Flow Temperature Field Research Articles

Numerical simulation of DC casting of large-size rare earth magnesium alloy ingot under low-frequency electromagnetic field

For studying the changes of macro-physical field in the casting process of large-scale rare earth magnesium alloy, through the numerical simulation method, a two-dimensional axisymmetric multi-physical field coupling model was established by using the multi-physical simulation software COMSOL Multiphysics. The changes of temperature field, flow field, Lorentz force, and liquid fraction of large-size rare earth magnesium alloy with diameter of 750 mm under different electromagnetic parameters (magnetic field frequency and current intensity) in steady state of direct-chill (DC) casting were studied. The results reveal that using a magnetic field can reduce the temperature gradient and greatly accelerate the melt flow, the depth of the sump is reduced by about 50 mm. As the current intensity rises, the flow rate in the melt becomes accelerated, the sump depth becomes shallower, while the melt area with a liquid fraction of 0.5 to 0.63 increases. The Lorentz force rises as the magnetic field frequency increases, but the skin depth of the magnetic field decreases from 64.9 to 36.4 mm.

Numerical and experimental investigation on heat transfer of multi-heat sources mounted on a fined radiator within embedded heat pipes in an electronic cabinet

In this paper, a finned heat sink for thermal management inside electrical cabinets was proposed to address the heat dissipation problem of some electronic equipment which are high heat flux and compact structure. The cooling system is mainly composed of four identical radiators, eight water-charged wickless L-shape heat pipes are embedded at the bottom of each radiator, and hollow fins are used in the condensation section of the heat pipes to enhance heat transfer. A thermal resistance network model of the heat pipe electric cabinet based on the superposition method was conceived. At the first part, experiments are conducted to measure the temperature of the heat source and the air in the air-flow passage. Transient and steady-state studies were carried out under different heating powers at the ambient temperature of 30 °C–35 °C. The maximum temperature difference between each heat source under forced convection is only 3.2 °C, which is more uniform than 10.36 °C in natural convection. The second part proposed a three-dimensional numerical model to further evaluate the heat transfer performance of the heat sink. The distribution characteristics of temperature field and flow field inside the cabinet are obtained by numerical calculation, and the variation of Ra number with height is further calculated. Taking the continuous junction temperature lower than 85 °C as the condition that the components in the cabinet can operate safely, the limit heat dissipation of the cooling device through natural convection can reach 763 W.

Research on flow field characteristics of nozzle plume for a solid rocket motor

In order to explore the flow field characteristics of nozzle plume for a small solid rocket motor (SRM), the two-dimensional axisymmetric N-S governing equations with component transports and chemical reaction terms are established. Then, the finite volume method of AUSM (Advection Upstream Splitting Method) spatial discretization scheme is applied for numerical discretization, and its solution is solved by time iteration to make the plume flow field steady. This technique is applied to research the flow field characteristics and characteristic parameter distributions of nozzle plume under different inflow conditions. The results are shown that: when considering the reburning reactions of nozzle plume, the temperature of the gas jet core area is much higher than the flow field temperature that the reburning phenomenon is not considered. In addition, the reburning phenomenon mainly occurs in the air-gas mixing and gas development areas, but in the initial core area of gas jet, this phenomenon is not obvious. What’s more, with the increase of altitude, the influence area of the gas jet gradually becomes larger. However, as the incoming flow Mach number increases, the intensity of the change of gas temperature along the nozzle axis weakens, and also the number of wave nodes and the influence region of gas jet gradually decrease.

Analysis of forced convection phenomenon in boundary layer flow of nanofluid over power-law rotating disk

In this paper, the thermal analysis is performed on the three-dimensional flow of viscous nanofluid over a rotating disk. The disk rotation is assumed as of the form [Formula: see text] where [Formula: see text], thus, there are different azimuthal velocities of the disk at different radii, and it increases with the increase in [Formula: see text]. Also, the effect of nanoparticles on the thermal conductivity of fluid is investigated by thermophoresis and Brownian motion features. For the governing problem, suitable similarity transformations are employed that convert the produced partial differential equations to ordinary differential equations. The flow field and energy transport are presented graphically and discussed in detail for the torsional exponent [Formula: see text], [Formula: see text] and [Formula: see text], respectively. It is noted that the temperature and concentration profile rise for the thermophoresis parameter while the opposite trend showed for Brownian motion. Moreover, the power-law index parameter [Formula: see text] declines the flow field temperature and concentration profiles.

