Size andtemperature of compressed gas slug.

[Prometheus21]

Hello everyone,

As my knowledge on this topic is lacking, and I'm heavily in need of a refresher course; some input from you as well will likely be very helpful.

For the sake of discussion I am looking at simple system; a DN6 flexible polymer-lined flex-hose with a distance/volume piece (heat-sink) at the end. The hose is to be pressurized with 360 bar oxygen (99,5%) at 60°C at a timespan of 20 ms or less. In other words we are looking at an adiabatic compression; or gas hammer effect.

Now designing this heat-sink is rather important to contain the initial slug of gas; and the first compression event is the most severe one; as subsequent compressions does not achieve nearly as great a temperature as the slug that is initially present. A key feature in this configuration is that there is very little mixing of the two gas slugs (the initial slug and the incoming one). Mathematically this compression is usually treated as if a piston was performing the compression (located at the interface of the to gas slugs).

Now there are guidelines in designing a heat-sink with regards to the volume of the initial compression; the consensus being that a short, large-diameter distance piece can be functionally equivalent to a long small-diameter distance piece.

If the slug of gas reaches a high enough temperature then this can lead to ignition of contaminants or the hose itself; meaning the length of this hot slug must be contained.

How would one estimate (with a high level of accuracy):

1. The minimum required length for a given internal diameter D of the heat sink?
2. The volume of the hot gas slug?

Any input on pointers or references that might help me out here? I could always run CFDs but I need to get a firmer grip on the theory behind it.

Thank you.

 

[LittleInch]

This is NOT a simple system.....

Can you sketch this out a bit for us to understand what s going on as it looks vey transient and quite complex to me. How do you get two gas volumes (slugs is usually referred to as liquids in a two or three phase system. It is also a strange unit of something (force, energy ?).

A time span of 20 milliseconds is going to be so fast all sorts of strange things are going on, including how much your flexible hose expands / moves etc

So is the incoming gas at 360 bar / 60C or is this what happens after?

In generly you want to maximise surface area to increase or maximise heat transfer, but what is this heat sink made of? whatever, very little is going to happen in 20 mSec

Volume of the slug? No idea as you haven't described your system well enough.

 

[Prometheus21]

To clarify: the system is simple compared to the systems actually seen in active filling stations for cylinders.

As for the sketch (rapid, simplified one):

A simple single-ended DPV - distance/volume piece is shown, connected to a hose.
Vu is the volume upstream of the flex hose including valve outlet, fittings ect.
Vh is the volume of the flex hose.
VDVP is the volume of the distance/volume region (heat sink + valve inlet, fittings ect. depending on what is connected - lets assume the majority of the heat shall be contained by the heat sink)

Slug in this setting refers to the "plug" of gas that occurs when rapid pressurization happens. The gas is compressed into a plug that will eventually reach the heat-sink.

Gas volume 1 (slug 1): the ambient volume of gas already in the hose.
Gas volume 2 (slug 2) : the volume that is released when the cylinder/ball valve configuration or similar opens and pressurization happens (typical ignition testing this is with 360 bar oxygen (99,5%) at 60°C, with an average valve opening time of 20 ms.)

Like stated in OP: A key feature in this configuration is that there is very little mixing of the two gas slugs.

"In generly you want to maximise surface area to increase or maximise heat transfer, but what is this heat sink made of? whatever, very little is going to happen in 20 mSec" - correct; the heat sink needs to be made of a very ignition resistant material; very often copper, nickel, monel or brass. The use of non-metals needs to be restricted and selected appropriately. The whole idea is to contain the volume of the warm slug of gas in the heat sink itself; so that the majority of the heat front do not come into contact with the hose (which contains non-metals).

I found a few good pointers in "FLAMMABILITY AND SENSITIVITY OF MATERIALS IN OXYGEN" volume 8, 9 and 13. Apparently the two main design ideas is to either contain the temperature increase; or contain the volume; but these are based purely on results of ignition testing. Guess I'm in for a lot of testing.

 

[LittleInch]

Well assuming the gas in the hose at t=0 is at atmospheric pressure? which "ambient" seems to mean? then the volume of the gas at 360 bar is going to be about 1/360th of the volume Vh+ VDVP. Soif the aim is to simply push this plug of gas out of the hose at what looks like close to sonic velocity and compress it at the far end then VDVP needs to be at least 1/360th of the total volume.

Then its pretty easy to work out length vs diameter, but note that the surface area is key to how long it then takes for the heat to dissipate.

Oh and its not usually possible to "estimate (with a high level of accuracy". Those are two different things.... Especially with such a transient thing going on.

BTW I would love to know which valve opens in 20msec....