Mixing Sections Research Articles

The effect of swirling shear blade on gas-liquid flow pattern and mass transfer performance in Venturi injector

To address the challenge of low mass transfer efficiency of gas-liquid reaction in traditional reactors, this study proposes a new method to improve the gas-liquid mass transfer efficiency by integrating swirl shear blades (SSB) into Venturi injectors. The gas-liquid flow patterns in the mixing and diffusion sections between the Swirl Venturi Injector (SVI) and the Traditional Venturi Injector (TVI) was compared, and it was found that the SSB significantly reduced the proportion of annular flow while increased the proportion of mist flow. The enhancement factor αE and the acceleration factor αA are applied to evaluate the mass transfer performance. Compared to TVI, the maximum αA of SVI was 29%, and the maximum αE was 43%. This work shows great potential in efficiently capturing gas components with low solubility in the liquid phase, thereby helping to achieve the goal of green chemical engineering.

A multi-stage enhanced flocculation reactor for the treatment of simulated shale gas hydraulic fracturing flowback fluid: Effect of aspect ratios for the intense mixing section

Flocculation is a significant process in treating shale gas hydraulic fracturing flowback fluids. To achieve a high removal rate of pollutant, a range of multi-stage enhanced flocculation reactors that incorporated sequential intense, moderate, and weak mixing sections along the flow direction were devised, in which the aspect ratios (ARs) in the intense mixing section were varied. The results of the computer simulation indicated that the reactor with AR of 3.5 exhibited a gradual decrease in velocity and energy along the flow direction, and the flow field distribution is highly uniform and symmetrical. The generated flow field promoted the formation of small flocs in the intense mixing section and facilitated the collision of small flocs in the moderate mixing section, while preventing the fragmentation of flocs in the weak mixing section, therefore resulting in the generation of large and compact flocs. Moreover, the generated flocs demonstrated a good resistance to shear and oscillation, facilitating their separation from the simulated flowback fluids. The obtained reactor achieved remarkable removal efficiencies of 80.15 % for COD and 89.06 % for turbidity. Furthermore, when treating bentonite suspension, the multi-stage flocculation reactor achieved a turbidity removal rate of 99.34 % with a processing time of only 2.38 min and a settling time of just 20 min. This demonstrated the broad applicability of this reactor to various types of wastewater. This work provides valuable insights into the design of efficient flocculation reactors by integrating multiple mixing intensities within a reactor.

Analysis of loss of cooling accidents in the spent fuel pools using system thermal hydraulics and computational fluid dynamic codes

The interest in the safety of on-site storage of Spent Fuel Assemblies (SFAs) has been increased after the accident on 11 March 2011, in Fukushima Daiichi Nuclear Power Plant. It is important to analyse the behaviour of not only integral effects in spent fuel pools, but also the local effects during heat removal from the SFAs. Usually, for the integral analysis the one-dimensional system computer codes were used. These codes allow to analyse whole pools and cover long time, with a relatively low cost of computational resources. The Computational Fluid Dynamic (CFD) codes could perform very detailed analysis, but this requires large computational resources. The strong side of CFD codes is the possibility to assess local effects which could occur.In the current work the analysis of heat transfer from the SFAs, placed in special baskets was performed using ANSYS FLUENT 17.2 and RELAP5 codes. The models of single basket with SFAs were developed using both codes. The comparison of calculations allowed to evaluate capabilities of both codes. It is shown, that using one-dimensional system thermal hydraulics code developed model allows to describe coolant mixing inside basket sections and received results fits well with CFD code calculation results. Based on gained experience, the loss of cooling accident in the spent fuel pools of Ignalina NPP was analysed using the model, developed by employing RELAP5 (one-dimensional system thermal hydraulics) code. The calculation results were used for safety evaluation of Ignalina NPP facilities.

