Falling Film Thickness Research Articles

Experimental study on the falling film thickness distribution on a horizontal tube under the cross airflow

Experimental study on the falling film thickness distribution on a horizontal tube under the cross airflow

Heat transfer enhanced surfaces for horizontal tube falling film evaporator characterized using laser interferometry

The falling film evaporators are a class of heat exchanger that is widely studied. The research for improving the efficiency is still of prime importance because of its widespread applications in many industries such as desalination, food processing, nuclear energy, refrigeration etc. Our motive in this work is to investigate the potential use of heat and mass transfer enhanced surfaces for horizontal tube falling film evaporator to scale up its effectiveness. Four different tube surfaces viz. bare copper tube, Al2O3 thermal spray coated tube, Al2O3 + TiO2 thermal spray coated tube and open cell copper metal foam wrapped tube were taken for the study. Scanning electron microscope, 3D surface profilometer, contact angle metre and energy-dispersive X-ray spectroscopy were utilized to study the surface texture, surface roughness, wettability and composition, respectively. Out of these four surfaces, a total of eight configurations were realized by varying the coating thickness, surface roughness and porosity. A new prototype of horizontal tube falling film evaporator was designed and fabricated with fully wetted flow. An in house developed novel method based on laser interferometry was utilized for the experimental investigation. This non-invasive technique was capable of measuring falling film thickness and falling film interface temperature simultaneously with unprecedented circumferential span. For the film thickness evaluation, an optical shadow method incorporating Otsu’s algorithm with precise interface tracking features was used. Mach–Zehnder Interferometer was employed to visualize the isotherm formation and to quantitatively infer the interface temperature. A 2D CFD study based on VOF model was carried out and found to be in good agreement with experimental as well as theoretical results. Further, the effect of flow rate, feed inlet water and body temperature were explored in detail and analysed statistically.

A comprehensive review on computational studies of falling film hydrodynamics and heat transfer on the horizontal tube and tube bundle

Falling film evaporation is a promising technology widely applied in refrigeration, desalination and beyond applications owing to the great advantages. However, the liquid film hydrodynamics and heat transfer performance are studied insufficiently. Compered with experiments, numerical simulations are economical and efficient especially in the study of liquid film, through which some microscopic understandings are expected to be extracted. In this paper, a comprehensive review for a large pool of computational studies about falling liquid film flow and heat transfer on the horizontal tube and tube bundle is presented. Computational methods of liquid–gas two-phase flow and mass transfer model, as well as relevant researches were first introduced. Then, the studies on the falling film hydrodynamics, film thickness, sensible heat transfer, flow pattern, evaporation and boiling outside the single tube, tube bundle, special-shaped tube and entire evaporator studied with 2D and 3D models are respectively reviewed in order. Then, the investigations on the falling film breakout and dryout are reviewed. And then, the numerical predictions of falling film thickness and heat transfer coefficient are involved. Afterthat, the benchmark data for falling film numerical simulation are summarized. Finally, some future needs and recommendation relating to crucial technologies that must be solved are proposed.

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.

Numerical investigation of the falling film thickness and heat transfer characteristics over horizontal round tube

The numerical investigation was performed to analyze the film thickness and heat transfer characteristics over the horizontal round tube of the falling film evaporator. A three-dimensional model for the falling film flow and heat transfer was established, and the volume of fluid (VOF) method was applied to track the gas-liquid interface. The effects of different spray densities, saturation temperatures, and longitudinal inter-tube spacings (Γ, Tsat, s) on the falling film thickness (δ) and local heat transfer coefficient (h) were analyzed. Asymmetric circumferential distribution of δ is captured by the simulation results. The h reduces first and then rises slightly as the φ increases, and the maximum value appears in the impact point. Moreover, in the circumferential direction, φ=70∘ is the critical value for the variation of δ under different s, and the increment of Γ and Tsat leads to the increase in δ and h. In the axial direction, with increasing the Γ, the δ and the h increase, and the axial size of the crest zone tends to narrow. In addition, the increase of s promotes the rise of h. Meanwhile, the δ varies slightly in the crest zone, and it decreases obviously in the stable zone.

