Thermal Flow Field Research Articles

“绿色”液体激光介质的热流场特性研究

“绿色”液体激光介质的热流场特性研究

노즐형상 및 대기조건에 따른 항공기의 플룸 IR 신호 연구

항공기 추진 시스템의 IR 피탐지성은 노즐형상 및 대기조건에 큰 영향을 받게 된다. 그 영향성을 분석하기 위해 대표적 스텔스 무인 항공기와 그 추진 시스템의 형상변형 수축노즐을 고려하였다. 먼저 압축성 CFD 코드를 이용하여 IR 신호 계산에 필요한 열유동장 및 노즐표면 온도 정보를 산출하였다. 플룸 IR 신호를 계산해 본 결과 축방향 신호수준은 상당히 감소하는 반면, 노즐의 좌우 측면에서는 노즐의 가로세로비 증가로 인해 플룸이 좌우로 확장되어 특정 가로세로비에서 상대적으로 증가된 플룸 IR 신호가 발생함을 확인하였다. 다음으로 LOWTRAN 7 코드와 연계하여 계절 및 관측거리 변화에 따른 대기 투과율을 분석하고 그 결과를 바탕으로 대기효과가 고려된 플룸 IR 신호를 계산하였다. 계산결과 계절이 여름일 경우와 비교적 근접의 관측거리에서 이산화탄소 밴드에서 IR 신호가 현저히 감소하는 것을 확인하였다. Nozzle configurations and atmospheric conditions play a significant role in the infrared signature level of aircraft propulsion system. Various convergent nozzles of an unmanned aircraft under different atmospheric conditions are considered. An analysis of thermal flow field and nozzle surface temperature distribution is conducted using a compressible CFD code. It is shown that the IR level in rear direction is considerably reduced in deformed nozzles, whereas the IR level in adjacent azimuth angles is increased in aspect ratios around 6 due to the plume spreading effect caused by high aspect ratio of nozzles. In addition, an analysis of atmospheric transmissivity for various seasons and observation distance is conducted using the LOWTRAN 7 code and subsequently plume IR signature is calculated by considering atmospheric effects. It is shown that the IR signature is reduced significantly in summer season and near the band of carbon dioxide in case of relatively close distance.

A microspray-based cooling system for high powered LEDs

We propose a microspray-based cooling system for the thermal management of high-power, light emitting diodes (LEDs). Experiments were performed by applying a single microspray to a single and a four LED system. The spray exited the nozzle using piezo-electric micropumping. The effect of cooling in a non-boiling regime was quantitatively studied within a range of relevant operating parameters. Furthermore, both μPIV flow visualizations, infrared thermal image observation and flow field measurements were first made to investigate the heat transfer mechanisms involved in this complex process.

3D Numerical Simulation on Thermal Flow Coupling Field of Stainless Steel During Twin-Roll Casting

The surface crack and lateral crack of the AISI 304 stainless steel thin strip produced by twin-roll casting were observed. The temperature at the center of outlet during twin-roll-casting process was determined by infrared thermometer. In order to avoid the surface cracks of the casting strip, the thermal flow coupling field of AISI 304 stainless steel during twin-roll-casting process was simulated by a 3D fluid-structure coupling model. According to the simulation result, the effect of the casting speed on thermal flow field was analyzed and the process parameters were optimized. Moreover, by studying heat flux curves, the heat transfer mechanism between molten pool and roll was analyzed. The results show that, with the increase of the casting speed, the temperature of the molten pool increases and the solidification point moves toward the outlet. Meanwhile, the whirlpool above gets larger. Based on the solidification front position, the optimized process parameters are 1500 °C and 0.37 m/s. The heat transfer mechanism between molten pool and roll contains direct contacting heat transfer and air gap heat transfer.

