A pinboard by
Anand Shivapuji

Research Associate, Indian Institute of Science


Analyze how engines designed for conventional fuels behave when fuelled with non regular fuels

Internal combustion engines, the mainstay of chemical to mechanical/electrical power generation have come under intense scrutiny owing to the availability issues associated with fossil fuels and more critically, their contribution to environmental pollution and green house gas emissions. In light of the same, intense efforts are being directed at moving away from business as usual scenario of fuelling internal combustion engines with fossilized hydrocarbons for motive and stationary power requirements. While there is a broad consensus towards moving away from fossils, non regular fuels derived from bio resources are yet to be established as viable alternatives on a commercial scale. While efforts are being directed in that direction, the unfavorable economics of scale in respect of engines for operation with non regular fuels has meant no or limited production of dedicated engines. This scenario is expected to persist for some more time in the near future. As such, the only alternative in the immediate future is to use existing engines designed for conventional fuels and operate them with non regular fuels of interest. Such an operation could lead to non-optimal performance due to differences in the properties of non-regular fuels of interest and the base fuel for which the engine is designed. Performance optimization is possible through (a) engine design modification to accommodate the properties of the new fuel, which however is not feasible/of interest to original equipment manufacturers due to the unfavorable economics of scale or (b) non intrusive optimization involving adjustments to properties like temperature, pressure, air-fuel ratio etc. In the broader ambit of non-intrusive optimization, conventional natural gas engines have been characterized for baseline performance and subsequently the engines have been adopted (non-intrusive) for fuelling with producer gas, a gaseous fuel generated from biomass. Optimization efforts based on in-cylinder analysis and full energy balance have resulted in doubling of the engine power output with producer gas rendering it economically attractive enough for original equipment manufacturers to sit up and take notice. Tail pipe emission characterization also suggests emissions to be well within prescribed standards permitting tailpipe treatment cost reduction. A generic philosophy (captured as a numerical model) for non-intrusive optimization has been adopted which can be extended to any other non-regular fuel.


Self-acceleration of cellular flames and laminar flame speed of syngas/air mixtures at elevated pressures

Abstract: Experimental study on the self-acceleration characteristics and laminar flame speed of CO/H2/air mixtures was conducted at elevated pressures up to 0.6 MPa with spherical outwardly expanding flames. Experimental conditions for the CO/H2/air mixtures of hydrogen fraction in syngas from 0.2 to 0.8, the pressures from 0.4 to 0.6 MPa and equivalence ratios from 0.5 to 1.0. At elevated pressures, the cellular structure occurs on the early stage of the flame development due to the significant influences of thermal diffusive and hydrodynamic instabilities and flame front was accelerated. Critical radius after which flame front becomes unstable decreases with H2 content in the fuel mixtures. For syngas mixtures with higher H2 content, critical radius increases with the increase of the equivalence ratio. Critical radius decreases with the increase of the equivalence ratio for the mixture with lower H2 content. Critical Peclet number, which is defined as the ratio of critical radius to flame thickness increases firstly and then decreases with H2 content for the mixtures with higher equivalence ratios and decreases all the time for the mixture with lower ones. In addition, the acceleration exponent which indicates the acceleration characteristics when flame front becomes unstable increases with the flame front propagation and is not the same with that of the turbulent flame. At last, an updated method, which excludes the acceleration effect of the cellular structure on the stretched flame speed at various flame radii has been proposed. It will help to obtain the laminar flame speed for fuel/air mixtures at elevated pressures with small time region between the end of the ignition spark and the onset of flame instability. This updated model replaces the experimental determined exponential term of the fractal structure and the updated intrinsic flame instability model. The measured laminar flame speed data are compared and analyzed with those predicted by Davis and Li mechanisms of syngas.

Pub.: 13 Aug '16, Pinned: 30 Aug '17

On Ignition Mechanisms of Premixed CH4/Air and H2/Air Using a Hot Turbulent Jet Generated by Pre-chamber Combustion

Abstract: An experiment was developed to investigate the ignition mechanisms of premixed CH4/air and H2/air mixtures using a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition process inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produced a jet comprising of only hot combustion products from pre-chamber combustion. The latter produced a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and spark position on the ignition mechanisms and probability. A global Damköhler number was defined to remove parametric dependency. The limiting Damköhler number, below which ignition probability is nearly zero, was found to be 140 for CH4/air and 40 for H2/air. Lastly, the ignition outcomes were plotted on the turbulent premixed combustion regime diagram. All non-ignition cases fell within the broken reaction zone regime, whereas flame and jet ignition mostly fell within the thin reaction zone regime. These results can provide useful guidelines for future pre-chamber design and optimization.

Pub.: 13 Jun '16, Pinned: 30 Aug '17