Horizontal-tube Falling Film Evaporator Research Articles

Surface with wettability gradient — A novel approach for crystallization fouling control on the horizontal falling film flow

To manage scaling on heat transfer surfaces, using chemical-based antifouling treatments takes a toll on the environment, and related maintenance costs are significant. A unique and environmentally friendly scale mitigation strategy is developed in this work. By manipulating the scale layer distribution using surface wettability gradients, stronger flow shear and weaker scaling adhesion are achieved, facilitating self-cleaning process. Experimental work utilizes a horizontal-tube falling-film evaporator to mimic multiple-effect distillation process. A millimetric wettability pattern – alternating hydrophilic and hydrophobic regions – is created by coating stainless steel tubes. The results show scale layers (calcium carbonate) were either thicker in hydrophilic regions and thinner in hydrophobic regions or had approximately uniform thickness in both regions but cracks in scale layers near wettability boundaries. Scale layers wash-off with flow is observed on patterned tubes, demonstrating self-cleaning process. In contrast to persistent deterioration in overall HTC (heat transfer coefficient) for baseline tube (77 % of initial HTC after 192 h), HTC on patterned tubes initially increases due to surface wetting effects, then maintains roughly constant (15 % higher than that of baseline after 192 h) along with lower scale mass. This work provides a novel scaling control method, a potential “green” alternative to current industry scale-control practices.

Analyzing wall adhesion effects on falling film hydrodynamics for aqueous lithium bromide application

Over the past decade, there has been a concentrated effort to accurately characterize the film thickness in horizontal tube falling film evaporators (HFFEs). Despite the experimental challenges in measuring thickness across the entire tube, researchers have made advances using adaptable operational settings, such as numerical modeling, to analyze falling films. One such condition is to analyze film hydrodynamics for different wall adhesion contact angles. In this study, a 2-D computational fluid dynamics (CFD) model is developed to quantify film thickness and wetting time and to analyze hydrodynamics for aqueous lithium bromide (LiBr) applications. The variation of liquid load (Γ1/2) from 0.01–0.05 kg/(m·s) and contact angle from 0–45° are studied for LiBr concentrations (C) of 0.45 and 0.65. Results indicate that the impact of contact angle diminishes at higher liquid loads and LiBr concentrations. At C = 0.45, the film thickens by 13.8 % for Γ1/2 = 0.05 kg/(m·s). Moreover, the effects of contact angle on wetting time are more pronounced than the average film thickness. With a higher contact angle, heat transfer may deteriorate due to the poor thermal resistance caused by higher average film thickness.

Experimental study on heat transfer of horizontal-tube falling-film flow in deviated columnar mode

Column flow is the most widely used flow pattern in horizontal-tube falling-film evaporators. Ideally, the liquid column is in the center plane of horizontal tube. In the actual falling-film evaporator, the installation deviation of nozzles, the steam sweep, as well as the disturbance caused by flash evaporation make the liquid column deviate from the center plane. The heat transfer characteristics of deviated columnar flow and its influencing factors were experimentally studied and analyzed in this paper. The results are as follows: The vertex of heat transfer coefficient curve shifts to the negative angle interval as the deviation distance increases. Meanwhile, the maximum value decreases with the increase of liquid column deviation distance. The heat transfer coefficients in the negative angle interval are greater than those at corresponding position in the positive angle interval. The heat transfer coefficients increase with the increase of evaporation temperature, spray density, and tube spacing, respectively.

Comparative analysis of heat transfer prediction for falling film evaporation on the horizontal tube based on machine learning methods

Precise prediction of heat transfer for falling film evaporation is necessary for the horizontal tube falling film evaporator design. In this study, models to predict the heat transfer are compared, which are developed by traditional Least Square method (LSM) and Machine Learning (ML) methods, including Support Vector Regression (SVR), Artificial Neural Network (ANN) and Random Forest Regression (RFR). The training data is obtained by a series of experiments, containing 510 experimental data of fresh water and seawater of salinities from 30 to 60 g/kg. The experiments were conducted with the test tube diameter of 19, 25.4 and 38 mm and with the heat flux of 7.7–12 kW/m2. All ML models behave better performances than the LSM model and the SVR model is suggested as the best model to predict the heat transfer, considering the prediction accuracy and generalization ability comprehensively. SHapley Additive exPlanations (SHAP) is used to explain the relationships between Nusselt number and input variables, which also helps to resolve controversial issues about the effects of spray density and tube diameter on the heat transfer. The importance of variables is ranked, in which the tube pitch and seawater salinity are regarded as the most influential variables. Besides, the limitations and performance of models are analyzed in terms of both dataset and algorithms.

