Thermal Flow Field Research Articles

Performance of tubular Solid Oxide Fuel Cell at reduced temperature and cathode porosity

The effect of decreasing the inlet temperature and the cathode porosity of tubular Solid Oxide Fuel Cell (SOFC) with one air channel and one fuel channel is investigated using Computational Fluid Dynamics (CFD) approach. The CFD model was developed using Fluent SOFC to simulate the electrochemical effects. The cathode and the anode of the cell were resolved in the model and the convection and conduction heat transfer modes were included. The results of the CFD model are presented at inlet temperatures of 700 °C, 600 °C and 500 °C and with cathode porosity of 30%, 20% and 10%. It was found that the Fluent-based SOFC model is an effective tool for analyzing the complex and highly interactive three-dimensional electrical, thermal, and fluid flow fields associated with the SOFCs. It is found that the SOFC can operate in the intermediate temperature range and with low porosity cathodes more efficient than at high temperatures given that the transport properties of the cathode, anode and the electrolyte can be kept the same.

The Experimental Investigation of Recirculation of Air-Cooled System for a Large Power Plant

The paper introduces thermal buoyancy effects to experimental investigation of wind tunnel simulation on direct air-cooled condenser for a large power plant. In order to get thermal flow field of air-cooled tower, PIV experiments are carried out and recirculation ratio of each condition is calculated. Results show that the thermal flow field of the cooling tower has great influence on the recirculation under the cooling tower. Ameliorating the thermal flow field of the cooling tower can reduce the recirculation under the cooling tower and improve the efficiency of air-cooled condenser also.

The carbon distribution in multicrystalline silicon ingots grown using the directional solidification process

In this study, we performed a numerical simulation of the growth of multicrystalline silicon ingots using the DSS method and compared the results with the experiments. The thermal flow field and the carbon concentration distribution during the growth process were analyzed under the same operating conditions. The carbon concentration distribution in the grown ingots was measured and the results compared with that of the simulation predictions. The simulation results are in good agreement with the experimental ones. The simulation shows that in a directional solidification furnace carbon impurities accumulate easily in the melt near the central region of the melt/crystal interface due to convection. This is the main reason for the non-uniformity of the carbon concentration in ingots grown in the DSS furnace. In order to improve the uniformity of carbon distribution in the melt, a higher convexity of crystalline front interface in the central region needs to be maintained during the growth process to reduce the strength of melt convection around the crystalline front interface.

On the microstructure of thermal and fluid flow field in a lasing medium of a high repetition rate dye laser

A high repetition rate, tunable dye laser suffers from thermal and flow field-induced inhomogeneity inside the laser-active zone of the dye solution, and shows a significant disparity in the spectral characteristics. The thermo–optical distortion developed in the lasing medium of dye solution, under high repetition rate pumping, perturbs the dynamics of the medium. The microstructures of the flow field in the hydrodynamic boundary layer, which are responsible for heat transfer, optical path length modulation, and divergence of the seeding radiation, transform the output characteristics of dye laser. The structure function, which represents the intensity of flow velocity fluctuations, and the eddy diffusivity of fluid flow help in design and development of a high-power tunable dye laser.

The effect of blade angle on the flow and pressure distributions in the vicinity of the diffuser blades of a room air conditioner

Many studies on air-conditioning systems are more focused on the individual thermal comfort rather than the thermal efficiency, due to an increase in health concerns. There are several factors influencing the thermal comfort, such as temperature, humidity, convection and air movement, etc. Numerical analyses were performed to investigate the effect of blade angle on the flow characteristics in the vicinity of diffuser blades of a room air conditioner (RAC), with three different blade discharge angles of 45.1°, 58.6° and 116°. We used the commercial code FLUENT to calculate the two-dimensional steady thermal flow fields with different impeller rotational velocities. The angular velocities were located within the range from 900 rpm to 1200 rpm. Turbulence closure was achieved using a standard k-ɛ model. A moving reference frame (MRF) approach was adopted to simulate the flow field generated by the impeller in an RAC. The results were graphically depicted with various geometrical configurations and operating conditions.

