Discrete Ordinate Model Research Articles

벤로형 비닐온실의 환기 성능에 관한 수치해석 연구

The ventilation performance of a Velo-typed greenhouse at high temperature atmosphere condition is investigated by means of computational simulations using a commercial software. The solar radiation is simulated using the Discrete Ordinates model, and the conduction through the covering material of the greenhouse and soil are calculated. The numerical scheme is validated by comparing the experimental result for the Almeria greenhouse. The natural ventilation performance through windows of the Velo-typed greenhouse is evaluated at free-stream velocity 0 m/s and 2.5 m/s. It is shown that the natural ventilation caused by the temperature difference is more efficient through windows on the ceiling than those on the sides.

비행 조건에서의 입자 분사에 따른 엔진 배기플룸 냉각 및 차폐 영향성 연구

Cooling and shielding effects of injecting micro-scale particles into the exhaust plume of an aircraft engine flying at high speed at an altitude of 6,096 m (=20,000 ft) were studied using computational methods. A discrete phase model was used to calculate a multiphase flow composed of exhaust plume and particles (liquid water mist and solid carbon). The IR radiation of the plume incident on the particle layer was first calculated, and then the radiation absorption coefficient of the injected particles was calculated using the discrete ordinate model. Furthermore, the change of the thermal flowfield of the exhaust plume according to the particle injection flow rate and the injection material, and the cooling and shielding effects were analyzed. The transmittance of the exhaust plume IR signal passing through the particle layer was determined through the maximum absorption coefficient at each point.

Assessment of weighted-sum-of-gray-gases models for gas-soot mixture in jet diffusion flames

The global radiative property models are widely used in combustion simulation due to its high computational efficiency. This study assesses the weighted sum of gray gases (WSGG) models for gas-soot mixture in the simulations of turbulent jet diffusion flame at atmospheric pressure with fully coupled combustion and radiation heat transfer. Non-adiabatic tabulated chemistry approach and the method of moments were employed for combustion modeling and soot particle dynamics. The radiative transfer equations are solved by the discrete ordinate model. Two kinds of WSGG model based on mixture modeling (MM) and superposition method (SM), and their combination with the gray and non-gray soot radiative property models are involved in the comparison. First, the radiative property models are evaluated on the one-dimensional parallel plate system using line-by-line model as the benchmark, and then the WSGG models are evaluated on the basis of the FSCK model in the combustion simulation. The results show that MM and SM provide similar prediction accuracy, while MM has much lower computational cost. The MM combined with the non-gray soot radiative property model (MMNS) provides the best accuracy for gas-soot mixture. Moreover, the relative error of MM combined with gray soot radiative property model (MMGS) is less than 10% for radiation source term, which is acceptable in engineering simulation. The gray implementation of WSGG model has low accuracy, which indicates that it is necessary to use the non-gray implementation of WSGG model in CFD simulations.

A new air recirculation system for homogeneous solar drying: Computational fluid dynamics approach

Improvement of air flow distribution, air velocity and temperature inside a mixed greenhouse dryer was numerically investigated using 3D CFD ANSYS FLUENT code. The study was performed considering six different locations of axial fans inside the greenhouse. Additional elements such as a false ceiling and front and back internal walls were included in some configurations to analyze their contribution in improving the air distribution. Numerical simulations were focused on dynamic fluid models in order to select the best configuration to achieve homogeneous air distribution and velocity among the six proposals. Then, the temperature distribution was analyzed considering the contribution of solar energy in the best-selected greenhouse configuration. Additionally, the Discrete Ordinate (DO) model was used to simulate the mechanism of heat transfer from solar radiation to the greenhouse. The results showed that installing an air recirculation system (including: axial fan, false ceiling, back and front walls), into the greenhouse can increase the air velocity in the drying chamber from 0.71 m/s to 1.5 m/s and the temperature from 315 K to 360 K, which represent an increase of approximately 111.26% y 11.11%, respectively, compared with the greenhouse without an air recirculation system. This improvement could result in the reduction of drying time and a homogeneous moisture content in dry products.

