What does compressible flow look like in a duct undergoing heating/cooling?

  • #1
s34n
3
0
Been trying to find out what compressible flows would look like, to picture why you get the not so intuitive property changes.

Started with looking into what might be happening in Rayleigh flow. Theres plently of examples on how to use maths to calculate property changes when going from inlet to outlet of a duct, but I've not been able to find anything that gives a visual picture of what is happening with the flow within the duct and at the heating/cooling point.

My reference material has been this book link to pdf copy of Fluid Mechanics: Fundamentals & Applications (link to copyright book removed by moderator) , Rayleigh flow starting at page 693 of the book (718 of the PDF).
And the table for Rayleigh flow property changes, Table 12-3 on page 696 (721 of pdf).

Should I be thinking of the heating/cooling causing a creation of an artifical nozzel out of the affected fluid at the wall of the duct?

Is the heating/cooling of the fluid meant to be equal across the cross section of the duct at the one point?

Cheers
 
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  • #2
Thread is in Moderation for Mentor review...
 
  • #3
After deleting a link to a PDF textbook copy of questionable origin, this thread is re-opened.
 
  • #4
If anyone has any suggestions on where to look or who/where to ask about visualising compressible flows, that would be welcomed as well.

Cheers
 
  • #6
Thanks onatirec.

Interesting video. Its those sorts of images and ideas that make me think maybe theres a nozzel being created out of the affected fluid, but thats just my crazy thoughts at the moment.

Cheers
 

1. What is compressible flow in the context of ducts undergoing heating or cooling?

Compressible flow refers to the movement of a gas in which the density of the gas can vary significantly due to changes in pressure and temperature. In ducts undergoing heating or cooling, this type of flow is particularly relevant because the temperature changes can alter the gas density and pressure, leading to variations in flow characteristics such as velocity and mass flow rate.

2. How does heating or cooling affect the speed of sound in a duct?

In compressible flow, the speed of sound is an important parameter that changes with the temperature of the gas. Heating the gas increases its temperature, which in turn increases the speed of sound. Conversely, cooling the gas decreases its temperature, reducing the speed of sound. This is crucial in ducts as it affects the Mach number, which is the ratio of the flow velocity to the speed of sound, influencing whether the flow regime is subsonic, sonic, or supersonic.

3. What happens to the flow properties as a gas is heated or cooled in a duct?

As a gas is heated in a duct, its volume tends to increase due to the decrease in density, assuming the pressure is constant. This can lead to an increase in flow velocity if the cross-sectional area of the duct remains constant. Conversely, cooling the gas increases its density, which can decrease the flow velocity under constant pressure conditions. Additionally, changes in temperature can also affect the viscosity and thermal conductivity of the gas, altering the flow characteristics further.

4. How do shock waves form in compressible flow within a duct?

Shock waves in compressible flows within a duct can form when the flow speed reaches or exceeds the speed of sound, transitioning from subsonic to supersonic speeds. This typically occurs when the duct geometry changes, such as in a converging-diverging nozzle, or when the gas is heated to high temperatures rapidly. The shock wave is a thin region where the gas properties such as pressure, temperature, and density change almost discontinuously, leading to energy losses and increased flow resistance.

5. What are the practical applications of understanding compressible flow in heated or cooled ducts?

Understanding compressible flow in heated or cooled ducts has significant practical applications in various engineering fields. For example, it is crucial in the design and operation of HVAC systems, aerospace engineering for jet engines and rockets, and in industrial processes where gases are transported at high temperatures. Accurate knowledge helps in optimizing performance, enhancing energy efficiency, and ensuring the safety and reliability of the systems involved.

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