPERATURE SCALES: Celcius( o C), Fahrenheit( o F), Kelvin(K), Rankine(R) In most of the world the Celsius scale is used for most temperature measuring purposes. SI Units o C (Celcius scale) Ice point... 0 o C Steam point o C English System o F (Fahrenheit scale) 32 o F 212 o F These are often referred to as two-point scales since temperature values are assigned at two different points Conversion: 9/5T( o C) + 32 = T( o F)

15 Temperature scale that is independent of the properties of any substance is called thermodynamic temperature scale. Kelvin(K) scale (lowest temperature on this scale is 0 K) SI Units K (Kelvin scale) English System R (Rankine scale) Conversion: T( o F) = 9/5T( o C) + 32 T(R) = 1.8T(K) T(K) = T( o C) T(R) = T( o F) Temp. Difference: T( o C) = T(K) T( o F) = T(R) SI system English system

16 IDEAL GAS TEMPERATURE SCALE: The temperatures on this scale are measured using a constant-volume gas thermometer, which is basically a rigid vessel filled with a gas, usually hydrogen or helium, at low pressure. This thermometer is based on the principle that at low pressures, the temperature of a gas is proportional to its pressure at constant volume. T = a + bp Ideal gas temperature scale can be developed by measuring the pressure of gas at ice and steam points ABSOLUTE GAS TEMPERATURE SCALE: Constant a is assumed to be zero. T = bp

17 4) PRESSURE A normal force exerted by a fluid per unit area (related to gas or liquid). In solids it is normal stress. It has the unit of newtons per square meter (N/m 2 ), which is called a pascal (Pa) Other pressure units: bar, standard atmosphere, kilogram-force per square centimeter

18 SI Units Pa (N/m 2 ), atm, bar, kgf/cm 2, mmhg English System psi(lbf/in 2 ), Psia, lbf/ft 2, in Hg Conversion between SI units: 1 bar = 10 5 Pa = 0.1 MPa 1 atm = 760 mmhg = kpa = bars 1 kgf/cm 2 = N/cm 2 = 9.807x10 4 Pa = bar = atm How a person can walk on fresh snow without sinking by wearing large snowshoes? And how a person cuts with little effort when using a sharp knife?

19 ABSOLUTE PRESSURE: The actual pressure at a given position. It is measured relative to absolute vacuum(absolute zero pressure) GAGE PRESSURE: The difference between the absolute pressure and the local atmospheric pressure. Example:The gage used to measure the air pressure in an automobile tire relative to atmospheric pressure VACUUM PRESSURE: Pressure below atmospheric pressure P vac = P atm - P abs P gage = P abs - P atm In thermodynamic calculations absolute pressure is used.

20 VARIATION of PRESSURE WITH DEPTH Pressure in a fluid increases with depth because more fluid rests on deeper layers. P 2 = P atm + gh A consequence of the pressure in a fluid remaining constant in horizontal directions is that the pressure applied to a confined fluid increases the pressure throughout the same amount This is called PASCAL S LAW

21 Force applied by a fluid is proportional to the surface area This enables us to lift a car easily by one arm, e.g. Hydraulic lifts Two hydraulic cylinders of different areas could be connected, and the larger could be used to exert a proportionally greater force than that applied to the smaller. P1 = P2 at the same levels, then; F1/A1 =F2/A2 F2/F1 = A2/A1

22 THE MANOMETER A manometer consists of a glass or plastic U-tube containing mercury, water, alcohol or oil and it is used measure the gas pressures P 2 = P atm + gh A manometer containing multiple immiscible fluids: P 2 = P atm + 1gh gh gh 2

23 THE BAROMETER Atmospheric pressure is measured by barometer (Italian Evangeliste Torricelli, ) Atmospheric pressure can be measured by inverting a mercury filled tube into a mercury container that is open to the atmosphere. P B = P atm, P c 0 P c + gh = P B = P atm P atm = gh Pressure units 760 mmhg = 1 atm 1 mmhg = 1 Torr 1 atm = kpa At sea level!!!

24 5) HEAT (Q) Heat is defined as an energy transfer to a body in any other way than due to work performed on the body SI Units :Joules(J) Energy transfer by heat can occur between objects by radiation, conduction and convection A red-hot iron rod from which heat transfer to the surrounding environment will be primaily through radiation.

