Ventilation Pipe 2

[flamby]

Hi folks,
I am designing a ventilation pipe for one underground tunnel. The pipe is a 500 mm dia 105 m high, made of seamless carbon steel. The volume it requires to cater is 47 cubic m/sec (cumec) which gives an air velocity of 239 m/sec.

I need your advise on aerodynamic behaviour of pipe, especially on vibrations considering Mach number is very close to 1.

Thanks in advance,

flame

 

[butelja]

You better double check your numbers. These flow values are OUTRAGEOUS for a 500 mm pipe in a ventilation system. To force that kind of flow, you will need a COMPRESSOR (not a fan) with ~6,000 kW (~8,000 hp) power input. Additionally, you better hope no personnel are in the vicinity of the discharge. Noise will be unbearable, and the thrust generated at the duct outlet will be ~14 kN (~3,000 lbf). A more reasonable flow velocity for ventilation ducts is in the 750-1500 m/min range. Granted, if you have to bore through solid rock, the economic flow velocity may be higher, but nowhere near sonic velocity.

 

[flamby]

Thanks Butelja for your comments. I have to provide a little more explanation to what I am up to. This is a water conveying tunnel to U/G power house. During initial filling of the tunnel, the discharge of 47 cumec is passed and the air is to be replaced by this 500 mm dia pipe (as the general setup stands today. I am open to revise dia or initial flow if the things dont work out to be sustainable).

I am not worried about the power demand as water will push the air through the system at normal pressure. So this is in fact air relief pipe, not a ventilation as I wrogly said.

Now my worries are exactly what you have warned, noise and perhaps vibrations. I have not dealt with pipes having to pass air at such speeds. How do I assess it? What is the better way out? As far as drag is concerned, I can take that into account in structural design.
Thanks.

 

[butelja]

OK, that is a different case entirely. I'm not an expert in hydropower by any means, but what you have stated raises some concern.

It would seem that the tunnel should be filled SLOWLY while all air is vented.

The airflow by itself can be handled without significant difficulty.

However, you absolutely must ensure that the water is not allowed near the entrance to the vent pipe if it is venting at a high flow rate. If the air/water interface hits this opening with any significant velocity, there will be a severe waterhammer that may fracture the tunnel. For an instantaneous velocity change, the waterhammer overpressure is on the order of 60 PSI/(ft/s), or 13.6 bar/(m/s).

 

[buzz41]

Would a diffusor at the exit be possible? This could reduce the velocity and noise. Head pressure for a give airflow would also decrease.

 

[ChasBean1]

Flame, I still don't have a clear picture of your application. Why do you fill the tunnel? What's at either end of the tunnel, is it open on both ends? Can air rush into the tunnel with the water and out with the water? If this ½ meter pipe is your only source for making up that kind of air volume, the size is way too small and could cause catastrophe in your application or could restrict water flow to a gurgle. If the vent is for initial fill only, you should significantly slow the rate of filling if at all possible.