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.

Contribution to the heat transfer analysis of natural and substitute refrigerants evaporated in a horizontal tube

Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles, requests a good knowledge of the heat transfer and pressure drop properties of substitute fluids. A contribution to this international effort is proposed with the study of two hydrofluorocarbon refrigerants (HFC-134a and the zeotropic mixture HFC-407C) and the study of the natural refrigerant ammonia. The HFCs have been experimentally tested on the first test rig developed in the Laboratory of Industrial Energy Systems (LENI) which was substantially modified to cope with ammonia, both in terms of safety requirements and operational conditions. The experimental test section is composed of two concentric tubes, with in tube evaporation of the refrigerant inside the inner tube of counter-current Annular water heating. A new database of the local heat transfer coefficients for the refrigerants HFC-134a, HFC-407C and ammonia together with pressure drop measurements has been collected and used to define and calibrate a new and more general heat transfer model. The two HFC refrigerants have been tested on the test rig developed by Kattan [36]; due to the chemical and the thermophysical properties of ammonia, a new test rig has been designed for the ammonia tests and a new calculation procedure based on the mean temperature measurement on the tube wall has been used. The experimental database covers evaporation tests on plain and on microfin tubes, and with nominal oil concentrations varying from 0 to 5 [%] (by wt.). Detailed modeling is concentrated on the tests on plain tube without oil. Each of the three refrigerants has been evaporated at 4[°C]. The other experimental conditions for the HFCs refrigerants were an inside tube diameter of 10.92[mm], a heat flux range of 2 – 5 [kW/m2] and three mass velocities of G = 100, 200, 300 [kg/(m2s)] visualized as Stratified-Wavy flows (G = 100[kg/(m2s)]), and mainly Annular for the higher mass velocities (G = 200, 300[kg/(m2s)]). The other global conditions for ammonia were an internal tube diameter of 14.00 [mm], a heat flux range of 5 – 70[kW/m2] and eleven mass velocities of G = 10,20,30,40,45,50,55,60,80,120,140[kg/(m2s)], corresponding to Stratified, Stratified-Wavy, Intermittent and Annular flow patterns; complementary tests, varying the mass velocity at constant vapor qualities (x = 20,50,80[%]), have been made in particular to better characterize boundaries between flow patterns. The flow patterns are visualized through glass sections at both ends of the 3.1 [m] long test tube. An extensive review of void fraction models with sensitivity analysis on the actual available flow pattern map has been made. From the 17 correlations reviewed, the models of Taitel-Dukler [71] and Rouhani [79] have been used. Other improvements to the threshold criteria of the Stratified-Wavy to Annular transition, lead to an accurate diabatic map to predict the flow patterns and their transition for very different families of fluids like HFC's and ammonia. A new approach in the prediction of two-phase flow heat transfer has been proposed through the study of each flow pattern separately, according to 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 function based on a pseudo Biot number allows, from the mean heat flux around the tube periphery, to base the model on two different mean heat fluxes applied respectively to the parts of the perimeter in contact with the liquid and the vapor in stratified types of flow. 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. The two-phase flow heat transfer coefficient has finally been obtained as a weighted mean function of the vapor and the liquid heat transfer coefficient with respect to their contact surface with the heated tube. Based on the database of refrigerants HFC-134a and ammonia, the standard deviation of the new heat transfer model is of σ = 27.9[%]. Even if the database showed that the flow conditions were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points were purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database. 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. Finally, a new method to model separated flows is proposed, based on partial hydraulic diameters and a mean interface velocity.

EPFL2000

Evaporation de mélanges d’ammoniac et d’huile dans des tubes.

Daniel Favrat, John Richard Thome, Olivier Zürcher

After an experimental study of refrigerants HFC-134a, HCFC-123, HFC- 404A and HFC-407C, and considerable modifications to the test stand, LENI has undertaken a new experimental program and has assembled a large new database for evaporation of ammonia and two ammonia-oil mixtures (1% and 3% wt. oil) inside horizontal tubes with smooth and enhanced surfaces. This work extends the current database to include a refrigerant in which there is a marked renewal of interest. The comparison of the ammonia results with the flow boiling heat transfer model of Kattan et al.(1997) will allow the model to be validated to a much broader range of application The experimental tests on smooth tubes without oil covered a wide range of mass velocity (20-140 kg/(m2s)) and heat flux (5-70 kW/m2). However some additional tests on evaporation for a wide range of mass velocities at constant vapor quality are still required as part of the database in order to investigate flow pattern transitions and to develop a new version of the Kattan et al.(1997) model with a wider field of application to fully stratified and stratified-wavy flow patterns. The predicitons of the heat transfer coefficient for the annular and intermittent flow patterns by the Kattan et al.(1997) model produced very convincing results (average deviation of -11.9% and standard deviation of 6.33%) calculated over 172 measurement points), in spite of the fact that the thermophysical characteristics of ammonia differ considerably from those of traditional refrigerants. The effect of 1% and 3% oil on plain and microfin tube heat transfer at 50 and 80 kg/(m2s) was similar to that found for annular flows with previous refrigerants, i.e. little influence at low to intermediate vapor qualities but a substantial fall off in performance at vapor qualities greater that 70%. At the lowest mass velocity of 20 kg/(m2s), the 3% oil tended to increase the plain tube performance while for the microfin tube the oil holdup in the test section was so severe for both 1% and 3% oil that the heat transfer coefficients fell towards zero at vapor qualities greater than 40%. Thus, very low mass velocities when using microfins should be avoided since a little oil will invariably be present in an operating unit. A study on two-phase pressure drops in tests without oil and with nominal oil concentrations of 1% and 3% showed that they remain the same or decrease in the presence of oil. The decrease is surprising result and were confirmed by several independent means of measurement, and merit more future in- depth study.

1998

INFLUENCE OF THE SATURATION TEMPERATURE ON THE HEAT TRANSFER COEFFICIENT FOR FLOW BOILING INSIDE HORIZONTAL SMOOTH TUBES

Ricardo Lima, John Richard Thome

This article presents experimental results of heat transfer from two different studies [1, 2] for horizontal flow boiling inside smooth copper tubes with different refrigerants. The main goal of this study was to verify the influence of the saturation temperature on the heat transfer coefficient. The experimental tests were carried for refrigerants R-134a, R-22 and R-404A, at saturation temperatures ranging from 5 to 20°C for two tubes of 12.7 and 13.84 mm ID and three heat fluxes of 5, 7.5 and 17.5 kW/m2. The vapor quality ranged from 5 to 90% and the mass velocity was 300 kg/s.m2. Under these conditions the heat transfer coefficient shows an interesting trend: it behaves differently, in function of the saturation temperature, in the nucleate and convective boiling dominated regions. In the nucleate boiling dominated region the heat transfer coefficient increases with the saturation temperature. In the convective boiling dominated region this phenomenon is inverted, that is, the heat transfer coefficient decreases with the saturation temperature. This phenomenon was verified through the concept of the thermal resistance of the liquid thickness. Since the heat transfer coefficient is proportional to the inverse of the thermal resistance of the liquid film, it increases with decreasing saturation temperature. The transition between nucleate and convective boiling dominated regions, denominated by [1] as a local minimum of the heat transfer coefficient, was also clearly observed with all the refrigerants.