Film Reynolds Number Research Articles

A numerical and experimental study of breakdown of falling film on horizontal circular tubes

Falling film horizontal tube heat exchangers are used in several industrial applications such as sea water desalination, multi effect distillation, chemical industries, ocean thermal energy conversion systems and refrigeration. Different flow patterns that occur during falling film evaporation on an array of horizontal tubes are droplet, droplet-columnar, columnar, columnar-sheet and sheet modes. From the heat transfer point of view, a sheet mode (continuous falling film spanning the intertube space) is the most desirable falling-film mode. Falling film breakdown leads to the occurrence of dry patches on tubes which cause heat transfer deterioration. In the present study, falling film breakdown on horizontal tubes under adiabatic condition has been investigated both numerically and experimentally. The study focuses on the influence of tube pitch to diameter ratio on the threshold film Reynolds number required to maintain continuous falling film. Results indicate that the threshold film Reynolds number increases with the increase in the pitch to diameter ratio for a given diameter of the tube. The study also shows that as the number of tubes in the column increases, the threshold film Reynolds number required to maintain continuous film over all the tubes increases. The study highlights the need for correlations for the column-sheet mode transition that take into account the effect of number of horizontal tubes in a vertical array.

Heat transfer and irrigating liquid hydrodynamics on one-row vertical bundle of inclined tubes in falling film heat exchanger

Spiral-wound heat exchangers widely used currently in various industries provide efficient heat transfer between media through spiral tube winds at evaporation and boiling of falling liquid film. The investigated bundle in the form of one-row vertical bundle of inclined tubes (5 degrees to the horizon) simulates a part of a spiral-wound heat exchanger. Heat transfer and hydrodynamics processes are studied at falling film irrigation by liquid R21 refrigerant at 3 bar of absolute pressure with water as heat carrier. The bundle is placed inside of rectangular heat exchanger with optical windows. Heat transfer measurements are performed for tubes at different positions on the bundle height. Irrigation densities corresponded to falling film Reynolds numbers 300 – 1100, heat flux values within the range of 5000 – 10000 W/m2.

On falling film evaporator – A review of mechanisms and critical assessment of correlation on a horizontal tube bundle with updated development

Optimization of the energy efficiency and reduction of the greenhouse gas emission for the heat pump system is imperative to meet the net-zero goals at 2050. Chillers with falling film evaporator design not only possess better system performance but also contain less refrigerant inventory. Hence, accurate prediction of the evaporator performance is pivotal especially when charged with low-GWP refrigerant. The present study reviews the correlations for falling film evaporator with a horizontal tube bundle configuration. The major efforts of this study include four tasks: (a) literature review of the experimental studies and available empirical correlations; (b) comprehensive discussion of the falling film evaporation heat transfer mechanism; (c) development of a new rationally based correlation based on available literature; and (d) comparison of different correlations based on the existing data. The collected data includes 4114 data points from 8 sources, 6 refrigerants (R-600a, R-290, R-245fa, R-134a, R-1234ze(E), R-123), 5 types of the tubes (Plain, Turbo-GII-HP, GEWA-B, Low-fin, High-Flux), liquid Weber number from 2.2 × 10−6 to 0.7, imposed vapor Weber number from 0 to 37.2, heat flux from 2.5 to 151.5 kW/m2, and film Reynolds number from 1 to 3159.8. The new correlation gives a MAD of 28.3%, and an R2 of 0.86. Yet, the developed correlation considers various heat transfer mechanisms, including the transition point from falling film evaporation to the nucleate boiling, local evaporation, dry-out, mist flow, imposed flow, and enhanced tube effects.

Experimental investigation on heat transfer of nitrogen flowing in a circular tube

Average and local convective heat transfer coefficients of nitrogen are measured experimentally in an electrically heated circular tube for a range of Reynolds number from 1.08 × 104 to 3.60 × 104, and wall-to-bulk temperature ratio from 1.01 to 1.77. The exit Mach number is up to 0.17, and the heat flux is up to 46 kW·m−2. The molybdenum test section has a 62 diameters heated section with an inside diameter of 5 mm and a 30 diameters entrance section to ensure the fully-developed flow. Uncertainty of Nusselt number is less than 1.6 % in this study. The results indicate that the average heat transfer correlations evaluated by both the bulk and the modified film Reynolds numbers agree well with the experimental data. The local heat transfer results based on bulk properties are compared with previous empirical correlations. New prediction correlations are recommended which are significantly affected by the property variation and heated length. The comparison between the proposed correlations and experimental points shows that 88 % of experimental data fall into an error of 10 %, and almost all data are within an error of 20 %.

Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures

Multiscale surface structures offer the opportunity to combine the heat transfer enhancement provided by microscale structures with the dryout benefits provided by some nanostructures, which is particularly attractive for falling film evaporators, who have to prevent dryout to ensure high heat transfer coefficients can be maintained.In this study commercially produced plain, low-finned and 3D enhanced commercially produced tubes were tested uncoated as well as coated with fibrous nanostructures that induce wicking on the surface to investigate the opportunity that multiscale enhancements might offer falling film boiling evaporators. Single tubes were heated internally by water and tested in the horizontal position with refrigerant R-134a drizzled over the outside of the tube surface at saturation temperatures of 5 and 25 °C, heat fluxes from 20 to 100 kW/m2 and film Reynolds numbers of up to 2000. The tubes tested were a roughened plain tube, a low-finned Gewa-KS tube and two different 3D enhanced tubes, an Gewa-B5 and an Turbo EHPII, that have networks of re-entrant cavities.The uncoated 3D enhanced tubes achieved the highest heat transfer coefficients, outperforming a roughened plain tube by up to 500 %. The uncoated low-finned tube had heat transfer coefficients that were up to 140 % higher than the roughened plain tube, despite having only an 80 % larger surface area, thought to be due to bubbles sliding down the channels between the fins, increasing the surface area of the bubbles in contact with the surface and thus allowing for increased microlayer evaporation.The application of the copper oxide coating benefitted the low-finned tube, with heat transfer enhancement of up to 60 %, possibly due to liquid wicked underneath the sliding bubbles and onto the fin tips. However, the nanocoating largely decreased the heat transfer of the roughened plain tube and the 3D enhanced tubes, thought to be due flooding of nucleation sites on the roughened tube and interference with the hydraulics of the re-entrant cavity network on the surface of the 3D enhanced surfaces.The low-finned tube also benefitted the most from the falling film boiling heat transfer mode compared to previous pool boiling results, with heat transfer increased by up to 80 and 460 % for the uncoated and coated low-finned tube respectively, again likely due to the bubbles sliding in the fin channels under falling film conditions.The Gewa-B5 tubes had the best dryout performance, followed by the Turbo EHPII tubes and then the low-finned Gewa-KS tube. The open pore structure of the Gewa-B5 likely aided it in dryout prevention, while the fins of the Gewa-KS are thought to have prevented lateral fluid distribution and thus worsen dryout.The dryout performance of the tubes we not markedly improved by the nanocoating, highlighting the difficulty in improving wettability on high wetting low surface tensions fluids such as refrigerants. Microscale enhancements were thus shown to have significant influence on dryout performance, while nanoscale enhancements had little effect in this study.

Intercolumn two-phase flow patterns across falling film tube bundles

Two-phase flow patterns are critical in falling film heat devices. Hydrodynamic characteristics of turbulent falling films across a horizontal triangular tube bundle were studied under a range of film Reynolds numbers and contact angles. A flow pattern map based on the k-means clustering approach was proposed after the intercolumn two-phase flow patterns were grouped. The results demonstrated that that the intercolumn liquid-vapor flow patterns in a horizontal tube bundle could be effectively and reasonably grouped using the k-means clustering approach according to the results of the void fraction and interface area. There are four intercolumn liquid-vapor flow patterns that were identified: bubbles flow, annular-slug-column flow, half-annular flow, and multi-bridges flow. The annular-slug-column flow pattern and the multi-bridges flow pattern are next to one another in the flow pattern map and are capable of transitioning one into the other. It is only through the two transitional flow patterns that the half-annular-column flow pattern and bubbles flow pattern can turn to one another. In the bubbles' flow pattern and half-annular-column flow pattern, respectively, the highest and least averaged heat transfer coefficients are attained. Simply raising the film Reynolds number or lowering the contact angle will not produce the best heat transfer results.

