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.In piping systems, it refers to the power lost overcoming the friction between the fluid and the walls of the pipe and between fluids layer. The friction between the adjacent fluid layer causes a velocity gradient across the cross-section of the fluid in the pipe, where the highs velocity is at the centre.
Friction loss has several causes, including:
- Frictional 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, kinks, and other sharp turns in hose or piping
This energy drop is dependent on the wall shear stress (τ) between the fluid and pipe surface. 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, while in laminar flow, the roughness effects of the wall are negligible. This is due to the fact that in turbulent flow, a thin viscous layer is formed near the pipe surface which causes a loss in energy, while in laminar flow, this viscous layer is non-existent
Bernoulli's equation states that the total head h along a streamline (parameterized by x) remains constant. This means that velocity head can be converted into gravity head and/or pressure head (or vice-versa), such that the total head h stays constant. No energy is lost in such a flow.
where:
- h = Head Loss due to friction, given in units of length
- f = friction factor (Darcy-Weisbach friction coefficient)
- L = Pipe Length
- D = Pipe Diameter
- V = Flow velocity
- g = Gravitational Constant
| where D is the pipe diameter. As the flow moves down the pipe, viscous head slowly accumulates taking available head away from the pressure, gravity, and velocity heads. Still, the total head h (or energy) remains constant. For pipe flow, we assume that the pipe diameter D stays constant. By continuity, we then know that the fluid velocity V stays constant along the pipe. With D and V constant we can integrate the viscous head equation and solve for the pressure at Point B, |
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| where L is the pipe length between points A and B, and Dz is the change in pipe elevation (zB - zA). Note that Dz will be negative if the pipe at B is lower than at A. The viscous head term is scaled by the pipe friction factor f. In general, f depends on the Reynolds Number R of the pipe flow, and the relative roughness e/D of the pipe wall, |
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| The roughness measure e is the average size of the bumps on the pipe wall. The relative roughness e/D is therefore the size of the bumps compared to the diameter of the pipe. For commercial pipes this is usually a very small number. Note that perfectly smooth pipes would have a roughness of zero. For laminar flow (R < 2000 in pipes), f can be deduced analytically. The answer is, |
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| For turbulent flow (R > 3000 in pipes), f is determined from experimental curve fits. One such fit is provided by Colebrook, |
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sources: en.wikipedia.org, .efunda.com
loss of energy in pipes
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