Conservation of mass - equation understanding

  • #1
Ketler
3
0
Homework Statement
Understanding conservation of mass equation
Relevant Equations
Hello All,

I have a problem to understand this equation:



It supposed to describe change in the mass of the control volume during a process.

Two terms on the left are the total mass flow rates in and out of the system. I struggle to understand RHS.

What means and why it is not equal to ?

Many thanks for all your help.

Lukas
 
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  • #2
The rhs is the rate of change of mass within the control volume. It's like a bank account. (Rate of money in) minus (rate of money out) equal (rate of accumulation of money within the account).
 
  • #3
Thanks for your answer. So if it is a rate of change, why is it not written as:


My assumption is, LHS can be also written as:
 
  • #4
Ketler said:
Homework Statement: Understanding conservation of mass equation
Relevant Equations:

Hello All,

I have a problem to understand this equation:



It supposed to describe change in the mass of the control volume during a process.

Two terms on the left are the total mass flow rates in and out of the system. I struggle to understand RHS.

What means and why it is not equal to ?

Many thanks for all your help.

Lukas
Can you define your variables, please?
 
  • #5
Ketler said:
Thanks for your answer. So if it is a rate of change, why is it not written as:


My assumption is, LHS can be also written as:
No. It's a notational thing. The over-dot does not mean a time derivative. the rate of flow in: where is the density of the inlet stream, is the velocity of the inlet stream (at the inlet to the control volume), and is the cross sectional area of the inlet flow conduit.
 
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  • #6
Chestermiller said:
No. It's a notational thing. The over-dot does not mean a time derivative. the rate of flow in: where is the density of the inlet stream, is the velocity of the inlet stream (at the inlet to the control volume), and is the cross sectional area of the inlet flow conduit.
what does CV mean here?
 
  • #7
pines-demon said:
what does CV mean here?
Control volume
 
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