Two-dimensional Computational Fluid Dynamics Research Articles

Computational fluid dynamics modeling of spatial atomic layer deposition on microgroove substrates

Spatial atomic layer deposition (ALD) is a promising high-throughput technique capable of producing ultrathin films on large substrates. Compared to flat wafers, deposition on substrates with microstructures has a wider range of applications such as photovoltaic cells, electronics, flexible displays, etc. However, spatial ALD on microstructure substrates is a complex and strong-coupled process of fluid flow, heat and mass transfer, as well as chemical reactions. The fluid dynamics and the precursor distribution in spatial ALD are of great importance to obtain conformal growth with high throughput. In this study, a two-dimensional model coupling computational fluid dynamics with chemical kinetics is established to quantitatively explore the effect of microgroove structures on the fluid dynamics, precursor distribution and film conformality in an atmospheric spatial ALD system. Slip boundary condition is adopted to model the slip flow regime in the micro-gap between the injector and the substrate surface, and dynamic layering method is implemented to simulate an entire ALD cycle with the in-line movement of the substrate. Results show that while the flow field is very smooth with a flat substrate, vortices always exist in the micro-gap with microgroove substrates. Due to the angle differences between the flow direction and the vertical microstructure surface, the inflow and outflow of precursors and purging gas are prevented at the microgroove corners. A relatively lower moving speed of the substrate is beneficial for the saturated film growth and better film conformality. Increasing the carrier gas flow rate can effectively reduce the non-uniformity and non-conformality of the film, but the precursor utilization also decreases. Compared with the nonoptimal conditions, within a moderate range of carrier gas flow rate and substrate speed, the precursor utilization and film conformality have been improved. Focusing on the effect of the micro-scale features of the substrate, this study is a valuable step in extending spatial ALD for ultrathin films on miniature devices.

Numerical Investigation on the Effects of Varying the Arc length of a Windshield on the Performance of a Highway Installed Banki Wind Turbine

Two-dimensional computational fluid dynamics (CFD) simulations are employed to investigate the effects of adding a circular-arc-shaped windshield on the performance of a Banki type vertical axis wind turbine (VAWT), particularly to the application where the VAWT is harnessing wind energy in highway caused by the passing vehicles. Unsteady Reynolds-Averaged Navier-Stokes (URANS) is the computational approach used to calculate the turbulent flow within the domain. Two sets of simulation cases based on two different vehicles (i.e., car and a bus) are performed with varying arc-length of the windshield. The results show that the windshield provides an increase in the energy captured by the VAWT by up to 16.14% compared to no windshield case when the car model is used. In contrast, windshield in all the simulation cases using a bus model gives a negative effect to VAWT performance where the worst case yields −64.77%.

Internal flow dynamics and performance of pulse detonation engine with alternative fuels

In this study, analytical and computational analysis is performed to determine the effect of thermodynamic detonation parameters on the performance of the pulse detonation engine. For analytical study along with pure fuels blend of hydrogen (50 %) + kerosene (50 %), hydrogen (50 %) + methane (50 %) and methane (50 %) + kerosene (50 %) are considered. The ANSYS FLUENT program is utilized two-dimensional computational fluid dynamics (CFD) simulation using a stoichiometric mixture of three pure fuels: hydrogen-air, methane-air, and kerosene-air. Time-dependent numerical simulations are used to explore the flow condition inside the detonation tube. Excellent performance is observed for hydrogen-air fuel. Hydrogen shows the highest velocity of 2524.36 m/s and a specific impulse of 6842.16 s. The lowest velocity and specific impulse are produced by kerosene of 1520 m/s and 1473.6 s, respectively. It is shown that measured parameters could vary significantly depending on the choice of fuels used. The results infer that hydrogen blends of methane and kerosene fuels are also suitable for pulse detonation engine (PDE) application. Finally, these analytical and simulated results are validated with the previously published literature and NASA CEA (national aeronautics and space administration -chemical equilibrium with applications).

