Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Dec 26th 2015, 05:49 PM #1 Junior Member   Join Date: Dec 2015 Posts: 4 Question about pressure, head I'm trying to get a better understanding of this. Say I have a sealed container, half full of fluid. A fluid line at the top penetrates the container down to the bottom, so that the lower end is submerged in liquid, while the upper end of the line extends a foot or more above the container. Now I pressurize the container, causing the fluid inside to be under similar pressure and causing it to rise up in the tube, say 1 foot. Let's assume the fluid line is steel or copper, with a wick inside, and assume the fluid is fuel. The head of liquid just reaches the top of the tube but doesn't spill over. Now I light the wick and fuel begins to burn. My question is, for how long will inside pressure continue to force fluid up to saturate the wick and replace fluid in the tube as it burns away?
 Dec 26th 2015, 08:20 PM #2 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 I think that there is a confusion between two effects here. the pressure afforded by the weight of the fuel in the pipe pushing down to balance the pressure in the container, and the capillary action of a wick drawing fluid up the pipe. If you were to replace the wick with a thin mesh or gauze over the top of the pipe, then the flame would burn for only a short time before the level of the fuel would drop below the gauze and the flame would go out. __________________ You have GOT to Laugh !
 Dec 26th 2015, 08:47 PM #3 Junior Member   Join Date: Dec 2015 Posts: 4 Thank you, well that gets to the heart of what I want to know, how long the pressure inside will continue pushing up the pipe as the fuel burns away. It sounds like you're saying the pressure is not constant, and once the fluid level drops, that's it, no constant pressure from within compensating for the fluid fuel burned off. (The idea is to make a low pressure burner that will burn a long time, vs. say, Colemans, that use maybe 30 psi.)
 Dec 27th 2015, 03:48 AM #4 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Another feature of a wick is that the friction between the fuel and the wick will allow a greater pressure to be applied before the fuel comes gushing out. Thus between capillary action pulling the low pressure fuel up and friction holding the higher pressure fuel back, a wick extends the pressure range over which your burner will work. __________________ You have GOT to Laugh !
 Dec 27th 2015, 09:47 AM #5 Junior Member   Join Date: Dec 2015 Posts: 4 Well actually I don't want the fuel gushing out but just sitting and remaining at a constant (high) level. So in effect what you're saying is the wick will burn up it's saturated fuel and then there's no more because pressure inside the tank isn't feeding it and the wick is too long to pull it to its full height. For comparison, Coleman and other pressure stoves/lanterns operate at a relatively medium pressure and direct fuel through a very small tube/ aperture, most of it filled, to obtain a high-pressure stream. That is not what I want to accomplish. Curious if a pressure regulator in the wick tubewould serve to accomplish my ends.
 Dec 27th 2015, 10:23 AM #6 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Do not burn yourself up! I think you mistook my last post, The presence of a wick will hold back a low pressure, and start to gently weep as the pressure builds above a certain point. You will have to maintain a small flow to supply the flame. The wick will help to regulate the flow over a modest range of pressures. If you are tempted to try some experiments I STRONGLY suggest you use WATER to see what works and what doesn't. Water should give a reasonable approximation for the purpose of the test, but will not burn and so will be MUCH SAFER. You want to get the water to just wet the top of the wick, with possibly just a slight dribble seeping out. FUEL UNDER PRESSURE IS *#'% DANGEROUS. (insert swearword of your choice) __________________ You have GOT to Laugh !
 Dec 27th 2015, 10:33 AM #7 Junior Member   Join Date: Dec 2015 Posts: 4 The experiment has been ongoing and I already have experience with some of those things, I need to arrive at some conclusive solution. Do you think a pressure regulator would let me maintain a higher level of pressure in the tank and keep a constant level at the wick? Granted would be micro psi. Needle valves haven't worked, there tends to be overflow or too much flame. I need to be able to regulate the flame down to zero.