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Compact Heat Exchangers and Enhancement Technology for the Process Industries - 2003

ISBN Druckformat: 978-1-56700-195-2

HEAT TRANSFER AND HYDRAULIC RESISTANCE IN COMPACT HEAT EXCHANGERS WITH HELICAL TUBES IN LONGITUDINAL FLOW

Abstrakt

The bundles of helical tubes or rods are used in space nuclear propulsion power plant intended for piloted expedition to Mars and are characterized by high heat transfer areas per unit volume (above 1250m2/m3) and by enhancement of heat transfer by modifying the flow patterns near the surface. In this case one of the urgent problems is the determination of coefficients of heat transfer and hydraulic resistance in bundles of helical tubes and rods of small diameters to be manufactured from the reactor materials: carbides of refractory metals, graphite and stainless steel. The generalization results of new experimental investigations of hydraulic resistance and heat transfer are presented for a bundles of helical tubes of oval, three-blade and four-blade profiles in the form of the criteria equations at flow of different heat carriers: air, water and liquid oxygen. It is discovered, that in bundles with flow swirling of helical tubes or rods the boundary layer thickness is smaller in ten times as equivalent diameter of channel. Therefore, the rough of surface of helical rods is shown up on the lesser values of Reynolds number as compared with a circular tubes to be manufactured from those materials. Moreover, the transition of coefficients of hydraulic resistance and heat transfer from laminar to turbulent flow covers a considerable Reynolds number range (80-104) and is smooth. The loss of stability of laminar flow is conditioned by nonstability of boundary layer on the curved walls of helical tubes and by additional turbulization of flow. The discovered specific features of heat transfer and hydraulic resistance in helical tube bundles allowed to determine the mechanisms of enhancement of transport processes in this channels. The representation of experimental data on heat transfer and hydraulic resistance in helical rod bundles in the form of the proposed criterial relations extends the modeling potentialities of heat transfer and diminishes the number of experimental studies necessary to establish these relations.
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