Falling Film Evaporation Research Articles

How surface modifications enhance vertical falling film evaporation

How surface modifications enhance vertical falling film evaporation

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.

A numerical investigation on the dynamics of particle deposition and fouling on a vertical falling film pipe

The zero liquid discharge process is designed to eliminate the discharge of industrial wastewater into the environment, addressing significant environmental concerns that have emerged in recent decades. Falling film evaporator is one of the primary technologies employed in these systems. As observed, there is a noticeable lack of research numerically simulating these phenomena in a vertical pipe including falling film flow. In the present study, an effort has been made to address the existing research gap by conducting numerical simulation of particles deposition and fouling phenomena in a falling film flow in a three-dimensional vertical pipe using computational fluid dynamics. Continuous phases which contain two non-penetrating fluids, i.e., air and water, are simulated using volume of fluid multiphase model and the large eddy simulation turbulence model. Meanwhile, particles motion has been tracked utilizing discrete phase model. Water film enters the pipe with a thickness of 3.7 mm and an inlet velocity of 1 m/s, and particles with a density of 3110 kg/m3 are injected through the water inlet surface in various cases, each with different diameters. As observed, with an increase in particles diameter from 5 to 15 μm, the asymptotic total fouling mass has increased over 14 times, reaching from 0.016 to 0.18 kg/m2, and the total number of deposited particles has also increased. By increasing the fluids inlet velocity from 1 to 1.5 m/s, the total number of deposited particles has decreased throughout the pipe wall, and the total fouling mass has experienced a reduction of 55.8 %.

Experimental study on the effect of tube surface modifications and in-channel baffle plates on falling film evaporator heat transfer properties for sever cabinets

To effectively alleviate the high energy consumption associated with traditional cooling strategies in data centers, this research synoptically designs an innovative and energy-efficient integrated structure combining a falling film evaporator with a cabinet. An experimental setup with a water film tube in a rectangular channel is constructed, and the impacts of liquid film temperature, spray density, and inlet air Reynolds number on heat transfer properties in the cases of the tube's surface structure modifications and baffle plates presence in the channel are explored, respectively. Two significant findings that enable low energy consumption-oriented cooling strategy development are derived. (1) Decreasing the spray density can reduce energy consumption when liquid film integrity is maintained. Upon quadrupling the inlet spray density for the smooth and finned tubes, the heat transfer rate and heat transfer coefficient between the tubes and airflow in the baffled and unbaffled channels only increase by 23.1 to 29.9% and 18.6 to 28.3%, respectively. (2) The tube surface modifications and baffles presence can significantly improve the heat transfer performance. The performance evaluation criterion between the airflow and finned tube in the channel with baffles is 2.5–4.0 times higher than those between the airflow and smooth tube in the baffle-free channel.

R-1234ze(E) 냉매를 이용한 유하액막식 증발기 성능 실험 연구

R-1234ze(E) 냉매를 이용한 유하액막식 증발기 성능 실험 연구

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.

Physics and correlations informed deep learning to foresee various regimes of the pool boiling curve

Pool boiling curve is the key feature for performance evaluation of phase-change equipment such as Kettle reboiler, flooded evaporator, falling film evaporator, immersion cooling, and the like. The performance evaluation typically relies on empirical correlations or some semi-analytically based theoretical approaches. However, it has not been possible to establish a good relationship between heat flux and temperature until now. The pool boiling curve of smooth and roughened surfaces, including the nucleate boiling region, critical heat flux (CHF), and transition boiling, have been precisely predicted for the first time using a generalized physics-based deep learning (DL) model that considers physics and correlation-based features. To predict the higher order pool boiling curve for a specific liquid-surface combination, the model is fed the initial experimental data from the lower order boiling curve in addition to the most important variables of the surface morphology, thermophysical properties of the working fluid, and pool boiling conditions. Various regimes of pool boiling curve, such as fully developed nucleate boiling, CHF, and transition boiling for the investigated liquid-surface combinations and testing conditions, are predicted with an accuracy of R2 (correlation coefficient) = 0.98. The reason for such a high accuracy to a wide range of surface morphologies, substrate materials, operating pressures, heater inclination angles, and working fluids is due to the inclusion of the initial pool boiling data along with the relevant, impactful surface, and liquid parameters into its framework.

