Natural convection flow transient

For prediction of subassembly coolant flow rate and temperature distributions a wide range of coolant flow and thermal convection regimes must be considered including laminar and turbulent flow natural, forced and mixed (forced + natural) convection and steady state and transient reactor conditions. 

Based on 2-D RANS and 3-D DNS simulations at meso-scale, it was concluded that 3-dimensional transient RANS simulations are probably the most promising approach for accurate and relatively inexpensive modelling at the macro (whole-body) scale. This approach allows the inclusion of various realistic and important phenomena, such as natural convection and chemical breakthrough. The results of this DNS simulation are nsed (i) to validate RANS and T-RANS simulations for the same conditions and (ii) to guide in choosing a turbulence modelling strategy for the RANS and T-RANS simulations of the flow underneath the porous layer. 

LEADIR-PS 200 has a graceful and safe response to all anticipated transients. For example, an overcooling event (as could be caused by loss of feedwater control or spurious opening of steam relief valves in combination with control system failure) causes the core inlet temperature (normally 350°C) to fall as the freezing point of 327°C is approached the coolant viscosity increases, coolant flow decreases, and in the absence of any control system action, the negative temperature coefficients of the fuel and moderator reduce reactor power. Heat removal is maintained by natural convection. 

The mechanisms of mass transport can be divided into convective and molecular flow processes. Convective flow is either forced flow, for example, in pipes and packed beds, or natural convection induced by temperature differences in a fluid. For diffusive flow we have to distinguish whether we have molecular diffusion in a free fluid phase or a more complicated effective diffusion in porous solids. Like heat transport, diffusion may be steady-state or transient. 

For a number of flow situations, the mass-transfer rate can be derived directly from the equation of convective diffusion (see Table VII, Part A). The velocity profile near the electrode is known, and the equation is reduced to a simpler form by appropriate similarity transformations (N6). These well-defined flows, therefore, are being exploited increasingly by electrochemists as tools for the kinetic characterization of electrode reactions. Current distributions at, or below, the limiting current, transient mass transfer, and other aspects of these flows are amenable to analysis. Especially noteworthy are the systematic investigations conducted by Newman (review until 1973 in N7 also N9b, N9c, H6b and references in Table VII), by Daguenet and other French workers (references in Table VII), and by Matsuda (M4a-d). Here we only want to comment on the nature of the velocity profile near the electrode, and on the agreement between theory and mass-transfer experiment. 

The present lecture summarizes some of tiie most recent joint research results from tiie cooperation between the Federal University of Rio de Janeiro, Brasil, and tiie University of Miami, USA, on tiie fransient analysis of both fluid flow and heat transfer within microchannels. This collaborative link is a natural extension of a long term cooperation between the two groups, in the context of fimdamental work on transient forced convection, aimed at tiie development of hybrid numerical-analytical techniques and tiie experimental validation of proposed models md methodologies [1- 9]. The motivation of this new phase of tiie cooperation was thus to extend the previously developed hybrid tools to handle both transient flow and transient convection problems in microchannels within the slip flow regime. 

The analytical model is shown in F ig. 1 for heat flow interaction with an environment consisting of Case I, heat transfer with an ambient through a convective heat transfer coefficient and. Case II, heat transfer with an ambient consisting of an imposed, constant heat flux. The system consists of a container (or, pipe as far as this analysis is concerned) initially filled with liquid at temperature T, Initially, both the wall and the liquid are at temperature Tj. At zero time, a pressurizing gas having temperature is introduced into the top of the container at x = 0. At this same time liquid discharge or flow is commenced such that the gas-liquid interface immediately starts to move downward at a velocity V. In addition, heat flow interaction of the nature of Case I or Case II between the outside of the container and the ambient starts. As a consequence of this, a transient process is introduced in the temperatures of both the wall and the gas. It is the purpose of this paper to present a solution for the thermal response of the gas and the wall. 

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