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35. Theoretical analysis on the transient ignition of premixed expanding flame in a quiescent mixture

Author

Yu, Dehai and Chen, Zheng

Subjects
Physics - Fluid Dynamics
Abstract

The present theory considers the transient evolution of the temperature and fuel mass fraction distributions, and it can determine the critical heating power and minimum ignition energy for successful ignition. The flame initiation process subject to central heating can be approximately decomposed into four stages, i.e., the fast establishment of the ignition kernel, the ignition-energy-supported flame kernel propagation, unsteady transition of the flame kernel, and quasi-steady spherical flame propagation. Comparison between present transient theory and previous quasi-steady theory indicates that the unsteady effects directly lead to the appearance of flame kernel establishing stage and considerably affect the subsequent flame kernel development by lowering the flame propagation speed. Time scale analysis is conducted, and it is found that the transient formulation completely degenerates to the quasi-steady theory either in the neighborhood of stationary flame ball or as approaching the planar flame. Previous quasi-steady theory shows that the critical heating power for successful ignition is proportional to the cube of the critical flame radius. However, the critical heating power predicted by the present transient formulation deviates this scaling law, which can be attributed to the unsteady thermal conduction from heating center to the flame front. Furthermore, in the transient formulation the central heating with finite duration is considered and the minimum ignition energy is obtained. For the first time, the memory effect that persistently drives flame propagation subsequent to central heating switching-off is examined. It is found that as the heating power grows, the memory effect becomes increasingly important and it can greatly reduce the minimum ignition energy.

Published
2021
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36. Theoretical analysis on droplet vaporization at elevated temperatures and pressures

Author

Yu, Dehai and Chen, Zheng

Subjects
Physics - Fluid Dynamics, Condensed Matter - Soft Condensed Matter
Abstract

Droplet vaporization has a great impact on combustion of liquid fuels and might greatly affects the engine performance. Physical understanding and theoretical interpretation of the droplet vaporization behavior are decisive to constitute appropriate models for spray combustion. In this work, we proposed a fully transient formulation for the droplet vaporization at wide ranges of temperature and pressure. The governing equations are solved analytically by means of suitable coordinate transformations. The droplet surface temperature and mass fraction of fuel vapor are determined by the matching conditions at the liquid-gas interface. Accordingly, a theoretical model, characterizing the time change of droplet size during its vaporization, is proposed in terms of the unsteadiness factor and droplet vaporization lifetime. The theoretical model successfully reveals the conventional recognition of droplet vaporization characteristics, and its predicted droplet size history agrees well with experimental results reported in the literature. Besides, based on the present theoretical analysis, the experimentally observed dual effect of pressure upon droplet vaporization is analyzed with help of an explicit formula for enthalpy of vaporization indicating its temperature-dependence.

Published
2020

46. A Theoretical Analysis on Enthalpy of Vaporization: Temperature-Dependence and Singularity at the Critical State

Author

Yu, Dehai and Chen, Zheng

Subjects
Condensed Matter - Statistical Mechanics, Physics - Applied Physics
Abstract

Accurate evaluation of enthalpy of vaporization (or latent heat of vaporization) and its variation with temperature is of great interest in practical applications, especially for combustion of liquid fuels. Currently, a theoretically rigorous formulation for latent heat is still not in place; and the existing fitting formulas for various fluids are, in nature, system-dependent. In this paper, the enthalpy of vaporization and its change with temperature are derived rigorously from first principles at the reference and critical states. A composite formula for enthalpy of vaporization is constituted from physical plausibility. The composite formula contains no fitting parameters; and it can accurately predict the enthalpy of vaporization from the reference temperature to the critical temperature.

Published
2019

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