Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: A comparative DNS study

Luong, Minh Bau,Yu, Gwang Hyeon,Chung, Suk Ho,Yoo, Chun Sang Elsevier 2017 Proceedings of the Combustion Institute Vol.36 No.3

<P><B>Abstract</B></P> <P>The ignition characteristics of a lean primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated using 2-D direct numerical simulations (DNSs) with a 116-species reduced mechanism of PRF oxidation. For RCCI combustion, <I>n</I>-heptane and <I>iso</I>-octane are used as two different reactivity fuels and the corresponding global PRF number is PRF50 which is also used as a single fuel for SCCI combustion. The 2-D DNSs of RCCI/SCCI combustion are performed by varying degree of fuel stratification, <I>r</I>, and turbulence intensity, <I>u</I>′, at different initial mean temperature, <I>T</I> <SUB>0</SUB>, with negatively-correlated <I>T</I>–<I>r</I> fields. It is found that in the low- and intermediate-temperature regimes, the overall combustion of RCCI cases occurs earlier and its mean heat release rate (HRR) is more distributed over time than those of the corresponding SCCI cases. This is because PRF number stratification, PRF′, plays a dominant role and <I>T</I>′ has a negligible effect on the overall combustion within the negative temperature coefficient (NTC) regime. In the high-temperature regime, however, the difference between RCCI and SCCI combustion becomes marginal because the ignition of the PRF/air mixture is highly-sensitive to <I>T</I>′ rather than PRF′ and <I>ϕ</I>′. The Damköhler number analysis verifies that the mean HRR is more distributed over time with increasing <I>r</I> because the portion of deflagration mode of combustion becomes larger with increasing fuel stratification. Finally, it is found that the overall combustion of both RCCI and SCCI cases becomes more like the 0-D ignition with increasing <I>u</I>′ due to the homogenization of initial mixture by turbulent mixing.</P>

DNSs of the Ignition of a Lean PRF/Air Mixture under RCCI/SCCI Conditions

Minh Bau Luong,Kwang Hyeon Yu(유광현),Chun Sang Yoo(유광현) 한국연소학회 2014 KOSCOSYMPOSIUM논문집 Vol.2014 No.11

A comparative DNS study of the ignition characteristics of dual-fueled reactivity controlled compression ignition (RCCI) and stratification charge compression ignition (SCCI) is investigated using a 116-species reduced primary reference fuel (PRF) mechanism. In the RCCI combustion, two PRF fuels (n-heptane and iso-octane) with opposite autoignition characteristics are separatedly supplied and in-cylinder blended such that spatial variations in fuel reactivity, fuel concentration and temperature are achieved. In the SCCI combustion, however, just a single fuel (PRF50) is used such that only fuel concentration and temperature inhomoginieties are obtained. Because three factors, rather than only two as in SCCI combustion, govern the overall RCCI combustion, combustion timing and combustion duration or heat release rate of RCCI combustion are flexibly and effectively controlled. It is found that the overall RCCI combustion occurs much earlier and its combustion duration is longer compared to SCC combustionI. Moreover, the negative temperature coefficient (NTC) has a positive effect on enhancing RCCI combustion by inducing a shorter combustion timing and a longer combustion duration as a result of the occurrence of a predominant low-speed deflagration-combustion mode.

A DNS study of ignition of lean biodiesel/air mixture with temperature and composition inhomogeneities

Minh Bau Luong,Chun Sang Yoo(유춘상) 한국연소학회 2013 KOSCOSYMPOSIUM논문집 Vol.2013 No.12

Two-dimensional direct numerical simulations of ignition of a biodiesel/air mixture with temperature (Τ) and equivalence ratio (Φ) stratifications are investigated using a 73-speices reduced chemistry. This study aims at three case studies (1) baseline cases With either only thermal stratification or only fuel concentration stratification: and the two highly feasible scenarios of the Τ-Φ distribution (2) uncorrelated Τ-Φ correlations; and (3) negatively correlated Τ-Φ correlations. The uncorrelated Τ-Φ distribution leads to a smooth combustion due to a mixed mode of combustion of deflagration and spontaneous ignition. The negatively-correlated Τ-Φ distribution results in a severe occurrence of heat release rate throught the whole domain because of spontaneous ignition.

A DNS Study of Ignition Characteristics of Lean PRF/Air Mixtures under HCCI Conditions

Minh Bau Luong,Chun Sang Yoo(유춘상) 한국연소학회 2012 KOSCOSYMPOSIUM논문집 Vol.- No.45

Direct numerical simulations (DNSs) of ignition of lean primary reference fuel (PRF)/air mixtures under homogeneous charge compression ignition (HCCI) conditions are performed using 116-species reduced chemistry. The influence of variations in the initial temperature field, imposed by changing the variance of temperature, and the fuel composition on ignition of lean PRF/air mixtures is studied using the displacement speed analysis.

