WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition...

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998

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ÇengelÇengelBolesBoles

Third EditionThird Edition

16CHAPTERCHAPTER

Thermodynamicsof High-Speed

Gas Flow

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(fig.16-1)

Steady Flow of a Liquid through an Adiabatic DuctSteady Flow of a Liquid through an Adiabatic Duct

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Three States of a Fluid on an h-s DiagramThree States of a Fluid on an h-s Diagram

(Fig.16-3)

The actual state, actual stagnation state, and isentropic stagnation state of a fluid on an h-s diagram

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Completely Arresting the Flow of an Ideal Gas can Raise its Temperature Completely Arresting the Flow of an Ideal Gas can Raise its Temperature

(Fig.16-5)

The temperature of an ideal gas flowing at a velocity V rises by V2/(2CP) when it is brought to a complete stop

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Propagation of a Small Pressure Wave Along a DuctPropagation of a Small Pressure Wave Along a Duct

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Control Volume Moving With the Small Pressure Wave Along a DuctControl Volume Moving With the Small Pressure Wave Along a Duct

(Fig. 16-8)

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The Velocity of Sound Changes With TemperatureThe Velocity of Sound Changes With Temperature

(Fig. 16-9)

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The Mach Number Can Vary With Different Temperatures Even With Equivalent VelocitiesThe Mach Number Can Vary With Different Temperatures Even With Equivalent Velocities

(Fig. 16-10)

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Throat: The Smallest Flow Area of a NozzleThroat: The Smallest Flow Area of a Nozzle

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Supersonic Velocities Cannot be Obtained by Attaching a Converging SectionSupersonic Velocities Cannot be Obtained by Attaching a Converging Section

We cannot obtain supersonic velocities by attaching a a converging section to a converging nozzle. Doing so will only move the sonic cross

section farther downstream

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Variation of Flow Properties in Subsonic and Supersonic Nozzles and DiffusersVariation of Flow Properties in Subsonic and Supersonic Nozzles and Diffusers

(Fig. 16-17)

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Effect of Back Pressure on the Pressure Distribution Along a Converging NozzleEffect of Back Pressure on the Pressure Distribution Along a Converging Nozzle

(Fig. 16-20)

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Effect of Back Pressure of a Converging Nozzle on Mass Flow Rate and Exit PressureEffect of Back Pressure of a Converging Nozzle on Mass Flow Rate and Exit Pressure

(Fig. 16-21)

The effect of back pressure Pb on the mass flow rate m and

the exist pressure Pe of a converging nozzle

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Variation of the Mass Flow Rate Through a Nozzle with Inlet Stagnation PropertiesVariation of the Mass Flow Rate Through a Nozzle with Inlet Stagnation Properties

(Fig. 16-22)

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The Effect of Back Pressure on the Flow Through a Converging-Diverging NozzleThe Effect of Back Pressure on the Flow Through a Converging-Diverging Nozzle

(Fig.16-26)

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The h-s Diagram for Flow Across a Normal ShockThe h-s Diagram for Flow Across a Normal Shock

(Fig. 16-29)

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Entropy Change Across the Normal ShockEntropy Change Across the Normal Shock

(Fig. 16-31)

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Isentropic and Actual (Irreversible) Flow in a Nozzle Isentropic and Actual (Irreversible) Flow in a Nozzle

(Fig. 16-33)

Isentropic and actual flow in a nozzle between the same inlet state and the exit pressure

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Schematic and h-s Diagram for the Definition of the Diffuser EfficiencySchematic and h-s Diagram for the Definition of the Diffuser Efficiency

(Fig. 16-35)

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The h-s Diagram for the Isentropic Expansion of Steam in a NozzleThe h-s Diagram for the Isentropic Expansion of Steam in a Nozzle

(Fig. 16-37)

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Schematic and h-s Diagram for Example 16-14Schematic and h-s Diagram for Example 16-14

(Fig.16-38)

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Chapter SummaryChapter Summary

• In this chapter the thermodynamic aspects of high-speed fluid flow are examined. For high-speed flows, it is convenient to combine the enthalpy and the kinetic energy of the fluid into a single term called stagnation (or total) enthalpy h0, defined as

(kJ/kg)

The properties of a fluid at the stagnation state are called stagnation properties and are indicated by the subscript zero.

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• The stagnation temperature of an ideal gas with constant specific heats is

which represents the temperature an ideal gas will attain when it is brought to rest adiabatically.

