࡯ᄺ䖯ሩ, 2014 ᑈ, ㄀ 44 ो : 201402 Recent progress and challenges in fundamental combustion research † Yiguang Ju Department of Mechanical and Aerospace Engineering, Princeton University, New Jersey, USA Abstract More than 80% of world energy is converted by combustion. Develop- ment of efficient next generation advanced engines by using alternative fuels and operating at extreme conditions is one of the most important solutions to increase energy sustainability. To realize the advanced engine design, the challenges in combustion research are therefore to advance fundamental understanding of com- bustion chemistry and dynamics from molecule scales to engine scales and to de- velop quantitatively predictive tools and innovative combustion technologies. This review will present the recent progresses and technical challenges in fundamental combustion research in seven areas including advanced engine concepts using low temperature fuel chemistry, new combustion phenomena in extreme conditions, alternative and surrogate fuels, multi-scale modeling, high pressure combustion kinetics, experimental methods and advanced combustion diagnostics Firstly, new engine concepts such as the Homogeneous Charge Compression Ignition (HCCI), Received: 2014-01-29;accepted: 2014-03-27;online: 2014-04-01 † E-mail: [email protected] $i teas: Yiguang Ju. Recent progress and challenges in fundamental combustion research. "Evances in Mechanics,2014,44: c 2014Advances in Mechanics. 2 ࡯ᄺ䖯ሩ ㄀ 44 ो : 201402 Reactivity Controlled Compression Ignition (RCCI), and pressure gain combus- tion will be introduced. The impact of low temperature combustion chemistry of fuels on combustion in advanced engines will be demonstrated. This is followed by the discussions of the needs of fundamental combustion research for new en- gine technologies. Secondly, combustion phenomena and flame regimes involving new combustion concepts such as fuel and thermal stratifications, plasma assisted combustion, and cool flames at extreme conditions will be analyzed. Thirdly, al- ternative fuels and methodologies to formulate surrogate fuel mixtures to model the target combustion properties of real fuels will be presented. A new concept of radical index and transport weighted enthalpy will be introduced to rank the fuel reactivity and to assess the impact of molecular structure on combustion prop- erties The success and limitations of the current surrogate fuel models will be discussed by using jet fuels and biodiesels as examples. Fourthly, the difficulty of modeling large kinetic mechanism of real fuel will be discussed The multi-time scale (MTS) method and the correlated dynamic adaptive chemistry (CO-DAC) method for kinetic model reduction and computationally efficient modeling will be compared and analyzed. Fifthly, the progress and challenges of high pressure combustion kinetics for hydrogen and larger hydrocarbons will be discussed. The important pressuredependent reaction pathways and key intermediate species at high pressure will be analyzed. Fundamental experimental methods for combus- tion and their uncertainties in acquiring combustion properties for the validation of kinetic mechanism will be discussed. Finally, recent progress in diagnostics of HO2,H2O2,RO2, ketohydroperoxide, and other key intermediate species for high pressure kinetic mechanism development will be summarized. Conclusions and opportunities of future combustion research will be made. Keywords alternative fuels, flame chemistry multiscale modeling, experimental methods and uncertainty, multi-species diagnostics Classification code: O341 Document code: A DOI: 10.6052/1000-0992-14-011 Ju Yiguang : Recent progress and challenges in fundamental combustion research 3 1 Introduction 1.1 Advanced engine design and multi-scale turbulent combustion modeling Combustion converts more than 80% of world energy and has played a dominant role in ground and air transportation. With the current difficulties in developing renewable energy, for a foreseeable future, combustion will remain to be the major energy conversion process in power generation and transportation. However, the energy conversion efficiency of existing combustion engines is low and combustion of fossil fuels is the major source contributing to climate change and air pollution (Chu et al. 2012). As such, there is an urgent need to develop advanced engine technology and new combustion concepts to drastically increase the engine efficiency and reduce emissions (DOE report, 2006). For ground transportation, recently, various new combustion engine technologies such the Homogeneous Charge Com- pression Ignition (HCCI) engines (Dec 2009, Lu et al. 2011, Reitz 2013) and the Reactivity Controlled Compression Ignition (RCCI) engines (Reitz 2013) have been developed. These engines