There is agreement in the open literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime. During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear when gravity is the controlling force. Very poor evidence about the effect of microfins on the flow patterns during condensation is given in the open literature. Thus, to investigate the two phase flow pattern during condensation a specific test section was built up; the fluid outlet pattern can be analysed and recorded. The heat transfer and pressure drop performances of microfin tubes during condensation of refrigerants have been studied experimentally and theoretically. In previous papers the present authors reported their own data condensing R134a, R410A and R236ea inside two different tubes: a 9.50 mm outer diameter microfin tube (7.69 mm inside diameter at the fin tip and 60 fins with 0.23 mm fin height and 13° helix angle) and a plain 8.00 mm inner diameter tube. The same operative conditions (saturation temperature, vapour quality and mass flux) used for heat transfer measurements are reproduced in the visualisation section in order to investigate the specific flow pattern for three different refrigerants with high (R410A), intermediate (R134a) and low (R236ea) saturation pressure. For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the visualisation experimental data are analysed with reference to parameters like dimensionless vapour velocity, Martinelli parameter and void fraction, that are adopted in the main available flow pattern maps. Since the available maps are designed only for smooth tubes, the results of this wide experimental work can be used in order to define a new flow pattern map specifically dedicated to microfin and enhanced tubes.

Flow pattern during condensation of three refrigerants: microfin vs. smooth tube

DORETTI, LUCA;ZILIO, CLAUDIO;FANTINI, FRANCESCA
2005

Abstract

There is agreement in the open literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime. During condensation inside horizontal tubes, the two-phase flow may be dominated by vapour shear or gravity forces. While annular flow pattern is associated with high vapour shear, stratified, wavy and slug flows appear when gravity is the controlling force. Very poor evidence about the effect of microfins on the flow patterns during condensation is given in the open literature. Thus, to investigate the two phase flow pattern during condensation a specific test section was built up; the fluid outlet pattern can be analysed and recorded. The heat transfer and pressure drop performances of microfin tubes during condensation of refrigerants have been studied experimentally and theoretically. In previous papers the present authors reported their own data condensing R134a, R410A and R236ea inside two different tubes: a 9.50 mm outer diameter microfin tube (7.69 mm inside diameter at the fin tip and 60 fins with 0.23 mm fin height and 13° helix angle) and a plain 8.00 mm inner diameter tube. The same operative conditions (saturation temperature, vapour quality and mass flux) used for heat transfer measurements are reproduced in the visualisation section in order to investigate the specific flow pattern for three different refrigerants with high (R410A), intermediate (R134a) and low (R236ea) saturation pressure. For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the visualisation experimental data are analysed with reference to parameters like dimensionless vapour velocity, Martinelli parameter and void fraction, that are adopted in the main available flow pattern maps. Since the available maps are designed only for smooth tubes, the results of this wide experimental work can be used in order to define a new flow pattern map specifically dedicated to microfin and enhanced tubes.
2005
Int. Conf. Thermophysical Properties and Transfer Processes of Refrigerants
9782913149434
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/2442985
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