CFD Model Research Articles

ANN and CFD driven research on main performance characteristics of solar chimney power plants: Impact of chimney and collector angle

Solar energy systems operate directly connected to the sun. Solar chimney power plants are privileged systems that can provide power output even in cloudy weather and during hours when there is no sun. The design and sizing of this system, which researchers focused on after its first application in the 1980s, is very effective on its performance. In this study, the collector slope and chimney slope that give maximum power output for the Manzanares pilot plant are investigated with a 3D CFD model. Simulations made using the RNG k-e turbulence model and the DO (discrete ordinates) solar ray tracing algorithm provide results that are in high compatibility with experimental data and literature. It is understood that the system provides maximum power at 0.6° collector slope and 1.5° chimney divergence angle. It is seen that the system, which gives a power output of approximately 46 kW in the reference case, exceeds the power output by 4.5 times and reaches 216.853 kW in the design that includes the collector and chimney slope. The effects of the main elements of the system on the performance are also included by changing the collector radius and chimney height while preserving these inclination angles. More than the power output in the reference case, 49.233 kW, can be achieved with the inclined design, with a collector radius of 73.2 m and a chimney height of 155.68 m. Although the effect of increasing the chimney height on power output continues after 1.2 floors, its effect decreases. In the study, it is seen that increasing the chimney height and changing the collector radius provide a greater increase in power output. Furthermore, the scope extends to the incorporation of an Artificial Neural Network (ANN) model, presenting a novel approach to predicting SCPP system performance. The findings ascertain the utilisation of 9 neurons in the hidden layer of the ANN, demonstrating a precise alignment with the study data.

Heat dissipation analysis and multi-objective optimization of Raspberry Pi CPU by natural convection

With the high-speed and high-frequency operation of CPU chips, as well as miniaturization and dense assembly of integrated circuits, the heat generated by the Raspberry Pi CPU is constantly increasing. To address the urgent need for efficient Raspberry Pi CPU heat dissipation and ensure normal operation within a safe temperature range, this study first established a model to simulate the CPU's thermal physics parameters during operation using CFD simulation software. An experimental platform was then set up to validate the CFD simulation results. Based on the validated CFD model, initial heat dissipation design was carried out. On this basis, the heat dissipation system was optimized by altering the heat sink’s length, width, fin height, thickness, spacing, and the thermal grease’s thermal conductivity between the heat sink and CPU chip. A surrogate model was established using the response surface methodology, and the NSGA-II genetic algorithm was employed for multi-objective optimization of the heat dissipation system’s cooling performance. Compared to the initial solution, the chip temperature was reduced by 6.19 °C, but the heat sink mass was increased by 0.65 g. By conducting multi-objective optimization design and making reasonable choices for the parameters, a suitable and an optimum design, in terms of both economics and efficiency, is obtained. These results demonstrate that this research can provide theoretical guidance and technical support for practical production. Furthermore, the methods and findings from this study can be applied to the design of heat dissipation systems for similar applications.

Direct multi-fidelity integration of 3D CFD models in a gas turbine with numerical zooming method

Multi-fidelity simulation improves the simulation accuracy and captures more detailed information about aero engines under limited computing resources, which is implemented by coupling different levels of models using numerical zooming methods. However, there is an obvious problem in traditional zooming methods such as the iterative coupled zooming method or mini-map method: both the convergence and accuracy depend highly on the component general characteristic maps. Based on the investigation of a micro gas turbine, a direct zooming method (Cycle with CFD in it, CWCFD) is developed. It directly embeds the 3D CFD compressor and turbine model into a 0D component-level model without component general characteristic maps. Then, the CWCFD zooming method is compared with the traditional 0D component-level model in terms of the throttle characteristics of the micro gas turbine, and the experimental data of the ground test is performed to verify the effectiveness of the CWCFD zooming methods. The results indicate that the CWCFD zooming method matches well with the test data better than the traditional 0D component-level model.

Multiscale CFD modelling of porous monoliths for heterogeneous catalysis of Knoevenagel condensation

In this research work, a multi-scale CFD model is developed for a catalytic heterogeneous reaction under continuous flow in a porous monolith. For this purpose, the Knoevenagel condensation was chosen as a model reaction. The model is based on a homogenization approach as well as pore scale modelling by coupling convection, diffusion and reaction within macro-meso porous hierarchical monoliths. The model's accuracy was determined through successful validation against previously available experimental data in the literature. Subsequently, the model was employed to examine the influence on reaction rates of structural and key process parameters like temperature, flow rates and macro/meso porosity. To analyze flow patterns and concentration profiles at the macro pore level, pore-scale simulations were conducted through image processing and CFD modeling of 25x25 µm Scanning Electron Microscopy (SEM) images of the monoliths. These simulations yielded an in-depth comprehension of pore flow and concentration behavior at the macro-meso level, offering valuable insights for optimizing pores in hierarchical structures.

