The discussion revolves around the discrepancies observed between predicted and actual head loss in a circuit involving a pump and pipes, particularly as the flow rate increases. Participants explore potential factors contributing to this difference, including assumptions about pipe friction, flow resistance, and the effects of flow rate on pressure drop.
Participants express multiple competing views regarding the relationship between flow rate and pressure drop, with no consensus reached on the underlying causes of the observed discrepancies. The discussion remains unresolved, with ongoing questions about the accuracy of measurements and assumptions.
Limitations include the lack of specific experimental details, assumptions about flow conditions, and the potential for measurement errors. The discussion also highlights the complexity of fluid dynamics and the need for precise calculations in varying flow conditions.
russ_watters said:You made an overly conservative assumption about the pipe friction and fitting loss, probably...or an error somewhere.
I would have expected the pressure drop to be proportional to the flow rate. After all, with no flow, there would be no pressure difference. A (correctly) calculated head loss would show this too so the error would also grow as flow increases. The electrical analogue to this would be a wrongly marked resistor and a consequential change in measured V/I slope.physea said:OK but this won't explain the increasing trend of the experimental/theoretical as flow rate increases? It would be a constant difference!
sophiecentaur said:I would have expected the pressure drop to be proportional to the flow rate. After all, with no flow, there would be no pressure difference. A (correctly) calculated head loss would show this too so the error would also grow as flow increases. The electrical analogue to this would be a wrongly marked resistor and a consequential change in measured V/I slope.
As far as I can see from what you wrote, you have a measured resistance to flow and you are comparing this with a calculated resistance. If that's what is happening then there will be a steady ratio between pressure drop and flow. You have to acknowledge that, with no flow, there will be no pressure drop (calculated or measured) so how can there be a constant difference?physea said:Why it would grow as flow increases?
It should neither be constant nor even linear: it should be a square function.physea said:OK but this won't explain the increasing trend of the experimental/theoretical as flow rate increases? It would be a constant difference!
I don't understand why you would think this. Does your car need the same power at any speed?Why it would grow as flow increases?
Pressure drop is proportional to the square of the flow rate.sophiecentaur said:I would have expected the pressure drop to be proportional to the flow rate.
Oh. Is that right for low flow speeds, without turbulence? Is there not a linear region?russ_watters said:Pressure drop is proportional to the square of the flow rate.
Let's see the actual calculation and schematic of the system.physea said:Hello! In an experiment (I don't have the details) the predicted head loss in a circuit of pump and pipes was higher as the flow rate increased, than the actual head loss (pressure differential). Can you tell me please what could result in that? What systematically acting factor created an increase in the difference of head loss between predicted and measured?
Laminar is linear because there is slipping along the wall, like pushing a block across the floor. Look up the Darcy-Weisbach equation.sophiecentaur said:Oh. Is that right for low flow speeds, without turbulence? Is there not a linear region?
OK. Not Totally Wrong but only right for low flow volume. It's a square law for the OP then, which gives even more departure from constant difference.russ_watters said:Laminar is linear because there is slipping along the wall, like pushing a block across the floor. Look up the Darcy-Weisbach equation.
physea said:I was thinking that there is a problem with the Uavg used to calculate Darcy losses.
A rotameter is used to measure flow rate at the beginning of the circuit. The liquid runs through the circuit that has bends, constrictions and other fittings. Would the U obtained from the rotameter be the Uavg? I expect it to be an overestimate.
1) Why would the velocity drop across the circuit? due to energy losses?
2) Why would that create an increasing trend of Darcy vs Experimental losses? Do higher rotameter values create higher Uavg?