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.     

Condensation in horizontal tubes, part 1: two-phase flow pattern map

A new flow pattern map and flow pattern based heat transfer model for condensation inside horizontal plain tubes are proposed in this two-part paper. In Part I, a new version of a two-phase flow pattern map, originally developed by Kattan et al. [J. Heat Transfer 120 (1998) 140] for flow boiling, is presented for condensation inside horizontal tubes while a new heat transfer model is presented in Part II. The new flow pattern map incorporates a newly defined logarithmic mean void fraction (LMε) method for calculation of vapor void fractions spanning from low pressures up to pressures near the critical point. Several other modifications are also made that are appropriate for condensation as opposed to evaporation. In the absence of void fraction data at high reduced pressures for these conditions, the new LMε method has been indirectly validated using the convective condensation model for annular flow and corresponding heat transfer test data at reduced pressures up to 0.8. The new map has also been successfully compared to some recent flow pattern observations for condensation and other existing flow transition criteria and maps.     

New theoretical models of evaporation heat transfer in horizontal microfin tubes

A stratified flow model and an annular flow model of evaporation heat transfer in horizontal microfin tubes have been proposed. In the stratified flow model, the contributions of thin film evaporation and nucleate boiling in the groove above the stratified liquid level were predicted by a previously reported numerical analysis and a newly developed correlation, respectively. The contributions of nucleate boiling and forced convection in the stratified liquid region were predicted by the new correlation and the Carnavos correlation, respectively. In the annular flow model, the contributions of nucleate boiling and forced convection were predicted by the new correlation and the Carnavos correlation in which the equivalent Reynolds number was introduced, respectively. The flow pattern transition curve between the stratified-wavy flow and the annular flow proposed by Kattan et al. was introduced to predict the heat transfer coefficient in the intermediate region by use of the two theoretical models. The predictions of the heat transfer coefficient compared well with available experimental data for ten tubes and four refrigerants.     

Prediction of two-phase condensation in horizontal tubes using probabilistic flow regime maps

A flow regime based condensation model is developed for refrigerants in single, smooth, horizontal tubes utilizing a generalized probabilistic two-phase flow map. Flow map time fraction information is used to provide a physically based weighting of heat transfer models developed for different flow regimes. The developed model is compared with other models in the literature, with experimentally obtained condensation data of R134a in 8.92 mm diameter tubes, and with data found in the literature for 3.14 mm, 7.04 mm, and 9.58 mm tubes with R11, R12, R134a, R22, R410A, and R32/R125 (60/40% by weight) refrigerants and a wide range of mass fluxes and qualities.     

Investigation of flow boiling in horizontal tubes: Part II—Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes

The new version of the flow pattern map presented in Part I of this paper has been used to modify the dry angle in the heat transfer model of Kattan-Thome-Favrat [J. Heat Transfer, 120 (1) (1998) 156]. This significantly improves the heat transfer prediction in stratified-wavy flow. Moreover, a new heat transfer prediction method has been developed for the dryout and mist flow regimes, which extends the applicability of the heat transfer model to these flow regimes. An extensive flow boiling heat transfer database has been acquired for R-22 and R-410A to develop and validate the new heat transfer prediction methods. The new model also shows good agreement with the independent heat transfer data of Lallemand et al. [M. Lallemand, C. Branescu, P. Haberschill, Local heat transfer coefficients during boiling of R-22 and R-407C in horizontal smooth and microfin tubes, Int. J. Refrigeration, 24 (2001) 57-72].     

Properties of new refrigerants and predictions for condensation heat transfer enhancement with low-finned tubes

The present article provides a review of thermophysical properties research of environmentally acceptable refrigerants. In recent years, it has become evident that common CFC type refrigerants can cause serious environmental damage when released into the atmosphere. The group of HFCs (HFC-32, HFC-125, HFC-134a, HFC-152a) with no chlorine atom in their chemical formulae and quite short atmospheric lifetime may be promising alternatives to CFCs. A compilation of properties is given which may be used to obtain arithmetic expressions for the vapour pressure, saturated liquid density, viscosity, latent heat of vaporization, saturated liquid thermal conductivity, and surface tension of these hydrofluorocarbons. For easily available comparisons, the results are presented in diagrams illustrating the dependence of thermophysical data on temperature at the saturated state. Heat-transfer predictions are specified for condensation of the HFCs on horizontal integral-fin tubes with rectangular fins. Parameters investigated include fin and tube geometry, etc. This study should be a useful input in developing new condensers with new refrigerants.     

