Pressure loss and heat transfer for single-phase turbulent flow in tubes fitted with wire-matrix inserts

Ritchie, John Murray (2009). Pressure loss and heat transfer for single-phase turbulent flow in tubes fitted with wire-matrix inserts. University of Birmingham. Eng.D.

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Abstract

Heat transfer enhancement devices have become widely accepted as a method of enhancing exchanger performance and changing duties to either improve output or meet new operating requirements. hiTRAN® wire matrix inserts consist of a number of loops wound around a central core consisting of two intertwined wires. These inserts see a number of applications inside industrial tubular heat exchangers. They work by removing the laminar boundary layer that is often a dominant resistance to heat transfer, and mixing it with the core flow. This thesis presents research undertaken into the performance characteristics of hiTRAN® inserts in single-phase turbulent flow. Cal Gavin Limited, the company that manufactures these inserts, identified a need for reliable heat transfer and friction factor data within the turbulent flow regime. In order to meet this need, a test rig was commissioned in the form of a double-pipe heat exchanger. This exchanger was used in order to obtain performance data for a wide range of the sponsoring company’s most common insert geometries, placed inside a number of tubes, with diameters ranging from 10 mm to 13/8 inch. The heat transfer and pressure drop data obtained from the test rig were analysed and empirical correlations drawn to describe performance for varying loop densities for each tube and insert geometry. These data were further analysed against the existing semi-empirical theory concerning the use of roughness and geometry parameters to describe friction factor and heat transfer in systematically-roughened channels. The current research has shown that the friction factor correlations may be adapted to incorporate a logarithmic relationship on the ratio of hydraulic diameter to coil pitch, in order to effectively determine the friction factor of hiTRAN® inserts for which this ratio is between 1 and 8. This represents the range of inserts for which the sponsoring company are regularly required to provide thermal designs. The heat transfer performance is shown to be effectively described by the existing analogy between friction factor and heat transfer, as applied to systematically-roughened channels. This thesis also proposes a number of positive commercial implications of the determination of these correlations for the sponsoring company. As well as giving a number of accurate empirical relationships and presenting a semiempirical correlation for the description of performance of hiTRAN® inserts, this work also investigates the effect of a number of geometrical parameters upon insert performance. These qualitative analyses provide an indication of how the optimum coil diameter varies with loop density for a given insert geometry, as well as considering the effect of both the number of turns applied in intertwining the core wire during fabrication, and of the strength of fit that the insert makes with the tube wall. A constant pumping power comparison is also presented, which considers the ratio of heat transfer for the enhanced tube to the heat transfer that would have resulted from the fluid being pumped with the same power through a plain empty tube. This analysis indicates the presence of an optimum pitch to coil wire thickness ratio, the presence of which is substantiated by consideration of the laminar boundary layer behaviour around hiTRAN® inserts. Finally, suggestions are made for how these qualitative analyses may be developed by future experimentation into determining an optimised insert, along with other proposals for further work on the test rig.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

Supervisor(s):