Mechanism of Speed Loss Reduction and Propulsion Efficiency Improvement of ONR Tumblehome with Active-Controlled Stern Flaps in Resonance Waves

The stern flap is a practical hull appendage equipment that enhances ship navigation performance and saves energy. The existing studies mainly focus on the fixed stern flap, other than an actively controlled one, so it is worth further exploring its effect and mechanism. By implanting the PID controller to the stern flap, this paper proposed a free-running CFD model on the ONRT (the Office of Naval Research Tumblehome) ship coupled with the active-controlled stern flap to investigate the hydrodynamic performance in resonance waves. The free-running performance in calm water and regular waves is numerically researched and verified versus the experimental and referenced results. Then, the effect of different PID coefficients and control strategies of the stern flap on the traveling speed, attitudes, and propulsion performance under the resonance wave condition is conducted, and the influence mechanism is explored. The results show that adopting a fixed flap controller and PID controller can reduce the original speed loss by 4.2% and 6.9%, respectively, and increase the average propulsive efficiency of the propeller by 1.0% and 1.4%, respectively. Further analysis reveals that the global effect of the suppressed motion attitudes due to the installation of the fixed flap effectively contributes to the resistance reduction. However, the local effect of the stern flap increases the resistance due to interaction with the propeller and stern. The PID-controlled stern flap exhibits similar average attitudes compared to the fixed one, which means the resistance reduction of the global effect is kept the same, and the active stern flap further improves the stern flow field, where the resistance increment of the local effect is weakened, enhancing the traveling speed and improving the propulsion efficiency.

Design and modeling of a co-flow reactor for turquoise hydrogen production

This work focuses on the design of a reactor for producing clean hydrogen from methane pyrolysis in the form of the so-called “turquoise hydrogen”. In addition to its simple geometry, the fundamental concept and the main novelty of the proposed method rely on using part of the methane to produce the required heat needed for the thermal decomposition of methane (TDM). The reactor configuration for hydrogen production is shown to produce significant advantages in terms of greenhouse gas (GHG) emissions. A reactive flow CFD model incorporating also soot formation mechanism has been first developed and validated with experimental results available in the literature and then used to design and characterize the performances of proposed reactor configuration. 3D CFD simulations have been carried out to predict the behavior of the reactor configuration; a sensitivity analysis is used for clearing the aspect related to key environmental parameters, e.g., the global warming impact (GWI). The real potential of the proposed design resides in the low emissions and high efficiency with which hydrogen is produced at the various operating conditions (very flexible reactor), albeit subject to the presence of carbon by-product. This suggests that this type of methane conversion system could be a good substitute for the most common hydrogen production technologies.

Experimental and numerical study on multiphase fluid dynamics of coal and blends with biomass (sugarcane bagasse) in a lab-scale full-loop circulating fluidized bed gasifier

ABSTRACT The current research presents a 3-D hydrodynamic simulation of the fluidized bed riser with a validated CFD model at different superficial gas velocities and inventory composition (coal of 2 kg is blended with sugarcane bagasse weights 0.2 kg and 0.8 kg). The experiments are performed by varying superficial gas velocity (0.5 Umf to 3 Umf), coal particle size (460 μm and 460 μm), and coal bed inventory size (1.5 kg and 2 kg) in a pilot-scale circulating fluidized bed riser of 100 mm diameter and 3000 mm height. The CFD model results are in good agreement with experimental pressure drop and solid volume fraction. The effect of drag and lift force in fluidization hydrodynamics has been studied. The validated CFD model is then extended to predict the hydrodynamics of the three-fluid, coal-sugarcane bagasse-air system. A mixing factor is defined to quantify the mixing behavior of coal and sugarcane bagasse. It is concluded that a 200 gm sugarcane bagasse blend has good mixing than the 800 gm blends. For efficient mixing, coal and sugarcane bagasse fluidized beds should operate in the 2 Umf to 4 Umf superficial gas velocity range. The results of axial and radial distribution of coal, sugarcane bagasse and air volume fraction, pressure drop, slip velocity, and fluidized bed heights are presented.