Modeling and simulation of a helical falling film evaporator to improve its performance and design

This work presents a computational model of a helical falling film evaporator of 13 tubes based on physical principles. The momentum and energy transfer of the system are modeled. The work aims to understand the behavior of a horizontal falling film evaporator and to improve the evaporator design by analyzing the heat transfer. In the evaporator, the wetting efficiency varies in each tube because the evaporation of the falling film is performed gradually. Therefore, the behavior of the evaporator variables, such as the wetting efficiency in each tube, is studied. Furthermore, the model granularity based on the analysis of the subcooling and saturation zones is investigated. The effect of the film breakdown onset Reynolds number and the mass flow rate on the evaporator performance is studied. This analysis shows that the maximum heat transfer rate of the simulation is reached when the falling film Reynolds numbers of the tubes are as close as possible and pass by the film breakdown onset Reynolds number value. Additionally, the results indicate that at a low mass flow rate, the heat transfer is carried out almost completely in the first five tubes, and at a higher mass flow rate, the heat transfer is more significant in the last tubes. Finally, a comparison between the evaporator of 13 tubes and an arrangement of two evaporators of five tubes each one is performed. From this analysis, the results show that the heat transfer rate of the arrangement of two evaporators is 21.16 % and 18.69 % higher than the evaporator of 13 tubes at 0.00529 and 0.00793 kg/s⋅m, respectively.

Numerical Simulation of Microscale Oblique Droplet Impact on Liquid Film

The oblique impact of microscale water droplets on liquid film is numerically investigated. Two-phase flow problems are simulated using three-dimensional incompressible Navier-Stokes equations, and the level-set method is employed for capturing the gas-liquid interface. The numerical model is verified using experimental results from a normal and oblique impact via the qualitative comparison of crown profile features and quantitative contrast of the crown height and radius varying with time. The article discusses the influence of tangential impact velocity, water film thickness, Reynolds number, and Weber number on the shape characteristics, tangential momentum, and kinetic energy of the annular crown. The results show that the decreasing momentum in the tangential direction can be divided into three clear stages: rapid decrease, slight increase, and continuous decrease. In addition, film thickness and Weber number have significant effects on the momentum decay rate.

Combining Microstructured Surface and Mesh Covering for Heat Transfer Enhancement in Falling Films of Refrigerant Mixture

The article presents the experimental results of combining a basic microstructure with partly closed pores and a mesh covering for heat transfer enhancement at the film flow of a refrigerant mixture. To reveal the effect of the combined structure, heat transfer on a microstructured surface without a covering as well as on a smooth surface with a mesh covering only has been studied. All experimental series were carried out using a binary mixture of R114 and R21 refrigerants. The mixture film flowed down the outer surface of a vertical cylinder in the undeveloped turbulence regime, when the film Reynolds number varied from 400 to 1300. It is shown that a microstructured surface with a fin pitch of 200 μm, fin height of 220 μm, and longitudinal knurling pitch of 160 μm, created by deformational cutting, demonstrates significant heat transfer enhancement: up to four times as compared to a smooth surface. However, adding a mesh covering with an aperture of 220 μm and a wire diameter of 100 μm reduces the intensification. The mesh covering overlaid on a smooth surface also does not provide heat transfer enhancement as compared to the smooth surface itself. The absence or even deterioration of heat transfer enhancement on surfaces with mesh covering can be primarily associated with the low thermal conductivity of the mesh material and shortcomings of the applied method of mesh mounting. The possibility of deteriorating vapor removal due to the incorrect selection of mesh covering parameters was also analyzed. The heat transfer coefficient values obtained for basic microstructured surfaces were compared with the dependencies available in the literature for predicting pool boiling heat transfer on microfinned surfaces.