Experimental study on break and velocity characteristics of liquid column between horizontal tubes

Column flow is one major flow mode in horizontal-tube falling-film evaporator. In this study, a horizontal-tube falling-film flow experiment platform is built up. A high-speed camera is used to shoot the process of falling and break of liquid column at the bottom of horizontal tube. Image edges are detected using the algorithm based on Sobel to quantify the liquid column features. Liquid column break process, falling velocity, variation pattern of break shape and break length, as well as the special phenomenon encountered in column falling process are discussed in detail. Results show that the break of liquid column undergoes three stages. The liquid column break shape has a periodic variation pattern, and the liquid column break length fluctuates within a certain interval. In addition, as the spray increases, break length increases, and the fluctuation interval of break length expands. The liquid falls downward spirally and its velocity rises in waves with time, and there exists an obvious uniform phase. In spray density range of 0.03–0.05 kg/m⋅s, liquid column falling velocity increases and the uniform phase is delayed as the spray increases. But when spray density varies in 0.05–0.06 kg/m⋅s, no significant increase in liquid falling velocity can be found.

Numerical simulation of flow characteristics of falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube

Horizontal-tube falling-film evaporator is regarded as an effective alternative to full-liquid evaporator because of its higher heat transfer coefficient and lower refrigerant charge. In the process of falling-film evaporation, the thin liquid film thickness (δ) is an important parameter for characterizing the hydrodynamics, and accurate prediction of δ can effectively prevent the reduction in heat transfer efficiency caused by local wall dryout. In this study, non-azeotropic refrigerant R32/R134a was taken as the research object. By incorporating the multi-component phase change model and contact angle model into the governing equations, a two-dimensional numerical model of falling-film evaporation outside a horizontal circular tube was established. The effects of the spray height (H), tube diameter (d), Reynolds number (Re), inlet temperature (Tinlet), and mass fraction of R32 in the liquid-phase mixture (MR32_liquid) on δ were analyzed under sheet flow. The results show that an increase in Tinlet, H, and d is favorable for increasing the average δ (δave), whereas an increase in MR32_liquid and Re leads to a reduction in δave. Besides, the decreasing rate of the δave decreases gradually as H and d increase. When H is 4 mm, the local δ (δlocal) first decreases and then increases as the circumferential angle (Φ) increases. When H ranges from 6 mm to 13 mm, as Φ increases, δlocal first increases slightly, then decreases, and finally increases significantly near the wake region. The δlocal of a larger tube diameter is thicker than that of a smaller tube diameter near the impact region, whereas in the subsequent regions, the δlocal of a larger tube diameter is thinner than that of a smaller tube diameter. The minimum δlocal occurs at Φ between 130° and 140°, and with an increase in H and d, Φ corresponding to the minimum δlocal decreases.

HEAT TRANSFER ENHANCEMENT AND SCALE FORMATION: EXPERIMENTAL STUDIES IN FALLING FILM EVAPORATORS USING COPPER METAL FOAM

Performance enhancement of horizontal tube falling film evaporators using thin metal foam was studied. A copper foam was brazed over the outer surface of a copper horizontal tube and used in the falling film evaporator of a desalination system. It was observed that the distillate production and heat transfer rate was enhanced by more than twice with the modified tube. Effect of feed rate, feed temperature, and subcooling on distillate production was studied for the copper foam-wrapped heat exchanger tube evaporator. A metal foam-wrapped heat exchanger tube was also used to study the effect of scale formation. The effects of temperature, flow rate, and seawater turbidity on scale formation was studied. Total scale deposit weight was observed after every 16 h of operation for a total run time of 128 h. It was observed that scale formation reduces with an increase in flow rate and increases with seawater temperature. It was also observed that scale formation is lower when the system is operated with seawater of low turbidity. The heat transfer enhancement surface promotes scale formation due to the increase in metal surface area in contact with seawater as in the copper foam-wrapped tube. Energy-dispersive X-ray spectroscopy and scanning electron microscopy were applied to investigate the mineralogy and microstructure of the deposits.

