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ABSTRACT
The combustion of liquid fuel droplets provides fundamental information that is relevant to spray combustion. A porous sphere setup has been used to investigate non-premixed combustion experimentally with three different liquid fuels, viz. methanol, diesel, and biodiesel (sunflower oil methyl ester). Experiments have been performed at atmospheric pressure with spheres of different diameters and over a range of free stream air velocities, resulting in different types of flames from envelope flame to wake flame. The measured mass burning rates are compared with the predictions from a theoretical model developed for envelope flames considering mixed convective transport. The model predictions are first validated for methanol and subsequently the model is applied for the other two fuels to evaluate their transfer numbers from the experimental mass burning rate data. A power law variation of nondimensional mass burning rate with effective Reynolds number (with contributions of forced and natural convection) is proposed for each of the fuels irrespective of sphere size and air velocity. The transition air velocities (from envelope to wake flames) for different fuels and the change in mass burning rate on transition are measured.
Acknowledgment
The authors wish to thank Prof. S. Ghoshal of the Mechanical Engineering Department, Jadavpur University, for his useful suggestions on the experimental setup.
Funding
The financial support from the Departmental Research Scheme of the Department of Power Engineering provided by the University Grant Commission, Govt. of India (UGC-DRS Phase I Programme) is gratefully acknowledged.
Nomenclature
| B | = | transfer number |
| Cpg | = | specific heat |
| d | = | diameter of sphere |
| g | = | gravitational force |
| Gr | = | Grashof number |
| hfg | = | latent heat of vaporization |
| kg | = | thermal conductivity |
| = | mass burning rate | |
| ms | = | mass of liquid sphere |
| = | normalized mass burning rate | |
| Nu | = | Nusselt number |
| NuF | = | forced convective Nusselt number |
| NuN | = | natural convective Nusselt number |
| Pr | = | Prandtl number |
| qi–l | = | energy required to heat liquid |
| r | = | radius of sphere |
| Re | = | Reynolds number |
| Reeff | = | effective Reynolds number |
| Tad | = | adiabatic flame temperature |
| Tb | = | boiling point |
| Tf | = | film temperature |
| T∞ | = | free stream temperature |
| = | average temperature | |
| u∞ | = | air velocity |
| Δhc | = | lower heating value of the fuel |
| ΔT | = | Tad – T∞ |
| β | = | coefficient of volumetric expansion |
| υ∞ | = | kinetic viscosity of air |
| γ | = | stoichiometric mass ratio of oxidizer to fuel |