Two-phase flow of refrigerants during evaporation under constant heat flux in a horizontal tube

This paper presents the results of simulations using a two-phase separated flow model to study the heat transfer and flow characteristics of refrigerants during evaporation in a horizontal tube. A one-dimensional annular flow model of the evaporation of refrigerants under constant heat flux is developed. The basic physical equations governing flow are established from the conservation of mass, energy and momentum. The model is validated by comparing it with the experimental data reported in literature. The present model can be used to predict the variation of the temperature, heat transfer coefficient and pressure drop of various pure refrigerants flowing along a horizontal tube. It is found that the refrigerant temperature decreases along the tube corresponding to the decreasing of its saturation pressure. The liquid heat transfer coefficient increases with the axial length due to the reducing thickness of the liquid film. The evaporation rate of liquid refrigerant tends to decrease with increasing axial length, due to the decreasing latent heat transfer through the liquid–vapor interface. The developed model can be considered as an effective tool for evaporator design and can be used to choose appropriate refrigerants under designed conditions.     

New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes

A new flow boiling heat transfer model and a new flow pattern map based on the flow boiling heat transfer mechanisms for horizontal tubes have been developed specifically for CO₂. Firstly, a nucleate boiling heat transfer correlation incorporating the effects of reduced pressure and heat flux at low vapor qualities has been proposed for CO₂. Secondly, a nucleate boiling heat transfer suppression factor correlation incorporating liquid film thickness and tube diameters has been proposed based on the flow boiling heat transfer mechanisms so as to capture the trends in the flow boiling heat transfer data. In addition, a dryout inception correlation has been developed. Accordingly, the heat transfer correlation in the dryout region has been modified. In the new flow pattern map, an intermittent flow to annular flow transition criterion and an annular flow to dryout region transition criterion have been proposed based on the changes in the flow boiling heat transfer trends. The flow boiling heat transfer model predicts 75.5% of all the CO₂ database within ±30%. The flow boiling heat transfer model and the flow pattern map are applicable to a wide range of conditions: tube diameters (equivalent diameters for non-circular channels) from 0.8 to 10 mm, mass velocities from 170 to 570 kg/m² s, heat fluxes from 5 to 32 kW/m² and saturation temperatures from −28 to 25 °C (reduced pressures from 0.21 to 0.87).     

Prediction of evaporation heat transfer coefficient based on gas–liquid two-phase annular flow regime in horizontal microfin tubes

A physical model of gas–liquid two-phase annular flow regime is presented for predicting the enhanced evaporation heat transfer characteristics in horizontal microfin tubes. The model is based on the equivalence of a periodical distortion of the disturbance wave in the substrate layer. Corresponding to the stratified flow model proposed previously by authors, the dimensionless quantity Fr₀ = G/[gd_eρ_v(ρ_l ? ρ_v)]^0.5 may be used as a measure for determining the applicability of the present theoretical model, which was used to restrict the transition boundary between the stratified-wavy flow and the annular/intermittent flows. Comparison of the prediction of the circumferential average heat transfer coefficient with available experimental data for four tubes and three refrigerants reveals that a good agreement is obtained or the trend is better than that of the previously developed stratified flow model for Fr₀ > 4.0 as long as the partial dry out of tube does not occur. Obviously, the developed annular model is applicable and reliable for evaporation in horizontal microfin tubes under the case of high heat flux and high mass flux.     

Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map

Several important modifications to the flow pattern map of Kattan-Thome-Favrat [J. Heat Transfer 120(1) (1998) 140-147] made, resulting in a significantly new version of the map. Based on the dynamic void fraction measurements described in [Int. J. Multiphase Flow 30 (2004) 125-137], the stratified-wavy region has been subdivided into three subzones: slug, slug/stratified-wavy and stratified-wavy. Furthermore, annular-to-dryout and dryout-to-mist flow transition curves have been added and integrated into the new flow pattern map, identified by distinct trends of the heat transfer coefficient as a function of vapor quality and by flow pattern observations to determine (and then predict) the inception and completion of dryout in horizontal tubes.     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Influence of Tube's Diameter on Boiling Heat TransferPerformance in Small Diameter Tubes

