Discrete Ordinates Radiation Model Research Articles

Numerical simulation for thermal distribution of air-conditioner outdoor units

Air-conditioners used in high-rise residential or office buildings often have outdoor units installed on the sidewalls or roofs in the confined space of the buildings. The heat released from the outdoor units causes the temperature rise in the confined installation space and leads to uncomfortable thermal environment and deteriorative performance of air-conditioners. The factors affecting the performance of air-conditioners are the heat released from the outdoor units, the solar radiation and the ventilation of the confined installation space, which are investigated in this study and simulated by FLUENT using the porous model and discrete ordinates (DO) radiation model. It is proved that the porous model is more reliable than the solid model. The optimal installation distance from the supporting wall is obtained. The average temperature of the exit without wind is 1.18% higher than that with wind. The results show that the heat released from the outdoor units and the ventilation of the confined installation space are the main factors affecting the thermal environment in the confined installation space. The influence of the solar radiation is negligible.

Development and experimental validation of a simulation model for open joint ventilated façades

The investigation of the thermal and fluid dynamical behaviour of open joint ventilated façades is a challenging task due to the complex airflows generated inside of the naturally ventilated cavity by the existence of open joints. For this reason, the use of advanced fluid measurement and simulation techniques is highly recommended. This paper focuses in the development and experimental validation of a simulation model for these façade systems. More specifically, different turbulence and radiation models available in the commercial computational fluid dynamic codes have been tested on a three-dimensional model and the results have been compared to particle image velocimetry measurements. The correlation between experimental and numerical data has been used in order to select the simulation procedure for this type of façades. Best fittings have been found when using the RNG k-epsilon turbulence model and the Discrete Ordinate radiation model. Using the selected scheme, parametrical simulations have been performed to investigate the effect of increasing the cavity height, and correspondingly, the number of slabs. Results show that ventilation air flow inside the cavity is enhanced by incident radiation as well as by the height of the façade.

Numerical Investigation on Thermal Environment inside the Passenger Compartment of a Highway Sleeper Coach

In order to verify thermal comfort of a 12-meter-long highway sleeper coach equipped with a four-ducted air-conditioning system, the thermal environment inside the passenger compartment was numerical investigated with RNG k-ε turbulence model. Also, discrete ordinates radiation model was applied to consider the effect of solar irradiation. The characteristic distribution of the airflow organization and temperature field were obtained and compared with experiment results. Results show that the air flow and temperature distribution inside the passenger compartment are not uniform and greatly influenced by solar radiation and air inlet parameters.

The Comparison of Experimental and Predicted Flame Temperature of Natural Gas Combustion

This present work was focused on the application of the probability density function method in conjunction with the k–∊ turbulence model in the goal to predict temperature fields of a non-premixed confined natural gas diffusion flame. A combined experimental and computational fluid dynamics modeling study of the turbulent non-premixed natural gas on a laboratory scale has been performed. An experimental study has been carried out on a fire-tube water heater. The flame and gas temperature in the combustion chamber were measured. The real geometry of the fire-tube water heater and the burner were modeled and analyzed by FLUENT code numerically. Turbulent diffusion flames were investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The boundary conditions were specified as the same as the measured values. The elemental analysis of the natural gas was taken as the mixture of hydrocarbon, and the air was the oxidizer. The standard k–∊ model was used for the modeling of the turbulence phenomena in the combustor. The non-premixed combustion model was chosen. In the conserved scalar approach, turbulence effects are accounted for with the help of an assumed shape probability density function or probability density function. The discrete ordinates radiation model is used for modeling of the radiative heat transfer in the combustion room. The model results were compared with the experimental results, in good agreement with the measurements.

Assessment of an unstructured exponential scheme discrete ordinates radiation model for non-gray media

For radiative transfer in complex geometries, Sakami and his co-workers have developed a discrete ordinates method (DOM) exponential scheme for unstructured meshes which was mainly applied to gray media. The present study investigates the application of the unstructured exponential scheme to a wider range of non-gray scenarios found in fire and combustion applications, with the goal to implement it in an in-house Computational Fluid Dynamics (CFD) code for fire simulations. The original unstructured gray exponential scheme is adapted to non-gray applications by employing a statistical narrow-band/correlated-k (SNB-CK) gas model and meshes generated using the authors’ own mesh generator. Different non-gray scenarios involving spectral gas absorption by H2O and CO2 are investigated and a comparative analysis is carried out between heat flux and radiative source terms predicted and literature data based on ray-tracing and Monte Carlo methods. The maximum discrepancies for total radiative heat flux do not typically exceed 5%.