A mechanistic model in annular flow in microchannel tube for predicting heat transfer coefficient and pressure gradient

A mechanistic model that focuses on annular flow is proposed in this work. The new model predicts pressure gradient, void fraction, and heat transfer coefficient in annular flow in a microchannel tube. Six fluids, namely R32, R1234yf, R134a, R1234ze(E), R1233zd(E), and R1336mzz(Z) were tested in previous studies. The tested fluids have a wide range of properties. The results of these six fluids are reviewed and compared to models in the literature. The existing heat transfer models fail to predict the measurements accurately. The proposed model starts from the shear balance at the interface of liquid film and vapor core. The new model uses film thickness to calculate the film Reynolds number. The Film Nusselt number is then calculated. A database approach is adopted to enhance prediction accuracy. Based on the measurements in the database, the model has an MAE of 11.7%, and ME of 0.5% when predicting the heat transfer coefficient. Li and Hrnjak (2022), a flow pattern map based on the same fluids and the same facility, is used to determine the flow pattern. The model is compared to measurements from varied sources in the literature, and it also shows good accuracy. The model can also be extended to other flow patterns in a microchannel evaporator.

Second law analysis and multi-objective optimization of a falling film absorber

A falling film absorber is studied in order to understand the phenomena that occur during its operation and to find a way to improve its performance. The present work is divided in two parts using a developed model from a previous article. The first part is focused on the study of the impact of design and operating parameters on the different sources of entropy generation but also on the absorbed mass flow rate and recovered heat. In the second part, a multi-objective optimization is used in order to screen the falling film absorber performance by plotting Pareto-optimal fronts. The objective functions considered are the recovered heat, the absorbed mass flow rate and the entropy generation. The film and HTF Reynolds numbers as well as the HTF inlet temperature are found to be the most interesting variables to control to obtain high absorbed mass flow rate and recovered heat while controlling the entropy generation. The existence of an optimal film Reynolds number is also underlined in this study.

Dynamics of condensate film in the vicinity of a pulling vapor stream

This article introduces eight possible film configurations that appear when a rising vapor stream condenses on a vertical surface. These configurations include hanging film, falling film, falling film with zero interface velocity, partly falling film, bidirectional film, partly rising film, transition film, and rising film. We develop a theoretical framework to predict condensate dynamics and heat transfer. The results show that the entire film morphology depends on a subcooling parameter (H) and a characteristic number (ϒN). Here, ϒN is the ratio of the relative Froude number and the film Reynolds number, which increases continuously from falling to rising configurations. Our analysis shows that a falling film's heat transfer and condensation rates increase as subcooling (H) increases. However, increasing H for partly falling films reduces the condensation rate, hindering the overall heat transfer. We demonstrate that the falling films produce the maximum condensate within a particular range of H; however, the maximum shifts to transition films beyond this range. As H increases, the transition films are attained at a smaller ϒN. We have identified five physical mechanisms that subtly govern the film dynamics: gravity, wall shear, interfacial shear, near wall mass-flux deficit, and interfacial mass entrainment. Concurrent participation of these mechanisms yields complex streamlines and spiral vortices.

Universal correlation for falling film evaporation on a horizontal plain tube

Falling film evaporators are widely used in many industrial sectors, such as desalination, refrigeration & air-conditioning, and chemical processes. The heat transfer correlations suitable for wider operating conditions and various refrigerants are useful for designing falling film evaporators. Although many researchers developed the correlation for their own experimental data, there is a limited universal correlation for falling film evaporators. In addition, the dry-out effects were not considered in the available universal correlations. Therefore, this study proposes a new correlation for falling film evaporation on a single horizontal plain tube, which includes nucleate boiling, film evaporation, and dry-out effects. The proposed correlation covers a wider range of operating conditions (Heat flux is of 2.5–168 kWm−2, and film Reynolds number is up to 3500), various refrigerants (R134a, R245fa, R1234ze(E), R123, R32, R410A, R290, and R600a), and the large number of experimental data (1101 data in total). The proposed correlation is validated against the available standard correlation with regard to mean absolute deviation (MAD) and R2 values. The MAD of the new correlation is about 10%, respectively, which is the lowest among other correlations. Similarly, the R2 showed a value of 0.92, which is the highest compared to the existing correlations. A simplified correlation is also proposed.