Power performance enhancement of vertical axis wind turbines by a novel gurney flap design

PurposeNowadays flaps and winglets are one of the main mechanisms to increase airfoil efficiency. This study aims to investigate the power performance of vertical axis wind turbines (VAWT) that are equipped with diverse gurney flaps. This study could play a crucial role in the design of the VAWT in the future.Design/methodology/approachIn this paper, the two-dimensional computational fluid dynamics simulation is used. The second-order finite volume method is used for the discretization of the governing equations.FindingsThe results show that the gurney flap enhances the power coefficient at the low range of tip speed ratio (TSR). When an angled and standard gurney flap case has the same aerodynamic performance, an angled gurney flap case has a lower hinge moment on the junction of airfoil and gurney flap which shows the structural excellence of this case. In all gurney flap cases, the power coefficient increases by an average of 20% at the TSR range of 0.6 to 1.8. The gurney flap cases do not perform well at the high TSR range and the results show a lower amount of power coefficient compare to the clean airfoil.Originality/valueThe angled gurney flap which has the structural advantage and is deployed to the pressure side of the airfoil improves the efficiency of VAWT at the low and medium range of TSR. This study recommends using a controllable gurney flap which could be deployed at a certain amount of TSR.

The Effect of Differentiating the Thermal Conductivity between Inner and Outer Walls on the Stability of a U-Bend Catalytic Heat-Recirculating Micro-Combustor: A CFD Study

The effect of differentiating the thermal conductivity between inner and outer walls on the stability of a U-bend catalytic heat-recirculating micro-combustor was investigated. To this end, a two-dimensional computational fluid dynamics (CFD) model was developed using the commercial code ANSYS Fluent (release 2020 R1) and, for different combinations of values for the inner and outer thermal conductivities, simulations of lean pre-mixed propane/air combustion were performed by varying the inlet gas velocity. Numerical results have shown that extinction is mainly ruled by the inner wall, whereas the outer wall controls blowout. Differentiating the thermal conductivity has been found to be an effective strategy to jointly exploit the better extinction resistance of low-conductive (i.e., insulating) materials, required by the inner wall, and better blowout resistance of highly conductive materials, required by the outer wall, thus enlarging the stable operating window of the catalytic micro-combustor compared to the use of the same material for both walls.

Correlations for Concentration Polarization and Pressure Drop in Spacer-Filled RO Membrane Modules Based on CFD Simulations.

Empirical correlations for mass transfer coefficient and friction factor are often used in process models for reverse osmosis (RO) membrane systems. These usually involve four dimensionless groups, namely Reynolds number (Re), Sherwood number (Sh), friction factor (f), and Schmidt number (Sc), with the associated coefficients and exponents being obtained by fitting to experimental data. However, the range of geometric and operating conditions covered by the experiments is often limited. In this study, new dimensionless correlations for concentration polarization (CP) modulus and friction factor are presented, which are obtained by dimensional analysis and using simulation data from computational fluid dynamics (CFD). Two-dimensional CFD simulations are performed on three configurations of spacer-filled channels with 76 combinations of operating and geometric conditions for each configuration, covering a broad range of conditions encountered in RO membrane systems. Results obtained with the new correlations are compared with those from existing correlations in the literature. There is good consistency in the predicted CP with mean discrepancies less than 6%, but larger discrepancies for pressure gradient are found among the various friction factor correlations. Furthermore, the new correlations are implemented in a process model with six spiral wound modules in series and the predicted recovery, pressure drop, and specific energy consumption are compared with a reference case obtained by ROSA (Reverse Osmosis System Analysis, The Dow Chemical Company). Differences in predicted recovery and pressure drop are up to 5% and 83%, respectively, highlighting the need for careful selection of correlations when using predictive models in process design. Compared to existing mass transfer correlations, a distinct advantage of our correlations for CP modulus is that they can be directly used to estimate the impact of permeate flux on CP at a membrane surface without having to resort to the film theory.

