Simplified Computational Fluid Dynamics Research Articles

Performance of a Simplified Computational Fluid Dynamics Model for a Phase Change Material–Water Finned Heat Exchanger Under Different Orientations

ABSTRACTThe prevalent numerical models for simulating axially finned heat exchangers with phase change materials (PCMs) and water as the heat transfer fluid rely on computational fluid dynamics (CFD) techniques, with a primary focus on phase change modeling. However, the computational demands of these models, incorporating phase change effects and resolving PCM movement in the liquid state, are substantial. From experiments suggesting that conduction in the solidified PCM around the finned tube dominates heat transfer during the heat discharge process, this article introduces a simplified CFD‐based model in which convective flow of the PCM is neglected. The model is experimentally validated using a 1‐m‐long axially finned heat exchanger prototype with four fins, recording temperatures under different water flow rates and orientations (horizontal and vertical). Results show that the proposed model predicts outlet water temperature satisfactorily, with absolute errors below 1.0°C and 2.2°C for the horizontal and vertical orientations, respectively. Additionally, the model can capture the temperature trend inside the PCM for the horizontal orientation.

Microclimate Investigation in a Conference Room with Thermal Stratification: An Investigation of Different Air Conditioning Systems

This paper investigates the microclimate in a conference room with thermal stratification, taking as a case study the chapel of Villa San Saverio, now the seat of the “Scuola Superiore” of the University of Catania (Italy). Surveys of the former chapel were conducted to monitor air temperature and relative humidity. Subsequently, the investigation relied on numerical simulations of a simplified computational fluid dynamics (CFD) model built with the DesignBuilder v7.0 software and validated by comparison with measured values. Simulations were then carried out considering three different scenarios: the current state without any HVAC system and two possible HVAC system configurations providing both air conditioning and ventilation. The results show that, from a comfort perspective, a lightweight radiant floor heating system, assisted by an appropriate ventilation system for air renewal placed at the floor level near the occupants, is preferable to floor-level fan coils and high ventilation channels. Furthermore, this was also confirmed by a preliminary energy analysis of the two HVAC options, where the ventilation effectiveness of the winter period, the temperature of the water the emitters are fed, the consequent COP value of the heat pump, and the electricity consumption were taken into consideration.

Whole-Body Cryostimulation: New Insights in Thermo-Aeraulic Fields inside Chambers

(1) Background: This article presents a study that aims to provide a precise understanding of the temperature distribution within a whole-body cryostimulation (WBC) chamber, whether it is empty or occupied by one or several individuals; (2) Methods: The study employs a mixed numerical and experimental approach, utilizing simplified computational fluid dynamics (CFD) simulations and experimental analysis; (3) Results: The results reveal a non-negligible temperature difference between the setpoint and actual temperature in the middle of the cryochamber. Furthermore, it is shown that the presence of individuals inside the chamber results in both an average temperature rise and a more heterogeneous thermal behavior associated with the number of individuals present. As the number of occupants in the cryochamber increases, the magnitude of the thermal gradient (up to 10 °C) and temperature heterogeneity (up to 13%) also increase; (4) Conclusions: The results suggest that when the cryotherapy chamber is occupied by three people, it becomes necessary to extend the duration of cold exposure to obtain a dose/effect ratio and analgesic threshold equivalent to those obtained when only one person is present. The findings of this study emphasize the need for further research to establish temperature guidelines and standardize measurement methods for effective WBC treatment.

A generalized zoning framework for development of CFD‐based reduced‐order compartment models

AbstractWhile computational fluid dynamics (CFD)‐based compartment models have gained popularity as a cost‐effective alternative to full CFD modeling of complex mixing systems, model development involves significant trial‐and‐error efforts. This work presents a generalized zoning framework for the streamlined development of CFD‐based compartment models with detailed characterization of the reduced‐order representation of the flow physics. With a stirred tank as an illustrative case, reduced‐order model for species transport and heat transfer with turbulent flow is derived, followed by introducing the generalized zoning framework to demonstrate how a reduced‐order compartment model can be constructed based on a simplified CFD simulation. A test case of mixing two miscible thermal fluids is used to evaluate the CFD‐based compartment model. The results demonstrate that the proposed zoning framework exhibits an accurate representation of the CFD simulation of hydrodynamics and demonstrates capabilities of capturing species and heat transfer in turbulent flow systems with complex geometric configurations.

