Flow Dead Zone Research Articles

Research on the Arrangement Scheme of Spirally Twisted Tape Inserts in a Grate Furnace

To eliminate the flow dead zone and homogenize the asymmetric flow field of a grate furnace, spirally twisted tape inserts (STTIs) with a pitch ratio of 1.5 were installed in the vertical flues of an SCL1000-13.5/450 grate boiler. The arrangement schemes found to be present inside the chosen 1000 t/d grate furnace, determined using the orthogonal experimental method, included separate installation in chamber II, separate placement in chamber III, and simultaneous arrangement in both chamber II and chamber III. The effects of row spacing H, column spacing W, and mounting angle φ were investigated by means of the practicable and feasible numerical simulation method. With a focus on the uniformity degree of the flue gas, the results showed that temperature distribution is directly correlated with the velocity field. When it comes to the uniformity of the flow field, the exclusive use of STTIs in chamber II was better than that in chamber III. Under the optimal combination scheme of STTIs in both chamber II and chamber III (scheme N323), the exhaust gas temperature reached the minimum value and the uniformity index of temperature increased to the range of 0.994~0.997. The findings in this work could provide a reference for the optimization of the flow field in a grate furnace.

Influence of irregular fiber filling on the performance of hollow fiber membrane modules for cold water production

The countercurrent hollow fiber membrane-based evaporative water cooler (MEWC) offers an eco-friendly and compact solution for cold water generation. This study introduces a random sequential addition algorithm to model the real-world irregular fiber filling within the MEWC. Inspired by the honeycomb structure, the developed 3-D numerical model adopts a calculation unit featuring a hexagonal prism comprising multiple fibers. Validation against experimental data reveals an average relative error of 2.81 % concerning outlet water temperature. The effects of fiber filling patterns (regular layout and random layout) on the velocity and temperature fields of the MEWC are investigated. Comparisons of outlet water temperature, cooling efficiency, consumptive electric power ratio, and heat and mass transfer resistance composition between these layouts under various operating conditions are conducted. The results indicate that the random layout fosters severe channeling effect and large flow dead zones, impairing air side heat and moisture transfer. The random layout exhibits over 15.9 % reduction in cooling efficiency and 36.3 % decrease in consumptive electric power ratio compared to the regular layout. Irregular fiber filling leads to a notable 158.6 % increase in air side heat transfer resistance and a 35.9 % rise in mass transfer resistance. Although irregular filling compromises the cooling performance, it demonstrates potential for energy savings under certain conditions. Design schemes should be carefully tailored to meet specific application requirements by considering these trade-offs.

Environmental simulation and refrigeration demand analysis for shield tunnel construction in hot summer and cold winter regions

The thermal hazards generated during the tunnel boring machine construction process significantly impact the physical and mental health of personnel. A field test was conducted on the tunnel of Metro Line 1 in Changsha, China, where the average temperature in the work zone reached up to 33.16 °C, and the air velocity was generally below 0.2 m/s. These conditions were unfavorable for ventilation and heat dissipation. In this paper, a full-scale CFD simulation model was established to simulate the construction environment and analyze the cooling demand. The results indicated that increasing the fresh air velocity could lower the temperature in the work zone; however, this effect was limited. When the fresh air temperature exceeded a certain threshold, it actually raised the average temperature of the work zone. Consequently, there is a need for supplementary cooling of the fresh air during the hot summer months. At a fresh air velocity of 15 m/s, the maximum temperature in the work zone was recorded at 27.8 °C when the fresh air was cooled down to 22.5 °C through auxiliary cooling measures. This approach effectively controlled the temperature in the work zone, reduced flow dead zones, and significantly improved air distribution and heat dissipation. Additionally, it ensured the physical and mental well-being of the construction workers and the safe operation of the equipment. This study provides valuable guidance for managing thermal environments in underground tunnel construction.