무인항공기 노즐 형상 변화에 따른 IR 신호 영향성 연구

The effects of various nozzle configurations on infrared signature are investigated for the purpose of analysing the infrared signature level of aircraft propulsion system. A virtual subsonic aircraft is selected and then a circular convergent nozzle, which meets the mission requirements, is designed. Convergent nozzles of different configurations are designed with different geometric profiles. Using a compressible Navier-Stokes-Fourier CFD code, an analysis of thermal flow field and nozzle surface temperature distribution is conducted. From the information of plume flow field and nozzle surface temperature distribution, IR signature of plume and nozzle surface is calculated through the narrow-band model and the RadThermIR code. Finally, qualitative information for IR signature reduction is obtained through the analysis of the effects of various nozzle configurations on IR signature.

An advancement in iterative solution schemes for three-dimensional, two-fluid modeling of two-phase flow in PWR fuel bundles

This paper outlines a fully three-dimensional two-fluid one-pressure model with a semi-implicit finite difference scheme coupled with heat conduction which can be applicable to thermal non-equilibrium two-phase flow field in subchannel geometry of Pressurized Water Reactors (PWR). The system of equations was linearized using the Newton–Raphson method and was collapsed into the pressure equations forming a system of the Poisson type. Then, two-phase flow modeling was combined with Krylov methods as advanced computing techniques to investigate the feasibility of implementing preconditioned Krylov subspace solvers as the numerical scheme to solve pressure equations. Six popular Krylov subspace solvers were considered: GMRES, FGMRES, DQGMRES, CGNR, BCG, and TFQMR combined with the block incomplete LU factorization with a dual truncation strategy (BILUT) preconditioner. These proposed iterative solvers were applied to the constructed linear pressure equations in the inner iteration in combination with the outer-Raphson iteration loop. Evaluation was performed in two stages. First, two-fluid numerical scheme capability was evaluated against OECD/NRC NUPEC PWR Bundle tests (PSBT Benchmark). The results for steady-state (PSBT) bundle show that an overall agreement can be found. At the second stage, convergency, stability, and accuracy of the proposed schemes were studied based on PSBT steady-state data through a comparison of utilized Krylov solvers and the direct inversion method as the pressure solution modeling options. It was found that the Krylov solvers do not introduce deviations to the two-phase flow results. Moreover, the convergence history of candidate Krylov solvers demonstrated that GMRES, FGMRES, and DQGMRES have a more efficient and stable convergence performance.

Numerical study of the characteristics of supercavitation on a cone in a stationary evaporator

Computation results of the supercavitation on a cone in a bounded axial cold water (20˚C) flow, obtained by ANSYS13 CFX and FLUENT, are found to correlate with experimental data. CFX solution has captured considerable backflow, and temperature gradient of 7˚C between free water stream and steam inside the supercavity, with the average water vapor temperature of supercavity volume equal to 17.8˚C, while FLUENT has revealed negligible temperature gradient, and the position of backflow vortex is different. Although we have used the same setup and appropriate meshes for both solvers, due to different cavitation models: mixture Rayleigh‐Plesset and Schnerr-Sauer, for CFX and FLUENT, respectively, we have found wide discrepancy in the resulted thermal, mass, and velocity flow fields. Understanding thermal, mass, and velocity fields of the supercavitating flow is important for the development of an industrial application of supercavitation for the design of the evaporators, open-type heat exchangers, coolers, deaerators, mixers, and chemical reactors. The CFX solver have been used for revealing the multiply factor response of the supercavitating flow on the temperature of source water, its inlet velocity, rate of steam extraction from the supercavity, and the degree of flow obstruction by the cavitator. This analysis goes beyond the presently referred experimental results, and the influences of the each factor on the supercavitating flow dynamics have been formulated in respect of the steam volume fraction, steam motion and effective temperature influence on the interphase heat mass transfer.