Coupling of Liquid and Surface Chemistry in Urea SCR Systems

Close-coupled selective catalytic reduction (SCR) systems are one method to deal with tightening emission legislation for NOx in internal combustion engines. Due to smaller mixing sections and at unfavourable boundary conditions, however, urea-water solution (UWS) droplets can impact on the SCR catalyst itself. To investigate this phenomenon further, this work develops a modeling capability of this process. Established mechanism for NH3-SCR and HNCO hydrolysis from literature is integrated into DETCHEMCHANNEL and a 2D COMSOL model to simulate the influence in the SCR Channel. Simulations are validated against end-of-pipe experiments from literature and spatially resolved concentration profiles from a hot gas test rig with very good agreement. Finally, a channel simulation is coupled with a model to describe the catalytic decomposition of an urea droplet. The coupled simulation is able to simulate the influence of UWS droplet impact onto a catalyst channel. Fast droplet decomposition causes a peak in NH3 and HNCO in the single channel and thus increases NOx conversion. However, the overall uniformity and efficiency are decreased, which is why droplet impact on the catalyst should be strictly avoided.

Entropy Analysis of Energy Losses in Heat-Exchanger and Mixing Sections of a Central Air Conditioner

Entropy Analysis of Energy Losses in Heat-Exchanger and Mixing Sections of a Central Air Conditioner

Deposition and decomposition of urea and its by-products on TiO2 and VWT-SCR catalysts

Exhaust gas aftertreatment devices such as selective catalytic reduction (SCR) catalysts are often positioned close to the combustion engine today. These close-coupled systems lead to rather short mixing sections of the reducing agent urea–water solution (UWS) and the exhaust gas in the case of SCR of nitrogen oxides (NOx) by ammonia. Due to higher temperatures, an increased UWS droplet evaporation is expected. However, startup procedures and unfavorable operating conditions can lead to a stronger impact onto exhaust pipes and the catalysts itself. The interaction of urea and its by-products with TiO2 and V2O5–WO3/TiO2 (VWT), as typical SCR catalyst materials, is investigated via thermogravimetric analysis. A surface reaction mechanism with associated kinetic expressions is developed. Complete decomposition of all deposited species occurs at temperatures as low as 450 °C. This behavior can be well represented by the model developed.

Гидродинамика абсорбционного барботажного аппарата

The article proposes an improved design of the energy-saving, compact absorption bubbling apparatus, which cleans the particles and gas mixtures in the exhaust gases of industrial enterprises, has a high absorption efficiency. As a result of theoretical research, an equation has been proposed that calculates the value of the height of the gas cushion “h”, which provides equal distribution of purified gas to the mixing sections of the apparatus and operation in a stable hydrodynamic mode. As a result, depending on this value, it is possible to calculate the gas velocities and the gas consumption supplied to the apparatus.

Design, manufacture, and testing of an innovative ridging device for controlling of wind erosion

Wind erosion is a serious problem in the world, especially in Iran. Several studies have used windbreaks and barriers to reduce wind erosion and wind-induced dust emissions; however, there is no study on the development of the ridge with a device. In this study, a new machine was designed and manufactured to create a ridge by mixing soil with biopolymer. The device was developed in three sections (machine, mixing, and ridge maker sections). Mechanisms that control movements are chain drives, brakes, and clutches that enable the device to operate in different topological regions. As part of a test phase of the device, three levels of polymer suspension (10, 20, and 30%) were added to the soil and a ridge was made by the ridging device. These ridges were created in one-hectare flat agricultural land. The ridging device efficiency was tested in 48 h field measurements during September 2018, where three measurements, including penetration resistance, aggregate stability, and ridge height and width were carried out. The results showed 1.5 kg cm−2 penetration resistance in the ridge using 30% volumetric suspension. The highest ridge was 43 cm with 30% suspension. Aggregate stability values of the ridges at 30% and 20% volumetric suspensions were 0.88 and 0.78, respectively. Overall, the results showed that creating a ridge with an optimum height could reduce wind velocity, soil transportation, and limit saltation in the ridge windward and leeward zones.

