Numerical Computational Fluid Dynamics Simulations Research Articles

Multiphase Numerical CFD Simulation of the Hydrothermal Liquefaction Process (HTL) of Sewage Sludge in a Tubular Reactor

This article presents multiphase numerical computational fluid dynamics (CFD) for simulating hydrothermal liquefaction of sewage sludge in a continuous plug-flow reactor. The discrete particle method (DPM) was used to analyze the solid particles’ interaction in liquid–solid high shear flows to investigate coupling computational fluid dynamics (CFD). Increasing solid particles’ interactions were observed with the increasing liquid velocity. The study examined the influence of parameters such as flow rate, temperature, and residence time on the efficiency of bio-oil production. An increase in temperature from 500 to 800 K caused an increase in the amount of biocrude oil produced from 12.4 to 32.9% within 60 min. In turn, an increase in the flow rate of the suspension from 10 to 60 mL/min caused a decrease in the amount of biocrude oil produced from 38.9 to 12.9%. This study offers insights into optimizing the flow channel of tubular reactors to enhance the HTL conversion efficiency of sewage sludge into biocrude oil. A parametric study was performed to investigate the effect of the slurry flow rate, temperature, and the external heat transfer coefficient on the biocrude oil production performance. The simulation data will be used in the future to design and scale up a large-scale HTL reactor.

Optimizing air inlet designs for enhanced natural ventilation in indoor substations: A numerical modelling and CFD simulation study

This paper investigates the ventilation and heat dissipation performance of a 110 kV indoor substation under natural ventilation conditions using computational fluid dynamics (CFD). The objectives are to evaluate the influences of air inlet design parameters including location and size, and transformer load, on the airflow distribution, temperature field, and cooling efficiency. The study finds that staggered opposite inlets optimize cooling uniformity without airflow attenuation. Compared to a single inlet, the maximum transformer temperature is reduced by 1.3 °C and energy utilization increases by 9.1 % with staggered inlets. Increasing the inlet length ratio initially improves cooling until an optimal point (The length ratio is 1.10), while reducing the inlet height ratio decreases airflow and efficiency. With load increasing, the intake airflow rises but at a reduced rate, and the temperature difference can exceed 15 °C under high loads. In summary, optimizing inlet design enhances natural ventilation performance in indoor substations, but limitations exist at high loads, indicating supplemental mechanical ventilation may be required.

A shape-predictive model for the spreading of photo-curable polymers in material extrusion additive manufacturing

Recently, there is a growing interest in material extrusion to print thermoset resins to manufacture large format and high-performance parts. However, the fidelity and mechanical integrity of the printed parts are limited by challenges such as uncontrolled spreading of individual beads (filament/droplet) after deposition and during cure. There is a considerable lack of experimental and theoretical studies on the spreading of reactive beads on solid substrates. In this work, we studied the simultaneous spreading and photo-curing of the photopolymerizable thermoset beads via experiment and numerical simulations. We used a novel experimental setup to track the spreading of droplets and filaments during photopolymerization and validate a moving mesh computational fluid dynamics (CFD) model. The CFD model was used to develop an approach (predictive model) to accurately predict the final spreading coefficient of cured resin beads without the need for full numerical simulations. The predictive model combines the generalized theory of a Newtonian spreading filament with a characteristic viscosity μave and time to gelation, τgel. Interestingly, μave is shown to be a material parameter that does not depend on processing conditions, but only on the material’s chemorheology. The predictive model is tested against a wide range of chemorheology and cure kinetic parameters and found to be in excellent agreement with the full numerical CFD simulations. This work will be very useful in estimating the final shape of beads during the material extrusion printing process, as well as a model to successfully parameterize extrusion-based 3D printers to control the shape of printed beads a-priori.