Numerical Simulation of Internal Fire Whirls in an Inclined Shaft

In order to understand the characteristics of fire whirls in the inclined shaft with corner gap, large eddy simulations (LES) of thermal flow field in a 9-meter-high inclined shaft were employed. In the inclined shaft, the conrner gap is 0.3m and 0.7m in width, and the inclination angle is 5°, 30°, 45°, 50° and 55° from the vertical orientation axes respectively. It is found that a whirling flow field could be formed under proper inclination angles and corner gap width. Rotational axis of fire whirls in the inclined shaft would have same inclination angles with the shaft. Similar to the case of fire whirls in a vertical shaft, the width of corner gap of inclined shaft also influences the whirling motion of the flame. In this paper, for the same fire and inclination angle, the 0.3-meter-wide corner gap is easier to form stable fire whirls than the 0.7-meter-wide corner gap. By comparison with buoyancy, the slightly greater Coriolis force was found above a height 3m for inclined fire whirls when the width of croner gap is 0.3m, however, the Coriolis force acts only on the lower part of the shaft when the width of croner gap is 0.7m.

Nonlinear auto-adjusting iterative reconstruction technique for interferometric tomography

A new algebraic reconstruction technique (ART), nonlinear auto-adjusting iterative reconstruction technique (NAIRT), is proposed and applied to reconstruct a section of an actual thermal air flow field. With numerical simulation, NAIRT was tested to reconstruct a complicated field to demonstrate its superior reconstructive capability. In contrast, three typical ARTs, the basic ART, simultaneous ART (SART), and a modified SART (MSART), were simulated to demonstrate the reconstructive capability improvement attained through the use of the proposed NAIRT. The calculated results were discussed with mean square error (MSE) and peak error (PE). A thermal air flow field was produced with an alcohol burner and was detected by a laser beam. With laser beam projections, a cross-section of the field was reconstructed by NAIRT. As a result, the reconstructive capability was improved much by NAIRT. The MSE decreased by 95.5%, and PE by 97.2% from that of the basic ART. Only NAIRT converged without filters while its reconstructive accuracy improved. By increasing the projections from 42 to 84, the accuracy of NAIRT without filters was improved significantly. NAIRT could effectively reconstruct the section of the thermal field. The proposed NAIRT needed no filter for its convergence and it had the highest reconstructive accuracy and simplest iterative expression of those analyzed.

Application of LBM in simulation of natural convection in a nanofluid filled square cavity with curve boundaries

A two-dimensional numerical study has been performed to investigate natural convection in a square cavity with curve boundaries filled with Cu–water nanofluid. Lattice Boltzmann Method (LBM) is used to simulate this problem. The effective thermal conductivity and viscosity of nanofluid are calculated by the Maxwell–Garnetts (MG) and Brinkman models, respectively. This investigation was compared with other numerical methods and was found to be in excellent agreement. Effects of nanoparticle volume fraction, Rayleigh numbers and inclination angle on flow and heat transfer are considered. The results proved that the change of inclination angle has a significant impact on the thermal and hydrodynamic flow fields. Also it can be found that maximum values of enhancement are obtained at Ra=103 and Ra=105 for γ>0° and γ<0°, respectively.

Computational Analysis of a Fixed Bed Thermal Oxidizer for Solid Wastes Disposal

Combustion control techniques have become a legal requirement to minimize pollution in municipal solid waste incinerators. Typically, incinerator destruction of pollutants is achieved when 2-second gas residence time at 8500 Celsius and about 6% O2 are guaranteed at exit. Performance of a fixed bed (two-stage) thermal oxidizer for solid waste is analyzed numerically using computational fluid dynamics (CFD) technique. The CFD analysis provides three-dimensional view of thermal and gas flow field inside the thermal oxidizer chamber. Localized zones of temperature and species concentration were analyzed and provided critical information for understanding the thermo-chemical processes taking place during incineration leading into design optimization and the operation strategy of the thermal oxidizer. Based on the CFD results, the original design of the thermal oxidizer was modified to optimize the flow characteristics and the residence time in the secondary chamber thereby achieving complete combustion of gases emanating from the lower chamber, hence less emissions of CO.

