Engineering Thermodynamics

Engineering Thermodynamics

DEFINITIONS AND FUNDAMENTAL IDEAS OF THERMODYNAMICSThermodynamics: The science of energy. It is the science that deals with various phenomena of energy and the related properties of matter, especially of the laws of transformation of heat into other forms of energy.The word thermodynamicsstems from the Greek words therme(heat) and dynamis(power).A thermodynamicsystemis a quantity of matter of fixed identity, around which we can draw a boundary.

The quantity of matter or region of space which is chosen for study is calledsystem. The mass or region which is outside our system is calledsurrounding. The imaginary or real surface which is used for separating system from surrounding is called boundaryand the boundary is the only layer which is shared by both the surrounding and the system.The Concept of a ``System''

Piston (boundary) and gas (system)

A close system is also called as control mass because in it there is no way from which the mass enters or leave the system. In it only energy which is in the form of heat or work is added or leaves the system.Closed systems exchange energy but not matter with an outside system.Though they are typically portions of larger systems, they are not in complete contact. The Earth is essentially a closed system; it obtains lots of energy from the Sun but the exchange of matter with the outside is almost zero.Open systems can exchange both matter and energy with an outside system.They are portions of larger systems and in intimate contact with the larger system. Your body is an open system.Open System:It is often called as control volume because it is a selected region over space from which the quantity of mass enters or leaves the system but the selected region over space remains same. The examples of control volume systems are turbines, compressors or nozzles.

Isolated System-in which there isno interaction between system and the surroundings. It is of fixed mass and energy, and hence there is no mass and energy transfer across the system boundary.Isolated systems can exchange neither energy nor matter with an outside system.While they may be portions of larger systems, they do not communicate with the outside in any way. The physical universe is an isolated system; a closed thermos bottle is essentially an isolated system (though its insulation is not perfect).

Macroscopic and Microscopic ApproachesBehavior of matter can be studied by these two approaches.In macroscopic approach, certain quantity of matter is considered, without a concern on the events occurring at the molecular level. These effects can be perceived by human senses or measured by instruments. eg: pressure, temperatureIn microscopic approach, the effect ofmolecular motion is considered. eg: At microscopic level the pressure of a gas is not constant, the temperature of a gas is a function of the velocity of molecules.Most microscopic properties cannot be measured with common instruments nor can be perceived byhuman senses.The Concept of a ``State''Thethermodynamic stateof a system is defined by specifying values of a set of measurablepropertiessufficient to determine all other properties.

For fluid systems, typical properties are pressure, volume and temperature. More complex systems may require the specification of more unusual properties. As an example, the state of an electric battery requires the specification of the amount of electric charge it contains.Characteristic quality of the entire systems depends not on how the system change state but only on the final particular state of the substance/system.State: It is the condition of asystem as defined by the values of all its properties. It gives acomplete description of the system.

Any operation in which one or more properties of a system changeiscalleda change of state.Intensive property: whose valueis independent of the sizeor extent ofthesystem.eg: pressure, temperature (p, T).Specific property: It is the value of an extensive property per unit mass of system.(lowercaselettersassymbols)eg:specificvolume, density (, ). It is a special case of an intensive property.