Impact of fluid flow on the dendrite growth and the formation of new grains in additive manufacturing

Manipulating the formation of new grains in metal additive manufacturing (AM) is critical for tailoring the grain structure in the as-built components. Temperature gradient and solidification velocity are believed the main factors controlling the formation of new grains, while the impact of fluid flow in AM has never been investigated. In this work, the multi-grid dendrite growth model is improved by two-way coupling with a computational fluid dynamics model to simulate the dendrite growth in the flowing liquid metal under various temperature gradients and solidification velocities found in AM condition, thereby studying the impact of fluid flow on the dendrite growth and the formation of new grains in front of dendrites with various orientations. The simulation results reveal that dendrite growth and the formation of new grains in AM are influenced by the fluid flow and solidification significantly and in similar manner as that in casting. Furthermore, molten-pool scale dendrite growth simulations are carried out with the temperature field and flow field data extracted from the thermal-fluid flow modeling results. The simulation results well explain the formation mechanism of the grain structure in the single-track experiments with electron beam and laser, and further demonstrate the remarkable impact of fluid flow on dendrite growth and the formation of new grains in AM.

Study on the characteristic of boiling expansion process of superheated LPG and its vapor cloud explosion

Liquefied Petroleum Gas (LPG) is stored in liquid form under high pressure conditions. Once the tank fails, LPG will leak in gaseous form hundreds of times its liquid volume. LPG inside the storage tank will undergo a violent boiling process leading to pressure rebound in the tank. In serious cases, boiling liquid expanding vapor explosion (BLEVE) will happen. And the leaking LPG will mix with air to form a flammable vapor cloud that can explode. Therefore, it is necessary to study the violent boiling process inside the LPG vessel after rupture, the leakage diffusion process in the surrounding environment and the vapor cloud explosion process. In this paper, CFD software FLUENT was used to conduct numerical simulation of the inter-phase transfer of mass and heat inside the vessel when LPG leakage happens, and the transient pressure inside the vessel was analyzed. The results obtained from the simulation are in good agreement with the experimental data under the same conditions which provides the validity verification of the phase transition during boiling process of LPG. A three-dimensional numerical model of LPG release process after the failure of a cylindrical vessel was established. The realizable k-ε model and eddy-dissipation model were used to simulate the explosion process of LPG after the release. The pressure, temperature and thermal radiation distribution of the explosion flow field were obtained so the damage range of LPG after explosion can be analyzed, which has certain guiding significance for the risk assessment of LPG.

Performance study on a thermoelectric generator with exhaust-module-coolant direct contact

The Automotive Thermoelectric Generator (ATEG) aims to recycle the heat in vehicle exhaust, and transforms it into electricity. This paper proposes a specific structure of an automotive direct contact thermoelectric generator (ADCTEG) system. With no space among the TEMs, exhaust gas and coolant water, this automotive direct-contact configuration works in favor of system integration, minimizing the contact thermal resistance, and maximizing thermal efficiency. In this new TEG system, the thermoelectric modules (TEMs) expose the hot side surface on the automotive waste gas, while another surface directly contact with the cooling water. For the purpose of revealing output characteristics of the ADCTEG and the heat transfer performance, several numerical simulations and bench test are carried out to predict the temperature distribution, flow field and output power generation. Meanwhile, the influence of experimental parameters on ADCTEG power generation performance is studied also, the output characteristics of ADCTEG under vehicle on-board environment are discussed. The net output power of ADCTEG under different engine rotation speed is calculated. The researches on output characteristics of liquid–solid–gas ADCTEG system has guiding significance for the structure design of heat exchanger and the optimization design of thermal resistance and heat transfer of the system, and also has reference value for the potential application of automotive TEG.

