Thermal convection, nonisothermal

Problem 12-13. Raleigh-Benard Convection - Nonisothermal Boundaries. The assumption of isothermal boundaries in the buoyancy-driven convection instability problem is generally an oversimplification. A more realistic picture is that the upper and lower surfaces are in contact with reservoirs that are maintained at a constant temperature so that the thermal boundary conditions are better approximated as... 

The simplest nonisothermal flowing system where processes associated with dissolution and deposition occur is one in which flow is induced by thermal convection. This is accomplished by heating one leg of a closed loop and cooling another leg. The flow rate is dependent on the height of the heated and cooled sections, on the temperature gradient, and on the physical properties of the liquid. Both single-phase (all-liquid) and two-phase (liquid-vapor) loops have been tested. In some cases, thermal convection loops are destructively examined after operation [55,67]. In others, specimens are removed and replaced numerous times for cumulative periods of 10 000 h or more [22,54,68],... 

In this paper we will first review some basic concepts and apply them to the design of isothermal reactors working in the diffusional regime.Then we will concentrate our attention on the problem of intraparticle convection in large pore catalysts.Several aspects of this question will be dealt with - effectiveness factors for iso -thermal and nonisothermal catalysts, measurement of effective diffu-sivities and the implication of intraparticle convection effects on the design and operation of fixed bed catalytic reactors. 

Even for gas reactions without change in mole number, forced convection caused by thermal expansion or contraction under nonisothermal conditions can significantly retard or accelerate the rate [29]. Moreover, it can possibly lead to... 

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