Parametric analysis of interfacial friction factor for liquid film dynamics sheared by turbulent gas flow

Water droplet impingement on a low-pressure steam turbine blade causing erosion has been recognized as a crucial issue. It is essential to elucidate a comprehensive droplet detachment mechanism, not only from the trailing edge but also from the liquid film surface. In the present paper, we investigate the influence of interfacial friction factor against liquid film dynamics on a wall sheared by a turbulent gas flow, including the liquid film thickness, liquid film velocity and entrained droplet detached from liquid surface for both pipe flow and plate flow conditions. We conduct the analyses by using a liquid film dynamics model, recently established, considering the three-dimensional destabilized waves and droplet entrainment from the liquid surface. As a result, the film thickness and velocity greatly depends on the interfacial friction factor. Interestingly, the rate of entrained droplet to initial liquid film has a minimum value when the interfacial friction factor equals to the inverse of the liquid film Reynolds number, while the remaining liquid film flow rate becomes maximum.

Numerical study of rising Taylor bubbles driven by buoyancy and additional pressure

The Taylor bubble driven by buoyancy and additional pressure is in a non-equilibrium state, which bubble and liquid flow have an accelerated tendency of rising, and the bubble behavior is significantly different with the Taylor bubble in stagnant liquid. In order to understand the dynamic properties of such bubbles, a numerical study was conducted using the coupled level-set and volume-of-fluid (CLSVOF) method combined with the stress-blended eddy simulation (SBES) model. The research results indicated that, due to the additional pressure, the difference between the bubble pressure head and the liquid column height is larger than zero, the pressure gradient force acting on the liquid column is greater than the force of gravity, this makes the bubble have an accelerating upward trend. The bubble shape and liquid film thickness are closely related to the relative bubble velocity, the correlation between dimensionless liquid film thickness and Reynolds number of relative bubble velocity was proposed. In addition, the article analyzed the influence mechanism of bubble shape, which is determined by the stress state of the fluid surrounding the bubble. Under the additional pressure, the axial stress on the bubble top increases significantly, while the radial stress remains almost invariable, which makes the bubble shape gradually become thinner.

Hydrodynamics and thermal performance of turbulent falling films through horizontal tube bundles

Turbulent falling films are commonly encountered in the practical operations of most falling film applications, but the relevant numerical study is very scarce. The hydrodynamics and thermal performance of falling films in horizontal tube bundles under the turbulent regime were numerically investigated in this study. The calculation ranges are: film Reynolds number from 3505 to 8034, heat flux of 47.3 kWm−2, inlet liquid temperature of 46 °C, tube diameter of 19.0 mm and liquid distributor height of 6.4 mm. Based on the results of the present numerical model, the Standard k-ω model is more suitable in reproducing the falling film heat transfer profile, as the present numerical model accurately predicted the turbulent falling film heat transfer characteristics. There are various hydrodynamics depending on the bundle arrangement and flow regime, including splashing, bridging, gas entrainment, Coanda effect, and liquid film thickness. The triangular bundle provides the highest heat transfer coefficient, but the square bundle provides the poorest one, and the square and rotated-triangular tube bundles behave the largest and smallest bundle effect, respectively. The bundle effect is generally independent of film Reynolds number, liquid feeder height and tube spacing.

Film thickness and heat transfer characteristics of R1233zd(E) falling film with nucleate boiling on an inclined plate

Falling film evaporation is an effective heat transfer method for achieving high heat flux and wide-area cooling with less refrigerant. As the film thickness and wall superheat increase, nucleate boiling occurs in the liquid film. In this case, the superficial liquid film thickness becomes thicker due to the existence of vapor bubbles. Although such superficial film thickness is an important parameter for expressing the film structure, the liquid film thickness of hydrofluorocarbon and hydrofluoroolefin refrigerants under nucleate boiling conditions has not yet been reported. This study experimentally investigated the liquid film thickness and heat transfer coefficients on an inclined plate with the width of 50 mm and the length of 75 mm. R1233zd(E) was used as the working fluid at a saturation temperature of 20°C. The film Reynolds number at the inlet was varied from 4.4 × 102 to 2.7 × 103, and the heat flux was varied from 5.6 to 138 kW/m2. As a result, the mean film thickness under adiabatic conditions increased when the flow rate increased or the inclination angle decreased from 30° to 15°, and ranged from 0.14 to 0.37 mm. The values were in good agreement with preexisting theories and correlations. Under heating conditions without boiling, the film thickness was almost equal to that under adiabatic conditions, and the heat transfer coefficients were slightly higher than the typical heat transfer correlation of the saturated falling film of water. With nucleate boiling in the high heat flux range, the superficial film thickness reached 1–3 mm at the location where vapor bubbles existed. The frequency of the thickness exceeding 1 mm increased with the heat flux as nucleate boiling became more active; thus, the average thickness also increased. The heat transfer coefficients under the developed nucleate boiling conditions were dominated by heat flux and were in good agreement with the existing pool boiling correlation.

