CFD Simulation Research Articles

Effect of arrangement patterns of spheres encapsulating phase change material in packed beds on thermal performance

The NaNO3-KNO3 salt has the disadvantage of poor heat transfer capacity and low heat transfer efficiency in single-tank heat storage systems. However, molten salt can be used as phase change materials (PCM) and encapsulated with highly conductive sphere-shaped metals as PCM spheres. Arranging these spheres can appropriately change the situation. In this study, a heat storage system of a multilayer packed bed of PCM spheres consisting of NaNO3-KNO3 salt and type 316 stainless steel sphere shell was designed. This system's heat transfer and heat storage performance were also investigated using CFD simulation. For the same sphere diameter, the heat storage efficiency of hexagonal close packing (HCP) was 1.1 times that of cubic close packing (CCP). With the same porosity, the heat charging efficiency for the case using the 47.5 mm sphere diameter was the highest, at 99.3 %. When the number of layers of spheres was increased from 3 to 7, the heat flux through the sphere wall and the heat storage density of PCM spheres decreased from 40.8 kW and 67.3 × 105 kJ/m3 to 35.7 kW and 59.2 × 105 kJ/m3, respectively. Therefore, the heat charging efficiency of HCP was better than that of CCP. Reducing sphere size could shorten the heat transfer time and improve the heat storage efficiency. These numerical simulation results are consistent with the commonly known fact that the volumetric heat transfer intensity in the porous media is proportional to the reciprocal particle size since the smaller the particle size, the larger the contact area between the packing and the fluid. The conclusion drawn from the present findings provides an idea for designing a PCM heat storage system with high temperature and high efficiency of molten salt. The idea could also be used as a reference for the optimal design of a packed bed reactor.

CFD modelling of a chemical looping combustion system − Exploring the potential of natural oxygen carriers

Research advancements in Carbon Capture technology has allowed for Chemical Looping Combustion (CLC) to emerge as a promising technique which can be feasibly implemented into existing power plant infrastructures. In this study, a 120 kWth interconnected Dual Circulating Fluidized Bed (DCFB) reactor has been studied, which uses methane fuel and NiO/Al2O3 particles as oxygen carrier. Two-dimensional transient CFD simulations have been conducted for the DCFB – CLC system using an Eulerian – Eulerian model coupled with heterogeneous reactions and the results have been compared to existing literature. Using the same methodology, two natural ore potential oxygen carriers in Pakistan: Hematite (Fe-based) and Hausmannite (Mn-based) have been evaluated for the same pilot scale apparatus. The fuel conversion and combustion efficiencies have been evaluated for the oxygen carrier particles and have been compared with the original system. The results show that Fe ore provides lesser combustion efficiency as compared to NiO, whereas Mn ore exhibits extremely high reactivity. In retrospect, the use of a mixed (Fe-Mn) oxide oxygen carrier can be considered as a viable option.

Simultaneous improvements of thermal efficiencies and smoke emissions in semi-premixed diesel combustion with dual-injectors and a spatially divided combustion chamber

To simultaneously establish the high thermal efficiencies and the low smoke emissions in the semi-premixed diesel combustion with a twin peaked heat release consisting of the first-stage premixed combustion and the second-stage spray diffusive combustion, a combination of dual-injectors and a newly designed divided combustion chamber was proposed and the combustion characteristics were investigated. The divided chamber with a lip at the middle of the side wall can prevent the second fuel spray from entering the burned region of the premixture from the first-stage injection, realizing a suitable spatial distribution of spray flames and suppressing the soot formation. The dual-injectors can independently provide the fuel sprays into the upper and the lower layers of the divided chamber with the optional quantities and timings for the first and second injections, establishing the optimum combustion phasing. The distributions of the equivalence ratios and the soot formation characteristics in the divided chamber as well as in an ordinary re-entrant chamber as a reference were analysed with CFD simulations, showing that the soot formation is smaller in the divided chamber. The second-stage spray diffusive combustion is improved as well as the late afterburning and the combustion duration are reduced with the suppression of interference between the spray flames in the divided chamber. The cooling loss in the divided chamber is smaller due to the weaker gas motion and the lower heat transfer coefficient. The experiments also showed that the thermal efficiencies are better and the smoke emissions are lower in the divided chamber at medium loads. However, at higher loads, the smoke emissions increase due to the offset injector arrangement, and the CFD analysis showed that lower smoke emissions can be expected with optimization of the injector arrangement to locate the injector for the second fuel injection at the centre of cylinder.

