9 Replies Latest reply on Mar 28, 2014 10:15 PM by Mark Keown

    Combustion in Flow Simulation

      Does teh flow simulation has combustion toos/

       

      I have a chemcial reaction between air an gas in a very long tube. The chemical reaction occurs at the beginning of the tube. I would like to predict by simulation the tube temperature. I know the gas pressure and the air / gas percentage and the burner geometery and characteristics.

       

      If there is no combustion module, what is the easiet way to predict the tube temperature/

       

      Please advise.

       

      Regards

       

      Samer Hassan

        • Re: Combustion in Flow Simulation
          Bill McEachern

          no combustion. You could do mixing though.

          Bill

          • Re: Combustion in Flow Simulation
            Rich Bayless

            Hello,

             

            It's possible to use Flow Sim to simulate the heat of combustion.

             

            Start with the gas mixture based on your combustion products.  For example, if you burn methane with air, you will end up with a mix of carbon dioxide, water vapor, oxygen, and nitrogen.  Work out the actual percentage of each component and define your incoming gas mixture with those percentages.

             

            Oxygen and nitrogen are transparent to radiant heat, so any simulation with just plain air will not need to account for gas radiation.  Carbon dioxide and water vapor both are active radiators and absorbers of radiant heat, which is why they are called the green house gases.  Simulations including carbon dioxide and water vapor will need manual calculation of how much radiative heat transfer to expect from the hot gas.

             

            Flo Sim does not simulate radiation to or from a hot gas, so you must estimate the radiation and add the amount of radiative heat transfer as a surface heat source.   

             

            Turn on radiation in the setup and set the emissivity of your materials as appropriate.  For example, any insulation next to the burner will probably be hot enough to be a significant source of radiation.

             

            Then set up the inlet conditions for your burner opening setting the appropriate incoming temperature of your combustion products mixture.

             

            Lastly, be sure to validate your simulation results against actual lab testing.

             

            Hope this helps,

             

            Rich.

              • Re: Combustion in Flow Simulation

                Hello:

                 

                The emission of Carbon Dioxide and Water Vapor depend on their temperature and their temperature drops down from 1700 F at the end of the burner to 400 F at the tube exit. Here, I mean a 70 Ft tube length

                 

                The objective is to predict the tube temperature along the centerline.Can we do it?

                 

                The known parameters are inlet air flow or pressure and inlet gas pressure.

                 

                What is obvious to me, unless there is combustion module, we cannot predict the percentage of Carbon Dioxide and Water Vapor and nitorgen and excess air.

                 

                Let us assume, I need to predict the stack temperature or the gas exiting the last tube, can we predict without combustion module?

                 

                Assuming I follow the following boundaries:

                 

                1- Inlet aIr pressure before the burner and before the begining of the tube.

                2- Outlet exhaust stack temperature at the tube exit.

                3- Incoming temperature of teh mixture before combustion.

                 

                Also the Radiative source from Carbon Dioxide and Water Vapor are still unkown to me.

                 

                The objective is:

                 

                1- Predict tube temperature and calculate the exhaust stack temperature at the last tube. We found experimentally there is a temperature gradient along the tube from 1200 F to 400 F and this decrease is non-linear.

                 

                 

                Please advise

                  • Re: Combustion in Flow Simulation
                    Rich Bayless

                    Hello,

                     

                    let's start with this statement:

                     

                    "What is obvious to me, unless there is combustion module, we cannot predict the percentage of Carbon Dioxide and Water Vapor and nitorgen and excess air."

                     

                    A quick internet search found this website:

                     

                    http://www.newworldencyclopedia.org/entry/Combustion

                     

                    This site shows how to calculate (predict) the percentages of CO2, H20, etc, in the combustion products.  Purchasing a combustion handbook will be a worthwhile investment, because it will also help you calculate radiation from the hot gases.  You do not need a combustion module to calculate the combustion products.

                     

                    Here's a great handbook:

                     

                    http://www.amazon.com/North-American-Combustion-Handbook-Industrial/dp/0960159622

                     

                    You should be able to predict the centerline temperature using Flow Sim.

                     

                    You may want to post some of your results.

                     

                    Rich.

                      • Re: Combustion in Flow Simulation

                        Hello:

                         

                        Thank you for your feedback.

                         

                        The problem is as follows: I have a burner which composes of the following:

                        Blower, gas, burner cup, and tuned air plates to obtain clean combustion products and zero carbon monoxide.

