Falling Film Evaporation Research Articles

Experimental Research on Heat Transfer Performance of CO2 Tube Bundle Falling Film Evaporator

Abstract CO2 is a promising refrigerant substitute, and falling film evaporator is widely used in high-capacity heat pump systems. In this article, a CO2 heat pump experimental device is built. According to experimental parameters, a CO2 falling film heat exchanger is fabricated. The falling film heat exchanger is used in heat pump evaporator, and its heat transfer characteristics are analyzed and tested. The outer diameter of the heat exchanger is 219 mm, outer diameter of the inner tube is 15.88 mm, and total outer surface area of the inner tube is 0.58 m2. Number of internal tubes was 12, each with a length of 0.97 m. During the test, change of heat transfer coefficient of each tube was analyzed when evaporation temperature, CO2 mass flowrate, and heat flow density changed. The influence of the position of heat exchange tube inside falling film evaporator on heat transfer coefficient is analyzed. The order of heat transfer coefficient of internal heat exchange tube is obtained. Finally, database of CO2 falling film evaporator is established. Through data sorting, heat exchange correlation formula is obtained. The deviation of 90% data in database is less than 5%. Heat exchange correlation formula has good accuracy.

Scaling trend of coal chemical saline wastewater in falling film evaporator: experimental solid–liquid equilibrium and numerical simulation

Abstract It was important to establish the scaling mechanism to slow down scaling, which is one of the important problems faced by saline wastewater treatment with falling film evaporators. In this paper, the phase equilibrium data of ternary system NaCl-CaCl2-H2O at 303.15–373.15 K were measured, and the prediction of the flow characteristics and scaling tendency of salt-containing wastewater in the evaporation tube was investigated by numerical simulation based on the phase equilibrium results. The results showed that the mesostable phase diagram of the ternary system was of simple type, and w(NaCl) and w(CaCl2) showed regular variation with temperature when w(NaCl) in the system was 15–28%. Moreover, the components of the four saline wastewaters (10% NaCl + 9, 10, 11, 12% CaCl2) with less salt content had a faster heating and a faster rate of evaporation of the liquid film in the evaporation tube. Based on the solubility in the experiments and simulations, it was concluded that NaCl crystals were precipitated at tube lengths of 718 mm, 942 mm, 966 mm, and 1,000 mm for four salt concentrations in the evaporation process by comparison, and the reliability of the predicted loci was also demonstrated by a falling film evaporation experimental setup.

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.

하이브리드 유하액막 증발기 성능에 관한 수치적 연구

The purpose of this study was to develop steady-state model of hybrid falling film evaporator, and to analyze performance of the heat exchanger. R1233zd(E) which is one of the low GWP refrigerants was selected as a working fluid. The steady state model was developed, based on the heat transfer correlations from previous studies for falling film evaporation and pool boiling using R1233zd(E). Based on the analysis results according to the number of nodes, the number os axial nodes was selected as 24. Performance analysis was conducted with respect to evaporator pressure, axial direction liquid refrigerant supply ratio from tray, rows of pipe for pool boiling region, and maximum heat transfer coefficient of falling film region. For the same heat capacity of the evaporator, the length of the pipe should extend approximately 3.6 times, when evaporator operation pressure increases from 49.2 to 58.4 kPa. The length of the pipe can be reduced approximately 18% as the number of rows for pool boiling region increases from one to three. The axial direction liquid refrigerant supply ratio from the tray also affects the performance of the evaporator. Compared to the equal supply ratio case, the pipe length must increase approximately 9%, when the axial direction liquid refrigerant supply ratio is 9:1 for the same cooling capacity. As the maximum heat transfer coefficient at falling film region increases from 5000 to 11000 W/m2K,the length of the pipe can be reduced approximately 27%.

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.

A numerical investigation into a long tube falling film evaporator in a zero liquid discharge (ZLD) process

• The evaporation of a two-phase turbulent falling film was investigated. • The oscillating structure of the falling film flow was studied. • Two new Local and Oscillating Reynolds Numbers were introduced. • The oscillations frequency remain almost constant in the fully developed region. • Care should be taken to select Re Γ while designing falling film vertical tubes. Brine concentrators of the falling film type are essential components of zero liquid discharge (ZLD) processes. In the present work, the evaporation of a two-phase turbulent falling film in a long vertical tube has been investigated. Volume-Of-Fluid (VOF) method along with the PLIC interface reconstruction technique is utilized to simulate the two-phase flow and free surface geometry capturing. The k-ω SST turbulence model and the Lee-Tanasawa condensation-evaporation model have been used to model turbulence and mass transfer, respectively. Furthermore, the oscillating structure of the falling film flow is studied, and two new local and oscillating Reynolds numbers are introduced. Results showed that the falling film inside the tube has an oscillating nature and, after passing through an entrance and a developing region, the oscillations frequency remain almost constant. Additionally, the oscillating Reynolds number based on the local wavelength establishes the best correlation with the falling film evaporation rate.