Evaluation of System Identification Methods for Free Vibration Flutter Derivatives of Long-Span Bridges

The significance of long-span bridges being susceptible to wind-induced vibrations and the need for evaluating their aerodynamic performance is the focus of this study. The main emphasis is on experimental methods for assessing the bridges’ aerodynamic stability, using sectional model tests with the free vibration technique. The dynamic properties of the model are determined from the measured response, using various system identification methods, including the modified Ibrahim time domain (MITD) and iterative least squares (ILS) for two-degree-of-freedom systems and the logarithmic decrement method (LDM) and the Hilbert transform method (HTM) for single-degree-of-freedom (SDOF) systems. A new dynamic testing setup was designed to facilitate single-degree-of-freedom (heave and pitch) and coupled two-degree-of-freedom (2DOF) motion in a wind tunnel section model. The vertical and torsional stiffnesses of the model were adjusted with elastic springs. A Great Belt Bridge section model was selected for testing due to its streamlined aerodynamic shape. The direct and crossflow derivatives were extracted from the measured response using the system identification methods mentioned. Additionally, analytical studies and numerical computational fluid dynamics simulations were conducted to validate the experimental results. The study found that HTM is most effective in SDOF due to its ability to extract both damping and frequency from the nonlinear response, whereas the MITD method is faster in converging system parameters in 2DOF system tests. The experimental and numerical results are comparable to the flat plate, which confirms the streamlined behavior of the Great Belt section from an aerodynamic perspective.

Modeling and validation of coarse-grained computational fluid dynamics–discrete element method for dense gas–solid flow simulation in a bubbling fluidized bed

Computational fluid dynamics (CFD) combined with the discrete element method (DEM) are powerful tools for analyzing dense gas–solid flows. However, the computational cost of CFD–DEM will be unfeasibly great when simulating large-scale engineering applications with billions of particles. Accordingly, the coarse-grained (CG) CFD–DEM method is applied to solve this problem. This investigated method replaces several smaller particles with larger ones called parcels, aiming to reduce the number of particles and fully consider the collision of particles between composition parcels and the collision of particles within composition parcels. First, high-speed photography verifies the numerical simulation's reliability. Then, the CG CFD–DEM was used to analyze the transient spatial distribution, transient average velocity, pressure drop, bed height, and the mixing state of particles in a dense gas–solid fluidized bed. The CG CFD–DEM was also compared with the CFD–DEM results, which showed a good agreement with the calculation results and proved the accuracy and applicability of the method. Finally, the computation time of the CG CFD–DEM was evaluated, showing a significant decrease in computation time with an increasing coarse ratio (k). This investigation can provide theoretical reference for the numerical simulation of the CG CFD–DEM method in dense gas–solid flow.

Influence of fuel distribution on co-combustion of sludge and coal in a 660 MW tangentially fired boiler

The distribution method of coal and sludge is an important factor affecting co-combustion, but few studies have been carried out in most papers. The influence of different fuel distribution methods for municipal sewage sludge and coal on co-combustion in a furnace was studied using numerical computational fluid dynamics simulations in this paper. In the considered cases, the burners that inject sludge were artificially altered, while the masses and contents of sludge and coal remained unchanged. The combustion temperature, combustion characteristics, and the heat of the boiler were analysed. The concentrations of CO, CO2, and volatile matter were used as characteristic parameters. The conclusion shows that the co-combustion temperature in the main combustion region inevitably decreases compared to coal mono-combustion regardless of the fuel distributions being used. When coal and sludge are premixed, the furnace temperature distribution is more uniform. However, premixing has a greater influence on the boiler heat flux. Feeding sludge into the furnace from the top layer of burners individually can effectively reduce the adverse impact of premixed combustion, and the total heat flux of the furnace is reduced by 4.8%, the heat flux decreased by about 6% for other distribution methods. Considering the broad prospects of sludge co-disposal in boilers, these findings provide guidance and suggestions for the co-combustion of sludge and coal, and they provide theoretical support for the efficient operation of boilers.

Numerical simulation of pneumatic throttle check valve using computational fluid dynamics (CFD)

The article presents a numerical CFD simulation of a throttle-check valve used in an innovative control system for two pneumatic drives. This type of control is used in an innovative rehabilitation device for lower limbs. In order to determine the boundary conditions, experimental tests were carried out. The throttle valves on the test stand were scaled and the air flow rate values were read for different valve opening heights. The purpose of this article is to present a CFD simulation of a pre-adjusted check valve throttle. Numerical simulation (CFD) makes it possible to study the flow phenomena inside a pneumatic throttle-check valve, with different sizes of flow gaps. The obtained results made it possible to determine the distribution of physical quantities of static pressure, the velocity of the medium flowing through the valve, or the vector velocity distribution. The throttle valve assembly has been scaled for a suitable degree of synchronization of the movement of the piston actuators independently of the different external loads acting on each of them. The authors investigated airflow phenomena for different valve opening heights. The simulation provided information on the occurrence of supersonic and subsonic flow velocities at specific valve opening heights.