Solar LFR system with new cavity reflector employing DO model

AbstractIn current context, to achieve lower heat loss in Linear Fresnel reflector (LFR) unit, innovative configuration for cavity has been offered. This cavity consists of one hot circular wall and two inclined adiabatic walls. The below wall is made of glass and connect with outside air flow. The operating fluid is air and its features were temperature dependent. Inclination angle (θ), wind speed and circular wall temperature are variable factors. Both thermal and irreversibility behavior were analyzed in outputs. Finite volume method (FVM) was utilized for such 2D problem involving radiation source term. The incident radiation contours were reported which were obtained by implement of discrete ordinates (DO) model. Verification was done with comparing data with that of previous numerical publication. Stream function augments about 87.5% with augmentation of θ while changing the hot temperature cannot affect the strength of the eddy. Radiation mode is directly relevant to temperature difference, so lower glass temperature provides lower incident radiation. At lowest Th, augment of θ offers reduction in glass temperature and incident radiation about 2.27% and 12.94%. Given θ = 120o, reduce of Tr makes cold temperature to increase about 20.59%. Owing to augmentation of heat loss with rise of Th, θ and wind speed; all factors have direct relation with Nu and Nur. Temperature of circular wall has highest influence on Nu. With assumption of minimum values of other factors, Nu augments 19.63% and 65.07% with rise of hext and θ while it declines about 89.59% with increase of Tr. Irreversibility decreases about 86.38% with rise of Tr when θ = 120o, hext = 20. Given Th = 673 K, hext = 20, total entropy augments about 6.75% with employing greater angle.

Investigations of Infrared Desktop Reflow Oven with FPCB Substrate during Reflow Soldering Process

This paper presents the study of infrared (IR) reflow oven characteristics for suitable operating conditions of the flexible printed circuit board (FPCB) in the reflow soldering process. A computer-based model that imitates a real-time oven was developed with practical boundary conditions. Since the radiation effect is dominant in the reflow process, a discrete ordinate (DO) model was selected to simulate the effect. The experimental work acts as a benchmark and the reflow profile was set to follow the standards of JSTD-020E. The simulation of the model has a great consensus between the experimental data. It was found that the temperature distribution was inhomogeneous along with the phases. The FPCB surface also has a higher surface temperature than oven air during the operating reflow profile. An in-depth study using the simulation approach reveals that the temperature distribution of the desktop reflow oven is dependent on several factors, namely fan speed, FPCB position, and FPCB thickness. The rotational fan generates an unsteady flow that induces inhomogeneous temperature at different positions in the reflow oven cavity. The results are useful for studying further improvements to achieve temperature uniformity within the oven chamber.

Investigations of the Emission Characteristics of a Dual-Fuel Gas Turbine Combustion Chamber Operating Simultaneously on Liquid and Gaseous Fuels

Abstract This study is dedicated to investigations of the working process in a dual-fuel low-emission combustion chamber for a floating vessel’s gas turbine. As the object of the research, a low-emission gas turbine combustion chamber with partial premixing of fuel and air inside the outer and inner radial-axial swirls was chosen. The method of the research is based on the numerical solution of the system of differential equations which represent the physical process of mass and energy conservation and transformations and species transport for a multi-component chemically reactive turbulent system, considering nitrogen oxides formation and a discrete ordinates model of radiation. The chemistry kinetics is presented by the 6-step mechanism of combustion. Seven fuel supply operating modes, varying from 100% gaseous fuel to 100% liquid fuel, have been analysed. This analysis has revealed the possibility of the application of computational fluid dynamics for problems of dual-fuel combustion chambers for the design of a floating vessel’s gas turbine. Moreover, the study has shown the possibility of working in different transitional gaseous and liquid fuel supply modes, as they satisfy modern ecological requirements. The dependencies of the averaged temperature, NO, and CO concentrations along the length of the low-emission gas turbine combustion chamber for different cases of fuel supply are presented. Depending on the different operating modes, the calculated emission of nitrogen oxides NO and carbon monoxide CO at the outlet cross-section of a flame tube are different, but, they lie in the ranges of 31‒50 and 23‒24 mg/nm3 on the peak of 100% liquid fuel supply mode. At operating modes where a gaseous fuel supply prevails, nitrogen oxide NO and carbon monoxide CO emissions lie in the ranges of 1.2‒4.0 and 0.04‒18 mg/nm3 respectively.