Heat transfer coefficient correlations of water subatmospheric vaporization in a channel of a smooth plate heat exchanger, based on Vaschy-Buckingham theorem

Water as an environmentally friendly, safe and cheap natural refrigerant (R-718) requires vaporization conditions at sub-atmospheric pressures (∼1 kPa). At these pressure ranges, studies on the specific vaporization of water are scarce and insufficient. For this reason, the engineering of heat exchangers using R-718 as working fluid remains empirical. The aim of this paper is thus to provide correlations to estimate water pool boiling and falling film evaporation heat transfer coefficient under subatmospheric pressure. Correlations developed are believed to be more widely used by providing dimensionless numbers consistent with phase-change phenomena occurring at sub and at-mospheric pressure. To develop these correlations, heat transfer phenomena during water vaporization at pressures ranging from 0.7 to 1.7 kPa in a scale-lab size (0.2 m wide × 0.5 m high) vertical smooth plate evaporator channel are studied. Filling ratio vary from 1/2 to 1/10 of the total channel height, canal thicknesses are set to 2 mm, 4 mm or 6 mm. Thus, raw experimental data from 139 tests performed under various operating conditions of a thermosyphon loop mimicking a sorption system are exploited. Each test consists of more than 1200 values. A graphical and statistical comparison between the heat transfer coefficient (HTC) obtained from the experimental database and the HTC estimated by empirical correlations from the literature is made, calculating the percentage of predicted data within an error band of ±30% (τ30), the mean absolute error (MAE), the mean relative error (MRE) and the correlation coefficient (r). Two methodical correlations for predicting the HTC of pool boiling and liquid film evaporation, are developed. They predict 86% (with MAE = 19%, MRE = 3%, r=0.79) and 83% (with MAE = 18%, MRE = 2%, r=0.84) of the experimental data in τ30 respectively. To extend their validity domains, the proposed correlations are evaluated using independent experimental data from the literature. Giving satisfactory results, it constitutes a first step towards the development of tools used for the sizing and the optimization of the design of two-phase heat exchangers.

Numerical modeling based machine learning approach for the optimization of falling - film evaporator in thermal desalination application

Scale formation that drastically increases thermal resistance and reduces freshwater production remains a critical challenge in thermal desalination. Novel designs of falling film evaporator and optimal operating condition hold great promise to mitigate scale formation, and increase heat transfer performance and fresh water production. In this work, CFD simulation based machine learning and multi-objective optimization are performed to identify optimal conditions and tube arrangement for evaporator. Non-dominated sorting genetic algorithm is adopted to determine and analyze the optimal pareto front for multiple objectives in desalination criteria. The errors of training, validation, and testing set are computed to identify an optimal hyperparameter set. For performance ratio, fouling resistance, and water production rate, the average relative error is 2.26%, 3.67%, and 3.24%. At pareto front, both performance ratio and water production rate increase at high temperature with fouling resistance (thermal resistance of the fouling layer) increasing as well. Tradeoffs between mitigating scale formation and enhancing desalination performance are evaluated in optimizations for different objectives. Potential optima are identified and can be applied as guidelines to determine evaporator design and system operating conditions.

Falling film evaporation of desulfurization wastewater with synergistic particle removal based on the dry-wet coupling electrostatic precipitator

Low temperature flue gas evaporation is an effective method for low-cost and efficient treatment of desulfurization wastewater (DW). However, due to the influence of low flue gas temperature and high salt content, it is easy to lead to scaling and blockage of evaporation flue, affect the stable operation of the system. In order to effectively use the waste heat of flue gas to treat DW and avoid the corrosion of DW to the flue, the falling film evaporation treatment technology of DW with synergistic particle removal is proposed. The effects of different operating parameters on the falling film evaporation and deposition characteristics of DW, and particle removal efficiency are studied. The results show that the increase of the electrostatic field voltage is conducive to the evaporation of DW, and inhibits its crystal deposition. The increase of the Cl- concentration is inconducive to the evaporation and crystal deposition of DW without electrostatic field, but is conducive to the evaporation and crystal deposition of DW in the electrostatic field. In the experiment of 120 ℃ flue gas, the evaporation capacity of liquid film decreases by 17.9% with Cl- concentration increasing from 0% to 5% without the voltage; but the evaporation capacity of liquid film increases by 30.9% with Cl- concentration increasing from 0% to 5% after applying 40 kV voltage. Replacing water with DW is conducive to the removal of particles by electrostatic field. The efficiency of particle removal increases from 79.9% to 84.8% with Cl- concentration increasing from 0% to 5% at 120 ℃ and 40 kV. In addition, increase on flue gas temperature is also conducive to the DW evaporation and particles removal by electrostatic field. Compared with other wastewater treatment technologies, this technology has the advantage of low treatment cost.