Flow in a large wind field with multiple actuators in the presence of constant vorticity

Study of wind farms is an area of active research. Researchers have proposed simplified wind farm models that define the wake structure in a wind farm and how they affect the performance of the wind turbines. Interestingly, these models do not take into account an important aspect of fluid flow, i.e., the fluid–structure interaction (FSI) between the turbines and the wind, which has an important role to play. This motivated researchers to implement numerical analysis tools to model the geometry of the wind turbines in computational fluid dynamics (CFD) based models of a wind farm in order to better understand the wake structure and study the performance of the wind farms. However, modeling the complex geometry of the blades and the turbines makes these models computationally expensive. In this paper, we propose an FSI methodology which can simplify the blade resolving CFD models and eliminate the requirement for modeling these complex geometries during preliminary engineering phase. As an example, we present simulations of up to three back-to-back wind turbines and compare the results with those obtained from wind engineering software, FLORIS. The proposed methodology demonstrates how the approach can be used to develop a relatively less computationally expensive wind farm model. The approach formulated in this paper follows an intermediate way between the analytical wind farm models and CFD models by introducing modifications to one of the most basic wind farm models (Jensen's model) and using it to develop a simplified CFD model using the decomposed immersed interface method strategy.

Controlling infectious airborne particle dispersion during surgical procedures: Why mobile air supply units matter?

The ventilation system in an operating room (OR) plays a vital role in reducing the risk of patients contracting an infection while undergoing a surgical procedure. The clean air supplied from the ceiling-mounted diffuser removes the airborne particles from the surgical site. The clean air, however, is often obstructed by the medical staff and other objects. Hence, some sterile instruments might remain outside the protected area. The present study aims to examine the effectiveness of a mobile air supply (MAS) unit in reducing the particle settlement on a patient under different airflow velocities supplied from the MAS unit. A simplified computational fluid dynamics (CFD) model of the OR was developed and validated based on published data. An RNG k-epsilon turbulence model, based on the Reynolds-Averaged Navier-Stokes (RANS) equations, was used to simulate the airflow, while a discrete phase model (DPM) was used to simulate the movement of the infectious airborne particles. The MAS unit was evaluated as an extension of unidirectional airflow ventilation. Results showed that the MAS unit successfully reduced the settlement of airborne particles by 78% from 45 particles/m 3 to 10 particles/m 3 . However, the operation of the MAS unit showed a reverse effect on the particle settlement (∼7 particles/m 3 ) on the patient when the MAS unit supplied air at a velocity of 0.6 m/s. The present study showed that air supply at a velocity of 0.5 m/s provided an optimum wiping effect that removed the airborne particles from the surgical zone. • Mobile air supply (MAS) unit evaluated as extended ceiling-mounted unidirectional airflow ventilation. • MAS successfully reduced airborne particles settlement from 45 to 10 particles/m 3 (or by 78%). • Air velocity of 0.5 m/s optimally removed (wiping effect) airborne particle from surgical zone.

Computational analysis of flow conditions in hydrodynamic cavitation generator for water treatment processes.

The research on the potential of cavitation exploitation is currently an extremely interesting topic. To reduce the costs and time of the cavitation reactor optimization, nowadays, experimental optimization is supplemented and even replaced using computational fluid dynamics (CFD). One of the approaches towards sustainable water treatment is the use of the cavitation reactor with bluff elements mounted on its stator and rotor. The experimental results show that, besides the rotational speed, the spacing of the rotor pins has the most significant effect on the cavitation intensity and effectiveness, while the pin diameter and the surface roughness are less significant design parameters. The present paper uses a simplified CFD approach to investigate the conditions in the reactor and to select the optimal among a number of geometry variations.

