Browsing by Author "Senda, Paul Tudieji"

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    Transient flow, flow pattern characterisation and performance evaluation of a two-phase natural circulation heat transfer loop.
    (Stellenbosch : Stellenbosch University, 2018-03) Senda, Paul Tudieji; Dobson, Robert T; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: This thesis project focusses on the experimental and theoretical analysis of the transient behaviour of a single and two-phase flow natural circulation heat transfer loop using water as the working fluid. The background theory provided by a literature survey shows that natural thermosyphon loops offer attractive solutions for passive cooling and heat transfer, and safety systems; especially in the nuclear and process industries. In particular the thesis considers a passive reactor cavity cooling system (RCCS) for next generation high temperature nuclear reactors such as “the pebble bed modular reactor (PBMR)”. A time-dependent mathematical computer simulation program of a 7 m high by 8 m wide, 32 mm inside diameter vertically orientated rectangular natural circulation loop was developed. The major assumption was that the flow is quasi-static; implying that although the mass flow rate might be changing with time, that at any instant in time that the mass flow rate is constant at axial position along the loop. To theoretically simulate the loop the working fluid was discretised into a series of one dimensional control volumes. By applying the conservation of mass, momentum and energy and suitable property functions to each control volume as series of time dependent partial differential equations were generated and then solved using an explicit finite difference method. The one vertical side of the loop was heated by a series of electrical heating elements and the other vertical side cooled using a series of water-cooling jackets. Transparent pipe lengths were inserted in the loop to observe the two-phase flow patterns. The loop was supplied with an expansion tank and operated in single and two-phase modes with water as the working fluid at a mass flow rate determined using an orifice-plate flow meter. A reasonable correlation between the experimental and theoretical simulations was found using a separated two-phase flow with frictional multipliers and vapour-liquid void fraction correlations, similar to those originally suggested by Martinelli. It was concluded that the as developed theoretical model adequately captured the actual transient and dynamic flow and heat transfer behaviour of the loop. It is thus recommended that a series of such loops could be used with confidence for a RCCS.