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• Analysis of cloud evaporation and autoignition based on an extended unit cell model. • Gas-phase transient effect is incorporated by considering evaporating boundary layer. • Ignition initially occurs in the relatively hot and fuel-lean region for normal cases. • A minimum ignition temperature is attained before infinite droplet spacing is reached. • Relations between critical ignition temperature and droplet spacings are revealed. Liquid clouds (sprays) are commonly encountered in nature and industrial applications, where droplets are subject to complex interactions. In this paper, an extended unit cell model is proposed to analyze the evaporation and autoignition of monodisperse, single-component, and quiescent droplet clouds. The extended model incorporates the transient development of the evaporating boundary layer attached to the droplet surface. Due to the inclusion of this gas-phase transient effect, the current theory can be applied to both dilute and dense clouds across a wide range of ambient pressures. In the absence of chemical reactions, the effects of multiple parameters, such as droplet spacing and ambient pressure, on the evaporation rate and lifetime of droplets are examined. The critical condition for fuel vapor saturation in the gas phase is re-obtained. Based on this, we further investigate the autoignition of droplet clouds exposed to hot oxidizing environments and derive the corresponding ignition criterion. Effects of various factors, including droplet size, droplet spacing, ambient pressure, initial oxygen concentration, and reaction order, on critical ignition temperatures are also considered. In comparison with transient numerical results, it is found that the current theoretical solutions predict well for a broad variety of parameters. [ABSTRACT FROM AUTHOR]

In order to achieve high efficiency and power density, modern combustors are generally operated at elevated pressures. As the pressure and gas-phase density increase, classical quasi-steady (QS) models of droplet evaporation, ignition, and burning introduce non-negligible errors. In this paper, we extend the QS model to incorporate the gas-phase transient (GT) effect by examining the development of the evaporating boundary layer near the droplet surface. Specifically, the evaporation and ignition of single droplets in an infinite, hot, and oxidizing environment are analyzed. The theoretical result agrees well with the transient numerical simulation for an extensive range of parameters. The proposed theory successfully predicts the GT effect on evaporation lifetime and critical ignition limit of droplets. It is demonstrated that the GT effect increases the droplet evaporation rate while tends to suppress ignition. In comparison with pure evaporation, the ignition of droplets is more severely affected. Additionally, the GT effect increases as the liquid-to-gas density ratio decreases or evaporation intensity increases. [ABSTRACT FROM AUTHOR]

The thermal ignition of a reacting boundary-layer flow over the hot surface of a wedge/cone with an unheated starting length, , is studied analytically and numerically. For the realistic situation of large activation energy reactions, a locally similar solution is derived and is found to agree well with the numerical solution within a wide range of , geometric angle and the Prandtl number, . An increase in is shown to reduce the minimum ignition distance, because the flow velocity is reduced by the viscous drag over the starting length so that the flow over the heated section is heated to a greater extent for the same distance. [ABSTRACT FROM AUTHOR]