Three-dimensional Transient Model Research Articles

A transient three-dimensional model for thermo-fluid simulation of cryogenic plate-fin heat exchangers

This work proposes a novel, three-dimensional model for transient simulation of cryogenic plate-fin heat exchangers (PFHE). This model is based on the combination of computational fluid dynamics with well-established correlations for calculating heat transfer and pressure drop occurring during fluid flow through various fin types. This reduces the complex geometry of a PFHE to a geometrically simple mesh, allowing for efficient simulation of large-scale industrial PFHE. Successful validation of the model is carried out using both data available in the literature and results calculated using commercial software. The necessity for a three-dimensional model is further demonstrated using a transient case study in which the design of inlet distributors is shown to cause unfavorable distribution of fluid flow and hence severe temperature maldistribution in the PFHE. These results show that the proposed model can be useful in both detailed engineering and lifetime investigations of flexible PFHE.

Insight into droplet formation in electroslag remelting process by numerical simulation

In this paper, a transient three-dimensional (3D) model is proposed to study the formation of metal droplets during electroslag remelting (ESR) process. The solution of the mass, momentum and energy conservation equations are simultaneously implemented with fully coupling electromagnetic field based on the finite volume method. The volume of fluid (VOF) model is used to capture the interface between slag and metal implicitly. Furthermore, the adaptive mesh refinement is adopted to improve the computational accuracy and efficiency. The model is firstly validated and then applied to industrial scale ESR process. Moreover, the simulation results illustrate that a two-stage process of the droplet formation occurs during the ESR process: the primary droplet formation and the satellite droplet formation. The primary droplet is formed by the gravity stretch, the pinch-off effect of the Lorentz force and surface tension. The formation process of the satellite droplet can be divided into two types: the tip pinch-off of the ligament controlled by the surface tension and the bifurcation of the ligament controlled by the Lorentz force. In addition, the effect of the metal-slag interface tension on the droplet formation is analyzed. The development of the model supports further understanding of the metallurgical reaction and non-metallic inclusion movement during ESR process.

Parametric Analysis and Optimization of Leaning Angle in Torque Converters

Abstract Torque converters are durable fluid couplings that can provide output torque multiplication. Blade leaning angle represents the angular position of a blade chord with respect to its radial reference line, and it is an important blade variable regarding both hydrodynamic performance and manufacturability of a torque converter. In traditional design processes, blade leaning angles are often determined based on experiences of engineers; hence, this study proposed a design approach using the combination of computational fluid dynamics (CFD) and optimization. Two CFD models were developed to design blade leaning angles. A steady-state periodic CFD model was employed for the parameter study and the optimization, and a transient full three-dimensional (3D) model was performed to study the flow mechanism and evaluate the performance with higher accuracy. Design of experiment (DOE) technique was employed to investigate the relationship between blade leaning angles and hydrodynamic performance, and a reduced cubic model was derived from the results. It was found that blade leaning angles had profound effects on torque converter performance; a large blade leaning angle intensified the flow blockage effect, thus resulting in a lower mass flowrate and torque capacity. Seven torque converters with different blade leaning angles were tested to validate the obtained numerical results, and the test data were found to be in good agreement with the CFD predictions. Finally, the hydrodynamic performance of the base model torque converter was optimized by a multi-objective genetic algorithm.

Influence of injection valve opening manner and injection timing on mixing effect of direct injection compressed natural gas–fueled engine

