Low Flow Rate Conditions Research Articles

Design of novel bracket structure for falling film devolatilizer and numerical simulation of its film-forming property

The falling film devolatilizer is a new type of devolatilization equipment, which has the advantages of high film-forming efficiency and no mechanical transmission. In this study, a devolatilizer with an open falling film substrate was designed, and the film-forming performance and surface renewal performance of high viscosity fluids under the falling film structure were numerically simulated. The results indicate that the opening of the falling film substrate makes the liquid film thinner, which is beneficial for the stretching of the liquid film. The advantages of the falling film under low viscosity and high flow rate conditions are more significant. Simultaneously, four bracket structures were designed. Compared with the rectangular bracket, the renewal frequency of fan-shaped brackets can be increased by more than 20%, and the flow rate can be increased by about 10%. The improvement of the two can directly improve the falling film devolatilization performance, so as to improve the polymer polymerization degree and improve the product performance. Therefore, exploring the falling film support frame with high devolatilization performance provides a certain engineering practical significance for the design of devolatilization equipment.

New Modeling Approaches for Ethylene Oxychlorination in Fluidized Bed Reactors: Industrial and Low Flow Rate Conditions

A simple model (Continuous Stirred-Tank Reactor) has been developed to predict the behavior of industrial ethylene oxychlorination fluidized beds operating in a turbulent regime. The approach showed good agreement both with results from industrial reactors and with those corresponding to the (Simple two phases-Plug bubble-Mixed flow emulsion approach) validated in the literature. For low flow rates, the use of the (Simple two phases - Plug bubble - Plug emulsion model) adapted to these conditions enabled us to highlight the location and extent of undesirable thermal hot spots for the process, and to propose actions to control them by acting on the temperature and/or on the feed gas flows. By comparing this model with the plug approach, the significant slowdown in ethylene conversion caused by resistance to mass transfer when feed flow rates are low is highlighted. Copyright © 2024 by Authors, Published by BCREC Publishing Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Research on Effect of Endwall Contouring of Vaned Diffuser on Stable Operating Range of Centrifugal Compressor

Abstract The study investigates the impact of diverse endwall contouring strategies on the operational stability of a high-pressure ratio centrifugal compressor stage. Employing numerical simulation techniques, this study examines the implications of contoured wall positions, specifically convex profiles at the pressure side and/or concave profiles at the suction side, as well as asymmetric endwall contouring, on both stage performance and internal flow field within the centrifugal compressor. And the stability enhancement mechanism of endwall contouring is clarified. The results demonstrate that employing full circular contoured vaned diffusers on the hub-side wall can achieve a maximum improvement in stall margin of 15.10%. Additionally, the diffuser featuring solely convex profiles at the pressure side on the hub-side wall exhibits an augmented stall margin by 8.39%, with a reduction in performance loss at low flow rate conditions. The asymmetric endwall contouring improves stall margin by 8.39%, and mitigates the performance loss across the entire operating range. At the near-stall point, the contoured endwalls redirect fluid towards the suction side, thereby mitigating the incidence at the vane leading edge and attenuating the interaction between the impeller trailing edge vortex and the diffuser leading edge vortex. Moreover, concave profiles serve to accelerate the fluid on the suction side along the flow direction, thereby suppressing the corner separation and weakening passage blockages. Simultaneously, the advantages of endwall contouring in vane diffusers extend to the improvement in the circumferential non-uniformity of flow fields and the enhancement in diffuser stability.

Conquering surfactant-induced partial wetting of commercial membrane in membrane distillation through in-situ water flushing

Surfactant-induced wetting impedes the practical implementation of membrane distillation (MD). Addressing this issue demands the development of an effective membrane cleaning strategy that can eliminate surfactants adhering to the membrane surface and restore the membrane hydrophobicity. However, current cleaning methods, such as direct drying and pressurized air backwashing, encounter challenges in thoroughly removing surfactants trapped within the pores while preserving the structural integrity of the membrane. This work presents a refined approach to conquer surfactant-induced wetting in MD by water flushing. Utilizing ultrasonic time domain reflectometry and optical coherence tomography techniques, we identified a critical cleaning depth and showed that the hydrophobicity of a partially wetted membrane can be fully recovered by water flushing when the wetting depth is below the critical threshold. Theoretical models evidenced that in instances of low water temperature and low flow rate conditions, relatively high critical cleaning depths can be realized, thereby expanding the operational scope for achieving complete hydrophobicity recovery. Our results demonstrated the applicability of water flushing to commercial membrane modules without necessitating any modification, emphasizing its substantial potential for advancing MD applications.

