Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics (776129), страница 8
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Thesystem can be considered to be perfectly insulated.Isentropic ProcessAn isentropic process is one in which the entropy of the fluid remains constant. This will be trueif the process the system goes through is reversible and adiabatic. An isentropic process can alsobe called a constant entropy process.Polytropic ProcessWhen a gas undergoes a reversible process in which there is heat transfer, the process frequentlytakes place in such a manner that a plot of the Log P (pressure) vs. Log V (volume) is a straightline.
Or stated in equation form PVn = a constant. This type of process is called a polytropicprocess. An example of a polytropic process is the expansion of the combustion gasses in thecylinder of a water-cooled reciprocating engine.Throttling ProcessA throttling process is defined as a process in which there is no change in enthalpy from stateone to state two, h1 = h2; no work is done, W = 0; and the process is adiabatic, Q = 0. To betterunderstand the theory of the ideal throttling process let’s compare what we can observe with theabove theoretical assumptions.An example of a throttling process is an ideal gas flowing through a valve in midposition.
Fromexperience we can observe that: Pin > Pout, velin < velout (where P = pressure and vel = velocity).These observations confirm the theory that hin = hout. Remember h = u + Pv (v = specificvolume), so if pressure decreases then specific volume must increase if enthalpy is to remainconstant (assuming u is constant). Because mass flow is constant, the change in specific volumeis observed as an increase in gas velocity, and this is verified by our observations.Rev. 0Page 29HT-01THERMODYNAMIC SYSTEMS AND PROCESSESThermodynamicsThe theory also states W = 0.
Our observations again confirm this to be true as clearly no"work" has been done by the throttling process. Finally, the theory states that an ideal throttlingprocess is adiabatic. This cannot clearly be proven by observation since a "real" throttlingprocess is not ideal and will have some heat transfer.SummaryThe important information from this chapter is summarized below.Thermodynamic Systems and Processes Summary•A thermodynamic system is a collection of matter and space with its boundariesdefined in such a way that the energy transfer across the boundaries can be bestunderstood.•Surroundings are everything not in the system being studied.•Systems are classified into one of three groups:Isolated system Closed systemOpen systemneither mass nor energy can cross theboundariesonly energy can cross the boundariesboth mass and energy can cross theboundaries-•A control volume is a fixed region of space that is studied as a thermodynamicsystem.•Steady state refers to a condition where the properties at any given point within thesystem are constant over time.
Neither mass nor energy are accumulating within thesystem.•A thermodynamic process is the succession of states that a system passes through.Processes can be described by any of the following terms:Cyclic process-Reversible process Irreversible process Adiabatic process-Isentropic process-Polytropic process Throttling processHT-01-a series of processes that results in the systemreturning to its original statea process that can be reversed resulting in no changein the system or surroundingsa process that, if reversed, would result in a change tothe system or surroundingsa process in which there is no heat transfer across thesystem boundariesa process in which the entropy of the system remainsunchangedthe plot of Log P vs. Log V is a straight line, PVn =constanta process in which enthalpy is constant h1 = h2, work= 0, and which is adiabatic, Q=0.Page 30Rev.
0ThermodynamicsCHANGE OF PHASECHANGE OF PHASEThe phase change of materials in a system is very important to thermodynamics.It is possible to design systems to take advantage of the phase changes betweensolid and liquid or between liquid and vapor to enhance the performance of thesystem.EO 1.13DISTINGUISH between intensive and extensiveproperties.EO 1.14DEFINE the following terms:a.Saturationb.Subcooled liquidc.Superheated vapord.Critical Pointe.Triple Pointf.Vapor pressure curveg.Qualityh.Moisture ContentEO 1.15DESCRIBE the processes of sublimation, vaporization,condensation, and fusion.Classification of PropertiesAs discussed earlier in this module, properties are classified as either intensive or extensive.Properties are intensive if independent of the amount of mass present and extensive if a functionof the amount of mass present. Properties such as pressure, temperature, and density areintensive, whereas volume and mass are extensive.
An extensive property may be made intensiveby dividing the particular property by the total mass. Total volume (V), which is an extensiveproperty, can be changed to specific volume, which is an intensive property, by dividing by themass of the system, ν = V/m. Any specific property (specific volume, specific enthalpy, specificentropy), is an intensive property, as indicated in Figure 3.The use of intensive and extensive properties is demonstrated in the following discussion.Consider as a system 1 lbm of water contained in the piston-cylinder arrangement of Figure 4.Suppose that the piston and weight maintain a pressure of 14.7 psia in the cylinder and that theinitial temperature is 60°F, part (a) of Figure 4.
As heat is transferred to the water, thetemperature increases. The specific volume increases slightly, and the pressure remains constant.When the temperature reaches 212°F, additional heat transfer results in a change in phase(boiling), as indicated in part (b).Rev. 0Page 31HT-01CHANGE OF PHASEThermodynamicsThat is, some of the liquid becomes vapor and both the temperature and pressure remain constant,but the specific volume increases considerably. When the last drop of liquid is vaporized, furthertransfer of heat results in an increase in both temperature and specific volume of the vapor, part(c). In this example, temperature and pressure are intensive, and therefore do not depend uponthe amount of mass present. By examining the specific volume (an intensive property) of thewater in the piston instead of the volume (an extensive property), we can examine how anyportion of the water in the piston changes.