Control of falling-film evaporators with modeling of fouling

Fouling is an undesirable phenomenon in milk evaporation, which affects the cost of production and the quality of the product. This phenomenon can be reduced by using different approaches, where process control is a feasible alternative. There are a large number of proposals for the control of a multi-effect falling-film evaporator (MEFFE), but the effect of process control on fouling has been omitted. In this work, a PID control strategy for MEFFE is developed under a dynamic model with computing of fouling and falling-film thickness. The strategy proposes several control configurations, from which the best option is selected by considering control stability, low fouling thickness and low demand of motive steam. The control strategy was applied to two case studies; the results were validated with experimental data and compared with other control techniques. The results show that this strategy is suitable for MEFFE, and guarantees the quality of the concentrated product.

A novel interferometric method for simultaneous measurement of film thickness and film interface temperature for a horizontal tube falling film evaporator for MED systems

Horizontal tube falling film evaporation spans a lot of applications, especially in multi-effect desalination application. Hence, the improvement of its gain output ratio is of prime importance, and for that, the studies on the characteristics associated with the flow, mainly falling film thickness and falling film interface temperature, are inevitable. As the falling film flow is a delicate fluid structure, non-intrusive measurement techniques have to be preferred for the experiment. In this work, we introduce a novel interferometric method to calculate film interface temperature and a shadowgraph approach to estimate film thickness simultaneously. A Mach-Zehnder interferometer with infinite fringe setting was used to capture the thermal fluctuations around the horizontal tube falling film evaporator. Interferometric method implemented in this work also serves as an image visualization technique and does the purpose of qualitative as well as quantitative analysis. An in house designed experimental setup was fabricated for producing a thin and uniform falling film with a fully wetted flow over the horizontal copper tube. A heater controller setup was incorporated into it for accurate temperature control. This method holds the advantage of scanning falling film thickness over circumferential angle for the range 5∘≤θ≤175∘, which is an improvement to state of the art. Furthermore, dynamic film thickness and instantaneous film interface temperature studies were also incorporated to prove the efficacy of the technique and to test the robustness of the algorithms implemented. CFD analysis was also carried out for the entire process by means of VOF scheme and was found to be in good agreement with the experimental results, which proves our technique to be a good one. A standard error of mean analysis was performed on every data set, which validates the reproducibility and the dynamic measurement capability of the technique used.

Numerical investigation on the longitudinal distribution characteristics of falling film flow outside a vertical tube

In order to design the vertical falling film evaporators applied in the sewage source heat pump systems, a numerical study employing the VOF (Volume of Fluid) method has been carried out to determine the thickness and the wave behavior of falling liquid film outside a vertical tube. The effects of the annular gap, tube diameter, tube length, spray density, viscosity and surface tension are discussed. The simulation results demonstrate that spray density and the fluid viscosity are the two most influential factors affecting the film thickness. Besides, when spray density is larger than 0.100 kg/(m∙s), the film thickness decreases slightly with the increase of the flow distance. So the influence of tube length on film thickness cannot be ignored and it also should be considered in the film thickness prediction model. On the other hand, the tube diameter and the surface tension have little effect on the film thickness. It mainly have a significant impact on the wave amplitude of the liquid film at low flowrate. Besides, the annular gap provides a limited effect on the outlet section of the falling film. On the basis of the simulation results, a new correlation incorporating the tube length is developed to effectively predict the falling film thickness. Compared with the experimental data, it is found that the prediction accuracy of the correlation can be controlled within ±15%. The present research provides a theoretical basis for designing falling film evaporators.

A comprehensive simulation and optimization on heat transfer characteristics of subcooled seawater falling film around elliptical tubes

In this paper, 2-D elliptical tube models based on the volume of fluid (VOF) method are established. The effects of tube perimeter, Reynolds number, inlet temperature, seawater salinity, and tube ellipticity (the ratio of the major axis to the minor axis) on the falling film thickness and heat transfer coefficients are studied. A mathematical algorithm combing the Kriging response surface method (RSM) and the multi-objective genetic algorithm (MOGA) is adopted to explore the parameter sensitivity and optimization results. The ranges for parameters are: tube perimeter from 31.4 to 94.2 mm, Reynolds number from 500 to 1500, inlet temperature from 10 to 90 °C, seawater salinity from 0 to 120 g kg−1, tube ellipticity from 1 to 4. The results show the local film thickness increases with Reynolds number and seawater salinity, but decreases with tube perimeter, inlet temperature and tube ellipticity. The increase of Reynolds number, inlet temperature, and tube ellipticity enhance heat transfer, while tube perimeter and seawater salinity have a negative effect. The tube ellipticity has a negligible impact on heat transfer when it exceeds 3.25. The tube perimeter and seawater salinity are the most and least influential factors, and three optimal parameter points are obtained. A correlation considering the effect of tube ellipticity is fitted to predict the overall external heat transfer coefficients, and 97% of prediction data are within 10% relative errors with the numerical results.

