# Determining Reverse Mass Flow Rate Formula for Multiple Diameter Tubes

• RealityMechanics
In summary, the equation for measuring the flow rate in a situation with air flowing in both directions is incorrect. The corrected equation is dimensionally correct and takes into account the direction of the flow.
RealityMechanics
Homework Statement
How would the mass flow rate equation in this scenario change if the flow is reversed?
Relevant Equations
See the image. A variation of Bernoulli's Equation.
Alright, this is more of a conceptual question than a HW question that will nonetheless help me design something. In the attached image, air is flowing through a large area tube and then a small area tube from right to left. A collaborator previously figured out the equation for measuring the flow rate in this way, which I placed beneath this diagram I made. I know this equation is correct because I tested out the measurements experimentally. In this image, the cylinders just represent the location of pressure sensors constantly logging the pressure at that location.

I would like to write another equation, however that represents the flow rate if instead, the air was sucked out from the right side. So essentially the air travels in reverse all the way from the open hole on the left to the right side of the tube. I attached an image I made representing this.

Here are a couple things I would like to point out that has been confusing me in figuring this out.
1. Pdiff is always going to be P1-P2 at any given instant. We do not know what P1 and P2 individually is. That's just the way the pressure sensor works.
2. The first equation was created from the perspective of the right hand side blowing air into the tube. I want this equation to still be from the same right side perspective, or something sucking the air out of the tube.
3. In the original scenario, Pdiff is always positive since P1 > P2. However, in this scenario I am trying to figure out, Pdiff is always going to be negative since P2 > P1. You cannot square root a negative. Perhaps, an absolute value is needed for Pdiff? Can we get these flow rates to be negative since the perspective is the same (right side) but now the air is being sucked out instead of blown in?

By the way, all horizontal measurements are symmetrical, if that matters. The length of both area tubes are the same, and the pressure sensors are equidistant from the ends of both area tubes and the center.

Any help will be much appreciated.

Hi @RealityMechanics. Welcome to PF. Since no one has yet replied, here are some comments...

I guess that you are treating air as incompressible and the flow as smooth (non-turbulent) - and that this is acceptable in the context you are working.

Your equation$$r = \sqrt {\frac {\frac {P_{diff}}{2 \rho_{air}}}{\frac 1{A_2^2} - \frac 1{A_1^2}}}$$is wrong. Check it dimensionally.

Once you understand the derivation of the correct equation, you should be able to work out what (if any) affect the direction of flow has on the parameters in the equation. You might like to remember that when ##r^2 = \text {some expression}##, then there are two possible values for ##r##.

Also, I’m not sure this should really be classified as “Advanced Physics Homework”. Maybe one of the Mentors will want to reclassify it.

## 1. What is the formula for determining reverse mass flow rate for multiple diameter tubes?

The formula for determining reverse mass flow rate for multiple diameter tubes is: Reverse Mass Flow Rate = (Area of Smaller Tube / Area of Larger Tube) * Mass Flow Rate of Larger Tube. This formula takes into account the different cross-sectional areas of the tubes and the mass flow rate of the larger tube.

## 2. How do I measure the cross-sectional area of a tube?

The cross-sectional area of a tube can be measured using the formula Area = π * (Radius)^2, where π is the mathematical constant pi and the radius is half the diameter of the tube. This formula works for both circular and rectangular tubes.

## 3. Can this formula be used for tubes with different shapes?

No, this formula is specifically designed for tubes with circular or rectangular shapes. Tubes with other shapes, such as triangular or oval, would require a different formula to determine the reverse mass flow rate.

## 4. How accurate is this formula?

This formula can provide a reasonably accurate estimate of the reverse mass flow rate for multiple diameter tubes. However, it may not account for factors such as friction, turbulence, and other fluid dynamics, which can affect the actual flow rate. For more precise measurements, it is recommended to use specialized equipment and techniques.

## 5. Can this formula be used for gases and liquids?

Yes, this formula can be used for both gases and liquids. However, it is important to note that the mass flow rate may differ for gases and liquids due to their different densities and viscosities. Therefore, the formula should be adjusted accordingly for accurate measurements.

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