MOOC Thermo_2014_Unit 2-1.pdf

8/10/2019 MOOC Thermo_2014_Unit 2-1.pdf

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Unit 2 Segment 1


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Work energy transfer across the system boundary

Work transfer is not a system property.

Work transfer depends on the process path.

There are many types or forms of work transfer.A. Expansion and compression work the work to raise or lower a piston


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Work energy transfer across the system boundary

Work transfer is not a system property.

Work transfer depends on the process path.

There are many types or forms of work transfer.A. Expansion and compression work the work to raise or lower a piston

What does a constant pressure compression process look like on a pressurevolume diagram?


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Shaft work, extension work, electrical work, film work, etc.


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Lets review - sign conventions for energy transfer:

Work outof a system is positive, W>0, and work is

bythe system.Work intoa system is negative, W


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If a turbine generates power via shaft work, what is the sthe work done by the system?


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Unit 2 Segment 2


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If a turbine generates power via shaft work, what isthe sign for the work done by the system?

Work done by the system is work out of the systemand should have a positive value (W>0).


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Let do an example of work transfer analysis with numbers.

We have air in a piston that is undergoes 2 processes in series.

In the first step, the air is compressed such a way that the process is described apolytropic, where PVn= constant. Th

of the air is 100 kPa, 0.04 m3/kg. The final specific volume at the end of the first process is 0.02 m3/kg. n = 1.3

In the second step, the air is subject to a constant pressure process where the final specific volume is equal to the initia

a) sketch the processes on a P-v diagram and label the states, 1, 2 and 3.

b)

determine the specific work (work per unit mass of air) for each step and the overall process.


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b)



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b)



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What would the P-v diagram look like if stepone was a polytropic expansionprocess?


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Unit 2 Segment 3


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What would the P-v diagram look like if stepone was a polytropic expansionprocess?


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The First Law of Thermodynamics relates changes in energy states heat transfer.

The First Law of Thermodynamics defines the conservation of energy

The conservation of mass (COM) states that mass can never be destro

The conservation of energy (COE) states that energy can never be des

E2-E1= Q-W


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E2-E1= Q-Wthe details of COE

an adiabatic process is one where Q = 0

E is the total energy of the system

E = KE + PE + U

U = the internal energy of the system

u = the internal energy per mass


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QQ is the energy transfer due to heat transfer

Q is NOT a propertyQ depends on the process path

Q>0 is heat transfer into the system

Q


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Whats wrong with this statement:!"Q = Q2- Q1?


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Unit 2 Segment 4


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Whats wrong with this statement:!"Q = Q2- Q1?

Heat transfer is NOT a system property! A system does not posseswork. The correct notation is!"Q = Q = 1Q2


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There are three types of heat transfer: conduction, conveand radiation.

Heat transfer is a discipline in an of itself.


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COE can be memorized using the simpleexpression:

In Out = Changenet heat transfer in net work transfer out =

change in system energy (for some timeinterval)

Can also write COE on a rate basis.


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So we have some ideas on how to quantify Q and W, what abMore specifically, U.

We need to know the stateof the system (at equilibrium), wh

described by the system properties.It was determined a long time ago, that you need 2 independ

intensive thermodynamic properties to completely describof a simple compressible substance.!whats that???


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What are the 3 phases of matter?


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Unit 2 Segment 5


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What are the 3 phases of matter?solid, liquid and gas of course. The phase of a substance can

determined or identified by the state of the substance.


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Lots of definitions now.

saturation state - state which begins or ends a phase change

critical point defined by the critical temperature and is the

temperature where liquid and vapor phases can co-exist atequilibrium.


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3D phase diagrams


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2D phase diagrams


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Unit 2 Segment 6


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2D phase diagrams


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2D phase diagrams: isobars and isotherms

Documents

MOOC Thermo_2014_Unit 2-1.pdf