Numerical simulation of the liquid film thickness distribution of horizontal tubes under cross-airflow

During the working process of the falling film evaporator, the airflow generated by evaporation process affects the falling film flow process. A three-dimensional model consisting of two horizontal tubes was conducted, which only considered the flow of one liquid column unit. The effect of cross-airflow on the liquid film thickness distribution of tube 1 and tube 2 in the jet mode was studied. The gas is air, the liquid is pure water, and the flow of gas and liquid begins simultaneously. The fluctuation and thickness distribution of the liquid film were analyzed. The research results indicate that liquid film fluctuation occur at the moment when the fluid falls from the first tube to the second tube, and continue to exist thereafter. When the airflow is applied, at l*=0.5, for the tube 1, the liquid film thickness from θ=45° to 90° areas is relatively flat; At l*=1, for the tube 2, the smallest liquid film thickness is located near θ=90° and 270°. The airflow has a certain inhibitory effect on the liquid film waviness on the windward side. The main reason why the liquid film thickness on the leeward side is greater than that on the windward side is due to the offset of the liquid column. The stability of the liquid column, which means that the liquid column does not offset, can ensure the stability of the falling film flow process.

Effect of oil concentration, viscosity, and surface tension on tube temperature and heat transfer coefficients of R1234ze(E)/oil spray evaporation on an enhanced tube bundle

Shell-and-tube evaporators are widely used in desalination, refrigeration, and petrochemical industries. The heat transfer process in spray-type evaporators combines the droplets impingement at the top rows of the bundles and falling film evaporation for the bottom rows, with a transitional region in the center of the bundle. Spray evaporators are a promising alternative to flooded-type shell and tube heat exchangers because they run with significantly lower refrigerant charge amounts.This study presents new data for the spray evaporation of low-GWP refrigerant/oil mixtures on an enhanced tube bundle. The experiments involved R134a (HFC) and R1234ze(E) (HFO) paired with synthetic Polyolester (POE68 and POE220) lubricants. The tests utilized a boiling enhanced tube (re-entrant cavity tube) and were conducted at saturation temperatures of −6.7 °C, 2.2 °C and 10 °C (20°F, 36°F and 50°F), with heat flux varying from 3 to 35 kW/m2 and oil circulation ratio (OCR) ranging from 0.4 % to 2 %. With the addition of oil, the bundle heat transfer coefficient (HTC) of refrigerant/oil mixtures decreased by 20–50 % at an OCR of 1.2 % and a heat flux of 13.5 kW/m2, across the tested saturation temperatures. The average wall superheat across the bundle exhibited at least a 48 % increase for all the refrigerant/oil mixtures tested at an OCR of 1.2 %. The viscosity of the refrigerant/oil mixture showed a limited influence on heat transfer compared to surface tension, mainly because the oil concentrations were quite small. Oil promoted a partial dry-out of the tubes, especially at the bottom of the bundle. Foaming was observed and increased with heat flux and oil concentration. Foam sheets on the tube bundle created an additional barrier for the liquid refrigerant to reach the tube heat transfer surfaces and thus decreased the overall bundle heat transfer coefficient.

Falling film flow and heat transfer on a horizontal tube under the shearing of vertical vapor

Falling film evaporators are commonly employed in a wide range of industrial applications, with a particular emphasis on the management of vapor generated during operations. It is widely recognized that vapor streams have a significant influence on falling film flow and heat transfer. In this paper, we conducted numerical investigations of this issue in the presence of vertical vapor streams and validated the present numerical results against previous experimental data. The results indicate that the length of the liquid tongues and the interaction time between two liquid films are affected by both downward and upward vapor streams. The effect of vertical vapor on the film thickness is dependent on the vapor’s orientation and position on the tube. Typically, is decreased by 25 % at z* = 0 and vg = 4.0 ms−1 and is increased by 20 % at z* = 0 and vg = 4.0 ms−1. The downward vapor enhances heat transfer on the top half of the tube’s periphery while weakening it on the bottom half, while the upward vapor contributes to heat transfer enhancement mainly in the detachment zone. Overall, the vertical vapor stream causes substantial redistributions of the heat transfer coefficient, velocity vector, and liquid film thickness in three distinct zones. Downward and upward vapor streams predominantly impact the upper and lower parts of the tube, respectively. Within the current velocity range, it is evident that vertical vapor streams consistently lead to an average enhancement in heat transfer.