A simplified CFD model to describe fluid dynamics, mass transport and breakthrough curves performance in cryogel supports for chromatographic separation

Cryogel-based chromatographic supports have a wide application in the bioseparation field due to the versatility of synthesis and functionalization. This study proposes a simplification of classical models applied to cryogel beds that analyze many geometrical and structural properties at the channel level (thickness, tortuosity, diameter distribution, monoliths number, etc.) resulting in an over-parameterized formulation. The proposed modeling allowed for simplified calculations and reduced parameterization by not establishing the microstructural morphology of the cryogel. Using homogeneous modeling in which the porous medium is continuous, we obtained fast-solving simulations with reduced computational requirements that can be used for real-time control system. Initially, a polyacrylamide cryogel was produced and characterized in terms of morphological structure, fluid dynamic behavior and mass transport. Next, breakthrough curves predicted by Computational Fluid Dynamics (CFD) simulations were compared with experimental data. The applicability of the modeling approach was extended to a new data set obtained from the literature under different operational conditions for complete model validation. The modeling approach represented the experimental data very well, presenting viable descriptions of protein bioseparation.

Hydrodynamic Behavior Analysis of Agitated-Pulsed Column by CFD-PBM

ABSTRACT Recently, a newly designed type of energy-input extraction column, agitated-pulsed column (APC), has been achieved high mass-transfer efficiency due to the small-size droplets and high dispersed-phase holdup. In view of the lack of feasible population-balance-model (PBM) kernel functions in APC, parameter optimization is conducted by a simplified PBM method. Then the optimized PBM kernel functions are implemented in the computational fluid-dynamics (CFD) code to investigate local two-phase flow behaviors in a 25 mm APC. The results show that the CFD-PBM successfully predicts the drop-size distribution measured in the experiments. CFD-PBM also gives good prediction of Sauter mean diameter and dispersed-phase holdup. The local flow behaviors are illustrated to understand the effects of operating conditions on the hydrodynamic performance. This work demonstrates a possibility for prediction of drop-size distribution by the combination of simplified PBM and CFD simulation.

Toward Vehicle-Level Optimization of Compound Rotorcraft Aerodynamics

This paper presents high-fidelity and simplified computational fluid dynamics modeling approaches within an optimization framework for compound rotorcraft configurations with rotor/propeller aerodynamic interactions. Simulations of an axial-flight ducted propeller and a rotor/wing test case are first presented to validate the in-house Helicopter Multi-Block 3 solver and to assess the actuator disk/line models for the simplified main rotor modeling. The actuator disk/line models are then used to represent the main rotor for simulations of a generalized rotor/propeller combination. The propeller performance is analyzed in detail, and large variations are observed in the single blade loading due to the main rotor wake. A simplified model for the rotor/propeller interaction simulation is also put forward, and an inflow distortion metric is proposed to quantify the aerodynamic interactions. With the help of a kriging surrogate model and the inflow distortion metric, aerodynamic interferences through the propeller disk are quantitatively visualized with variations in the propeller position, propeller thrust, and main rotor advance ratio. Optimization of the propeller position under the main rotor for minimized interference with rolling/pitching moment constraints is also attempted using both gradient-based (adjoint) and gradient-free (efficient global optimization) approaches. The optimization results are verified using blade resolved simulations, and fluctuations of the propeller single blade loading were effectively reduced due to the optimization. The work is a first step toward high-fidelity methods for vehicle and configuration optimization.

Experimental and numerical investigation of swirling H[formula omitted]O2 and polypropylene hybrid rocket motor with regenerative cooling

