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Amin2014
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Can an expanding balloon be considered a control volume?
I meant a balloon that can be blown intoruss_watters said: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.
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:I meant a balloon that can be blown into
Did the volume change as well, or just the shape of the control surface?Chestermiller said: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.
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: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...
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.
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: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 ?
That is the fixed control volume, such as flow through a pipe.Amin2014 said: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.
Thanks, I thought of this myself. Was wondering why no book suggested these. Good to know White has taken care of it.256bits said:That is the fixed control volume, such as flow through a pipe.
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/~fluids/posting/Lecture_Notes/Control Volume and Reynolds Transport Theorem_10-11-2013_Final.pdf
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:Can an expanding balloon be considered a control volume?
The volume of a control volume can change with time, as it is a dynamic system that can experience changes in its boundaries or the material within it. This change in volume is known as volume flux.
The volume of a control volume can change due to various factors such as fluid flow, chemical reactions, and heat transfer. These processes can cause a change in the boundaries of the control volume or the material within it, resulting in a change in volume.
The change in volume of a control volume can be calculated by taking the difference between the initial and final volumes of the system. This can be represented mathematically as ΔV = Vf - Vi, where ΔV is the change in volume, Vf is the final volume, and Vi is the initial volume.
Yes, it is possible for the volume of a control volume to remain constant if there are no external influences or processes causing a change in volume. This is known as a steady-state system where the volume remains constant over time.
The change in volume of a control volume can affect its properties, such as density and pressure. As the volume changes, the density and pressure of the material within the control volume may also change. This can have an impact on the overall behavior and dynamics of the system.