Pressure Elevation of High-Performance Steam-Water Condensing-Injector

The steam-water condensing injector (SI) is a device capable of producing high-pressure subcooled liquid streams by combining steam and subcooled liquid jet at a higher pressure than the inlet streams. The present study investigated the SI's pressure elevation mechanism using supersonic steam and a subcooled water jet. The experiment was carried out using central-water jet type SI equipped with an overflow port at the inlet steam pressures of 0.3 to 0.7 MPa and inlet liquid mass flow rates of 0.4 to 0.7 kg/s. Axial pressure and temperature distributions were measured at the mixing nozzle and diffuser sections to study the pressure elevation mechanism. Based on the analytical foundation laid by Grolmes (1968), the one-dimensional analytical model was implemented by treating SI's two-phase flow regimes as inverted annular and dispersed flows. For the current SI geometry and operating conditions, the present one-dimensional model could predict the axial pressure distribution of the SI and the maximum discharging pressure with reasonable accuracy for sufficiently high inlet flowrate ratio conditions.

Study on effect of geometry on performance of pulse detonation rocket engines

In order to investigate the effect of the geometry on the performance of pulse detonation rocket engines under valveless self-adaptive working mode, the multi-cycle experiment was carried out on the pulse detonation rocket engines with different mixing sections and obstacles, while the operating frequency was 20 Hz. Gasoline was utilized as fuel, and oxygen-enriched air as oxidizer. Experimental results indicate that the DDT distance and DDT time of PDRE can be shortened by using the mixing section of gradual-expansion structure. The effect of the spiraling grooves and annular grooves on the DDT process is weak. The mixture-based specific impulse of the mixing section with gradual-expansion structure is 4.2%-7.2% lower than that of the mixing section with sudden-expansion structure. The mixture-based specific impulse of PDRE with sudden-expansion structure and Shchelkin spirals is the largest, about 116.4 s.

Steam ejector performance considering phase transition for multi-effect distillation with thermal vapour compression (MED-TVC) desalination system

The multi-effect distillation with thermal vapour compression (MED-TVC) desalination system is efficient to produce freshwater. The steam ejector performance is not fully understood as the phase transition has been ignored in many studies. The present work develops a two-phase condensing flow model to assess the steam ejector performance considering nonequilibrium condensation phenomena. The transition of the flow structure from an under-expanded flow to an over-expanded flow in the steam ejector is investigated in detail. We present that the maximum Mach number can reach 4.02 in the under-expanded flow, which is weakened to 2.88 in the over-expanded flow. The steam undergoes several expansion-compression processes in the steam ejector in the under-expanded flow, which induces the formation and evaporation of massive droplets. In the over-expanded flow, the steam is compressed and then expanded after leaving the primary nozzle and the condensation process is not observed in mixing and constant sections. The increasing suction chamber pressure significantly improves the entrainment ratio while leading to an increasing entropy loss coefficient. The entrainment ratio is improved from 0.25 for the under-expanded flow to 1.69 for the over-expanded flow, while the entropy loss increases from 0.081 for the under-expanded flow to 0.29 for the over-expanded flow. This indicates that the transition of the flow structure from an under-expanded flow to an over-expanded flow can entrain more steam from the last effect while causes more entropy losses in a steam ejector for the MED-TVC desalination system.

Extension‐dominated improved dispersive mixing in single‐screw extrusion. Part 2: Comparative analysis with twin‐screw extruder

AbstractIn Part 1 of this work, the possibility of improving single‐screw extruders (SSE) better dispersive mixer was explored by harnessing extensional flows provided by the hyperbolic contracting–diverging channels of extensional mixing elements (EME). Addition of the EME to the pin screw generated enhanced breakup for polymer blends and nanocomposite systems without significant penalty in flow rate. In Part 2, experiments are performed on immiscible polymer blends (low‐viscosity ratio and high‐viscosity ratio) and nanocomposites on both SSE and twin‐screw extruder (TSE) with the same rotation speed and throughput. Morphological results show tremendous improvement in dispersive mixing capability of SSE when equipped with EME that are mainly comparable to conventional TSE that is, with kneading blocks as mixing sections, although not as good as TSEs equipped with EMEs. Mechanical results also show enhanced modulus when EME is used in SSE operations.

