Summary

Oil-free compressors enable different architectures in refrigeration system design, however some heat exchanger arrangements enable occurrence of condensate flow reversal to the compressor. Liquid presence in the compressor should be avoided, since it is one of the main factors in mechanical failure. The condensate back-flow occurs in counter-current to the compressor discharge vapor, the transitions between co-current and counter-current flow are studied in the counter-current flow limitation (CCFL) topic. This phenomenon is studied since 1960, aiming to predict the condition in which the liquid film starts to be dragged partially by the gas, point which is known as onset of flooding. Most of the works on this topic deal with adiabatic studies with separated airwater injection, only recently experimental work have been developed with refrigerant fluids and condensation. Despite all experimental work the flooding phenomena is still not fully understood today, and testing and empirical correlations are the primary means to predict its occurrence.Thus it is difficult to extrapolate transition curves to different fluid and operation conditions. In order to assure that no condensate will flow back to the compressor, the discharge operation condition should be above the flow reversal transition line. Due to technical motivation in nuclear power plants and reflux condensation columns, the emphasis in studying CCFL was in the flooding point, therefore the reflux condensation point was less studied. This work aim is to investigate experimentally the onset of flow reversal in upflow condensation of R134a refrigerant in vertical and inclined small diameter tubes. A experimental test facility was designed and constructed to allow measurements and high-speed visualization of flow reversal condition. The vapour superheat, condensation pressure and mass flow were controlled at the test section inlet and due condensation rate variation in the test section different operation conditions were obtained at the outlet, location where the visualization is realized. 264 experimental point in 17 operation conditions were obtained, where measurements and visualization were realized simultaneously for all experiments. Experimental results were analysed and compared to correlations to predict CCFL transitions, also different dimensionless group approach to comprise fluid properties were tested. New empirical constants were suggested to Wallis correlation and the Re group was added in the approach of fluid properties influence.

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