Simulated stress mitigation strategies in embedded bioprinting

Extrusion-based bioprinting is a powerful tool for fabricating complex cell-laden constructs. Embedded ink writing (EIW) is an extrusion-based printing technique wherein a nozzle embedded into a support bath writes continuous filaments. Because it allows for low-viscosity inks, EIW is particularly useful for bioprinting. One of the largest challenges in extrusion-based bioprinting is limiting the damage that cells experience inside the nozzle. Longer shear stress durations and higher shear stress magnitudes lead to more damage. Shape fidelity is also critical for bioprinting. Filaments in EIW can exhibit defects such as sharp edges and large aspect ratios, which can lead to porosity, surface roughness, and poor mechanical properties in the final part. We use numerical computational fluid dynamics simulations in OpenFOAM to evaluate whether common shear stress mitigation techniques improve cell viability without causing shape defects. Critically, we find that using a conical nozzle, increasing the nozzle diameter, decreasing the print speed, and decreasing the ink viscosity can improve the viability of stress magnitude-sensitive cells, but using a conical nozzle, increasing the nozzle length, and decreasing the print speed can increase damage in stress duration-sensitive cells. Additionally, using a conical nozzle or a larger nozzle can lead to larger shape defects in printed filaments. Material selection and printing parameter selection in embedded bioprinting should take into account allowable shape defects, allowable cell damage, and cell type.

Thermal uniformity enhancement of the motorcycle exhaust thermoelectric generator—Part 2: Muffler outlet optimization

AbstractFollowing the development of the muffler with the optimal guide fins (MOGF) in the companion paper (Part 1), this study optimizes the muffler outlet in terms of structure and geometry to further improve the thermal uniformity of the hot‐side heat exchanger (HHE). Two outlet structures and two outlet geometry parameters (i.e., outlet angle φ and outlet position y) are proposed. The exhaust flow uniformity of HHE serves as an assessment criterion in numerous computational fluid dynamics simulations to determine the optimal outlet structure and optimal values of φ and y. The outlet of the MOGF is then modified based on the simulation results to create the muffler with the optimal guide fins and outlet (MOGFO). Various experiments on the MOGFO are conducted to verify the simulation results and evaluate the improvement of the HHE's thermal uniformity. The simulation results prove that the optimal outlet improves the exhaust flow uniformity by 1.78% on average compared with the original outlet. In experiments, the combination of optimal muffler's outlet and optimal muffler's guide fins on the MOGFO raises the thermal uniformity of HHE by an average of 22.71% and 50% compared with the MOGF and the muffler model with nonoptimal guide fins and outlet in Part 1, respectively. The remarkable outcomes of this paper, together with Part 1, are expected to enhance the efficiency and lifespan of the thermoelectric generator unit.

How is a turbidite actually deposited?

The deposition of a classic turbidite by a surge-type turbidity current, as envisaged by conceptual models, is widely considered a discrete event of continuous sediment accumulation at a falling rate by the gradually waning density flow. Here, we demonstrate, on the basis of a high-resolution advanced numerical CFD (computational fluid dynamics) simulation and rock-record examples, that the depositional event in reality involves many brief episodes of nondeposition. The reason is inherent hydraulic fluctuations of turbidity current energy driven by interfacial Kelvin-Helmholtz waves. The experimental turbidity current, with realistic grain-size composition of a natural turbidite, used only 26 to 33% of its in-place flow time for deposition, while the remaining time went to the numerous episodes of sediment bypass and transient erosion. The general stratigraphic notion of a gross incompleteness of sedimentary record may then extend down to the deposition time scale of a single turbidite.