Thermodynamic analysis in laminar falling film evaporator

• 2D analysis of heat transfer in a laminar falling film evaporator. • The sources of irreversibilities were identified, located and quantified. • The local entropy generation varies with the operating and design conditions. • The overall entropy generation is impacted by the local one. • The thermal and evaporation efficiencies also impact the overall entropy generation. An evaporative laminar falling film flowing by gravity on a vertical plate heated by a heat transfer fluid is studied to determine the local temperature, entropy generation and evaporated mass flow, assuming constant saturation pressure at the free interface, in co-current and counter-current configurations. The heat transfer fluid and film energy equations are solved simultaneously through a 2D model using an implicit finite difference scheme. Local and global analysis were performed in order to study the impact of parameters such as the heat transfer fluid and film Reynolds numbers and inlet film temperature on entropy generation (thermal and viscous irreversibilities) and on thermal and evaporation efficiencies. Results show that the increase of the overall thermal entropy generation leads to the increase of the evaporated mass flow rate. Both increase with the increase of the heat transfer fluid Reynolds number and decreases with the increase of the film Reynolds number for both configurations. The inlet film temperature – overheated or subcooled inlet fluid flow - also impacts the evaporation process. The influence of the Reynolds number and temperature difference on the heat transfer coefficients of both fluids and for both configurations is also discussed.

Effect of Structural Optimization of Scrubbing Cooling Rings on Vertical Falling Film Flow Characteristics.

In order to study the influence of the structural optimization of the scrubbing cooling ring in the scrubbing cooling chamber on the flow characteristics of the vertical falling film, the flow characteristics of the turbulent falling film in the rising section of the development region at different internal platform heights of the scrubbing cooling ring and a high Reynolds number were studied by FLUENT software. First, the correctness of the model was verified by the maximum error of simulation and experimental results of no more than 9.836%. Then, the distribution of liquid film thickness (δ), velocity (V), and turbulence intensity (Iz) at 0° of the tube in the axial direction x = 0–500 mm were calculated and obtained when the platform height (H) was 0–30 mm and the liquid film Reynolds number (Rel) = 1.1541 × 104–3.4623 × 104. The results showed that δ in the entrance region increased sharply due to the “jet” effect with solid wall constraints formed by the structure of the water inlet pipe and the scrubbing cooling ring. On the contrary, the liquid film in the fully developed region showed a stable fluctuation trend due to the weakening of the “jet” effect. When H = 30 mm, the change of δ was relatively stable and the change of Iz was small, indicating that this platform height is conducive to the stable and uniform distribution of the liquid film. In addition, when Rel < 1.1541 × 104, the liquid film was unstable due to the low flow rate and insufficient cohesion of the liquid film, but V increased slightly. In addition, with the increase of Rel, δ did not change significantly along the axial direction, that is, the Plateau–Rayleigh hindered the growth of δ. Finally, the empirical formula for δ applicable to Rel = 1.1541 × 104–3.4623 × 104 at the axial fixed position was fitted for the first time.

흡수식 냉동기용 유하액막식 증발기의 전열관 형상이 열전달 성능에 미치는 영향

To understand the mechanism and obtain data that can be used for design, an experimental apparatus was constructed to investigate the heat transfer characteristics of a falling film evaporator for an absorption chiller. Evaporators with plain tubes or three enhanced tubes were prepared. The enhanced tubes were designed to improve surface wetting rate with surface protrusions or a floral pattern shape. The performance change of the evaporator was investigated according to the flow rate of chilled water, the inlet temperature of the chilled water, the liquid film Reynolds number, and the evaporation temperature. In the case of using the enhanced tube, the average heat transfer rate was approximately 16.3% higher than that of the plain tube, and the difference in heat transfer rate between the enhanced tubes was as small as approximately 6%. The Nusselt number of end-cross, end-dimple, and end-dimple floral tubes was higher by 13.1%, 26.5%, and 46.9%, respectively, than in the plain tube.