Comprehensive study of heat transfer enhancement in tubular heat exchangers

Abstract

E. K. Kalinin G. A. Dreitser N. V. Paramonov A. S. Myakochin A. I. Tikhonov S. G. Zakirov E. S. Levin L. S. Yanovsky Moscow Aviation Institute, Moscow, USSR Tashkent Polytechnical Institute, Tashkent, USSR Research and Production Association at Central Boiler-Turbine Institute, Leningrad, USSR IIAt the Moscow Aviation Institute a highly effective method was developed to enhance heat transfer in tubular heat exchangers, and comprehensive studies were made of its effectiveness when applied to gas and liquid flow in tubes, circular channels, and in-flow tube bundles over a wide range of performance parameters as well as to boiling and condensation of heat carriers. The essence of the method is as follows. Equidistant annular grooves are rolled on the outer surface of a heat transfer tube. In this case, annular smooth-configuration diaphragms are formed on the inner tube surface. Annular grooves and diaphragms turbulize the flow in a wall layer and provide heat transfer enhancement outside and inside the tubes. Experimental investigations were conducted at the Moscow Aviation Institute, Moscow Aviation Technology Institute, Tashkent Polytechnical Institute, Research and Production Association at the Central Boiler-Turbine Institute, and elsewhere. The results of introducing this method in various types of heat exchange apparatuses are also presented. The theoretical analysis of a turbulent flow structure in channels and in a separation zone and the experimental studies in channels of various configurations make it possible to reveal a hitherto unknown law for heat transfer on the channel wall in the presence of discrete flow turbulization at forced convection: At a certain ratio of turbulizer sizes and locations, heat transfer increases faster than hydraulic resistance, as compared to a smooth channel. New investigations conducted on 20 types of tubes with turbulizers of various shapes have shown that the use of smooth-shape turbulizers provides, at the same Nu/Nusm , a 30-35% reduction of ~/~sm (Nu is the Nusselt number, ~ is the hydraulic resistance coefficient, subscript s m refers to a smooth channel) and enable one to extend the region of advancing growth of heat transfer. A study was conducted of heat transfer enhancement in a heated tube in which a supercritical-pressure kerosene and oil flowed under the conditions of carbon deposit. Heat transfer increased more than fivefold and hydraulic resistance, 3.75-fold. Compared to smooth tubes, there was a substantial decrease in deposits in turbulizer-equipped tubes. This method proved to be very effective when applied to boiling of heat carriers. At disperse and slug film boiling inside tubes, heat transfer increases 3-8-fold, and at surface boiling of water and water-glycerine mixtures, by 30-40%. Heat transfer enhancement at film and drop condensation of vapors of water, benzene, acetone, and their mixtures on the outer tube surface as well as at vapor condensation from vapor-air mixtures was investigated. Decreasing groove pitch, making a projecting part of the tube convex, and making a smooth junction between groove surfaces and projecting tube parts (Author's Certificate USSR. No 1307211) enable one to augment film condensation heat transfer by factors of 1.7-2.8 on vertical tubes and 2 .5-3 on horizontal tubes due to effective separation of the condensate film or its running down into grooves. Annular grooves and diaphragms decrease salt deposits outside and inside the tubes and make heat exchangers serviceable at carbonate hardness of water up to 20 mg-eq/liter. The experimental data outlined in the present article were supported by tests of commercial heat exchange apparatuses. In single-phase medium apparatuses,