Numerical investigation of Thermo-aerodynamic characteristics of gas foil thrust bearing

• Distribution of flow and temperature field in micro-clearance of GFTB are studied. • Temperature caused by viscosity dissipation accounts for more than 90% of aerodynamic heat. • The film temperature distribution is different when with or without viscous dissipation. • Temperature varies directly with rotate speed and inversely with film thickness. • Increase of wedge factor lowers the temperature and leads to reflux at the inlet. This study aims to reveal the thermo-aerodynamic characteristics of gas foil thrust bearings in the presence or absence of viscosity dissipation. The lubrication models in the cross-section of the gas film have been described, and the term of viscosity dissipation in energy equations is decoupled. The influence of different factors on gas film temperature distribution is discussed and compared. The results show when considering viscosity dissipation, the peak temperature is located near the circumferential outlet and the side of the outer diameter where has more shear stress. However, with the absence of viscosity dissipation, the peak value takes place near the end of the convergence channel where the gas compression is the most severe. Contrary to the case that without viscosity dissipation, gas film temperature decreases with the increase of wedge factor in the presence of viscosity dissipation. As wedge height increases, backflow will occur at the bearing entrance, reducing inlet temperature. When rotational speed increases, the gas film temperature caused by viscous dissipation increases much faster. The increase of film thickness reduces film temperature and bearing performance rapidly, but has no effect on the temperature distribution. This study confirmed that viscosity dissipation plays a significant role in the temperature rise and flow field of gas foil thrust bearings. More specially, temperature rise in the presence of viscosity dissipation accounts for more than 90% of the aerodynamic heat. The results obtained in this study are significant because they provide basic design guidelines for a temperature rise of the gas film.

Evaluation of flamelet/progress variable model for the applications in supersonic combustion using hybrid RANS/LES approach

The Flamelet/Progress Variables (FPV) model coupled with large eddy simulation (LES) has been performed to appraise its applicability and performance for supersonic hydrogen combustion, based on the simulations of strut based scramjet and cavity based combustors. For the two typical schemes involved in the present work, the validation of the numerical solver compiled by in-house code is realized by the comparisons with the experimental measurements including temperature, velocity, and wall pressure. The methods for calculating mixture fraction variance in the framework of LES are also investigated and examined in both cases. It can be observed that the diffusion combustion mode is in the dominant state for the whole reactive zone in both cases, where the FPV model is physically compatible. Through the comparisons of the scale similarity method and the transport equation method for calculating mixture fraction variance, the importance of calculating this sub-grid turbulent scalar flux is emphasized. The results show that the mixture fraction variance calculated by the scale similarity model is locally high, but its diffusion is insufficient, which makes the local temperature of the combustion flow field overpredicted and concentrated. On the contrary, the transport equation of the mixture fraction variance can overcome this problem and improve the combustion prediction accuracy, thus, it is more recommended if the computing resources are guaranteed. Qualitatively, the Flamelet/Progress Variables (FPV) model with the large eddy simulation approach can accurately capture the structure and characteristics of the supersonic combustion flow field.

Direct background-oriented schlieren tomography using radial basis functions.

Background-oriented schlieren tomography (BOST) is effective for flow field measurement; however, different from general computed tomography (CT), the BOST utilizes the deflection of rays passing through an inhomogeneous field for measurement. It is sensitive to the refractive index gradient. Therefore, an additional integration step is typically employed to obtain the refractive index. In this article, a calculation method of projection matrix is proposed based on the radial basis function (RBF). The 3D distribution of the refractive index can be reconstructed directly. This method was first verified by numerical simulation. Then, the 3D instantaneous refractive index field above a candle flame was measured. The reprojection error was calculated by ray tracing. The results illustrate the accuracy and stability of the proposed method. This research provides a new and complete solution for the 3D instantaneous flow field (refractive index, density, or temperature) measurement.