STUDY ON CFD SIMULATION METHOD FOR ANALYSING FLOOD LOADS IN OPEN-CHANNEL STEADY FLOW

STUDY ON CFD SIMULATION METHOD FOR ANALYSING FLOOD LOADS IN OPEN-CHANNEL STEADY FLOW

Transient Flow-Induced Stress Investigation on a Prototype Reversible Pump–Turbine Runner

Pump–turbine units with high heads are subjected to strong pressure pulsations from the unsteady transient flow in fluid channels, which can produce severe vibrations and high stresses on the pump–turbine structural components. Therefore, reducing transient flow-induced stresses on prototype reversible pump–turbine units is an important measure for ensuring their safe and efficient operation. A high-head prototype reversible pump–turbine with a rated head of 440 m was used to investigate the transient flow characteristics and the flow-induced-stresses in this study. First, the flow passages of the pump–turbine unit and the structure of the reversible pump–turbine runner were constructed with CAD tools. Next, CFD simulations at the full load were performed to investigate the pressure pulsation characteristics of the pump turbine in both the time domain and the frequency domain. After this, the pressure files calculated by the CFD were exported and applied to a finite element model of the pump–turbine runner to calculate the transient flow-induced dynamic stresses. The results show that the pressure pulsations in the flow passage are closely related to the rotational speed, the guide vane number, and the runner blade number of the pump–turbine unit. The maximum flow-induced stresses on the pump–turbine runner at the full load were below 2 MPa and lower than the allowable value, which reveals that the designs of the pump–turbine runner and the flow passage are acceptable. The conclusions can be used as a reference to evaluate the design of high-head pump–turbines units. The approaches used to carry out the transient flow-induced stress calculations can be applied not only to pump–turbines units but also to other types of fluid turbomachinery such as pumps, turbines, fans, compressors, turbochargers, etc.

Facade flame depth coming out from the fire compartment opening subject the external sideward wind

When a fuel leakage/energy release (i.e., combustion, fire) inside a compartment, the external facade flame characteristics through opening are of great significance for protecting human life and property, as well as assessment of its risk and impact to urban environment. This work investigates facade flame depth coming out from fire compartment opening with fuel combustion/energy release inside the compartment for under-ventilated condition under sideward wind, which is a fundamental parameter in describing facade morphological characteristics in particular building fire, meanwhile still no work can be found in previous studies. Results show that, as sideward wind speed increases, the flame depth first increases a bit then decreases significantly. A CFD simulation is carried out to understand flow field controlling facade flame depth under windless and sideward wind situations. A physical analysis is presented on account of mechanisms of complex interaction among sideward wind inertial, outflow momentum through opening, and flame buoyancy flux are proposed based on controlling mechanisms, which well represents the facade flame depth by a non-dimensional function under sideward wind condition. This study makes an important new contribution comparing with previous knowledge, and has potential practical value for fuel combustion inside confined space and the corresponding numerical model verification.

A Surrogate Model of Heat Transfer Mechanism in a Domestic Gas Oven: A Numerical Simulation Approach for Premixed Flames

This paper introduces an innovative analytical model to compute flame velocities and temperatures within a premix burner in a domestic gas oven. This model significantly streamlines the heat transfer simulation process by simplifying the modeling of the thermo-chemical energy release during combustion, effectively reducing complexity and computation time. Accelerated solutions are essential at the initial design stages when comparing the effect of the oven parameter variation on the overall performance. The validation of the proposed analytical model involved experimental assessments of the temperature of the false bottom plate in a natural gas oven. The resulting data were then compared against CFD simulations performed utilizing the proposed model. The results revealed a marginal discrepancy of 4% between the experimental measurements and the outcomes generated by the model. Simulations were executed under differing conditions, encompassing scenarios with and without radiation effects. This exploration identified natural convection as the predominant heat transfer mechanism, with heat radiation contributing only modestly to the heating of the false bottom plate. Among its advantages, the proposed model offers a notable reduction in the numerical complexity of the modeling of the combustion process. Furthermore, its straightforward integration into numerical simulations involving premixed flames underscores its practical utility and versatility in evaluating design performance at the early stages of the design. Highly accurate models can be left for the final oven configuration validation.

