Post by finiteparts on Feb 19, 2017 15:36:34 GMT -5
In response to this,
I thought that I would just start a new thread to discuss all things "combustor design" related, starting with this one on swirl stabilization.
So to start from the absolute fundamentals, let's talk about some combustion basics. We know that to have a combustion reaction occur, we need a fuel and oxidizer and a source of ignition. So if we go to something simple that we all know, a candle, we can gain some basic understanding and compare it to our combustor design.
When we light a candle, we set-up a very similar system to a combustor. If we think of the wax as a very viscous liquid hydrocarbon, we can draw a parallel to our kerosene liquid fuel. The wax itself will not burn in it's liquid state, it has to be converted to a gas to burn. So when we apply a flame to the wick to start the process, we are melting the upper most layer of wax on the candle and any wax that is still in the wick. As the wax is further heated, it phase changes to a gas and then mixes with the updraft of air to produce the combustible mixture that feeds the flame. Once initiated, the radiational heating of the flame continues the process of melting the wax into a low viscosity liquid that flows by capillary action up the wick into the hotter region where it flashes to a gas state and then flows into the flame...and the cycle continuously repeats.
Now to compare this to our combustor designs. The wax is the kerosene fuel flowing in, which must be heated to a point that it flashes to a gas so that it can participate in the combustion. The rate of vaporization is a function of the ambient temperature that the fuel is spraying into. This is where the two major fuel processing methods diverge.
The first major fuel processing method is spray atomization. The goal of atomization is to break up a stream of fuel into as many smaller droplets as possible to increase the surface area from which the fuel can evaporate. Since evaporation occurs between a liquid to gas interface, increasing the area of that interface increases the evaporation rate. As can be seen in the curve below, if you break up a given volume into a number of droplets, you increase the surface area
As an example, if we started with an 0.080 inch diameter jet and "took" a .125 inch long section of it, to get to a spray of 30 micron diameter droplets (which is roughly considered a fog and thus would be suspended in air over time), we would have over 750,000 droplets and we would have increased the available surface area from which to evaporate from by about 350 times. If we could get the droplet sizes down to 12.5 microns, we could increase the surface area over 1000 times it original value!
But, evaporation is not only a function of surface area, it is more complex. Diffusion and evaporation both compete to transport the mass across the gas-liquid boundary. If we take these as the rate limiting processes, then two equation illustrate the variables involved. You can read more about this in J.J. Bikerman's "Surface Chemistry, Theory and Application".
If we look at these equations, we can see that there are other things that can help us get the fuel into a gaseous state...a larger temperature gradient, diffusion coefficient, lower latent heat of vaporization, etc...but the easiest thing to control is the droplet diameter. By inspection, we can see that if we took the same volume of fuel and broke it up into a thousand smaller droplets, we would get a 100 fold increase in evaporation or diffusion depending on which one was dominant. (remember that volume is a cubic function, so if we break it into a 1000 droplets, the droplet diameters would be = r/10)
Another equation is provided to calculate the rate of change of the droplet mass (dm/dt) assuming that it is moving through the ambient air is shown below,
Since this equation assumes that the droplet is large enough to fall through the air, it is considered larger than a fog. This one is tougher to use, but it may be a better fit if the coefficients can be found for a kerosene type fuel at our operating point conditions.
We now know that we want to have a larger number of smaller droplets, but how do we do this? The issue we have to face with atomizing liquid is that it takes work to create the surface tension need to hold the smaller droplets together. So, in order to make smaller and smaller droplets, we HAVE to do work on it. We can do work on it by the same way we do work on other fluids, with pressure, temperature or velocity (potential or kinetic work). We will come back to this for each method we discuss, but this a key thing to keep in the back of your mind, that it takes work to increase the liquids surface area.
The second major method of processing the fuel into the vapor state is probably very familiar to most on this site, that being heating. Vaporizers are simple way to push the internal energy of the liquid over the hurdle of phase change, but durability and sizing are challenges. I will differ this discussion for a few posts, but I promise to come back to it...there is some very nice information out there on this.
We now have a large number of drops that are evaporating just upstream of the flame, just like the candle. The fuel vapor and air are mixed and ready to burn, but they need something else to burn, they need the ignition energy to get them over the chemical potential energy hump before they can self propagate. Let's assume that we are already lit off and running in a steady state manner. The way the reactants usually get lit off in the candle is that the reaction rate is just in balance with the reactant flow speed and as such you get a stable flame front.
But, the turbulent flame speed for kerosene-air flames is under 16 ft/s, so if the mean flow speed is larger than that, the reaction front and thus the flame cannot propagate, it just gets blown off. If you think about a candle, you know that as you blow more and more on it, the flame gets more noisy, unstable and eventually, it just can't "hold" on and blows off when the reaction rate is slower than the reactant flow speed.
This is the reason that we need to design the combustor to have flame anchoring regions or devices...which I will put in the next post.