Distribution characteristics of falling film thickness on horizontal tube evaporators

Distribution characteristics of falling film thickness on horizontal tube evaporators

Interferometric analysis of flow around a horizontal tube falling film evaporator for MED systems

In this work, an optical shadow method (non-intrusive) was used to evaluate the falling film thickness around the circumference of a horizontal tube. A Mach-Zehnder Interferometer was used to visualize the isotherm formation. Most of the well established non-intrusive techniques for measuring film thickness around the cylinder fails to measure the film thickness near to entry region (θ<30∘) and towards the exit region (θ>140∘) because of the limitation and complexity of the apparatus. Relavance of this work lies on the simplicity of the measurement and the range of circumferential angles that can be investigated (10∘≤θ≤170∘). This technique involves image visualization, thus, serves as a tool for analyzing the film both quantitatively and qualitatively. A lucid algorithm was developed for image processing along with a set of intensity profile tracking procedures making the whole process of film thickness measurement more obvious. The measured film thickness is showing good agreement with the commonly used empirical formula, and the effectiveness of using those empirical formulas for the small diameter tubes was analyzed. Further, this study was extended to determine the effects of flow rate and feed inlet temperature on film thickness. Film thickness variation around the circumference of the cylinder is showing an increase in trend with increasing Reynolds number, but a change in trend with the varying feed inlet temperature is found to be marginal. This study is further extended for different positions of the tube in the order of impingement of film or feed inlet as top, middle and bottom tubes and the film thickness variation was again found to be marginal. Dynamic characterization at θ=10∘ was studied and the maximum film thickness variation was found to be 11.8%. A standard error of mean analysis was performed on every data set which validates the reproducibility and the dynamic measurement capability of the technique used.

Simulation of the hydrodynamics and mass transfer in a falling film wavy microchannel

The flow in a liquid falling film is predominantly laminar, and the liquid-side mass transfer is limited by molecular diffusion. The effective way to enhance the mass transfer is to improve the liquid film flow behavior. The falling film behaviors of water, ethanol and ethylene glycol in nine different wavy microchannels were simulated by Computational Fluid Dynamics. The simulation results show that the falling film thickness exhibits a waveform distribution resulting in a resonance phenomenon along the wavy microchannel. The fluctuation of liquid film surface increases the gas–liquid interface area, and the internal eddy flow inside the liquid film also improves the turbulence of liquid film, the gas–liquid mass transfer in falling film microchannels is intensified. Compared with flat microchannel, the CO2 absorption efficiency in water in the wavy microchannel is improved over 41%. Prediction models of liquid film amplitude and average liquid film thickness were established respectively.

Droplet entrainment within the evaporator to the suitable volume-filling ratio of a vertical two-phase closed thermosyphon

The present study on vertical two-phase closed thermosyphon (TPCT) is aimed to determine the suitable volume-filling ratio, as a function of geometries, heat flux and vapor temperature, in order to avoid the potential local dryout in the evaporator section and hence to maximize the performance. In the study, the droplet entrainment, which is caused by the internal counter flow during the nucleate boiling within the falling liquid film in the evaporator section, is introduced to improve the existing TPCT model, so a comprehensive model with considering all three heat transfer regimes in the evaporator is further established. The suggested lower and upper limits of volume-filling ratio (CFR and EFR) are then determined by utilizing the criterias for local dryout, flooding limit and boiling limit. Furthermore, the effects of geometries, heat flux and vapor temperature on the range of volume-filling ratio are analyzed in details, a simplified correlation of CFR is then proposed based on the numerical modeling results. Particularly, the predictions of the distribution of falling film thickness and onset of flooding are validated with the published experimental data and other numerical simulation results. It is also found that the droplet entrainment significantly influences the thickness and distribution of the falling film. Increasing vapor temperature and inner diameter and decreasing the evaporator length significantly enlarge the volume-filling ratio range (between CFR and EFR), operation envelope (corresponding to boiling limit and flooding limit) of TPCT. Increasing the condenser length increases the volume-filling ratio range and operation envelope to a small extent. The influence mechanisms of the optimum filling ratio by the heat flux and geometries are complicated; however, a correlation of CFR can be obtained in a good agreement with the numerical modeling results within ±30% deviation for the whole limited scope.

