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Civil Engineering Questions
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Ali Hamza
Ali Hamza
Mechanical
28 Jan 2019

Energy of a Liquid in Motion :

ENERGY OF A LIQUID IN MOTION

KINETIC ENERGY

A body of mass m when moving at a velocity v possesses a kinetic energy, K.E = Β½ mv2

Kinetic head = 𝐾.πΈπ‘€π‘’π‘–π‘”β„Žπ‘‘=12π‘šπ‘£2𝛾 (π‘‰π‘œπ‘™)=𝑣22𝑔

units (metres or ft)in a flow of real fluid the velocities of different particles will usually not be the same, soit is necessary to integrate all portions of the stream to obtain true value of kineticenergy. It is convenient to express the true value in terms of mean velocity v and afactor Ξ±. Hence,π‘‘π‘Ÿπ‘’π‘’ 𝐾. πΈπ‘€π‘’π‘–π‘”β„Žπ‘‘= 𝛼𝑣22𝑔

First law of thermodynamics: Energy can neither be created nor destroyed.

POTENTIAL ENERGY

The potential energy of a fluid particle depends on its elevation above any arbitrary datum plane. We are usually interested only in differences of elevation, and thereforeΒ the location of the datum plane is determined solely by considerations of convenience.Β 

A fluid particle of weight W situated a distance z above datum possesses a potential energy of Wz. Thus its potential energy per unit weight is z.Β 

Potential head = 𝑃.𝐸 π‘Šπ‘’π‘–π‘”β„Žπ‘‘ =Β Β π‘šπ‘”π‘§Β π‘šπ‘”Β = zΒ 

PRESSURE ENERGY

It is energy possessed by a liquid particle due to

its pressure. If a liquid particle is under pressure

P, then pressure energy head is P/Ξ³

Pressure head = 𝑃

𝛾

z + P/Ξ³ is static head or piezometric head

v

2/2g velocity head

z + P/Ξ³ + v2/2g is total energy head given

by pitot tube

Head is energy per unit weight.