Transition from pipe flow to open channel flow

In summary: I assume you're asking about a liquid-liquid setup. If the top is open to the atmosphere then the liquid will evaporate and the pressure will drop in the system. Over time the equilibrium will be reached and the system will be in equilibrium.
  • #1
heb26
1
0
I'm working on a fluid flow problem that doesn't seem very complex but the solution is eluding me at the moment. Given a length of smooth tubing of known ID and roughness with water running through it, is it possible to predict at what velocity the flow will transition from "pipe" flow (fully wetted perimeter) to "open channel" flow (less than fully wetted perimeter with some open surface)?

Any assistance is appreciated.
 
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  • #2
Hi heb,
... is it possible to predict at what velocity the flow will transition from "pipe" flow (fully wetted perimeter) to "open channel" flow (less than fully wetted perimeter with some open surface)?
I assume you're referring to a gas that gets into a pipe otherwise filled with liquid as opposed to two phase flow. If that's correct, then it's not a function of velocity. A pipe without a source of air can't entrain air regardless of velocity. The air has to get 'sucked in' at some point.

If you're talking about two phase flow, then the thermodynamics of the pipe is responsible for converting liquid to gas, not velocity.
 
  • #3
I took the OP's question to mean that the flowrate has dropped sufficiently to not have enough "stuff" to keep the pipe full.
 
  • #4
Related problem

I have a closely related question: I have a horizontal pipe open at one end filled with a high density fluid. The open end is connected to a large reservoir containing a lower density fluid. Both fluids are at the same pressure. How can I calculate the time it takes for the high density fluid to run out and be replaced by the lower density fluid?
 
  • #5
ariespc said:
I have a closely related question: I have a horizontal pipe open at one end filled with a high density fluid. The open end is connected to a large reservoir containing a lower density fluid. Both fluids are at the same pressure. How can I calculate the time it takes for the high density fluid to run out and be replaced by the lower density fluid?

Assuming my mental picture of your setup is correct, since the pressure is the same it won't flow. A little more detail or a drawing might help clear that up though.

CS
 
  • #6
I'm not sure how to post a figure, but think of a glass full of water that gets tipped on its side. The pressure at the interface is equal to atmospheric pressure in both fluids (water and air), and yet the water flows out of the glass and is replaced by air. What's the explanation? How long does it take for the water to flow out if the glass is a long tube?
 
  • #7
ariespc said:
I'm not sure how to post a figure, but think of a glass full of water that gets tipped on its side. The pressure at the interface is equal to atmospheric pressure in both fluids (water and air), and yet the water flows out of the glass and is replaced by air. What's the explanation? How long does it take for the water to flow out if the glass is a long tube?

So the pipe is not closed on one end, but rather open to the atmosphere, and on the other end the pipe is connected (and open) to a reservoir with a lower density fluid...correct?

Is the reservoir top open to the atmosphere?

CS
 

What is the difference between pipe flow and open channel flow?

Pipe flow refers to the movement of fluid through a closed conduit, such as a pipe, where the flow is primarily driven by pressure differences. Open channel flow, on the other hand, occurs in open channels such as rivers, canals, or streams, where the flow is primarily driven by gravity.

What factors determine the transition from pipe flow to open channel flow?

The transition from pipe flow to open channel flow is primarily determined by the relative depths of the fluid in the pipe and the channel. When the fluid depth in the channel exceeds that in the pipe, the flow will transition from pipe flow to open channel flow.

How is the flow rate affected by the transition from pipe flow to open channel flow?

The flow rate typically increases when the transition from pipe flow to open channel flow occurs. This is because open channel flow has less resistance than pipe flow, allowing for a greater volume of fluid to flow through.

What are some common applications of open channel flow?

Open channel flow is commonly used in irrigation systems, wastewater treatment plants, and stormwater management. It is also studied in the fields of hydrology and hydraulics to understand and predict the behavior of rivers and streams.

What are some challenges in studying the transition from pipe flow to open channel flow?

One of the main challenges in studying this transition is accurately measuring the flow rate in both pipe and open channel systems. Additionally, the transition can be affected by various factors such as the shape and roughness of the channel, making it difficult to generalize the behavior across different systems.

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