Hydrogen/carbon monoxide syngas burning rates measurements in high-pressure quiescent and turbulent environment

A high-pressure, double-chamber, fan-stirred, large-scale explosion facility is proposed for measurements of laminar and turbulent burning velocities, S_L and S_T, of centrally-ignited hydrogen and carbon monoxide syngas/air mixtures over an initial pressure range of p = 0.1-1.0 MPa. Results show that lean syngas laminar flames at elevated pressure are highly unstable resulting in cellular structures all over the expanding flame front surface, where S_L∼p^−0.15 having a relatively modest decrease with pressure as compared to lean methane flames where S_L∼p^−0.50. Contrarily, as to lean syngas turbulent flames, values of S_T increase with increasing pressure (S_T∼p^0.15) at a fixed r.m.s. turbulent fluctuating velocity (u′ ≈ 1.4 m/s). Moreover, it is also shown that increasing u′/S_L is still a way much more effective in increasing values of S_T/S_L than increasing pressure. Finally, discussions are offered and area for further studies identified.     

Effects of H2 or CO2 addition, equivalence ratio, and turbulent straining on turbulent burning velocities for lean premixed methane combustion

Using hydrogen or carbon dioxide as an additive, we investigate the bending effect of turbulent burning velocities (ST/SL) over a wide range of turbulent intensities (u^′/SL) up to 40 for lean premixed methane combustion at various equivalence ratios (?), where SL is the laminar burning velocity. Experiments are carried out in a cruciform burner, in which a sizable downward-propagating premixed CH₄/diluent/air flame interacts with intense isotropic turbulence in the central region without influences of ignition and unwanted turbulence from walls. Simultaneous measurements using the pressure transducer and pairs of ion-probe sensors at various positions of the burner show that effects of gas velocities and pressure rise due to turbulent combustion on ST of lean CH₄/H₂/air flames can be neglected, confirming the accuracy of the ST data. Results with increasing hydrogen additions (δ=10, 20, and 30% in volume) show that the bending of ST/SL vs u^′/SL plots is diminished when compared to data with δ=0, revealing that high reactivity and diffusivity of hydrogen additives help the reaction zone remaining thin even at high u^′/SL. In contrast, the bending effect is strongly promoted when CO₂ is added due to radiation heat losses. This leads to lower values of ST/SL at fixed u^′/SL and ?, where the slope n can change signs from positive to negative at sufficiently large u^′/SL, suggesting that the reaction zone is no longer thin. All ST data with various δ can be well approximated by a general correlation (ST−SL)/u^′=0.17Da^0.43, covering both corrugated flamelet and distributed regimes with very small data scatter, where Da is the turbulent Damköhler number. These results are useful in better understanding how turbulence and diluents can influence the canonical structures of turbulent premixed flames and thus turbulent burning rates.     

Hysteresis loops of methane catalytic partial oxidation for hydrogen production under the effects of varied Reynolds number and Damköhler number

Hysteresis loops of catalytic partial oxidation of methane (CPOM) for hydrogen production under the effects of varied Reynolds number and Damköhler number are investigated numerically in this study. The physical phenomena are predicted using the indirect mechanism, which consists of the total oxidation (or combustion), steam reforming and CO₂ reforming of methane in a catalyst bed. Numerical results reveal that, when the Damköhler number is relatively low, a hysteresis loop of CPOM from varying Reynolds number develops. Increasing the Damköhler number leads to the loop shifting toward the regime of high Reynolds number. However, once the Damköhler number is large to a certain extent, the chemical reactions are always exhibited for the Reynolds number less than 2000. A closed loop is thus not observed. Alternatively, for a given Reynolds number, an ignited Damköhler number and an extinguished Damköhler number can be obtained. Accordingly, three different regions in the plot of Damköhler number versus Reynolds number are identified. Physically, when the role played by Damköhler number on CPOM is much more important than by the Reynolds number (Region I), the thermal effect governs the chemical reactions. In contrast, if the Reynolds number plays a key role in determining the CPOM (Region III), the chemically frozen flow prevails over the catalyst bed. When the residence times of the total oxidation and convection in the catalyst bed are in an equivalent state (Region II), CPOM is characterized by a dual-solution, rendering the hysteresis loops. From the distributions of ignited and extinguished Damköhler numbers, the catalytic reactor and operation of partial oxidation of methane and other fuels can be designed accordingly.