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Eulerian-Granular Mass transfer UDF problem

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Old   April 24, 2011, 22:34
Default Eulerian-Granular Mass transfer UDF problem
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John
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I am simulating a fixed bed wood gasifier. I am using the Eulerian multiphase model with 2 DEFINE_MASS_TRANSFER UDFs to model the pyrolysis. However, whenever I turn on the mass transfer mechanisms, it completely messes up my results. I get reversed flow on the pressure outlet, negative pressures, and large velocities flowing down instead of up. When I view the contours of the mass transfer rate, it is listed as 0. It is as though the mass is being transferred to the solid phase instead of from, but I cannot figure out how that could have happened. Here is the UDF code:

#include"udf.h"
DEFINE_MASS_TRANSFER(co_source,cell,thread,from_in dex,from_species_index,to_index,to_species_index)
{
Thread *gas = THREAD_SUB_THREAD(thread, to_index);
Thread *solid = THREAD_SUB_THREAD(thread, from_index);
real x [ND_ND];
real k2 = 0;
real k3 = 0;
real rate = 0;
real char_factor = 0;
real y = 0;
C_CENTROID(x, cell, solid);
y = x[1];
if (y < 0.76) {
char_factor = 0;
}
elseif (y < 2.44) {
char_factor = (y - 0.76) / 1.68;
}
else {
char_factor = 0;
}
k2 = 25100000 * exp( -117000/(8.3144 * C_T(cell,solid)));
k3 = 13800000000 * exp( -161000/(8.3144 * C_T(cell,solid)));
rate = char_factor * k2 * k3 * 163.33;
return (rate);
}

If anybody might know what is wrong I'd be very grateful. As you can probably tell by my number of posts recently I'm getting close to a deadline!
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Old   April 25, 2011, 17:58
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John
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I found the problem:

Basically I had the wrong density model for the fuel mixture turned on so the density of my surrogate fuel was smaller than air. Oops....

Now I have another problem. Although these mass transfer rates are on the order of 30 or 40 kg/m^3-s, I am getting low mass fractions of CO and CO2, the species I am transferring to. The maximum mass fraction is 0.016, which seems to not make sense. Also, the mass is being transferred in the fuel bed region. The concentration is 0 in the section of the reactor below the fuel bed region as expected but for some reason it is also 0 in the region above the fuel bed. Also, my velocities become very large at the top of the fuel bed. This could be because of the mass transfer, but if this much mass is being transferred why is the CO mass fraction still almost 0?
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