EES example Ch 7

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27 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

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EES example
Ch

7

7
-
42 and 7
-
43 “
Thermodynamics, an
Engineering Approach” 7
th

edition,
Cengal

and Boles

and

EES
http://fchart.com



7
-
42 A heavily insulated piston
-
cylinder
device contains 0.02 m^3 of steam at 300kPa
and 200C. Steam is now compressed in a
reversible manner to a pressure of 1.2
MPa
.
Determine the work done on the steam
during this process.


H
2
O

300 kPa

200

C

Assumptions


1
The kinetic and potential energy changes are
negligible.



2
The cylinder is well
-
insulated and thus heat
transfer is negligible.



3
The thermal energy stored in the cylinder
itself is negligible.


4
The process is stated to be reversible.


Start a new EES file


File
-
> New


Give the file name
Cengal

Prob

7_42


Set Preferences


Options
-
> Preferences
-
>


Autosave

every 5 minutes


set units automatically when possible

Analysis


If the process is both reversible and Adiabatic,
then the entropy is constant. S1=S2



Enter

Knowns

into

EES


"
Knowns
:"

P_1 = 300 [
kPa
]

T_1 = 200 [C]

V_sys

= 0.02 [m^3]

P_2
= 1200 [
kPa
]

S_2=S_1


Analysis


We take the contents of the cylinder as the
system. This is a closed system since no mass
enters or leaves. The energy balance for this
adiabatic closed system can be expressed as










Type into EES


W_b_in
=
m_sys
*(u_2
-
u_1)


To determine the work, we need:


m, u1, u2



Mass = volume*density


m_sys
=
V_sys
*
rho_1

Thermodynamic
Properites


Within EES we can use the command



Options
-
> function info


Fill in the table


Select paste


u[1]=
IntEnergy
(
Steam_IAPWS,T
=T[1],P=P[1]
)



Since we are using the same fluid again and
again we can define a string variable


String Variables


EES supports both numerical and string
variables. String variable are identified by a $
as
the last
character in the variable name, e.g.,
Fluid$. String constants are enclosed in single
quote marks
.


Define the string Fluid$ as
'
Steam_IAPWS


Fluid
$='
Steam_IAPWS



Steam_IAPWS



The note on
pg

187 of the EES manual states


Steam_IAPWS

implements high accuracy
thermodynamic properties of water substance
with the
1995 Formulation for the
Thermodynamic Properties of Ordinary Water
Substance
for General
and Scientific Use,
issued by The International Association for the
Properties
of Water
and Steam (IAPWS). This

Thermodynamic Properties


In EES type

u_1
= INTENERGY(
Fluid$,P
=P_1,T=T_1)

rho_1 = density(
Fluid$,P
=P_1,T=T_1)

s_1 = entropy(Fluid$,P=P_1,T=T_1)

u_2 = INTENERGY(Fluid$,P=P_2,s=s_2)

T_2_isen = temperature(Fluid$,P=P_2,s=s_2)

Window
-
> Formatted Equations

Calculate
-
> Solve

Compare to hand calculation solution



“Thermodynamics, an Engineering Approach”
7
th

edition,
Cengal

and Boles


7
-
43 Reconsider Prob. 7
-
42. Evaluate and
plot the work done on the stream as a
function of final pressure as the pressure
varies from 300
kPa

to 1.2
Mpa


Start a new EES file


Save 7_42


File
-
> Save


Rename 7_42 as 7_42


File
-
> Save As


Type in 7_43 as the new file name

Create a table


Since we are
calulating

W as a function of P2,
we need to create a table with P2 and W as
the variables.


First, comment out the original P2 definition

"P_2 = 1200 [
kPa
]”


Next set the units for P2

Options
-
> Variable Info

Create a Table

Table
-
> New Parametric Table

Plot
-
> new plot
-
> X
-
Y Plot

File
-
> Save