Experimental study on horizontal tube falling film evaporation in geothermal powered organic Rankine cycle generation

Geothermal energy has become one of the indispensable renewable energy in the energy development in the 21st century, and it is also one of the most realistic and competitive energy. The orc power generation test system with low temperature heat source (60 ∼ 90 °C) is designed, and the key component horizontal tube falling film evaporator is designed. The experimental results show that the heat transfer coefficient increases with the increase of evaporation temperature in the range of 40 ∼ 65 °C. The experimental results show that when the spray density is in the range of 0.010 ∼ 0.025 kg/m K, the heat transfer coefficient increases with the increase of spray density. The experimental results show that when the hot water mass flow rate is in the range of 0.06 ∼ 0.18 kg/s, the heat transfer coefficient increases with the increase of hot water mass flow rate.

Three-dimensional heat transfer coefficient distributions in horizontal tube falling film evaporation

Three-dimensional heat transfer during falling film evaporation was numerically investigated with spray densities varying from 0.0183 kg/(m⋅s) to 0.17 kg/(m⋅s). The flow characteristics and the heat transfer coefficient distribution were investigated for droplet flow, column flow and sheet flow over the tube. The simulations agree well with previous experimental data. The heat transfer coefficient with droplet flow is improved by each droplet impact and the resulting saddle-shaped waves. At lower spray densities, dryout patches appear that reduce the heat transfer until parts of the dryout patches are rewet. The local heat transfer coefficients for column flows have non-uniform three-dimensional distributions with the smallest heat transfer coefficients at φ = 165 ∼ 180° for l*=0 or 1. For sheet flows, the local heat transfer coefficient first decreases and then increases with increasing circumferential angle, while it varies only a small amount in the axial direction.

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 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.

Effects of Film Flow and a Surfactant on Scale Formation in Falling Film Evaporators for Seawater Desalination

Horizontal tube falling film evaporators are commonly used in multiple-effect distillation (MED) plants for seawater desalination. Scale formation on the evaporator tubes strongly depends on film flow characteristics. The thickness of the liquid film on a horizontal tube was measured in a test rig with a high-resolution optical micrometer. Furthermore, scale formation was studied with artificial seawater in a horizontal tube falling film evaporator at pilot plant scale. The effects of a polyoxyalkylene triblock copolymer surfactant on falling film flow and scale formation were investigated at different wetting rates. Film thicknesses on the bottom of the tube exceed film thicknesses on the top by an order of magnitude. The thinnest film is formed on the tube sides. The mean film thickness increases with increasing wetting rate. The scale formation on the top of the tube is stronger than that at the bottom. The highest scale thickness was measured at the tube sides. Scale mass and scale layer thickness increase with decreasing wetting rate. As the minimum and the mean film thickness increase and surface waves are mostly damped in presence of the surfactant, the mass transfer resistance is higher and, thus, scale formation is reduced.

Modeling of crystallization fouling on a horizontal-tube falling-film evaporator for thermal desalination

Scale formation within horizontal-tube falling-film evaporators is an important issue for thermal desalination, because of the deterioration of heat transfer performance and added maintenance costs. Predicting the fouling process is crucial for desalination plant design, operation, and maintenance. In this paper, a model is developed to predict the crystallization fouling process during seawater falling-film flow over a horizontal tube bundle. The scaling is estimated based on deposition theory, with couple heat transfer including the in-tube steam condensation, conduction through the tube wall and the scale layer, and falling-film evaporation. The spatial and temporal variations of temperature, heat transfer coefficient, and scale thickness are predicted, and the effects of variations in process parameters of steam, feed seawater, and tube properties on the scale thickness, evaporation rate, and heat transfer coefficient for falling-film evaporation are analyzed. Comparisons to existing experimental data show the scaling layer thickness falls in the general range of reported measurements, with a slight overestimation due to neglecting the delay of scaling onset under real conditions. The scale layer thickness increases dramatically as the in-tube steam pressure is increased or the seawater flow rate is decreased. In general, the scaling layer becomes thicker on the lower tubes in the tube bundle, due to the heat and mass transfer of the falling film. The effect of tube material on scaling appears mainly dependent on thermal conductivity; thus, using polymer rather than a stainless steel tube decreases scaling and evaporation rate 96% and 88%, respectively. This work has potential to guide thermal desalination plant design, material selection, and operating parameter optimization.