설비공학 분야별 최근 연구 동향: 2021년 설비공학논문집 발표논문에 대한 종합적 고찰

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2021. The purpose of this article is to understand the status of current research issues in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Summary of this paper is follows.<BR> (1) The topics of the paper in the field of building mechanical system were the energy consumption, indoor environment analysis, building energy saving, ventilation system, renewable energy system and law of mechanical facilities.<BR> (2) The topics of the research in the field of indoor environment were the IAQ, ventilation, air cleaning, cooling/heating load, renewable energy, zero-energy building, building energy performance etc.<BR> (3) The research topics in the field of refrigeration in 2021 include robust temperature control of variable speed refrigeration system, R32 liquid-injection showcase vapor compression system, hybrid dessicant dehumidifying system, compressor torque calculation by differential pressure measurement, FAPO zeolite adsorbent, compression-adsorption hybrid refrigeration system and so on.<BR> (4) The topics of the papers published in the field of heat were heat transfer, a heat exchanger and a battery, etc. <BR> (5) The topics of the papers published in the field of heat transfer were the air flow rate inside drum of clothes dryer, the dynamic behavior and thermal performance of a solar water heater, a higher energy density storage system using sensible heat or phase change of water, factors affecting performance characteristics of falling film evaporation using R-1233zd(E) refrigerant and the underground seawater intake system in a power plant.

Experimental investigation of the adsorbents using pressure and thermal swing for adsorption and desorption

In this study an experimental investigation was conducted on a single stage falling film evaporator together with a single adsorbent bed. The evaporator produced the water vapour which was flooded into the adsorbent bed using the mechanism of the vacuum pressure swing. After the adsorbent had captured the adsorbate, which was water vapour in this study, it was then released using thermal swing to trigger the desorption of the previously captured adsorbate. The tests were performed at controlled varying pressures conditions below atmospheric pressure. The accumulative mass of the adsorbate on the adsorbent was monitored with an aid of a specially built and installed scale within the adsorbent bed. The adsorbent materials - silica gel and zeolite-that were used in this study were locally supplied by a South African vendor. It was observed that silica gel had the highest adsorption capability of 65 g of water vapour per 200 g of silica gel at controlled experimental testing conditions.

Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

Abstract As an efficient heat and mass transfer technology, falling film evaporation has attracted more and more scholars' attention. In this paper, a falling film evaporation platform was established to discuss the mechanism of two-phase flow in narrow channels. The width of the channel varied from 0.5 to 2.0 mm. We focused on the relationship between heat transfer characteristics with flow patterns, channel width, and pressure loss. Three kinds of flow patterns were observed: bubble flow, restricted bubble flow, and dry area. In channels with the same width, the initial heat flux of subcooled evaporation increased linearly with the growth of flow rate. But at the same flow rate, the initial heat flux of subcooled evaporation decreased when the channel width became larger. The heat transfer coefficient on the wall was influenced by the bubble generation frequency and bubble separation diameter. Higher bubble generation frequency led to smaller bubble separation diameter. Through mathematical analysis, it was found that the heat transfer coefficient changed with the channel size. And a boundary was observed between the strengthening and weakening status. The comparison of theoretical and experimental values showed good agreement.

Sustainable olive-oil mill wastewater treatment by distillation using a parabolic trough solar collector

The olive oil industry, although among the most important sectors in the Mediterranean basin, causes huge environmental problems due to the discharge of polluting wastewater. The treatment of olive-oil mill wastewater is therefore at the crossroads of many concerns and is a major challenge for sustainable development. This study focuses on the exploitation of a parabolic trough solar collector as a clean source of energy for sustainable olive-oil mill wastewater treatment. A conical coil heat exchanger immersed in an olive-oil mill wastewater distillation reservoir is coupled with a parabolic trough solar collector. The productivity, environmental impact, and profitability of the proposed prototype are experimentally evaluated. A comparative study and a Physico-chemical characterization of filtered and unfiltered olive-oil mill wastewater are conducted to assess their potential reuse. A simulation using a spray-type falling film evaporation approach is realized to achieve numerical optimization. The obtained results show that the use of a parabolic trough solar collector allows more heat gain to be transferred into the distiller, enhancing system productivity, and decreasing temperature fluctuation compared to the traditional solar distiller. A significant reduction of 95 % to 97 % of phenolic compounds is observed for the distillate. The overall analysis reveals that the 26.6 m2 parabolic trough solar collector aperture area has generated 19 MWh of thermal energy throughout the olive oil post-production phase, which led to the avoidance of about 12,660 kg (eq) of CO2 emissions, while the Levelized cost of heat was 0.3 $/kWhth.

