physics question regarding flammable liquids

S

SkinnyJoe

BANNED
Joined
Jun 9, 2007
Messages
7,236
Does surface area in any way determine how fast a given volume of liquid burns up completely?

For instance, if I am burning 20 cc of a liquid in a dish 2 inches in diameter, and 20 cc of same liquid in a dish with a 1 inch diameter, will the 20 cc take the same amount of time to completely burn up, in both dishes?

Thanks.
 
In my professional opinion as a records manager, yes.

Imagine taking a glass of gasoline and lighting it, versus spreading that same amount over your entire driveway. The gas on the driveway would just burn off in one big FOOF, while the stuff in the glass would take a while to burn down.
 
YES, because in most cases the liquid is NOT flammable...ie...gasoline....the vapors are...there for more area spread over...the more vapors are burned and the quicker the liquid goes away...
 
so the liquid spread over a larger area will burn faster than the same volume spread over a smaller area, correct?

then the answer is no.
 
Simple answer is yes. You need three things for fire: heat, fuel and oxygen. Oxygen cannot be supplied interior to the fuel, unless by a mechanical system like a fuel injection system of your car or the spray nozzel of your portable stove. In a normal condition, oxygen is supplied from the air above the fuel, the rate of oxygen delivery is determined by the 1) surface area and 2) wind speed.
 
SkinnyJoe said:
Does surface area in any way determine how fast a given volume of liquid burns up completely?
Click to expand...

Yes, due to (as others have said) evaporation of the liquid to a flammable vapour.

SkinnyJoe said:
For instance, if I am burning 20 cc of a liquid in a dish 2 inches in diameter, and 20 cc of same liquid in a dish with a 1 inch diameter, will the 20 cc take the same amount of time to completely burn up, in both dishes?
Click to expand...

No, the 2 inch diameter cup gives a larger exposed surface of liquid, so more vapour, more quickly > quicker burning > more heating up > more evaporation etc.
 
There are some physicists on here.

Until they chime in, my answer is "C".
 
LOL. I was just told of an idiot who decided to remove ice from his driveway by pouring gasoline on it and lighting it up !! Surprise !! He did barely manage to save his car and house !!....Physics - a fuel injected engine is more efficient than a carburator version because with the fuel injected the fuel droplets are smaller thus a better air to fuel ratio [more surface exposed to the air] permitting more complete combustion => more power,less polution.
 
Based on what I know, surface area does change the rate of combustion. If you increase the surface area and more of the flammable vapors will ignite creating more fire. Understand that the increase will be proportional to the surface area.

Some of the things that will impact the rate of combustion are:
1) Vapor pressure of flammable liquid and the ambient pressure (if the vapor pressure is equal to ambient pressure the fluid is boiling, thus the evolution of vapors is greater. More vapors, more fire. If an only if you are below the Upper Flammability Limit for the given compound)
2) Temperature of the fire and the ambient temperature (the closure the temperatures are together the more efficiently the fire will burn)
3) Surface area
4) The amount of Oxygen available (The amount of oxygen will drive the level of combustion i.e. complete or incomplete. The presence of oxygen will also dictate the formation of other compounds and at what rate i.e. NOx, SOx. In addition if there is too much oxygen the temperature will be driven and reduce combustion. This will occur for the same reasons detailed in 6 and 7)
5) The presence of a catalyst (the catalyst will not directly participate in combustion, but will increase the rate)
6) Moisture content, in the air or the fuel (energy will be transferred into increasing their temperature and not creating more fire)
7) Presence of ancillary non-combustible materials (energy will be transferred into increasing their temperature and not creating more fire)

That’s my 2-cents using a B.S. in Chemical Engineering
 
5) The presence of a catalyst (the catalyst will not directly participate in combustion, but will increase the rate)


This is one I have been working with on the alcohol stoves. I would have thought that a larger can (5.5 oz. catfood, 3 3/8" dia.) would burn faster/hotter than a smaller can (3 oz. catfood 2 9/16" dia) with the same one ounce of fuel. But it doesn't work that way. The burn rate is dependent upon the boiling of the fuel turning it from liquid state to gas. The smaller can heats more readily and thoroughly causing the more concentrated volume to boil faster than in the larger container. I am guessing that the hot metal of the can is the non-consumed catylist mentioned.

Now I wonder if the rate might be accelerated by increasing the surface area of the catylist such as by adding a coil of metal to the interior of the container. The idea being to obtain the highest BTU output in the shortest time by further increasing boil/evaporation rate. Or adding a mesh above the can to work like a catalytic converter for a more complete, possibly hotter burn.
 
You are right in thinking that the rate can be increased by increasing the surface area of the catalyst. However there is a trade off that you will need to consider. When you increase the surface area of a catalyst that the fuel must flow through, there will be an increased pressure drop across the system. Therfore, you will loose total through put and yeild overall reduction in combustion unless you over come the pressure drop.
 
All things being equal (this is a HUGE disclaimer), yes. A chemical reaction occurs on the surface, more surface area, more chemical reaction.

A sugar cube melts much slower than if you grind that cube up and sprinkle it into the liquid (coffee, tea, whatever). Grinding it up greatly increases the surface area.
 
In this case, the original catylst, the metal of the can, acts as a heat sink using some heat from the flame contained to boil/vaporize more fuel. An added coil might absorb/transfer more heat to increase the boil rate, therefore the amount of vapor available for combustion (time-wise, not overall). In other words, in the open can, pressure is not a factor so much as evaporation rate? This type of can stove is non-pressurized.
 
The common sense answer is: Compare how big the flame is in both dishes. If one is bigger its burning more fuel and faster.

Skam
 
Alcohol flames are nearly invisible blue unless seen in total darkness. I'll do an experiment using two identical cans with equal fuel in each, one can with a coil of aluminum from a coke can placed inside. We shall see which one burns out quicker.
 
I've studied physics and my guess is that the rate should scale with the surface area as mentioned by others before. But I would suspect that at some point, the vapor and flame covering the surface of the liquid would exclude oxygen and there may be some limit. This is why you want to atomize the gas in a car engine before igniting it.
Why don't you go ask the local fireman? They probably have more practical real knowledge.
 
There is one scenario that I can think of that might be interesting. If the "flamable liquid" is something like Nitromethane, where the fuel carries all or almost all of its OWN oxygen within itself. This would be more of an explosion than a normal combustion, but in that instance, the surface area would be much less important. At that point, you would want to stand clear of the reaction anyway!!!
 
You must log in or register to reply here.
Back
Top