25 HEAT TRANSFER

26 6) WORK (W) Form of energy; force times distance, (N.m) or joules(j) Definition of work In metric system: The amount of energy needed to raise the temperature of 1 g of water at 14.5 o C by 1 o C is defined as 1 calorie (cal) 1 cal = J In English system: Energy unit is the Btu (British thermal unit) : energy required to raise the temperature of 1 lbm of water at 68 o F by 1 o F 1 Btu = kj

27 SYSTEMS AND CONTROL VOLUMES The mass or region outside the system Quantity of matter or region in space chosen for study The real or imaginary surface that seperates system from its surroundings (has zero thickness)

28 SYSTEMS Closed Systems (Control mass) Open Systems (Control volume) Control surface Consists of fixed amount of mass No mass can cross its boundary Energy (heat&work) can cross the boundary Volume does not have to be fixed Adiabatic system: A kind of closed system No mass transfer Energy can cross the boundary only in the form of work Encloses a device that involves mass flow such as a compressor, turbine or nozzle Both mass and energy(heat&work) can cross the boundary A control volume may also involve a moving boundary Special Case: isolated system Rigid walls. No mass and energy (heat&work) transfer!

29 Examples of Open Systems (Control Volumes) Water heater Air compressor Car radiator Wind Turbine Gas Turbine

30 PROPERTIES OF A SYSTEM Any characteristics of a system is called property e.g. Pressure, temperature, volume, mass, viscosity, thermal conductivity, thermal expansion coefficient 1) Intensive Properties: Independent of the mass of a system Temperature (T); ( o C, K, o F, o R) Pressure (P); (Pa or Psi) Chemical potential ( i ); (kj/mole, Btu/mole) 2) Extensive Properties: Depend on the size or extent of the system Mass(m); (kg or lbm) volume ( ); (m 3 or ft 3 ) Momentum Energy Enthalpy Entropy Extensive properties per unit mass are called specific properties e.g. Specific volume; = /m; (m 3 /kg or ft 3 /lbm) Specific total energy, e = E/m; (kj/kg)

31 STATE AND EQUILIBRIUM The stationary condition of the system at which properties are defined is called state of system. Expansion of gas Equilibrium implies a state of balance. A system in equilibrium experiences no changes when it is isolated from its surroundings Thermal equilibrium (temp. is same) Mechanical equilibrium (pressure is same) Phase equilibirum (mass of each phase reaches an equilibrium level) Chemical equilibrium (chemical composition does not change with time)

32 THE STATE POSTULATE When sufficient number of properties are specified the rest of the properties assume certain values automatically. The number of properties required to fix the state of a system is given by the system postulate The state of a simple compressible system (absence of electrical, magnetic, gravitational, motion and surface tension effects) ; Specified by two independent, intensive properties (temperature and specific volume)

33 PROCESSES AND CYCLES Process: changes that occur when a system changes its state from one equilibrium state to another (Series of states that system passes during a process) Property A State 2 Cycle: a system is said to have undergone a cycle if it returns to its initial state at the end of process State 1 Property B

34 PROCESSES Isothermal Process Isobaric Process Isochoric (or isometric) Process Temperature remains constant Constant pressure Constant specific volume Quasistatic or quasi-equilibrium process: System is close to equilibrium Example: Very slow compression of a gas Gas molecules will have sufficient time to redistribute and there will not be a molecule pile up in front of the piston

35 STEADY-STATE A system in a steady state has numerous properties that are unchanging in time In steady-state processes fluid flows through a control volume steadily Example: Turbines, pumps, boilers, refrigeration systems During steady-state flow fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process Therefore, volume ( ), mass(m) and total energy (E) remain constant

36 TEMPERATURE AND ZEROTH LAW OF THERMODYNAMICS When two materials at different temperatures contact each other heat is transferred from the body at higher temperature to one lower temperature until both bodies attain the same temperatures (thermal equilibrium) Zeroth Law of Thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other

37 Third body may be replaced by a thermometer (object#1) Note: they are not in contact

38 Assignments: Examples 1-6, 1-7, 1-8, 1-9