Self-Ignition Delay

In order to better understand H2 + air mixture self-ignition, calculations of T/, for a practical range of pressure and temperature, have been performed using a modern kinetic scheme (42 direct and reverse reactions with corrected values of the constants [19, 20]). 

Analysis of self-ignition in a practical range of pressure and temperature has revealed some characteristics requiring revision of steady-state reactivity concepts of hydrogen and its mixtures with other fuels, for example, hydrocarbons. 

Traditional representation of self-ignition delay time Tj dependence on temperature Ti = (T) gave little information because of the nontrivial impact of pressure. The 

The dependence of t, = f(P, T) is plotted in Fig. 6.5 for T = 1,000-1,400 K and P — 0.01-10 MPa. Traditionally, a plane projection of a represented surface t,- = fi(T) [13-17, 21-42] is used for analysis. This analysis is applicable for the general dependence of the drooping curve at low pressure and high temperature. Additional rising or falling surface segments point out the limited applicability of known 

The first zone, at low pressure, P 0.2-0.3 MPa is well known, it is characterized by a delay time/pressure inverse dependence. It is precisely this branch that has been discussed and used for predictions many times. 

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Fig. 15. Effect of initial ethane concentration on the self-ignition delay values (7ign) in oxidation of methane-ethane mixtures modeling at different values of reaction (28) rate constant (T = 773 K, P = 70bar, [O2]0 = 15.4%). (1) k28 = 3 x 105s (2) k28 = 6 x 105s (3) k28 = 1.2 x lOV1.

The practice of using hydrogen as a fuel in RJE, LPE, ICE has revealed some characteristic features incompatible, at the first glance, with the apparent high hydrogen reactivity. The increased combustion stability of H2 + O2 mixtures in CC and GG of LPE, compared with kerosene -1- O2 mixture combustion stability, has not been explained. Difficulties encountered in managing the steady combustion of H2 + O2 mixtures in RJE at 0.5-1.0 MPa and T 1,000 K are still not understood. Attempts to describe the reaction zone structure of the detonation, on the basis of many empirical relations for H2 + air mixture self-ignition delay, have been unsuccessful. 

The calculated boundary of the change in the dominant chemical reaction mechanism is confined inside the shaded area between fines A and B in Fig. 6.4. To the left of the separating zone (sometimes called the forth ignition limit [12]), branching-chain reactions dominate. For the first time, the possibility of the growth of a self-ignition delay with a pressure rise in a combustible mixture was indicated by the improved expression for t, in [17]. 

Fig. 6.5 Self-ignition delay time in relation to pressure and temperature for H2 + air stoichiometric mixture t, c P, MPa T, K...

Fig. 6.6 Characteristic zones of self-ignition delay time dependence on pressure at fixed temperatures curve...

The three characteristic zones of Ha + air self-ignition delay time/pressure dependence at fixed temperature require revision of the data obtained by many... 

Regretfully, some works [43] have been published where authors, based on controversial measured results [37], interpreted reaction models that predict 100 times greater self-ignition delay time than the observed values. The outlines of transit zone II have been qualitatively described by modem chemical kinetics [44, 45] and experiments performed have not proved the sharp growth of the delay time along the ascending branch. The greatest difference between the theoretical and experimental values have been observed at T < 1,100 K and P > MPa [45-50]. 

Fig. 6.9 Main self-ignition regime diagram for stoichiometric HAM in temperature-pressure coordinates / - strong regime II - transition regime III - mild regime - no self-ignition. Dashed curves -denote constant self-ignition delay...

Fig. 6.10 (a) Calculated (curves) and measured data for self-ignition delay time versus temperature and pressure for a 5% H2 -i- 95% O2 mixture, (b) Calculated (curves) and measured data for self-ignition delay time versus temperature and pressure for a 15% H2 + 85% O2 mixture... 

Comparison of calculated and experimental self-ignition data is presented in Fig. 6.10a, b. AH of the presented t, values were obtained in the 800-1200 K temperature range. The diagrams present the experimental points and indications of the temperature at which self-ignition delay has been measured. 

Conclusions made in [46,47] about HAM and HOM self-ignition characteristics have multiple proofs. Figure 6.11 presents summary collating the experimental data for measured self-ignition delay time [62] and compares them with the data... 

Table 6.2 Self-ignition delay time in pre-mixed and stratified H2 + air mixtures...

Fig. 6.15 Effect of NO additives on the self-ignition delay time at various temperatures. P = 2 MPa...

It is seen that a small amount of NO reduces the self-ignition delay time. The maximum promoting effect was measured at about 0.5% NO by volume. At more than 5% NO by volume, the enhancing effect on self-ignition was not seen. At pressures lower than 1 MPa the NO effect on the process is negligible. 

Figure 6.16 demonstrates the effect of NO2 additives on self-ignition of a 15% H2 + 85% air mixture at 1,100 and 1,200 K temperatures. The 50-time reduction in the self-ignition delay time can be expected at some initial pressures and temperatures. 

The influence of a pressure perturbation on the self-ignition of hydrogenous mixtures, with or without nitrogen oxides, can be characterized by the relation O = dXjJdP (Fig. 6.17a, b). At O > 0 the self-ignition delay time decreases due to... 

Fig. 6.17 (a) Self-ignition delay time versus pressure for a 15% H2 + 85% air mixture with a nitrogen monoxide additive (7) and without this additive (2). (b) Self-ignition delay time pressure index versus pressure for a 15% H2 + 85% air mixture with a nitrogen monoxide additive (i) and without this additive (2)... 

Fig. 6.18 Comparison between H2 and CH4 oxygen mixtures self-ignition delay time at various initial pressures...

Fig. 6.19 Hydrogen/heptane -1- air self-ignition delay time versus temperature and pressure...

Experiments with the hybrid mixtures (hydrogen -i- atomized kerosene -i- oxygen) did not show additional characteristics and proved the insignificant effect of gaseous hydrogen additives on liquid fuel self-ignition delay time. 

Thus, wall perforation [7, 79] leads to formation of attached ignition centers and qualitatively alters the character of t,- = t,- (7 dependence. While investigating self-ignition phenomena in ducts with focusing and perforating elements, standard dependences like t = t (T) are not applicable, because the gas temperature behind the reflected wave is unknown. The dependence of the self-ignition delay time on the Mach number of the incident shock wave t, = t, (M) is the only one that can be used. That dependence is not general and shows partial characteristics of the selected method of observation. 

The water steam effect on H2 + air mixture self-ignition has been analyzed using calculations with up-dated kinetic schemes and is illustrated by the diagrams in Fig. 6.32 (1,000-1,200 K temperature) [7]. The increase in self-ignition delay time Tr2o, for mixtures with water steam, compared to t, for a dry mixture, at several temperature levels, is presented in Fig. 6.33 as a function of the initial pressure. 

Fig. 6.34 Experimental data for self-ignition delay of XH2O + (1 - x) (15%...

The temperature dependence of t, in Fig. 6.35 proves the weak pressure effect on the self-ignition delay at temperatures exceeding 1100 K. For practical assessment of the water steam effect on H2 + air self-ignition within the temperature range 950-1100 K, it is advisable to use the kinetic scheme from [93]. 

Fig. 6.35 Pressure and temperature effect on 11.25% H2 + 63.75% air + 25% H2O (steam) mixture self-ignition delay [44]...

Gaseous carbon dioxide is considered as a potentially efficient diluent causing suppression of blast processes in H2 + air/02 mixtures. In connection with that, by way of example, estimates of H2 + CO2 + air self-ignition delay within practicable pressure and temperature ranges have been made. 

Fig. 6.38 The effect of inert diluents on self-ignition delay time behind the detonation wave in H2 -I- air mixtures at 0.1 MPa initial pressure and 298 K...

There must be a space for forming some volume of a reactive mixture (RM) inside the duct. The formed RM volume has to preserve a high temperature for some time (not less than the self-ignition delay time). 

Expansion ratio at combustion Self-ignition delay time... 

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