Membrane distillation is a novel membrane-based separation technology with the potential to produce pure water from high-salinity brine. It couples transport behaviors along the membrane and across ...the membrane. The brine in the feed is gradually concentrated due to the permeate flux across the membrane, which is a significant factor in initiating the scaling behavior on the membrane surface along the feed flow direction. It is of great interest to investigate and estimate the development of scaling on the membrane surface. This work specifically focuses on a long-distance membrane distillation process with a sodium chloride solution as the feed. A modeling approach has been developed to estimate the sodium chloride scaling development on the membrane surface along the flow direction. A set of experiments was conducted to validate the results. Based on mathematical simplification and analytical fitting, a simplified model was summarized to predict the initiating position of sodium chloride scaling on the membrane, which is meaningful for scaling control in industrial-scale applications of membrane distillation.
Scholars usually ignore the non-equilibrium condensing effects in turbulence-model comparative studies on supersonic steam ejectors. In this study, a non-equilibrium condensation model considering ...real physical properties was coupled respectively with seven turbulence models. They are the k-ε Standard, k-ε RNG, k-ε Realizable, k-ω Standard, k-ω SST, Transition SST, and Linear Reynolds Stress Model. Simulation results were compared with the experiment results globally and locally. The complex flow phenomena in the steam ejector captured by different models, including shock waves, choking, non-equilibrium condensation, boundary layer separation, and vortices were discussed. The reasons for the differences in simulation results were explained and compared. The relationship between ejector performance and local flow phenomena was illustrated. The novelty lies in the conclusions that consider the non-equilibrium condensing effects. Results show that the number and type of shock waves predicted by different turbulence models are different. Non-equilibrium condensation and boundary layer separation regions obtained by various turbulence models are different. Comparing the ejector performance and the complex flow phenomena with the experimental results, the k-ω SST model is proposed to simulate supersonic steam ejectors.
It has been identified that temperature polarization and concentration polarization are typical near-surface phenomena limiting the performance of membrane distillation. The module design should ...allow for effective flow, reducing the polarization effects near the membrane surfaces and avoiding high hydrostatic pressure drops across and along the membrane surfaces. A potential route to enhancing the membrane distillation performance is geometry modification on the flow channel by employing baffles as vortex generators, reducing the polarization effects. In this work, various baffles with different structures were fabricated by 3D printing and attached to the feed flow channel shell in an air gap membrane distillation module. The hydrodynamic characteristics of the modified flow channels were systematically investigated via computational fluid dynamics simulations with various conditions. The membrane distillation tests show that adding the baffles to the feed channel can effectively increase the transmembrane flux. The transmembrane flux with rectangular baffles and shield-shaped baffles increases by 21.8% and 28.1% at the feed temperature of 70 °C. Moreover, the shield-shaped baffles in the flow channel not only enhance the transmembrane flux but also maintain a low-pressure drop, making it even more significant.
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•We performed experiments concerning a single drop impact on wetted spheres.•The sphere-drop curvature ratio can greatly influence the splashing thresholds.•The spreading factor can ...be increased by decreasing the curvature ratio.•Weber number influences on the spreading factor are minor.•A linear model for the spreading factor is proposed.
Numerous experiments were performed to investigate a heptane drop impact dynamics on wetted spheres using a high speed camera. Outcomes after impact include spreading at a low impact Weber number and splashing at a high value. Limits between the two outcomes can be greatly affected by the sphere-drop curvature ratio ranging in 0.090–0.448. Additionally, the spreading process on wetted spherical surfaces is discussed in detail. The spreading factor defined as the ratio between the spreading area and the drop surface area can be increased by increasing the curvature ratio or by reducing liquid viscosity, while the effect of the increment in the Weber number is minor. It is found that the spreading factor follows a linear law with dimensionless time, which is confirmed by the butanol drop spreading as well. Finally, concerning different curvature ratios and fluids, many coefficients with respect to the linear law are obtained to predict the spreading scale by regressing the experimental data.
•Experimental tests address the pure water static flash evaporation phenomenon.•The impact of static pressure distribution in the liquid film is analyzed.•The minimum liquid level will be changed ...above the 14 K superheated during the flash evaporation.•The interface temperature declined and then increased during the mild evaporation.
