Fluids- More Pipe Losses Flashcards
Major losses
A pressure (or head) loss due to frictional losses at the pipe walls
Minor losses
Due to an interruption of the smooth flow of fluid
Examples of causes of minor losses
Valves, bends, elbows, inlets, exits, enlargements and contractions. Basically pipe components.
What to do when finding a flow rate down a pipe
Can’t calculate Re so assume it is 10^5. Read off from Moody diagram in normal way to get f. Rearrange equation for pressure loss (given) to find V. Based on this V calculate the Re it suggests. If the two values of are are within 10%of each other, the assumption was right and go on to calculate volumetric flow rate. If not, second Re calculated becomes new assumption and do whole thing again.
Why do pipe components cause minor losses?
They interrupt the smooth flow of fluid and cause flow separation and induce mixing
What is the symbol for minor loss coefficient?
KL
Where L is subscript
It is a function of geometry
Formula for minor loss
Ploss=0.5KLρV^2
Formula for KL for sudden expansion
KL=(1-A1/A2)^2
Where A2>A1
The velocity used in the minor loss formula is the initial velocity, V1
What is the angle φ for a diffuser?
The inclusive angle between the walls (<180)
Equal to twice the angle one wall makes with the horizontal
What does KL depend on for a diffuser?
The angle,φ. The area ratio d/D where D>d.
Greater φ means more pressure loss so greater KL
Greater area ratio lower KL
What happens during sudden contraction of a fluid?
The flow leaves the large pipe at an angle and doesn’t fill the small pipe completely. This narrowing of the flow path is known as the vena contracta.
What does KL depend on for sudden contraction?
Area ratio d2/d1
As this ratio increases, value of KL decreases
When ratio is about 0, KL=0.5
When ratio is about 1, KL=0
What affects KL for a bend?
The angle the bend goes through and the sharpness of the bend. Value for KL increases with both
How does KL vary for valves open and partially closed?
Large head loss in partially closed valve due to irreversible deceleration, flow separation and mixing of high-velocity fluid coming from the narrow valve passage. However the head loss through a fully open valve is quite small.
How do KL values for different components in series combine when diameter of pipe remains constant?
They add up to get overall KL value