Improving the performance of a condensing heat exchanger for biomass combustion at household scale

A computational CFD model representing the behavior of a condensing heat exchanger used in domestic heating and hot water applications has been presented and validated, obtaining a flow output gas temperature error of 7%. An analysis of the heat exchanger internal aerodynamic performance revealed the need for geometry enhancements which involved increasing the number of combustion gas passages, reducing the cannon diameter, and incorporating turbulence generators.The geometric modifications produced a significant increase in variables of interest for the performance of the equipment when compared to the baseline case. The improvement is quantified as the 68% increase in turbulent intensity, 57% increase in velocity, doubling of flue gas residence time and 96% increase in total heat transfer rate.The design that contains turbulence generators presents the best performance, obtaining the highest heat transfer rate equal to 3.5 kW, which produces a decrease in the flue gas temperature equal to 62% and an increase in the water outlet temperature equal to 53% compared to the base case. In contrast, the prototype without turbulence generators showed a relatively lower performance in terms of heat transfer, but with a 9 times lower pressure drop.

STAR-CCM+ simulation of debris bed formation in the unheated DEFCON-S experiment

In a severe accident of light-water reactor, molten corium can be formed and discharged into the reactor cavity. If the resulting debris bed is not adequately cooled, it could threaten the containment integrity by causing a molten core-concrete interaction. This study analyzes a non-heating debris bed formation experiment of DEFCON-S using the STAR-CCM+ code. Relevant CFD models are established to reflect the interaction between particles and fluids to describe the particle behavior and flow of fluids. The shape and size of debris bed are predicted through the CFD calculation. The rolling resistance becomes an important parameter in this simulation. The prediction with the default values of STAR-CCM+ forms a spread shape of particle bed compared to the experimental result. The shape of debris bed obtained with modified rolling resistance coefficients gives consistency with the DEFCON-S experiment, having a deviation in diameter of 4.6 % and a deviation in height of 3.8 %.

Plugging of pipes by cohesive particles. Computed tomography investigation and theoretical analysis

Flow regimes leading to clogging of pipes exist for most multiphase flows with cohesive particles. Although plugging is critical in many industrial and medical applications, there are few records describing the details of the process. To address the problem, we conduct a computed tomography (CT) study of plugging in a cohesive ice slurry. We run experiments for ice concentration of 15%, Re∼3500, particle size 0.2-0.4 mm, and their surface energy ∼150 mJ/m2. The CT scans, combined with experimental logs of the main flow parameters, revealed the formation of deposits around the orifice inserted into the pipe, with a complete blockage formed downflow the orifice. The deposition efficiency of ice was relatively low ∼ 10−3. Reproducing the experimental deposition efficiency with the Lagrangian CFD model, we extracted the probability of particle capture at the orifice, which was about 0.5%. The simulation results illustrated how inter-particle interactions hindered the plugging.

Multiphase CFD modelling of Head loss characteristics of Iron-Ore solid-liquid suspension flowing inside 90° pipe elbow section of slurry conveying system

Multiphase CFD modelling of Head loss characteristics of Iron-Ore solid-liquid suspension flowing inside 90° pipe elbow section of slurry conveying system

Analysis Of Hybrid Solutions For Coastal Protection Combining Physical And Numerical Cfd Modeling

Historically, flooding and erosion challenges in coastal areas have been addressed through conventional gray/rigid structures such as breakwaters, dikes, and walls (Morris et al., 2017). As a consequence of the context of climate change, which is accompanied by a global biodiversity crisis and an increasing risk to coastal systems and communities, Nature-based Solutions (NbS) have emerged in recent years as an alternative to conventional engineering options. NbS can completely or partially mitigate coastal flooding and erosion problems while offering several additional co-benefits, such as carbon sequestration, improved water quality, habitat creation, and more (Sutton-Grier et al., 2015). Nevertheless, NbS may not be effective on their own in areas where there is insufficient available space for their development or in high-risk areas. In such cases, combining conventional engineering with nature-based solutions to obtain a so-called hybrid solution can represent an optimal approach, capable of providing the necessary risk reduction and realizing the benefits associated with natural solutions (Vuik et al., 2016). This makes hybrid solutions a highly attractive option that is currently gaining increasing interest. However, due to the limited number of real cases implemented, their relatively novel nature, and existing gaps in our knowledge about their hydrodynamic behavior, there is a pressing need to study hybrid solutions in greater detail. To this end, an experimental campaign is being conducted and is complemented by numerical CFD modeling, coupling IH2VOF and OpenFOAM models, to better understand the coastal protection services provided by the combined solution and to assess the suitability of the method of process superposition, commonly used to analyze the interaction between the flow and the hybrid solution. The main variables analyzed for this purpose are the evolution of wave height and wave run-up.