Most widely referred properties in thermodynamicsPressure; Volume; Temperature; Entropy; Enthalpy; Internal energyExtensive property: whose value depends on the size or extent of the system(upper case letters as the symbols). eg: Volume, Mass (V,M).If mass is increased, the value of extensive property also increases.Properties may beextensiveorintensive.Phase:It is a quantity of mass that ishomogeneous throughout in chemical composition and physical structure. e.g. solid, liquid, vapour, gas.Phase consisting of more than one phase is known as heterogeneous system.Change of Phase: Solid to Liquid- Melting or Fusion Liquid to Solid- Solidification or Freezing Liquid to Gas- Evaporation or Vaporization Gas to Liquid- Condensation of Liquefaction Solid to Gas- Sublimation Path And ProcessThesuccessionofstatespassedthroughduringachangeofstate is called the path of the system.A system is said to go through a process if it goes through a series of changes in state.Consequently: A systemmay undergo changes insome orall ofits properties. A process can be construed to be the locus of changes of state.Types of Processes Isothermal- constant temperature Isobaric- constant pressure Isometric (Isochoric)- constant volume Isentropic- constant entropy (PVk = C) Isenthalpic- constant enthalpy Adiabatic- no heat addition or removal Polytrophic- PVn = CCycle is a series of processes one after the other such that the initial and final states are the same.Properties and UnitsSystems of UnitsCGSMKS (Metric) or SIFPS (English/Imperial)The SI unit prefixes are used in all branches of engineering.Newtons law states that the acceleration of a particular body is directly proportional to the resultant force acting on it and inversely proportional to its mass.a F/m;a = kF/m;k = ma/Fwhere k is the proportionality constant; k is unity but not dimensionlessCGS : 1 dyne force accelerates 1 gram mass at 1 cm/s2 ; k = 1 gm . cm/dyne . s2 MKS/SI : 1 Newton force accelerates 1 kg mass at 1 m/s2 ; k = 1 kgm . m/Newton . s2 English : 1 lb force accelerates 1 slug mass at 1 ft/s2 ; k = 1 slug . ft/lbf . s2 From: F = maFrom: F = m.aMKS/Metric/SI: 1 Newton (N) = 1 kgm . m/s2 CGS:1 dyne = 1 gm . m/s2 FPS (English)1 lbf = 1 slug . ft/s2 1 slug = 1 lbm . ft/s2 ; 1 lbf = 32.174 lbm . ft/s2 Weight, W = the force of gravity acting on a body.W = mg; g gravitational accelerationg = 9.807 m/s2 (metric/SI) = 9807 cm/s2 (CGS) = 32.174 ft/s2 (FPS/English)Volume, V space occupied by matter. (m3 ; cm3 ; ft3 ) Density, mass per unit volume. = m/V;(kg/ m3 ; g/ cm3 ; lb/ ft3 )Specific volume, volume per unit mass. = V/m = 1/;(m3/kg; cm3/g; ft3/lb)Specific weight: The weight of a unit volume of a substance. = W/V = mg/(m/) =g

MKS:(kg/ m3) (m/s2) = kg.m/s2. m3 = N/ m3 CGS: (g/ cm3) (cm/s2) = g.cm/s2. cm3 = dyne/ m3 FPS: (lbm/ ft3) (ft/s2) = lbm .ft/s2. ft3 = (lbm .ft/s2)/ft3 = (1/32.174) lbf /ft3 Density of Water at Standard Condition

English:62.4 lbm/ ft3 8.33 lbm/ galMKS/SI:1000 kgm/ m3 1 kgm/ liSpecific gravity: The ratio of the density of a substance to the density of some standard substance at a specified temperature(usually water at 4C)For Solid and Liquid:S.G. = /H2O = / H2OFor Gasses:S.G. = MW/MWgMWg = 28.97= 29Mass FundamentalsNewtons Physics mass is constant anywhere in the universe.

Law of Conservation of mass states that mass is indestructible, provided that there is no nuclear process involved.

min = mchange + mout and if m = 0; min = moutFor fluid passing through a given section;VF = A.VmF = VF/= AV/ = AVwhere:VF = volume flow rate, m3/sA = cross-sectional area of given section, m2 V = velocity (average speed) of fluid, m/smF = mass flow rate, kg/s= specific volume, m3/kg = density, kg/m3 Examples:1. Two liquids of different densities (1 = 1500 kg/m3 and 2 = 500 kg/m3) are poured together into a 100 L tank, filling it. If the resulting density of the mixture is 800 kg/m3, find the respective quantities of liquids used. Also find the weights of the mixture.2. Two gaseous streams enter a combining tube and leave as a single mixture. These data apply at the entrance section.A1 = 75 in2, V1 = 500 fps, 1 = 10 ft3/lb; A2 = 50 in2, mF2 = 16.67 lb/s, 2 = 0.12 lb/ft3 At exit: A1 = 75 in2, V3 = 350 fps, 3 = 7 ft3/lbFind the speed at section 2, the flow rate and area at section 3.3. A pump discharges into a 3-m-per-side cubical tank. The flow rate is 300 liters/min, and the fluid has a density 1.2 times that of water. Determine (a) the flow rate in kg/s; (b) the time it takes to fill the tank.4. A tank contains a mixture of 20 kg of nitrogen and 20 kg of carbon monoxide. The total tank volume is 20 m3. Determine the density and specific volume of the mixture.5. A cylindrical tank is 50 in. Long, has a diameter of 16 in., and contains 1.65 lb water. Calculate the specific volume and density of water.