Airway resistance variation correlates with prognosis of critically ill COVID-19 patients: A computational fluid dynamics study

ObjectiveTo evaluate the quantitative changes of respiratory functions for critically ill COVID-19 patients with mechanical ventilation, computational fluid dynamics (CFD) analysis was performed based on patient-specific three-dimensional airway geometry. Methods37 cases of critically ill patients with COVID-19 admitted to the ICU of Huangshi Traditional Chinese Medicine Hospital from February 1st to March 20th, 2020 were retrospectively analyzed. 5 patients whose clinical data met the specific criteria were finally cataloged into death group (2 patients) and survival group (3 patients). The patient-specific three-dimensional airways were reconstructed from the central airways down to the 4th-5th bifurcation of the tracheobronchial tree. The volume changes of bronchi were calculated during the disease progression according to the comparison of two CT scans. Additionally, the changes of air flow resistance were analyzed using numerical simulation of CFD. ResultsPearson correlation analysis demonstrated that there was negative correlation between the change of volume (ΔV) and the change of resistance (ΔR) for all COVID-19 patients (r=-0.7025). For total airway volume, an average decrease of -11.41±15.71% was observed in death group compared to an average increase of 1.86±10.80% in survival group (p=0.0232). For air flow through airways in lower lobe, the resistance increases for death group by 10.97±77.66% and decreases for survival group by -45.49±42.04% (p=0.0246). ConclusionThe variation of flow resistance in the airway could be used as a non-invasive functional evaluation for the prognosis and outcome of critically ill patients with COVID-19. The ‘virtual’ pulmonary function test by integrating follow-up CT scans with patient-derived CFD analysis could be a potentially powerful way in improving the efficiency of treatment for critically ill patients with COVID-19.

Enhancing vortex-induced vibrations of a cylinder with rod attachments for hydrokinetic power generation

A new converter consisting of a circular cylinder on an end-spring is proposed to harness hydrokinetic power from water flow and tides. Two cylindrical rods with a small diameter are attached to both sides of a cylinder parallel to the axis of symmetry of the cylinder above and below the stagnation point. It is inspired by the trembling of cables with ice attached in barely noticeable winds. Numerical CFD (Computational Fluid Dynamics) simulations are carried out using a two-dimensional Reynolds-Averaged Navier-Stokes equations solver, which is based on ANSYS Fluent using User-defined Function. It is found that two small-diameter cylindrical rods attached to the main cylinder at 55° to 65° with respect to the incoming flow significantly expand the aeroelastic instability range for the main cylinder thereby increasing the generated power. The water flow energy harvester with this structure harvests energy exceeding the onset speed of VIV and is sustained over a wide water speed range.

A bi-fidelity surrogate modeling approach for uncertainty propagation in three-dimensional hemodynamic simulations

Image-based computational fluid dynamics (CFD) modeling enables derivation of hemodynamic information (e.g., flow field, wall shear stress, and pressure distribution), which has become a paradigm in cardiovascular research and healthcare. Nonetheless, the predictive accuracy largely depends on precisely specified boundary conditions and model parameters, which, however, are usually uncertain (or unknown) in most patient-specific cases. Quantifying the uncertainties in model predictions due to input randomness can provide predictive confidence and is critical to promote the transition of CFD modeling in clinical applications. In the meantime, forward propagation of input uncertainties often involves numerous expensive CFD simulations, which is computationally prohibitive in most practical scenarios. This paper presents an efficient bi-fidelity surrogate modeling framework for uncertainty quantification (UQ) in cardiovascular simulations, by leveraging the accuracy of high-fidelity models and efficiency of low-fidelity models. Contrary to most data-fit surrogate models with several scalar quantities of interest, this work aims to provide high-resolution, full-field predictions (e.g., velocity and pressure fields). Moreover, a novel empirical error bound estimation approach is introduced to evaluate the performance of the surrogate a priori. The proposed framework is tested on a number of vascular flows with both standardized and patient-specific vessel geometries, and different combinations of high- and low-fidelity models are investigated. The results show that the bi-fidelity approach can achieve high predictive accuracy with a significant reduction of computational cost, exhibiting its merit and effectiveness. Particularly, the uncertainties from a high-dimensional input space can be accurately propagated to clinically relevant quantities of interest (e.g., wall shear stress) in the patient-specific case using only a limited number of high-fidelity simulations, suggesting a good potential in practical clinical applications.