CFD Air Flow Evaluation of Finned Tube Evaporator for Refrigerated Display Cabinet Application

This study is aimed to develop a simulation to improve the performance of the finned tube evaporator which is applied to the refrigerated display cabinet. CFD model was developed to be able to analyse the characteristics of air flow inside the fin gap and air side heat transfer coefficient. Geometry of the model of overall finned tube evaporator is considered covering two aluminium wavy fins with an air flow in between, combination of staggered cooper tubes with refrigerant flow inside. Fin gap is designed 4 mm to anticipate frost on the fin surface that can block air flow. Turbulence models used in the study is the realizable k-ε turbulence which had the best performance turbulence model and it was validated with secondary data from previous studies and shows the lowest error only 5.9 %. The use of CFD was found to be sufficiently representative of the heat transfer characteristics of evaporators, and acted as an effective simulation tool to determine the heat transfer coefficient in order to improve efficiency in terms of improved design. The characteristics of air flow between the fin gap and around the tube was obtained various and complex. In the case study the entry velocity of 1.7 m /s at the highest turbulence condition of the first row can reach speeds of 2.75 m/s. Hight turbulence regime in flow can indicate higher the heat transfer coefficient of the evaporator.

CFD modelling of Close-Coupled Gas Atomisation (CCGA) process by employing the Euler-Lagrange approach to understand melt flow instabilities

This paper presents a CFD analysis of the melt and flow-field instabilities in a close-coupled gas atomisation process (CCGA) caused by the gas–melt interactions. The melt mass flow is coupled to the atomiser internal pressure, which varies over time due to the unsteady flow field. A two-phase flow of Argon gas and melt particles is modelled using a coupled Euler-Lagrange framework. Three different initial gas-to-melt ratios (GMRs) of 5.5, 2.6 and 1.32 are considered to study the gas–melt interaction. The results show that in the case GMR = 1.32, sustained instability both in the melt and in the flow-field are distinctly observed, resembling physical atomisation process. This melt fluctuation corresponded to an alternating flow-field fluctuation; alternating between open- to closed-wake condition, where the local maxima of the melt corresponded to an open-wake condition, and the local minima of the melt corresponded to a closed-wake condition.

Multiphysics performance evaluation of thermoelectric generator arrays

Thermoelectric generators (TEGs) are widely used nowadays in heat recovery and power generation applications. TEGs are connected in series and/or parallel configurations in an array to meet the voltage and/or current requirements of electric load. Therefore, it is necessary to examine the electrical performance of different TEG array configurations employed in TEG systems. In the present study, an experimentally validated hydro-thermoelectric multiphysics computational model is employed by integrating the computational fluid dynamics (CFD) approach with the TEG model to test the electrical performance of various TEG array configurations and determined load voltage, current, associated power, and conversion efficiency. Each TEGs array configuration’s performance is evaluated by varying the hot water temperature from 27 °C to 42 °C as well as hot and cold water flowrate from 0.5 L/min to 2 L/min. Various configurations including series and a combination of series and parallel connections were designed. The findings show that the series configuration of the TEGs array generates higher electric power than series and parallel combined configurations whereas the latter achieves the maximum power point at a higher electric current than the series configuration. The comparative analysis of CFD model with the experimental results indicates a maximum discrepancy of 7 %. For thermoelectric power-generating systems, the proposed model might serve as a benchmark for optimizing TEGs array configurations according to the electric load requirements.

Exergy performance analysis of hydrogen recirculation ejectors exhibiting phase change behaviour in PEMFC applications

A comprehensive analysis of flow behavior and exergy performance was conducted in hydrogen recirculation ejector, taking into account the phase change of humid hydrogen for proton exchange membrane fuel cell (PEMFC) systems. A two-phase nonequilibrium condensation CFD model integrating the entropy transport equations was established. The effect of phase change on ejectors’ performance under various primary pressure, secondary pressure and temperature, and back pressure were analyzed. When primary pressure was adjusted into 3.0 bar, a liquid fraction of 4.56 % was observed at the outlet, while the liquid fraction will increase to 15.35 % when the inlet pressure got to 5.0 bar. Then, the values of mass flow rate and liquid mass fraction on different sections under variable secondary temperature and back pressure were calculated. Finally, it is found that a larger primary pressure brought an increase in exergy destruction. So was exergy destruction ratio. However, increasing secondary pressure leads to the opposite result. With the increase of secondary temperature from 60 to 80 °C, exergy destruction increases from 330.28 to 390.23 kJ/kg but the destruction ratio decreases from 29.85 % to 26.19 %.

