Head losses

Head losses
Major Losses
Minor Losses
Definition • Dimensional Analysis • Types • Darcy Weisbech Equation • Major Losses • Minor Losses • Causes Head Losses
3. • Head loss is loss of energy per unit weight. • Head = Energy of Fluid / Weight • Head losses can be – Kinetic Head – Potential Head – Pressure Head 6/10/2015 4Danial Gondal Head Loss
4. • 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 6/10/2015 5
5. • (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. 6/10/2015 6 Dimensional Analysis
6. • 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) 6/10/2015 7 Head Loss
7. • For Inviscid Flow EGL1 - EGL2= 0 • For Viscous Flow EGL1 - EGL2= Hf 6/10/2015 8 Head Loss
8. MAJOR LOSSES IN PIPES
9. •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. 6/10/2015 10 Friction Loss
10. •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. 6/10/2015 11 Friction Loss
11. 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.

1. Danial Zafar Gondal 13-ME-027 Ehtisham Qaiser 13-ME-031 Faizan Shabbir 13-ME-032 Asif Mehboob 13-ME-034 6/8/2015 2 Group Members

2. 6/8/2015 3 Definition Dimensional Analysis Types Darcy Weisbech Equation Major Losses Minor Losses Causes Head Losses

3. Head loss is loss of energy per unit weight. Head = Energy of Fluid / Weight Head losses can be Kinetic Head Potential H ead Pressure Head 6/8/2015 4 Danial Gondal Head Loss

4. 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 6/8/2015 5

5. (P / ρ g) + ( v² / 2g ) + ( z) = constant ( FL -2 F -1 L 3 LT -2 L -1 T 2 ) + (L 2 T -2 L 1 T 2 )+(L) = constant (L) + (L) + (L) = constant As L represent height so it is dimensionally L. 6/8/2015 6 Dimensional Analysis

6. 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 ( P 1 / ρ g) + ( v 1 ² /2g ) + ( z 1 ) = EGL 1 (Energy Grade Line At point 1) ( P 2 / ρ g) + ( v 2 ² /2g ) + ( z 2 ) = EGL 2 (Energy Grade Line At point 2) 6/8/2015 7 Head Loss

7. For Inviscid Flow EGL 1 - EGL 2 = For Viscous Flow EGL 1 - EGL 2 = Hf 6/8/2015 8 Head Loss

8. Major Losses In Pipes

9. 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. 6/8/2015 10 Friction Loss

10. 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. 6/8/2015 11 Friction Loss

11. 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.    6/8/2015 12 Friction Loss

12. For Laminar Flow 6/8/2015 13 Friction Loss For Turbulent Flow

13. Minor Losses In Pipes

14. 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 (h Lm ) . 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.

15. Losses caused by fittings, bends, valves, etc…

16. 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).

17. where , H Lm = minor loss K = minor loss coefficient V = mean flow velocity 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

18. 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

19. 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

20. Analytical expression of K - If the velocity v 1 < 1.2 m/s or 4 ft/s, the K values can be given as : K = [ 1-(A 1 /A 2 ) ]² = [ 1-(D 1 /D 2 )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.

21. Decrease in pipe diameter Note that the loss is related to the velocity in the second (smaller) pipe! Sudden Contraction

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

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

24. Comparison

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

26. 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

27. 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

28. Fluid moves from zero velocity in tank to v². Entrance Losses

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30. 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 6/8/2015 33 Causes

31. 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. 6/8/2015 34 Causes

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