Numerical Simulation for Falling Film Thickness around Horizontal Tube in MVC and MED Evaporators

Falling film on horizontal tube evaporators, of both Mechanical Vapor Compression (MVC) and the Multi-Effect Distillation (MED) desalination systems, plays an important role in the heat and mass transfer (evaporation) and accordingly the systems productivity. Falling film thickness is mainly influenced by the intertube space, circumferential angle and the film’s Reynolds number. This paper presents two-dimensional numerical study of falling film thickness around horizontal tube in MVC and MED evaporators. The study is based on computational fluid dynamics (CFD) using volume of fraction (VOF) as a multi-phase technique in ANSYS Fluent. The numerical model is developed in order to study the heat and mass transfer charactristics, the liquid falling film behaviour and thickness distribution around circular horizontal. Four CFD study cases are developed to simulate the falling film behaviour at circumferential angle range from 150 to 1650 with inter-tube spacing of 10 mm, 16 mm, 33 mm and 40 mm and for constant value of flow rate and at the same surrounding conditions. Simulations are conducted using a domain of only two tubes with 20 mm outer diameter.The results from the numerical models are compared with the published experimental correlations, showing a comparatively reasonable agreement. In addition, a parametric study is carried out to illustrate the effect of flow Reyonlds number (Re) and intertube space on the average circumferential film thickness and heat transfer rates.

Distribution characteristics of falling film thickness around a horizontal corrugated tube

Falling-film evaporation has an essential method for desalination. The heat transfer tube is the core component of falling film evaporator. Liquid film thickness is one of the most important parameters for predicting heat and mass transfer coefficient of falling film evaporation. In this paper, a set of experimental apparatus for measuring the film thickness of a horizontal tube is set up. The experiments have been undertaken under different parameters, i.e. the tube diameter varying from 25 to 45 mm, the tube spacing ranging from 10 to 60 mm, the corrugated radius varying from 0.25 to 1 mm, and the film Reynolds number ranging from 150 to 1000. The liquid film falling around a horizontal tube is performed to explore the distribution characteristics of the film thickness. The liquid film thickness is measured by the conductance probe measurement method. The experimental results showed that the film thickness of the corrugated tube increases with the increasing of the film Reynolds number, which is the thinnest from 90° to 120°. It decreases as the tube spacing increases and increases with the tube diameter and corrugated radius. The liquid in the wave troughs can be replenished to the wave crests at any time because of the corrugated structure. The falling film thickness was thinner at the wave crests and thicker at the wave troughs. The phenomenon of dryout hardly appears in the falling film flow of the corrugated tube. Based on the experimental data, a new correlation has been given to predict the film thickness of the corrugated tube.

Evaluación del espesor de una película de agua descendente vertical con visualización y técnicas de reconocimiento de imágenes

The present work is focused on the experimental study of a vertical downward annular flow to determine the thickness and stability of a falling film water flow. For this purpose, it was designed and implemented an experimental set up to generate the annular two-phase flow pattern, provided with an injection head with a special geometry to induce a liquid film inside a cylinder. Due to the small dimensions, the film was visualized using a pulsated laser to illuminate the region and the pictures were taken with a high-speed camera. This technique allowed the determination of the falling film thickness by means of an algorithm to recognize image contours. In some of the studied cases, a concurrent air flow was injected in the center of the cylinder in order to evaluate its influence on the interfacial hydrodynamics of the liquid film. Average film thickness were obtained for different Reynolds numbers in different axial observation points, and it was observed that the liquid film annular area and the shape of the header to inject the water, are important factors for the surface characteristic and thickness of the film, and its stability as well. The experimental results show that the standard deviation increases in proportion to the average film thickness, especially in the turbulent regime.

Impact of surface tension and viscosity on falling film thickness in multi-effect desalination (MED) horizontal tube evaporator

Falling film evaporators are extensively used in many applications because of their higher heat and mass transfer coefficient over low temperature difference. Falling film thickness, which depends on liquid spray density and thermophysical properties, is an important parameter in determining heat and mass transfer performance as it represents thermal resistance. In multi-effect desalination (MED) evaporators, the thermophysical properties such as surface tension and viscosity change due to operating temperature differences, i.e. higher in the first evaporator/effect and lower in the last evaporator/effect. The surface tension variation with temperature is often neglected in most of the film thickness characterization and modeling studies. Therefore, it is needed to analyze surface tension effects distinctly on film thickness. In this study, a 2D CFD model was developed by implementing volume of fluid (VOF) multiphase model to distinguish liquid and gas phases. The effects of viscosity and surface tension were analyzed separately, and it was found that with constant surface tension, viscosity effects account for 66.1% variation in film thickness for operating temperature range of 85 °C–5 °C. However accounting both viscosity and surface tension dependence, the results showed 72% increment in the film thickness. In addition, CFD results exhibited lower conduction thermal resistance of 0.2 m2 K/W at 85 °C against 0.4 m2 K/W at 5 °C, which reflects better evaporator performance at higher temperature. Hence, more focus and detailed analysis are recommended given in increasing the first effect temperature from 65 °C to 85 °C as it would improve thermal performance due to lower thermal resistance rather than decreasing last effect temperature from 40 °C to 5 °C as the thermal resistance would increase.