In order to improve the control precision of in-cylinder mixing and combustion process, and to avoid the engine power drop because of the port fuel injection mode, it becomes a tendency to inject gas fuel into cylinder directly, with the help of the high-pressure gas-fueled injection device. However, considering that the mixing speed of gas fuel with air is usually slower than that of gasoline or diesel, the gas fuel direct injection mode tends to cause poor mixing performance and insufficient combustion in engine. Based on this situation, in-cylinder mixing process of direct injection gas–fueled engine is taken as the research object in this article. The three-dimensional transient computational fluid dynamics model of the injection and mixing process in gas-fueled direct injection engine is established to analyze the effects of the poppet valve opening manner and injection timing on the in-cylinder mixing homogeneity. The results indicate that delaying the injection timing can improve the wall impact strength and help to form a tumble flow in the cylinder. The stronger wall impact and tumble flow can reverse the natural diffusion law and greatly improve the in-cylinder mixing effect. Under the same injection timing conditions, although the pull-open valve has a larger injection penetration distance, the in-cylinder mixing effect is still worse than the push-open one. This is because, for the push-open valve, the fuel jet will be around the slope of the valve, which is beneficial to improve the mixing effect.

Experimental and numerical study of the heat transfer process during the startup of molten salt tower receivers

During the startup operation of molten salt solar tower plants, the tubes of the receiver are subjected to a high nonhomogeneous heat flux along with a low heat transfer coefficient to the air inside the tube, which results in a highly uneven angular temperature distribution. This uneven temperature distribution causes thermal stresses and tube deflection in the receiver. The most important constraint for the design and operation of central receivers is to keep the intercepting solar flux within the tube mechanical safety limits. In this work, an experimental facility consisting of a molten salt loop that simulates a solar tower receiver tube is used to measure the outer surface temperature and the deflection of the tube under the startup operating conditions of a solar tower power plant. An inverse heat transfer problem is applied to obtain the heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient three-dimensional numerical model of an empty circular pipe subjected to a nonhomogeneous heat flux is developed. A good agreement between the experimental and calculated tube temperatures and deflection is observed, with differences of 7% and 10%, respectively. Moreover, the thermal stresses are calculated. It has been found that higher thermal stresses are obtained when the tube is preheated compared to the stress when the molten salt is flowing under similar heat flux conditions.

Numerical investigation on the influences of molten pool behavior on weld penetration depth in pulsed gas metal arc welding

A three-dimensional transient numerical model with a Gaussian surface heat source was built based on the volume of fluid method to delve into the dynamic behavior of free weld pool surface in a stationary pulsed gas metal arc welding. When other conditions remain unchanged, the height fluctuation of pool surface and the variation in arc voltage in the peak current period decrease with the increase in the penetration depth. With the increase in the wire feed speed, the finger-like penetration can enhance the electromagnetic force to accelerate the molten metal flow, thereby leading to a reduced relative velocity between the droplet and the molten metal. As a result, the effect of droplet transfer on the growing molten pool is reduced, and a smaller amplitude oscillation is induced.

Numerical analysis of temperature uniformity of a liquid cooling battery module composed of heat-conducting blocks with gradient contact surface angles

Battery thermal management has an important significance for electronic vehicles to maintain a suitable temperature range and reduce local temperature differences. In this study, a novel liquid cooling battery thermal management system (BTMS) that uses a straight microchannel flat tube and heat-conducting blocks with gradient contact surface angles is proposed to improve the temperature uniformity of cylindrical lithium-ion battery module. A three-dimensional transient heat transfer model is conducted to investigate the thermal performance of the proposed battery module with 48 cells arranged on both sides of a straight microchannel flat tube. The effects of contact surface angle (αi), gradient angle increment (Δα) and inlet velocity of the fluid medium on the cooling performance are discussed. The results indicated that the module with a gradient contact surface angle, which implies gradually increasing the heat transfer area, further improved the temperature uniformity of the battery module compared to that with an unchanged αi. In addition, the increase of the inlet velocity does have a positive effect on both the reduction of maximum temperatures and enhancement of uniform temperature distribution. When Δα is 15° and the inlet velocity of the fluid medium is 0.015 m/s, the temperature difference (ΔT) of the battery module can reach 2.58 °C at the end of discharge. In addition, the preheating performance of the battery module at subzero temperature is also discussed. The results show that the temperature uniformity increase with the increase of the gradient angle, thus when Δα is 15°, the temperature difference of the battery module can be controlled to 8.05 °C.