Numerical Investigation of Aerodynamic Characteristics of NACA 4312 Airfoil with Gurney Flap

This paper presents a two-dimensional Computational Fluid Dynamics (CFD) analysis on the effect of gurney flap on a NACA 4312 airfoil in a subsonic flow. These numerical simulations were conducted for flap heights 1.5%, 1.75%, 2% and 3% of chord length at fixed Reynold Number, Re (5×105) for different angle of attack (0o ~16o). ANSYS Fluent commercial software was used to conduct these simulations. The flow was considered as incompressible and K-omega Shear Stress Transport (SST) model was selected. The numerical results demonstrate that lift coefficient increase up to around 12o AoA (angle of attack) for NACA 4312 with and without gurney flap. For every AoA lift coefficient and drag coefficient presented proportionate behavior with flap height. However, lift co-efficient was decreased after around angle of attack due to flow separation. Maximum lift to drag ratio was found at around 4o AoA for every flap length and airfoil with flap of 1.5%C (chord length) had shown the most optimized aerodynamic performance through the analysis. This study concluded that airfoil with gurney flap displayed enhanced aerodynamic performance than the airfoil without gurney flap due to the delay in flow separation.

Numerical examination of wave power absorption by the Edinburgh Duck wave energy converter device

A two-dimensional computational fluid dynamics (CFD) numerical model is developed to study the power takeoff (PTO) efficiency of an Edinburgh duck wave energy converter (WEC) device by using the OpenFOAM source package. After the numerical validations and the convergent verifications, the characteristics of the duck WEC device for the power takeoff in various incident wave periods are examined. The present numerical investigation indicates that the maximum absorbed power of the duck WEC device is attained in its resonant period, while the maximum efficiency is achieved in a lower period. The influence of the damping coefficient on the PTO is also investigated. The optimal damping coefficient for acquiring more power and better efficiency is determined from a numerical examination.

Effect of Operating Parameters on Carbon Dioxide Depressurized Regeneration in Circulating Fluidized Bed Downer using Computational Fluid Dynamics

Typically, heating or high-temperature treatment has been used to regenerate solid sorbent. In this study, the depressurized regeneration using a circulating fluidized bed downer was proposed and the significance of its operating parameters was identified. Two-dimensional computational fluid dynamics were employed to systematically investigate the effects of operating parameters on carbon dioxide depressurized regeneration with potassium carbonate solid sorbent particles. The simulated model was based on a laboratory scale circulating fluidized bed downer. The chemical equilibrium model for predicting the highest outlet carbon dioxide mass fraction was then used. A central composite design was employed to identify the main, quadratic, and interaction effects of operating parameters to the regeneration process. The operating parameters consisted of the outlet system pressure, inlet gas velocity, and inlet solid circulation rate, while the response variable was the released outlet carbon dioxide mass fraction. Among the multiple operating parameters, there were two main operating parameters and their combinations, namely the inlet gas velocity, outlet system pressure, square of inlet gas velocity, and interaction between inlet gas velocity and outlet system pressure, which had great impacts on the regeneration. All the main, quadratic, and interaction effects were explained. Then, the optimal operating conditions were obtained through the response surface method.

Numerical simulation of LNG tanks exposed to fire

The increasing use of Liquefied Natural Gas (LNG) as a fuel for ships and vehicles poses relevant safety concerns, extended to the entire LNG supply chain and distribution network. Understanding the phenomena associated with the behavior of LNG tanks exposed to severe heat sources is thus a fundamental issue to identify potential safety-critical scenarios. The experimental data and modeling approaches currently available, mainly referring to small-scale pilot vessels, feature relevant limitations when extended to large-scale applications. In the present study, a two-dimensional non-equilibrium computational fluid dynamics model (2D CFD) of LNG tanks exposed to fire engulfing scenarios was developed. The 2D CFD model was validated against experimental bonfire data and was extended to simulate the behavior of large-scale vessels used in specific industrial applications, as the road transportation of LNG and the fuel supply of ships. A set of Key Performance Indicators (KPIs) was defined to support the safety assessment of LNG tanks, and to identify the potential transition to safety critical regions during fire exposure. The CFD results obtained allowed investigating the influence of operative parameters and geometry on the pressure build-up in the tanks, as well as on the transient evolution of complicating phenomena, such as the thermal stratification. The KPIs defined provide a useful support for the design of safety systems and for decision making in emergency response.