Chapter SummaryChapter Summary16-22

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• The (isentropic) stagnation properties of an ideal gas are related to the static properties of the fluid by

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• When stagnation enthalpies are used, the conservation of energy equation for a single-stream, steady-flow device can be expressed as

where h01 and h02 are the stagnation enthalpies at states 1 and 2, respectively.

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• The velocity at which an infinitesimally small pressure wave travels through a medium is the velocity of sound (or the sonic velocity). It is expressed as

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• For an ideal gas the velocity of sound becomes

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• The Mach number is the ratio of the actual velocity of the fluid to the velocity of sound at the same state:

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• The flow is called sonic when M = 1, subsonic when M < 1, supersonic when M > 1, hypersonic when M >> 1, and transonic when M 1.=~

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• The nozzles whose flow area decreases in the flow direction are called converging nozzles. Nozzles whose flow area first decreases and then increases are called converging-diverging nozzles. The location of the smallest flow area of a nozzle is called the throat.

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• The highest velocity to which a fluid can be accelerated in a convergent nozzle is the sonic velocity. Accelerating a fluid to supersonic velocities is only possible in converging-diverging nozzles. In all supersonic converging-diverging nozzles, the flow velocity at the throat is the velocity of sound.

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• The ratios of the stagnation to static properties for ideal gases with con-stant specific heats can be expressed in terms of the Mach number as

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• When M = 1, the resulting static-to-stagnation property ratios for the temperature, pressure, and density are called critical ratios and are denoted by the superscript asterisk:

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• The pressure outside the exit plane of a nozzle is called the back pressure. For all back pressures lower than P*, the pressure at the exit plane of the converging nozzle is equal to P*, the Mach number at the exit plane is unity, and the mass flow rate is the maximum (or choked) flow rate.

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• Under steady-flow conditions, the mass flow rate through the nozzle is constant and can be expressed as

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• The variation of flow area A through the nozzle relative to the throat area A* for the same mass flow rate and stagnation properties of a particular ideal gas is

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• The parameter M* is defined as the ratio of the local velocity to the velocity of sound at the throat (M = 1):

It can also be expressed as

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• In some range of back pressure, the fluid that achieved a sonic velocity at the throat of a converging-diverging nozzle and is accelerating to supersonic velocities in the diverging section experiences a normal shock, which causes a sudden rise in pressure and temperature and a sudden drop in velocity to subsonic levels. Flow through the shock is highly irreversible, and thus it cannot be approximated as isentropic.

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The properties of an ideal gas with constant specific heats before (subscript x) and after (subscript y) a shock are related by

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• The entropy change across the shock is obtained by applying the entropy-change equation for an ideal gas across the shock:

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• The deviation of actual nozzles from isentropic ones is expressed in terms of the nozzle efficiency N, nozzle velocity coefficient CV, and the coefficient of discharge CD, which are defined as

where h01 is the stagnation enthalpy of the fluid at the nozzle inlet, h2 is the actual enthalpy at the nozzle exit, and h2s is the exit enthalpy under isentropic conditions for the same exit pressure.

Actual kinetic energy at nozzle exit

Kinetic energy at nozzle exit for isentropic flowfrom the same inlet state to the same exit pressure

Actual velocity at nozzle exit

Velocity at nozzle exit for isentropic flowfrom the same inlet state to the same exit pressure

Actual mass flow rate through nozzle

Mass flow rate through nozzle for isentropic flowfrom the same inlet state to the same exit pressure

Chapter SummaryChapter Summary16-40

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• The performance of a diffuser is expressed in terms of the diffuser efficiency D the pressure recovery factor FP, and the pressure rise coefficient CPR. They are defined as

Actual stagnation pressure at diffuser exit

Isentropic stagnation pressure

Actual pressure rise

Isentropic pressure rise

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• Steam often deviates considerably from ideal-gas behavior, and no simple property relations are available for it. Thus it is often necessary to use steam tables instead of ideal-gas relations. The critical-pressure ratio of steam is often taken to be 0.546, which corresponds to a specific heat ratio of k = 1.3 for superheated steam.

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• At high velocities, steam does not start condensing when it encounters the saturation line, and it exists as a supersaturated substance. Supersaturation states are nonequilibrium (or metastable) states, and care should be exercised in dealing with them.

WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition...

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