Thermal Contact Resistance between Aero Engine Compressor Blade and Flexible Fixture

In the process of two-pass micro plasma arc welding of titanium alloy sheet, based on the thermal contact resistance theory, a three-dimensional transient heat transfer finite element model of contact heat transfer between titanium alloy sheet and flexible fixture was established. The effects of contact upper surface temperature, micro-contact thermal resistance, micro-gap thermal resistance, DC and pulse welding on thermal contact resistance were investigated, and the simulation and experimental results were compared. The results show that the thermal contact resistance increases as the temperature increases when the temperature of the contact upper surface (titanium alloy sheet) is in the range of 290–350 K. The effect of micro-contact thermal resistance on the thermal contact resistance is 26.1%–46.3% larger than the micro-gap thermal resistance. The thermal contact resistance of pulse welding is 3.1%–3.5% larger than DC welding. The error of simulation and experimental results of weld pool depth is 8.6%, which verifies the reliability of the model.

3D CFD modelling of liquid dispersion in structured packed bed column for CO2 capture

Structured packing has been used for gas-liquid reactions in several industrial processes, including amine-based CO2 scrubbing. Accurate prediction of flow behaviour within the columns provides a better understanding for process intensification and optimisation, leading to more efficient systems and reduction in capital cost. In this work, three-dimensional (3D) transient Eulerian-Eulerian two-phase flow models were developed to assess mechanical dispersion models for prediction of the liquid dispersion. The present model showed very good agreement with experimental results, but also provided much better predictions compared to two-dimensional (2D) models, indicating the anisotropic behaviour of flow. It was found that an increase in inlet velocity of liquid resulted in a wider dispersion, but with overconcentration on its border. Moreover, it was shown that multiple liquid inlets showed an increase in liquid hold-up, but slightly increased pressure drop along the column, compared to the single liquid inlet.

Transient Response of Microfluidic Fuel Cell

Diffusion boundary layer on the electrode surface of microfluidic fuel cells limits the mass transfer to the catalyst layer. Thus, investigation of the transient response of microfluidic fuel cell associated with the change in operating condition is not a trivial work. In this paper, transient characteristics of a microfluidic fuel cell upon changes in voltage load is investigated by applying a transient three-dimensional, two-phase and non-isothermal numerical model. Proposed model is developed with COMSOL multi-physics and governing equations are formulated based on the conservation laws for mass, momentum, species and electrical potentials. This model evaluates the effect of a dominant dynamic aspect of MFFC operation (mass transfer effect), accounts for transient convective and diffusive transport, and allows prediction of species concentration. Simulation results show that during step voltage change, the transient current lags behind voltage due to the time constant of reactant species transport. Also, simulation results show that the over/under shot of the current density occurs during the step change in the fuel cell voltage. The limited mass transport in the diffusion boundary layer causes a time delay between the voltage step change and the change of reactant distribution on the electrode surface. During the step voltage decrease/increase, the initial higher/lower reactant concentration on the electrode surface causes the over/under-shoot of current density. Simulation results show that the properties of the current over/undershoot can be varied according to the different values of functional parameters of the fuel cell.

Experimental and numerical study on the thermal performance of ventilated roof composed with multiple phase change material (VR-MPCM)

In the hot summer, roof is the prime location absorbed the most solar radiation for both residential and commercial buildings. Hence, enhancing the thermal performance of roofs is essential for reducing the heat gain thus building load. In this study, a novel ventilated roof composed with multiple phase change material (VR-MPCM) was proposed and its thermal performance was investigated experimentally and numerically. A contrast experiment was conducted in two full-scale chambers. The research results indicated that VR-MPCM can reduce the indoor peak temperature by 16.9–18.8%, and push back the peak temperature occurrence time by 30–50 min compared with the conventional ventilated roof (CVR). In addition, the cumulative heat flux during 4320 min demonstrated that the energy conservation rate of VR-MPCM can go up to 97.1% compared with the CVR. A verified three-dimensional transient heat transfer model was introduced for further research. Through the simulation research, intermittent opening of vents was found to be the best operating strategy for reducing heat gain. Regional comparison indicated that VR-MPCM in severe cold and cold climate zones had obvious energy saving potential. In conclusion, VR-MPCM has a potential of energy saving by reducing the diurnal heat gain and utilizing the nightly free cooling.