In this paper, the experimental and numerical investigations of both swirling hybrid rocket motors without and with the design of regenerative cooling (RC hereafter), using 90%-wt H2O2 and polypropylene (PP) as the liquid oxidizer and the solid fuel, respectively, were performed and compared. We implemented a new design of regenerative cooling using the oxidizer to transfer the heat generated from combustion. A series of ground horizontal hot-fire tests, which include the catalytic-bed only and the motor without/with RC were performed to measure the propulsion performance related parameters. The resulting C* efficiency of the RC design with catalytic-bed only remained in comparison with the one without RC. With a similar initial combustion chamber geometry, the RC implementation resulted in a better engine Isp efficiency from 96.3 to 98.6 with a lower O/F ratio from 6.49 to 5.94. In addition, the topologies of the burned fuel grain surface were optically measured by a 3D laser scanner. The resulting variation of regression rates between the upstream and downstream fuel for the chamber with the RC was smaller than case without the RC. Moreover, a simplified semi-empirical computational fluid dynamics (CFD) model based on the measured regression rates was constructed for investigating the physical phenomenon inside the hybrid rocket combustion chamber. The simulations show that a higher swirling number in the later portion of the port was found for the RC case. By concerning about the different O/F ratios and port size of the burned fuel, a series of simplified numerical CFD investigation with a fixed O/F and a fixed port size varying the regression rates from upstream to downstream fuel systematically with the same average region rate was performed to complement the semi-empirical CFD simulations. The results showed that there were two distinct regions including an upstream non-classical region dominated by the gas impingement of gas and a downstream classical region caused by the swirling effect of gases. In the non-classical region, the calculated distributions of the swirling numbers of the former are essentially the same for all the cases no matter what the magnitude of the regression rate is, while in the classical region higher swirling numbers are found for the case of more uniform distribution of regression rates. Heat generated near the upstream of the fuel grain was transferred more efficiently through the design of RC, which makes the resulting fuel regression more uniform than that without RC. The resulting swirling efficiency in the RC case was higher than that without RC. The HRM with RC hence had a better engine Isp efficiency with a lower O/F ratio.

Numerical modelling of oxy-coal combustion to access the influence of swirl strength, combustion environment and gasification reactions on the flow and combustion behaviour

A simplified 2D axisymmetric computational fluid dynamics simulation of oxy-coal combustion is performed to address the influence of swirl strength and the combustion environment on the flow field and combustion characteristics. A quantitative analysis of the internal recirculation zone length and flame length under various operating conditions is studied by employing developed numerical models. The influence of char-CO2 and char-H2O gasification reactions on the species consumption and temperature profile is also studied through numerical modelling. The swirl strength considerably influences both the flow field and temperature profile inside the furnace. At higher swirl strength, radially dispersed shorter flames are obtained due to spreading of the secondary stream. Results showed that the flame shifted towards the burner at higher oxygen content due to rapid devolatilisation at higher flame temperature. The ignition of pulverised coal particles is enhanced at higher oxygen concentration due to increased stoichiometry near the burner. Oxy-35% O2 has 300 K higher flame temperature than oxy-21% O2. Endothermic gasification reactions significantly influence the temperature profile in the furnace. CO2-char gasification reaction has a more pronounced influence than the H2O-char gasification reaction. The consideration of gasification reactions has resulted in an approximately 5% reduction in peak temperature.

Optimizing the Egress Route Using a New Smoke Emulator IoT System

The ability to find optimal egress routing is critical for safe and effective firefighting processes. This article presents a novel smoke emulator that can be used to provide this ability in firefighting processes. It has two fundamental components: 1) a sensor network based on the Internet of Things (IoT) and 2) a simplified computational fluid dynamics (CFD) smoke simulator based on Navier-Stokes equations. Supported by IoT, real-time events, such as door opening and window breaking, can be detected, and the relevant information can be used to update the variables in CFD to ensure high accuracy and relevancy. A long short-term memory (LSTM) is employed to evaluate the values of any sensors temporarily malfunctioning. A two-level of A* routing algorithm (Nosrati et al., 2012) has also been developed to optimize the egress routing for evacuees with an aim to minimize the time of smoke exposure. Simulation studies demonstrated that this smoke emulator helps significantly improve the firefighting process's safety and effectiveness.

On the Calculation of Propulsive Characteristics of a Bulk-Carrier Moving in Head Seas

The present work describes a simplified Computational Fluid Dynamics (CFD) approach in order to calculate the propulsive performance of a ship moving at steady forward speed in head seas. The proposed method combines experimental data concerning the added resistance at model scale with full scale Reynolds Averages Navier–Stokes (RANS) computations, using an in-house solver. In order to simulate the propeller performance, the actuator disk concept is employed. The propeller thrust is calculated in the time domain, assuming that the total resistance of the ship is the sum of the still water resistance and the added component derived by the towing tank data. The unsteady RANS equations are solved until self-propulsion is achieved at a given time step. Then, the computed values of both the flow rate through the propeller and the thrust are stored and, after the end of the examined time period, they are processed for calculating the variation of Shaft Horsepower (SHP) and RPM of the ship’s engine. The method is applied for a bulk carrier which has been tested in model scale at the towing tank of the Laboratory for Ship and Marine Hydrodynamics (LSMH) of the National Technical University of Athens (NTUA).

