The role of composition in the combustion of n-heptane/iso-butanol mixtures: experiments and detailed modelling

Abstract

Experimental data and detailed numerical modelling are presented on the burning characteristics of a model gasoline/biofuel mixture consisting of n-heptane and iso-butanol. A droplet burning in an environment that minimises the influence of buoyant and forced convective flows in the standard atmosphere is used to promote one-dimensional gas transport to facilitate numerical modelling of the droplet burning process. The numerical model includes a detailed combustion kinetic mechanism, unsteady gas and liquid transport, multicomponent diffusion inside the droplet, variable properties, and non-luminous radiative heat transfer from the flame. The numerical simulation was validated by experimental measurements in the standard atmosphere which showed good agreement with the evolutions of droplet and flame diameters. The iso-butanol concentration had a strong effect on formation of particulates. Above ~20% (volume) iso-butanol, flame luminosity was significantly diminished anddecreased with increasing iso-butanol concentration, while CO₂ emissions as a representative greenhouse gas were not strongly influenced by the iso-butanol loading. The soot shell was located near a 1350 K isotherm for concentrations up to 20% (volume) iso-butanol, suggesting this value as a possible soot inception temperature for the mixture droplet. The combustion rate decreased with increasing iso-butanol concentration which was attributed to iso-butanol's higher liquid density. No evidence of a low temperature burning regime, or of extinction, was found (in experiments and simulations) for the small droplet sizes investigated.

Keywords:

• droplet combustion
• biofuel
• droplet
• combustion
• surrogate
• n-heptane
• iso-butanol

Acknowledgements

The authors are pleased to acknowledge the interest in our work by Wing Tsang of NIST, Yuhao Xu of Prairie View A&M University, Anthony Reeves and Perrine Pepiot of Cornell University, and Dr. Xia Zeng of Cornell who provided assistance with chemical analysis of the heptane/iso-butanol mixtures.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental data

Supplemental data for this article can be accessed http://10.1080/13647830.2020.1800823.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

Support for this work was provided by the Co-Optimization of Fuels & Engines (Co-Optima) program sponsored by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies and Vehicle Technologies Offices under project DE-EE0007978. Partial support was also provided by NASA grants NNX08AI51G and 80NSSC18K048.