Temperature Scales

All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice pointand the steam point.Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0C or32F).Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure(100C or212F).Celsius scale: in SI unit systemFahrenheit scale: in English unit systemThermodynamic temperature scale: A temperature scale that is independent of the properties of any substance.Kelvin scale(SI) Rankine scale(E)A temperature scale nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on this scale are measured using a constant-volume gas thermometer.

A constant-volume gas thermometer would read -273.15C (0K) at absolute zero pressure.

The reference temperature in the original Kelvin scale was the ice point, 273.15 K, which is the temperature at which water freezes (or icemelts).The reference point was changed to a muchmore precisely reproducible point, the triple pointof water (the state at which all threephases of water coexist in equilibrium), which is assigned the value 273.16 K.

ConversionTEMPERATURE AND THE ZEROTH LAW OFTHERMODYNAMICSThe zeroth lawof thermodynamics: If two bodies are in thermal equilibrium with a third body, theyare also in thermal equilibrium with each other.

By replacing the third body witha thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have thesame temperature reading even if they are not incontact.

Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure.If two systems (say A and B) are in thermal equilibrium with a third system (say C) separately(that is A and C are in thermal equilibrium; B and Care in thermal equilibrium) then they are in thermal equilibrium themselves (that is A and B will be in thermal equilibriumPRESSURE:Pressure(the symbol:P) is theforceper unitareaapplied in a directionperpendicularto the surface of an object.

where:

Pis the pressure,Fis thenormal force,Ais the area of the surface area on contactTheabsolute pressure-pabs- is measured relative to theabsolutezero pressure- the pressure that would occur at absolute vacuum. All calculation involving the gas laws requires pressure (and temperature) to be in absolute units.Absolute pressureis zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. The actual pressure at a given position. It is measured relative to absolute vacuum(i.e., absolute zero pressure).Pabs= pg + patmGauge pressureis the pressure relative to the local atmospheric or ambient pressure.- is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. Most pressure-measuring devices are calibrated to read zero in theatmosphere, and so they indicate gage pressure.Most gauges read the excess of pressure over atmospheric pressure and this excess is called "gauge pressure". While a useful measurement for many practical purposes, it must be converted to absolute pressure for applications like theideal gas law.Since a partial vacuum will be below atmospheric pressure, the phrase "negative pressure" is often used.When a system is at atmospheric pressure, the gauge pressure is said to be zero.Agaugeis often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called thegauge pressureand can be expressed as

pg= ps- patm wherepg=gauge pressureps= system pressurepatm=atmospheric pressure Atmospheric pressure is the force from the weight of the atmosphere above us pushing down on a unit area. - In a fluid, which is a liquid or a gas, the pressure increases with the depth of the fluid because there is a greater weight of fluid pushing down on each unit of area. The surface of the earth is at the bottom of an atmospheric sea. The standard atmospheric pressure is measured in various units: 1 atm = 1.01325 bar = 101.3 kPa = 14.696 psi (lbf/in2)= 760 mmHg =10.33 mH2O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kgf/cm2= 33.90 ftH2OThemercury barometer is the standard instrument for atmospheric pressure measurement in weather reporting. The decrease in atmospheric pressure with height can be predicted from thebarometric formula.The unit mmHg is often called torr, particularly in vacuum applications: 760 mmHg = 760 torr

Pascal's principlestates that any pressure applied to an enclosed fluid is transmitted to every point in the fluid. This principle is the foundation for all hydraulic devices.

Atmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure vary with temperature and altitude above sea level.Fluid pressureis the pressure at some point within afluid, such as water or air.Fluid pressure occurs in one of two situations:1. an open condition, called "open channel flow"a. the ocean, orb. swimming pool, orc. the atmosphere.2. a closed condition, called closed conduitsa. water line, orb. gas line.The concepts of fluid pressure are predominantly attributed to the discoveries ofBlaise PascalandDaniel Bernoulli. Bernoulli's Equation can be used in almost any situation to determine the pressure at any point in a fluid. The equation makes some assumptions about the fluid. Such as the fluid being ideal and incompressible. The equation is written between any two points in a system that contain the same fluid.