Thermal and Stress Analysis of Glazing in Fires and Glass Fracture Modeling with a Probabilistic Approach

The aim of the study is to predict the thermal and stress behavior of a framed glass subjected to typical fire conditions, and the initial glass fracture time and locations using a probabilistic approach as an alternative to Pagni's deterministic criterion. Thermal stresses in glass have been little researched. The probabilistic approach has the advantage of taking into account some uncertainties such as the edge conditions. The model employed is based on stress and conduction heat transfer models, a spectral discrete ordinates radiation model, and a failure probability model. Some results of its verification and applications are reported here.

Numerical simulation and comparison with experiment of natural convection between two floors of a building model via a stairwell

The paper presents a numerical study of three-dimensional buoyancy-driven flow in a half-scale model of a two-floor building model. The model consists of an upper compartment and a lower compartment with a stairway connecting the two floors. The model forms a closed system, with no inlet or outlet. The flow is driven by a single heat source placed in the lower compartment. The study is linked closely to a previously published experimental study by the present author, which provided the details of the geometry and the boundary conditions as well as data for comparison with the present numerical results. The numerical method is large eddy simulation with the dynamic kinetic energy transport subgrid model. Radiation exchange is modelled using the discrete ordinates (DO) radiation model. The thermal boundary conditions on the model walls are set as heat flux. It is shown that the air temperature level is sensitive to the initial conditions for temperature, but air velocity is unaffected. In order to study this effect further, with the aid of the k–ε model, the measured wall temperatures are set as boundary conditions, which removes the dependency on initial temperature. For the cases studied, comparisons are made between the measured and computed wall temperatures, wall heat fluxes, air temperature and air velocity. There is a general agreement between the two results.

CFD Analysis of Turbulent Natural Ventilation in Double-Skin Façade: Thermal Mass and Energy Efficiency

Double-skin façade provides many opportunities for energy conservation in buildings. This work summarizes the results of a computational fluid dynamic (CFD) analysis, which was carried out to investigate the effect of blind and thermal mass concrete on the thermal performance of a double-skin façade. Navier-Stokes equations, turbulent k-ε model, and discrete ordinates radiation model have been used to simulate the airflow and to study the airflow pattern. It was shown that the airflow patterns for natural ventilation with a blind and thermal mass concrete were very complex with multiple vortex structures. On the other hand, the heat flux and the airflow velocity at the inlet have a significant impact on the natural ventilation airflow rate. The Rayleigh number for turbulent flow was in the range of 107≤Ra≤108 using a blind. However, the Rayleigh number was less than 107 using thermal mass concrete. On the other hand, thermal mass increases the energy performance of natural ventilation. A good agreement was performed by comparing the experimental data and the simulation results.

Computational fluid dynamics modelling and optimal configuring of a channelled optical fibre photoreactor

A computational fluid dynamics model, coupled with a discrete ordinate radiation model, was developed for a channelled optical fibre reactor (COFR) and used to optimise its channel configuration. Air channels were grouped according to their location in the COFR into: inner arc, intermediate arc and outer arc. The flow field in the COFR was laminar with similar velocity vector distributions in all channels irrespective of their location in the COFR cross-section. The computational results demonstrated TiO 2 coatings on the channel walls received light increasingly in the order: outer arc>intermediate arc>inner arc. The radiation distribution on the TiO 2-coated channel walls was governed by the spatial distance. The ethylene conversion distribution corresponded well with the radiation distribution with channels in the inner and the outer arcs showing the lowest and the highest conversion, respectively. The simulation results agreed well with the experimental data possessing Pearson correlation coefficients of 0.995, 0.997 and 0.997 for variations in the incident radiant power, gas flow rate and initial ethylene concentration, respectively. Six alternate channel configurations were considered for optimising overall ethylene conversion in the COFR with an improvement of up to 33% being achieved.

Computational fluid dynamics (CFD) modeling of an electrical heating oven for bread-baking process

Radiation is the most dominant heat transfer mode in an electrical heating oven. A 3D CFD model for an electric heating baking oven was developed. Three different radiation models namely, discrete transfer radiation model (DTRM), surface to surface (S2S) and discrete ordinates (DO) were employed for the simulation of the electrical baking oven. All models predicted almost similar results, which tallied well with the experimental measurements. During the full heating cycle, the oven set-point temperature was reached after 360 s. Lower temperature zones occurred near oven wall due to lower air flow. Based on preliminary evaluation of applicability, the DO radiation model was selected for bread baking simulation and validated with the experimental measurement of bread temperature. Bread simulation was carried out to study the profiles of temperature and starch gelatinization of crust and crumb of the product. This study indicated the baking process to be complete at 1500 s when the temperature of bread-center reached 100 °C.