Comparative computational analysis of dispersive mixing in extension‐dominated mixers for single‐screw extruders

AbstractEnhanced dispersive mixing can be achieved by harnessing extension‐dominated flows, because they are more efficient in agglomerate/droplet breakup than shear‐dominated flows. Herein, we compare computationally the performance of two single‐screw extrusion mixing sections with significant extensional flow components, the Chris Rauwendaal dispersive (CRD) mixer and the extensional mixing element (EME), recently proposed by the authors. Tapered slots in the CRD and hyperbolic contractions in the EME attempt to create extensional stresses through significant velocity gradients. Our studies confirm that EMEs are a better dispersive mixer than the CRD mixer, as they impose more intense and uniformly distributed extensional flow in a higher volume fraction of the material, and with less specific mechanical energy consumption. Overall, the EME design with a biaxial contraction was found to be the best dispersive mixer for single‐screw extruders (SSE). The manuscript also summarizes all the existing dispersive mixers for SSE and ranks them relatively for their dispersive mixing capability.

Enhanced Dispersion and Mechanical Behavior of Polypropylene Composites Compounded Using Extension-Dominated Extrusion

Abstract The process of dispersing filler in polymer matrix is vital to the behavior of polymer composites. The current study involves understanding the extent of dispersion of filler by only varying the nature of mixing during the process. Identical polymer composite materials are processed via two different kinds of mixing sections on the screw in a twin-screw extruder, differing in the type and amount of stress they impose on the filler agglomerate. An aggressive (900) Kneading Block (KB) mixing section is compared with recently developed Extensional Mixing Elements (EMEs), which impart extension dominated mixing while KB imparts shear dominated mixing. Various EME geometries of different levels of aggressiveness were computationally studied and validated. Composites obtained from KB are compared with composites processed using five different EME geometries. Three composites of Polypropylene (PP) filled with carbon black, graphene nano platelets and carbon nanotubes were studied independently. Composites processed through EMEs display about an order of magnitude better dispersion of filler agglomerate over the composites processed through KB. In addition, enhanced modulus and yield stress is observed for composites processed through EMEs. An improvement of 63% to 266% in the strain achieved for EME processed composites is seen under biaxial film stretching.

Design and Evaluation of General Purpose, Barrier, and Multichannel Plasticating Extrusion Screws

Extrusion screw designs and validation are presented for three multiple channel, fractal screws for comparison with common general purpose, and barrier screws using an instrumented single screw extruder with high impact polystyrene (HIPS) and low density polyethylene (LDPE) at varying screw speeds. The fractal screws are designed with multiple channels and pressure–volume–temperature relations to control shear heating with cooling by adiabatic decompression. The general‐purpose design had the highest throughput but did not provide sufficient mixing and so resulted in excessive variation in the melt temperature and pressure at screw speeds above 40 RPM. The barrier screw was a capable design with good performance for LDPE and HIPS with screw speeds from 20 to 60 RPM. However, it tended to provide excessive shear heating at higher screw speeds due to the large surface area of the barrier and mixing sections. The first fractal screw design was a multichannel variant of the general‐purpose design and exhibited good consistency but excessive heating due to the large bearing area between the flights and barrel. The second fractal screw design provided decompression in the feed zone and metering zone to improve throughput but was limited by a poor transition section design. The third fractal screw design remedied these deficiencies with an improved transition section and intermittent clearances for dispersive mixing. Its performance rivaled that of the barrier screw with respect to volumetric output and energy efficiency but provided better melt pressure consistency. Cold screw freezing experiments were performed for all five screws with 5% black, blue, and violet colorants serially added to neat HIPS. The cold screw pulls showed that the general purpose and barrier screws exhibited significant racing of the materials within their screw channels and, thus, broad residence time distributions. Examination of the material cross sections indicated persistent coiled sheet morphologies, which were best dispersed with the third fractal screw. POLYM. ENG. SCI., 60:752–764, 2020. © 2020 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.

