FORCED CONVECTION CONDENSATION OF PURE REFRIGERANTS AND BINARY MIXTURES INSIDE A HORIZONTAL TUBE

Abstract

Forced convection condensation inside horizontal tubes occurs in several engineering applications, e.g. refrigeration and air- conditioning systems, paver plants, chemical process industries, etc. Studies of flow patterns and heat transfer during two-phase flow is of great importance for heat exchanger design. It is also suggested in literature that use of binary mixtures in refrigeration systems may result in several direct and indirect advantages. An extensive review of existing literature revealed that for the case of condensation inside horizontal tubes (i) flow pattern studies are rather limited, (ii) flow pattern data for mixtures are non-existent, (iii) there is no, unanimity over a single heat transfer coefficient correlation for design of heat exchangers, and that (iv) the heat transfer data for mixtures are scarce. In view of the above, in the present work the problem of prediction of flow pattern and heat transfer characteristics during forced convection condensation of pure refrigerants 12 and 22, and their binary mixtures, inside a horizontal tube have been investigated. To carry out the experimental investigation, a test set-up was designed, fabricated and installed. The experimental test facility used was essentially a. well instrumented vapour compression refrigeration unit. The system included a pre.. condenser, an aftercondenser, and a refrigerant bypass line which permitted a wide range of operating conditions in the test condenser. The test condenser consisted of three lengths of hori-zontally mounted annular coaxial double copper tubes with refrigerant flowing through the inner tube and cooling water flowing in the outer annulus countercurrently. Each length of test section was 1 m long, the inner tube was 10 mm I.D. and 12.5 mm 0. D. while the outer tube was 50 mm I. D. The three test sections were joined together smoothly in series with sight glasses between them and also at the inlet of the first section and exit of the last section. The inside diameter of the glass tube of sight glasses was the same as that of the inner tube so as not to disturb the condensate flow. The apparatus was provided with necessary instrumentation for the measurements of cooling rates, mass flow rates, vapour quality, mass concen-tration, average heat transfer coefficient of each test section, etc. The range of operating variables covered in this investi-gation were t 1. Working fluid R-129 R-22 and their mixtures of three different compositions (25,50 and 75 mass percent of R-22) 2. Inlet superheat t 21 - 36°C 3. Refrigerant mass t 172 - 560 'kes m2 velocity Average condensing t 19.6 - 40°C temper a tur e Vapour quality at visual 0.00 - 0.99 section 6.• Average cooling hea.t t 350 - 31000 W/m2 flux in a. test section (iv) Coolant water flow rate 0.111- 0.250 kg/s (1+00 - 900 kgthr Coolant water s 7 - 19°C temperature The thermodynamic properties of pure refrigerants 12 and 22 were evaluated from the equations given by Downing [38]. For binary mixtures of R-12 and R-22, the thermodynamic and thermo-physical properties were computed by the equations and combining rules suggested by Kandlikar [67]. A general flow regimes map has been developed on the basis of the existing condensing flow pattern data of previous investigators over a wide range of operating conditions. It was. found that the data were best correlated on a. map with dimension-less gas velocity jg* as ordinate and (i-a)/a as abscissa, where 9 r a is .void fraction evaluated from amith s L114] correlation. The proposed flow regimes map was in excellent agreement for annular, semia.nnular and wavy flows, which exist over. a major part of the length during condensation inside a horizontal tube. For these flow patterns, the prediction of proposed map was considerably better than those of the existing maps [11+123,118]. In the experimental flow pattern studies, for pure refrigerants 12 and 22 over 350 flow pattern data points resulting from 99 test runs and for binary mixtures of R-12 and R-22 over 1+30 data points resulting from 109 test runs were obtained. For R-22 and binary mixtures of R-12 and R-22, flow pattern data were generated, perhaps, for the first time. In the present investi-gation, the six major flow patterns identified were , spray, annular, semiannulsr, wavy, slug and plug flows.