Quote:
Originally Posted by steve s
Please, believe me, the large vapor bubble seen in my pictures above is swept all the way DOWN HILL from the sediment bowl to the carb. Part of that pathway is vertical; part is slightly sloped. It makes no difference against the liquid flow when the motor is running; you might think bubbles always go up, but not so.
Here's a picture of my fuel line showing its slope--taken just after I grasped at the insulation straw.
Steve
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Steve:
I'm sorry. When I read this thread initially, your very clear point that you observed the bubble to exit toward the carburetor, and thus downhill, did not jump out at me as it should have. Thank you for bringing it to my attention and answering my question about your gas line slope thoroughly with the photo.
My initial guess was that the Model A fuel line was a little too big to be fully obstructed by bubbles, but it looks like I was quite wrong. As I'm sure you are aware, a small tube will be obstructed by bubbles, held together by surface tension, yet in a large tube gravity will stratify the flow and allow countercurrent liquid and vapor flow. In the absence of substantial inertial or viscous effects, these tube sizes would be separated by a critical Bond number (
http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_number).
I just pulled out very long, clear drinking straw which I have measured as 5.5 mm inside diameter. As one might expect for drinking straw, I can suck water and air successively until the straw is filled with little plugs of water and air. This mimics the type of behavior that would allow a bubble to transit your fuel line downward. Interestingly, I can't do the same with denatured alcohol (ethanol) which has a somewhat lower surface tension. In terms of Bond number, the Model A fuel line (7.1) falls right between the ethanol in the straw (9.5) and the water in the straw (4.2). That's based on the first credible number that I found for the surface tension of gasoline from a journal paper published several years ago (
http://www.sciencedirect.com/science...6943320400090X). In seems that ethanol blending reduces surface tension (
http://dx.doi.org/10.1115/1.4003177), which I would presume is due to the same reasons that you nicely explained the vapor pressure.
The reason that I go into all of this is that I believe what you describe is possible, but just barely so, raising interesting possibilities. A small difference in the inside diameter of the fuel line between cars or in the surface tension of the fuel could very much change the behavior in the fuel line. This might contribute to the mysterious distinction between vapor locking cars and non-vapor locking cars.
-- Brian d'Entremont
To clarify, for the Model A fuel line, I estimate:
Bo=(730kg/m^3)(9.81N/kg)(0.00467m)^2/(0.022N/m)=7.1