Conservation of mass - equation understanding

  • #1
Ketler
3
0
Homework Statement
Understanding conservation of mass equation
Relevant Equations
$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt}$$
Hello All,

I have a problem to understand this equation:

$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt} $$

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 $$ \frac{dm_{CV}}{dt} $$ means and why it is not equal to $$\dot{dm_{CV}}$$?

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:
$$ \dot{dm_{CV}} $$

My assumption is, LHS can be also written as:
$$ \frac{{dm_{in}}}{dt} - \frac{{dm_{out}}}{dt} = ... $$
 
  • #4
Ketler said:
Homework Statement: Understanding conservation of mass equation
Relevant Equations: $$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt}$$

Hello All,

I have a problem to understand this equation:

$$ \dot{m}_{in} - \dot{m}_{out} = \frac{dm_{CV}}{dt} $$

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 $$ \frac{dm_{CV}}{dt} $$ means and why it is not equal to $$\dot{dm_{CV}}$$?

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:
$$ \dot{dm_{CV}} $$

My assumption is, LHS can be also written as:
$$ \frac{{dm_{in}}}{dt} - \frac{{dm_{out}}}{dt} = ... $$
No. It's a notational thing. The over-dot does not mean a time derivative. ##\dot{m}_{in}## the rate of flow in: $$\dot{m}_{in}=\rho_{in}v_{in}A_{in}$$where ##\rho_{in}## is the density of the inlet stream, ##v_{in}## is the velocity of the inlet stream (at the inlet to the control volume), and ##A_{in}## is the cross sectional area of the inlet flow conduit.
 
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Likes Ketler
  • #6
Chestermiller said:
No. It's a notational thing. The over-dot does not mean a time derivative. ##\dot{m}_{in}## the rate of flow in: $$\dot{m}_{in}=\rho_{in}v_{in}A_{in}$$where ##\rho_{in}## is the density of the inlet stream, ##v_{in}## is the velocity of the inlet stream (at the inlet to the control volume), and ##A_{in}## 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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