Analysis of stream flow and its impact on thermal performance in shell-side of heat exchangers based on flow dead zone

The performance of shell-and-tube heat exchanger (STHE) is primarily determined by different flow paths on the shell-side. The flow path and its impact on the shell-side flow in segmental baffle shell-and-tube heat exchanger with clearance (SC-STHE) is investigated qualitatively and quantitatively, by adopting stream classification and dead zone analysis method. By utilizing Residence Time Distribution (RTD) curves, the influence of different streams on fluid flow and heat transfer is discussed to reveal the intricate interconnections within the shell-side of SC-STHE. Then the cases with baffle-tube clearance (0.2–1.0 mm) and baffle-shell clearance (1.0–5.0 mm) ranges are studied. The study identifies that optimal comprehensive performance is achieved with a baffle-tube clearance of 0.6 mm, resulting in a 3.5 % improvement compared to a clearance of 0.2 mm. An increase of baffle-shell clearance results in a reduction in comprehensive performance by 8.23–19.54 %. The interactions among stream flows and potential mechanisms are discussed by analyzing the influence of each flow path on the flow dead zone volume. It is revealed that the presence of baffle-tube and baffle-shell clearances reduces the volume fraction of flow dead zones compared to structures without clearances. A-stream and E-stream are identified as pivotal, mutually influencing B-stream and C-stream while maintaining relative independence. The result indicates that an increase in baffle-tube clearance results in a 7.60 % expansion of the volume of the dead zone within the B-stream, while in the C-stream, it decreases by 2.58 %. Conversely, an increase in baffle-shell clearance reduces the volume of the dead zone in the B-stream by 5.04 % but increases it in the C-stream by 31.95 %. The flow dead zones are mainly distributed in the B-stream and C-stream regions, and the diversion effect of leakage flows improves the fluid stagnation but also leads to a decrease in turbulence intensity and flow velocity. This study reveals the complex interaction mechanisms among the stream flows, facilitating further analysis and optimization in STHE design.

Investigation on the thermal-fluidic characteristics of ice slurry in sinusoidal corrugated plate heat exchangers

The sinusoidal plate heat exchangers (SCPHEs) using ice slurry as coolant have a critical role for building air conditioning. However, the thermal-hydraulic properties of ice slurry in SCPHEs are still unclear. Therefore, its flow and heat transfer performances in SCPHEs are studied using Euler-Euler model in this paper. The results show that with increasing corrugation pitch (P), the velocity of the mainstream direction and the flow dead zone decrease. As enlarging the corrugation amplitude (H), the low-velocity flow zone and flow dead zone increase. The average resistance coefficient (fave) of ice slurry reduces with increasing P and volume flow rate. When volume flow rate is between 0.144 m3/h and 1.44 m3/h, fave decreases more significantly than other stages. In addition, fave increases with increasing H and ice concentration (αin). The mass transfer of ice particles is increased at higher H and smaller P. The average Nusselt number (Nuave) of ice slurry, heat transfer rate (Qw) and total heat transfer coefficient (U) of SCPHEs increase as enlarging H, αin and volume flow rate, and decrease with increasing P. While the U and Qw are less affected by ice particle concentration. In addition, the empirical correlations of fave and Nuave in SCPHEs are developed.

Design and performance analysis of a discontinuous spiral baffle heat exchanger

In this paper, a new type of heat exchanger with discontinuous spiral baffles is designed, and its performance on heat transfer is analyzed by numerical simulation. The research results show that the airflow in shell-side flows by a similar helical way, “Flow dead zone” is not found. The high enthalpy airflow’s temperature decreases about 38% of the inlet air temperature by cooling down with a convective heat transfer coefficient of 344.4 W/(m2·K), while the decrease of the airflow pressure is 30%.

Study on the Flow Dead Zone in the Shell of an Industrial Tubular Fixed Bed Reactor

Study on the Flow Dead Zone in the Shell of an Industrial Tubular Fixed Bed Reactor

Predictable Local Polarization inside Electrodes By Three-Dimensional Multi-Scale Model in Flow Batteries

