Chapter 2 Lecture 3 - Energy Equation-2

7/25/2019 Chapter 2 Lecture 3 - Energy Equation-2

1/21

Energy Equation


2/21

Energy EquationChapter 2 Lecture 3 2

Mechanical Energy?

Forms of energy that can be converted toMECHANICAL !"# com$letely and

directly by mechanical device%s&

#inetic energy %#E& ' (otential energy %(E&

are forms of Mechanical energy %emech&

)hermal energy is not in the form of emech


3/21


Mechanical Energy? %Contd*&

(ressure can be associated +ith emech

( , (-A , N-m., %N*m&-m/, 0-m/

It1s energy $er unit volume2

3ystems used to trans$ort fluid maye4erted or e4tracted energy


4/21


Flo+ or5

It1s the +or5 effect $roduced due to$ressure acting over the distance

3tated in the amount of $er unit mass %(-6&Convenient to be e4$ressed in fluid

$ro$erties terms as $art of the fluid energy

It1s called as FL! ENE"78


5/21


For a flo+ing fluid9 emechcan be +ritten:

emech, eflo+ ; #E ; (E

, %(-6& ; %


6/21


Flo+ or5 ' Flo+ Energy %Contd*&

If emech> , +or5 is su$$lied to the fluidIf emech@ , +or5 is e4tracted from the fluid

If emech, , flo+ $ro$erties constant

Consider:

%Cengel ' Cimbala9 .&


7/21Energy EquationChapter 2 Lecture 3 7

Flo+ or5 ' Flo+ Energy %Contd*&

or5 generated $er unit mass is same for

to$ and bottom generation i*e*:

emech

to$

, emech

bottom

From $revious Figure also:



Energy )ransfer and Efficiency

emechis transferred by rotating devices such

as $um$ and turbine

(um$ , )ransfer emechfrom shaft to fluid

)urbine , )ransfer emechfrom fluid to shaftEfficiency of emechconversion is Bmech



Mechanical Efficiency

Mechanical Efficiency is defined as:

, ,

, ,

1mech out mech loss

mech

mech in mech in

E E

E E

= =

here:

Emech9out, Emech9in Emech9loss



(um$ ' )urbine Efficiency

In fluid system9 attention is given to increasethe $ressure9 velocity and elevation

)his is done by su$$lying mechanical energy

to the fluid by $um$ of fan

Also9 by reversing the $rocess to reduce the

$ressure9 velocity9 and elevation of the fluid

)his is done by e4tracting mechanical energy

from the fluid by turbine



(um$ ' )urbine Efficiency %Contd*&

(um$ efficiency is defined as:

here:



hile turbine efficiency is defined as:

here:

(um$ ' )urbine Efficiency %Contd*&



Motor ' 7enerator Efficiency

3hould not be confused +ith BmechMotor Efficiency:

7enerator Efficiency:

M t ' 7 t Effi i



Motor ' 7enerator Efficiency

%Contd*&

Electrical

(o+er

Flo+ing

Fluid

Motor

%(um$&

7enerator%)urbine&Flo+ingFluid Electrical(o+er

Bmotor

Bturbine

B$um$

Bgenerator



Combined Efficiency

"atio of the increase in the mechanical energy

of the fluid to the electrical $o+er consum$tion

of the motor

(um$DMotor 3ystem

)urbineD7enerator 3ystem

"atio of the decrease in the mechanical energy

of the fluid to the electrical $o+er generation of

the generator



Combined Efficiency %Contd*&

Mathematically:

|| fluidmech,

outelect,

outmech,inmech,

outelect,

generaorturbinegen-turbineE

W

EE

W

=

==

inelect,

pump

inelect,

fluidmech,

inelect,

inmech,outmech,

motorpumpmotor-pump W

W

W

E

W

EE

=

=

==

For turbineDgenerator

For $um$Dmotor



Energy 3im$lification

3im$lification in term of emechcan be +ritten as:

emechin emechout, emechsystem; emechloss

For steady o$eration9 energy balance turns to be:

emechin , emechout; emechloss

)hat1s steady flo+ analysis2



E4am$les ' )utorials

Consider a river flo+ing to+ard a la5e atan average velocity of / m-s at a rate of

m/-s at a location G m above the

la5e surface* etermine the totalmechanical energy of the river +ater $er

unit mass and the $o+er generation

$otential of the entire river at that

location



E4am$les ' )utorials %Contd*&

Electric $o+er is to be generated by installinga hydraulic turbineDgenerator at a site m

belo+ the free surface of a large +ater

reservoir that can su$$ly +ater at a rate of

J 5g-s steadily* If the mechanical $o+er

out$ut of the turbine is K 5 and the

electric $o+er generation is 59

determine the turbine efficiency and thecombine turbineDgenerator efficiency of this

$lant* Neglect losses in the $i$es



E4am$les ' )utorials %Contd*&

At a certain location9 +ind is blo+ing steadily

at J. m-s* etermine the mechanical energy

of air $er unit mass and the $o+er generation

$otential of a +ind turbine +ith a m

diameter blades at that location* Also

determine the actual electric $o+er

generation assuming an overall efficiency of/ $ercent* )a5e air density to be J*. 5g-m/


21/21

Next Lecture?

ernoulli1s

Equation

Chapter 2 Lecture 3 - Energy Equation-2

Documents

Transcript of Chapter 2 Lecture 3 - Energy Equation-2