head losses

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• Danial Zafar Gondal 13-ME-027

• Ehtisham Qaiser 13-ME-031

• Faizan Shabbir 13-ME-032

• Asif Mehboob 13-ME-034

Group Members

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• Definition

• Dimensional Analysis

• Types

• Darcy Weisbech Equation

• Major Losses

• Minor Losses

• Causes

Head Losses

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• Head loss is loss of energy per unit

weight.

• Head = Energy of Fluid / Weight

• Head losses can be

– Kinetic Head

– Potential Head

– Pressure Head

Head Loss

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• Kinetic Head

– K.H. = kinetic energy / Weight = v² /2g

• Potential Head

– P.H = Potential Energy / Weight = mgz /mg = z

• Pressure Head

– P.H = P/ ρ g

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• (P/ ρ g) + (v² /2g ) + (z) = constant

• (FL-2F-1L3LT-2L-1T2) + (L2T-2L1T2)+(L) = constant

• (L) + (L) + (L) = constant• As L represent height so it is dimensionally L.

Dimensional Analysis

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• However the equation (P/ ρ g) + (v² /2g ) + (z) = constant

Is valid for Bernoulli's Inviscid flow case. As we are studying viscous flow so

(P1/ ρ g) + (v1² /2g ) + (z1) = EGL1(Energy Grade Line At point 1)

(P2/ ρ g) + (v2² /2g ) + (z2) = EGL2(Energy Grade Line At point 2)

Head Loss

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• For Inviscid Flow

EGL1 - EGL2= 0

• For Viscous Flow

EGL1 - EGL2= Hf

Head Loss

MAJOR LOSSES IN

PIPES

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•Friction loss is the loss of energy or “head” that occurs in pipe flow due to viscous effects generated by the surface of the pipe.• Friction Loss is considered as a "major loss" •In mechanical systems such as internal combustion engines, it refers to the power lost overcoming the friction between two moving surfaces.•This energy drop is dependent on the wall shear stress (τ) between the fluid and pipe surface.

Friction Loss

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•The shear stress of a flow is also dependent on whether the flow is turbulent or laminar. •For turbulent flow, the pressure drop is dependent on the roughness of the surface. •In laminar flow, the roughness effects of the wall are negligible because, in turbulent flow, a thin viscous layer is formed near the pipe surface that causes a loss in energy, while in laminar flow, this viscous layer is non-existent.

Friction Loss

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Frictional head losses are losses due to shear stress on the pipe walls. The general equation for head loss due to friction is the Darcy-Weisbach equation, which is where f = Darcy-Weisbach friction factor, L = length of pipe, D = pipe diameter, and V = cross sectional average flow velocity. This equation is valid for pipes of any diameter and for both laminar and turbulent flows.

Friction Loss

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For Laminar Flow

Friction Loss

For Turbulent Flow

MINOR LOSSES IN

PIPES

• In addition to head loss due to friction, there are always other

head losses due to pipe expansions and contractions, bends,

valves, and other pipe fittings. These losses are usually known

as minor losses (hLm).

• In case of a long pipeline, the minor losses maybe negligible

compared to the friction losses, however, in the case of short

pipelines, their contribution may be significant.

• Losses caused by fittings, bends, valves, etc…

• Minor in comparison to friction losses which are considered major.

• Losses are proportional to – velocity of flow, geometry of device.

HL = K (v² /2g)

• The value of K is typically provided for various devices.

• Energy lost – units – m or ft• K - loss factor - has no units (dimensionless).

where ,

HLm = minor loss

K = minor loss coefficient

V = mean flow velocity

g

VKhLm 2

2

Type K

Exit (pipe to tank) 1.0

Entrance (tank to pipe) 0.5

90 elbow 0.9

45 elbow 0.4

T-junction 1.8

Gate valve 0.25 - 25

Typical K values

Minor Losses Are Due to

• As fluid flows from a smaller pipe into a larger pipe through

sudden enlargement, its velocity abruptly decreases; causing

turbulence that generates an energy loss.

• The amount of turbulence, and therefore the amount of

energy, is dependent on the ratio of the sizes of the two pipes.

Sudden Enlargement

• Energy lost is because of turbulence. Amount of turbulence

depends on the differences in pipe diameters.

HL = K (v² /2g)

• The values of K have been experimentally determined and

provided in Figure and Table

• Analytical expression of K - If the velocity v1 < 1.2 m/s or 4

ft/s, the K values can be given as :

K = [ 1-(A1/A2) ]² = [ 1-(D1/D2)² ]²

• As previous table consist of practical values therefore

theoretical formulas are different for different values &

above mentioned formula is applicable at 1.2 m/s velocity.

• Decrease in pipe diameter

Note that the loss is related to the velocity in the second (smaller) pipe!

Sudden Contraction

• The section at which the flow is the narrowest – Vena Contracta• At vena contracta, the velocity is maximum.

Energy losses for sudden contraction are less than those for sudden enlargement.

Comparison

• Again a gradual contraction will lower the energy loss (as opposed to sudden contraction). θ is called the cone angle.

Gradual Contraction

• Case of where pipe enters a tank – a very large enlargement.

• The tank water is assumed to be stationery, that is, the

velocity is zero.

• Therefore all kinetic energy in pipe is dissipated, therefore K

=1.0

Exit Loss

• If the enlargement is gradual (as opposed to our previous

case) – the energy losses are less.

• The loss again depends on the angle of enlargement.

Gradual Enlargement

Fluid moves from zero velocity in tank to v².

Entrance Losses

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Head loss has several causes, including:

• Losses depend on the conditions of flow and the physical

properties of the system.

• Movement of fluid molecules against each other

• Movement of fluid molecules against the inside surface of a

pipe or the like, particularly if the inside surface is rough,

textured, or otherwise not smooth

• Bends and other sharp turns in piping

Causes

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In pipe flows the losses due to friction are of two kinds:

• Skin-friction – This is due to the roughness of the inner part of the

pipe where the fluid comes in contact with the pipe material

• Form-friction – It is due to obstructions present in the line of flow

perhaps a bend, control valve, or anything that changes the course of motion of the flowing fluid.

Causes

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THANK YOU…

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