Film Reynolds Number Research Articles

Numerical simulation of flooding characteristics in steam-water countercurrent inside an inclined pipe under condensation conditions

In this paper, a three-dimensional CFD model, which is established on VOF (Volume of Fluid) model, Lee model together with CSF (Continuum Surface Force) model and RNG k − ɛ turbulence model, is adopted for the condensation heat and mass transfer of steam in an inclined circular tube with the occurrence of flooding and liquid bridge. The entire process of liquid bridge formation is depicted with a three-dimensional space-time plot of the wave amplitude of the interfacial wave with time and axial distance. The effects of different pipe inclination angles and different contact angles on the flow behavior and heat transfer characteristics are investigated and the results demonstrate that under high liquid film Reynolds number and condensation conditions, as the inclination angle increases, the liquid bridge begins to form at a position which moves upward along the pipe. The reason is that as the inclination angle increases, the liquid film is more uniformly distributed in the radial direction, and a vortex symmetrical about the axis is formed on the leeward side of the flooding wave.

Experimental and Numerical Study of Hydrodynamic and Heat Transfer Characteristics of Falling Film Over Metal Foam Layered Horizontal Tube

AbstractA novel nonintrusive technique based on an air-coupled ultrasonic transducer was used to study the hydrodynamic behavior of falling film over metal foam layered horizontal tube. Copper foam having a porosity of 90.5%, brazed over a copper tube of 25.4 mm diameter was used in this study. Falling film thickness distribution in the circumferential direction and the dynamic characteristics of falling film were studied in the falling film Reynolds number range of 356–715, and at a tube spacing of 5 mm and 15 mm. The falling film characteristics over metal foam layered horizontal tubes were compared with that over a plain horizontal tube surface. Heat transfer studies of falling film over metal foam layered tube were studied in an evaporator of a multi-effect desalination system by experiment. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and were found to be in good agreement. Metal foam properties required for the computational model were obtained using a microcomputed tomography-based study.

An experimental study on the heat transfer performance characteristics of horizontal tube falling film absorbers for single-stage absorption heat transformer

The absorber is one of the most important components in the LiBr-water absorption heat transformer (AHT) because it has the largest heat transfer area, and its performance significantly influences the overall system efficiency. Nevertheless, studies on absorbers used at the high-pressure level have not progressed. In this study, the horizontal tube falling film absorbers were constructed by introducing three types of heat transfer tubes (copper bare, copper-nickel bare, copper-nickel floral). The combined effects of the tube structure and material on the heat transfer performance of the absorber were experimentally investigated under the operating conditions of the single-stage AHTs. The experimental results showed that the film Reynolds number of 50 was optimal for the absorbers used in this study. At that point, the overall heat transfer coefficients of the copper bare tube, the copper-nickel bare tube, and the copper-nickel floral tube were 1,484, 1,448, and 1,299 W/m2 K, respectively. In addition, the heat transfer performance of the bare-type tubes was superior to that of the floral tube, except the under the low film Reynolds number. The empirical correlations of the Nusselt number for three types of heat transfer tubes were derived based on the experimental results and were predicted the data within the standard errors of 3.79–5.57 %.

Precise determination of R134a boiling bundle effect on a column of reentrant cavity tubes

The boiling bundle effect was investigated on a column of reentrant cavity tubes to reveal the heat transfer coefficient (HTC) variation for a tube which is affected by other tubes. All experimental data were obtained on the specific tube, which is the distinct difference between the adopted homologous method and the routine method. Two columns of eight horizontal tubes were submerged in R134a liquid, and each tube was specifically heated by an ethylene glycol–water mixture. The saturation temperature was 6.0 °C, the heat flux ranged from 10.0 kW/m2 to 28.0 kW/m2, and the row number ranged from 1 to 7. The tube length in the test section was 500 mm, and the pitch–diameter ratio was 1.32. Results showed that the uncertainty of the bundle effect using the homologous method was less than ± 4.4%, which decreased by at least 80% compared with that using the routine method. The values were from 1 to 1.32, indicating that the upward bubbles from the lower heated tubes positively influenced the boiling HTC. Moreover, the bundle effect increased with the increase in row number and decreased with the increase in heat flux. Correlations of the single-tube HTC and the bundle effect with vapor film Reynolds number and effective row number were established to predict the experimental data. The current study can offer a reference for further investigations of film or convective boiling on tube bundles.

