Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

The present study presents new flow boiling heat transfer results of R-134a flowing inside a 13.84 mm internal diameter, smooth horizontal copper tube. The heat transfer measurements were made over a wide range of test conditions: saturation temperatures of 5, 15 and 20 C, (corresponding to reduced pressures of 0.08, 0.12 and 0.14), vapor qualities ranged from 0.01 to 0.99, mass velocities of 300 and 500 kg/m(2) s, and heat fluxes of 7.5 and 17.5 kW/m(2). The experimental results clearly show that a local minimum heat transfer coefficient systematically occurs within slug flow pattern or near the slug-to-intermittent flow pattern transition. The vapor quality x(min) at which the local minimum occurs seems to be primarily sensitive to mass velocity and heat flux. Thus, it is influenced by the competition between nucleate and convective boiling mechanisms that control macroscale flow boiling. The experimental results were compared to four types of predictive methods: (a) strictly convective, (b) superposition, (c) strictly empirical and (d) flow pattern based. Generally, all the methods tend to underpredict the experimental data and the higher errors occur in two particular regions: low and high vapor qualities. These vapor qualities correspond to slug and annular patterns, respectively. For slug flow, methods that require the identification of nucleate boiling related regions tend to predict the heat transfer coefficient accurately. This emphasizes that for slug flows, heat transfer is not a simple juxtaposition of nucleate and convective boiling contributions, but that the integration of these two heat transfer mechanisms is also a function of flow parameters. The comparisons between experimental and predicted data show that the best overall results are obtained with superposition and flow pattern based methods. (C) 2008 Elsevier Ltd. All rights reserved.

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.

2008

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

Flow pattern maps for convective boiling of CO2 and R410A in a horizontal smooth tube: Experiments and new correlations analyzing the effect of the reduced pressure

John Richard Thome

In this work flow visualizations and measurements are made and analyzed to identify flow regime transitions, slug-to-intermittent, intermittent-to-annular and the dry-out inception, during the flow boiling of CO2 in a horizontal smooth tube of 6.00 mm of internal diameter, varying the reduced pressure between 0.57 and 0.64, the mass velocity between 150 and 500 kg/m(2) s and the heat flux between 5 and 20 kW/m(2). Additional experiments for R410A show the effect of the reduced pressure over a wider range, from 0.19 to 0.52, varying the other operating parameters in the same ranges. All together, the new experimental dataset of 1420 observations and heat transfer measurements were utilized to determine the location of the flow pattern transitions, which showed a strong dependency of the vapor quality on the mass velocity for each transition line, for fixed reduced pressure. The dry-out inception line was influenced by the heat flux, as expected. The influence of the reduced pressure was also identified as an important parameter with a remarkable impact. This flow pattern database was then statistically compared with well established methods (Wojtan et al. (2005) [1] for R410A, Cheng et al. (2008) [2] for CO2) and recent methods [3,4], showing poor agreement in the determination of the intermittent-to-annular flow regime transition for all the methods and in the prediction of the dry-out inception for all the methods, except for Wojtan et al. (2005) [1]. Finally, new easy-to-use correlations are proposed to provide better agreement with the experimental dataset and to explicitly illustrate the effect of the reduced pressure in an effort to generalize this diabatic flow pattern map for broader application. (C) 2011 Elsevier Ltd. All rights reserved.

2012

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

Olivier Zürcher

EPFL2000