MHD natural convection in a corrugated enclosure with discrete isothermal heating

AbstractIn this study, we numerically examine the unsteady two‐dimensional natural convective heat transfer flow problem in a corrugated enclosure containing an electrically conducting fluid whose top wall is nonuniformly heated. The vertical sidewalls of the corrugated enclosure are kept isothermally cold, while square‐shaped undulations at the bottom wall are discretely heated. Five different cases are considered depending upon the location of the heat sources. A higher‐order compact scheme is used to discretize the governing equations, and an advanced iterative solver, like the hybrid biconjugate gradient stabilized method, is used to solve the system of algebraic equations generated by the numerical discretization. We first validate our results with existing experimental and numerically computed results for the case of a square cavity with and without a heat source. Then, the computed results are analyzed over a range of key parameters, like Rayleigh number (), Hartmann number (), and Prandtl number (), to study the effect of these parameters on the characteristics of heat transfer and the flow field in the corrugated enclosure. Computed results are presented in the form of streamlines, isotherms, local and averaged Nusselt number plots, and so forth. It is found that, with the increase in the heat sources, the convection rate becomes stronger in the core region of the corrugated enclosure at higher Rayleigh numbers. Variations in the Hartmann and Prandtl numbers also have significant effects on the temperature distribution and flow field. Our simulated results show various flow features that have not been studied before.

Design and numerical investigation of multi-channel cooling plate for proton exchange membrane fuel cell

Proton exchange membrane fuel cells need cooling plates with strong heat exchange capacity to maintain temperature balance. In order to obtain better cooling performance, four new types of flow field distribution of cooling plate are designed, including local serpentine channel, local parallel channel and the flow field in which the four channels are symmetrically distributed along the diagonal and horizontal centerline of the cooling plate. The maximum temperature, temperature difference, temperature uniformity index and pressure drop of the multi-channel cooling plate under different working conditions are analyzed by numerical simulation. The results show that the multi-channel design can effectively enhance the heat transfer effect of the cooling plate and reduce the pressure drop. The local serpentine channel design can increase the fluidity of the coolant, avoid local reflux, make the temperature distribution more uniform and avoid local overheating. The maximum temperature of local serpentine channel flow field is 1.2 K lower than that of local parallel channel flow field. The pressure drop of internal parallel flow field design is 2.58 kPa, which is 62% lower than that the design of internal serpentine flow field. Increasing the inlet coolant flow can strengthen the heat transfer capacity of the cooling plate, when the flow rate increases from 2e-6 m3/s to 4e-6 m3/s, the maximum decrease is 8.53% of Model6. But it will increase the pressure drop of the channel with long cooling channel, the maximum pressure drop increase is 924 kPa when the inlet flow increases from 6e-6 m3/s to 8e-6 m3/s.

Heat Flow Field Analysis on Heat Dissipation Features of High-Wattage Power Cabinets

With the rapid development of electronic science and technology, power electronic devices are increasingly efficient, intelligent, small, and multi-functional. To ensure the thermal reliability of these devices, it is particularly important to make a reasonable heat dissipation design. However, there is insufficient research into the precise measurement of the flow field and temperature field within real high-voltage power electronic devices under the heat dissipation state of liquid cooling. To fill the gap, this paper carries out a heat flow field analysis on the heat dissipation features of high-wattage power cabinets. Out of the multiple power electronic device groups in a typical high-wattage power cabinet, a row of power electronic device units was taken as the objects. Then, several power electronic devices were analyzed numerically to establish a thermal model of the power electronic device group. Next, we designed the dissipation structure of the group, obtained the correlation between the flow field uniformity and temperature distribution of power electronic devices, and analyzed the effects of the ambient temperature of the group, the flow rate of cooling air, and other factors on the temperature features of power electronic devices. The proposed thermal model and dissipation system design strategy were proved feasible through experiments.