Formation of the periodic material flow behaviour in friction stir welding

A two-dimensional computational fluid dynamics model is established to verify the correlation between periodic material flow and tool eccentricity during friction stir welding (FSW) process. In the model, the marker material is tracked by employing the Volume of Fluid technique. Besides, a novel boundary condition is proposed, which is not only non-uniformly distributed around the pin but is also connected with the real-time orientation of tool eccentricity. Finally, the dynamic evolution of marker material with the pin rotating is analysed, and the comparison between calculated and measured distribution of deposited marker material is carried out. The result demonstrates that tool eccentricity can be considered as the original mechanism for the formation of periodic material flow behaviour in FSW.

Numerical study on fuel-NO formation characteristics of ammonia-added methane fuel in laminar non-premixed flames with oxygen/carbon dioxide oxidizer

Ammonia (NH3) is considered as an attractive carbon-free fuel although a technical obstacle involves the formation of large amounts of nitrogen oxides (NOx) due to its N-atom. The study focuses on the fuel-NO formation characteristics in ammonia-added methane flame with an oxygen/carbon dioxide (O2/CO2) oxidizer to elucidate the key elementary reaction steps. Two well-known reaction mechanisms were employed in two-dimensional axisymmetric computational fluid dynamics (CFD) and one-dimensional opposite-diffusion (OPPDIF) flame simulations, and the results were compared with those of experimentally measured NOx emission trends. Hence, a region with a significant difference in nitrogen monoxide (NO) formation characteristics calculated using the two reaction mechanisms was determined via local distributions and statistical analysis of the 2D CFD results. Additionally, the multidimensional effect was observed while comparing the axial profiles of 1D OPPDIF and radial distributions of 2D CFD at various axial positions from the fuel nozzle. Reaction pathways were extracted at the representative positions, and the major NO production and destruction steps were analyzed via the dimensionless conversion rate of NH3 to NO species. The results demonstrated that the fast N-related reactions in the NH – N – NO pathway, N + OH → NO + H in particular, highly influenced the predicted fuel-NO formation trends with increases in the oxygen concentration in the oxidizer.

Numerical simulation of gasification with a one-dimensional particle submodel for char structure evolution

The exact knowledge of reaction rate, diameter, and density of a char particle is important for precise computational fluid dynamics (CFD) simulations of gasifiers. Most of the present research treats char particles as homogeneous with respect to density and composition. Diameter and density evolve according to power law relations, which are empirical and hard to validate. In this work, char particles are subdivided into multiple homogeneous shell–like volume elements. It allows the calculation of a density profile within the particle and hence profiles of specific surface area and effective diffusivity. This one–dimensional approach is directly compared to the common modeling approach based on Thiele modulus and power law relations. Both have been implemented into a two-dimensional CFD simulation of a lab–scale fixed bed gasifier. Simulations are carried out with particle sizes of 100 µm and 1 mm at temperatures of 800 °C up to 1800 °C with carbon dioxide and water vapor at 1 atm and 50 atm total pressure. The smaller diameter is relevant for entrained flow gasifiers. The reaction rate can accurately be described by the common approach with slight modifications. However, larger deviations of up to 13% and 42% remain for diameter and density in regimeII. Future work can either employ the presented shell model or adopt the proposed modifications to the common approach.

