[Show abstract][Hide abstract] ABSTRACT: The effects of combined natural and forced convection on thermal explosion in a spherical reactor are studied. Upward natural convection arises from internal heating caused by a chemical reaction, whilst downward forced convection is driven by injecting fluid at the top and removing it at the bottom of the reactor. It is shown that explosive behaviour is favoured by a balance between the natural and forced flows. Such a balance establishes a nearly stagnant region close to the centre of the reactor which quickly heats up to explosion. In fact, counter-intuitively, explosion may occur in an otherwise stable reactor by injecting cold fluid or enhancing natural convection. A scaling relation predicting the physico-chemical conditions for which explosion occurs at minimum heat release is developed. The work concludes with a quantitative three-dimensional regime diagram, accounting for the effects of heat transport by conduction, natural convection and forced flow for systems of similar geometry, where the regions of stability and explosion are delimited.
Combustion and Flame 01/2013; 160(1):191–203. DOI:10.1016/j.combustflame.2012.08.012 · 3.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: During an exothermic reaction in a fluid, convection may ensue on a local scale and then develop to the scale of the entire vessel. In this work, we study the effects of both localised and global convection on thermal explosions occurring between parallel plates. Analytical relations are derived for the various transitions in regimes of convective and thermal behaviours. We show that these relations agree well with previous numerical work and with new simulations in the present investigation. We also determine analytically the time for onset of convection, as well as the temperature increase at that time, for stable and explosive systems. The effects of the Prandtl number of the fluid on the transitions between regimes are noted.
[Show abstract][Hide abstract] ABSTRACT: Whether or not a chemical reaction in a fluid leads to an explosion is shown to depend on four timescales: that for the chemical reaction to heat up the fluid containing the reactants and products, for heat conduction out of the reactor, for natural convection in the fluid, and finally for chemical reaction. This approach is developed for an irreversible, nth-order chemical reaction, A→B occurring exothermically in a closed spherical vessel, whose wall is held at a fixed temperature. These four timescales are expressed in terms of the physical and chemical parameters of the system. A new three-dimensional regime diagram is proposed, in which the three effects inhibiting explosion, viz. the consumption of reactant, and heat removal both by thermal conduction and by natural convection, appear separately. Numerical simulations are performed for laminar natural convection occurring, so that the development of temperature, composition and velocity throughout a reacting gas is computed for increasing times. The results are compared with previous experimental measurements in the gas phase for the decomposition of azomethane. The criterion for an explosion is considered in some detail; it appears that these systems explode if and when the maximum dimensionless rise in temperature exceeds a value close to 5.
Combustion and Flame 02/2010; 157(2):230-239. DOI:10.1016/j.combustflame.2009.10.016 · 3.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new way of ascertaining whether or not a reacting mixture will explode uses just three timescales: that for chemical reaction to heat up the fluid containing the reactants and products, the timescale for heat conduction out of the reactor, and the timescale for natural convection in the fluid. This approach is developed for an nth order chemical reaction, A --> B occurring exothermically in a spherical, batch reactor without significant consumption of A. The three timescales are expressed in terms of the physical and chemical parameters of the system. Numerical simulations are performed for laminar natural convection occurring; also, a theoretical relation is developed for turbulent flow. These theoretical and numerical results agree well with previous experimental measurements for the decomposition of azomethane in the gas phase. The new theory developed here is compared with Frank-Kamenetskii's classical criterion for explosion. This new treatment has the advantage of separating the two effects inhibiting explosion, viz. heat removal by thermal conduction and by natural convection. Also, the approach is easily generalised to more complex reactions and flow systems.