Solving Air Lock / Vapor Lock Puzzle

[rudy]

Hello-

First post here, I can't seem to wrap my head around this concept and how to overcome it.

I read here: https://www.physicsforums.com/threads/air-lock-concept.83124/
that air's compressibility is what stops flow, why is this? I also read that a stronger pump (in a vehicle's fuel system, for example) can theoretically overcome vapor lock. What is the tipping point? Does the air bubble need to be compressed to the same pressure as the liquid to keep moving?

I am trying to think of an analogy which would help myself understand but not coming up with anything at the moment. Thank you for any insight.

 

[sophiecentaur]

I take issue with that thread you are quoting; it doesn't seem to make sense to me. But I see it's long dead.
Compressing bubbles in a fluid can account for hydraulic brakes failing but that's not a pumped fuel system. Braking systems use a very small amount of displaced fluid when you press the pedal and you can run out of travel if there are bubbles in the pipes and they get compressed. The compressibility of the air makes the pedal feel 'spongy', even when the brakes eventually bite. Also, brake fluid is designed to absorb a certain amount of air under pressure - to harden up the system.
Air lock:
For a pumped fluid system, it's a dynamic thing, often, with a bubble in the tube being forced downward along the tube and its buoyancy keeping it up and partially blocking the tube and 'subtracting' from the head produced by the pump. I had a central heating system with the same problem. There are (at least) two ways of dealing with this; One is to have a bleed system which let's trapped air out at a high point (one Central Heating System technique) or to have a pump with a very high throughput volume (not a lot of pressure needed as we are only dealing with less than 1m of head) and just enough head to 'wash' the bubble down, despite it still rising up the liquid column. Of course, if you do manage to wash the bubble out of the system, it may turn up somewhere else. For this reason, many fuel systems recirculate unused fuel back to the tank, along with bubbles.
Vapour lock:
A local hot spot on the fuel line can cause the fuel to boil locally and generate a lot of back pressure. No fuel gets through and only vapour can get into the carburettor (old engines). Another advantage of a recirculating system is that the larger volume of fuel flow will keep it cooler and reduce the possibility of boiling. I think vapour lock is a thing of the past - except in simple systems with gravity feed - like lawnmowers.

 

[rudy]

That's pretty helpful, might take me a few re-reads and wiki articles to get it still. The part that made the most sense to me from your reply is that the bubbles buoyancy keeps it in place at a local high spot in a system. I can see how this would cause a restriction in the pipe or hose limiting flow.

Does there have to be a "high spot" for air to get trapped or can it get stuck anywhere? Can an air bubble stop flow in the middle of a flat section of line/hose/pipe? I see how the boiling gas can cause back pressure, thanks, I'm more wondering about unheated air pockets.

 

[sophiecentaur]

Yes, a vapour lock will only exist at a hot section but air bubbles can coalesce in any local high point. It can take a long time for a big enough bubble to form to give you trouble. When it's big enough, the bubble acts like a piston, pushing upwards against the downward flow.
Air will only find itself, naturally, at a high spot - bubbles will rise to the top. If you have a long horizontal run at the highest level with lots of air in it, you will get what happened to my CH system. The pump would drive the air to the far end of the horizontal run and start to force it down the vertical run until there was a vertical section of pipe with mostly air in it pushing back at the pump pressure. Then you could hear a small flow of water dropping through about half a metre of drop; The pipe formed a sort of weir at the downward bend and a slight flow was just making it through the airlock but not enough to heat the top of the house. Things could be different with smaller bore tubing and I would expect the flow to be cut off completely perhaps. I moved house before I got the urge to sort it out!

I found that Wiki has a bit about why you need to bleed brakes and how to do it. (But that's a different issue)

 

[rudy]

Different purpose, but similar concepts it seems. Anyways, thanks for explaining that, I'm sure I'll be back soon.