INTaoDUCTIONReclitlydry-troeeVaPoratorofairconditioningmaChineandreffigeratorhavebeendevefoPinginthedirectionofusingsmalldiamtertube.TheHITACHICo.conductedaserlesofeVaPoratfonhe8ttransferexperimeats,usingthethinwallcoppertubesofinnerdiamter9.52nun,8mm,7mmand5nunre-spectivelytheworkingmediawasHCFC22(qualityx=o.6)I'].TheresulthasshoWnthatheeVaPo-rationheattransfercoefficielltsweresghcatlyincreasedwiththedecreasingoftubediameter.EVaThorationheattransfercoefficientofa5.onuninnerdiare…     

Influence of Tube‘s Diameter on Boling Heat Transfer Performance in Small Diameter Tubes

This paper reports the experiments of evaporation study in 6 mm inner copper diameter tubes using HFC-134a, HCFC-22 and CFC-12 as working fluid. The results show that the evaporation heat transfer coefficient increases with the decreasing of inner diameter of tubes. A new concept of non-dimensional tube diameter U is proposed in this paper for correction of the influence of the tube diameter on the evaporation heat transfer coefficient. And further, a convenient empirical correction method is presented.     

Energy efficiency analysis of a new ternary HFC alternative

In this paper, energy efficiency results are presented for a new HFC ternary blend proposed as a substitute for CFC 502 and HCFC 22. The blend is composed of R-23/R-32/R-125. Performance evaluation test results were obtained after an experimental heat pump set up with a 3 kW rotary compressor. The refrigerants tested in this study under different conditions were HCFC-22, as a reference base refrigerant and R-410a (HFC-32/HFC-125), R-407c (HFC-32/HFC-125/HFC-134a), as well as quaternary mixture; HFC-32/HFC-125/HFC-143a/HFC-134a. The experimental data showed that our proposed HFC ternary blend R-23/R-32/R-125 has superior performance compared to other proposed HFC alternatives such as R-410a and R-407c, under the same conditions. Pressure ratios and head pressures were compatible with new compressors to be used in new systems. Furthermore, experimental results demonstrated that the ternary blend R-23/R-32/R-125 is the best performing replacement for R-22 in heat pump applications and low temperature equipment. Experiments also showed that the heat pump system using R-23/R-32/R-125 was stable and experienced reasonable head pressures. © 1997 John Wiley & Sons, Ltd.     

Thermo-hydraulic predictions for multi-pass evaporators by orthogonal collocation using a new flow pattern map

A recently published paper by this author [S. Thyageswaran, Analysis of multi-pass evaporators using orthogonal collocation, Int. J. Refrigeration doi:10.1016/j.ijrefrig.2007.06.011 (in press)], shows that orthogonal collocation is an effective alternative to traditional integration for the thermal analysis of multi-pass evaporators. The steady rate of heat exchanged (Q) and overall pressure drop (Δp), for an R-22 based chiller having one shell and eight tube passes, were predicted using the Kattan–Thome–Favrat and the Müller-Steinhagen and Heck models for the boiling R-22. While Q was over-predicted by 0.95%, Δp was over-predicted by 20.3%. In the present work, results have been obtained using state-of-the-art, unified heat transfer and pressure drop sub-models based upon an improved flow pattern map by Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes, part 1: a new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969; L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes, part 2: development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985], and Moreno Quibén and Thome [J.M. Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes, part 1: diabatic and adiabatic experimental study, Int. J. Heat Fluid Flow 28 (5) (2007) 1049–1059; J.M. Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes, part 2: new phenomenological model, Int. J. Heat and Fluid Flow 28 (5) (2007) 1060–1072]. The new predictions for Q and Δp are 141.76 kW and 13.3 kPa, respectively, compared to their rated values of 140.67 kW and 13.789 kPa.     

