ESG-9 Thermal Contact and Gap Resistances of Interference-Fit Bimetallic Finned Tubes - Background Report

K. A. Gardner

This report deals with the additional thermal resistance in interference-fit finned tubes due to less than perfect metal-to-metal contact between tube and fins, as influenced by the thermal operating conditions. The objective is to provide some idea of the current state of the art and how it came about, as well as to indicate areas of uncertainty requiring further study.

A survey of the pertinent literature is presented in condensed form, followed by a narrative (qualitative) description of the mechanism of introduction of contact and gap resistance.

Quantitative theoretical relations from the literature are set forth and discussed in some detail; it is found necessary to correct some of them and this is done. Attention is called to the need for experimental determination of the initial contact pressure established by the finning operation for use in these relations.

Test results of several investigators of the contact and /or gap resistance are shown graphically. One of these also presented results on initial contact pressure determined by a non-heat-transfer method (internal strain gauges) which could be construed to mean that load bearing contact is the as-fabricated condition extended over only a small fraction of the fin pitch. The same investigators also observed an axial displacement at the temperature of the tips of footed tension wound fins which led them to question one aspect of the theoretical derivations.