- #1
Amin2014
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Can an expanding balloon be considered a control volume?
Can the volume of a control volume change with time?
- Context: Undergrad
- Thread starter Amin2014
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Discussion Overview
The discussion revolves around whether an expanding balloon can be considered a control volume in fluid dynamics and thermodynamics. Participants explore the definitions and characteristics of control volumes, particularly in relation to mass flow and volume changes over time.
Discussion Character
- Debate/contested
- Conceptual clarification
- Technical explanation
Main Points Raised
- Some participants argue that a balloon is a closed system and therefore should be classified as a control mass rather than a control volume.
- Others suggest that a balloon can be treated as a control volume if it is capable of being blown into, allowing for mass flow in and out.
- A participant raises the example of water evaporating from a glass, proposing that the liquid water's volume could be considered a control volume even if it changes over time.
- Some participants note that control volumes are typically treated as fixed, but there are instances where portions of control volume interfaces can move without mass passing through.
- Concerns are expressed about the distinction between control volumes and open systems, questioning if they become redundant if the volume can change.
- Participants mention that textbooks often assume a constant volume for control volumes, leading to confusion about scenarios where this assumption may not hold.
- One participant highlights the importance of considering diffusion rates of gases in balloons when discussing mass inside a control volume.
Areas of Agreement / Disagreement
Participants do not reach a consensus on whether an expanding balloon can be considered a control volume. Multiple competing views are presented, with some supporting the idea and others contesting it based on definitions and characteristics of control volumes.
Contextual Notes
The discussion reveals limitations in the definitions and assumptions surrounding control volumes, particularly regarding their ability to change in volume and the implications for mass flow analysis.
- #2
dRic2
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Yes
- #3
russ_watters
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I would answer no. The point of a control volume is that it is open, enabling you to track mass flows in and out. A balloon is closed: it is a control mass.
- #4
dRic2
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What about water evaporating from a a glass (for example). I'd take the control volume to be the liquid water's volume rather than the volume of the glass, even if it is changing in time...
I'm mean it's just a definition
I'm mean it's just a definition
- #5
Amin2014
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I meant a balloon that can be blown intoruss_watters said:
- #6
dRic2
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Why didn't I think of that... it's even in the title... I feel so dumb 
- #7
russ_watters
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No, I don't think so - I read it the same way you first did: as expanding without changing mass. E.G., heating a balloon. Because...dRic2 said:Why didn't I think of that... it's even in the title... I feel so dumb
I don't think so, but I'm not certain. A balloon that can be blown into is a transient process, not a continuous one, which seems to me to defeat the definition. The throat on the other hand is a control volume.Amin2014 said:
- #8
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Usually, a control volume is treated as fixed. However, I had seen developments where a portion of a control volume interface (across which no mass passes) can be moving. I have also seen moving and deforming (material) control volumes across which no mass passes.
- #9
Amin2014
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Did the volume change as well, or just the shape of the control surface?Chestermiller said:
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- #10
Amin2014
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Control volumes are used to analyze transient and continuous processes alike; Refer to an engineering thermodynamics textbook. The point is that it's a mathematical tool that provides a different point of view from the lagrangian/control mass viewpoint. A transient process can lend itself to either analysis. For transient processes, changes in quantities are normally expressed with Δ, such as ΔU, where as in continuous processes time derivatives are used; ##dU/dt##russ_watters said:
My real concern is that if the volume of a control volume can change, then what's the distinction between "control volume" and "open system"? Seems redundant to me.
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- #11
dRic2
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I think you are overthinking it. I mean, what's the problem in practice ? Do you have in mind a scenario where you don't know how to choose the control volume ?
- #12
Amin2014
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The derivations of formulas in textbooks on thermodynamics and fluid mechanics seem to repeatedly make the assumption that the volume of whatever chosen control volume is constant, that's why I had to ask.dRic2 said:
- #13
256bits
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That is the fixed control volume, such as flow through a pipe.Amin2014 said:
One can have a moving control volume, such as around an airplane.
One can also have a deformable control volume, as in your balloon case. Here this case the control surface moves with the surface of the balloon.
The control volume separates your area of interest from the surroundings.
Read this
http://user.engineering.uiowa.edu/~...ynolds Transport Theorem_10-11-2013_Final.pdf
- #14
Amin2014
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Thanks, I thought of this myself. Was wondering why no book suggested these. Good to know White has taken care of it.256bits said:
- #15
Don jennings
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Sure but the diffusion rate of molecules out of the balloon would have to be taken into account if you rely on the mass inside the balloon to be static. It won't be. For instance if the inflating gas is helium you know full well helium gets through very small cracks and the plastic of most balloons is porous and even a metallic mylar balloon will still have some porosity.Amin2014 said: