Laminar Burning Velocities of Hydrogen-Blended Methane–Air and Natural Gas–Air Mixtures, Calculated from the Early Stage of p ( t ) Records in a Spherical Vessel

: The ﬂammable hydrogen-blended methane–air and natural gas–air mixtures raise speciﬁc safety and environmental issues in the industry and transportation; therefore, their explosion characteristics such as the explosion limits, explosion pressures, and rates of pressure rise have signiﬁcant importance from a safety point of view. At the same time, the laminar burning velocities are the most useful parameters for practical applications and in basic studies for the validation of reaction mechanisms and modeling turbulent combustion. In the present study, an experimental and numerical study of the effect of hydrogen addition on the laminar burning velocity (LBV) of methane–air and natural gas–air mixtures was conducted, using mixtures with equivalence ratios within 0.90 and 1.30 and various hydrogen fractions r H within 0.0 and 0.5. The experiments were performed in a 14 L spherical vessel with central ignition at ambient initial conditions. The LBVs were calculated from p ( t ) data, determined in accordance with EN 15967, by using only the early stage of ﬂame propagation. The results show that hydrogen addition determines an increase in LBV for all examined binary ﬂammable mixtures. The LBV variation versus the fraction of added hydrogen, r H , follows a linear trend only at moderate hydrogen fractions. The further increase in r H results in a stronger variation in LBV, as shown by both experimental and computed LBVs. Hydrogen addition signiﬁcantly changes the thermal diffusivity of ﬂammable CH 4 –air or NG–air mixtures, the rate of heat release, and the concentration of active radical species in the ﬂame front and contribute, thus, to LBV variation.


Introduction
In recent years, dependence on fossil fuels as the main energy source has led to a worldwide crisis, due to fossil fuel depletion, and environmental problems due to pollutant emissions have become severe. To respond to these problems, continuous attempts have been made in the exploration of clean, renewable alternatives for sustainable development [1,2]. The research on alternative fuels is mainly focused on natural gas [3][4][5][6][7], hydrogen [8][9][10][11][12][13][14][15], and alcohols [16,17], able to be mixed with gasoline, diesel, or to form mutual blends.
The extended use of natural gas (NG) as fuel in internal combustion (IC) and gas turbines prompted numerous research groups to examine its combustion in air, under various initial conditions (NG/air ratio, pressure, temperature, additives) [4][5][6]. A common practice in NG use is its blending with hydrogen. Hydrogen has attracted the attention of researchers as a renewable clean energy source. Hydrogen is easily ignitable and its flame spreads quickly, which is essential for IC engines to order to work under an improved combustion efficiency [7][8][9][10][11][12][13][14][15]. Hydrogen addition to NG is a good option for ambient initial conditions. For this, p(t) data obtained in experimental investigations were used. These measurements, performed according to European Standards EN 15967 [46], were originally conducted to determine the maximum explosion pressure p max , the maximum rate of pressure rise (dp/dt) max , and the deflagration index K G . The paper reports LBVs of hydrogen-blended methane-air and natural gas-air mixtures, using mixtures at ambient initial pressure and temperature (p 0 = 0.97-1.04 bar, T 0 = 22-25 • C). The mixtures being studied consist of single fuel-air (CH 4 -air) and multifuel-air mixtures, where the multifuel was CH 4 blended with H 2 , or natural gas blended with H 2 , both under various amounts of added hydrogen, within 0-50 mol% (in respect to the initial fuel amount). The studied systems had either a constant fuel (CH 4 or NG)/H 2 ratio and a variable equivalence ratio or a variable fuel (CH 4 or NG)/H 2 ratio and a constant equivalence ratio. Their laminar burning velocities were determined from the cubic law constants, evaluated from p(t) records obtained in the early stage of closed vessel explosions. Thus, the LBV of H 2 -blended CH 4 -air and NG-air were determined from single p(t) records, without the additional optical methods of investigation, and were found to describe well the behavior of these multifuel mixtures.
The present results represent a contribution to the data pool regarding the flame propagation in hydrogen-blended CH 4 /air and NG/air mixtures. Earlier studies [47,48] provided data for risk assessment at gas processing facilities, especially those involving the introduction of hydrogen into existing natural gas networks under project NATURALHY financed by the EU. The unwanted consequences of such events, i.e., loss of assets, human injuries, fatalities, or severe environmental impacts, can be avoided by a proper safety analysis based on experiments, as discussed by Amin et al. [49].

Experimental Procedure
The experiments were conducted at BAM as part of a systematic investigation on safety-related characteristics of natural gas-hydrogen mixtures [11,43]. The pressure-time curves obtained during the determination of explosion limits, explosion pressures, and deflagration constants (K G ) could be used directly to calculate the LBVs using the method described below. The laboratory set-up was previously described in detail [11]. The setup affords the monitoring of closed vessel explosions for gaseous mixtures of variable composition, pressure, and temperature. A 14 L spherical vessel with central ignition was used to perform experiments with quiescent gas mixtures. An exploding wire igniter (nickeline wire, diameter: 0.12 mm, distance of electrodes: 5 mm) delivered about 15 J for ignition. The ignition systems used are in accordance with EN 1839 [50].
The pressure measurements were made with piezoresistive pressure transducers, type PAA10 (Keller). For mixture preparation pressure transducers with a measuring range between 0 bar (abs) and 2 bar (abs) or between 0 bar (abs) and 5 bar (abs) were used. For measuring the explosion pressure, pressure transducers with a measuring range between 0 bar (abs) and 10 bar (abs) were used. The measuring frequency was 20,000 Hz. The digitized signals of the measuring sensors were connected to an A/D converter, type MCL-USB (16 channels, 16 Bit A/D, sampling frequency: 500 kHz, from Jet Systemtechnik GmbH) and a PC for displaying, storing, and evaluating the data.
Two types of European natural gas were tested: one representing a typical Russian natural gas composition, nearly pure methane, and one of wet type containing CH 4 and a significant amount of C 2 -C 4 alkanes, similar to Northern Sea natural gas [11]. The composition of the studied natural gas is given in Table 1. Using pure CH 4 and wet natural gas blended with H 2 , multifuel-air mixtures of variable hydrogen content and equivalence ratio were studied at ambient initial pressure and temperature. Their composition is given in Table 2. The hydrogen content in the initial flammable mixture is expressed as the fraction r H , defined as: The equivalence ratio, ϕ, of any flammable mixture is defined as: [fuel] [oxygen] st where the index "st" refers to the stoichiometric concentration of the fuel-air mixture. The mixtures of CH 4 or NG with different fractions of hydrogen and air were prepared in a pressure resistant stainless-steel vessel, according to the partial pressure method at total pressure of 2 bar or 5 bar, assuming an ideal gas behavior for all gaseous components.

Computing Program
The burning velocities of fuel-air and multifuel-air systems, at ambient initial conditions, were delivered by the kinetic modeling of their flames using COSILAB software (version 3.0.3) and Gas Research Institute (GRI) mechanism version 3.0 [51], where 53 chemical species and 325 elementary reactions are taken into account. The input data were taken from the thermodynamic and molecular databases of Sandia National Laboratories, USA, according to the international standard (format for CHEMKIN). The runs were performed for premixed 1D adiabatic laminar free flames of stoichiometric CH 4 -air, NG-air, and H 2 -blended mixtures at ambient initial conditions. In these runs, a steady Newton solver (25 iterations, relative tolerance 10 −5 ; absolute tolerance 10 −8 ), an unsteady Newton solver (15 iterations, relative tolerance 10 −4 ; absolute tolerance 10 −6 ), and an unsteady Euler solver were used, as mentioned earlier [52,53]. The parameters of the adaptive grid were GRAD = 0.1, CURV = 0.2, with a maximum ratio of adjacent cell size between 1.3 and 1.1.

Data Evaluation
The pressure rise in the early stage of closed vessel explosions, ∆p, was found to increase with time from ignition, t, according to so-called "cubic law" [52,54]: The equation is valid as long as the spherical propagation of the combustion wave is not disturbed. In small-scale explosions, the flame front of reactive mixtures does not develop significant cellular structures and no gas turbulence appears. Indeed, the pressure rise ∆p was found proportional to t 3 in quiescent conditions of combustion (CH 4 -air, at p 0 = 1 bar (Dobashi [55]) and aviation grade JP-4 fuel with air, at p 0 = 135-275 bar (Knapton et al. [56]) and to t 3.6−6.0 in turbulent conditions (CH 4 -air, at p 0 = 1 bar (Dobashi [55]). In large-scale explosions (vessels with a volume between 1 and 10 m 3 ), where the buoyancy effect determines the distortion of the flame, even in the early stage, the time exponent varied between 3.3 and 5.0 for stoichiometric CH 4 -air mixtures diluted with various amounts of nitrogen (Sapko et al. [57]).
Assuming the isothermal compression of unburned gas ahead the flame front, the laminar burning velocity, S u , was related to k, the cubic law coefficient of pressure rise by the relationship [54]: where R is the radius of the explosion vessel, ∆p max is the peak pressure rise in the explosion at the initial pressure p 0 , and p max = ∆p max + p 0 . The measured values of ∆p max and p max are used as input values for S u calculation.
The cubic law coefficient can be determined for each experiment over a restricted pressure range (p 0 ≤ p ≤ 2p 0 ) using a nonlinear regression method with a relationship of the form: where a and c are corrections for pressure and time, respectively, necessary for eliminating the signal shift of the pressure transducer and the possible delay in signal triggering [52]. Smaller residuals and a random error distribution, indicating better correlations, were obtained by using this modified Equation (4), instead of Equation (2). The method was successfully used for determining LVBs of various hydrocarbon-oxidizer mixtures [52][53][54][58][59][60][61] under various initial conditions of pressure and/or temperature.

Results and Discussion
A set of experimental burning velocities of CH 4 -H 2 -air mixtures obtained from p(t) measurements in a closed spherical vessel at ambient initial pressure, temperature, and constant r H , is given in Table 3. The influence of the initial composition (characterized by r H and ϕ) on experimental laminar burning velocities is shown in Figure 1, where data referring to CH 4 -H 2 -air and NG-H 2 -air mixtures are given. The burning velocities of all mixtures increase by increasing the equivalence ratio and reaching their peak values, then start to decrease at higher equivalence ratios. The LBV variation due to H 2 addition is observed already at r H = 0.05 for lean CH 4 -air mixtures. In the range of rich CH 4 -air and NG-air mixtures, the LBV changes significantly only at r H > 0.10. For higher hydrogen contents (r H = 0.25 and 0.50), a marked increase in LBV appears over the whole examined range of equivalence ratios.  [5], and 38 cm s −1 [53]. For this stoichiometric CH4-air mixture, the earlier reported LBVs vary within 35 and 41.5 cm s −1 [4][5][6]62] at ambient initial conditions, a domain where the present values range well. For the stoichiometric NG-air mixture, the LBV is Su = 45.7 cm s −1 , close to 44.0 cm s −1 , previously reported [6,37].
Hydrogen addition to CH4 or NG results in an increase in Su at all initial concentrations of CH4 or NG. The blended mixtures (CH4-H2 or NG-H2) with a constant [H2] having various equivalence ratios develop the maxima of their burning velocities at slightly rich mixtures, with φ = 1.05 ± 0.05.
Both datasets were well fitted by 2nd order polynomials, plotted as well in Figure 1. The points are scattered around the best-fit lines; a possible cause is the turbulence created already in the early stage of explosion by the ignition, made by a strong initiation source (exploding wires, which delivered 15 J). This turbulence could determine deviations from the spherical propagation and errors of the model used. Other methods were also used to extract the laminar burning velocity from transient pressure measurements [63][64][65][66][67]. These methods require equilibrium calculations of unburned and burned gas states to determine LBV using the transient burned mass fraction. They are rigorous methods, which also deliver the temperature gradients in the burned gas and afford the evaluation of the flame stretch effect on burning velocity. In comparison with them, the actual method for determining LBVs from pressure measurements in the early stage of explosion propagation has the advantage of delivering LBVs without additional computations and without additional experimental techniques for measuring the flame radius. This aspect is an advantage when flammable mixtures of multifuels or multi-oxidizers are examined, and the thermodynamic calculations of the intermediate states are impossible or very difficult.
Hydrogen addition to CH 4 or NG results in an increase in S u at all initial concentrations of CH 4 or NG. The blended mixtures (CH 4 -H 2 or NG-H 2 ) with a constant [H 2 ] having various equivalence ratios develop the maxima of their burning velocities at slightly rich mixtures, with ϕ = 1.05 ± 0.05.
Both datasets were well fitted by 2nd order polynomials, plotted as well in Figure 1. The points are scattered around the best-fit lines; a possible cause is the turbulence created already in the early stage of explosion by the ignition, made by a strong initiation source (exploding wires, which delivered 15 J). This turbulence could determine deviations from the spherical propagation and errors of the model used. Other methods were also used to extract the laminar burning velocity from transient pressure measurements [63][64][65][66][67]. These methods require equilibrium calculations of unburned and burned gas states to determine LBV using the transient burned mass fraction. They are rigorous methods, which also deliver the temperature gradients in the burned gas and afford the evaluation of the flame stretch effect on burning velocity. In comparison with them, the actual method for determining LBVs from pressure measurements in the early stage of explosion propagation has the advantage of delivering LBVs without additional computations and without additional experimental techniques for measuring the flame radius. This aspect is an advantage when flammable mixtures of multifuels or multi-oxidizers are examined, and the thermodynamic calculations of the intermediate states are impossible or very difficult.
Examination of literature on LBV of H 2 -blended CH 4 -air and NG-air mixtures provides a wide database of comparison. All datasets show the same variation of burning velocities when examined as a function of the equivalence ratio (at constant r H ) or function of r H (at constant ϕ) [3,7,19,21,23,[27][28][29][30]37,38,68,69]. Differences arise among measurements performed using different techniques, involving various corrections of the recorded information.
A closer examination of present results compared with literature data can be made by examining the LBV of stoichiometric CH 4 -air and NG-air mixtures blended with hydrogen at various fractions of added H 2 , as shown by the plots given in Figure 2. With respect to CH 4 -H 2 -air mixtures (Figure 2a), the present results are found to match well with those obtained as stretch-free LBV, using the technique of counter-flow flames [23]. For the Energies 2021, 14, 7556 7 of 16 examined range of the fraction of added hydrogen, r H ≤ 0.5, they seem to depend linearly on it. The same holds for the other datasets, obtained mostly with stationary flames using the heat flux technique [4,24,28,31]. However, by extending these plots to the whole range of hydrogen composition (0 ≤ r H ≤ 1.0), one can see the fast increase in LBV at hydrogen fractions higher than 0.5. At all fractions of added H 2 , the datasets obtained using the heat flux method consist of lower LBV than the present data, which were not stretch-corrected, but their trend is the same. The H 2 -blended NG-air mixtures can be characterized by similar plots (Figure 2b); their LBV follows the same marked increase with r H at hydrogen fractions higher than 0.3.
A closer examination of present results compared with literature data can be made by examining the LBV of stoichiometric CH4-air and NG-air mixtures blended with hydrogen at various fractions of added H2, as shown by the plots given in Figure 2. With respect to CH4-H2-air mixtures (Figure 2a), the present results are found to match well with those obtained as stretch-free LBV, using the technique of counter-flow flames [23]. For the examined range of the fraction of added hydrogen, ≤ 0.5, they seem to depend linearly on it. The same holds for the other datasets, obtained mostly with stationary flames using the heat flux technique [4,24,28,31]. However, by extending these plots to the whole range of hydrogen composition (0 ≤ ≤ 1.0), one can see the fast increase in LBV at hydrogen fractions higher than 0.5. At all fractions of added H2, the datasets obtained using the heat flux method consist of lower LBV than the present data, which were not stretch-corrected, but their trend is the same. The H2-blended NG-air mixtures can be characterized by similar plots (Figure 2b); their LBV follows the same marked increase with at hydrogen fractions higher than 0.3.
(a) (b) A comparison of LBV for the stoichiometric CH4-H2-air and NG-H2-air mixtures, using the present data, is shown in Figure 3 (experimental LBV) and Figure 4 (computed LBV) where the dimensionless (relative) , defined as: is plotted against the mole fraction of hydrogen, . Yu et al. [23] Coppens et al. [24] Hermanns et al. [28] Hu et al. [31] Dayma et al. [72]   A comparison of LBV for the stoichiometric CH 4 -H 2 -air and NG-H 2 -air mixtures, using the present data, is shown in Figure 3 (experimental LBV) and Figure 4 (computed LBV) where the dimensionless (relative) LBV rel , defined as: is plotted against the mole fraction of hydrogen, r H .       For both experimental and calculated datasets, H2 addition influences to a greater extent the LBV of CH4-air, as compared to NG-air.
For the restricted range of hydrogen fractions 0 ≤ ≤ 0. At any hydrogen concentration, the burning velocities of NG-H2-air are smaller than those of CH4-H2-air, as a consequence of the presence of heavier hydrocarbons in NG. Both experimental and modeling studies on the effect of hydrogen addition to the C2, C3, or C4 alkanes [3,8,[19][20][21]69,70] showed that the enhancement of LBV upon the addition of 50% H2 was about 20-40% lower compared to when the fuel was pure methane. The computed LBV can be compared to present experimental LBVs when plotted as functions of the equivalence ratios of H2-blended CH4-air and NG-air mixtures, as shown in Figure 5. The shape of the experimental ( Figure 1) and computed LBVs plots is similar. The plots obtained for CH4-H2-air have marked maxima appearing around φ = 1.10 for all . In contrast to them, the plots obtained for NG-H2-air are rather flat; their maxima appear in the range φ = 1.0-1.10.  Both experimental and modeling studies on the effect of hydrogen addition to the C 2 , C 3 , or C 4 alkanes [3,8,[19][20][21]69,70] showed that the enhancement of LBV upon the addition of 50% H 2 was about 20-40% lower compared to when the fuel was pure methane.
The computed LBV can be compared to present experimental LBVs when plotted as functions of the equivalence ratios of H 2 -blended CH 4 -air and NG-air mixtures, as shown in Figure 5. The shape of the experimental ( Figure 1) and computed LBVs plots is similar. The plots obtained for CH 4 -H 2 -air have marked maxima appearing around ϕ = 1.10 for all r H . In contrast to them, the plots obtained for NG-H 2 -air are rather flat; their maxima appear in the range ϕ = 1.0-1.10. Another comparison between the experimental and the computed burning velocities is shown in Figure 6 for the stoichiometric CH4-H2-air and NG-H2-air mixtures with various hydrogen contents. Another comparison between the experimental and the computed burning velocities is shown in Figure 6 for the stoichiometric CH 4 -H 2 -air and NG-H 2 -air mixtures with various hydrogen contents.
Another comparison between the experimental and the computed burning velocities is shown in Figure 6 for the stoichiometric CH4-H2-air and NG-H2-air mixtures with various hydrogen contents.
The relationship is similar to Le Châtelier's rule for predicting the flammability limits of multifuel gases, using the flammability limit of each fuel from the mixture [71]. For the For both CH 4 and NG mixtures, the computed LBV are lower than the experimental LBVs, and the differences increase in parallel with r H . Figure 6a contains an additional plot of predicted LBVs, calculated from the experimental LBVs of pure H 2 -air and CH 4 -air mixtures and the composition of blended mixtures by using an equation proposed by Di Sarli et al. [25]: The relationship is similar to Le Châtelier's rule for predicting the flammability limits of multifuel gases, using the flammability limit of each fuel from the mixture [71]. For the examined range of hydrogen concentrations, the predicted LBVs are lower than those directly obtained from experiments. However, one must consider that S u,H 2 used to determine the predicted LBVs were taken from the literature [71] and do not belong to the present set of experimental burning velocities.
The deviations between experimental and computed LBV can be assigned to the fact that GRI 3.0 Mech is adequate for modeling the combustion of C1-C4 alkanes (components of natural gas) but less adequate for modeling the combustion of multifuel-air mixtures with a high H 2 content. Better predictions were obtained after using improved mechanisms [19,25,28,30].
The enhancement effect of hydrogen upon flame propagation in H 2 -blended mixtures can be better understood by examining the flame structure obtained from the kinetic modeling of CH 4 -air and CH 4 -H 2 -air flames. In this purpose, the stoichiometric composition of these flammable mixtures was chosen for systems at ambient initial pressure and temperature.
The influence of H 2 concentration on temperature and the volumetric rate of heat release profiles of CH 4 -H 2 -air flames is shown in Figures 7 and 8 at 0 ≤ r H ≤ 0.5. The temperature of burned gas is not influenced by hydrogen addition; only the flame front thickness (evaluated as the half of the reaction zone) is decreased by H 2 addition. This variation was assigned by Salzano et al. [69] to the increased amount of NO x produced by the combustion of H 2 -blended CH 4 -air ternary mixtures. In contrast to this trend, H 2 addition to CH 4 -air results in a marked change in the heat release, as shown by data from Figure 8. Similar plots were obtained for H 2 -blended NG-air ternary mixtures. temperature of burned gas is not influenced by hydrogen addition; only the flame front thickness (evaluated as the half of the reaction zone) is decreased by H2 addition. This variation was assigned by Salzano et al. [69] to the increased amount of NOx produced by the combustion of H2-blended CH4-air ternary mixtures. In contrast to this trend, H2 addition to CH4-air results in a marked change in the heat release, as shown by data from Figure 8. Similar plots were obtained for H2-blended NG-air ternary mixtures. The change in the volumetric rates of heat release with the fraction of added hydrogen is shown in Figure 9 for both CH4-H2-air and NG-H2-air flames. Similar plots are obtained by examining 's influence on the sum of peak mass fractions of the most abundant radical species: H, OH, O, and HO2, shown in Figure 10. The differences between CH4-H2-air and NG-H2-air flames are observed only for mixtures with a hydrogen content below 0.25.
The modeling, thus, indicates a combined (synergetic) influence of these two factors: the rate of heat release and the abundance of reactive radical species. Their individual influence on LBVs is shown in Figures 11 and 12.  The change in the volumetric rates of heat release with the fraction of added hydrogen is shown in Figure 9 for both CH4-H2-air and NG-H2-air flames. Similar plots are obtained by examining 's influence on the sum of peak mass fractions of the most abundant radical species: H, OH, O, and HO2, shown in Figure 10. The differences between CH4-H2-air and NG-H2-air flames are observed only for mixtures with a hydrogen content below 0.25.
The modeling, thus, indicates a combined (synergetic) influence of these two factors: the rate of heat release and the abundance of reactive radical species. Their individual influence on LBVs is shown in Figures 11 and 12.       The literature data show that H 2 blending of CH 4 -air or NG-air results in the enhancement of their reactivity [3,7,8,19,21,23,[36][37][38]69,70,72,73], determining the LBV increase even if the temperature of their burned gases is little influenced by H 2 addition. The results of the present study, thus, show good agreement with previous studies on these flammable mixtures.
A less examined factor contributing to the observed variations of LBV is the thermal diffusivity of blended mixtures, which increases many folds by hydrogen addition. In accordance with the thermal theory of flame propagation, the laminar burning velocity can be written as: where D = λ ρ·c P is the thermal diffusivity of the flammable mixture, C 0 is the total initial fuel concentration, T f is the end temperature in the flame front, and n and E a are the overall activation parameters (reaction order and activation energy) of the oxidation reaction [74]. The thermophysical properties of CH 4 , H 2 , O 2 , and N 2 are listed in Table 4. Close values are expected for NG, when compared to pure CH 4 . Indeed, the high thermal conductivity of hydrogen influences the thermal diffusivity of H 2 -blended CH 4 -air and NG-air mixtures, resulting in higher LBVs compared to non-blended flammable mixtures.

Conclusions
The flammability characteristics of methane or natural gas enriched with hydrogen are frequently studied to ensure the safe use of these mixtures in various activity branches. Although, in practical applications, hydrogen concentration does not exceed 20%, the present study was focused on hydrogen percentages between 0 and 50% in the blended CH 4 -H 2 or natural gas-H 2 mixtures.
In the study, the propagation of H 2 -blended methane or natural gas-air flames in a closed spherical vessel was monitored by means of p(t) variation recorded in the early stage of spherical propagation. Its novelty consists of the possibility to determine LBV from single p(t) records, without additional optical methods of investigation. This simple method for evaluating data does not consider the flame stretch in this incipient stage of propagation and the flame width. However, the delivered results can be used for a preliminary characterization of the propagation stage by the LBV, extrapolated to the initial moment of experiment, i.e., at (p 0 ; T 0 ). An additional reason is based on the possibility to use the present method for multifuel-or multioxidizer-flammable mixtures, where the exact calculation of thermodynamic conditions for the burned gas, under transient conditions, is difficult, even impossible.
The experimental laminar burning velocities were examined in comparison with the computed burning velocities obtained from the numerical modeling of 1D laminar flames with the GRI 3.0 mechanism.
A fair agreement between the experimental burning velocities and reference values from the literature was observed not only for CH 4 -air mixtures but for H 2 -blended CH 4 -air mixtures as well. The same good agreement found for present LBVs of NG-air mixtures with literature values confirmed the possibility of using the actual simple method for extracting laminar burning velocities from the measurements of transient pressure in the early stage of explosion propagation.
The addition of increasing amounts of H 2 to CH 4 or to natural gas was found to determine the increase in laminar burning velocity for all investigated compositions of the methane-air or natural gas-air mixtures. In parallel, a strong influence of added hydrogen on the rate of heat release and concentration of active radical species in the flame front was observed. The results are assigned to the higher thermal conductivity and thermal diffusivity of hydrogen, resulting in a higher reaction rate and a higher heat dissipation rate of H 2 -blended mixtures.
The reported data can further help to identify the degree of danger of explosions and develop more effective measures to prevent explosions of flammable mixtures.