Redox flow batteries (RFBs) are regarded as one of the most promising grid-scale energy storage technologies, which can improve the applicability of wind and solar energy by leveling their fluctuating power output. In RFBs, the gas side reactions including hydrogen evolution reaction and oxygen evolution reaction reduce the efficiency and capacity of the batteries and the gas produced may even cause safety problems. Even if the voltage is controlled in practical applications to restrict the gas side reactions, it is still inevitable due to flow dead zones or insufficient reactants locally inside the electrode. Gas side reactions are usually caused by too high local overpotential, i.e., local over polarization. Therefore, in this work, we propose a multi-scale model that allows the rapid prediction of local polarization to limit and manage the gas side reactions. The multi-scale model contains a deep neural network, a pore network model, and a three-dimensional continuum model with the advantages of both accuracy and extensibility. By learning from the sample data provided by the cell-scale model and the pore-scale model, the deep neural network can quickly predict the local polarization under different operating conditions and give the distribution map of the entire electrode. Compared with conventional models, multi-scale models can take into account the geometry of electrode fibers and are suitable for electrode design and modification. Due to the calculation superiority of the pore network model, the multi-scale model can realize three-dimensional local polarization prediction, which enables a more accurate analysis of the internal uniformity of the electrode. Through the developed model, we explore the effects of the interdigitated flow field, flow rate, initial concentration and voltage on local polarization. It is shown that the multi-scale model can accurately predict local polarization. However, the promotion of flow rate cannot completely eliminate the occurrence of local over polarization.

Hydrodynamics and Mass Transfer in an Airlift Loop Reactor: Comparison between Using Two Kinds of Spargers

The effects of different spargers on the hydrodynamics and mass transfer of an airlift loop reactor were investigated. The gas holdup, liquid loop velocity, and volumetric mass transfer coefficient of the reactor were tested using a ring orifice distributor and a jet nozzle. The study was conducted in a 6 m high airlift loop reactor at a superficial gas velocity of 0.01~0.04 m/s, and the superficial liquid velocity was maintained at 0.0154 m/s. The results showed that using the jet nozzle provided a higher gas holdup, liquid loop velocity, and mass transfer. When the superficial gas velocity was less than 0.0325 m/s, the liquid loop velocity generated by the jet nozzle was approximately 1.1-fold higher than that generated by the ring orifice distributor, and the disparity in gas holdup between the riser and downcomer enhanced the power of liquid circulation. When the superficial gas velocity was more than 0.0325 m/s, the jet kinetic power dominated the improvement in the liquid loop velocity, and the energy input from the nozzle to the airlift loop reactor was greater than 10.8 J/(s·m2). This indicated a threshold of energy input for overcoming the friction loss. In this situation, the liquid loop velocity in the jet form increased considerably, thus favoring the mixing performance and temperature uniformity of the reactor. It was also of significance for avoiding the formation of a flow dead zone in scale-up airlift loop reactors.

Optimization of flow field distribution to improve desalination performance of battery electrode deionization by designing flow channel structure

Battery electrode deionization (BDI) is a technique for removing salts from brine. It works with the aid of electrochemical reactions between ions and active compounds in electrodes under the electric field. In the BDI device, the function of the flow channel is to transport the electrolyte into the cell and assign the reactants to the electrode surface. The BDI flow field with classical plate structures typically suffers large flow dead zones, which greatly reduces the uniformity of the reactants supplied to the electrodes. This will affect the desalination performance of the cell. In particular, the flow field distribution will have an increasing impact on the desalination performance, considering the demand for commercialization and large scaling. In this work, five flow channel proposals of the BDI device are investigated numerically. Flow patterns and intercalation-Na clouds are used to analyze the distribution of the flow field and the intercalation-Na degree. The uniformity factors are introduced to describe the uniformity of Na+ ion concentration and intercalation-Na degree distribution. The salt adsorption capacity (SAC), average salt adsorption rate (ASAR), and volumetric energy consumption are used to evaluate the desalination performance of the system. The results showed that proposals with flow field design can effectively reduce the dead zone within the flow channel and utilize the electrodes more uniformly and fully, and the SAC and ASAR are also improved as a result. Reducing the charging current density and increasing the inlet flow rate can optimize the uniformity of ion distribution within the flow channel and the uniform utilization of the electrode, as well as reduce the volumetric energy consumption.