The Optimization of Thermal Flow Field in a Large-Size MOCVD Reactor

In this paper, the effects of operating conditions in a large-size, vertical and close-spaced reactor for metalorganic chemical vapor deposition are investigated and described by simulation and analysis. The parameters are involved such as space between gas inlet and susceptor (11-20 mm), reactor wall temperature (50-200℃), gas inlet temperature (50-200℃),and chamber pressure (100-760 torr). These listed parameters show that an optimization ideal stagnation flow can be achieved in a large-size close-spaced reactor.

Computational Modeling of the Effects of Viscous Dissipation on Polymer Melt Flow Behavior During Injection Molding Process in Plane Channels

The present finite volume method based fluid flow solutions investigate the boundary-layer flow and heat transfer characteristics of polymer melt flow in a rectangular plane channel in the presence of the effect of viscous dissipation and heat transfer by considering the viscosity and density variations in the flow. For different inflow velocity boundary conditions and the injection polymer melt temperatures, the viscous dissipation effects on the velocity and temperature distributions are studied extensively to analyze the degree of interactions of thermal flow field dominated by the viscous heating and momentum diffusion mechanism with varying boundary conditions. The modified forms of Cross constitutive equation and Tait equation of state are adopted for the representation of viscosity variations and density change, respectively, in the polymer melt flow. These models together with the viscous dissipation terms are successfully incorporated into the finite volume method based fluid flow solutions to realistically represent the heat effects in the plane channel. The numerical results presented for two different commercial polymer melt flows, namely, polymer Polyacetal POM-M90-44 and polypropylene (PP), demonstrate that proposed mathematical formulations for viscosity and density variations including viscous heating terms into the energy equations, which are fully coupled with momentum equations, lead to more accurate representation of the fluid flow and heat transfer phenomena for the polymer melt flows in plane channels.

Numerical study of magnesium (Mg) production by the Pidgeon process: Impact of heat transfer on Mg reduction process

In this paper, a magnesium (Mg) reduction model considering both heat transfer and chemical reaction kinetics was firstly developed. The reduction model was carried out on a single retort to investigate the impact of different constant heating temperature on the magnesium reduction process in the retort. And then, the reduction model was incorporated into the conventional coal combustion model to describe the process of magnesium production in a coal-fired furnace. The comprehensive model was solved according to the actual industrial operation. The characteristics of magnesium production in the coal-fired furnace, such as thermal-flow field around the retorts in the furnace, the distribution of magnesium reduction extent in the retorts and the total extent of magnesium reduction with time were in detail studied. Especially, the impact of external thermal-flow around the retorts on the extent of magnesium reduction was investigated and discussed.

Development of High Precision Thermo-Anemometer

An improved system of thermo-anemometer was developed. Temperature compensator was designed based on more accurate equations. Assumptions in the temperature compensation equation were minimized to make minute adjustment in each part of the circuit. An improved corrector circuit reduced the error of the velocity signal to be less than ±1% up to about 120°C. A new digital delay device was designed. Deviations from the specification of delay device could be adjusted by employing an automatic amplitude and mean voltage adjuster. Consequently, it can regenerate the original velocity waveform quite accurately, free from wave deformation and electrical noise up to 6kHz. Calibration time was reduced. The present device can obtain higher correlation terms in the thermal flow fields more accurately.

Nanofluid Raleigh-Benard convection in rectangular cavity: simulation with lattice Boltzmann method

The lattice Boltzmann method is used to simulate the thermal field and flow field of nanofluid Raleigh-Benard convection in a rectangular cavity. The heat transfer characteristics of nanofluid are compared under different Raleigh numbers, volume fractions of nanoparticles and particle sizes. The results show that under the same Raleigh number and volume fraction, the convection heat transfer of nanofluid becomes weakened by increasing the particle size. Under the same Raleigh number and particle size, the convection heat transfer of nanofluid becomes strengthened by increasing the volume fraction of nanoparticles.