Application of Coolant Diffusion Equation in Numerical Simulation of Flow in Air-cooled Turbines

This article deals with time-averaged coolant diffusion equation in arbitrary, body-fitted coordinates. Schmidt number and Lewis number are used to obtain the mass diffusion coefficient. Other than the existing N-S solver, a new solver for the diffusion equation and the influences of cooling air on the thermal parameters, such as gas constant, constant-pressure specific heat, is established. Capable of simulating the flow of multi component fluid, this solver is used to simulate the flow in air-cooled two-stage turbines. The comparison of test results from the modified solver to those from the traditional N-S solver has evidenced that a marked difference present is in thermal parameters and flow field structure.

G114 デルタ翼群を用いたT形合流管内の乱流混合促進(対流伝熱促進・制御I)

Experimental study was made on promotion and control of turbulent mixing of hot and cold airflows in a T-junction, which simulated an HVAC unit for automobile A/C system. Two kinds of delta wings were used as a mixing promoter, and the influence of their attack of angles on the thermal mixing and flow field were examined in detail. It was found that the thermal mixing is promoted by longitudinal vortices and strong turbulence produced by the delta wings, and the degree of mixing promotion can be controlled by changing the angle of attack of the wings.

Influence of material characteristics on plastomechanics of the FSW process for T-joints

The potential of friction stir welding (FSW) has thoroughly been investigated by several authors, however their focus has primarily been on butt joints. T-joints are of paramount interest for transportation industries especially due to their capacity to straighten panels. Welding of T-joints is very challenging due to thin walls, poor location of the rib–web interface and the requirements for corner-fillets. This paper investigates FSW of T-joints of two popular aluminum alloys, i.e. 2024-T4 and 6082-T6, and the role played by the material characteristics on joining. First, an experimental study is carried out with specially designed fixture to determine the effect of process conditions. Then, the joints are metallurgically and mechanically evaluated. Finally using a numerical model of the process previously developed by the authors, the thermal and plastic flow fields for the two alloys are calculated and compared. It is found that the material dependent thermal and plastic fields affect the state of TMAZ, HAZ and nugget-region in the joint and that the low-strength high-work hardening alloy 6082 provides a much better joint integrity than the higher-strength low-hardening 2024 primarily due to the greater penetration of the plastic zone in the former.

Convective heat transfer and the pattern of thermal emission on the gas giants

Jupiter and Saturn emit nearly twice the thermal energy they receive from the Sun. Although insolation decreases toward the poles, the large-scale outward heat flux is nearly uniform, with smaller-scale latitudinal undulations that correlate with the zonal jet streams. Here we present numerical models of rapidly rotating, turbulent 3-D convection in geometrically thin, uniformly forced layers of Boussinesq fluid that approximate the deep convection zones of Jupiter and Saturn. In previous studies we have demonstrated that such models generate zonal flows comparable to those observed on the gas giants. By analysing the simulated patterns of convective heat transfer, we show here that deep convection in the gas giants can explain the anomalously uniform large-scale thermal emissions as well as the jet-scale variations. In particular, we find that convective heat transfer by quasi-geostrophic thermal plumes in relatively thin spherical shell geometry generates an outward heat flow pattern with a broad equatorial minimum and peaks at the poles. The results suggest an alternative to the hypothesis that insolation controls the large-scale patterns of heat flux and zonal flow on the gas giants. Instead, we propose that the large-scale thermal and zonal flow fields originate deep within the planets' molecular envelopes.