Micromixing efficiency of non-Newtonian fluid in rotating packed beds with novel tube-tube premixers

Rotating packed beds (RPBs), as the typical chemical intensification devices, have been widely applied in liquid-phase fast reactions and precipitation processes. In this study, three novel tube-tube premixers were designed, and three-dimensional CFD models of the flow and mixing processes for non-Newtonian fluids in tube-tube premixers were established. Meanwhile, the micromixing efficiency in RPBs with different tube-tube premixers was experimentally investigated. The results show that TG-2 premixer exhibits the best premixing performance. The segregation index decreases as the rotational speed or flow rate increases, indicating the increase in micromixing efficiency. With the increase of CMC mass fraction, the segregation index initially decreases and then increases. Moreover, the micromixing efficiency of TG-0-RPB is the highest in aqueous solution, the micromixing efficiency of TG-4-RPB is the highest in 0.2 wt.% CMC system, and the micromixing efficiency of TG-2-RPB is the highest when the CMC concentrations is equal to or greater than 0.4 wt.%. The micromixing times of non-Newtonian fluid in RPBs range from 2 to 10 ms.

Effect of particle size distribution variability on the permeability of hydrate-bearing sediments: A CFD study

Natural gas hydrates (NGH) are considered as a future clean energy in the era of carbon neutrality. The seepage behaviour of hydrate-bearing sediment (HBS) and its controlling factors are significant for developing effective production strategies. In this study, we aim to elucidate how the heterogeneous spatial distribution of clayey-silty sediment particles, particularly the non-uniform particle size distribution (PSD) affects the permeability of HBS. An algorithm was developed to generate the heterogeneous spatial distribution of sediment particles based on the PSD of hydrate-bearing cores extracted from Japan Nankai Trough (D50 = 122 μm). A series simulation of fluid flow in porous media based on CFD was conducted for the HBS with varying hydrate saturations (SH). The watershed segmentation algorithm was employed for the identification of pore structure, including pore space and pore throat with a detailed analysis on the pore structure evolution with SH. Host sediments with higher uniformity result in initial high permeability but the permeability reduction with increasing SH is more significant compared with that of lower uniformity. Pore throat width reduced to less than 5 μm with SH above 50% compared with its original width of ∼30 μm. Tortuosity exhibits a linear increase with SH, while permeability decreases exponentially with increasing tortuosity. We propose an improved permeability model (KCpro) based on the classical Kozeny-Carman model accounting for the change in specific surface area for non-uniform PSD in HBS with good agreement. This study confirmed the applicability of CFD modelling as a promising tool to study the seepage behaviour of HBS complementary to classical permeability experimental tests. Results of this study provide fundamental understanding on how permeability evolves in HBS with significant PSD variations in natural gas hydrate reservoirs in nature.

Bleed air CFD modelling in aerodynamic simulation of A heavy duty gas turbine compressor

This study investigates the effects of different bleed air CFD modelling methods on the aerodynamic simulation of a heavy-duty gas turbine multistage compressor with bleed airflow. Two methods, the local source term method and the simplified bleed off-take geometry method, are compared. The results reveal that while overall compressor performance shows minimal variation between the methods, significant differences are observed near the blade tip region. The bleed source term method predicts a more uniform flow field in the bleed cavity, leading to a larger surge margin for the compressor. Therefore, in the analysis of multistage compressors, it is advisable to adopt simplified geometric modeling method for simulating bleed air extraction. Moreover, when utilizing this approach, careful consideration should be given to the choice of the slot inclination angle. This design parameter presents a crucial trade-off between losses within the exhaust structure and those arising from the entry of mainstream flow into the exhaust slot cavity.

Large Scale Physical Model Study On Clay Erosion With Gras Cover On Primary Coastal Defence Structures

Sea dikes in the Netherlands consist of sandy or clay core with a clay layer with grass. The lower outer slope is often covered with a hard revetment, such as asphalt or a placed block revetment, to protect the dike material against wave impact. Since the strength of the clay layer with a grass subject to severe wave impact is unknown, the hard revetment needs to be constructed on a large part of the outer slope to ensure that the flood defence meets the safety requirements. Therefore, a research programme was started to determine the strength of the clay layer with grass on the upper slope of a dike. As a first step, large scale physical model tests have been performed. In the second step the CFD model OpenFOAM has been used to extend the physical model results by computing the peak pressure of the hydraulic load on the clay for the tested geometries and additional geometries with varying parameters that may influence the erosion rate. With all results, it was possible to derive formulas for failure probability calculations in the final step. The knowledge development on the erosion of a clay layer with grass on the outer slope by wave attack resulted into formulas that are currently used in the safety assessment of dikes contributing towards safer deltas and resilient designs of dikes.