Combustion Calculator - Applet Manual

Tutorial (TOC) > Daemons > Combustion > Manual

a. Combustion Daemons: Combustion daemons appear both under the Specific Closed Process and Specific Open Steady branches, each of which comes in two categories: premixed fuel and oxidizer and unmixed (separated) fuel and oxidizer. The premixed daemons are used mostly for generic reactions and combustion involving closed processes, while the unmixed daemons are more suitable for steady combustion chambers, where fuel and oxidizer enter the chamber through separate inlets.

It is assumed that you are familiar with the closed process and open steady daemons, discussed in different sections of this tutorial.

b. Balancing Reactions: A number of combustion problems involve simply balancing the overall chemical reaction. All combustion daemons have Reaction Panel as the default panel. For unmixed fuel and oxidizer the panel image is shown Fig. 1 below.


Fig. 1 Reaction Panel of the combustion daemon.

In addition to the usual choices of SI and English units, the daemon offers Mass or Molar basis for balancing a reaction. Reaction can be balanced on molar basis (e.g. 1 kmol of fuel reacts with...), mass basis (e.g. 1 kg of fuel reacts with...) or converted from one basis to another with any combination of these radio buttons.

The local control panel contains an Action Menu with a number of action items such as Balance Reaction, Initialize, Theoretical Air, etc.

Below the control panel, there are three blocks of variables where the reactants - fuels and oxidizers - and the products are specified. Notice that the default choice of oxidizer is air and that of products are CO2, H2O(g) and N2. For premixed combustion daemons, there are two blocks - reactants and products. You choose the reactants and products, enter the known amounts (mostly for reactants) and select Balance Reaction to balance the reaction. Unmixed daemons allow quite a few more operations.

To illustrate some of the frequently used features of an unmixed combustion daemon, suppose we are interested in the theoretical (stoichiometric) reaction between methane and air. Choose CH4 from the species menu of the fuel block. Simply choose Theoretical Air from the Action Menu and the reaction gets balanced. 1 kmol of CH4 reacts with 9.52 kmol of air - 1 kmol of air is treated as a mixture of 0.21 kmol of O2 and 0.79 kmol of N2 - producing 1 kmol CO2, 2 kmol H2O and 7.52 kmol N2. The air fuel ratio appears on the Message Panel. To know the breakup of air between O2 and N2, choose Air->O2,N2 and the oxidizer panel now contains the amount of O2 and N2 in kmols.

Click on the Mass button and all the kmols are converted to the mass basis (kg or lbm depending on the choice of units, SI or English). The reaction is now expressed in terms of 16 kg of CH4. To convert it to the basis of 1 kg of CH4, choose Normalize from the Action Choice.

To balance a reaction with known amount of excess or deficient air, enter the percent excess air (or deficient air) in the Excess/Def Air widget and choose Excess Air from the Action Choice. Notice that the daemon automatically posts left over oxygen on the products panel. Products of a complete reaction (C going to CO2, S going to SO2, etc.) are automatically chosen by the daemon when you select Theoretical Air or Excess Air from the Action menu. Remember that in a reaction where a fuel reacts with 300% theoretical air, the excess air is only 200%.

Suppose we would like to balance a reaction between 1 kmol of CH4 and 5 kmol of O2 producing CO2, H2O and O2. Super-Init all the panels (note the default species), choose CH4 as the fuel, enter 1 kmol by clicking the check box, get rid of Air as the oxidizer by clicking the Air checkbook twice, choose O2 as oxidizer, enter 5 kmol as the O2 amount, choose O2 as an additional product of the reaction, and select Balance Reaction. The same method can be used if the amounts are known in terms of species mass.

For ideal (therefore, perfect) gases, species volumes are proportional to the corresponding molal amount. If a volumetric product analysis is given, the same method can be used for balancing the reaction (see the Examples section).

To evaluate chememical properties of a gas mixture, the mixture can be set up (with masses or moles specified) as a products mixture. In that case the mixture composition can be read using Read As Is item in the Action Menu.


Fig. 2 The States Panel for the steady-state combustion daemon.

c. States Panel: If you are familiar with Generic Closed Process and Generic Open Steady daemons, the corresponding combustion daemons will appear familiar with a few noticeable differences.

