Two-phase local heat transfer correlations for non-ozone depleting refrigerant-oil mixtures.
This thesis describes the work undertaken over a 3 year period under a
Department of the Environment, Transport and the Regions (DETR) Partners in
Technology programme in collaboration with four industrial partners.
The aim of the work was to investigate the two-phase flow heat transfer and fluid
dynamic performance of the non-ozone depleting refrigerants and lubricating oil
mixtures in a horizontal tube. A single tube test facility was designed and built to
determine local heat transfer coefficients and overall pressure drop for evaporation
and condensation. A circulating pump system pressurised through an accumulator
charged with nitrogen was selected to enable accurate setting of the evaporation
and condensation conditions. The system was validated using R22. Tests were
undertaken for R404A, R407C, Isceon 59 with a Polyol-ester lubricating oil. The
experimental data consist of two-phase local heat transfer coefficients and overall
pressure drops inside a smooth copper tube. Numerous data were obtained for
evaporation and condensation, and constituted a large database of two-phase heat
The experimental results were compared with several existing correlations
traditionally used in heat exchanger design, to assess their suitability with the
new refrigerants. As a result of these comparisons offering a poor agreement, a
model characterising the variation of the local heat transfer coefficient was
developed. The evaporation model consists in a modification of the Kattan et al.
model (1998a, b & c). The new model predicts the experimental results well, with a
standard deviation of 6.1% for refrigerant R407C and Isceon 59. The effects of
lubricating oils on two-phase flow heat transfer are also discussed qualitatively.
This work provides a design tool when dealing with the non-ozone depleting
refrigerants, and recommendations are made on the use of several correlations.