CFD Simulation to Assess the Effects of Asphalt Pavement Combustion on User Safety in the Event of a Fire in Road Tunnels

This paper presents a specific 3D computational fluid dynamics model to quantify the effects of the combustion of asphalt road pavement on user safety in the event of a fire in a bi-directional road tunnel. Since the consequences on tunnel users and/or rescue teams might be affected not only by the tunnel geometry but also by the type of ventilation and traffic flow, the environmental conditions caused by the fire in the tunnel under natural or longitudinal mechanical ventilation, as well as congested traffic conditions, were more especially investigated. The simulation results showed that the combustion of the asphalt pavement in the event of a 100 MW fire, compared to the case of a non-combustible road pavement, caused (i) an increase in smoke concentrations; (ii) a greater number of users exposed to the risk of incapacity to escape from the tunnel; (iii) a more difficult situation for the firefighters entering the tunnel upstream of the fire source in the case of natural ventilation; (iv) a higher probability of the domino effect for vehicles queued downstream of the fire when the tunnel is mechanically ventilated.

CFD simulation of novel adaptive pin-fins microchannel heat sink to improve thermal management of electronic chips

Frequent hotspots pose a significant challenge for chip thermal management. Conventional microchannel with fixed structure cannot deal with random hotspots, which appear in irregular time and space. To address random hotspots and reduce microchannel energy consumption, a novel adaptive pin-fins microchannel (A-MCHS) is proposed in this paper to intelligently tackle with the hotspot region, which consists of a number of pin-fins with property of phase transition and volume shrinkage when the temperature is raised to 313.15 K. A CFD method is applied to study its thermal performance and user defined function (UDF) is used to realize the adaptive thermal response of pin-fins. It’s found that the adaptive pin-fins of the hotspot undergo volume reduction and enlarged local coolant flow under higher hotspot heat flux, illustrating the automatic and adaptive flow regulation effect. Compared with the fixed pin–fin microchannel, the local heat transfer performance of A-MCHS in the hotspot region is enhanced by 37.9 %, average Nu of the microchannel is advanced by 7.4 %, and the maximum chip surface temperature is reduced by up to 3.12 K. Moreover, it also shows a significant reduction of 10.7 % in pressure drop, suggesting a great energy saving in pumping cooling fluid. It’s indicated that the adaptive pin-fins microchannel provides efficient cooling and intelligently enhanced thermal management and energy saving for high heat flux electronic chips.

Transonic Flow Field Analysis of a Minimum Nozzle Length Rocket Engine

The aim of this paper is to develop a profile of axisymmetric minimum length nozzle giving a uniform and parallel flow at the exit section. The study is done at high temperature, lower than the dissociation threshold of the molecules. The design is made by the method of characteristics (MOC). The variation of the specific heats with the temperature is considered. The numerical results have been validated with CFD simulation Ansys-Fluent software. The second part of this study is to calculate and analyze the transonic flow field of this supersonic nozzle. The computation of the flow field characteristics at the throat is thus essential to the nozzle developed thrust value and therefore to the aircraft or rocket it propels. An investigation was conducted to analyze the effects of parameters on the position of the sonic line. These parameters include stagnation temperature T0, radius of the nozzle, types of gases, and exit Mach number ME.

