Figure 5 shows the net rates of production of CH4, CO, O2, and H2, at Tst=2000,ξst=0.055. Other conditions employed in the computations are marked in the figure. For YCO,1=YCO,2=0YCO,1=YCO,2=0, CO is not added to the fuel stream or to the oxidizer stream, YCO,1=0.16(YCO,2=0) refers to CO addition to the fuel stream, and YCO,2=0.12(YCO,1=0), and YCO,2=0.19(YCO,1=0) refer to CO addition to the oxidizer stream. The cases for CO addition to the oxidizer stream are on either side of the peak strain rate shown in Fig. 2 and have nearly similar PD 0325901 strain rates. All cases show that consumption of CH4 and O2 take place in different layers of the reaction zone. This is indicated by the location of the peak value of the rate of consumption of CH4 and O2. These layers are called the CH4-consumption layer, and O2-consumption layer. In the CH4-consumption layer fuel reacts with the radicals to form CO and H2. In the O2-consumption layer H2 and CO are consumed and CO2 and H2O are formed. Radicals are also produced in the O2-consumption layer from the principal chain-branching step, H + O2 ?? OH + O. This step together with the fast reaction H2 + O ?? OH + H, and two times the fast reaction OH + H2 ?? H2O + H, gives the overall step 3H2 + O2 ?? 2H + 2 H2O. Thus consumption of H2 and O2 is chain branching and leads to formation of radicals. Carbon monoxide is consumed by the chain propagating step CO + OH ?? CO2 + H. Thus consumption of CO is not chain-branching. For the case where no CO is added to the reactant streams, the rates of consumption of O2 and H2 are larger than the rate of consumption of CO. When CO is added to the fuel stream indicated by YCO,1=0.16(YCO,2=0), the relative order of the rates of consumption of O2, H2, and CO are similar to that with no CO addition. The CO added to the fuel stream does not react in the CH4-consumption layer. It moves into the O2-consumption layer where isotonic is oxidized to CO2. Thus the influence of CO addition to the fuel stream on the overall reactivity can be expected to be small as indeed is shown in Fig. 2 and Fig. 3. The flame structure with moderate amounts of CO addition to the oxidizer stream for YCO,2=0.12(YCO,1=0), shows that the rate of consumption of CO is larger than that of H2, but it is still lower than that of O2. Thus chain-branching reactions continue to be dominant. The flame structure with relatively high levels of CO addition to the fuel stream for YCO,2=0.19(YCO,1=0) shows significant qualitative differences from the other cases. Here the rates of consumption of CO is higher than the rate of consumption of O2 and H2. Thus chain-branching reactions are less pronounced in comparison to the case with moderate levels of CO addition. This decreases the radical level and as a consequence the overall reactivity.