Volume by itself tells us nothing about the water inthe piston. However, by knowing the specific volume we can tell if the water is a liquid orsteam.It is customary to define some intensive propertiesassociated with extensive properties. For example, thevolume per unit mass is called the specific volume,v≡VMand the internal energy per unit mass is called thespecific internal energy.u≡UMIntensive properties are useful because they can betabulated or graphed without reference to the amountof material under study.Figure 3 Intensive PropertiesHT-01Page 32Rev.
0ThermodynamicsCHANGE OF PHASEFigure 4Piston-cylinder ArrangementSaturationThe term saturation defines a condition in whicha mixture of vapor and liquid can exist togetherat a given temperature and pressure.Thetemperature at which vaporization (boiling) startsto occur for a given pressure is called thesaturation temperature or boiling point. Thepressure at which vaporization (boiling) starts tooccur for a given temperature is called thesaturation pressure. For water at 212°F, thesaturation pressure is 14.7 psia and, for water at14.7 psia, the saturation temperature is 212°F.For a pure substance there is a definiterelationship between saturation pressure andsaturation temperature.
The higher the pressure,the higher the saturation temperature. Thegraphical representation of this relationshipFigure 5 Vapor Pressure Curvebetween temperature and pressure at saturatedconditions is called the vapor pressure curve. Atypical vapor pressure curve is shown in Figure 5. The vapor/liquid mixture is at saturation whenthe conditions of pressure and temperature fall on the curve.Saturated and Subcooled LiquidsIf a substance exists as a liquid at the saturation temperature and pressure, it is called a saturatedliquid.Rev.
0Page 33HT-01CHANGE OF PHASEThermodynamicsIf the temperature of the liquid is lower than the saturation temperature for the existing pressure,it is called either a subcooled liquid (implying that the temperature is lower than the saturationtemperature for the given pressure) or a compressed liquid (implying that the pressure is greaterthan the saturation pressure for the given temperature). Both terms have the same meaning, soeither term may be used.QualityWhen a substance exists as part liquid and part vapor at saturation conditions, its quality (x) isdefined as the ratio of the mass of the vapor to the total mass of both vapor and liquid. Thus,if the mass of vapor is 0.2 lbm and the mass of the liquid is 0.8 lbm, the quality is 0.2 or 20%.Quality is an intensive property.
Quality has meaning when the substance is in a saturated stateonly, at saturation pressure and temperature. The area under the bell-shaped curve on figure 6shows the region in which quality is important.xmvapor(mliquid(1-20)mvapor)Figure 6 T-V Diagram Showing the Saturation RegionHT-01Page 34Rev. 0ThermodynamicsCHANGE OF PHASEMoisture ContentThe moisture content of a substance is the opposite of its quality. Moisture (M) is defined asthe ratio of the mass of the liquid to the total mass of both liquid and vapor. The moisture ofthe mixture in the previous paragraph would be 0.8 or 80%.
The following equations show howto calculate the moisture of a mixture and the relationship between quality and moisture.mliquidM(1-21)(mliquid mvapor)M=1-xSaturated and Superheated VaporsIf a substance exists entirely as vapor at saturation temperature, it is called saturated vapor.Sometimes the term dry saturated vapor is used to emphasize that the quality is 100%. Whenthe vapor is at a temperature greater than the saturation temperature, it is said to exist assuperheated vapor. The pressure and temperature of superheated vapor are independentproperties, since the temperature may increase while the pressure remains constant.
Actually, thesubstances we call gases are highly superheated vapors.Constant Pressure Heat AdditionConsider the plot on thetemperature-volume diagram ofFigure 7, viewing theconstant-pressure line thatrepresents the states throughwhich the water of theprevious discussion passes as itis heated from the initial stateof 14.7 psia and 60°F. Letstate A represent the initialstate and state B represent thestart of the saturated liquid line(212°F).
Therefore, line ABrepresents the process in whichthe liquid is heated from theinitial temperature to thesaturation temperature.Figure 7 T-V DiagramRev. 0Page 35HT-01CHANGE OF PHASEThermodynamicsPoint C is the saturated vapor state, and line BC is the constant-temperature process in which thechange of phase from liquid to vapor occurs. Line CD represents the process in which the steamis super-heated at constant pressure. Temperature and volume both increase during the process.Now let the process take place at a constant pressure of 100 psia, beginning from an initialtemperature of 60°F. Point E represents the initial state, the specific volume being slightly lessthan 14.7 psia and 60°F. Vaporization now begins at point F, where the temperature is 327.8°F.Point G is the saturated-vapor state, and line GH is the constant-pressure process in which thesteam is superheated.In a similar manner, a constant pressure of 1000 psia is represented by line IJKL, the saturationtemperature being 544.6°F.Critical PointAt a pressure of 3206.2 psia, represented by line MNO, there is no constant-temperaturevaporization process.
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