Mass flow rate for compressible flow

In summary: The maximum airflow limit that occurs when the Mach number is equal to one is called choking of the flow. The limiting of the mass flow rate is called choking of the flow. When you have a tube with changing area, like the nozzle shown on the slide, the maximum mass flow rate through the system occurs when the flow is choked at the smallest area.
  • #1
LoopInt
36
1
I am confused about the relation of mass flow rate with different mach numbers in a compressible flow.
In wikipedia (http://en.wikipedia.org/wiki/Choked_flow), I read the following:

"Although the gas velocity reaches a maximum and becomes choked, the mass flow rate is not choked. The mass flow rate can still be increased if the upstream pressure is increased as this increases the density of the gas entering the orifice."

This makes me think that:
- The maximum velocity will be mach 1 and won't increase (at the minimum tube area, or throat)
- The mass flow rate will not be limited, because the higher the pressure, the higher the density, so mass flow rate will increase (mdot = mach1 * A * rho).

But then I read from NASA (http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html):

"There is a maximum airflow limit that occurs when the Mach number is equal to one. The limiting of the mass flow rate is called choking of the flow."

And:

"If we have a tube with changing area, like the nozzle shown on the slide, the maximum mass flow rate through the system occurs when the flow is choked at the smallest area."

This seems contradictory. I might be missing something.
 
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  • #2
For isentropic processes, the maximum velocity will be reached in the throat, where M=1, so the maximum velocity will be the speed of sound.
Also, the maximum mass flow rate will be reached in the throat, but it is given by the isentropic relations.
When you modify the inlet pressure, the maximum velocity will still be the speed of sound, but the maximum mass flow rate can have a different value. Both maxima will still occur in the throat.
Note that the speed of sound can change if you modify the pressure, so even though the maximum velocity will always be the speed of sound, the value of the velocity can change.
 
  • #3
That's very helpful, thanks!

You said that the speed of sound can change, right? But looking at this equation:
$$a = \sqrt{\gamma R T}$$
What changes when the pressure increases?
$$R_{air}=287 \frac{J}{kg K}$$
$$\gamma_{air}=1.4$$
$$T= 300 K$$ (of the atmospheric temperature? Or do I need to calculate it somehow?)

Also, the NASA site mass flow rate equation uses the Mach Number. And to calculate the Mach Number I need to calculate the dynamic pressure, but to calculate the dynamic pressure I need the Mach Number (http://en.wikipedia.org/wiki/Mach_number). So how can I calculate the Mach Number, without knowing the velocity? For example, can I calculate the Mach Number for a pressure difference in a tube with area A and length L?
 

1. What is mass flow rate for compressible flow?

Mass flow rate for compressible flow is the amount of mass that passes through a given cross-sectional area per unit time in a system where the fluid is compressible. This is an important parameter in studying the behavior of gases and other compressible fluids in various engineering applications.

2. How is mass flow rate for compressible flow calculated?

The mass flow rate for compressible flow can be calculated using the equation: m = ρ * A * V, where m is the mass flow rate, ρ is the density of the fluid, A is the cross-sectional area, and V is the velocity of the fluid. This equation takes into account the compressibility of the fluid by including the density in the calculation.

3. What factors affect the mass flow rate for compressible flow?

The mass flow rate for compressible flow is affected by several factors, including the density of the fluid, the cross-sectional area of the system, and the velocity of the fluid. Additionally, factors such as temperature, pressure, and the properties of the fluid (such as viscosity and compressibility) can also impact the mass flow rate.

4. How does compressibility of the fluid affect the mass flow rate?

The compressibility of the fluid plays a significant role in determining the mass flow rate for compressible flow. As the compressibility of the fluid increases, the density decreases, resulting in a lower mass flow rate for the same cross-sectional area and velocity. This is because the fluid molecules are more spread out and take up more space, reducing the number of molecules passing through a given area in a given time.

5. Why is mass flow rate for compressible flow important in engineering?

The mass flow rate for compressible flow is an essential parameter in engineering as it helps in understanding the behavior of gases and other compressible fluids in various systems. It is used in the design and analysis of systems such as gas turbines, compressors, and pipelines. It also helps in determining the efficiency of these systems and identifying potential issues such as flow restrictions or blockages.

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