A DNS Study of Ignition of a Lean PRF/Air Mixture under RCCI/SCCI Conditions

Minh Bau Luong,Gwang Hyeon Yu(유광현),Chun Sang Yoo(유춘상) 한국연소학회 2015 KOSCOSYMPOSIUM논문집 Vol.2015 No.5

A comparative DNS study of the ignition characteristics of dual-fueled reactivity controlled compression ignition (RCCI) and stratification charge compression ignition (SCCI) is investigated using a 116-species reduced primary reference fuel(PRF) mechanism. It is found that the overall RCCI combustion is greatly enhanced and flexibly controlled, thereby significantly reducing the peak HRR. The negative temperature coefficient has a synergistic effect on advancing the overall combustion and smoothing RCCI/SCCI combustion by inducing more low-speed deflagrations. Compared to SCCI, the overall combustion of RCCI is more advanced in time regardless of T0. However, it is also found that the effects of reactivity and/or equivalence ratio stratifications on reducing the peak are nearly eliminated with T0 in the high temperature regime for both RCCI/SCCI due to the dominant spontaneous ignition mode.

A DNS Study of RCCI Combustion

Minh Bau Luong,Gwang Hyeon Yu(유광현),Chun Sang Yoo(유춘상) 한국연소학회 2015 KOSCOSYMPOSIUM논문집 Vol.2015 No.12

The chemical aspects of primary reference fuel (PRF)/air mixture under RCCI conditions are investigated to provide fundamental insights into the ignition characteristics of RCCI combustion. Chemical explosive mode analysis (CEMA) is adopted to understand the ignition process of the lean PRF/air mixture by identifying controlling species and elementary reactions at different locations and times.

Effects of the NTC Regime on the Ignition of a Lean n-Heptane/Air Mixture with Temperature and Composition Inhomogeneities

Minh Bau Luong,Kwang Hyeon Yu(유광현),Chun Sang Yoo(유춘상) 한국연소학회 2014 KOSCOSYMPOSIUM논문집 Vol.2014 No.5

The effect of the negative temperature coefficient (NTC) on the ignition of a stratified lean n-heptane/air mixture of temperature and equivalence ratio was investigated using 2-D direct numerical simulations (DNSs) with a 58-species reduced mechanism. It was found that for T? in the NTC regime, fuel stratification is more effective than thermal stratification in controlling the ignition delay and mitigating the heat release rate (HRR). One of the key finding in this study is that for the intermediate T?, the overall combustion becomes more advanced and mean heat release rate (HRR) is temporally more distributed with increasing Ø′ regardless of the negatively-correlated T-Ø relations. It is primarily because the deflagrative mode prevails at the reaction fronts for large Ø′ and hence the combustion occurs subsequently, rendering mean HRR more distributed over time.

Ignition of a lean PRF/air mixture under RCCI/SCCI conditions: Chemical aspects

Luong, Minh Bau,Yu, Gwang Hyeon,Chung, Suk Ho,Yoo, Chun Sang Elsevier 2017 Proceedings of the Combustion Institute Vol.36 No.3

<P><B>Abstract</B></P> <P>Chemical aspects of the ignition of a primary reference fuel (PRF)/air mixture under reactivity controlled compression ignition (RCCI) and stratified charge compression ignition (SCCI) conditions are investigated by analyzing two-dimensional direct numerical simulation (DNS) data with chemical explosive mode (CEM) analysis. CEMA is adopted to provide fundamental insights into the ignition process by identifying controlling species and elementary reactions at different locations and times. It is found that at the first ignition delay, low-temperature chemistry (LTC) represented by the isomerization of alkylperoxy radical, chain branching reactions of keto-hydroperoxide, and H-atom abstraction of <I>n</I>-heptane is predominant for both RCCI and SCCI combustion. In addition, explosion index and participation index analyses together with conditional means on temperature verify that low-temperature heat release (LTHR) from local mixtures with relatively-high <I>n</I>-heptane concentration occurs more intensively in RCCI combustion than in SCCI combustion, which ultimately advances the overall RCCI combustion and distributes its heat release rate over time. It is also found that at the onset of the main combustion, high-temperature heat release (HTHR) occurs primarily in thin deflagrations where temperature, CO, and OH are found to be the most important species for the combustion. The conversion reaction of CO to CO<SUB>2</SUB> and hydrogen chemistry are identified as important reactions for HTHR. The overall RCCI/SCCI combustion can be understood by mapping the variation of 2-D RCCI/SCCI combustion in temperature space onto the temporal evolution of 0-D ignition.</P>

On the effect of injection timing on the ignition of lean PRF/air/EGR mixtures under direct dual fuel stratification conditions

Luong, Minh Bau,Sankaran, Ramanan,Yu, Gwang Hyeon,Chung, Suk Ho,Yoo, Chun Sang Elsevier 2017 Combustion and Flame Vol.183 No.-