A Study of Thermal Response and Flow Field Coupling Simulation around Hayabusa Capsule Loaded with Light-Weight Ablator

The numerical simulation of flow field around Hayabusa capsule loaded with light-weight ablator thermal response coupled with pyrolysis gas flow inside the ablator was carried out. In addition, the radiation from high temperature gas around the capsule was coupled with flow field. Hayabusa capsule reentered the atmosphere about 12 km/sec in velocity and Mach number about 30. During such an atmospheric entry, space vehicle is exposed to very savior aerodynamic heating due to convection and radiation. In this study, Hayabusa capsule was treated as a typical model of the atmospheric entry spacecraft. The light-weight ablator had porous structure, and permeability was an important parameter to analyze flow inside ablator. In this study, permeability was a variable parameter dependent on density of ablator. It is found that the effect of permeability of light-weight ablator was important with this analysis.

Simulation and Analysis on Mechanical Strength of Reinforced Concrete Beam Undergoing a Fire

Reinforced concrete is a common structure in buildings in Taiwan. Steel bars, concrete materials and the structural strength of the reinforced concrete deteriorate due to high temperature deriving from a fire. Therefore, it is essential to assess its structural safety and analyze whether the architectural structure will remain its design strength undergoing a fire. This study employs Fire Dynamics Simulation (FDS), fire simulation software, to construct model of thermal flow field. By integrating FDS with PHOENICS, thermal flow software, this study also calculates the effect of the fire’s thermal transmission on the building, investigates the effect of the fire size and the mode the beam undergoing a fire on change of the structural strength, and provides quantified data for safety assessment for buildings which have undergone fires.

NUMERICAL SIMULATION OF MIXED CONVECTION FLOW OVER HEAT SOURCE MODULES MOUNTED ON A HORIZONTAL PLATE

Mixed convection flow over heat source modules mounted on a horizontal plate has been studied numerically. The present analysis is valid when the buoyancy force effects are small compared to forced convection effects. The mixed convection flow problem is formulated by two-dimensional incompressible flow with the buoyancy term represented by the Boussinesq approximation. The governing equations are solved in the stream function and vorticity formulation using high accuracy finite difference schemes. Results are reported for single and two heat source modules mounted on a horizontal plate with and without thickness. The effects of induced velocity, heat input, thickness, and location of heat source modules on heat transfer rate and thermal field in the vicinity of heat sources are reported. Results have shown that there is a decrease of maximum temperature on heat source modules with increase in free-stream velocity of the fluid. Results show that the change of location and thickness of heat source modules has a significant effect on thermal flow field characteristics. The maximum temperature and average Nusselt number on heat source modules are reported at different parameters. The reported results agreed well with experimental results available in the literature.

Research on Thermal Flow Field Synergy and Heat Dissipation Performance of Electric Vehicle Cooling System

It is applied field synergy principle to research the heat dissipation performance of different electric vehicle cooling mode,the results indicate that: the heat dissipation capability of both sides outlet mode of wind is worse,up-outlet mode of wind can improve heat dissipation capability and aerodynamic capability;the aerodynamic capability of up-outlet mode of wind with one hole is better,the heat dissipation capability of up-outlet mode of wind with two holes is superiority,it is balance to choose up-outlet mode of wind with one hole between aerodynamic capability and heat dissipation capability;along with vehicle velocity increased,the maximum temperature rising and maximum temperature difference of battery pack will descend,and it is linearity relation along with vehicle velocity;based on field synergy principle,the sequence of different flow path's heat dissipation capability from low to high: single-inlet and single-outlet,double-inlet and double-outlet,three-inlet and three-outlet,six-inlet and six-outlet,it validates that the judgment is accuracy by combined of simulation and experimental.Then the reference basis for choosing electric vehicle cooling mode is offered.