CFD simulations of MHD effects on mixed convectional flow in a lid-driven square cavity with square cylinder using Casson fluid

Enclosure design significantly impacts thermal engineering processes and devices, including electronic devices, thermal exchangers, power engines, heating systems, solar panels, and nuclear power plants. The square cavity with variable aspect ratios is utilized for multi-objective optimization and improved thermal performance of microchannels. Cold cylinders in square enclosures are often utilized to reduce energy loss in micro heat sinks and heat exchangers. So, article’s main objective is to investigate concurrent characteristics of external and internal forces on a Casson fluid contained in a square cavity with magnetohydrodynamics (MHD). A square cylinder is embedded in the middle of the enclosure, and an inclined magnetic field with angle φ is applied. The upper lid is assumed to be inertially driven by a constant amount of slipping velocity, and a thermal state is disturbed by assuming uniform temperature at the left wall while the right wall and embedded cylinder are cool. The finite element method (FEM) based open-source software renowned as COMSOL Multiphysics is utilized to solve the non-dimensionalized momentum and energy equations. This study uses the square enclosure to study essential factors, such as the Casson parameter β , Hartmann number Ha , Reynolds number Re , and Grashof number Gr . The findings reveal that the kinetic energy at β = 10 is 12.22 times greater than that of β = 0.1 at Ha = 0 . It is observed that Ha , β , and Gr have a direct impact on the local Nusselt number. Fluid kinetic energy increases as Gr increases while it decreases as Re increases. This study shows how obstacles of various shapes and temperature conditions have an essential impact on handling heat transfer characteristics.

CFD simulation of a laboratory-scale monolithic reactor for the combustion of VOC

This study presents initial findings from simulating a laboratory-scale monolithic reactor for VOC (Volatile Organic Compounds) combustion. The reactor, made of stainless steel, measures 45 cm (height) and 2.3 cm (diameter). It contains a catalyst supported by a ceramic structure, tested for toluene combustion. Using ANSYS CFD simulation software, fluid dynamics inside the reactor were accurately modeled. The simulation then focused on a specific channel within the monolith, based on comprehensive reactor data. The final stage simulated toluene combustion using the Mars-Van Krevelen mechanism. ANSYS CFD results for toluene combustion, measured by CO2 yield, closely matched experimental outcomes. The average CO2 yield of 1.16 showed a strong alignment between the simulation and the experiment. ANSYS CFD not only aids in obtaining hard-to-gather information about fluid dynamics and chemical reactions but also enables simulations of geometry changes and catalyst types once validated. It also supports exploring more complex reactions.

ANSYS Fluent-CFD analysis of a continuous single-slope single-basin type solar still

The present study endeavours to substantially contribute towards alleviating the global water scarcity problem. The task entailed designing a computational model of a renewable energy-based evaporator. Using ANSYS Fluent, the CFD simulations of a three-dimensional conventional continuous single slope, single basin solar still were carried out in summer at 23.79°N, 86.43°E coordinates. With the optimized inclination condensation angle of 29° and water depth at 1 cm, the solar still recorded the highest hourly drinking water, about 1.5 kg m−2, at 11:00 h. The continuous production resulted in a water collection rate of 8.6 kg m−2 day −1, encompassing the production of all previous models. Additionally, compared with the literature correlation for solar still simulated mass, the Power model calculation was the closest, with a 12.4% variation. Furthermore, the study showcases that CFD is an economical, efficient, and easily diagnosable technique for designing solar stills.

3D numerical study of NH3/H2 MILD combustion in a reversed flow MILD combustion furnace

Combustion of fuels to generate energy is essential for numerous residential and industrial human endeavors. In contrast to fossil fuels, the demand for carbon-free fuels such as ammonia and hydrogen to meet climate commitments is rapidly growing. This study conducts a numerical investigation of NH3/H2 MILD combustion. A three-dimensional CFD simulation model of a 90-degree sector of a reversed flow MILD combustion furnace is presented. The SAGE detailed chemical kinetics solver featuring the CEU-NH3 mechanism is employed with dynamic mechanism reduction for computational efficiency. The effects of varying the hydrogen mole fraction in the inlet fuel mixture from 0 to 50 % on the thermal characteristics, reaction zone, and emissions are investigated. The findings show that temperature homogeneity, assessed through temperature uniformity parameters and standard deviation, reveals a diminishing trend in temperature homogeneity with increased hydrogen concentration. The boundaries of the reaction zone are determined according to the NNH and N mole fractions, and both criteria are approximately identical. All the investigated cases match the MILD combustion definition; however, the cases with H2 concentrations ranging from 10 to 40 % fall under the MILD-like combustion regime. Emissions analysis shows that for all investigated cases, the ammonia slip emissions and the exhaust N2O emissions are below 1 and 0.5 ppm, respectively. The NOx emissions are found to increase abruptly with increasing hydrogen concentration from 0 to 35 % and subsequently undergo minor fluctuations.

