Enhancement of heat and mass transfer in heat exchangers with helical tubes

At present a enhancement of heat and mass transfer in different channels due to flow swirling is among one of the more promising way to design compact heat exchangers. This method of enhancement for helical tube bundles in longitudinal flow enables to substantially decrease the size and mass of heat exchangers and to augment interchannel mixing of the heat carrier in intertube space. The heat exchangers on the base of oval twisted tubes have been developed and used in power engineering and food industry. For suppression of azimuthal nonuniformities of velocity and temperature that develop in the lateral entrance of heat carrier flow the helical-tube twisted bundle heat exchanger can be used. In this device, a helical tube bundle is twisted relative to longitudinal axis of bundle. The goal of this paper is generalization of the comprehensive data on the heat transfer enhancement and improvement of interchannel fluid mixing as well as makes a comparison of heat and mass transfer effectiveness for the channels formed by typical helical tube and twisted bundles. For investigating heat and mass transfer in these bundles were used the original methods of zone heating and of heat diffusion from point source. The heat transfer from a system of linear finite-size heat sources is considered for determining an effective diffusion coefficient using the homogenized flow model and statistical methods. Heat transfer in bundles was investigated by method of local similitude. The experimental data on heat transfer in a typical helical tube bundles is generalized in the form of second order multiterms in logarithm coordinates for geometrically nonsimilar bundles. The data on the interchannel flow mixing for helical tube bundles and for helical-tube twisted bundles are represented by the same relations. The cross mixing coefficients in these bundles exceed by 10 times the turbulent diffusion coefficients near the axis of circular tube. The twisting bundle of helical tubes enhances heat transfer compared with to a typical helical tube bundle. It are concluded, that a use of twisted tube allows enhancing heat transfer and transverse flow mixing and decreasing of temperature nonuniformity over the bundle cross-section.