Controller design for optimal operation of Multiple Effect Evaporator of paper mills

The Kraft recovery process in the paper mills is highly complex and nonlinear. The evaporation of black liquor using Multiple Effect Evaporators (MEE) in the recovery unit of the Kraft process is an energy-consuming procedure. The challenge lies in designing a proficient control strategy to conserve energy and guarantee good product quality. In this article, the 2-DOF-PI controller is designed to enable efficient control of MEE and eliminate process uncertainties. The Heptads’ Effect Falling Film Evaporator in the backward feed flow configuration is used as a working platform. The steady-state and transient behavior of MEE is modeled and analyzed to assist in controller design. The steady-state process parameters, that guarantee optimum energy efficiency, are estimated using Moth–Flame Optimization (MFO). MFO is also employed to estimate the optimal 2-DOF-PI controller parameters to attain improved product quality and energy-efficient performance of MEE. The competence of MFO towards controller tuning is shown by a fair comparison with some well-known optimization techniques. The quantitative investigation of the statistical results demonstrates that MFO outperforms the other algorithms. To validate the proficiency of the designed control scheme, its performance is compared with basic PI/PID control strategy for set-point tracking, noise suppression, and process uncertainties. The simulation results indicate that the designed MFO-tuned 2-DOF-PI controller offers efficient control action and, therefore, proves to be an appropriate control algorithm to ensure sustainable production.

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.

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.

Effects of tube shape on the distribution of film thickness and heat transfer performance in falling film evaporation

Effects of tube shape on the distribution of film thickness and heat transfer performance in falling film evaporation

Experimental study of the falling film evaporative cooling on horizontal tubes plates

Evaporative cooling equipment has advantages of energy and water conservation and compact structure, is widely used in various industrial sectors. To better characterize the heat transfer capacity of the evaporative cooling process, the falling-film evaporation heat transfer process outside horizontally closed arranged tubes (horizontal tubes plate) was analyzed. A theoretical model was established to divide the total heat transferred by the tube wall into two parts: the convective heat transfer of the air and that of the liquid film. The latter was divided into three parts: the heat transferred by the convective heat transfer between the liquid film and the air, the mass and latent heat transfer between the liquid film and the air, and the heat absorbed by the rise in temperature of the liquid film. Experiments were performed with both smooth and ribbed tubes. Correlations of the heat transfer coefficients between the tube wall surface and the air and between the tube wall surface and the liquid film and the coverage rate of the liquid film were obtained. The experimental results indicated that using ribbed tubes effectively enhanced the liquid film coverage on the tubes, resulting in a maximum reinforcement of 122.73% compared with that of smooth tubes. Moreover, the experimental results verified the correlation equations, with the deviation between the calculated and experimental results was within ±10%.

Effect of fouling on falling film evaporator performance in industrial conditions of fruit juice concentrate production

In food processing, falling film evaporators are commonly used and their energy consumption is a significant operating cost for the production of juice concentrate. This cost is influenced not only by the evaporator design but also by the method of operation, particularly the fouling of the heat exchanger walls formed during the operation of the evaporator on the inner surface of the tubes from the juice side. These deposits are a significant component of the total wall thermal resistance, thus hindering evaporation. This issue results in a production decrease and requires evaporator cleaning. The publication presents an analysis of the calculation methods of fouling formation and their thermal resistance based on a literature review on evaporators for the production of sugar and milk. The obtained results were then processed by adjusting the coefficients to the experimental results obtained during the actual production of fruit juice concentrate. The decrease in actual evaporator efficiency was used to experimentally verify the theoretical calculations. This analysis resulted in a final simple formula for the estimation of the fouling influence on evaporator performance under industrial conditions. Simulations have shown that a sharp drop in evaporator efficiency may occur with insufficient recirculation or prolonged operation without cleaning. In this case, the amount of sediment can increase and cause a change in the boiling character and thus a rapid decrease in evaporation. • Effect of fouling on falling film evaporator for fruit juice concentrate production is estimated. • Fouling build-up speed on the inner surface of the tubes for fruit juice concentrate production is estimated. • Industrial fouling conditions in the production of fruit juice concentrate are analysed.