A one-way coupled numerical magnetic field and CFD simulation of viscoplastic compressible fluids in MR dampers

Recent developments in employment of magnetorheological (MR) fluids in different applications heighten the need for better understanding of their non-linear flow characteristics. In this paper, the modelling of the dynamic hysteretic behaviour of a twin-tube MR damper via a novel one-way coupled numerical approach is presented. The approach couples the Finite Element Analysis (FEA) of the MR damper magnetic circuit with the Computational Fluid Dynamics (CFD) analysis of the fluid flow field. A User-Defined Function (UDF) predefines the fluid shear-rate-dependent apparent viscosity into the CFD solver according to the magnetic field density attained from the FE solver. Two transient CFD cases are presented, one of them studies the flow in the damper as an incompressible single-phase flow, while the other accounts for the effect of fluid compressibility in the liquid-gas domain as a two-phase flow. The validation of the proposed numerical approaches is achieved via the direct comparison with the published experimental measurements for the same MR damper. It has been found that the fluid compressibility affects the hysteretic behaviour of MR dampers greatly. Moreover, the flow field shows the distributions of pressure, velocity and viscosity contours. In particular, the results show higher non-Newtonian viscosity in the throttling area and lower Newtonian viscosity elsewhere. Furthermore, it is shown that the variation of some design parameters of the damper, such as the width of magnetic poles, input current and frequency of the piston motion, has significant effects on the damper behaviour.

Optimization Of The Flow Distribution In V-Groove Flat Plate Solar Collector using Cfd Simulation

Flat plate solar collector is the major component of a solar heating system that converts solar radiation to thermal energy. It provides clean energy at no operating cost, however, its poor performance constitutes a serious drawback to adopt it for small application. This inefficiency is the result of involved thermal losses and the lack of full exploitation of the available energy. To exploit the maximum potential, the working fluid flow should be uniformly distributed through the collector to extract the heat from the hot absorbing surface. This study addresses the uniformity of the flow distribution for v-groove flat plate solar collector for water heating to optimize the performance of the collector. The study investigated the effect of the manifold geometry and the number of the side riser channels on the flow distribution by using numerical computational fluid dynamics simulation on Ansys Fluent Software. The mass flow rate was optimized for maximum thermal performance and then the optimum point was used for investigating the flow distribution. The simulation was validated against experimental data from literature with 99% confidence. The study found that the circular manifold gives uniform flow distribution with a standard deviation of 5% at the optimum mass flow rate of 11.5g/s. the study concluded that the tapered circular manifold is the optimum geometry for uniform flow distribution as it provides the least pressure difference inside the manifold.

Blade shape optimization of an aircraft propeller using space mapping surrogates

Propeller performance greatly influences the overall efficiency of the turboprop engines. The aim of this study is to perform a propeller blade shape optimization for maximum aerodynamic efficiency with a minimal number of high-fidelity model evaluations. A physics-based surrogate approach exploiting space mapping is employed for the design process. A space mapping algorithm is utilized, for the first time in the field of propeller design, to link two of the most common propeller analysis models: the classical blade-element momentum theory to be the coarse model; and the high-fidelity computational fluid dynamics tool as the fine model. The numerical computational fluid dynamics simulations are performed using the finite-volume discretization of the Reynolds-averaged Navier–Stokes equations on an adaptive unstructured grid. The optimum design is obtained after few iterations with only 56 computationally expensive computational fluid dynamics simulations. Furthermore, an optimization method based on design of experiments and kriging response surface is used to validate the results and compare the computational efficiency of the two techniques. The results show that space mapping is more computationally efficient.

Numerical CFD Simulations and Indicated Pressure Measurements on a Sliding Vane Expander for Heat to Power Conversion Applications

The paper presents an extensive investigation of a small-scale sliding vane rotary expander operating with R245fa. The key novelty is in an innovative operating layout, which considers a secondary inlet downstream of the conventional inlet port. The additional intake supercharges the expander by increasing the mass of the working fluid in the working chamber during the expansion process; this makes it possible to harvest a greater power output within the same machine. The concept of supercharging is assessed in this paper through numerical computational fluid dynamics (CFD) simulations which are validated against experimental data, including the mass flow rate and indicated pressure measurements. When operating at 1516 rpm and between pressures of 5.4 bar at the inlet and 3.2 bar at the outlet, the supercharged expander provided a power output of 325 W. The specific power output was equal to 3.25 kW/(kg/s) with a mechanical efficiency of 63.1%. The comparison between internal pressure traces obtained by simulation and experimentally is very good. However, the numerical model is not able to account fully for the overfilling of the machine. A comparison between a standard and a supercharged configuration obtained by CFD simulation shows that the specific indicated power increases from 3.41 kW/(kg/s) to 8.30 kW/(kg/s). This large power difference is the result of preventing overexpansion by supercharging. Hence, despite the greater pumping power required for the increased flow through the secondary inlet, a supercharged expander would be the preferred option for applications where the weight of the components is the key issue, for example, in transport applications.