Multiobjective hydraulic optimization of the diffuser vane in an axial flow pump

Separation flows tend to induce a chaotic flow field that eventually leads to energy losses and reduced efficiency. The present study performed a multiobjective optimization to improve the hydraulic performance of an axial flow pump at the best efficiency point (BEP) and critical stall point based on the diffuser vane (DV) geometry. Computational fluid dynamics were applied to predict the hydraulic performance of a series of DV models with design points generated through design of experiment. Six different surrogate models were evaluated based on the R-squared criteria. The nondominated sorting genetic algorithm II was also employed to search for optimum solutions for design variables. Hydraulic performance balance between low and high flow rate conditions was analyzed based on the velocity triangle. After optimization, the efficiency and total head at the BEP of the optimum model were increased by 2.341% and 2.779%, respectively, compared to the reference model. Despite the minimal changes to the hydraulic performance at the critical stall point, the optimal operating range was notably expanded in the high flow rate region. Thorough evaluation of losses attributed to horseshoe, corner, and trailing-edge vortices was conducted in meridional planes, multiple spans, and various cross sections in the DV domain. Additionally, the formation and development of turbulent flow were analyzed in detail by transient simulation. Vibration and noise caused by instabilities in the flow characteristics of the reference model were substantially reduced by 36.76% and 67.342% at the first higher-harmonic frequencies at the BEP and the critical stall point, respectively.

Thermal-hydraulic analysis of a full-scale lead-bismuth cooled fast reactor core considering inter-wrapper flow

The Lead-bismuth cooled Fast Reactors (LFR) utilizes wrapped hexagonal fuel assemblies arranged in a honeycomb pattern. The inter-wrapper flow (IWF) between adjacent assemblies has a significant influence on the core thermal-hydraulics. In this paper, the LFR core assembly analysis model, inter-wrapper flow and heat transfer model, and multi-scale coupling analysis model were developed and were implemented into the previously self-developed subchannel-level three-dimensional thermal hydraulic analysis code CorTAF-LBE for whole core of LFR, which is based on the OpenFOAM computational fluid dynamics simulation platform. The inter-wrapper flow and heat transfer model was validated against the KALLA-IWF experiment. Whole core steady state operating condition and low flow rate condition simulations were performed with reference to the 19-rod bundle geometry from KALLA laboratory and typical LFR core assembly arrangement. The whole core steady state distribution laws of key thermal-hydraulic parameters were obtained, and the influence of low flow rate operation on safety parameters such as cladding and pellet temperatures was analyzed. Under low flow rate operating conditions, the IWF heat transfer was accounted for 4.87% of the core thermal power, which was 315.6 kW higher than that under normal operating conditions, playing an important role in core heat removal and safe operation. This work provides important insights into LFR core design and the assessment of the role of IWF under low flow rate conditions.

Improved desalination performance of flow-electrode capacitive deionisation by a novel drop-shape channel

ABSTRACT As an emerging desalination technology, flow-electrode capacitive deionisation (FCDI) has the advantages of theoretically infinite adsorption capacity and applicability to high-concentration brine. However, during the operation of FCDI, the flow electrode in the S-shape channel is prone to sedimentation and clogging the channel. This undesirable phenomenon brings low efficiency and security issues. Therefore, a drop-shape channel was designed for FCDI to improve the flow regime of the flow electrode. The flow simulation of the drop-shape channel was performed to select the appropriate geometry to avoid the formation of the vortex and low-velocity region. The simulation results showed that the streamlined design of the drop-shape channel has insignificant velocity gradients. It significantly reduces the low-velocity region and improves the phenomenon of particle sedimentation. The desalination performance with varieties of electrode flow rate, AC content, and voltage was used to investigate the advantage between S-shape and drop-shape channels. It was found that under the conditions of low flow rate, high AC content, and high voltage, the drop-shape channel FCDI system could still obtain better ASRR and CE.