Experimental and numerical simulations of compound heat transfer in an internally circulating fluidized bed

In this paper, the compound heat transfer of convection and radiation in an internal circulating fluidized bed for low temperature oxidation of pulverized coal was numerically simulated. The numerical prediction about the compound heat transfer characteristics and its relationship of convection and radiation was compared with the experimental measurements of low temperature radiation. The discrete ordinate model (DOM) was employed to investigate the radiation heat transfer in the low temperature oxidation internally fluidized bed, and user-defined function (UDF) was compiled by considering the effect of the particle flow emissivity on radiation heat transfer. The convection heat transfer coefficient increases with the increase of gas inlet volume flow rate and initial bed material height, however, thermal radiation coefficient was opposite. An increase of the thermal flux results in an increase of the surface temperature of the fluid and the heater surface, and the thermal transfer coefficient between bed material and heater surface is also increased. The results have the application reference to complex thermal transfer design of circulating fluidized bed systems wherein thermal radiation transfer must be considered as a significant mode.

A Coupled Modelling Method for the Evaluation of the Impact of Pavement Solar Collector on Urban Air Temperature and Thermal Collection

The Urban Heat Island (UHI) effect is a phenomenon whereby urban areas become warmer than their surrounding rural areas, due to the replacement of vegetation and soil with surfaces such as asphalt and concrete. The asphalt pavement surfaces tend to absorb a large amount of heat through solar radiation and increase the air temperature, which affects the operation of building heating and cooling systems, causing environmental problems and thermal discomfort. However, this energy can be collected by water circulated through buried copper pipes to cool down temperatures and be stored for other usages. This work aims to develop a method for determining the optimum areas to locate pavement solar collector (PSC) systems and simulate the reduction of ambient air and surface temperature by using a coupled computational modelling approach. Discrete ordinate model and solar-ray tracing were utilised for solar radiation effect modelling in the 3D simulation. Furthermore, the PSC prototype was developed, and lab-scale experiments were carried out for validation. Based on the simulated conditions, in the unshaded area, the asphalt slab’s near-surface temperature was reduced by up to 10℃ and the outlet water temperature increased by about 5℃. At the pedestrian height level, the air temperature was reduced up to 4.6℃. This study further expands the investigation of the variation of outdoor conditions such as air temperature and solar radiation. The results showed that the proposed method could be used to optimise the pavement solar collector’s positioning to reduce urban surface and air temperature.

Effect of burner angle on the heat transfer of a frit furnace

In this work, a numerical analysis was performed about the effect of a flat-flame burner incidence degree on the heat transfer of an industrial scale frit melting furnace, which uses a flat-flame natural gas oxy-combustion burner. The thermal performance of the furnace was evaluated by predicting the temperature distributions, the recirculation of the combustion gases, and the heat flow to the load, using three different geometrical configurations, differing in the inclination of the burner at 0°, 3.5°, 7° with respect to the longitudinal axis. The simulations were carried out using the ANSYS® Fluent software. The Steady Laminar Flamelet (SFM) model, the k-epsilon realizable model, and the discrete ordinates model were used to model combustion, turbulence, and radiation, respectively. The weighted model of the sum of gray gases (WSGGM) was used for the coefficient of absorption of the combustion species. It was observed that the furnace temperature estimated with the simulations is similar to that found in the actual process. Additionally, the simulations showed that for the angle of 7°, the flame collides with the frit, which could generate deposition of frit particles in the internal walls of the furnace; this would affect the emissivity of the refractory material. The 3.5degree angle showed a better distribution of heat flow to the frit and recirculation rate compared to the burner at 0° and 7°.

