Calculating Loss Coefficient in 4-way Junctions
 

Dear friends,

One of the projects that I am dealing with is about the calculation of loss coefficients (DP/(0.5*rho*V^2)) in a 4-way junction. (three inlets and one outlet)
Also, I have tried to compare my results with those mentioned in the following book;
"Internal Flow Systems" , "Ds Miller"
Unfortunately, there is a discrepancy in the results. (40% almost). In detail, in my simulation the magnitude of Yplus is under 100 and the drop pressure is calculated in the regions that the flow is developed totally. Moreover, the drop pressure that comes from friction is calculated according to Moody diagram and it is subtracted from the drop pressure in the junction.
By the way, SST turbulence model is used and Reynolds number is around 1e5.
Does anybody have any idea about this problem or know other references for the validation?

Any hint is appreciated,
Best Regards
Sasan Ghomi

There are lots of studies of the pressure losses of pipework and fittings. Google is your friend.

Don't forget that simple empirical models like the Moody diagram often have massive errors. So you can't validate a CFD model against a Moody diagram style model, you need something more accurate than that.

But before you do external validation to sources experimental and empirical results, you should make sure you have done a thorough internal validation and made sure your mesh, time step and convergence criteria sensitivities are acceptable.

I have always found it very difficult to compare pressure drop to empirical stuff because of static pressure differences due to it being converted to/from dynamic pressure. The empirical stuff ignores dynamic/static pressure conversion I believe.

I agree with Evelica, that using Total Pressure gives a better agreement. Mostly...... ;-)

This seems like a good time to explain a similar situation:

TASK: Calculate Pressure Drop through a junction box. Flow comes in one size duct (odd shape) into a largish volume (junction box), turns around, and exits through a different sized (smaller) cross section duct.

Since the outlet was smaller, there was a static pressure loss as it was converted to dynamic pressure. For my answer though, I don't want to include this loss, and the designer should account for it elsewhere.

You know that K loss value for an exit = 1 that you learned in school? That isn't real. Nope, it's not. What that really is, is an accelerational loss at the beginning of the pipe, K=1 exactly matches the dynamic head: DP = ((K*rho)/2)*(V^2). Each time the pipe changes cross sectional area or flow is added, subtracted, this dynamic head will change, and in turn the static pressure. But we don't consider it in the beginning. Just to make the math easier, instead of starting at the beginning, and keeping track of all the changes in dynamic/static head, we just tack it on at the end once we have the final pipe size and velocity. So even though in my situation, that static pressure loss was real, I didn't want to include it because it would be accounted for in the exit loss equation, which isn't really an exit loss as I just explained.
So Total Pressure difference from a CFD should more closely agree with a simplified empirical correlation. But this gets more confusing as well, as the total pressure needs time (distance) from the junction to develop/stabilize, and it depends on where you make this measurement spatially in the pipe.

So I think the most proper answer I could give for "Junction box Pressure Drop" would be to include extra length on the inlet/outlet in my simulation so I had fully developed flow at my result planes away from the junction box. Use the areaAve total pressure difference between the two planes in order to neglect the dynamic loss. Then also subtract the empirical straight line pressure drops for the extra length I included on the inlet and outlet lines. Goofy as heck, but this would be needed to match the empirical calcs, or for someone top throw into their spreadsheet of pressure drops.
Converting CFD results to be used in Empirical situations can be tricky, including DP, HTC and other results as well.