<P><B>Abstract</B></P> <P>The ignition characteristics of lean primary reference fuel (PRF)/air/exhaust gas recirculation (EGR) mixture under reactivity-controlled compression ignition (RCCI) and direct duel fuel stratification (DDFS) conditions are investigated by 2-D direct numerical simulations (DNSs) with a 116-species reduced chemistry of the PRF oxidation. The 2-D DNSs of the DDFS combustion are performed by varying the injection timing of <I>iso</I>-octane (<I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB>) with a pseudo-<I>iso</I>-octane (PC<SUB>8</SUB>H<SUB>18</SUB>) model together with a novel compression heating model to account for the compression heating and expansion cooling effects of the piston motion in an engine cylinder. The PC<SUB>8</SUB>H<SUB>18</SUB> model is newly developed to mimic the timing, duration, and cooling effects of the direct injection of <I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB> onto a premixed background charge of PRF/air/EGR mixture with composition inhomogeneities. It is found that the RCCI combustion exhibits a very high peak heat release rate (HRR) with a short combustion duration due to the predominance of the spontaneous ignition mode of combustion. However, the DDFS combustion has much lower peak HRR and longer combustion duration regardless of the fuel injection timing compared to those of the RCCI combustion, which is primarily attributed to the sequential injection of <I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB>. It is also found that the ignition delay of the DDFS combustion features a non-monotonic behavior with increasing fuel-injection timing due to the different effect of fuel evaporation on the low-, intermediate-, and high-temperature chemistry of the PRF oxidation. The budget and Damköhler number analyses verify that although a mixed combustion mode of deflagration and spontaneous ignition exists during the early phase of the DDFS combustion, the spontaneous ignition becomes predominant during the main combustion, and hence, the spread-out of heat release rate in the DDFS combustion is mainly governed by the direct injection process of <I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB>. Finally, a misfire is observed for the DDFS combustion when the direct injection of <I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB> occurs during the intermediate-temperature chemistry (ITC) regime between the first- and second-stage ignition. This is because the temperature drop induced by the direct injection of <I>i</I>-C<SUB>8</SUB>H<SUB>18</SUB> impedes the main ITC reactions, and hence, the main combustion fails to occur.</P>

Direct numerical simulations of ignition of a lean <i>n</i>-heptane/air mixture with temperature and composition inhomogeneities relevant to HCCI and SCCI combustion

Luong, Minh Bau,Yu, Gwang Hyeon,Lu, Tianfeng,Chung, Suk Ho,Yoo, Chun Sang Elsevier 2015 Combustion and Flame Vol.162 No.12

<P><B>Abstract</B></P> <P>The effects of temperature and composition stratifications on the ignition of a lean <I>n</I>-heptane/air mixture at three initial mean temperatures under elevated pressure are investigated using direct numerical simulations (DNSs) with a 58-species reduced mechanism. Two-dimensional DNSs are performed by varying several key parameters: initial mean temperature, <I>T</I> <SUB>0</SUB>, and the variance of temperature and equivalence ratio (<I>T</I>′ and <I>ϕ</I>′) with different T − ϕ correlations. It is found that for cases with <I>ϕ</I>′ only, the overall combustion occurs more quickly and the mean heat release rate (HRR) increases more slowly with increasing <I>ϕ</I>′ regardless of <I>T</I> <SUB>0</SUB>. For cases with <I>T</I>′ only, however, the overall combustion is retarded/advanced in time with increasing <I>T</I>′ for low/high <I>T</I> <SUB>0</SUB> relative to the negative-temperature coefficient (NTC) regime resulting from a longer/shorter overall ignition delay of the mixture. For cases with uncorrelated T − ϕ fields, the mean HRR is more distributed over time compared to the corresponding cases with <I>T</I>′ or <I>ϕ</I>′ only. For negatively-correlated cases, however, the temporal evolution of the overall combustion exhibits quite non-monotonic behavior with increasing <I>T</I>′ and <I>ϕ</I>′ depending on <I>T</I> <SUB>0</SUB>. All of these characteristics are found to be primarily related to the 0-D ignition delays of initial mixtures, the relative timescales between 0-D ignition delay and turbulence, and the dominance of the deflagration mode during the ignition. These results suggest that an appropriate combination of <I>T</I>′ and <I>ϕ</I>′ together with a well-prepared T − ϕ distribution can alleviate an excessive pressure-rise rate (PRR) and control ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. In addition, critical species and reactions for the ignition of <I>n</I>-heptane/air mixture through the whole ignition process are estimated by comparing the temporal evolution of the mean mass fractions of important species with the overall reaction pathways of <I>n</I>-heptane oxidation mechanism. The chemical explosive mode analysis (CEMA) verifies the important species and reactions for the ignition at different locations and times by evaluating the explosive index (EI) of species and the participation index (PI) of reactions.</P>