Where:p = pressure of the fluid = g= density*acceleration of gravity v = velocity of the fluid =specific weightof the fluid.z = elevationg =acceleration of gravity

= pressure head= velocity headPressure of an ideal gasPressure varies linearly with temperature, volume, and quantity according to theideal gas law,

where:Pis the absolute pressure of the gasnis theamount of substanceTis the absolute temperatureVis the volumeRis theideal gas constant.

Real gasesexhibit a more complex dependence on the variables of state.Vapor pressure is the pressure of avaporinthermodynamic equilibriumwith its condensedphasesin a closed system. Allliquidsandsolidshave a tendency toevaporateinto a gaseous form, and allgaseshave a tendency tocondenseback to their liquid or solid form.Vapor pressure

The main study used for the pressure within a pipe is the application of a simple device called a manometer. Manometers appear in many shapes and sizes (simple manometer, differential manometer, U-shape manometer, barometer, etc.) and serve different purposes (single pipe pressure, difference in pipe pressures, atmospheric pressure). The different manometer devices measure gage pressures; however, through a simple calculation knowing the atmospheric pressure, the absolute pressure of the desired point of points can be determined:

Pgas>PatmGas pressure =atmospheric pressure +h(height of the mercury)

Pgas< PatmGas pressure = atmospheric pressure -h(height of the mercury)Closed-tube manometers look similar to regular manometers except that the end thats open to the atmospheric pressure in a regular manometer is sealed and contains a vacuum. In these systems, the difference in mercury levels (in mmHg) is equal to the pressure in torr.

Closed tubeOpen TubeOpen TubeDifferential Manometer

The same concepts of fluid statics are used in solving a differential manometer; however, the difference between the two types is that in a differential manometer the pressures are not always known. Therefore, the result will sometimes come out as a difference in pipe pressure. The dimension, Rm, is the manometer reading, which is the height difference between the two surfaces of the manometer fluid, M. The dimensions, hA and hB, are the height differences between the pipe fluid/manometer fluid interface and respective pipe location (A or B). The equation for the difference in the pressure of A and B for this manometer is as follows:

As in simple manometers, the pressure or pressure difference can be determined as a gage pressure (or pressure head), or as an absolute pressure (or pressure head).

ProblemsA skin diver wants to determine the pressure exerted by the water on her body after a descent of 35 m to a sunken ship. The specific gravity of sea water is 1.02 times that of pure water. What is the pressure?A hiker is carrying a barometer that measures 101.3 KPa at the base of the mountain. The barometer reads 85 KPa at the top of the mountain. The average air density is 1.21 kg/m3. Determine the height of the mountain.A diver descends 100 m to a sunken ship. A container is found with a pressure gage reading of 100 KPa (gage). Atmospheric pressure is 100 Kpa. What is the absolute pressure of the gas in the container? (The density of water is 1000 kg/m3). A vertical frictionless piston cylinder contains air at a pressure of 300 KPa with atmospheric pressure of 100 KPa. The diameter of the piston is 0.25m, and g = 9.8m/s2. Determine the pistons mass.A piston-cylinder contains 2 lbm of water. The initial volume is 0.1 ft3. The piston rises, causing the volume to double. Determine the final specific volume of the water.A beer barrel has a mass of 10 kg and a volume of 20 liters. Assuming the density of beer is 1000 kg/m3, determine the total mass and weight of the barrel when it is filled with beer.A tank has a vacuum gage attached to it indicating 20 KPa (vacuum) where the atmospheric pressure is 100 KPa. Determine the absolute pressure in the tank.A pressure cooker operates by cooking food at a higher pressure and temperature than is possible at atmospheric condition. Steam is contained in the sealed pot, with a small vent hole in the middle of the cover, allowing steam to escape. The pressure is regulated by covering the vent hole with a small weight, which is displaced slightly by the escaping steam. Atmospheric pressure is 100 Kpa, the vent hole area is 7mm2, and the pressure inside is 250 Kpa. What is the mass of the weight?Determine the pressure of these gasses in mmHg.

ProblemsFind the gage and absolute pressure at point A of the system.

Find pressure at B, and pressure head at B in length units of water.

Problems