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Tomas October 31, 2005 06:04

help setting up boundary conditions for combustor
Dear All,

I am modelling a simple can combustor and need help setting up initial boundary conditions. The physical conditions are: compressor delivers air mass flow of 0.32 kg/s at comp. ratio 2 (pabs = 202650 Pa), static temp. = 378 K (inlet velocity calculated to be approx. 60 m/s). Pressure loss at outflow is expected to be 2-3%. For fuel delivery, there are 16 fuel holes arranged in a circular pattern, each hole is 1 mm in diameter. Fuel velocity should be approx. 80 m/s. Fuel mass flow (CH4) should be 0.00232 kg/s divided by 16 holes = 1.4e-04 kg/s, which means calculated fuel pressure relative to operating pressure (202650 Pa) should be 350420 Pa at fuel static temp of 293 K. Since the combustor operates at pabs 202650, pabs at the fuel holes should be 350420 + 202650 = 553078 Pa. I set up the boundary condition in CFX-pre as follows: OPERATING PRESSURE: pref + prel = pop => 202650 + 0 = 202650 Pa INLET: -airin- velocity = 60 m/s, static temp = 378 K -fuelin- velocity = 80 m/s, static temp = 293 K OUTFLOW: -outlet- static pressure = 0 Pa. THE PROBLEM AFTER CFX-post: The sum of fuel mass flow at fuel inlet is not 0.00232 kg/s but 0.00180!? (pabs is only approx 203000 Pa at fuel inlets when it should be 553078 Pa . What am I doing wrong? With less mass flow of fuel, I get incorrect temperature fields and as a result, incorrect NOx ang CO predictions. Any help would greatly be appreciated! Tom

Joe November 3, 2005 03:31

Re: help setting up boundary conditions for combus
Hi, check first the simple things. Look at the balances in the outfile and the solver are they converged?

Regards Joe

Tomas November 3, 2005 04:34

Re: help setting up boundary conditions for combus
Hello Joe, Thanks for your reply. I just want to know if I've setup the correct boundary conditions at the inflows and outflow. Maybe there is a better way to set them.

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