Numerical simulation on fluid flow behavior during 3-dimensional dendrite growth with random preference angle

The flow behavior of three-dimensional (3D) dendrite growth with random preferred angle under natural convection was studied by using the Lattice Boltzmann-Cellular Automata (LB-CA) method with dynamic and static grids. In this model, the temperature field, flow field and solute field calculated by Lattice Boltzmann method (LBM) and dendrite growth calculated by CA method were carried out in static and dynamic grids respectively, and the coupling between LBM and CA was performed by interpolation of calculation parameters between dynamic and static grids. Results show that the asymmetry of solid phase distribution makes the streamline distribution more complex. At the initial stage of multiple dendrites growth, the fluid flow is relatively free. When dendrites grow close to each other, the fluid flow is blocked and can only flow along the gap between dendrites. During the wall equiaxed-columnar-central equiaxed crystals transformation (ECET) process, dense eddy current is formed at the wall equiaxed crystals at first. Then, when the wall equiaxed crystals gradually develop into columnar crystals, the eddy current moves with the solid-liquid interface. When the central equiaxed crystals are formed, the eddy current at the front of the columnar crystals gradually disappears. New eddies appear as the central equiaxed crystal grows.

Numerical Analysis of Physical Characteristics and Heat Transfer Decoupling Behavior in Bypass Coupling Variable Polarity Plasma Arc.

A novel bypass coupling variable polarity plasma arc was proposed to achieve the accurate adjusting of heat and mass transfer in the welding and additive manufacturing of aluminum alloy. However, the physical characteristics and decoupled transfer behavior remain unclear, restricting its application and development. A three-dimensional model of the bypass coupling variable polarity plasma arc was built based on Kirchhoff’s law, the main arc and the bypass arc are coupled by an electromagnetic field. The model of current attachment on the tungsten electrode surface is included for simulating different heating processes of the EP and EN phases in the coupling arc. The distribution of temperature field, flow field, and current density of the bypass coupling variable polarity plasma arc was studied by the three-dimensional numerical model. The heat input on the base metal under different current conditions is quantified. To verify the model, the arc voltages are compared and the results in simulation and experiment agree with each other well. The results show that the radius of the bypass coupling arc with or without bypass current action on the base metal is different, and the flow vector of the bypass coupling arc plasma with bypass current is larger than the arc without bypass current. By comparing the heat transfer on the electrodes’ boundary under different current conditions, it is found that increasing the bypass current results in the rise in heat input on the base metal. Therefore, it is concluded that using bypass current is unable to completely decouple the wire melting and the heat input of the base metal. The decoupled degree of heat transfer is one of the important factors for accurate control in the manufacturing process with this coupling arc.

Influence of laser-Sn powder coupling on microstructure and mechanical properties of fusion welded AZ31B-alloy to DP590-steel

Microstructure and mechanical properties were studied in fusion welded AZ31B-alloy to DP590-steel at different weld conditions such as with varying laser power, and Sn powder added to lap joint with prefabricated width-settled concave groove. Temperature field and flow field for laser welding including chemical potential of each element are also calculated. The results suggested that Al atom tends to diffuse toward Fe side which contributes to form a FeAl compound reaction layer at interface. Meanwhile, Al accelerates the diffusion of Sn and Fe as a catalyst. The shear force of joint climbs at first, then declines, and the maximum value reaches 1711 N when laser power is 1 kW, which is improved by 38% compared with that of 0.9 kW. The improved joint properties are dominated by metallurgical regulation of Sn power, which benefits from the accelerated flow of molten pool and increased temperature leading to the phase composition transformation from α-Mg + Mg2Sn + FeAl to α-Mg + Mg2Sn + FeSn + Fe3Sn + FeSn2 + FeAl in direct laser irradiation region. However, under the condition of excessive laser power, overreaction occurs at side boundary interface, which leads to the formation of thick IMC and the production of hot crack defect, thus the joint performance decreases.