Enhancing control disorder and implementing V2X-Based suppression methods for electric vehicle CO2 thermal management systems

In recent years, the development of electric vehicles (EVs) thermal management systems has underscored the crucial role in ensuring driving safety and optimizing driving range has become increasingly prominent. However, the inherent dynamic complexity of EV operation coupled with automatic control systems, can sometimes lead to unstable behavior, resulting in performance degradation and safety risks for compressors and batteries. To effectively address this issue, an evaluation was conducted on the dynamic control characteristics of an EV thermal management system utilizing CO2 as the refrigerant in this study. Through mathematical modeling and experimental analysis, the erratic nature of the dynamic thermal process was first identified. The underlying reasons were elucidated, focusing on system control characteristics and intrinsic mechanisms. It was found that control disorder could induce abnormal actions in thermal management system components like compressors and expansion valve, leading to significant performance decline and issues such as liquid carryover in compressor suction. Furthermore, specific control disorder regions of CO2 heat pumps for EVs were delineated, providing a framework for assessing the likelihood of system control disorder. Notably, control disorder was more likely to occur under conditions of low indoor air flow rate, high ambient temperature, and low supply air temperature. Given the widespread nature of this issue and the lack of suitable solutions, two control disorder suppression schemes were developed using V2X technology and validated through simulation. Results showed that adoption of V2X communication technology prevented an average of 70.1 % COP degradation, ensuring stability and safety of compressors and batteries under various operating conditions. The research provides useful information for exploring the dynamic characteristics of CO2 thermal management systems, offering a novel approach to enhance the system stability and efficiency.

An Investigation of Contaminant Transport and Retention from Storage Zone in Meandering Channels

Contaminant trapping by recirculation zones occurring at the apex of natural meandering channels induces a long tail in the contaminant cloud, thereby complicating the prediction of mixing behaviors. Thus, the understanding of the interaction between solute trapping and recirculating flow is important for responding to and mitigating water pollution accidents. In this research, the EFDC model was employed to reproduce three-dimensional flow structures of recirculating flow at the channel apex and investigate the influence on contaminant mixing. To investigate the contaminant transport characteristics from the storage zone in meandering channels, simulations were conducted using various discharge values to assess the impact of storage zone development on the concentration–time curves. The analysis of the relationship between the storage zone size and mixing behaviors indicates that an increase in discharge could result in a shorter tail and larger longitudinal dispersion even with the larger storage zone size. On the other hand, the enlarged recirculation zone size contributes to reducing transverse dispersion, evidenced by flatter dosage curves under lower flow rate conditions. These findings suggest that the increase in longitudinal dispersion with a larger flow rate is primarily caused by the reduction in transverse dispersion resulting from the formation of the recirculation zone.

Experimental microwave assisted CO2 desorption of a solid sorbent in a fluidized bed reactor

In this study, a novel method has been applied to regenerate CO2 from a solid sorbent (Zeolite 13X) using a mono-mode microwave unit under fluidization conditions. Several parameters have been varied to investigate their effect on the desorption characteristics as well as on the energy consumption. While flow rate varied between 200 Nml/min to 1,000 Nml/min, regeneration temperature changed from 40 °C to 100 °C. Microwave initial forward power also varied from 15 W to 30 W.Volumetric heating of microwave led to desorption of CO2 from the sorbent effectively while fluidization enhanced the heat transfer within the sorbent by providing homogeneous heat transfer with lower desorption time and absorbed energy. The most dominant parameters on the desorption time and energy consumptions were found to be flow rate and regeneration temperature, respectively. Under different conditions, the energy consumptions to regenerate a kg CO2 varied between 2.84 MJ and 27.17 MJ. Even though the lowest energy consumption was obtained when the flow rate was 200 Nml/min at 40 °C; the time for a complete desorption found to be quite high with this flow rate. Therefore, a minimum energy consumption of 3.87 MJ/kg CO2 within a short period of regeneration time can be achieved under the fluidization regime with the flow rate of 1,000 Nml/min at 40 °C. While an increase in the flow rate also caused a slight increase in the energy consumption for low regeneration temperatures, it resulted in a significant decrease under higher regeneration temperatures. Furthermore, the main drawback of low flow rate conditions is that most of the energy was consumed to remove the last 10 % of CO2 from the sorbent. Therefore, fluidization plays a significant role on the removal of CO2, especially between the 90 % and 100 % regeneration percentages.This work is the first study in the literature that has investigated the effects of fluidization on the desorption characteristics of CO2 under mono-mode microwave conditions. It was proved that fluidization reduced the energy consumption of CO2 desorption with higher microwave efficiency.