Guidelines for the computational domain size on an urban-scale VAWT

Focusing on the H-type vertical axis wind turbines (VAWTs) widely used in urban, the SST k - ω turbulence model was selected to establish a two-dimensional Computational Fluid Dynamics (CFD) model, the moment coefficient (Cm), and power coefficient (Cp) was taken as the evaluation indicators. The CFD model was used in different domain sizes. The simulation results under the sizes were used to investigate the effect of the computational domain size on the numerical simulation results of the aerodynamic performance of the VAWT. Furthermore, a distance from the turbine center to the domain inlet and outlet of 10D (D: diameter of the turbine) each, a domain width of 10D and a diameter of the rotating core of 1.5D are found to be safe choices to minimize the effects of blockage and uncertainty in the boundary conditions on the results. It provides a reference for the selection of the domain size when CFD simulation is implemented in a vertical axis turbine to get its prediction of the performance in the future accurately.

Multi-condition optimization of a cross-flow fan based on the maximum entropy method

Cross-flow fans are widely used in heating, wind-curtains, and air-conditionings, as well as other ventilation systems. A single or double arc is generally used as the camber line of cross-flow fans, but this design leads to constraints in the geometry of the blade profiles. In this study, the camber line of a cross-flow fan blade was parameterized by five parameters based on the fourth-order Bezier curve. A two-dimensional computational fluid dynamics (CFD) simulation was conducted to predict the aerodynamic characteristics and the internal flow field. It is necessary in multi-condition optimization, to evaluate the relative importance of the performance parameters under different working conditions and determine their weight factors. Here, a novel maximum entropy method (MEM) was proposed to quantify of volume flow rate, because the method avoids the subjectivity in the selection of the weights. Subsequently, a multi-island genetic algorithm (MIGA), combined with numerical simulation, was used to search the global optimum in the given design space. The results indicated that the optimum combination of the structural parameters reduced the blade channel vortex in a particular location of the impeller and changed the position and size of the eccentric vortex. The volume flow rate of the optimized impeller was 4.28% higher at the minimum rotation speeds and 12.87% higher at the maximum rotation speeds.

Analysing the transport phenomena of novel dew-point evaporative coolers with different flow configurations considering condensation

This paper entails a study on the transport phenomena of a novel dew-point evaporative cooler with counter-flow closed-loop configuration, a potential alternative to the conventional mechanical vapor compression system. In contrast to the conventional indirect evaporative cooler and the regenerative indirect evaporative cooler, the proposed novel dew-point evaporative cooler is capable of achieving the simultaneous goals of pre-cooling, energy recovery and dehumidification. The novel dew-point evaporative cooler is classified as either type 1 or type 2 based on the relative flow direction between the water and the supply air. A two-dimensional computational fluid dynamics model is judiciously developed and employed to conduct a comparative study which evaluates the performance of the conventional indirect evaporative cooler vis-a-vis both novel type 1 and novel type 2 configurations. Experimental data is then employed to validate the predictive accuracy of the mathematical model. The distributions of both temperature and humidity ratio are plotted for a typical case of the novel cooler incorporating both flow configurations and considering the effects of moisture condensation. Additionally, the impacts made by key parameters on the cooling performance under condensation state are analyzed by using three evaluation indexes, namely, latent efficiency, wet-bulb efficiency, and enlargement coefficient. By regulating several operating parameters, the variations of the three evaluation indexes are plotted via the proposed numerical model. Key results from this study revealed that the novel type 1 flow configuration is capable of achieving the highest latent and wet-bulb efficiencies, and enlargement coefficient. It is also observed that the relative flow direction between the water and the supply air exerts a stronger influence on the cooler performance compared to the changes of the water mass flow rate.

Implementation of ventilation towers in a greenhouse established in low altitude tropical climate conditions: numerical approach to the behavior of the natural ventilation

Deficient ventilation rates and airflow patterns in naturally ventilated greenhouses are the main causes for the generation of inadequate and heterogeneous thermal conditions inside this type of structure. In this research, a numerical study was developed using a two-dimensional computational fluid dynamics model validated in a Colombian greenhouse established under warm climate conditions (PG). The objective of the research was to evaluate the effect on natural ventilation generated by the coupling of two ventilation towers on the PG greenhouse, scenario called MPG. The results indicated that airflow patterns over the region where crops are established in MPG are between 25.9% and up to 142.5% faster compared to PG, which generates ventilation rates up to 57.3% higher in MPG, which is why the temperature distribution under this scenario is more homogeneous inside the structure and its average thermal differential with the outside environment does not exceed 1.1 °C.