Geometrical Properties of the Resolved-Scale Velocity and Temperature Fields Predicted using Large-Eddy Simulation

In this paper, local geometrical properties of the velocity and temperature fields of combined forced and natural convection in a vertical slot are studied using large-eddy simulation based on both numerical and analytical approaches. Previous studies on turbulence geometrical statistics appearing in the literature have primarily focused on either isothermal or isotropic turbulent flows; whereas in this work, we extend the scope of research to investigation of a wall-bounded thermal flow. In particular, we focus on studying the resolved helicity, enstrophy generation, local vortex stretching, and a variety of characteristic geometrical alignment patterns between the resolved velocity, vorticity, temperature gradient, subgrid-scale heat flux and the eigenvectors of the resolved strain rate tensor. In order to quantify the effect of buoyancy on the geometrical properties of the thermal flow field, a systematic comparative analysis has been conducted based on three different flow regimes (i.e., viscous sublayer, buffer layer and logarithmic layer) in both the hot and cold wall regions. The near-wall restriction on the geometrical property of the thermal flow field has been analyzed and some interesting wall-limiting geometrical alignment patterns in the form of Dirac delta functions are also reported.

Performance Simulation and Thermal Stress Analysis of Ceramic Recuperators formed by SiC and MAS

This study conducted performance simulations for ceramic recuperators and performed thermal stress analysis to provide reliable guidance for heat recuperator designers. A numerical simulation was conducted utilizing STAR-CD to predict thermal and pressure flow fields inside two 50 W ceramic compact heat exchangers, one made of SiC and the other made of MAS. The predicted temperature and pressure profiles from performance simulations were then used to compute thermal stress analysis of the ceramic plate recuperators. Thus, three issues were discussed—the impact of fluid pressure and the thermal expansion on the ceramic recuperators, the effects of the constraint condition of the model boundary on the ceramic plates, and the different effects of the two different ceramic recuperator cores could made to the outer metal shells.

Characteristics of Thermal-Flow Fields in a PECVD Reactor with Various Operating Conditions

Plasma-enhanced chemical-vapor deposition (PECVD) is a chemical process used to produce highpurity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. The understanding in the region from the gas inlet to the wafer surface is a crucial factor for successful thin-film deposition. The gaseous flow behavior and in turn, the growth of the thin film can be influenced by the reactor geometry, the thermo-flow conditions, and the operation procedure. In this study, the effects of various operating conditions and the shapes of the PECVD reactor are considered to elucidate the thermal-flow fields and the deposition rate. The noticeable feature of the model used in this study is that we can control the operating conditions such as the inlet velocity, the rotational speed of substrate in the PECVD reactor, and the design features of the PECVD reactor, such as the height of the susceptor and the distance between the susceptor and the wall of PECVD reactor chamber.

Calculation of temperatures in enclosed switchgears with regard to the closed circulation flow of gaseous dielectrics

In connection with the temperature calculation for enclosed gas isolated switchgears (especially in the case of constant load) problems arise at the mathematical modelling of the convective heat transmission, when the vertical temperature difference in the gas is important due to the circulating flow. In this case the model of the thermal network analogous to a simple electrical network is not applicable immediately. Therefore the convective heat flow within the switchgear will be determined by the calculation of the thermal flow field basing on the physical fundamental equations exclusively. A simple example demonstrates in which way the result of the field calculation will be inserted to the model of thermal network. For example the exact consideration of the convective flow field has significant influence on the result of the temperature calculation.

An input/output approach to the optimal transition control of a class of distributed chemical reactors

An input/output approach to the optimal concentration transition control problem of a certain type of distributed chemical reactors is proposed based on the concept of residence time distribution, which can be determined in practice by using data from experimental measurements or computer simulations. The main assumptions for the proposed control method to apply are that the thermal and fluid flow fields in the reactor are at pseudo-steady-state during transition and that the component whose concentration is to be controlled participates only in first-order reactions. Using the concept of cumulative residence time distribution, the output variable is expressed as the weighted sum of discretized inputs or input gradients in order to construct an input/output model, on the basis of which a constrained optimal control problem, penalizing a quadratic control energy functional in the presence of input constraints, is formulated and solved as a standard least squares problem with inequality constraints. The effectiveness of the proposed optimal control scheme is demonstrated through a continuous-stirred-tank-reactor (CSTR) network and a tubular reactor with axial dispersion and a first-order reaction. It is demonstrated through computer simulations that the proposed control method is advantageous over linear quadratic regulator (LQR) and proportional-integral (PI) control in terms of control cost minimization and input constraint satisfaction.