Cavitation diagnosis method of centrifugal pump based on characteristic frequency and kurtosis

Centrifugal pumps are important equipment in industrial production. At present, vibration signals are often used to diagnose cavitation in centrifugal pumps, but the vibration signals are easy to be disturbed and the fault characteristics are unstable to be detected. In this paper, a single stage centrifugal pump is taken as the study object, and the vibration signals of various parts of the centrifugal pump cavitation state are collected under different flow conditions. The short-time Fourier transform and one-third octave analysis are performed on the filtered signals, and the characteristic frequency of cavitation and the energy near the characteristic frequency with the development of cavitation are obtained. Based on vibration signals, the vibration root mean square (rms) and kurtosis values of different cavitation states are obtained. Flow state, kurtosis, and rms are used as input variables in the double-layer backpropagation neural network model to identify and classify the cavitation states of centrifugal pumps. The results show that the trained neural network model can accurately identify and classify the cavitation state of the centrifugal pump under the conditions of low flow rate, rated flow rate, and large flow rate, and the accuracy is more than 99.5%. This study provides a new technique for diagnosing cavitation in centrifugal pumps.

Investigation on unsteady characteristics of flow separation in a supercritical carbon dioxide centrifugal compressor

As one of the core components of supercritical carbon dioxide (S-CO2) closed Brayton cycle, the efficiency of S-CO2 centrifugal compressor plays a crucial role. Affected by the special thermophysical properties of S-CO2 fluid, the flow mechanism of S-CO2 centrifugal compressor is different from the centrifugal compressor of conventional fluid, such as air. Previous studies have found that the flow separation within the flow domain of this kind of compressor is easily to occur downstream the pressure surface. Many steady computational fluid dynamics (CFD) studies have been conducted on the S-CO2 centrifugal compressors, but few studies focused on the unsteady evolution of this flow separation. In this paper, the unsteady CFD simulation is carried out in the flow passage of a S-CO2 centrifugal compressor. The solution domain of CFD simulation includes compressor blades, diffuser and volute. The performance and the unsteady flow behavior of S-CO2 centrifugal compressor is obtained. Under low flow rate conditions, the flow separation on the pressure surface of the blade of S-CO2 centrifugal compressor is more likely to occur, causing the decrease of compressor performance. This flow separation has strong unsteady characteristics, which deteriorates several times during a rotation cycle, meanwhile the vortex shedding happens. At the time steps of vortex shedding, the pressure near the trailing edge of the impeller fluctuates greatly, indicating that this unsteady flow separation brings a large flow loss to the S-CO2 centrifugal compressor.

Study on the Unsteady Flow Characteristics of a Pump Turbine in Pump Mode

Extensive research has been conducted on the performance of pump turbines, particularly focused on understanding the generation mechanism of S-shaped characteristics. However, there has been a lack of research on unsteady flow characteristics in hump characteristics with small guide vane openings. This study focuses on the hump characteristics of a pump turbine in pump mode. The unsteady numerical simulation method is used along with experimental testing to examine the internal flow characteristics and induced pressure fluctuations under pump operating conditions. The results indicate that flow separation occurs in the impeller when the flow rate decreases to the valley operating condition, and recirculation flow occurs near the impeller inlet at the partial flow rate. Moreover, the unstable flow on the positive slope exhibits a low-frequency characteristic of 0.15fn. The pressure fluctuation from the hub to shroud areas of the guide vane region diminishes sequentially. Notably, distinct vortex structures emerge at the draft tube cone section under the valley operating condition. These structures extend toward the elbow section of the draft tube as the flow rate decreases. This phenomenon generates low-frequency pressure fluctuation originating from the primary frequency of the vortex and dean vortex on the surface, located at 0.4 D of the draft tube under conditions of low flow rate.