                         

                        You are absolutely right in one part of your answer regarding the percentage of carbon dioxide, water vapor, and excess air.

                         

                        But regarding the tube temperature, we need to understand the flow speed of the air-gas mixture inside the tube. Please note that I have a variety of tube lengths. Let us consider for the time being: 70 Ft tube length. Each tube length is around 10 Ft.

                         

                        Determining the flow speed is very important to predict the air-gas mixture temperature flowing through the tube and thus predict the surface tube temperature. Thus, I can provide total and static pressure measurement near the blower as you could see them in the attached picture where there is pitot pressure tube. The total and static pressures change depending on the tube length. Let us now focus on the 70 Ft tube length.

                         

                        We are not asking for an exact tube temperature measurement but at least within 10 % to 15 % compared to the experimental data.

                         

                        I am attaching the surface tube temperature for the 70 Ft tube length where the maximum temperature is around 1170 oF (mid of the first tube) and the minimum temperature is around 340 oF at the exit of the last tube. Also the upper surface of the tube is slightly higher than the bottom tube surface.

                         

                        Please note that the axial tube temperature increase slightly from 300 oF to 1170 oF (approximately mid of the first tube) and then decreases significantly to 340 to the last tube.

                         

                        I am not sure how to insert the gas emissions for carbon dioxide and water vapor as boundaries conditions inside the model.

                         

                        Please advise on how to predict the surface tube temperature at least the bottom surface. Attached are the experimental data.

                          • Re: Combustion in Flow Simulation
                            Rich Bayless

                            Hello Samer,

                             

                            The input firing rate is usually a 'given' in this type of analysis.  What was the burner firing rate and percent excess air during the surface temp testing?

                             

                            Using the combustion handbook you can then calculate the mass flow of the combustion products.

                             

                            Enter the combustion products as an inlet mass flow boundary condition, such as the attached picture shows.  This screenshot assumes methane and 40% excess air, at 100,000 Btu/Hour input.

                             

                            You can then set the temperature of the inlet mass flow as appropriate, until the incoming energy matches your firing rate.  Use a surface parameter on the inlet face to find out the incoming Btu/Hour at various temperatures.

                             

                            Post your models to show your progress as you try things.

                             

                            Rich.

                              • Re: Combustion in Flow Simulation

                                Helo Rich:

                                 

                                Thank you for the feedback.

                                 

                                Please see attached the flow simulation model. I run the file many times but unfortunately, the maximum temperature that it reaches is 360 F then shoot up and terminates the solver abnormally. The results look strange.

                                 

                                Attached is the experimental temperature measurement.

                                 

                                The input firing rate is: 200,000 BTU/H and I use 220 lb/h, and 40 % excess air and then calculate he percentage of carbon dioxide, water vapor, and nitrogen using gas handbook.

                                 

                                Please look to the model and send me your comments on how to improve to validate the experimental data.

                                 

                                The measured outlet flow flue gas temperature is: 420 F.

                                 

                                The objective is to calculate the tube temperature. Please note the upper tube surface should be higner than the bottom tube surface.

                                 

                                See attached file tube temperature file in Excel and word file.

                                 

                                Could you please answer me as soon as you can.

                                 

                                Regards

                                 

                                Samer

                                  • Re: Combustion in Flow Simulation
                                    Rich Bayless

                                    Hello Samer,

                                     

                                    I looked at your model set up and have a few comments.

                                     

                                    1. You should define a fluid subdomain, rather than accepting the default.  Also, I think you should add air outside of the tube, since this heater is not operating in a vacuum.  Convective heat transfer would then be included from the tube to the reflector and from the reflector to the room.

                                     

                                    2. you may want to build a box around the heater to simulate an air filled room. The goal is to heat a room, so this makes sense.

                                     

                                    3. You do not define hot gas entering the tube, but only 220F.  It would make sense (to me at least), to enter the tube with hot gas (around 3000F) and to then let Flo Sim run from there.

                                     

                                    4. you should start with a short model, such as with 1 foot long.  I lowered the length to 1 foot, initial mesh to 3, and turned off min wall thickness and that lowered the cell count from several million to about 150k cells.  The one foot model runs, but soon goes to very negative temperatures.  not sure why, but fix the above and try again.

                                     

                                    Try some of these suggestions, and then repost your model.

                                     

                                    Rich.