CFD assisted engineering of spiral microfluidic reactors for room temperature synthesis of ZnO nanoparticles

In microfluidic reactors designed for the synthesis of nanomaterials, hydrodynamics and mixing performance play a crucial role in the characteristics of the synthesized nanoparticles. In the current study, CFD simulation is utilized to shed light on flow characteristics inside a spiral microfluidic device and the way it affects the mixing during ZnO synthesis. Before microfabrication, the optimal number of spiral loops and flow rate in the microfluidic reactor was determined through mixing index calculation according to the CFD results in terms of the concentration distribution. Furthermore, this synthesis method is compared with batch nanoparticle fabrication techniques. In the microfluidic system, the concentrations of precursors were 50 times more diluted rather than batch methods. The droplet-based microfluidic ZnO synthesis was eight-fold faster than batch synthesis. The ZnO nanoparticles were characterized by XRD, SEM, and FTIR techniques. The outcomes of the FTIR analysis demonstrated that both the batch and microfluidic samples exhibit identical vibrational peaks. The experiments showed that the microfluidic system is reliable for controlling size. Besides, the CFD results indicated that a spiral micromixer with five loops provides sufficient mixing. The acquired findings from the characterization analyses can lead us toward a clue that once the ZnCl2/NaOH concentration ratio is greater than unity, a narrower size distribution of ZnO particles is probable. Consequently, the microfluidic reactor can be an efficient method for nanomaterial construction owing to the minimization of energy consumption and independence of the operational synthesis temperature.

Alleviation of SWBLI over the payload of a launch vehicle by change of nose shape

Wind tunnel tests on a typical heavy-lift launch vehicle configuration featuring a core vehicle flanked by two strap-on boosters showed very high levels of unsteady pressures over the payload region at transonic Mach numbers when the nose had a cone-cylinder shape. This was due to very high levels of Shock Wave Boundary Layer Interactions (SWBLI) associated with the conical shape, which caused alternating flow featuring the formation of a unsteady λ-shock system over the payload region and a pair of counter-rotating vortices. The phenomenon was found to strongly depend on the proximity of the noses of the strap-on boosters to the boat tail downstream of the payload region. When the conical nose was replaced by an ogive-shaped nose, significant reduction in SWBLI occurred, with dramatic reduction of shock oscillations, pressure fluctuations, replacement of alternating flow by steady flow and disappearance of the vortex-pair. Time-averaged pressure fluctuations indicate reduction in pressure gradients due to a more gradual expansion of flow over the ogive nose compared to conical nose, an observation also supported by CFD simulations. The alleviation of SWBLI with ogive nose was so effective that the presence or absence of strap-on boosters had practically no effect on pressure fluctuations over the payload region. Unlike the conical shape, the ogive nose cone shape seems to meet both the shape and pressure parameter-criteria of NASA to ensure buffet-free environment.

Effect of non-uniform circumferential spallation of seal coating on the aerodynamic performance in a transonic axial rotor: A steady numerical simulation

Seal coating is a critical component of a compressor, and its function is to minimize the blade tip clearance and prevent fluid leakage. Non-axisymmetric spallation of the seal coating can compromise its effectiveness and reduce the compressor's efficiency. In this study, a numerical simulation was performed on a transonic axial rotor to study the effect of non-axisymmetric spallation of seal coating on the aerodynamic performance. Circumferential deep grooves were used to represent the irregular peeling of the seal coating in the full-annulus grids CFD simulation. Results show that as the circumferential length of spallation of seal coating increases, the total pressure ratio and isentropic efficiency decrease, and the surge margin decreases at first but then increases. After seal coating spallation, the flow loss changes near the suction surface and the tip passage. The flow loss changes on the suction surface relate to the interaction between the leading-edge shock/passage shock and the boundary layer. At the tip passage, the tip leakage flow and secondary flow induced low-velocity region and the effect of spallation on blockage dominate the flow loss and instability. The change in surge margin is contributed to the amount of and the chordwise variation of tip leakage flow, and the surge margin drops to the minimum when the spallation length is 9 passages where a “double-twist” flow behavior of TLF occurs substantially.