Analysis of two-phase gas-liquid flow in an Electric Submersible Pump using A CFD approach

This study comprehensively analyzes a mixed-flow Electric Submersible Pump's (ESP) performance under challenging conditions of low flow rates and high gas fractions. The research leverages a computational model built on the Eulerian-Eulerian methodology, adeptly solving the 3D incompressible Navier-Stokes equations for each phase. To ensure an encompassing depiction of the flow dynamics within the ESP, the model considers critical parameters such as the bubble diameter and interphase forces, including drag, lift, and virtual mass force.Key findings reveal that the ESP's performance deteriorates markedly under the studied conditions. One of the primary influencers is the phenomenon of gas locking, where gas accumulation within the impeller hampers the consistent flow of liquid, thereby degrading the pump's efficiency. In addition, the model delineated the occurrence of the surging phenomenon across diverse gas volume fractions and rotational speeds, which further exacerbates the reduction in the ESP's performance. Intriguingly, the bubble diameter was identified as a decisive factor, especially at minuscule flow rates. Large bubble diameters instigate the formation of gas pockets on the suction side of the vane, culminating in a segregated flow pattern and further declining pump efficiency.On the other hand, the drag force is the primary interaction force between phases of gas-liquid flow. Other interfacial forces, such as lift and virtual mass force, change the pump's performance but do not affect the gas flow and distribution patterns for a one-stage configuration. Results show that the Schiller-Neuman drag coefficient offers a better fit than the Bozano-Dente coefficient.Conclusively, the study underscores the indispensable role of nuanced computational models in elucidating the intricate flow dynamics within ESPs, especially under conditions fraught with operational challenges. The insights gleaned enrich our understanding of ESP performance dynamics and provide a roadmap for designing more resilient and efficient pumping systems in the future.

Research on optimization scheme for boiling heat transfer models of mini-channels casing-pipes once-through steam generator

The mini-channels casing-pipes once-through steam generator(MCOTSG) is used for a certain type of small modular reactor. The boiling heat transfer model is crucial to the accuracy of simulating the heat transfer process on the secondary loop of MCOTSG. In this paper, the heat transfer tubes of MCOTSG are taken as the research object. The thermal–hydraulic model of primary and secondary loops is established based on the RELAP5 program. Then the different nucleate boiling heat transfer models are calibrated and analyzed based on the experimental results of the steady state under low flow rate conditions. On this basis, the secondary loop heat transfer characteristics and the overall operating characteristics of OTSG under the transient state of feedwater flow rate reduction are simulated. The results of the steady state and transient state analysis show that the revised model that the Bertsch correlation is used to correct the Chen correlation performs the best in terms of computational stability and computational accuracy. The results of the study can provide a basis for the safety analysis and optimal design of MCOTSG and small modular reactor under low flow rate conditions.

Study on Unsteady Flow Characteristics of Cooling Water Pump for Nuclear Power Plant Equipment under Low Flow Rate Conditions

During the operation of cooling water pumps, it is necessary to operate them under conditions of low flow rate. In order to improve the unstable performance of the cooling water pump under low flow rate conditions. Taking the cooling water pump as the research object, the internal flow and pressure pulsation characteristics of the cooling water pump under 0.4Q to 0.6Q conditions were investigated, and the influence of different operating conditions on the performance and vibration of the cooling water pump was analyzed. The ANSYS CFX 2022 software and the SST k-ω turbulence model were used to perform a three-dimensional numerical simulation of the cooling water pump. After analyzing the simulation results, the velocity and pressure cloud and streamline diagram within the semi-spiral suction casing and impeller were obtained. The internal flow state of the cooling water pump was then analyzed in detail under low flow rate conditions. At the same time, a series of monitoring points were set up within the impeller, and the pressure pulsation within the impeller was analyzed using the frequency domain diagram and the radial force polar coordinate diagram. The results show that at flow rates between 0.4Q and 0.6Q, a certain amount of vortex has been generated in the suction casing, which affects the flow state when entering the impeller. Furthermore, significant vortices have been generated in the middle and back part of the blade and mainly concentrated in the pump cover of the mid-open pump. At the same time, when in low flow rate conditions, the primary frequency of pressure pulsation is mainly the axial frequency, with three times the axial frequency and blade frequency following. The amplitude of the pressure surface (PS) of the blade is greater than that of the suction surface (SS) and increases as the flow rate decreases. The internal radial force corresponds with the result of pressure pulsation and exhibits a certain pattern. This study outlines the coolant pump’s internal flow and pressure pulsation characteristics under low-flow conditions. It proposes a solution to stabilize the cooling water pump at low flow rates and provides theoretical guidance for optimizing its design.

