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Simulating turbulators in fire tubes

Question asked by Dave Reichart on Mar 12, 2015
Latest reply on Mar 13, 2015 by Amit Katz

Hello,

 

I have recently begun using Flow Simulation to analyze fire-tube boiler designs.  For those not familiar, a fire-tube boiler is basically a shell-and-tube heat exchanger that uses hot combustion gasses in the tubes to heat water in the shell.

 

The tubes are simple straight pipes, but they also have Brock turbulators (wavy strips of metal) in them.  I fear that including the turbulator geometry into my model will make the calculation never ending.  I would like to model the simple straight tubes in such a way as to have them have the heat transfer characteristics of tubes with turbulators.  My goal in using Flow Simulation is to study the water side flow, so I'm not concerned with flow trajectories in the fire tubes, as long as they are transferring heat to the water side realistically.

 

Through lab testing a prototype, I have the static pressures and temperatures for the combustion gas at the inlet and outlet (note: the turbulators cause a pressure drop across the tubes as well as causing turbulence).  So far I have run simulations using these pressures and the inlet temperature as my boundary conditions, but the results are giving me much higher outlet temperatures than measured in lab testing.  Also, I have been selecting the "Turbulent only" option for the fluid subdomain that includes the tubes.

 

These are the lab measured values for the combustion gasses:

 

Inlet:  1440 F, 408.102 in.H2O absolute

Outlet:  120 F, 407.102 in.H2O absolute

 

So I'm looking for any tips, tricks, or tactics to get these fire-tubes to compute as though they had turbulators.  By using the pressures attained through testing, the simulation seems to calculate a much higher gas flow rate trough the tubes, and therefore less heat taken out of the gas.

 

Also looking for any advice on choosing values for turbulence intensity and length.

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