Akter Hossain

A Numerical Study on Time-Dependent Melting and Deformation Processes of Phase Change Material (PCM) Induced by Localized Thermal Input

Deep understanding of fire damage triggered by combustion of electric wires is one of important issue in terms of the fire safety of automated facilities, such as factories, power plant etc. Combustion process of electric wire is, nevertheless, quite complicated since it consists of multi-phase, multi-dimensional time-dependent heat and mass transfer with chemical reactions in each phase and several fundamental processes are equally important so that it is relatively hard to simplify the system. Unlikely the conventional solid combustion, the metal rod in the wire could play an important role to determine the combustion process of the wire (Bakhman et al., 1981a, 1981b). In this way, the thermal interaction between the metal rod and surrounded insulated matter (mainly polymeric materials) is a leading key process in understanding the fire character of the wire (Umemura et al., 2002). However, this is not only the matter of concern; we have much serious problem to face with. As pointed, the combustible of electric wire is mainly the polymeric material surrounded by the (conductive) metal rod and the polymer does “melt” during the fire event. The shape of liquefied polymer freely modifies and sometimes tremendously deforms, thus, the precise tracking of the liquid-gas interface is one of important task since the major chemical reaction at the interface should govern the overall gasification rate (Blasi, 1991; Ross, 1994), i.e., gas-phase combustion behavior. As summarized, in order to predict the precise process of electric wire, the following three processes need to be modeled properly: 1) three-phase induced by chemical reactions (e.g., degraded pyrolysis), 2) deformation of interface of liquid phase, 3) heat transfer between metal rod and surrounded polymer (either melted and solid). Our ultimate goal is to understand each contribution properly and develop simplified model of wire combustion. To this date, the precise experimental observation of combustion behavior of electric wire has been performed by authors’ group (Nakamura et al., 2008a, 2008b, 2009), as shown schematically in Fig. 1. It shows the magnified still image of spreading flame over the specially-designed “model” wire (polyethylene-coated on the several kind of metal wires.

Numerical simulation of flame dynamics associated with negative velocity induced by deformed flame shape

In this study, the influence of the negative velocity field formed ahead of an abruptly deformed flame tip on the propagation behaviour of a laminar premixed flame is numerically investigated. A strong deformation in the flame front is induced by imposing a very narrow, in-line pre-heating zone in the unburned region. The simulation is performed under low Mach number approximation by using a multi-scale multi-physics Computational Fluid Dynamics (CFD) solver FrontFlow/Red with one-step finite rate chemistry in order to track the time-dependent flame dynamics. The computed results unveil that the flame front is deformed significantly within a short time due to the narrow in-line pre-heating effect. The flame deformation induces a strong negative velocity field ahead of the deformed flame tip, acting in the direction of propagation, which gives rise to a strong pair vortex. This strong pair vortex interacts with the flame tip and then slides down along the flame surface in the upstream direction during propagation. This flame-vortex interaction causes further deformation in the flame surface in the upstream direction, and consequently, the flame exhibits a wave-like surface, which enhances the flame propagation speed. The auto-generation of a strong pair vortex ahead of the flame front due to the localised thermal input could be applied as one of the methods to control the combustion externally. It is also expected that the results obtained in this study could have a significant impact on the detailed understanding of the local thermo-fluid dynamical interaction process of turbulent combustion in practical combustors.

Conjugate Heat and Mass Transfer on Fluctuating Mixed Convection Flow along a Vertical Wedge with Thermal Radiation

Abstract We study the boundary layer characteristics of heat and mass transfer flow past a vertical wedge in the presence of thermal radiation. The surface temperature and the species concentration are assumed to be oscillating in the magnitude but not in the direction of oncoming flow velocity. The governing equations have been solved by two distinct methods, namely, the straightforward finite difference method for the entire frequency range, and the series solution for the low frequency range and the asymptotic series expansion method for the high frequency range. Numerical solutions have been presented in terms of the amplitudes and phase angles of the skin friction, the rate of heat transfer and the mass transfer with the variations of Richardson’s number, the Prandtl number, the conduction–radiation parameter, the surface temperature parameter and the Schmidt number. Furthermore, the effects of these parameters are examined in terms of the transient skin friction, heat transfer and mass transfer.