Thermal cooling of photovoltaic panels using porous material

An effective photovoltaic thermal regulation technique using porous material confined at the backside of the panel is proposed. Numerical thermal analysis was conducted to study the case of a steady-state mixed convection heat transfer with solar radiation using ANSYS® Fluent. Discrete Ordinate (DO) model was validated and used to simulate actual solar radiation on a 2-D model. The investigation addressed the effect of the convective parameter (λ), Darcy number (Da), inclination angel (θ), porous thickness ratio (S p/S), and emissivity (ϵ) on the average Nusselt number at the tedlar layer and the conversion efficiency (η) of the PV module. Compared to the clear domain, it was found that for three porous fins, five porous fins and porous layer cases the percent increase in efficiency was 6.73%, 9.19%, and 8.34%, respectively. Also, the buoyant effect overcame the friction effect in the natural-dominant regime and the percent decrease in the friction was 138.52%, 206.22%, and 118.64%, respectively for the same porous configuration order. It was also found that increasing the emissivity (ϵ) of the duct base wall enhances the cooling effect as it dissipates heat from the domain.

Solar energy application for LFR unit with trapezoidal cavity receiver considering radiative mode

In current article, influence of radiation and convection modes on behavior of air inside an absorber in trapezoidal shape has been investigated. Such cavity is part of Linear Fresnel Reflector (LFR) unit and it is important to analyze such geometry to reach higher performance. Properties of carrier fluid were assumed as function of temperature. To involve radiation effect, discrete ordinates model was implemented. Influences of external convective factor (hext), Angle of adiabatic wall (θ), temperature ratio (Tr) on irreversibility and heat transfer were depicted in outputs. Augment of Tr, θ and hext cause incident radiation to decline about 85.95%, 20.97% and 20.68% with assuming lowest values of other factors. Strength of eddy, temperature difference augments about 180% and 9.77% when θ enhances at Tr = 0.8. With considering highest level of other factors, as values of wind speed and θ intensify, Nu enhances 9.28% and 17.36% while it declines about 52.2% with rise of Tr. Given minimum values of other parameters, irreversibility intensifies about 45.45% and 8.3% with augment of hext and θ but it declines about 84.44% with augment of Tr.

Numerical Simulation of a Lab-Scale Molten-Salt External Solar Receiver and Its Experimental Validation

In this work, a lab-scale external receiver with a xenon lamp set was designed to study optical and thermodynamic properties, and its benchmark was also introduced. Eighteen stainless-steel tubes were placed on the receiver to absorb radiant energy from the xenon lamps and achieve light-to-heat conversion. The indirect method was used to obtain the optical efficiency of the xenon lamp setup, and the heat-flux distribution on the surface of the receiver had an average heat flux of 99.4 kW/m2. Furthermore, the experimental method was used to study the temperature change of the receiver during the startup, stabilization, and shutdown of the receiver. A numerical simulation was conducted using commercially available software with the realizable k-ε model and the discrete ordinates model as the radiative transfer equation to valid the experimental result. The temperatures of the back and front of the receiver were obtained by thermocouples and an infrared thermal imager, respectively. The temperature of the back of the receiver rose rapidly after the lamps were turned on, and the temperature of the heat receiver remained unchanged when it was stable. The velocity of the molten salt was also studied, and the maximum speed was 1.9 m/s. The temperature increases of the experiment and simulation were approximately 4°C and 4.2°C, respectively, and the temperature of the tube wall was 40°C higher than that of the molten salt. The comparison of the experiment results indicate that the deviation between the receiver’s back temperatures in the simulation and experiment was basically within 5%, indicating consistency, and the thermal efficiency of the receiver was approximately 85%.

Numerical Simulation of Solar Radiation and Conjugate Heat Transfer through Cabin Seat Textile

Abstract The solar radiation and the conjugate heat transfer through the cabin seat fabric were investigated numerically with a focus on a comparative analysis of various fabric solar reflectance or reflectivity (SR) and inlet cooling air velocity. For this purpose, 3D compressible Reynolds-averaged Navier–Stokes equations with the low Reynolds number turbulence model were utilized to simulate the airflow in the cabin. The discrete ordinate radiation model was adopted to describe the solar radiation. The conjugate heat transfer between the airflow and the fabric seats was included. The airflow temperature, radiative heat flux, and radiative heat transfer through the fabrics in a fixed cross section were studied. The results demonstrate that the increase in fabric SR leads to the increase in energy reflected to the atmosphere, which will bring about a lower temperature on the seat fabric. The decrease in emissivity and the energy absorbed results in the lower heat transfer and heat radiation and leads to the improvement of the cabin thermal environment. The high-temperature gradient near the seat causes the forced air circulation and is beneficial for the improvement of the thermal comfort. However, the cooling effect is not so obvious near the cabin seats when the inflow speed is increased.