Simulation of Mechanical Stresses in BaTiO3 Multilayer Ceramic Capacitors during Desoldering in the Rework of Electronic Assemblies Using a Framework of Computational Fluid Dynamics and Thermomechanical Models.

Multilayer ceramic capacitors (MLCCs) are critical components when thermal processes such as reflow desoldering are used during rework of electronic assemblies. The capacitor's ferroelectric BaTiO3 body is very brittle. Therefore, thermomechanical stresses can cause crack formation and create conductive paths that may short the capacitor. In order to assess the thermally induced mechanical stresses onto an MLCC during reflow desoldering, simulations were carried out, which make use of a framework of computational fluid dynamics and thermomechanical models within the ANSYS software package. In the first step, CFD simulations were conducted to calculate the transient temperature field in the surrounding of the MLCC component, which was then used as an input for FEM simulations to compute the arising mechanical stresses inside the MLCC. The results of the simulations show that the major contribution to mechanical stresses within the MLCC component comes from the mismatch in thermal expansion between the printed circuit board and the MLCC. The temperature gradients along the MLCC component are rather small and account only for moderate internal stresses within the brittle BaTiO3 body.

Airflow Analysis of the Bus Under Various Velocities Using CFD Simulation

This project investigated the aerodynamic effects of various airflow velocities around a bus. Key parameters included airflow velocity and turbulence models, with dependent variables being the velocity and pressure on the bus body. The bus model, sourced from an existing GrabCAD file, was imported into SimFlow 4.0 for analysis. A steady-state simulation was employed, incorporating symmetry conditions, turbulence modeling, boundary conditions, and a baseline post-processing method to visualize the velocity and pressure distributions around the bus. Mesh refinement was carefully adjusted to accommodate the bus's large size and accurately capture the flow gradients. The simulation results were analyzed using Paraview post-processing to evaluate the final velocity and pressure contours. The results showed that the airflow velocities crucially affect the velocity and pressure distribution around the bus. This study yielded significant and insightful results by varying the independent parameters, advancing the understanding of bus aerodynamics under different airflow velocities.

Impact of multi-walled carbon nanotubes (MWCNTs) on hybrid biodiesel blends for cleaner combustion in CI engines

This study investigates the influence of multi-walled carbon nanotubes (MWCNTs) and hybrid biodiesel blends on the performance, combustion, and emission characteristics of a compression ignition engine. The hybrid biodiesel comprises waste frying oil methyl ester (WFOME), sesame oil methyl ester (SOME), and ultra-low sulfur diesel (ULSD) blended at 20 %:80 % by volume (B20). MWCNTs were added into B20 at 25 ppm and 50 ppm concentrations (B20-25MWCNT and B20-50MWCNT). Experimental analysis was conducted under varying engine loads and injection pressures, complemented by CFD simulations. Results demonstrate that the addition of MWCNTs significantly enhances B20 blend performance, with B20-50MWCNT exhibiting a 25 % increase in brake thermal efficiency and a 17 % reduction in brake specific fuel consumption compared to neat B20. Additionally, the incorporation of MWCNTs resulted in notable reductions in harmful emissions. NOx emissions for the B20 + 50MWCNT blend were reduced by up to 36 % compared to diesel and 43 % compared to B20. Smoke opacity and particulate emissions also showed significant decreases, highlighting the cleaner combustion facilitated by the presence of MWCNTs. The CFD simulations provided detailed insights into the combustion process, validating the experimental findings and elucidating the mechanisms behind the observed improvements. The enhanced fuel atomization, better fuel-air mixing, and catalytic properties of MWCNTs were identified as key factors contributing to the superior performance and reduced emissions.