Numerical Studies into Hydrodynamics and Heat Exchange in Heat Exchangers Using Helical Square and Oval Tubes

Rinat Shaukatovich Misbakhov¹, Viktor Mihaylovich Gureev², Nikolai Ivanovich Moskalenko¹, Andrey Mihaylovich Ermakov² and Ilyas Zul’fatovich Bagautdinov¹

¹Kazan State Power Engineering University , Russia, 420066, Kazan, ul. Krasnoselskaya., 51. ²Kazan National Research Technical University Named After A.N. Tupolev, Russia, 420111, Kazan, ul. Marksa, 10

DOI : http://dx.doi.org/10.13005/bbra/2252

ABSTRACT: Increase in efficiency of heat exchangers is an urgent task which facilitates development of compact heat exchangers for various industries. The efficiency can be increased by various methods: superficial intensification, use of insertion made of twisted tapes, use of membranes in intra-tube space, flow whirl etc.. One of the approaches to flow whirl is the use of helical tubes of various cross sections. This work investigates into heat exchange and hydrodynamics of heat exchanger with helical tubes of square and oval cross sections. The range of studies of flow modes has been selected involving the most widely applied in practice Re numbers from 3000 to 20000. Numerical studies have been aided with ANSYS CFX solver technology and the calculated Menter’s Shear Stress Transport Turbulence Model. As a consequence, the hydraulic resistance and heat efficiency of helical tubes of square and oval cross sections have been obtained as a function of flow mode. The use of helical tubes of square cross section makes it possible to increase heat transfer from 80 % at lower Re = 3000 to 40 % at Re = 20000. More promising is the application of oval tubes, for which the heat transfer increases by 90-100 % in all studied range of Re numbers. Analysis of heat thermal-hydraulic efficiency demonstrates dominancy of oval helical tubes at higher Re numbers (from 10000), at lower Re numbers (3000-7500) square helical tubes provide higher thermal-hydraulic efficiency. In addition, the flow structure has been determined in tubes and in intra-tube space. Analysis of liquid flow structure in intra-tube space demonstrates better miscibility upon application of helical tubes of oval cross section, which permits to increase heat transfer by 90 %. The flow structure with helical tubes of square cross section is close to flow around of plain tubes.

KEYWORDS: Helical tubes; oval tubes; square tubes; heat exchange; simulation; heat transfer; flow structure

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