Numerical and experimental analysis of the single droplet evaporation in a ultrasonic spray pyrolysis device

ABSTRACTThe present study deals with the numerical analysis of the water droplet evaporation in the carrier gas inside an ultrasonic spray pyrolysis (USP) device. Droplet evaporation is studied through numerical computational fluid dynamics simulation using Ansys Fluent version 16.1 software. The governing equations for mass, momentum, and energy contain source terms for the effects of droplet evaporation. The results are provided as time dependent evaporation rate, temperature and diameter of droplet. Additional experimental evaporation of HAuCl4 solution droplets with temperatures of 80, 100 and 120°C was performed on a USP device. The obtained dried particles of gold chloride were characterized with TEM and analysed for their size and shapes to determine the effect of evaporation rate on the dried particle morphology. This provides insight into selecting optimal parameters for gold nanoparticle synthesis with HAuCl4 in USP, for targeted sizes and shapes of the nanoparticles.

Impact forces on the drive spoon of a large cannon irrigation sprinkler: Simple theory, CFD numerical simulation and validation

The service life and stability of the large cannon sprinklers depends on the stress distribution that occurs on the drive spoon caused by the impact force of the driving water jet. A simple theory of the forces caused by the impact of the water jet on the drive spoon was developed, and a three-dimensional (3D) model numerical simulation using computational fluid dynamics (CFD) was constructed. An experiment was carried out to verify the accuracy of the simple momentum-based theory and the numerical CFD simulations. The results showed a large deviation between the simulated and measured values under low pressure conditions, with a maximum deviation of 15.98%. However, the simulations had good accuracy under high pressure conditions. The difference between simple theory and experimental values also decreased with increasing working pressure when considering both horizontal and vertical components. The simple theory was adjusted by the use of correction factors that were regressed with working pressure using experimental data. The coefficient of determination of the correction factor expressions for both the horizontal and vertical components were 0.9799 and 0.9289 respectively. After adjustment of the simple theory, the calculated values of the vertical and horizontal impact forces were compared to the experimental values and had a difference

Design of a Pressurized Smokeproof Enclosure: CFD Analysis and Experimental Tests

Pressure differential systems have the purpose of maintaining tenable conditions in protected spaces for different types of building safe places, like escape routes, firefighting access routes, lobbies, stairwells and refuge areas. The aim of pressure differential systems is to establish airflow paths from protected spaces at high pressure to spaces at lower or ambient pressure, preventing the spread of toxic gas released during a fire. This strategy ought to be supported by a detailed design of the necessary air supply, considering also the cycle of opening and closing doors during the egress phase. The paper deals with the design of a simple pressure differential system intended to be used in a building as a pressurized smokeproof enclosure. Specifically, experimental tests and numerical modelling are conducted with the objective of characterizing the pressure evolution in a small compartment under different conditions and through a cycle of door opening. Experimental tests are conducted in a simple 3-m side cubic enclosure with two doors and no vent openings. While a centrifugal fan blows constant airflow inside the structure, the pressure trend in time is recorded during steady state and transient conditions; additionally, the velocity of the airflow across the doors has been measured by means of an anemometer. Numerical CFD (computational fluid dynamics) simulations are carried out to reproduce the same smokeproof enclosure configuration (both geometrical and boundary conditions) using the fire dynamics simulator (FDS). Furthermore, specific attention is paid to the modelling of the leakage across the doors, directly inserted in the model through a localized HVAC (heating and venting air conditioning) advanced leakage function. Comparisons between experimental tests and numerical simulations are provided. Once the model was correctly calibrated, other geometrical and mechanical configurations have been studied, looking for convenient and efficient positions of the fan in order to fulfill the requirements of the pressure differential, airflow velocity and door handle force. The paper highlights some fundamental aspects on the pressurization and depressurization during steady state and transient phases, trying to identify if there are airflow profiles typical of some geometrical configurations.