SF 6 arc plasmas modelling for compact and environmental-friendly gas circuit breakers

In order to calculate the complex arc plasma combined thermal flow dynamics with electromagnetics, we firstly should define the governing equations on these fields and interface them with multidisciplinary simulation techniques, which include thermal flow and electromagnetic fields simultaneously. Through the processes it is very important to make appropriate models on turbulence, radiation, ablation etc. for this type of interruption technique. Also, the study of geometric effects (i.e., nozzle shape) is needed to optimize for the better performance of interruption and compact and environmental-friendly gas circuit breakers.

A numerical study on propagation of premixed flames in small tubes

A premixed flame in a tube suffers strong variation in its shape and structure depending on boundary conditions. The effects of thermal boundary conditions and flow fields on flame propagation are numerically investigated. This study employs eight combinations of thermal and velocity boundary conditions. Navier–Stokes equations and species equations are solved with a one-step irreversible global reaction model of methane–air mixtures. Finite volume method using an adaptive grid method is applied to investigate the flame structure. In the case of an adiabatic wall, friction force on the wall significantly affected the flame structure while in the case of an isothermal wall, local quenching near the wall dominated flame shape and propagation. In both cases, variations of flow fields occurred not only in the near field of the flame but also within the flame itself, which affected propagation velocities. Near the quenching conditions, strong similarity in the flame structure was found regardless of the boundary velocity profiles due to self-induced velocity deformation. This study provides an overview of the characteristics of flames in small tubes at a steady state.

Numerical simulation of the internal flow in microwave plasma thruster in magnetic field

Recent experiments show that the performance of microwave plasma thruster is a little too low because the energy absorbed by plasma depends on the relationship of plasma and microwave frequency. One potential solution is to use external magnetic field to improve the transfer process of microwave in plasma. This paper presents numerical study about how a magnetic field affects the thermal plasma flow field and its temperature in microwave plasma thruster. Out purpose is to see if the magnetic field can enhance the thermal plasma core temperature. Assuming that the plasma within the thruster is under the condition of local thermal equilibrium and the external magnetic fields is uniform, coupled Navier-Stokes, Maxwell and Saha equations are set up to describe the plasma flow under microwave electromagnetic and external magnetic fields. Solving Navier-Stokes equations with semi-implicit method for pressure-linked equations and Maxwell equations with finite-difference time-domain method, the result shows that when the magnetic field intensity is 0.5 T the temperature of plasma core is increased by 24% compared with that at zero magnetic field. Thus the magnetic field improves the performance of microwave plasma thruster.

Surface Deformation and Convection in Electrostatically-Positioned Droplets of Immiscible Liquids Under Microgravity

A numerical study is presented of the free surface deformation and Marangoni convection in immiscible droplets positioned by an electrostatic field and heated by laser beams under microgravity. The boundary element and the weighted residuals methods are applied to iteratively solve for the electric field distribution and for the unknown free surface shapes, while the Galerkin finite element method for the thermal and fluid flow field in both the transient and steady states. Results show that the inner interface demarking the two immiscible fluids in an electrically conducting droplet maintains its sphericity in microgravity. The free surface of the droplet, however, deforms into an oval shape in an electric field, owing to the pulling action of the normal component of the Maxwell stress. The thermal and fluid flow distributions are rather complex in an immiscible droplet, with conduction being the main mechanism for the thermal transport. The non-uniform temperature along the free surface induces the flow in the outer layer, whereas the competition between the interfacial surface tension gradient and the inertia force in the outer layer is responsible for the flows in the inner core and near the immiscible interface. As the droplet cools into an undercooled state, surface radiation causes a reversal of the surface temperature gradients along the free surface, which in turn reverses the surface tension driven flow in the outer layer. The flow near the interfacial region, on the other hand, is driven by a complimentary mechanism between the interfacial and the inertia forces during the time when the thermal gradient on the free surface has been reversed while that on the interface has not yet. After the completion of the interfacial thermal gradient reversal, however, the interfacial flows are largely driven by the inertia forces of the outer layer fluid.