Numerical modeling of heat losses in a line focusing solar compound parabolic concentrator with planar absorber

In this work, a three-dimensional steady state thermal model is developed for a compound parabolic concentrator of 2× concentration considering the effects of surface radiation and wall conduction. The performance of the compound parabolic concentrator is modeled to obtain the convective and radiative heat loss under the influence of different parameters such as absorber temperature, absorber emissivity, ambient temperature, external heat loss coefficient, aspect ratio and concentrator tilt. The turbulent natural convection in the concentrator is modeled using standard two equation k-ε turbulent model with enhanced wall treatment. For the surface radiation inside the concentrator, discrete ordinate radiation model is used. The thermo physical properties of air inside the cavity are assumed to be constant except the density which is modeled as incompressible ideal gas. The conduction at the walls is accounted by setting suitable wall thickness for reflectors, glass cover and end walls in the solver. This work clearly demonstrates the need for a three dimensional numerical simulation and coupled natural convection and radiation to exactly reproduce the velocity fields as would be obtained in experiments. The convective Nusselt number, radiative Nusselt Number and thermal stratification are obtained from the parametric study. This study on estimation of heat losses in compound parabolic concentrator is applicable to both solar thermal and solar photo-voltaic collectors with a view of modifying the concentrator design to optimize its thermal performance.

Numerical and Experimental Study of the Effect of Injected CO2 Flow on the Stability of Flameless Combustion

Abstract The effect of the injection of externally sourced carbon dioxide (CO2) on the stability of the flameless combustion regime was evaluated numerically and experimentally, taking temperature uniformity and pollution emissions (NO and CO) as criteria. The flameless combustion regime was studied in a lab-scale furnace fueled with natural gas (NG) at a thermal power of 20 kW based on the low heating value (LHV). The CO2 was injected into the lower part of the furnace to directly affect the reaction zone. Computational fluid dynamics (CFD) simulations were performed using the ansys-fluent software. The models used to describe the turbulence, the radiation heat transfer, and the turbulence–chemistry interaction were the standard k–ɛ model, discrete ordinate model (DOM), and eddy dissipation concept (EDC) model, respectively. The NG oxidation was described with a seven-step global reaction mechanism with the EDC model. Three excess air conditions were analyzed, 20%, 25%, and 30%, combined with various CO2 injection flows. At 30% excess air, the flame exhibited destabilization without any CO2 injection. Adding CO2 attenuates the destabilization because of the dilution effect. Increasing either the CO2 or excess air flow resulted in a considerable decrease in the global temperature of the process, consequently producing an increase in CO emissions and a decrease in NO emissions. Finally, for the conditions studied, increasing the mass flow of externally sourced CO2 did not destabilize the flameless combustion regimen. This result shows the potential of the implementation of flameless combustion in industrial processes where CO2 is releasing as a result of a reaction external to the combustion process, such as cement, ammonia, or lime production among others.

Study of turbulent natural convection coupled with thermal radiation in a vertical cavity having a large form factor

AbstractIn this study, a numerical simulation study of turbulent natural convection coupled with thermal radiation in a vertical cavity differentially heated and filled with air assumed as a transparent fluid was carried out. The cavity has a variable form factor which can reach large values. The vertical walls are subjected to constant temperatures (Tc and Tf), whereas the horizontal walls are assumed adiabatic. The flow inside the cavity is turbulent and turbulence was modeled by using the model, and to take into account of the radiative transfer, the discrete ordinate model (DO) was introduced. To solve the different equations, Ansys‐Fluent software based on the finite volume method was used. Some numerical results obtained for the Rayleigh number value of 1011 have been validated by some existing results in the theory. It is found that the thermal radiation has a significant influence on the flow structure and temperature variation where the flow becomes reinforced. It accelerates the airflow inside the cavity and gives the formation of significant velocity and temperature gradients along the walls of the cavity. Taking into account of the surface, thermal radiation is essential in the correct evaluation of temperature in the cavity.

