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stanley650586031
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- TL;DR Summary
- Huge pressure drop after changing pipe length ( I don't think it is due to head losses)
Hi All,
Recently, I have been struggling to seek a solution for a seemingly simple flow question and try to rationalize my thoughts with mechanical engineering knowledge I have acquired from school but none of them can make me feel comfortable about answering the phenomenon I encountered in the lab. So, I am here to seek for help.
The whole idea is just trying to physically observe the pressure and flow rate some point downstream before the air leaves the pipe. However, I accidentally discovered something weird but interesting in the experiment-As I replaced one of the sections of the pipe with a different length of pipe, there was a significant pressure change inside the flow system!
Below are the pictures that could better describe the experiments I did and observations I made. There are two configurations.
As shown in the picture above, the first configuration is that I have a total of four sections of pipes in this set up with two pressure gauges and one flow meter. The inlet condition is compressed air with pressure of 6 bar and outlet condition is atmosphere ( 1 bar) at sea level. The first pressure gauge is at the location of 50 mm from the source. The second pressure gauge is 5000 mm or 5 m downstream from the first gauge. For the flow meter, it is placed at 500 mm further downstream from the second pressure gauge. Regarding the pressure and flow rate values on the instruments, pressure value measured on gauge is 6 bar and value on the second gauge is 3 bar and flow rate is 2050 LPM. So, we could clearly see here that 3 bar of pressure became dynamic pressure (kinetic energy), leaving the system as high speed air jet and static pressure was 3 bar as measured by the pressure gauge 2.
For configuration 2 which is shown in the picture above, all the conditions stay the same except for the third portion of the pipe which I replaced it with a longer pipe with length of 6 m. The wired things happened here..... The pressure value for the second gauge became 5.5 bar and flow rate became 1200 LPM. What the heck just happened...
If we are just talking about the dynamic and static pressure here, it totally makes sense to me that as the static pressure goes up (What was measured by the gauge 2), the dynamic pressure goes down (flow rate dropped shown on the flow meter). However, what I don't understand here is why changing the length of the pipe only could lead to such a significant pressure drop in the system. I thought about major head loss or minor loss or materials of the pipes but I don't think any of these would contribute to this degree of loss in pressure.
The intuitive way to explain this phenomenon, from my perspectives, is that in configuration 1, the volume of air in the third portion of the pipe (500 mm in length) is relativity less compared to that in configuration 2 due to length differences. We know in physics that if we exert a 100 N force on an object, it is going to push us back with 100 N considering no losses. The same works for air. Since the volume of air in the third portion of the pipe in the Config 1 is less than Config 2, the force required move that volume of air by a certain distance is less too meaning it will push us back with less forces leading to lower pressure at that section. Moreover, since the volume of air is less, if we still apply 6 bar of pressure on it, it will move relatively faster compared to Config 2 which has a longer pipe and thus more volume of air in that section.
For Config 2, we have a longer pipe in the third portion which means more volume of air which means it requires larger forces to move it by the same distance as Config 1. Larger force means air will push us back with that force too which could kind of explain why we have higher pressure there. Also, Larger volume of air is more difficult to move compared that of Config which explains why we have lower velocity at the outlet.
This is just my intuitive thinking but does not really have any scientific theories and equations to backup it.
What I need are some theories or equations that could perfectly explain this. If anyone has any thoughts, insights, ideas, or anything, please let me know. I will really appreciate it.
Recently, I have been struggling to seek a solution for a seemingly simple flow question and try to rationalize my thoughts with mechanical engineering knowledge I have acquired from school but none of them can make me feel comfortable about answering the phenomenon I encountered in the lab. So, I am here to seek for help.
The whole idea is just trying to physically observe the pressure and flow rate some point downstream before the air leaves the pipe. However, I accidentally discovered something weird but interesting in the experiment-As I replaced one of the sections of the pipe with a different length of pipe, there was a significant pressure change inside the flow system!
Below are the pictures that could better describe the experiments I did and observations I made. There are two configurations.
As shown in the picture above, the first configuration is that I have a total of four sections of pipes in this set up with two pressure gauges and one flow meter. The inlet condition is compressed air with pressure of 6 bar and outlet condition is atmosphere ( 1 bar) at sea level. The first pressure gauge is at the location of 50 mm from the source. The second pressure gauge is 5000 mm or 5 m downstream from the first gauge. For the flow meter, it is placed at 500 mm further downstream from the second pressure gauge. Regarding the pressure and flow rate values on the instruments, pressure value measured on gauge is 6 bar and value on the second gauge is 3 bar and flow rate is 2050 LPM. So, we could clearly see here that 3 bar of pressure became dynamic pressure (kinetic energy), leaving the system as high speed air jet and static pressure was 3 bar as measured by the pressure gauge 2.
If we are just talking about the dynamic and static pressure here, it totally makes sense to me that as the static pressure goes up (What was measured by the gauge 2), the dynamic pressure goes down (flow rate dropped shown on the flow meter). However, what I don't understand here is why changing the length of the pipe only could lead to such a significant pressure drop in the system. I thought about major head loss or minor loss or materials of the pipes but I don't think any of these would contribute to this degree of loss in pressure.
The intuitive way to explain this phenomenon, from my perspectives, is that in configuration 1, the volume of air in the third portion of the pipe (500 mm in length) is relativity less compared to that in configuration 2 due to length differences. We know in physics that if we exert a 100 N force on an object, it is going to push us back with 100 N considering no losses. The same works for air. Since the volume of air in the third portion of the pipe in the Config 1 is less than Config 2, the force required move that volume of air by a certain distance is less too meaning it will push us back with less forces leading to lower pressure at that section. Moreover, since the volume of air is less, if we still apply 6 bar of pressure on it, it will move relatively faster compared to Config 2 which has a longer pipe and thus more volume of air in that section.
For Config 2, we have a longer pipe in the third portion which means more volume of air which means it requires larger forces to move it by the same distance as Config 1. Larger force means air will push us back with that force too which could kind of explain why we have higher pressure there. Also, Larger volume of air is more difficult to move compared that of Config which explains why we have lower velocity at the outlet.
This is just my intuitive thinking but does not really have any scientific theories and equations to backup it.
What I need are some theories or equations that could perfectly explain this. If anyone has any thoughts, insights, ideas, or anything, please let me know. I will really appreciate it.