Investigation on the Combustion Characteristics and Flame Inhibition of Bio-derived Oxygenated 2-methyltetrahydrofuran


Student thesis: Doctoral Thesis

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Awarding Institution
  • Kim Meow LIEW (Supervisor)
  • Yong Jiang (External person) (External Supervisor)
Award date29 Jun 2022


Bio-fuels play a unique constructive role in reducing carbon emissions due to their carbon-neutral and renewable properties. The second-generation oxygenated biofuel 2-methyltetrahydrofuran (2-MTHF) is cyclic ether. Many investigations have demonstrated that 2-MTHF can be a promising alternative fuel due to its renewable nature and environmentally friendly properties. Recent foremost breakthroughs in 2-MTHF’s production routes will increase its widespread use. Nevertheless, the fuels’ production, transportation, and application require high-pressure operating conditions. If accidental leakage occurs, it will result in solemn fire and explosion accidents. Therefore, it is necessary to perform studies on 2-MTHF’s combustion characteristics and flame inhibition at higher pressures concerning the fire protection and control perspective.

Pressure displays a crucial effect on combustion by affecting the reaction rate and laminar burning velocity (LBV) and modifying the cellular instability perturbation of the flame. Many cracks, wrinkles, and cellular structures will form on the flame front due to the inherent instabilities and external disturbances. These structures will split and enlarge, leading to a rapid increase in flame surface area and self-acceleration as the flame expands. Cellular instability is a precursor to the flame’s self-turbulence and can even cause deflagration and explosion. The spherical flame method is one of the essential methods to investigate the laminar premixed combustion characteristics, enabling the precise measurement of LBV under elevated pressure, temperature, and wide equivalence ratio conditions. Besides, it can trace the flame’s cellular instability via a high-speed camera. The LBV over a broad pressure range is critical for validating and revising the reaction mechanism and is also a crucial input parameter for turbulent combustion and fire simulation.

Fundamental experimental data on the LBV and cellular instability of 2-MTHF under high pressures are scanty. Moreover, only a few studies address 2-MTHF’s flame inhibition. Thus, this study investigated 2-MTHF’s laminar flame characteristics and flame inhibition under varied initial pressures. There are improvements in the experimental and numerical methods. The combustion bomb system was modified and upgraded by expanding the boundary conditions and improving the measurement accuracy, and the data analysis methods were further complicated. The LBVs and Markstein lengths (Lb) were measured under varied conditions. The MLOC-S reduction method was proposed because of the difference in reaction time scale and hardship in the computational convergence of the detailed mechanism after coupling with the fire extinguishing agent. The MLOC-S method allowed nonlinear mapping and multi-point sampling, combining species correlation and sensitivity analysis to reduce the detailed mechanism. The study addressed the following essential issues:

It investigated the effect of pressure on the LBVs and inherent instabilities in spherically expanding 2-MTHF flames. Also, it established an underlying experimental database of LBVs and Lbs. Updating some third-body reactions’ coefficients enabled a broadly applicable mechanism. The study also assessed the critical conditions where the flame status changed from stable to unstable. It also studied 2-butanone (MEK), a straight-chain oxygenated bio-fuel, for comparison to further analyze the common behaviors presented by different types of oxygenated fuels with different chain structures. The propagation of MEK/air flames was similar to 2-MTHF: The increase in pressure efficiently suppressed flame propagation and hastened the flame front cellularization. The onset of cellular instabilities of premixed mixtures advanced to a smaller radius at higher initial pressures. Yet, the MEK flame entered the self-acceleration propagation earlier than the 2-MTHF flame.

This study also explored the physical suppression effect of carbon dioxide (CO2), nitrogen (N2), and helium (He) dilution on 2-MTHF/air flames at elevated pressure. The results illustrated that CO2 displayed the highest ability to decrease the LBVs, followed by N2 and He. Moreover, it delved into the suppression effects of diluents and diluent ratio on cellular instabilities. The relative magnitudes of these diluents’ stabilizing abilities were in the order of He > CO2 > N2.

The study also addressed the fire extinguishing agent’s (represented by DMMP) suppression efficiency in 2-MTHF/air premixed flames. The detailed mechanism of 2-MTHF was reduced, depending on the MLOC-S method, and generated Mech-F with 96 species and 333 elementary reactions. The DMMP’s fire extinguishing efficiency was investigated with the Mech-F’s and DMMP’s coupled mechanism. When the DMMP addition Xd>"1.0%" , the reduction of the flame speed decreased gradually, and the fire extinguishing efficiency of DMMP was saturated. The stability analysis revealed that the DMMP addition efficiently suppressed the cellular instability on the flame front. Furthermore, the results disclosed that DMMP was more efficient for delaying the cellularization of fuel-lean flames.

This thesis’ primary innovations and contributions were as follows. An advanced experimental and theoretical platform was instituted, studying the 2-MTHF spherical flames’ propagation and suppression. The 2-MTHF’s fundamental experimental dataset was established; The detailed mechanisms corrections were made; The cellular instability progress of 2-MTHF flame was explored at higher pressures; The study also addressed the 2-MTHF flame’s physical dilution and chemical suppression; The MLOC-S reduction method was proposed to construct a compact mechanism with high reliability and computational efficiency, providing crucial input files for turbulent combustion simulations.

    Research areas

  • Oxygenated biofuels, 2-methyltetrahydrofuran, Laminar burning velocity, Cellular instability, Combustion bomb, Mechanism reduction