New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns

Corresponding to the updated flow pattern map presented in Part I of this study, an updated general flow pattern based flow boiling heat transfer model was developed for CO₂ using the Cheng–Ribatski–Wojtan–Thome [L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside horizontal tubes, Int. J. Heat Mass Transfer 49 (2006) 4082–4094; L. Cheng, G. Ribatski, L. Wojtan, J.R. Thome, Erratum to: “New flow boiling heat transfer model and flow pattern map for carbon dioxide evaporating inside tubes” [Heat Mass Transfer 49 (21–22) (2006) 4082–4094], Int. J. Heat Mass Transfer 50 (2007) 391] flow boiling heat transfer model as the starting basis. The flow boiling heat transfer correlation in the dryout region was updated. In addition, a new mist flow heat transfer correlation for CO₂ was developed based on the CO₂ data and a heat transfer method for bubbly flow was proposed for completeness sake. The updated general flow boiling heat transfer model for CO₂ covers all flow regimes and is applicable to a wider range of conditions for horizontal tubes: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m² s, heat fluxes from 1.8 to 46 kW/m² and saturation temperatures from ?28 to 25 °C (reduced pressures from 0.21 to 0.87). The updated general flow boiling heat transfer model was compared to a new experimental database which contains 1124 data points (790 more than that in the previous model [Cheng et al., 2006, 2007]) in this study. Good agreement between the predicted and experimental data was found in general with 71.4% of the entire database and 83.2% of the database without the dryout and mist flow data predicted within ±30%. However, the predictions for the dryout and mist flow regions were less satisfactory due to the limited number of data points, the higher inaccuracy in such data, scatter in some data sets ranging up to 40%, significant discrepancies from one experimental study to another and the difficulties associated with predicting the inception and completion of dryout around the perimeter of the horizontal tubes.     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

Flow boiling in horizontal flattened tubes: Part II – Flow boiling heat transfer results and model

Experiments of flow boiling heat transfer were conducted in four horizontal flattened smooth copper tubes of two different heights of 2 and 3 mm. The equivalent diameters of the flattened tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids were R22 and R410A. The test conditions were: mass velocities from 150 to 500 kg/m² s, heat fluxes from 6 to 40 kW/m² and saturation temperature of 5 °C. The experimental heat transfer results are presented and the effects of mass flux, heat flux, and tube diameter on heat transfer are analyzed. Furthermore, the flow pattern based flow boiling heat transfer model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part I – A new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969; L. Wojtan, T. Ursenbacker, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II – Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985], using the equivalent diameters, were compared to the experimental data. The model predicts 71% of the entire database of R22 and R410A ±30% overall. The model predicts well the flattened tube heat transfer coefficients for R22 while it does not predicts well those for R410A. Based on several physical considerations, a modified flow boiling heat transfer model was proposed for the flattened tubes on the basis of the Wojtan et al. model and it predicts the flattened tube heat transfer database of R22 and R410A by 85.8% within ±30%. The modified model is applied to the reduced pressures up to 0.19.     

水平管降膜蒸发器中不同形式的布液器对内部流场流动影响的数值模拟

应用Fluent两相流VOF模型,数值模拟水平管降膜蒸发器内部流场流动的过程。提出3种不同结构的布液器,分别对蒸发器内部流动特性进行了比较分析,得到气液两相图、速度云图、XY图等。模拟结果表明:中间开孔布液器使两部分蒸发管阵产生的蒸汽通过中间蒸汽排气口疏导出去,换热面大,使液体分配较均匀,更为合理,有利于蒸发器内部整体流场的优化设计。研究结果可为降膜蒸发器的优化设计提供理论依据。     

A two-phase flow pattern map for annular channels under a DC applied voltage and the application to electrohydrodynamic convective boiling analysis

An experimental study of tube side boiling heat transfer of HFC-134a has been conducted in a single-pass, counter-current flow heat exchanger under an electric field. By applying 0–8 kV to a concentric inner electrode, the mechanics of EHD induced flow and heat transfer augmentation/suppression have been investigated for flow conditions with inlet qualities of 0% to 60%, mass fluxes from 100 kg/m² s to 500 kg/m² s, and heat flux levels between 10 kW/m² and 20 kW/m². A theoretical Steiner type two-phase flow pattern map for flow boiling in the annular channel under applied DC high voltage is also developed. The flow regimes encountered in the convective boiling process have been reconstructed experimentally and compared with the proposed EHD flow regime map. The results show that when the proposed dimensionless criterion M_d Re² is satisfied, EHD interfacial forces have a strong influence on the flow pattern which is considered to be the primary mechanism affecting the increase in pressure drop and the augmentation or even suppression of heat transfer.