Prediction of heat transfer in evaporation of saturated falling films on bundles of horizontal tubes

Falling film evaporators consisting of bundles of horizonal tubes are widely used in many applications including refrigeration, chemical processing, desalination, etc. While many experimental studies have been done, as yet there is no well-verified method for the prediction of heat transfer. The research reported here was done to fulfill this need. A correlation is presented for heat transfer before occurrence of CHF which has been verified with a wide range of data from 12 sources. The data include plain and enhanced tubes of many types and materials, diameters 15.9 to 25.4 mm, P/D 1.1 to 2.0, liquid film Reynolds number from 43 to 6449, and heat flux from 1 to 94 kWm −2 . Included are eight diverse fluids (water, halocarbon refrigerants) with reduced pressures from 0.000046 to 0.172. The correlation predicts 2708 data points with mean absolute deviation of 17.4%. The correlation and its verification are presented and recommendations are made for its use.

A general correlation for heat transfer during evaporation of falling films on single horizontal plain tubes

Falling film evaporators are widely used in many industries including desalination, petrochemical, and refrigeration. Hence ability to correctly predict heat transfer in these evaporators is important. While many prediction methods have been proposed, none has been validated with data from many sources covering a wide range of parameters. Hence there is a need for a well-verified general correlation. This research was done to fulfill this need. A correlation has been developed for heat transfer coefficient of single horizontal tubes with falling film evaporation. It is shown to be in good agreement with data from 22 sources that cover a very wide range of parameters. Included in the data are 11 fluids (water, ammonia, halocarbon refrigerants, hydrocarbons), tube diameters from 12.7 mm to 50.8 mm, heat flux from 1 to 208 kWm−2, and liquid film Reynolds number 19 to 10,734. A total of 1237 data points are predicted with a mean absolute deviation of 17.4%. The new correlation is presented together with details of its verification.

DEVELOPED LIQUID FILMS FALLING AROUND TAYLOR BUBBLES INSIDE VERTICAL STAGNANT COLUMNS

The present work reports an experimental study of developed liquid films falling around single Taylor bubbles inside vertical tubes containing stagnant liquids. Experiments were carried out in acrylic tubes with 2.0 m length and inner diameters of 0.019, 0.024 and 0.034 m. Five water-glycerin mixtures were used, corresponding to film Reynolds number（Ref）ranging from 2 to 7650. A pulse-echo ultrasonic technique was applied to measure the rise velocity of the bubble and the equilibrium thickness of the liquid film. These parameters together with the calculated standard deviation of the equilibrium film thickness provided information about the development of waves on the gas-liquid interfaces, which could be related with the laminar-turbulent transition of liquid films falling around Taylor bubbles. The results indicated that the wave amplitudes increased sharply for Ref> 1000. This value of Ref is in agreement with literature concerning the laminar-turbulent transition for free falling films on vertical surfaces.

Numerical Study of Liquid Film Formation around Tubes of Horizontal Falling Film Evaporator

Falling film evaporative heat exchangers are extensively used in processing industries; broad areas of application being refrigeration, desalination and food processing industries. The fundamental aspect of this type of heat transfer process is to extract the process heat in the form of latent heat by liquid which is sprayed over the surface of the process tubes. Formation of liquid film over a fully wetted horizontal round tube of falling film evaporator has been numerically simulated here. Two numerical approaches, Volume of Fluid (VOF) technique and the Eulerian multiphase model are applied to compare their results. The effect of varying flow and geometrical parameters on the film thickness is investigated. Two horizontal tubes of diameter 19.05mm and 25.04mm with three different uniform spacing have been selected for simulation. Film Reynolds numbers 650, 950 and 1250 are considered for the above set of parameters. Ii is observed that the geometrical and flow parameters considerably influence the film thickness. Transient analysis of the film formation has been carried out and parameters like pathline of liquid film and the velocity profile have been obtained for understanding the flow behavior in a better manner. All the simulated results agree well with the published data.

흡수식 히트트랜스포머용 고온 흡수기의 열전달 성능 특성에 관한 실험적 연구

The high-temperature absorber is one of the largest components in the absorption heat transformer. It has a decisive effect on the overall system efficiency. Nevertheless, studies analyzing the efficiency of high-temperature absorber are incomplete and scarce. In this study, we designed an experimental apparatus similar to the conventional single-stage absorption heat transformer. The high-temperature absorber was used as the horizontal falling film with the copper floral tubes. The comprehensive heat transfer performance of the high-temperature absorber was analyzed based on its inlet condition parameters such as the film Reynolds number, strong solution concentration, hot water velocity, hot water temperature, and temperature of evaporation. The results indicate that the heat transfer performance of the high-temperature absorber was rapidly decreased after the film Reynolds number exceeded 50. Accordingly, we decided to use the film Reynolds number of 50 as an inflection point to determine the heat transfer performance in our experiment. At this point, the heat transfer rate and overall heat transfer coefficient were 4.42 ㎾ and 1,367 W/㎡K, respectively.