take the advantage of high compression ratio of diesel engines and low emissions of gasoline engines by using highly diluted, premixed and/or highly stratified fuel/air mixtures with excessive exhaust gas recirculation (EGR). As such, to control engine knock, heat release rate, and ignition timing at different engine loads, understanding the combustion process at high pressure and low temperature conditions involving the negative temperature coefficient (NTC) and cool flame chemistry (Curran et al. 1998) becomes extremely impor- tant. Moreover, the low temperature and high pressure combustion processes coupled by strong fuel and temperature non-uniformities in engines are controlled by both large-scale turbulent mixing and sub-grid-scale turbulence-chemistry interactions. Therefore, detailed understanding of combustion processes in HCCI and RCCI engines requires not only an accurate turbulent combustion model which can appropriately predict sub-grid turbulent- chemistry interaction but also a validated high pressure and low temperature chemistry for real transportation fuels. Unfortunately, strictly speaking neither a validated high pressure and low temperature kinetic mechanism for real fuels nor an accurate and computation- ally efficient sub-grid turbulent-chemistry model is available for advanced engine modeling (Chen 2011, Pope 2012). Moreover, previous turbulent combustion experiments and model- ing are mainly focused on high temperature thin flame regimes and few studies are carried to understand how low temperature combustion chemistry and autoignition affect turbulent 4 ࡯ᄺ䖯ሩ ㄀ 44 ो : 201402 flame regimes and propagation speeds (Won et al. 2014) Therefore, the first challenge in combustion is how we can develop validated high pressure and low temperature combustion models for advanced engine modeling. In air transportation, to increase the fuel efficiency and meet the stringent CAEP-6 and NASA (N+3) emission standards of the Committee on Aviation Environmental Protection (CAEP) and NASA, new lean burn aircraft combustor concepts such as the twin annular premixing swirled (TAPS) burner (Mongia 2010), lean-premixed pre-vaporized (LPP), lean direct injection (LDI) burners (Tacina et al. 2003), trapped vortex combustion (TVC) burn- ers (Hsu et al. 1998), and pressure gain combustors (Schwer and Kailasanath 2011) have been developed. To achieve high speed propulsion, supersonic ramjet engines such as X-43 and X-51 have been developed and tested (Moorthy et al. 2012, Yu et al. 2013). Moreover, new advanced gas turbine engines have higher compression ratios and thus have changed the conventional rich-quench-lean diffusion combustion to fully and partially premixed com- bustion. In addition, due to the increase of ignition Damk¨ohler number at elevated tem- perature, the thin flame front flame propagation process in conventional engines is replaced substantially by volumetric ignition. Especially, at ultra-lean fuel conditions, local flame extinction, re-ignition, and ignition to flame as well as ignition to detonation transitions will occur. As such, premixed turbulent flame regimes at high ignition Damk¨ohler may become very different from that of the classical wrinkled and corrugated flamelet regimes (Bradley 1992, Driscoll 2008, Peters 2000) and the conventional incompressible flow, flamelet, and pre-assumed probability density function (PDF) based turbulent combustion modeling ap- proaches may not be appropriate (Peters 1988, Pitsch 2006, Pope 2013) for the new engine modeling. As shown in Fig. 1 (Gou et al. 2010), combustion in engines involves many orders of magnitudes of different time- and length-scales ranging from electronic excitation, molecular diffusion, soot particle formation, sub-grid turbulent mixing, and engine scale flow motion and instability. The main factors affecting the combustion phenomenon depend on the combustion process. For example, for near limit combustion the time scales involving elementary combustion chemistry is important. For engine instability, the timescales of sub- grid turbulent mixing, heat release rate, and acoustic waves are more important. For flame extinction, the molecular diffusion is important. Therefore, the second challenge of combus- tion is how to develop a new turbulent combustion modeling approach which can address the multi-time scale, multi-length scale, and multi-physics combustion processes accurately with detailed kinetic mechanisms. For high speed propulsion such as supersonic combustion and Scramjet engines, vitiated Ju Yiguang : Recent progress and challenges in fundamental combustion research 5 Atom Molecular Microflow Flames Engine combustion Physical process Molecules collisions Nanoparticles Molecular and turbulent transport scales Thermo- Soot growth, Mixing, ignition,