1. University of Wisconsin -- Engine Research Center slide 1 Negative pressure dependence of mass burning rates of H2/CO/O2 diluent flames at low flame temperature by Michael P. Burke, Marcos Chaos, Frederick L Dryer, Yiguang Ju Review by P Krishna

2. University of Wisconsin -- Engine Research Center slide 2 Content Introduction Experimental Methods Result  Pressure dependence of mass burning rates  Flame temperature dependence of mass burning rate  Effect of CO/CO 2 addition on mass burning rate Analysis and discussion  Analysis of controlling reactions and kinetic pathways  Implications for kinetic modelling Conclusion

3. University of Wisconsin -- Engine Research Center slide 3Introduction Motivation  Syngas(Mixture of H2 pure or with CO, CO2 and H2O) is used in Integrated gasification combined cycle. Conditions high pressure and low temperature for better efficiency lower No x Better models needed for use in fully premixed combustion due to problem of flashback, blowout etc. Current models are unable to predict well in region of interest. Objective  Test performance of recent kinetic model  Explore changing kinetic path ways for high pressure flame  Outline the possible sources of error model and experimental results

4. University of Wisconsin -- Engine Research Center slide 4 Experimental Methods Combustion chamber can be treated a constant volume cylindrical bomb. Diluents chosen to reduce instability. He for lean case to increase Lewis no. Ar/CO 2 for rich case to decrease Lewis no. Initial temperature 295 +/- 3 K

5. University of Wisconsin -- Engine Research Center slide 5Cont.. Table 1 Experimental conditions

6. University of Wisconsin -- Engine Research Center slide 6Cont.. High speed Schlieren photography used for measurement Automated detection process used to calculate r f  Unstreched flame speed found out by linear interpolation to Zero  Results with instability or effected by gravity or non linear behavior not taken into consideration

7. University of Wisconsin -- Engine Research Center slide 7 Results

8. University of Wisconsin -- Engine Research Center slide 8 Pressure dependence of mass burning rate Pressure dependence of mass burning rate Pressure depends n=2{∂ln(f o )/∂ln(p)} T n = order of reaction f o mass burning rate For pressure upto 10 atm. order is positive and above order is negative. Lower pressures the reaction mechanism match experimental result.

9. University of Wisconsin -- Engine Research Center slide 9 Flame temperature dependence of mass burning rates of H 2 flames Negative reaction order was only observed for temperature range of 1500 -1700 K Pressure of maximum burn rate decreases with decreasing temperature The temperature dependence of mass burning rate increase with increasing pressure Can be given by Zeldovic number Ze = ((T f –T u )E a /R 0 )/(T f 2 ) Ze is higher more dependence 

10. University of Wisconsin -- Engine Research Center slide 10 Effect of CO 2 /CO of burning rate

11. University of Wisconsin -- Engine Research Center slide 11 Analysis and Discussion

12. University of Wisconsin -- Engine Research Center slide 12 Analysis of controlling reactions and kinetic pathway (R1)H + O2(+M) = HO2(+M) (R2)H + OH + M = H2O + M (R3)O + H + M = OH + M (R4)H + H + M = H2 + M (R5)H + O2 = OH + O (R6)O + H2 = OH + H (R7)OH + H2 = H2O + H (R8)HO2 + H = H2 + O2 (R9)HO2 + H = OH + OH (R10)HO2 + OH = H2O + O2 (R11)CO + OH = CO2 + H (R12)HCO + H = CO + H2 (R13)H + CO + M = HCO + M (R14)HCO + O2 = CO + HO2 (R15)O + OH + M = HO2 + M (R16)H + HO2 + M = H2O2 + M (R17)HO2 + HO2 = H2O2 + O2  Primary cause of Pressure dependency is H consumption pathways  Two competing path ways R1 and R5 responsible for second limit  At high pressure (above third limit) and low temperature  H 2 O 2 (formation from HO2 and decomposition)pathway at this condition is explosive but only chain carrying in contrast to chain branching of second limit  For these condition a “extended” second limit is defined  (k8+k9)/2k8*(2k5/(k1[M]) =1  Reactions involved play an important part in negative pressure dependency.

13. University of Wisconsin -- Engine Research Center slide 13  Pressure increases HO2 concentration increases and more flux through HO2 channel instead of strong branching channels.  Reactive zone restricted to higher temperature and narrower region ( higher Zeldovich number)

14. University of Wisconsin -- Engine Research Center slide 14 As seen from above graph the sensitivity analyses of A-factor of elementary  Increase considerably with temperature specifically reaction R5, Using CO 2 has higher third body efficiency which increase recombination reaction (R1) Lower values of H, narrower reactive region  Higher concentration of H 2 O at high pressure which has even more higher third body efficiency

15. University of Wisconsin -- Engine Research Center slide 15 Implications for kinetic modeling  Even the usage of the lower and upper limit of elementary reaction constant yields 30 % variation at region of interest.  Third body efficiency limits for H 2 O 16 to 24 relative to Ar, upper limit is used burning rate is used 30% less than compared to lower limit.  One or more elementary reaction could be important at higher pressure  Eg. O + OH +M =HO 2 + M

16. University of Wisconsin -- Engine Research Center slide 16Conclusion 1.At low pressure the mass burning rate is experimentally observed to increase with it, but at higher pressure it found to decrease with pressure. This phenomenon is observed at lower pressure with lower flame temperature. Addition of CO addition to fuel does not significantly alter the pressure dependence. Dilution with CO2 strengthens the pressure and temperature dependence. 2.The models are not able to predict satisfactorily at high pressure and low flame temperature regime. 3.Preheating the mixture with fixed dilution yields weakened burning rate pressure dependence and sensitivity result, in a manner similar to raising flame temperature through decreasing dilution. Additionally, the strength of the burning rate pressure dependence of H2/ O2/diluent flames can be ranked by diluent as He<N2<CO2<H2O. 4.As the pressure is increased, the extended second limit, which demarcates straight- chain kinetics from chain-branching kinetics, is shifted to higher temperatures. The portion in flame where branching occurs is narrower and towards higher temperature. 5.The sensitivities of mass burning rates to elementary rate constants increase considerable pressure, this amplifies the effect of uncertainty in the reaction rate. Additionally the rates are also sensitive to the collision efficiency. 6.There appear to be one or more elementary reactions that could be important at high pressure that are missing from most of current H2 mechanism. Example R15 (O + OH + M = HO2 + M) demonstrated significantly to impact lean high pressure flames. 7.The sensitivity of the prediction of prediction to rate parameters for branching and recombination increases dramatically with increasing pressure and decreasing flame temperature.

17. University of Wisconsin -- Engine Research Center slide 17 Questions?

18. University of Wisconsin -- Engine Research Center slide 18