Combustion instability of ethanol and n-heptane fuels under different combustor geometries

Combustion instability has become one of the most significant challenges faced in designing modern industrial gas turbines and aero-engines. In this study, the influence of combustor structure on the combustion instability of ethanol and n-heptane is explored by adjusting the plenum length (200–550 mm) and the swirler vane angle (60° and 75°). The ethanol combustion instability maps show that the oscillation frequency increases as the equivalence ratio and plenum length decrease with the stabilization range of n-heptane being broader. The maximum pressure pulsation amplitudes of ethanol and n-heptane during combustion instabilities are close to 495 and 415 Pa, respectively. The flow field temperature measurements show that the fuel properties change the flame shape and temperature distribution in the combustor, thus potentially affecting the intensity of the oscillations. Short, compact ethanol flames and more uneven temperature distribution are more likely to lead to unstable combustion. This work provides more experimental observations on the effects of fuel properties and combustion configuration on combustion stability.

Numerical simulation for dynamic behavior of molten pool in cable-type welding wire GAMW

The cable-type welding wire gas (CWW) metal arc welding (GMAW) is an innovative arc welding process with high efficiency and high welding quality. However, there is still a lack of deep research on the molten pool of CWW-GMAW. The behavior of molten pool of CWW-GMAW was studied by both numerical simulations and experiments. The temperature field and flow field in the longitudinal section and across cross-section under the current of 450 A in CWW-GMAW were simulated. The result shows that the isotherm in front of the molten pool is denser than that in the back on the longitudinal section. The arc rotation force causes the velocity of the molten pool in the direction of weld penetration becoming more significant than that of weld width. The rotating momentum of the droplet affects the temperature distribution of the weld width and penetration on the cross-section of the molten pool. The coupling of electromagnetic force and rotating arc force makes the flow field present a clockwise vortex shape. The simulated results of weld geometries of CWW GMAW are in good agreement with the experimental results, indicating the applicability and accuracy of the established model.

Thermal Characteristics Simulation of an Energy-Conserving Facade: Water Flow Window

In this paper, a 3D numerical simulation was completed to explore the thermal characteristics in a water flow window in-depth. CFD was used to carry out the analysis on top of successful validation. By changing the solar intensity, water supply temperature and velocity, the temperature distribution and flow field in the window cavity, as well as the water heat gain, were analyzed and compared. This is meaningful for improving the energy-conserving performance in building applications. Simulation results reveal that the variation of solar intensity and water supply temperature affects directly the temperature distribution and the water heat gain but has little impact on the overall velocity field. Local vortices are generated in the window cavity, and their formation and location are largely affected by the varied temperature rise in the water layer. The water heat gain increases and then decreases with the increase in water supply velocity. In addition, a large-enough water supply velocity can disorder the uniform upward flow. These are detrimental to effective thermal extraction. Therefore, in practical application, the vortex should be eliminated, and the flow velocity should be determined properly to maximize the water heat gain.

The influence of process parameters on the preparation of closed-cell aluminum foam by friction stir processing

In order to obtain high-quality aluminum foam components, a new preparation method for a 7075 closed-cell aluminum foam based on friction stir processing (FSP) was proposed. The influence of process parameters on microstructures of closed-cell Al foam precursor was investigated by optical metallographic microscope and scanning electronic microscope (OM/SEM). The temperature field and flow field of the Al foam precursor during FSP were simulated by Fluent CFD software. The formability of the Al foam precursor was analyzed by the cupping test. It is found that the rotating speed has great influence on the uniformity and density of the precursor prepared by FSP, but the welding speed has little influence. Numerical simulation shows that the maximum flow velocity of material in the tool shoulder increases by 1.5 times when the rotating speed increases from 800 to 2000 r/min, and the peak temperature at the stirring pin reaches 491 °C, which is consistent with the experimental results. The powder ring is continuous and uniform due to sufficient plastic deformation and flow at travel speed of 50 mm/min and the rotating speed of 2000 r/min. The formability of the precursor increases with the increase of deformation temperature. Especially the cupping test value of precursor is close to that of base metal at 450 °C. In the foaming process, the foam pores first appear at the bottom and then expand upwards with the extension of the foaming time. The expansion rate of the whole foam increases rapidly, and the pores with highly spherical are homogeneously distributed when the foaming time is 110 s at 680 °C.