Keywords: Methanol; Short kinetics mechanism; Interface boundary conditions: Non-luminous flame radiation; Heterogeneous flames; Mass burning rate; Wall flames; Turbulence model

Methanol, an alcohol fuel, is envisaged as a key alternative fuel in India for its application in transport, energy and household cooking devices. It is a light oxygenated fuel, produces non-sooty flames with less emissions such as CO. The major source for production of methanol includes natural gas, reduction conversion of atmospheric carbon dioxide and syngas obtained from gasification of coal and biomass. Methanol is used in several devices, where multiple modes of combustion, such as non-premixed, premixed and partially premixed, are expected to occur. Systematic analyses of flame extents, mass burning rates, flow and species fields assist in fine development of these combustion systems. This can be carried out using a comprehensive numerical model. Experimental data on canonical methanol flames in different configurations are available in literature and such data can be used to validate the numerical results. In order to model these heterogeneous (in the presence of liquid methanol surface) flames under various configurations, solutions that implement coupled conditions at the interface of liquid and gas phases would be valuable. Based on the survey of literature, different modes of combustion have been modelled using global single-step finite rate or a reduced global multi-step reaction scheme. Numerical models using short or detailed chemistry in multi-dimensional domains are scarce. The use of a detailed mechanism with elementary reactions and intermediates would be computationally expensive, especially for multi-dimensional configurations. This has led to the development of short reaction mechanisms as an upgrade on the global single-step reaction.
The scope or global objective of this work is to incorporate ashort chemical kinetics mechanism and heterogeneous liquid-gas interface condition in a comprehensive numerical model to simulate canonical methanol flames. The specific objectives are to validate the results predicted from the short kinetics mechanism by using appropriate model parameters and user defined functions in Ansys Fluent, against available experimental data. This also includes prediction of the mass burning rate of canonical methanol flames and a parametric study to understand the formation of important reaction zones and species distribution in these flames under forced and natural convective conditions, and laminar and turbulent regimes.
The short reaction mechanism is validated for homogenous flame in opposed flow configuration with an analysis of the flame structure, extinction and auto-ignition characteristics. Validation of the coupled interface boundary condition along with the short reaction mechanism has been carried out for heterogeneous flames in forced convection modes in cross-flow, co-flow and porous sphere configurations. Comparison of the predictions from the numerical model with experimental data has been presented in terms of temperature and velocity profiles in cross-flow flames. The predicted flame height and temperature profiles are compared with the experimental data for co-flow flames. In the case of porous sphere configuration, where the fuel surface is non-orthogonal to the coordinate system, validation is presented in terms of the mass burning rate comparison. In the natural convection mode, laminar and turbulent wall flames from vertical methanol wicks are validated using profiles of wall heat flux, temperature and velocity.
Parametric study is conducted on the effects of free stream velocity on the flame characteristics in cross-flow, co-flow and porous sphere configurations. The flame anchoring, mass burning rate and reaction mode are dependent on the freestream velocity. A cross-flow flame that anchors upstream of the leading edge of the pool at lower air velocities transitions to a separated flame that sustain further downstream at a critical air velocity. Partial premixing of fuel and oxygen enhances the mass burning rates as well as the reaction rates. In co-flow flames, mass burning rates increases with air velocity and asymptotically reaches a constant value. In porous sphere flames, two regimes of envelope and wake flames are seen depending on the sphere diameter and freestream velocity. Analyses of the effects of the interaction of two porous spheres in tandem arrangement, on the flame shape and mass burning rates are carried out with detailed descriptions of reaction zones, temperature, velocity and species fields, as predicted by the numerical model. In the case of wall fires, the predicted flow field and reaction zones are used to study the flame development along the wall in laminar and turbulent regimes.
In summary, the numerical model incorporated with short kinetics mechanism and interface boundary conditions has been well validated for its application in simulations of canonical methanol flames in various configurations and in different regimes of burning including forced and natural convection. The model is also validated for its prediction of turbulent wall flame, as compared with available data in natural convective flow. This model can be successfully used in simulating methanol flames in various applications, where different modes of combustion are envisaged.