Flow state analysis of molten salt in shell and tube heat exchanger with perforated baffles

Addressing the issue of flow dead zones in molten salt heat exchangers in concentrating solar power generation systems, this study focuses on the conventional shell and tube heat exchanger using molten salt and heat transfer oil as the working medium. The flow dynamics of molten salt within the heat exchanger are analyzed. To quantify the volume fraction of the molten salt flow dead zones, the residence time distribution curve is employed. Four baffle salt flow configurations are comparatively assessed. Findings indicate that the four opening configurations effectively enhance the reduction of molten salt flow dead zones, with volume fraction reductions ranging from 57.8% to 68.21%. Notably, configuration 4 yields the most optimal results. Furthermore, molten salt flow states in varying regions were examined: the innermost flow dead zone exhibited the highest improvement, followed by the middle area, with the edge area showing the least enhancement. Additionally, the impact of the opening diameter on the flow dead zone was explored. The volume fraction of the molten salt flow dead zone diminishes as the opening diameter expands, with the rate of this change also decelerating. Given that molten salt at the opening manifests as a jet, enlarging the opening diameter lessens the pressure differential across the baffle, subsequently weakening the jet's intensity and its influence on the flow dead zone.

Velocity effect on the impact dynamics of high-speed granular avalanches based on centrifuge modeling and DEM simulations

The design strategies for barrier structures against high-speed granular impacts are still lacking, because the granular impact dynamics is poorly understood. We have investigated the impact behavior of high-speed granular avalanches with different velocities (7–20 m/s) using the centrifuge modeling and DEM simulation. The results indicate that the peak total impact force should be calculated as the sum of the dynamic and static components, and the impact force distribution mode can be treated as having a rectangular shape. For granular impacts with velocities of 7–20 m/s, the dynamic component can be calculated based on the hydraulic dynamic model fed the maximum values of the flow depth, velocity, and density. The static component can be estimated using Coulomb's earth pressure theory using the dead zone weight based on an assumption of the static failure mode of the dead zone and with the flow–dead zone interaction-induced force being implicitly considered.

Comparison of hemodynamic features and thrombosis risk of membrane oxygenators with different structures: A numerical study

PurposeThe geometric structure of the membrane oxygenator can exert an impact on its hemodynamic features, which contribute to the development of thrombosis, thereby affecting the clinical efficacy of ECMO treatment. The purpose of this study is to investigate the impact of varying geometric structures on hemodynamic features and thrombosis risk of membrane oxygenators with different designs. MethodsFive oxygenator models with different structures, including different number and location of blood inlet and outlet, as well as variations in blood flow path, were established for investigation. These models are referred to as Model 1 (Quadrox-i Adult Oxygenator), Model 2 (HLS Module Advanced 7.0 Oxygenator), Model 3 (Nautilus ECMO Oxygenator), Model 4 (OxiaACF Oxygenator) and Model 5 (New design oxygenator). The hemodynamic features of these models were numerically analyzed using the Euler method combined with computational fluid dynamics (CFD). The accumulated residence time (ART) and coagulation factor concentrations (C[i], where i represents different coagulation factors) were calculated by solving the convection diffusion equation. The resulting relationships between these factors and the development of thrombosis in the oxygenator were then investigated. ResultsOur results show that the geometric structure of the membrane oxygenator, including the location of the blood inlet and outlet as well as the design of the flow path, has a significant impact on the hemodynamic surroundings within the oxygenator. In comparison to Model 4, which had the inlet and outlet located in the center position, Model 1 and Model 3, which had the inlet and outlet at the edge of the blood flow field, exhibited a more uneven distribution of blood flow within the oxygenator, particularly in areas distant from the inlet and outlet, which was accompanied with lower flow velocity and higher values of ART and C[i], leading to the formation of flow dead zones and an elevated risk of thrombosis. The oxygenator of Model 5 is designed with a structure that features multiple inlets and outlets, which greatly improves the hemodynamic environment inside the oxygenator. This results in a more even distribution of blood flow within the oxygenator, reducing areas with high values of ART and C[i], and ultimately lowering the risk of thrombosis. The oxygenator of Model 3 with circular flow path section shows better hemodynamic performance compared to the oxygenator of Model 1 with square circular flow path. The overall ranking of hemodynamic performance for all five oxygenators is as follows: Model 5 > Model 4 > Model 2 > Model 3 > Model 1, indicating that Model 1 has the highest thrombosis risk while Model 5 has the lowest. ConclusionThe study reveals that the different structures can affect the hemodynamic characteristics inside membrane oxygenators. The design of multiple inlets and outlets can improve the hemodynamic performance and reduce the thrombosis risk in membrane oxygenators. These findings of this study can be used to guide the optimization design of membrane oxygenators for improving hemodynamic surroundings and reducing thrombosis risk.