The CFD studies on the influence of un-wetted area on the heat transfer performance of the horizontal tube falling film evaporation

This paper discusses about the effect of un-wetted area of tube on the heat transfer performance of horizontal tube falling film evaporation. A 2-D CFD model was developed to perform simulations and investigate the output and validated them with published data available in the literature. In the present study the volume of fluid method is used to track the boundary of the liquid vapour from the contours of volume fraction. Effect of varying tube wall temperature or wall super heat (6-11?C) on un-wetted area, heat transfer coefficients, and mass transfer coefficients of the circular tube were obtained from the simulation model and the results were analysed and reasons were identified and discussed here. The threshold value of wall super heat above which phase change occurs between liquid film and tube surface is identified as 6?C. Also it is noted that mass transfer rate increases and then decreases with increase of wall super heat and heat transfer coefficient showed declining trend.

Experimental study of heat transfer performance of horizontal-tube falling film evaporator

Experimental study of heat transfer performance of horizontal-tube falling film evaporator

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.

Improved Heat Transfer Performance of Falling Film Evaporator for Desalination Industry by Balanced Liquid Distribution

Horizontal-tube falling film evaporators (HFE) are widely used in the thermal desalination industry due to their increasing advantages. The liquid distribution state of the tube bundle directly affects the heat transfer performance of the evaporator. To improve the uneven liquid distribution among tube rows, a balanced distribution structure (BDS) that can balance the liquid distribution state in the HFE was proposed. A general distributed parameter model was built to describe the steady-state performance of the HFE with and without the BDS, taking into account different liquid distribution of odd and even rows. The model is applied to simulate the multieffect distillation (MED) system of a desalination power plant, and the numerical results show a good arrangement with the operating results. The results indicate that the BDS slightly improves the overall heat transfer performance and the total evaporation rate. This work and the designed BDS certainly could help in the design and optimization of HFE for desalination processes.

Numerical studies on the thermal regimes of the horizontal tube falling film evaporation under varying feeder height

ABSTRACT This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m2 resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.

Investigation on heat transfer performance of corrugated tubes in low-temperature multi-effect falling-film evaporation

ABSTRACT The enhanced heat transfer performance of falling film evaporation during desalination through altering smooth tubes by corrugated tubes was investigated. An experimental research on corrugated and smooth tubes was carried out using a horizontal-tube falling-film evaporation heat transfer experimental apparatus. The changes in heat transfer coefficient with the process parameters (sprinkling density, heat flux, heat transfer temperature difference, and evaporation temperature) were obtained according to the experimental data. The influence of tube type on liquid-film distribution outside the tube was also examined by numerical simulation. Results showed that within a certain scope increasing of any one variable of sprinkling density, heat flux, and evaporation temperature has a positive impact on the heat transfer coefficient. When the sprinkling density exceeded 0.178 kg/(m·s), the heat transfer coefficient tended to be stable. Compared with the simulation results, the liquid film coverage and thickness outside the tube significantly influenced heat transfer efficiency. When the heat flux exceeded 130kW/m2, the growth of the heat transfer coefficient significantly slowed down. Under the same experimental conditions, the heat transfer coefficient of corrugated tubes increased by 30% compared with that of smooth tubes. In addition, the content of non-condensable gas within the seawater affected the heat transfer coefficient significantly.

Numerical Investigation on the Effect of Tube Geometry and Feeder Height on the Heat Transfer Performance of Horizontal Tube Falling Film Evaporation

Abstract This paper discusses about the effect of tube geometry and liquid feeder height on the heat transfer performance of falling film evaporation over the horizontal heated plain tubes. To investigate this, a two-dimensional computational fluid dynamics (CFD) model was developed, compared, and validated with published data available in the literature. A numerical simulation was carried out for varying liquid load, tube diameter, liquid feeder height, and corresponding changes in the heat transfer co-efficient (HTC), and mass transfer rate was recorded and analyzed. An attempt was also made to measure the thickness of the film around the tubes from the simulation model. Mechanisms that control the factors such as HTC, film thickness, and mass transfer were numerically investigated and discussed in this work. Numerical results indicated that low value of liquid film thickness appears approximately at the angular position of the range between 90 deg and 125 deg. Also the numerical investigation revealed that liquid film thickness decreases and HTC and mass transfer rate increases with the increase of feeder height. No remarkable change in film thickness was observed with increase in the tube diameter. This numerical study also proved that the prediction of thermally developed boundary region on the circumference of the tube could be possible in terms of mass transfer rate. It was also observed from the numerical study that the highest mass transfer rate takes place between the angle 135–165 deg near to the bottom of the tube.