Heat flux estimation using simplified models by means of deviation time

Most of the thermal problems in engineering involve transient three-dimensional models of complex numerical or analytical solution. In some cases, the three-dimensional problems can be reduced. The use of simplified models to the inverse problem's solution has some advantages, such as: reduction in computational time and disregard some boundary conditions (convection heat transfer coefficient or prescribed temperature). To the procedure presentation, some heat fluxes were simulated in one-dimensional finite models. The Transfer Function Based on Green's Functions (TFBGF) method was used as inverse problem solution tool and excellent estimates were realized using the semi-infinite model. In a controlled laboratory experimental test, this procedure of model reduction was also applied. The heat flux was successfully estimated by using two different models: an 1D semi-infinite and an 1D finite. These simplifications are only valid for times shorter than deviation time.

Experimental and computational fluid dynamics modeling of a novel tray humidifier column in humidification dehumidification desalination; evaluation of hydrodynamic and heat transfer characteristics

In this article, a novel tray humidifier column for humidification dehumidification desalination was proposed. The performance of the humidifier column has been investigated with experimental and computational fluid dynamics simulations. The hydrodynamics and heat transfer characteristics of this tray humidifier has been studied. A stainless steel sieve tray with a rectangular cross section with a dimension of 20 × 50 cm was used in the experimental study. In computational fluid dynamics modeling, a transient three-dimensional model has been developed based on the volume of fluid framework by using standard k-epsilon model. The effect of air and seawater flow rate and inlet seawater temperature on the exit air temperature has been investigated. The results show that the humidifier effectiveness of the tray humidifier column varies between 0.67 and 0.87 depending on operating conditions. Then, tray column can be used in humidification dehumidification desalination systems with advantages such as compact equipment, low-pressure drop, and handling solids or other sources of fouling.

Single and Multiscale Bubble Motions Beneath an Inclined Downward-Facing Surface in the Aluminum Reduction Cell

Three-dimensional (3D) transient mathematical models are developed to explore the single and multiscale bubble dynamics beneath the inclined downward-facing surface of the anode in an aluminum redu...

Forecasting leachate generation from pilot woodchip stockpiles using a three-dimensional transient flow model

Leachate generation from open stockpiles of recycled woodchip materials is potentially harmful to aquatic ecosystems. There is growing interest in using numerical models to simulate leachate generation from outdoor piles, but this requires information about the hydraulic properties of the materials. The objectives of this study were to simulate leachate from woodchip piles with the numerical model HYDRUS-3D and to optimize subsets of parameters for single (SPM) and dual (DPM) pore flow models with the Bayesian Markov Chain Monte Carlo algorithm DREAMZS. Three experimental piles, each approximately 30 m3, were setup with mixtures of either once (coarse) or twice (fine) ground woodchips. Leachate continuously collected over a period of six months was similar across piles. As a result, subsets of optimized flow parameters for the coarse and fine woodchips were not different. Leachate predictions by the two pore flow models were similar and agreed reasonably with the field measurements, as indicated by Nash-Sutcliffe efficiency values greater than 0.6. This result suggests the simpler SPM is adequate for field predictions of leachate. However, leachate was consistently under-predicted by both pore models by 13–27% during rainfall events with more than 1 cm in 6 h. The optimized flow models can be used as a tool for studying pile management strategies.