CFD simulation on flow and mixing process in different horizontal self-cleaning single-shaft kneaders

The horizontal self-cleaning single-shaft kneader possesses excellent mixing performance and has many industrial applications in bulk polymerization, polycondensation and polymer devolatilization. Hence, the objective of this research is to investigate the hydrodynamics and mixing performance of different new types of self-cleaning single-shaft kneaders by using the finite element method (FEM) simulation and particle tracking technique. Both the distributive mixing process of randomly distributed material points between halves of kneaders and the local distributive mixing process of a cluster in selected positions were analyzed. The effect of kneader geometry structure on hydrodynamics and mixing performance was also analyzed. The FEM simulation were validated by using a small visual experimental device. There hardly exists the flow dead zone in the self-cleaning kneader. The area of region with high velocity magnitude increases with the number of dynamic kneading bars and decreases with the number of static kneading bars. The area with high shear rate increases with the number of both dynamic and static kneading bars due to small clearance. There exists the periodical intermeshing interaction between the static kneading bars on kneader wall and the dynamic kneading bars on rotating shaft. What counts is that the distributive mixing process and mixing efficiency can be enhanced and improved by the periodical intermeshing interaction.

Effect of the novel combined internal devices on gas−solid flow behavior in spouted beds

To reduce the flow dead zone in spouted beds (SBs), a novel multi-jet–axial swirl blade spouted bed (MJ-ASB SB) is proposed. Experimental verification and simulation analyses of the MJ-ASB SB, a conventional SB (CSB), and an integral multi-jet spout-fluidized bed (IMJSFB) were carried out. The simulation results showed that the combined structure resulted in more powerful turbulence, better radial mixing of particles in the spouting and annulus regions, and improved the flow−field uniformity. The intensification factor of the MJ-ASB SB was more than 20%, which was better than that of the IMJSFB, and the enhancement factors of the gas and particle velocities were four and five times those of the IMJSFB, respectively. Optimal structural parameters were obtained by analyzing the different structural parameters of the novel internal devices, and it was found that particle fluidization in the MJ-ASB SB was best when the height ratio ξ was 3, the width ratio was 1.3 and the rotation was clockwise.

Topological structures for microchannel heat sink applications – a review

The microchannel heat sink (MCHS) has the advantages of small heat transfer resistance, high heat transfer efficiency and small size, which exhibits good heat transfer performance in the field of active heat dissipation of electronic devices integrated with high heat flux density. In this paper, the application of MCHS in thermal management is reviewed in recent years, and the research progress of microchannel topology on enhancing heat transfer performance is summarized. Firstly, the research progress on the cross-sectional shape of the microchannel shows that the heat transfer area and fluid flow dead zone of the microchannel is the keys to affecting the heat transfer performance; Secondly, the microchannel distribution and the bionic microchannel structure have a great role in enhancing heat transfer performance, especially in microchannel temperature uniformity; Thirdly, the disturbing effect caused by interrupted structures in microchannels such as ribs and concave cavities has become a hot topic of research because it can weaken the thermal boundary layer and increase heat dissipation. Finally, the commonly used MCHS materials and cooling media are summarized and introduced. Based on the above reviews of MCHS research and applications, the future trends of MCHS topologies are presented.

Modelling of gas-liquid-solid distribution in the cocurrent three-phase moving bed

The three-phase moving bed reactors (TMBRs) can supply a potential solution for the multi-phase reactions with deactivating catalysts to realize Online Catalyst Replacement (OCR). In this work, computational fluid dynamics (CFD) simulations were employed to investigate the flow behavior of each phase in a cocurrent TMBR. The Euler-Euler multi-phase flow model and two different liquid-solid force models were employed to describe the dense-phase particle flow and liquid-solid interaction, respectively. The results revealed that the solid flow rate increased due to the gas-liquid flow, but radial solids distribution had not changed significantly. The effect of gas-liquid flow on the solid volume distribution was mainly reflected in the wall and inlet region, which led to lower solid volume fraction and higher granular temperature in the local area. Meanwhile, compared with trickle beds, with the increasing gas flow rate or decreasing liquid flow rate, both the liquid holdup and flow dead zone decreased in TMBRs. With the increase in liquid flow rate, the effect of liquid migration (from the wall to the center) on the liquid distribution was weakened. Besides, due to the competition between the pressure gradient force and gas-liquid drag force, the variations of volume and velocity of each phase depended on operating conditions, which were divided into three typical ranges. The gas-liquid-solid distribution could significantly advance the understanding of flow behaviors in TMBRs, based on which the reactor development could be further promoted.