Hydrodynamic behavior of liquid falling film over horizontal tubes: Effect of hydrophilic circular surface on liquid film thickness and heat transfer

Falling film configurations have widely been used in various types of refrigeration systems, and wettability has played an important role in characterizing the heat and mass transfer due to varying hydrodynamic behavior. The present paper focuses on a three-dimensional computational fluid dynamics (CFD) simulation to investigate the effect of hydrophilic circular surface on hydrodynamic behavior of liquid falling film over the heated horizontal circular tubes. First, the thickness of the liquid falling film and the liquid distribution around the circumferential and axial directions were analyzed at a feed water temperature of 20 °C for three film Reynolds numbers of 12, 24, and 36 (50 ≤ Q̇feed ≤ 150 ml·min-1) and three heat fluxes of 5, 10, and 15 kW m-2. The influences of the contact angle on the liquid film hydrodynamics and heat transfer were then analyzed. The results showed that at a low feed film Reynolds number, even for super-hydrophilic nature (e.g., contact angles of 0° or 10°), the liquid film failed to cover the first-row tube surface. Increasing the feed film Reynolds number enhanced the liquid film distribution over the tubes and the dry spot decreased, which enhanced the heat transfer performance. At relatively higher contact angles of 30° and 60° (but still hydrophilic nature), the wetted area decreased and dry spots covered most of the second- and third-row tubes, which significantly influenced the thermal performance. Validation of the CFD model based on the volume-of-fluid approach was achieved by comparison with the experimental data, and the experimentally measured values of local heat transfer coefficient were employed for the comparison to support the validity of the present study.

Using Thermodynamic Availability to Predict the Transitional Film Reynolds Number between the Droplet and Jet Modes in Falling Liquid between Horizontal Tubes

The objective of this paper is to propose and validate a model for predicting the transitional film Reynolds number between the droplet and jet modes of falling film flows. The proposed model relies on thermodynamic availability with no fitting to mode transition data. The model directly predicts the numerical values of the transitional film Reynolds numbers at different operating conditions. The results of the model are validated against 43 experimental data points from the literature, which cover 10 working fluids and a wide range of tube geometries. The accuracy of the proposed model in predicting the validation data is -on average- more than 20 % better than that of the widely used Re=a(Ga)b and Re=a(Ga)b(S/ξ)c models. The model also achieves low computation time (less than 30 seconds for the 43 validation data points). The model presented in this paper has the potential of integration with heat transfer, mass transfer, and pressure drop models for optimizing the design and operation of falling film heat and mass exchangers.

Falling film boiling of refrigerants over nanostructured and roughened tubes: Heat transfer, dryout and critical heat flux

Falling film evaporators offer an attractive alternative to flooded evaporators as the lower fluid charge reduces the impact of leaks to the environment and associated safety concerns. A study was conducted of saturated falling film boiling of two refrigerants on one polished, one roughened and three nanostructured copper tubes in order to evaluate the potential of nanostructures in falling film refrigerant evaporators. Tubes were individually tested, placed horizontally within a test chamber and heated by an internal water flow with refrigerant distributed over the outside of the tubes. Wilson plots were used to characterise the internal water heat transfer coefficients (HTCs). A layer-by-layer (LbL) process was used to create the first nanostructured tube by coating the outside of a tube with silica nanoparticles. A chemical bath was used to create copper oxide (CuO) protrusions on the second nanostructured tube. The third tube was coated by following a commercial process referred to as nanoFLUX. R-245fa at a saturation temperature of 20 °C and R-134a at saturation temperatures of 5 °C and 25 °C were used as refrigerants. Tests were conducted over a range of heat fluxes from 20 to 100 kW/m and refrigerant mass film flow rates per unit length from 0 to 0.13 kg/m/s, which corresponds to a film Reynolds number range of 0 to approximately 1500 to 2500, depending on the refrigerant. Heat fluxes were increased further to test whether the critical heat flux (CHF) point due to a departure from nucleate boiling (DNB) could be reached. The CuO and nanoFLUX tubes had the lowest film Reynolds numbers at which critical dryout occurred at heat fluxes near 20 kW/m2, but as the heat fluxes were increased towards 100 kW/m2, critical dryout occurred at the highest film Reynolds numbers of the tubes tested. Furthermore, in some higher heat flux cases, CHF as a result of DNB for the CuO and nanoFLUX tubes was reached before critical dryout occurred, and DNB became the limiting operational factor. The refrigerant condition that had the worst dryout performance in terms of film Reynolds number was R-134a at 25 °C, followed by R-134a at 5 °C and R245fa at 20 °C. Tests across the heat flux range and refrigerant conditions revealed that compared to the polished tube, the roughened tube had HTCs between 60 and 100% higher, the LbL tube had HTCs between 20% lower and 20% higher, the CuO tube had HTCs between 20% lower and 80% higher and the nanoFLUX tube had HTCs between 40 and 200% higher than the polished tube. The falling film enhancement ratios for the plain and nanostructured tubes were found to be of a similar order of magnitude, typically between 1.3 and 0.8.

