Olivier Zürcher

Development of a Diabatic Two-Phase Flow Pattern Map for Horizontal Flow Boiling

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. Based on new flow pattern data for three different refrigerants covering a wide range of mass velocities, vapor qualities and heat fluxes. The new flow pattern map includes the prediction of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters and is an extension to the flow pattern map model of Kattan et al. [J. Heat Transfer 120 (1998) 140]. The proposed modifications allow an accurate prediction of the flow pattern for very different fluids which are the substitute refrigerants (HFC-134a and HFC-407C) and the natural refrigerant R-717 (ammonia).

In-Tube Flow Boiling of R-407C and R-407C/Oil Mixtures Part II: Plain Tube Results and Predictions

Intube evaporation tests for R-407C and R-407C/oil are reported for a plain copper tube. The tests were run at a nominal inlet pressure of 6.45 bar (93.5 psia) at mass velocities of 100, 200 and 300 kg/m2s (73581, 147162 and 220743 lb/h ft2) over nearly the entire vapor quality range. Pure R-407C performed very similar to pure R-134a run previously in similar tests at all three mass velocities, differing only at high vapor qualities where the peaks in href vs. x were shifted slightly. For local vapor qualities from 10-70%, oil tended to have little effect on local R-407C/oil heat transfer coefficients at the lowest mass velocity while at the higher mass velocities the effect was to increase or decrease the coefficients within ±20% of the pure R-407C values. At vapor qualities higher than 70%, the effect of the oil was very dramatic, decreasing performance by as much as 80-90% even with small amounts of oil. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. Two methods for predicting local boiling coefficients of refrigerant-oil mixtures were presented. Using the refrigerant-oil mixture viscosity in place of the pure refrigerant viscosity in the recent Kattan-Thome-Favrat flow boiling model and flow pattern map without further modification predicted the R-134a/oil and R-407C/oil data quite accurately. The Friedel two-phase friction multiplier was found to work adequately for pure R-134a and pure R-407C. Finally a new local refrigerant- oil viscosity ratio was developed that accurately predicted two-phase pressure drops of R-134a/oil and R-407C/oil mixtures at high vapor qualities.

Evaporation of refrigerants in a horizontal tube: an improved flow pattern dependent heat transfer model compared to ammonia data

Few experimental test data are available for evaporation of ammonia inside tubes and numerous new data have been measured and presented here. An improved approach to the prediction of flow boiling heat transfer in horizontal tubes has been proposed through the study of each flow pattern separately, incorporating a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A new function, based on a pseudo-Biot number delineates two different mean heat fluxes on the perimeter of the tube in stratified types of flow, one in contact with the liquid and one in contact with the vapor. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. Even if the database showed that the flow conditions in the annular liquid film were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points where purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database of refrigerants HFC-134a and ammonia. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation.

In-Tube Flow Boiling of R-407C and R-407C/Oil Mixtures Part I: Microfin Tube

In-tube evaporation tests for R-407C and R-407C/oil are reported for a microfin tube. The tests were run at a nominal inlet pressure of 645 kPa (93.5 psia) at mass velocities of 100, 200 and 300 kg/(m{sup 2}{center{underscore}dot}s) (20.5, 41, and 61 lb/s{center{underscore}dot}ft{sup 2}) over nearly the entire vapor quality range. At similar operating conditions, pure R-407C performed similarly to previous pure R-134a tests at the highest mass velocity, but lower than R-134a for the other mass velocities. Any amount of oil tended to decrease local R-407C microfin heat transfer coefficients, especially at high vapor qualities were degradations of as much as 50% or more occurred. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. Buildup of the local oil mass fraction in the microfin test sections was observed at high vapor qualities, together with the formation of slowly flowing viscous liquid films, a phenomenon that became more acute at lower mass velocities.

An onset of nucleate boiling criterion for horizontal flow boiling

An extensive database has been collected for the evaporation of two hydrofluorocarbon refrigerants (HFC- 134a and the zeotropic mixture HFC- 407C) and the natural refrigerant ammonia (R-717). A model to predict the onset of nucleate boiling has been successfully developed to differentiate pure convective evaporation from mixed convective and nucleate boiling heat transfer during horizontal evaporation inside a horizontal tube of 14mm I.D. The analysis of the Stratified, Stratified-Wavy and mainly Annular flow patterns during evaporation with differente heat flux ranges showed very accurate results in terms of mean local heat transfer coefficient using this new onset of nucleate boiling criterion.