An experimental investigation on thermodynamic phenomena of liquid film flash evaporation is presented. During experiments the pure waterfilm is flashed at an initial temperature range of 60–85 °C, an initial liquid film height range of 3–20 mm, and a superheat range of 2–22 K. The results show that the enthalpy difference is the dominant parameter for flashing phenomena and process. The static pressure distribution in the liquid film has an obvious impact on flashing evaporation process since the water static pressure can cause the change of saturation temperature. The liquid film height during the flash can be reached many times of the initial heightand it represents the intensity of the flashing. The flashing evaporation can be stepped more rapidly to the mild evaporation due to the lower water film. The NEF(non-equilibrium fraction) and the intensity of the flashing increased with the height of water film.
•Main factors affecting the boiling point rise of salt solutions are discussed.•Experimental measurement of BPE in single-salt and multi-component salt solutions.•Using ion concentration to assess ...BPE reduces the impact of electrolyte ionization.•Examined the impact of ion types on the boiling point rise of salt solutions.•The relationship between BPE and temperature and ion concentration is obtained.
This paper applies thermodynamics theory to analyze the main factors that affect the boiling point elevation (BPE) of salt solutions. By using the vapor pressure measurement method, boiling point experiments were conducted on several common inorganic salts used in industrial wastewater treatment and desalination at different concentrations and temperatures. The experimental data shows that the BPE of a salt solution is approximately a quadratic function of the solution temperature at constant concentration, and the BPE of tri-ion salt solutions tends to be higher than that of NaCl solution at the same concentration. Based on theoretical analysis and a summary of experimental results, it was found that BPE is not only related to solute concentration but also to the number of ionized solute ions. It was also found that replacing the compound concentration with the ion concentration as the independent variable of concentration can reduce the impact of electrolyte ionization and reduce variable parameters. Furthermore, the impact of ion types on BPE was studied in mixed salt solutions. The results show that the higher the average ion activity coefficient of the solute, the higher the boiling point of the solution. Through nonlinear fitting analysis of experimental data, a correlation for calculating the BPE of multi-component salt solutions in industrial water treatment is proposed based on solution temperature and ion concentration.
The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature ...difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.
•An experimental system was built up to measure the outer circumferential distributions of local heat transfer coefficient of horizontal tube falling film flow.•The local heat transfer coefficient ...along the angle were measured.•The effects of spray density, tube spacing, heat flux and fluid temperature on local heat transfer coefficient were investigated.•New correlations of local heat transfer coefficient of falling film flow outside a horizontal tube were proposed.
An experimental system was built up to measure the outer circumferential distributions of the local heat transfer coefficient of horizontal tube falling film flow. The wall temperature along the angle were measured. Based on the experimental data, the local heat flux distribution on the tube surface were obtained by numerical calculation. The local heat transfer coefficient distribution outside the tube were obtained. The effects of spray density, tube spacing, average heat flux and fluid temperature on local heat transfer coefficient were investigated. The results show that the local heat transfer coefficient decreases sharply with the increase of angle, then decreases steadily, and finally increases slightly. According to the distribution of the local heat transfer coefficient and heat transfer mechanisms, the tube surface is divided into the impingement zone, the heat diffusion transfer zone and the tail detachment zone. The increase of spray density has little influence on the heat transfer in the impact zone, but make the heat transfer increases slightly in the heat diffusion transfer zone and the tail detachment zone. The influence of tube spacing is mainly concentrated in the impingement zone. The heat transfer is enhanced with the increase of tube spacing in the impingement zone. The average heat flux has no obvious effect on heat transfer. With the increase of fluid temperature, the heat transfer is greatly increased all over the cycle. Besides, based on the experimental data, new correlations of local heat transfer coefficient of falling film flow outside a horizontal tube are proposed.
A set of experimental facilities were set up to measure overall heat transfer coefficient of horizontal-tube falling film evaporator with square-pitch bundle. Effect of spray density, saturation ...temperature, total temperature difference, and inlet steam velocity on the overall heat transfer coefficient K is studied. The tubes are made of HAL77-2A aluminum brass with an outer diameter of 25.4 mm. Fluids inside and outside the tubes are steam and fresh water respectively. The results indicate that growth of spray density and saturation temperature helps to increase the K. The K could also be increased by reducing the total temperature difference. However, the impact of the inlet steam velocity on the K is less significant. Furthermore tube bundle-depth effect and space distribution of local overall heat transfer coefficient K˜ in the evaporator are also presented. Based on this investigation, basic engineering design information will be provided to establish the governing parameters for horizontal-tube falling film evaporator in the field of seawater desalination.
•An open liquid distributor is designed to realize a stable deviated columnar flow.•The heat transfer distribution of deviated columnar flow is analyzed.•Influencing factors on heat transfer of ...deviated columnar flow are analyzed.
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.