Analysis of the Flow Characteristics of the Liquid Film on the Wall Surface of the Corrugated Plate Dryer Based on PSD Method: A Short Communication

Corrugated plate dryer is an important steam-water separation equipment in steam generator of secondary loop of nuclear power plant. Its steam-water separation efficiency has a certain influence on economic index of nuclear power plants. It is meaningful to understand flow characteristics of liquid film on corrugated plate wall. Besides, the fluctuation of the liquid film may cause the occurrence of liquid film rupture, thus the research on the fluctuation of the liquid film on the wall of the corrugated plate is very important. This paper uses PLIF (Plane laser induced fluorescence) method to measure liquid film thickness. Curve of film thickness with time is obtained. Calculation formula of average value of liquid film thickness is given. Based on power spectral density (PSD) method, nonlinear characteristics of liquid film thickness are studied. Results show that the relationship between film thickness and Reynolds number conforms to Nusselt theory. Nevertheless, the results show deviation from Nusselt as well as Karapantsios estimation. When Reynolds number is in the range of 600 to 3000 and surface roughness is 0.03, the fluctuation of the power spectrum of the liquid film thickness caused by gravity can be ignored, which provides a reference for study on steam-water separation.

Using Thermodynamic Availability to Predict the Transitional Film Reynolds Number between the Jet and Sheet Modes in Falling Liquid between Horizontal Tubes

The purpose of this paper is to develop and validate a theoretical framework -based on thermodynamic availability- that directly predicts the transitional film Reynolds numbers between the jet and sheet modes of falling films on horizontal tubes. The prediction of the prevailing flow mode is important for the design and operation of falling film heat and mass exchangers. The proposed model accounts for fluid properties as well as tube geometry (diameter and spacing). The transitional film Reynolds numbers calculated using the proposed model were validated against 52 experimental measurements from the literature. More than 73 % of the results from the proposed model are within ± 25 % from their respective experimental values. Moreover, the proposed model has a mean absolute percentage error of 20.4 % which is lower than the 30.4% obtained by the widely-used empirical model (Re=a(Ga)b), for the same validation data. The proposed model offers a convenient tool for predicting the mode transitions, provides insight into the role of availability in the mode transitions, and has the potential of integration with other models (e.g. mode-based heat transfer models, evaporator models, and chiller or desalination system models).

A critical review of analytical and numerical models of condensation in microchannels

Understanding condensation in microchannels is critical for designing optimized and compact microchannel condensers, which have the potential to increase system level efficiency and reduce system size. Analytical and numerical models can yield insights into local condensation phenomena. This study provides a review of models of condensation in microchannels. These models are categorized based on the applicable, typically annular and intermittent, flow regimes. Annular-flow-based models are further subdivided into analytical models with closed-form solutions, analytical models with numerical solutions, reduced-order CFD models, and full-fledged CFD models. Intermittent-flow-based models are subdivided into single-bubble, repeated-unit-cell, and overall-bubble-train models. The characteristics, use, and range of applicability of each of these models is highlighted. A criterion for the film Reynolds number is proposed for internal forced convective condensation in microchannels, to assess the validity of the modeling assumptions. Annular flow models using the laminar film assumption appear to be valid for Re δ0 < ~ 360, beyond which turbulent effects must be modeled. Intermittent flow models using the laminar film assumption, in turn, appear to be valid for Re δ0 < ~ 130.

Flow behavior of liquid falling film on a horizontal corrugated tube

The intertube flow pattern presents droplet, droplet-column, column, column-sheet, and sheet with the increase of flow rate on the falling film flow process outside a horizontal corrugated tube. The film Reynolds number of each flow pattern transition of a horizontal corrugated tube is less than that of a horizontal smooth tube. The corrugated tube surface is more favorable to axial spreading of the liquid film compared with the smooth tube. The droplet or column departure-site spacing is smaller than in a smooth tube. As the intertube spacing of the horizontal corrugated tubes increases, the film Reynolds number of each flow pattern transition and the droplet or column departure-site spacing are increased. Based on the experimental data, a new correlation has been suggested to predict the column departure-site spacing of a horizontal corrugated tube.

