MODELING OF CONDENSATION PROCESS INSIDE SMOOTH HORIZONTAL TUBES

Abstract

The present dissertation work consists of the development of a mathematical model for the condensation of pure saturated vapours, inside a smooth horizontal tube. The condensation heat transfer coefficient based on the flow regime inside the tube has been obtained by computation. A new generalised heat transfer model for the condensation of pure vapours inside a horizontal smooth tube has been developed. In fact the model by Thome et al. (2003) has been modified based on simplified flow structures of the flow regimes. The proposed model can predict local condensation heat transfer coefficient for the following flow regimes: annular flow, intermittent flow, stratified-wavy flow, fully stratified flow and mist flow. Further, the model has been developed for a large number of refrigerants. like R-11, R-12, R-22, R-32, R-113, R-125, R-134a, R-404A, R-410A, and some of the hydrocarbons like propane, n-butane, iso-butane and propylene. The proposed model has been tested for mass velocity of 24 - 1022 kg rn-2S-1, vapour quality 0.03 - 0.97, reduced pressure 0.02 - 0.8 and tube internal diameter 3.1 - 21.4 mm. The flow pattern map of Taitel and Dukler (1976), modified by Hajal et al. (2003), has been used in present work to modify the model of Thome et al. (2003). The effect of various parameters i.e. mass velocity, vapour quality, void fraction, reduced pressure, the difference between the saturated temperature and the wall temperature etc. on the flow pattern map predictions and on the heat transfer coefficient are studied by using the present model. The proposed model is also tested on extreme values of vapour quality and different flow regime transition zones. The flow pattern predictions are compared with the predictions of flow pattern from methods provided by Tandon et al. (1982), Dobson and Chato (1998), Soliman (1982) and Cavallini et al. (2002). The different void fraction models applied previously for flow pattern map prediction are studied and compared with present logarithmic mean void fraction model. Finally the condensation heat transfer coefficients obtained by present model for different refrigerants and hydrocarbons at different test conditions are iii compared with the experimental results of different investigators. The predictions of present model are also compared with the predictions from other models for condensation inside smooth horizontal tubes viz. Cavallini et al. (2002), Shah (1979), Dobson and Chato (1998), and Aprea et al. (2003), which are widely accepted. A comparison of the results from the proposed model shows better agreement with the experimental values in comparison to the predictions from the widely accepted Thome's model. The proposed model predicts the heat transfer coefficient within an error band of ± 17.5 percent for 80 percent of experimental data, whereas, the widely accepted Thome's model (2003) predicts the heat transfer coefficient within an error band of ± 20 percent. iv