Scale-Up Effects of Depleted Uranium (DU) Beds for Hydrogen Isotopes Storage

The purpose of this study is to investigate the influence of design parameters for the scale-up of the depleted uranium (DU) bed. The actual DU bed chosen for this study has a DU loading of 1.86 kg for a tritium capacity of 70 g and is cylindrical in shape and equipped with copper foam to enhance internal heat transfer. Based on the reference DU bed geometry, three different scale-up bed geometries to increase the amount of DU loading up to 9.3 kg were designed under different aspect ratios for comparison purposes and simulated using a three-dimensional transient DU hydride model developed in our previous studies. The simulation results are compared in terms of the evolution of the DU hydride temperature and H/U atomic ratio during the DU hydriding process. This study helps to identify key design parameters (e.g., it is critical to scale up the DU bed geometry).

Vertebral Artery Stenoses Contribute to the Development of Diffuse Plaques in the Basilar Artery.

Vertebral artery (VA) stenosis is relevant to a high early risk of recurrent stroke and basilar artery (BA) is the most common intracranial site of atherosclerotic lesions. It is important to show predictive risk factors for transient ischemic attack (TIA) or posterior infarctions. The aim of the study is to investigate morphometry and hemodynamics in intracranial vertebral and basilar arteries of health and diseased patients to enhance the risk assessment. Based on the geometrical model reconstructed from CTA images in 343 patients, a transient three-dimensional computational model was used to determine the hemodynamics. Patients were classified in symmetric, asymmetric, hypoplastic, and stenotic groups while patients in the stenotic group were divided into unilateral, bilateral, bifurcation, and tandem stenotic sub-groups. Patients in bilateral, bifurcation, and tandem stenotic sub-groups had significantly lower basilar artery diameters than other groups. Patients in the stenotic group had significantly higher surface area ratio (SAR) of high time-averaged wall shear stress gradient (TAWSSG) and higher incidence of TIAs or posterior infarctions than other groups while patients in the tandem stenotic sub-group had the highest values (SAR-TAWSSG of 57 ± 22% and TIAs or posterior infarction incidence of 54%). The high SAR-TAWSSG is predisposed to induce TIAs or posterior infarction.

The Influence of Barrel Offset on Cylinder Liner-Piston Ring Lubrication State

The shape of the barrel surface is an important parameter of the piston ring, and it is also one of the key factors determining the lubrication state of the cylinder liner-piston ring friction pair. To research the influence of barrel surface shape on the lubrication performance of cylinder liner piston ring, a transient three-dimensional lubrication model of cylinder liner piston ring is established by combining the average Reynolds equation and green wood formula. The lubrication performance of this friction pair under different working conditions and different cylinder surface shape is analysed, which provides the basic numerical research for the shape optimization of piston ring.

Numerical analysis of the TIG arc preheating effect in CMT based cladding of Inconel 625

A three-dimensional transient numerical model considering metal transfer process is established to analyze the effect of TIG arc on the formation of CMT cladding process. The temperature of the solid metal around the weld pool increases by about 250-300 K during hybrid welding. The liquid flow velocity behind the weld pool decreases, and the backward flow trend is weakened obviously. The hybrid welding promotes the spread of the molten cladding material and the stability of the weld pool. The contact angle decreases from 102° to 77°, and the dilution ratio increases from ∼2.5% to ∼5.4% when the TIG is added. The preheating of TIG arc can decrease the contact angle of CMT cladding without increasing the dilution significantly.

Real-Time Estimation of Molten Steel Flow in Continuous Casting Mold

To realize precise flow control of molten steel in the continuous casting mold, we developed a real-time flow estimation algorithm based on a three-dimensional transient model. Conventional model-based estimation suffers from the estimation error caused by disturbances, e.g., submerged entry nozzle (SEN) clogging. To incorporate the influence of the disturbances into the model calculation and reduce the estimation error, the developed algorithm adjusts the virtual external force at the SEN ports in the model. As a result of the simulation experiment, the asymmetrical flow caused by the nozzle clogging was successfully estimated, and the estimation error of the flow ratio of the left and right SEN ports was only 3 pct.