Numerical investigation for steam tubes temperature reduction in a four fuels tangentially fired boiler

The combustion of by-products in boilers for steam generation plays an important role for heat recovery in industrial plants. However, the use of different fuels at the same time may cause flame imbalance, flame impingement and overheating at some points of the furnace. A numerical study of the reactive flow within a tangential fired boiler, to investigate the phenomenon of overheating in the furnace wall tubes, was developed in this work. The capacity of the simulated boiler is 255 t/h of superheated steam, co-firing natural gas, liquid methanol and other two gases in 24 burners distributed on the four corners of the furnace. The flow in each burner was simulated in detail and the velocity, temperature and pressure profiles at the outlets were imported into the furnace. The numerical approach involved the use of the SST k-w, Eddy Dissipation Concept with combustion kinetics mechanisms and Discrete Ordinate Model to describe the turbulent reactive flow and the radiation heat transfer. The liquid methanol multiphase atomization process with compressed air was treated by the Mixing Model. The performed simulations identified the high temperature points, verified on the field by a thermometry system installed inside the furnace. The introduction of air flow guiding fins was simulated, reaching up to 18% mean temperature reduction. Meshes with more than 18 million elements were created and the calculations were conducted on a high-performance computer (HPC) using up to 400 cores at the same time.

Sensitivity analysis via adjoint Monte Carlo calculations of plasma focus irradiation of micro-silica beads in phantoms

One of the main issues in the analysis of the performances of radiation detectors via Monte Carlo methods is their sensitivity with respect to the various irradiation parameters: source intensity and position, detector effectiveness and sensitivity, etc. In the present work, the capability of a quite new Trueinvivo© micro-silica beads TLD has been analyzed from the point of view of the Monte Carlo modeling of the irradiation process and the possibility of a speed up of the calculations using accurate bias techniques based on the adjoint solution of the transport process. The source considered is a Plasma Focus device explicitly designed for cell cultures or “in vivo” tests. The experimental data and the pure analogic Monte Carlo simulation previously obtained results coming from dosimeters at different depth and positions in a PMMA phantom composed by slabs and built for hosting the TLDs, have been benchmarked with respect to the adjoint bias approach. The results obtained through different biasing techniques, backward particle transport analysis, Weight Windows and a discrete ordinate model solution, show how it is possible to optimize the Monte Carlo calculations with great effectiveness.

An accurate and efficient radiative transfer model for simulating all-sky images from Fengyun satellite radiometers

Forward radiative transfer models (RTM) are an indispensable tool for quantitative applications of satellite radiometers, e.g., for data calibration, instrument development, retrieval, and so on. In this study, we develop an accurate and efficient RTM for radiometers onboard Fengyun satellites, namely FYRTM (RTM for Fengyun Radiometers). Correlated k-distribution models are developed to improve the computational efficiency for gas absorption, and the effects of cloud and aerosol multiple scattering and emission are accelerated with pre-computed look-up tables. FYRTM is evaluated with a rigorous simulation based on discrete ordinate radiative transfer model (DISORT) as well as a popular fast forward model, i.e., the Community Radiative Transfer Model (CRTM). Results indicate that FYRTM-based simulations are two to three orders of magnitudes faster than the DISORT-based simulations. Compared to the rigorous model, FYRTM relative errors are within 2% at solar channels, and brightness temperatures (BT) differences are within 1 K at infrared channels. Compared with CRTM, FYRTM is computationally similar at solar channels, but three times faster at infrared channels. Furthermore, simulated reflectances/BTs using FYRTM are in a good agreement with the satellite observations. Overall, FYRTM is capable to simulate satellite observations under different atmospheric conditions, and can be extended to other radiometers onboard the Fengyun satellites (both geostationary and polar-orbiting satellites). It is expected to play important roles in future applications with Fengyun observations.