Validation of the Hybrid Turbulence Model in Detailed Thermal-Hydraulic Analysis Code SPIRAL for Fuel Assembly Using Sodium Experiments Data of 37-Pin Bundles

In the study of safety enhancements on advanced sodium-cooled fast reactors (SFRs) by the Japan Atomic Energy Agency (JAEA), it has been essential to clarify the thermal hydraulics under various operating conditions at high and low flow rate conditions in a fuel assembly (FA) with wire-wrapped fuel pins to assess the structural integrity of the fuel pin that achieves a high-performance core with high burnup ratio and high power density. A finite element thermal-hydraulic analysis code named SPIRAL has been developed by JAEA to analyze the detailed thermal-hydraulic phenomena in the FA of a SFR. In this study, numerical simulations of 37-pin bundle sodium experiments at different Reynolds (Re) number conditions, including a transitional condition between laminar and turbulent flows and turbulent flow conditions, were performed to validate the developed hybrid k-ε/kθ-εθ turbulence model equipped in SPIRAL to consider the low Re number effect near the wall in the flow and temperature fields. The temperature distributions predicted by SPIRAL were consistent with those measured in the sodium experiments at the Re number conditions. Through the validation study, the applicability of the hybrid turbulence model in SPIRAL to the thermal-hydraulic evaluation of sodium-cooled FAs in a wide range of Re numbers was confirmed.

Study on structure optimization and distribution characteristics of centrifugal refrigerant distributor for air conditioner

The refrigerant distributor helps resolve non-uniform flow distribution issues in multi-path evaporators. Using the CFD method, this paper investigates the distribution mechanism of R410A in a centrifugal distributor under air-conditioning conditions and explores the impact of structural factors on its distribution performance. The Taguchi method is employed to identify the optimal combination of structural parameters and optimization direction for achieving distribution uniformity. Furthermore, the flow distribution characteristics of the optimized distributor are investigated under different conditions. The results show that, under the working conditions of an inlet mass flow rate of 50 kg·h−1 and an inlet quality of 0.1, the non-uniformity of the mass flow rate in the outlet branches of the optimized distributor is only 1.07 %, which represents a decrease of 7.64 % compared to the original distributor. Additionally, the non-uniformity of the outlet branch quality also decreased by 9.41 %. These findings indicate that the optimized distributor exhibits superior distribution performance. Nonetheless, the degradation of the optimized centrifugal distributor's uniform distribution performance with increasing refrigerant inlet mass flow rate and quality suggests that it is more suitable for low mass flow rate and quality conditions.

Flow Loss Mechanism in a Supercritical Carbon Dioxide Centrifugal Compressor at Low Flow Rate Conditions

Flow Loss Mechanism in a Supercritical Carbon Dioxide Centrifugal Compressor at Low Flow Rate Conditions

Numerical and Experimental Study on the Process of Filling Water in Pressurized Water Pipeline

As an important working condition in water conveyance projects, the water filling process of pipelines is a complex hydraulic transition process involving water–air two-phase flow with sharp pressure changes that can easily cause pipeline damage. In light of the complex water–air two-phase flow during pipeline water filling, this study explores the water filling process of right-angle elbow pressure pipelines using CFD numerical simulations and physical model experiments, analyzing changes in water phase volume fraction, water-gas two-phase flow patterns, and hydraulic parameters in the pipeline under low flow rate conditions of 0.6 m/s and high flow rate conditions of 1.5 m/s. Results show that under low flow rate conditions, there is more local trapped gas at the top of the pipeline, causing negative pressure at local high points in the pipeline and forming a vacuum. Under high velocity conditions, water-gas two-phase flow changes more frequently in the pipeline, with a large number of bubbles collapsing at the top, resulting in large fluctuations in pipeline pressure. Finally, through physical experiments, the main flow patterns during water filling in right-angle elbows are verified and analyzed. These results have certain reference significance for formulating safe and efficient water filling velocity schemes for pressurized pipelines.