Numerical approach to hole shape effect on film cooling effectiveness over flat plate including internal impingement cooling chamber

Numerical approach have been conducted on a flat, three-dimensional discrete-hole film cooling geometries that included the mainflow, injection tubes, impingement chamber, and supply plenum regions. The effects of blowing ratio and hole’s shape on the distributions of flow field and adiabatic film cooling effectiveness over a flat plate collocated with two rows of injection holes in staggered-hole arrangement were studied. The blowing ratio was varied from 0.3 to 1.5, while the density ratio of the coolant to mainstream is kept at 1.14. The geometrical shapes of the vent of the cooling holes are cylindrical round, simple angle (CYSA), forward-diffused, simple angle (FDSA) and laterally diffused, simple angle (LDSA). Diameter of different shape of cooling holes in entrance surface are 5.0 mm and the injection angle with the main stream in streamwise and spanwise are 35° and 0° respectively. Ratio of the length of the cooling holes and the diameter in the entrance surface is 3.5. The distance between the holes in the same row as well as to the next row is three times the diameter of hole in the entrance surface. The governing equation is the fully elliptic, three-dimensional Reynolds-averaged Navier–Stokes equations. The mesh used in the finite-volume numerical computation is the multi-block and body-fitted grid system. The simulated streamwise distribution of spanwise-averaged film cooling effectiveness exhibited that low Reynolds number k– ε model can give close fit to the experimental data of the previous investigators. Present study reveals that (1) the geometrical shape of the cooling holes has great effect on the adiabatic film cooling efficiency especially in the area near to the cooling holes. (2) The thermal-flow field over the surface of the film-cooled tested plate dominated by strength of the counter-rotating vortex pairs (CRVP) that generated by the interaction of individual cooling jet and the mainstream. For LDSA shape of hole, the CRVP are almost disappeared. The LDSA shape has shown a highest value in distribution of spanwise-averaged film cooling effectiveness when the blowing ratio increased to 1.5. It is due to the structure of the LDSA is capable of reducing the momentum of the cooling flow at the vent of the cooling holes, thus reduced the penetration of the main stream. (3) The structure of the LDSA can also increase the lateral spread of the cooling flow, thus improves the spanwise-averaged film cooled efficiency.

Numerical simulation of nonequilibrium flow in high-enthalpy shock tunnel

The flow field of a nozzle starting process with thermal and chemical nonequilibrium has been simulated. This flow is produced in high enthalpy impulse facilities such as the free piston shock tunnel. The governing equations are the axisymmetric, compressible Navier–Stokes equations. In this study, Park’s two-temperature model, where air consists of five species, is used for defining the thermodynamic properties of air as a driven gas. The numerical scheme employed here is the hybrid scheme of the explicit and implicit methods, which was developed in our laboratory, along with AUSM + to evaluate inviscid fluxes. In the present simulation, the Mach number of an incident shock wave is set at M s=10.0. It corresponds to a specific enthalpy, h 0, of 12 MJ/kg. The results clearly show the complicated thermal and chemical nonequilibrium flow field around the end of the shock tube section and at the nozzle inlet during the initial stage of the nozzle starting process. They also suggest that the phenomenon of nozzle melting might be associated with a flow separation at the nozzle inlet.