The analysis on the battery thermal management system with composite phase change materials coupled air cooling and fins

In this study, paraffin (PA) and expanded graphite (EG) were used to prepare composite phase change materials (CPCM). Through experimental study, the optimal axial thickness (45 mm) and radial thickness (8 mm) of CPCM have been found. Thus, based on the above results, a structure of BTMS using CMCP (PA/EG) coupled air cooling and fins (III-BTMS) was proposed to reduce the overall weight of the BTMS. It has been validated that the II-BTMS could meet the cooling performance requirements with 0 m/s air velocity in Li-ion battery at the 20 °C environmental temperature at 2C discharge rate. It means that in this condition, no extra energy is needed to cool the Li-ion battery of II-BTMS. The high discharge rate simulation shows that the cooling performance of III-BTMS is still better than the I-BTMS. In simulation results, it could be clearly observed that the fins could effectively decrease the flow dead zone in the III-BTMS model. Take Tesla automobile (model S) as an example, compared with using the I-BTMS, the b-type fin of III-BTMS module could at least reduce the weight of 45.84 kg in the study. The research results of III-BTMS are hoped to provide research ideas for the application of CPCM to EVs.

Experiment and CFD study on the hydrodynamics in novel internal-intensified spouted beds

To overcome the limitations of traditional spouted beds, two novel internal-intensified spouted beds were proposed: swirl-nozzle anticlockwise-axial-swirler (SNAAS) and swirl-nozzle clockwise-axial-swirler (SNCAS) spouted beds. The hydrodynamics of the novel internal-intensified, conventional, and swirl-nozzle spouted beds (SBs) were experimentally and numerically investigated. Adding internal-intensified structures could reduce the minimum spouting velocity and accelerate particle fluidization. The gas radial velocity in novel spouted beds with ξ = 3 is the largest, which is the optimal height ratio. Compared with conventional and swirl-nozzle SBs, adding novel internal-intensified structures can effectively promote the radial velocity of gas and particle, reduce the flow-dead zone in the bed, obtaining larger gas turbulent kinetic energy. The flow field uniformity of gas and particle within beds in descending order as SNCAS SB > SNAAS SB > swirl-nozzle SB > conventional SB. The enhancement factor I of flow field uniformity in novel internal-intensified SBs is 2 to 3 times that in the swirl-nozzle SB. Mesh of (a) Conventional SB, (b) Swirl-nozzle SB and (c) Swirl-nozzle axial-swirler SB. • Internal-intensified spouted beds were proposed by introducing axial swirler and swirl nozzle. • The minimum spouting velocity can be reduced by novel internal-intensified structures. • Internal-intensified improves gas-solid contact throughout the spouted bed. • The enhancement factor I in novel internal-intensified SBs is 2 to 3 times that in the swirl-nozzle SB.

Numerical simulation research on suction process of concrete pumping system based on CFD method

In this paper, a numerical model of the suction process of the pumping system was established, and the accuracy of the numerical model was verified by the rheological experiment of fresh concrete and the suction experiment of the pumping system. By analyzing the numerical results, the distribution characteristics of the internal flow field of the hopper and the change law of the bubble shape in the fresh concrete during the suction process were explored. The results show that the established numerical model can effectively simulate the material suction process of the pumping system, and the flow velocity of the fresh concrete in the mixing area presents a periodic variation behavior. The simulation results show that there is a “flow dead zone” at the bottom of the hopper. Smaller flow speed of fresh concrete was found around the S valve, and the bubbles located inside the fresh concrete have a negative impact on the suction effect of the pumping system. The research results of this paper provide valuable references for designers to improve the hopper structure and optimize the operational parameter of the suction process of pumping system. Distribution characteristics of the internal flow field of the hopper in the suction process. • The model of suction process was established by the CFD method. • Flow velocity of concrete in hopper presents the law of periodic variation. • Flow velocity of concrete decreases when it flows through the S valve. • Air bubbles gather at the end face of the piston at the end of the suction.