Heat Transfer Across Tube Banks With a Passive Control Vortex Generator in Steady One-Directional and Oscillatory Flows

Fluid can flow in one-directional (normal flow) or oscillatory conditions. Fluid flow in some energy system involved oscillatory flow condition. The use of vortex generator has been proven to improve heat transfer in the case of one-directional flow but the impact of vortex generator in oscillatory flow condition is yet unknown. This study focusses on the heat transfer performance across a heated tube banks using a Computational Fluid Dynamics (CFD) model. Two flow conditions were modelled: steady one-directional and oscillatory flow conditions. Two-dimensional CFD models of steady flow and oscillatory flow were solved using the SST k-? turbulence model for two different cases of heated tube banks with and without the vortex generators. The heat transfer performance for both flow conditions were analysed by considering a heat transfer parameter known as Colburn-j factor. Results showed that the use of a vortex generator increased the heat transfer enhancement, regardless of the flow conditions. However, it is also noted that the heat transfer behaviour in a steady flow and an oscillatory flow is not the same, especially with the appearance of secondary flows in the system. The difference is discussed with respect to dimensionless quantity of Colburn j-factor, the non-dimensionless quantity, and the amplitude of temperature field. The result indicates that the heat equation in the steady flow condition is not very suitable to be directly used in oscillatory flow conditions. Appropriate heat equation needs to be properly addressed for situations that involve oscillatory flow motion.

An experimentally validated multifidelity hybrid model for analyzing the pressure variation of a relief system

In a relief system, the pressure variation in the system is often caused by the operations of the pressure control elements such as the pressure relief valve and ball valve, in addition to the pressure fluctuations in the pressure resources. This pressure variation couples with the acoustics or the motion of passive pressure control elements, which may result in system malfunction. To investigate the principles of the pressure variation in a relief system including a reservoir, a pipeline and a spring-loaded pressure relief valve, a multifidelity hybrid model combining the method of characteristics (MOC) and computational fluid dynamics (CFD) method is proposed. In the multifidelity hybrid model, the characteristics of the pressure resources are modeled using their performance curves, the compressible gas transmitted in the pipe is calculated by one-dimensional MOC, and the air flow in the valve as well as the valve motion is simulated by a two-dimensional CFD model. To validate the model, analyses are conducted with a 1:1 scale test rig and a full CFD model. The comparison of the results shows that both the multifidelity hybrid model and the full CFD model can accurately capture the pressure fluctuations in straight pipelines induced by valve closure and predict the forms of valve motion, but the calculation speed of the multifidelity hybrid model is four times faster than that of the full CFD model.

Full-loop study of a dual fluidized bed cold flow system: Hydrodynamic simulation and validation

The unsteady behaviors of the air-silica sand flow in a lab-scale dual fluidized bed gasification cold flow system have been studied. A two-dimensional computational fluid dynamics full-loop model with poly-size distribution in solid phases was developed as an innovation of this study to investigate the effects of crucial parameters on system hydrodynamics. The results showed a decrease in the mixture static pressure from the bottom to the upper regions of the system, which maintained the system operations stable. The riser air inlet velocity and the gasifier static bed height were found to play considerable roles in enhancing the total sand flow rates. The same tendencies in the prediction and experiment of both the mixture pressure and the sand flow rate showed the feasibility of the proposed model. Besides, the residual evaluation enhanced data reliability and supported model validation. Especially, undesirable phenomena possibly occurring in the system operation under improper conditions could also be predicted. Accordingly, the inventory of bed material and the fluidizing gas flow rates should be suitably regulated to maintain pressure balances, trouble-free continuous flow, optimal sand circulation rate, and low solids loss. Furthermore, the obtained results in this study can be used as a reference for optimizing the designs and operational conditions of large-scale plants.