[PSUBS-MAILIST] air flow vs. water flow

Fri Mar 30 20:50:16 EDT 2018

 Cliff, your right, you would figure it out during a commercial watching the football game on Sunday. I would drive to the lake, annoy a fisherman, lie to a tree hugger and probably be late for supper. But in the end we would both have it.Hank
    On Friday, March 30, 2018, 6:32:59 PM MDT, Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 To get a first order approximation, assume turbulent pressure drop in a pipe nipple.  Write two cases, one for air and one for water at same temperature and pressure near surface and same pressure drop .  This will give you that the Vair is equal to Vwat times the square root of ratio of water over density over air  and ratio of  friction factor of water over the friction factor of air.  Assume friction factors are the same for first order approximation.   Va=Vw*(density_w/densityair * fw/fa)^0.5 At the surface this reduces to Va=29*Vw for the same pressure drop.  So if took 60 sections for water, it would take 2 seconds for air.  This assumes the limiting factor is the turbulent pressure drop through the MBT vent valve.  If the MBT flood valve was to small, then, it would start to impact how quickly the MBT would fill with water.  In the real world it is a combination of the turbulent frictional resisence through both the MBT vent and flood valves.   For better approximation use CFD code to model the MBT including the MBT vent and flood ports.
What hank would have already done.  Find a 55 gal oil drum from behind his shop.  Remove the 2" cap from the top and turn the drum upside down. Cut out a hole to fit the new MBT valve.  Put the drum and attached MBT valve in the back of his truck and head to the nearest lake.  Suspend a 20 lb weight below the drum so it floats.  Open the MBT and measure how long this MBT takes to completely flood.  Prorate this time based on volume of Psub MBT compared to 55 gals.  
Cliff
On Thu, Mar 29, 2018 at 11:35 AM, Alec Smyth via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Hi everyone,
I was testing one of my new MBTs yesterday, removed from the sub, by turning it upside down and filling it with water, then opening the mushroom valve to let the water out. It takes 60 seconds to empty. In actual operation it will be air instead of water coming out that valve. The pressure will be the same, but because it will be air instead of water the tank should empty much faster. The question is, does anyone know more or less how much faster? One should be able to calculate this with Bernoulli's formula, but this is very turbulent flow so not sure if it would really hold. Any rules of thumb?

Thanks,Alec
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