Modeling and Optimization of Rectangular Latent Heat Storage Elements in an Air-Guided Heat Storage System

The thermal performance of macro-encapsulated latent heat storage elements in form of thin, rectangular plates within an air-driven heat storage system is investigated. The plates are made of high-density polyethylene and filled with phase change material. Several plates are stacked together and inserted into a box which can be installed in a heating / air conditioning system. First, an experimental test rig was designed. Measured temperature and pressure drop data retrieved from the rig was then used to validate a simplified computational fluid dynamics model describing a section of the test set-up including a stack of ten plates. Afterwards, the model was further reduced in complexity in order to perform a parametric study by varying geometric parameters of the storage plates. In total, 217 different geometries were simulated and their thermo-hydraulic efficiency was evaluated. As a result, geometries were proposed, which can potentially increase the thermo-hydraulic efficiency of the storage plates by over 50%.

Visualização da ventilação natural em ensaios na mesa d´'água comparado a simulações computacionais

No processo de projeto nota-se a existência de uma lacuna entre a teoria e a prática, principalmente com relação à ventilação natural. Isso requer o uso de métodos que auxilie a visualização do fluxo de ar nos edifícios e sua transferência para os projetos. Muitas das ferramentas confiáveis de predição da ventilação natural, como o túnel de vento e os softwares de Dinâmica dos Fluídos Computacional (Computational Fluid Dynamic – CFD), são complexas e caras, dificultando o seu uso durante o ensino e na prática dos projetistas. Diante disso, o objetivo dessa pesquisa é verificar se ferramentas simplificadas representam o escoamento do ar no projeto arquitetônico de maneira fiel. Para isso, a visualização da ventilação natural através de experimentações simplificadas é comparada com ferramentas complexas e de alta confiabilidade, como simulações CFD. O método foi dividido em 3 etapas. Primeiramente, foi construído um modelo físico modular, baseado na literatura especializada. Em seguida, ensaios na mesa d'água foram executados. Por fim, simulações computacionais em uma ferramenta simplificada (software Fluxovento) e em uma mais complexa baseada na Dinâmica dos Fluídos Computacional (software CFX) foram realizadas. Os resultados demonstram a compatibilidade entre as simulações CFD e os ensaios na mesa d´água, indicando que o ensino com essa ferramenta torna acessível o entendimento de conceitos básicos da ventilação natural. Já no Fluxovento, diferenças significativas foram registradas na análise do fluxo de ar, dificultando o entendimento da ventilação natural. Ressalta-se a característica modular do modelo para elaborar novos casos em estudo de ventilação natural.

A numerical study of the impact of vegetation on mean and turbulence fields in a European-city neighbourhood

Vegetation in the urban environment has various impacts on microclimate. While optimal strategies for investigating these impacts are the subject of ongoing research, most approach rely on Computational Fluid Dynamics (CFD) simulations. We evaluate mean wind and turbulence fields simulated using the fast-running Quick Urban & Industrial Complex (QUIC) Dispersion Modeling System, a simplified CFD tool that resolves buildings and vegetation. We use QUIC to investigate the role of deciduous trees in modifying the airflow of a real neighbourhood in Bologna, Italy by running large ensembles of simulations per case study, obtained by varying the input wind direction. This approach can minimise intrinsic model uncertainty as well as uncertainties associated with real environments. Model validation is performed using measurements from an experimental field campaign focused on a vegetated urban street canyon in Bologna. Ensemble simulation results show good agreement with the experimental data for various conditions (i.e., simulation ensembles overlap experimental variability in most cases). The role of trees is investigated by comparing simulations with and without trees. Trees are found to reduce airflow by constraining local circulation and reducing turbulence intensity. Finally, the combined effect of building morphology and vegetation is investigated by adopting a formalism to represent the presence of vegetation using area-fraction coefficients. An area-fraction coefficient threshold of 0.225 has been identified that separates flow behaviours and is present in cases with and without vegetation. Below this threshold, a constant wind and turbulence regime exists, while above the threshold, winds and turbulence vary with area-fraction coefficient.