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.

The simulation of droplet impact on liquid film evaporation in horizontal falling film evaporator based on smoothed particle hydrodynamics

PurposeIn this paper, the complex flow evaporation process of droplet impact on the liquid film in a horizontal falling film evaporator is numerically studied based on smoothed particle hydrodynamics (SPH) method. The purpose of this paper is to present the mechanism of the water treatment problem of the falling film evaporation for the high salinity mine water in Xinjiang region of China.Design/methodology/approachTo effectively characterize the phase transition problem, the particle splitting and merging techniques are introduced. And the particle absorbing layer is proposed to improve the nonphysical aggregation phenomenon caused by the continuous splitting of gas phase particles. The multiresolution model and the artificial viscosity are adopted.FindingsThe SPH model is validated qualitatively with experiment results and then applied to the evaporation of the droplet impact on the liquid film. It is shown that the larger single droplet initial velocity and the smaller single droplet initial temperature difference between the droplet and liquid film improve the liquid film evaporation. The heat transfer effect of a single droplet is preferable to that of multiple droplets.Originality/valueA multiphase SPH model for evaporation after the droplet impact on the liquid film is developed and validated. The effects of different factors on liquid film evaporation, including single droplet initial velocity, single droplet initial temperature and multiple droplets are investigated.

The effect of turbulence on the minimum thickness of a liquid film flowing down a vertical tube

When a liquid flows down a vertical tube at a low flowrate, it may not be able to completely cover the surface as a film, and the flow will comprise a number of rivulets separated by unwetted areas. Predictions of the minimum film thickness and corresponding minimum flowrate below which films cannot be sustained are needed in many industrial heat and mass transfer operations. In this paper a model is developed using the minimum energy method to calculate the minimum film thicknesses that can be maintained for flows down a vertical tube. It is concluded that inclusion of the effect of turbulence in the model can significantly increase the values of the calculated minimum film thicknesses. In these calculations (for 100⁰C water flowing down a 6 mm inside diameter tube) the effect of turbulence increased as the tube to liquid contact angle increased. For a flow at the highest contact angle of 90⁰, the minimum film thickness increased by 22% above the laminar value and the corresponding minimum flowrate increased by 75%. The model presented in this paper can be used to calculate the thermal performance of falling film evaporators over the whole range of flowrate.

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.

Machine learning-based Nusselt number prediction for falling-film evaporators in absorption refrigeration systems

The evaporator serves as a pivotal component of an absorption refrigeration system (ARS), bolstered by improved wettability and mitigation of the irregular arrangement of the liquid film. The complexities of the evaporation heat transfer characteristics of falling films have led to numerous empirical correlations for falling-film Nusselt number (Nu) estimation, demanding substantial computational resources and experimental expenses and limiting their universal applicability. This study used machine learning to predict Nu of a falling-film evaporator. Artificial neural network (ANN) and random forest regression (RFR) models were used to predict heat transfer characteristics and establish a novel Nu correlation using an experimentally derived dataset. Correlations of falling-film Nu were established for six tube types using empirical data, ANN, and RFR predictions and then compared based on experimental falling-film Nu. The ANN and RFR provided high prediction performance within 10 % error and an R2 of 0.985 and 0.999, respectively. The Nu correlations derived from ANN and RFR demonstrated satisfactory agreement, typically within ±15 % error of experimental results lower than that of the empirical correlations. The optimized ANN and RFR demonstrates their promising ability to predict heat transfer characteristics and establish Nu correlations, thereby reducing the need for immense experimental efforts.

Dynamic modeling of a centrifugal chiller with hybrid falling film evaporator including pressure drop computation

Over the last decade, the introduction of new technologies such as falling film evaporators and advanced control techniques has significantly enhanced chiller efficiency while reducing refrigerant charges. However, the complexity of the involved transport phenomena poses significant challenges in simulating the behavior of these devices, resulting in very few validated models that encompass such advances. This study introduces a novel dynamic model for centrifugal liquid chillers with a hybrid evaporator (one that combines features of falling film and flooded designs). Employing the moving boundary method, the model incorporates the intricacies of the transport phenomena, including the pressure drop across the tube bank, with relatively low computational requirements. Implemented in OpenModelica, the model was validated using performance data from a 550 TR centrifugal chiller, yielding normalized residual errors of less than 0.5% for values of supply water temperature and power consumption. This provides a comprehensive tool suitable for analyzing chiller performance as a function of the evaporator geometric characteristics and enabling optimal dynamic control of central cooling systems incorporating this kind of chillers, as long as contributes to the development of dynamic and steady state models of vapor compression centrifugal chillers with falling film evaporators.

