Auto-ignition characteristics of methane/n-heptane mixtures under carbon dioxide and water dilution conditions. (15th November 2020)

• Record Type: Journal Article
• Authors: Gong, Zhen; Feng, Liyan; Qu, Wenjing; Li, Lincheng; Wei, Lai
• Is Part Of: Applied energy. Volume 278(2020)
• Publication Date: 2020-11-15
• Subjects: N-heptane -- Methane -- Carbon dioxide -- Water -- Ignition delay time -- Shock tube; IMO international maritime organization -- RCCI reactivity controlled compression ignition -- HCCI homogeneous charge compression ignition -- PPC partially premixed combustion -- MILD moderate or intense low-oxygen dilution -- IDT ignition delay times -- PMT photomultiplier tube -- NTC negative temperature coefficient -- ROP rate of production -- Φ overall equivalence ratio of mixture -- P initial pressure -- T initial temperature -- C specific heat capacity -- EGR exhaust gas recirculation -- TDC top dead center -- CH4 methane -- n-C7H16 n-heptane -- CO2 carbon dioxide -- H2O water -- Ar argon -- He helium -- N2 nitrogen; Power (Mechanics) -- Periodicals; Energy conservation -- Periodicals; Energy conversion -- Periodicals; 621.042
• DOI: 10.1016/j.apenergy.2020.115639
• Languages: English
• ISSNs: 0306-2619

Abstract

Highlights: Influence of exhaust gases on the ignition of CH4 /n-C7 H16 mixture were revealed. Thermal, chemical and third-body collision effects are qualitatively compared. CO2 suppressed ignition due to its thermal and chemical effect. H2 O accelerated ignition due to its third-body and chemical effect. The addition of CO2 /H2 O mixture slightly fastened auto-ignition of CH4 /n-C7 H16 . Abstract: To further optimize the combustion performance of natural-gas/diesel engine under exhaust gas recirculation (EGR) condition, influence of physicochemical impacts of H2 O and CO2 on the ignition characteristics of n-heptane/methane mixture ( Φ = 0.5/1.0) under various thermodynamic conditions ( p = 2 bar/1199 K < T < 1568 K, p = 60 bar/700 K < T < 1200 K) were investigated by shock tube and NUI mechanism. Experiments indicated at p = 2 bar/1199 K < T < 1568 K, CO2 and H2 O additions increased and shortened ignition delay times (IDT) respectively. Mixture of CO2 and H2 O slightly accelerated ignition. Original and minor modified NUI mechanism well captured the inhibition and acceleration effect of CO2 and H2 O on ignition respectively. Both thermal and chemical effects of CO2 (R36:CO + OH = CO2 + H) were responsible for its ignition inhibition impacts at higher temperatures. Whereas thermal effect of CO2 became the dominant factor at lower temperatures. Chemical effect of H2 O (H2 + OH<=>H + H2 O&O + H2 O<=>2OH) promoted OH formation and enhanced whole system reactivity, which suppressed its thermal effect and accelerated ignition process. At p = 60 bar/700 K < T < 1200 K, CO2 addition significantly retarded ignition due to its thermal effect. At 900 K < T < 1200 K, the higher third-body efficiency of H2 O promoted ignition (R21:H2 O2 (+M) = 2OH(+M)&R34:H + O2 (+M) = HO2 (+M)). At 700 K < T < 900 K, thermal effect of H2 O, which suppressed its third-body effect, inhibited ignition progress. These observations implied that the impact of exhaust gases on the ignition of n-heptane/methane depended on the coupled influence mechanisms of physicochemical effects, the composition of exhaust gases and located thermodynamic conditions. At intermediate-temperature high-pressure conditions (typical thermodynamic conditions of dual-fuel engine), raising the concentration of H2 O or CO2 in exhaust gases accelerated or delayed the combustion progress of natural-gas/diesel engine at EGR condition by enhanced third-body effect or thermal effect respectively.