On the States panel (Fig. 2), you will find in the fluid selector Fuel, Oxidizer and Products in the case of unmixed daemons and Reactants and Products for premixed daemons. As you select a state number (say, State-3) make sure that you also select the type of the mixture (say, Products). Typically you should use State-1 and State-2 for Reactants and Products in the case of premixed daemons, and State-1, 2, and 3 for Fuel, Oxidizer, and Products for unmixed daemons. Enter the known properties (p, T, etc.) and
Calculate a state in the same manner as with any State Daemon. The default amount of mass (m or mdot) are imported from Reaction Panel and should not be overwritten if possible. Starting with Version 7.0 the combustion daemons support aglebraic equations; for instance, one can write h3 as '=(m1*h1+m2*h2)/(m1+m2)' for evaluation of the adiabatic flame temperature.

For steady flow combustion chambers (steady flow combustion daemon), generally, p1=p2=p3. Notice that kinetic energy need not be assumed negligible in order to evaluate the states completely. Similarly, in a constant-volume, premixed reaction, Vol2=Vol1.

Using expressions such as =(m1*h1+m2*h2)/(m1+m2) for h3 one can directly evaluate the adiabatic flame temperature T3. Similarly, isentropic state can also be evaluated (say, for a nozzle in which complete combustion reactions takes place). Use of the Process or Device Panel makes it easier to analyze the complete system.

d. Process/Device Panel: On the Process Panel , there are two begin states - bA- and bB-States for unmixed daemons and just one begin state - b-State - for the premixed daemon. Both types of daemons have a single finish or f-State. Load the completely or partially evaluated states. Enter the known values of Q and/or W and calculate. Super-Calculate to update all the answers. For the calculation of adiabatic flame temperature, for instance, Q=W=0 and T3 is calculated as e3 is evaluated from the energy equation (see the custom equation on the Process Panel, Fig. 3) and posted back to State-3.

The Device Panel of the steady state combustion daemon work in a similar fashion. There are two inlet states for Fuel and Oxidizer for unmixed daemons- i1- and i2-States - and a sinle inlet state -i-State - for the premixed daemon. Both types have a single exit or e-State . Load the completely or partially evaluated anchor states. For an isentropic device with no work output (say, a nozzle) enter Qdot=Wdot_ext=Sdot_gen=0. Super-Calculate to completely evaluated the states including Vel3, which may be the desired unknown.

e. I/O Panel: On the I/O Panel you can enter expressions such as =m1*h1+m2*h2-m3*h3 to evaluate the heat of combustion, for instance. In addition the Panel recongnizes any algebraic expression using the Microsoft Excell format. Super-Calculate produces a detailed output for the recently calculated solution on this panel.

f. Parametric Study: The Super-Calculate buttons updates all calculations. So, after evaluating a desired quantity, say, the adiabatic flame temperature, the solution can be repeaed for a different fuel quite easily. Simply uncheck the fuel in the Reaction Panel, select the new fuel(s) and Super-Calculate. The new reaction will be balanced using the action displayed on the Action Menu, all states and analyses will be repeated and a new value of T3 will be found.


Fig. 3 The Device Panel for the steady-state combustion daemon, (b) the closed-process combustion daemon.

g. Perfect Gas/Ideal Gas Models: Mixtures are modeled by using the perfect gas or ideal gas mixture models. Obviously, the ideal gas model is more accurate as the specific heats are assumed to vary with temperature. The data used are quite accurate up to 6000 K for most species. The perfect gas model evaluates cp at 298 K for each species. You will note that cp, therefore, is different for the fuel, oxidizer and products mixtures.

h. Presence of Solids or Liquids: The fuel mixture can cotain liquid or even solid speicies. The products may contain liquid form of H2O. In a mixture that contains gases and solids/liquids, the volume occupied by the condensed phase is neglected.

i. Mixture Properties: A mixture of up to eight species can be created in the Reaction Panel as products. A fuel can also be composed by mixing different components. The amounts can be specified in mass or molar basis. Using the Read As Is item in the Action Menu, the mixture can be read in. In the States Panel states 3, 4... etc. can be used to model the products mixture. The Gibb's function g for the mixture is also evaluated as part of the state.