Convective heat transfer of falling film around the horizontal half-oval tube with reverse airflow in an evaporative condenser

Horizontal falling-film evaporation is frequently used in industrial air conditioning system to improve its thermal performance. Transient heat transfer characteristics on the surface of a circular tube have been investigated, while the non-circular tube such as half-oval tube has been rarely reported. This study aims to investigate the thermal performance of liquid film around a half-oval tube by adopting transient simulations with laminar and SST k-ω models, respectively. Simulations consider varying wind velocity, liquid Reynolds number, heat flux and length-width ratio of the tube. Simulated results are quantitively validated by comparing with reported experimental results, and the average differences are below 9.5 %. Results indicate that the local convective heat transfer coefficient (CHTC) increases with wind and the average CHTC is at least 5.98 % higher than the case without wind. Although the influences of liquid Reynolds number and heat flux show slight differences, they still can improve the convective heat transfer. The local CHTC exhibits an escalating trend with length-width ratio of 1–5. Meanwhile, the average CHTC for the four tested half-oval tubes is at least 6.8 % higher than the values of the circular tube. An empirical correlation for predicating the average CHTC is established by regression fitting with a mean deviation of 4.6 %. These findings help to design the falling-film evaporation system by adopting the half-oval tube in industrial air conditioning system.

Intensification of evaporative precipitation of lignin in a spinning disc evaporator

A continuous process for the evaporative precipitation of lignin derived from Organosolv black liquor was successfully developed and implemented in a spinning disc evaporator (SDE). A maximum of 95 % of the lignin was recovered at a disc speed of 1200 rpm, black liquor feed flow rate of 1 ml s−1 on a smooth disc surface heated to 100 ⁰C. These optimal flow rate and disc speed values corresponded to conditions which maximised residence time to <1 s in 4 disc passes without compromising on the rate of evaporative heat transfer. The lignin precipitate did not have any detectable impurities present after being washed and dried. The particle size for the lignin precipitate was in the region of 3 – 9 µm with minor agglomeration occurring, providing good filterability. A scaled up version of the process was modelled for benchmarking purposes and it was found that the SDE had a better theoretical performance than a falling film evaporator (FFE) system developed for the same process, with the energy consumption reduced by 23% and a higher lignin recovery rate per volume of black liquid processed once adjusted for processing time (38.1 g L−1 h−1 for the SDE vs. 3.23 g L−1 h−1 for the FFE).

Investigation of the inlet useful temperature difference effect on the distribution of hydrodynamic characteristics of the vapor-liquid flow inside the falling-film evaporator heating tube designed for a nitric acid solutions evaporation

We present the results of mathematical modeling of effect of inlet useful temperature difference on the vapor- liquid flow in the heat exchange tube of a vertical evaporating film apparatus, which is designed for evaporation of nitric acid solutions. Distributions obtained along the heat exchange tube length of local values of the: mass flow rate of the fluid and secondary steam, the velocity of the fluid film, the velocity of the secondary steam, the Reynolds criterion of the liquid and vapor phases, the absolute pressure depending on the values of this useful temperature difference. The study was carried out using numerical simulation. To study the effect of the initial useful temperature difference on the hydrodynamics of the vapor-liquid flow, the vertical evaporator equipped with the tube ø38x3 mm and length of L = 4 m was adopted. The linear nature of the distribution of hydrodynamic parameters along the length of the heat exchange tube is established. The absolute pressure in the tube space decreases along the heating tube due to pressure drop. The limiting factor in reducing the absolute pressure is the friction losses of the secondary vapor on the solution film surface. A decrease in the pressure in the tube space leads to decrease in the boiling point of the solution. This leads to increase useful temperature. This has a positive effect on the evaporation efficiency The obtained data should be used in the design of evaporation plants, which are designed for evaporation of nitric acid solutions.