Falling-film evaporation over horizontal rectangular tubes: Part I—Experimental results

Abstract An experimental study on falling-film evaporation on horizontal rectangular tubes, the external geometry of extruded flat, microchannel tubes, was conducted. Previous falling-film evaporation studies have focused on horizontal and vertical round tubes, and the usage of microchannel tubes provides a promising new heat exchanger design option. The test sections used in this study have external dimensions of 203 × 1.42 × 27.4 mm (length × width × height) and were tested with water as the refrigerant. Heat transfer coefficients were measured for saturation temperatures from 10 to 30 °C, test section spacings from 5 to 15 mm, heat fluxes from 10 to 20 kW m−2, and film Reynolds numbers of 48 to 544. This paper, Part I, reports the experimental methodology and the results, while in the companion paper, Part II, models for predicting heat transfer are presented.

Experimental study of the falling film evaporation coefficients of R290 in a horizontal enhanced tube array

In this study, the falling film evaporation heat transfers of R290 on an array composed of five enhancement horizontal tubes (groove tubes) are studied. The tests are performed at constant saturation temperatures 5.5 °C with change of heat flux from 10 to 40 kW/m2. The film Reynolds number ranges from 200 to 2200 and the film flow rate of refrigerant is between 100 and 660 kg/h.The results show that the film flow rate and heat flux have significant effects on R290 falling film evaporation heat transfer coefficients (HTCs) of the tubes in the tube array. With decreasing the film flow rate on the five tubes the tube HTCs display two stages, a plateau stage and a sharp drop stage. The heat transfer coefficients firstly keep more or less constant at the plateau stage and then decreasing rapidly. At the same nominal film Reynolds number the tube averaged heat transfer coefficients of tube No. 1 to tube No. 5 decrease in order of the increasing tube number from top to bottom of the array. The falling film evaporation HTC of R290 on single enhanced tube is about 4.5 times of single smooth tube HTC, and the HTCs of the enhanced tube array is higher than single smooth tube by more than 2.5 times. In addition, the R290 HTCs of the tube array are higher than those of R134a for the same tube array in the plateau region by about 25%. It is found that at high heat flux of 30–40 kW/m2, the heat transfer coefficient variation with film Reynolds number of the lower enhanced tubes in the tube array exhibits severe undulating characteristics.

Experimental and numerical study on filmwise condensation of pure propane and propane/methane mixture

The condensation process of pure propane and propane/methane mixture on the vertical plate were experimentally and numerically researched to clarify the condensation characteristics at the first stage of natural gas liquification. In this work, a visualization experimental system was designed to study the flow pattern and heat transfer characteristics of propane vapor condensation on the vertical plate. The condensate film on the plate is wave laminar flow on the experimental conditions of liquid film Reynolds number from 59 to 321. With the increase of the liquid film Reynolds number, the flow patterns turn from straight isolated waves (about 8-10 mm length) to dense and short waves, which enhances the condensation heat transfer coefficient. At the same time, a 2-D numerical model was proposed to reveal the condensation mechanism of propane and propane/methane mixture with different wall subcooled temperature (5-40 K) and methane mole fraction (80%-95%). The numerical results agree well with the experimental data. And the results show that 80% methane reduces the liquid film thickness approximately by half comparing with the pure propane. The increase of methane mole fraction could reduce the liquid film thickness, but its contribution to the heat transfer is negligible compared with the mass diffusion resistance caused by the gas boundary layer. The heat transfer coefficient decreases beyond ninety percent compared with pure propane at the methane mole fraction of 80–95%.

Surface microstructures for boiling and evaporation enhancement in falling films of low-viscosity fluids

The effect of surface microstructures created by the precision method of processing plastic materials (method of deformational cutting) on heat transfer was studied on falling films of refrigerant mixture R114/R21. The binary mixture film flowed along the outer surface of the vertical heated cylinders in the laminar-wave regime within the range of the film Reynolds numbers from 300 to 1400. The heat flux density varied from zero to critical values. It has been shown that surfaces with semi-closed pores have an advantage in heat transfer enhancement at boiling (up to four times compared to a smooth surface), while micro-pin structures contribute to an increase at evaporation regime (up to two times compared to a smooth surface). Both types of microstructures demonstrate an increase in the values of critical heat flux in comparison with a reference smooth surface (more than two times) and significantly exceed the values obtained for some of modern commercially available surfaces of “GEWA” and “Turbo” series. The parameters of surface microstructures, most effective for heat transfer enhancement in various regimes, have been identified. The experimental results were compared with the available correlations for calculating the heat transfer coefficient and the critical heat flux.