Urea derived deposits in diesel exhaust gas after-treatment: Integration of urea decomposition kinetics into a CFD simulation

Formation of solid urea by-products in mixing sections of SCR systems is a well-known problem in development and design of exhaust gas after treatment systems. Spray impingement induces liquid film formation on mixer blades and tailpipe walls, from which urea by-products can be derived in dependence on temperature and residence time. This work presents a comprehensive modeling approach for prediction of by-product formation. An existing kinetic model for urea decomposition is integrated into CFD simulations by user coding. Implementation is evaluated by 3D simulations of thermogravimetric decomposition of urea and its by-products. Results are compared to experimental data and simulation results from a 0D model. Furthermore, the procedure is applied to simulations of a flow setup including urea injection for demonstration of the model capabilities.

Investigation of a microchannel heat and mass exchanger for absorption systems

An investigation of a microchannel heat and mass exchanger for vapor absorption systems is presented. A sheet with 88 microchannels of depth 0.5 mm and width 0.76 mm is investigated via visual and thermal measurements. The flow in the microchannels is predominantly in the slug-flow regime, with distinct vapor and liquid regions. High rates of heat transfer were observed in the absorber. However, poor liquid mass transfer rates limit the performance of the microchannel absorber. The performance of the microchannel absorber is compared with that of an absorber with serpentine microscale passages containing micro-pin fins. A physics-based model for the absorption of ammonia into a dilute solution of ammonia-water in a microchannel absorber with mixing sections is developed using insights from flow visualization. The model is validated by comparing its predictions with the data. The potential of surfactants to enhance absorber performance is also investigated.

Effect of Spray-Exhaust Gas Interactions on Ammonia Generation in SCR Mixing Sections

Effect of Spray-Exhaust Gas Interactions on Ammonia Generation in SCR Mixing Sections

Characterization of oilseeds mechanical expression in an instrumented pilot screw press

This work studies oil seeds mechanical expression in a screw press. For this purpose, a pilot-scale screw press (0–40 kg/h) was instrumented by installing sixteen pressure sensors and three temperature probes throughout the screw. Canola (Brassica napus L) and flax (Linum usitatissimum) seeds were used for investigation. The impact of the screw rotation speed on the press performances and the matter flow inside the press was investigated. The screw press capacity, passage time, extraction yield and specific energy consumption were studied is the range of 0–18.2 rpm. For each experiment the radial pressure, the internal residual oil content and the matter velocity evolution were measured all along the screw.Results show that increasing the rotation speed enhances the press capacity and decreases the passage time, reduces the extraction yield and the specific energy consumption. The recorded data allowed the identification of different functional sections of the screw press (feed, compression and mixing/relaxation sections) in relation with the screw geometry. In the compression sections, high pressure leads to oil expression and the formation of hard cake. In the mixing sections, pressure falls to zero and press-cake becomes friable. The matter velocity in the mixing sections is quick as compared to that in compression sections. Furthermore, results show the existence of oil reflux phenomena inside the screw press cage.Based on the obtained results, the continuous screw press behavior was schematized as a succession of several individual batch presses with intermediates steps of press-cake mixing and oil refluxes in order to facilitate process modeling.

Preparation of Ammonia from Liquid AdBlue – Modeling Approaches and Future Challenges

AbstractSustainable mobility is a legal and public demand. An important measure to clean the exhaust gas of diesel engines is the conversion of nitric oxides with selective catalytic reduction (SCR). The development of SCR systems can be strongly supported by numerical methods. This review article is to document the status of the CFD‐simulation method with respect to the preparation of the injected urea‐water solution, the turbulent transport of ammonia and the assessment of solid deposits coming from liquid film. Comparisons with measurements for realistic sprays and mixing sections are given.