Experimental and theory investigations on falling film flow characteristics and heat extraction performance of spray heat exchanger for sewage heat pump system

The flow characteristics of sewage directly affect the technical and economic performance of sewage-source heat pump systems in heat extraction from sewage. To prove there is significant improvement in heat extraction from sewage with the spray heat exchanger for sewage heat pump system (SHPS): (1) The flow states of the falling film on and between the multiple horizontal tubes were investigated experimentally. The correlations of Reynolds number (range from 100 to 600) and the tube distances to the flow were derived; (2) A model was developed for the falling film flow state analysis of the horizontal elliptical tube of the spray heat exchanger using the ellipse plotting method, which was constructed using approximately three circular arcs. The laser-induced fluorescence technology was used to measure the horizontal tube falling film thickness of water. This experiment focused on the film thickness distribution characteristics of Al-brass tubes with outside diameters of 19 mm and 25.4 mm in column flow with the tube spacing ranging from 20 mm to 40 mm and the Reynolds number (Re) ranging from 184 to 368. The images of the gas-liquid interface were captured with a digital camera. The image processing technology was used to quantitatively describe the liquid film thickness around the horizontal tube. The comparisons between the falling film thickness calculation results and experimental data were in good agreement for the variation of the angle of the elliptical surface and the specific liquid rate. Furthermore, a novel correlation was developed to describe the Nusselt number which was subjected to the examined tube shapes; (3) The heat extraction performance of SHPS with spray heat exchanger was experimentally investigated. The temperature of the sewage ranged from 10 to 30℃. The heat supply, input power, COP, and output water temperature of the condenser of the SHPS were discussed. The results of the proposed calculation method were in conformity with the experimental data and gave good predictions with maximum deviation being lower than 8.6%. Thus, the proposed method is helpful in improving the design and the performance of the SHPS.

Falling film thickness measurement using air-coupled ultrasonic transducer

In this work, a new, non-intrusive, increased sensitivity technique based on air-coupled ultrasonics was developed to measure the falling liquid film thickness. Air-coupled transducer of frequency 1 MHz was used to make static and dynamic measurements of water level/film thickness. The air-coupled method were compared with a metal-side contact ultrasonic approach as well as calculating water levels in the collection tank. The falling film thickness measurement over inclined plate was used in this study. Measurement of film thickness was taken at flow rates in the range of 0.34–0.9 lpm. Relatively good agreement was found between the readings, which validates the dynamic measurement capability of the new technique. The technique was applied to measure the horizontal tube falling film thickness for falling film Reynolds number ranging from 130 to 350, at different circumferential angles ranging from 40° to 130° and at tube spacing of 5 mm and 15 mm. Measurements were compared with correlations available in the literature and found to be in good agreement.

Numerical study on falling film flowing characteristics of R113 inside vertical tube under different structural conditions

Abstract To develop an appropriate falling film evaporation device for organic fluid cogeneration, a numerical study on the gas–liquid two-phase counter-current flow characteristics of R113 inside a vertical tube under different structural conditions was conducted using the Fluent software. The effects of the tube length, tube diameter, and annular gap on the falling film flow characteristics were determined, respectively. The results indicated that under a certain spray density, the falling film thickness in the region of the steady section was almost constant with different structural parameters for the tube diameter, tube length, and annular gap. In addition, a smaller tube diameter resulted in a steadier film flow. When the tube diameter decreased to a specific value, the film thickness showed a uniform distribution along the wall surface. This indicated that a best falling tube diameter exists. Meanwhile, the film fluctuation was enhanced with an increase in the tube length. When the tube length was greater than 1.2 m, the falling film splashed and could not completely wet the wall surface. The film fluctuation was enhanced by decreasing the annular gap, and the film could not be formed when the annular gap was smaller than 1.2 mm.