The effects of medical staff turning movements on airflow distribution and particle concentration in an operating room

The present study aims to examine the effects of medical staff’s turning movements on the number of particles falling onto the patient. A simplified computational fluid dynamics (CFD) model of the operating room was developed and validated based on the published data. An RNG k-ε turbulence model based on the Reynolds-Averaged Navier-Stokes (RANS) equations was used to simulate the airflow, while a discrete phase model was used to simulate the movement of the airborne particles. The medical staff’s turning movements were controlled by integrating a user-defined function (UDF) code and using a dynamic mesh method. Results show that medical staff’s turning movements have a significant influence on the airflow velocity distribution and particle concentration around the patient. A turning movement generally causes more particle deposition; and bending arms also increase the particle depositions when compared to straight arms.

Fluidized bed CFD using simplified solid-phase coupling.

The performance of a bubbling fluidized bed largely depends on the dynamics of solids. Therefore, it is very important to predict the bubble size and shape for modelling the degree of mixing of solid particles. The complicated non-linear characteristics of a bubbling fluidized bed make it difficult to study and model the hydrodynamic behaviour. Hence, designing of a bubbling fluidized bed is often limited to the calculation of mass balance and estimation of bed pressure drop. The aim of the present work is to evaluate a new simplified multiphase Computational Fluid Dynamics (CFD) approach for studying the hydrodynamics of a gas-solid bubbling fluidized bed. In this work, the viability of a simplified coupling of solid-phase with the gas phase over the traditional multiphase approach has been demonstrated. The available traditional multiphase CFD technique is not suitable for capturing the particulate phase in the dense stagnant regions of fluidized bed. This is because, under the traditional CFD multiphase process, solid phase is treated like a fluid. Shear stress is higher when fluid moves and zero when it is stagnant. In contrast, friction force is maximum when solid particles are stagnant. Therefore, a suitable approach is necessary to model the stagnant particulate phase in the calculation domain. In the present simplified coupling process, mass fluxes for the solid phase are obtained explicitly from the solid phase momentum equation and solid pressure is calculated from the solid phase continuity equation. These solid phase flow parameters are incorporated into the commercial CFD software single-phase gas flow by writing user-defined subroutines. The simplified coupling approach is validated against the available experimental digital images and the traditional CFD modelling results of the fluidized bed. The comparison shows, the available traditional multiphase model predicts less realistic or sometimes unrealistic results while the present numerical model captures more realistic results.

Medical Staff’s Posture on Airflow Distribution and Particle Concentration in an Operating Room

During a surgical procedure, each of the medical staffs would have different postures. Supporting medical staff such as anaesthesiologist would stand in upright condition with straighten-forearm, while medical staff that is performing surgical procedures is in bent-forearm posture. The positioning of forearm might interrupt the air supplies from the ceiling-mounted diffuser, that serves to remove the airborne particles from the surgical zone. Consequently, the movement of particles in the surgical zone is affected, and the tendency of particles to fall onto the patient’s wound is increased. This situation could elevate the chances of a patient contracting surgical site infections and could increase the risk of death. The present study aims to examine the effects of medical staff’s forearm posture on the number of particles falling onto the patient. A simplified computational fluid dynamics (CFD) model of the operating room was developed and validated based on the published data. An RNG k-ε turbulence model based on the Reynolds-Averaged Navier-Stokes (RANS) equations was used to simulate the airflow, while a discrete phase model was used to simulate the movement of the airborne particles. Results show that bent-forearm of medical staff obstructed the downward airflow to remove the particles released by the medical staff. Approximately 37 particles/m3 accumulated in the chest region of the medical staff. A high particle accumulation is also observed at the gap between the staff’s legs due to the stagnant airflow.