Experimental and numerical modeling of the effect of solid surface on NO x emission in the combustion chamber of a water heater

A combined experimental and CFD modeling study of the turbulent non-premixed natural gas on a laboratory scale has been performed. Effect of solid surface enhancement in combustion chamber on the flame temperature and NO emission was investigated. The solid surface called as filling material (FM) was cylindrical and was placed coaxially in the center of combustion chamber. The temperature and NO distribution in the combustion chamber were compared for different geometries of the filling material. The diameters of the filling materials were 25 and 30 cm with two lengths of 20 and 40 cm. Experimental study has been carried out on a fire tube water heater. The flame temperature on the center line of the combustion chamber, gas temperature and NO emission in the combustion chamber were measured. The actual geometry of the fire tube water heater and the burner were modeled and then analyzed by the FLUENT code. Turbulent diffusion flames were investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The measured values were specified as the boundary conditions. The elemental analysis of the natural gas was taken as a mixture of hydrocarbon and air was the oxidizer. The standard k– ε model was used for the modeling of the turbulence phenomena in the combustor. The non-premixed combustion model was chosen. In the conserved scalar approach, turbulence effects were accounted for with the help of an assumed shape probability density function or PDF. The discrete ordinates (DO) radiation model was used for modeling of the radiative heat transfer in the combustion room. The model results were compared with the experimental results. The model results were in good agreement with the measurements. The filling material provided the recirculation of the cooler gases into the flame. The recirculation reduced the oxygen concentration in the flame and controlled the flame temperature. It was found that the filling material with the diameter bigger than the flame diameter increased the heat transfer rate in the back flow around the flame.

Evaluation of reflection and refraction in simulations of ultraviolet disinfection using the discrete ordinates radiation model

Simulations of UV disinfection systems require accurate models of UV radiation within the reactor. Processes such as reflection and refraction at surfaces within the reactor can impact the intensity of the simulated radiation field, which in turn impacts the simulated dose and performance of the UV reactor. This paper describes a detailed discrete ordinates radiation model and comparisons to a test that recorded the UV radiation distribution around a low pressure UV lamp in a water-filled chamber with a UV transmittance of 88%. The effects of reflection and refraction at the quartz sleeve were investigated, along with the impact of wall reflection from the interior surfaces of the chamber. Results showed that the inclusion of wall reflection improved matches between predicted and measured values of incident radiation throughout the chamber. The difference between simulations with and without reflection ranged from several percent near the lamp to nearly 40% further away from the lamp. Neglecting reflection and refraction at the quartz sleeve increased the simulated radiation near the lamp and reduced the simulated radiation further away from the lamp. However, the distribution and trends in the simulated radiation field both with and without the effects of reflection and refraction at the quartz sleeve were consistent with the measured data distributions.

Numerical Simulation of Combustion Characteristics in High Temperature Air Combustion Furnace

The influences of air preheating temperature, oxygen concentration, and fuel inlet temperature on flame properties, and NO, formation and emission in the furnace were studied with numerical simulation. The turbulence behavior was modeled using the standard k-ɛ model with wall function, and radiation was handled using discrete ordi-nate radiation model. The PDF (probability density function)/mixture fraction combustion model was used to simulate the propane combustion. Additionally, computations of NO x formation rates and NO x concentration were carried out using a post-processor on the basis of previously calculated velocities, turbulence, temperature, and chemical composition fields. The results showed that high temperature air combustion (HiTAC) is spread over a much larger volume than traditional combustion, flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. The temperature profile becomes more uniform when oxygen concentration in preheated air decreases, especially at low oxygen levels. Increase in fuel inlet temperature lessens the mixing of the fuel and air in primary combustion zone, creates more uniform distribution of reactants inside the flame, decreases the maximum temperature in furnace, and reduces NO x emission greatly.

SOLAR RADIATION DISTRIBUTION IN A TUNNEL GREENHOUSE

SOLAR RADIATION DISTRIBUTION IN A TUNNEL GREENHOUSE

Turbulence Modeling in a Model Combustor

As a benchmarking of turbulence modeling in diffusion flame combustors, the flow field of a propane-air diffusion flame combustor with interior and exterior conjugate heat transfers was investigated numerically. Solutions obtained from four turbulence models together with a laminar flamelet combustion model and a discrete-ordinates radiation model were compared with a comprehensive database obtained from a series of experimental measurements. It was found that the Reynolds stress model (RSM) illustrates superior performance over three popular two-equation eddy-viscosity models. Although the main flow features were captured by all four turbulence models, only the RSM was able to successfully predict the lengths of both recirculation zones and the turbulence kinetic energy distribution in the combustor chamber. In addition, the RSM provided fairly good predictions for all Reynolds stress components, except for the circumferential normal stress at downstream sections. However, the coupling between the RSM and combustion models needs to be improved to extend its application in practical combustion systems.

A Study of the Heating Section of a Gas Heated Steam Reformer

A gas heated steam reformer (GHR) which converts natural gas to synthesis gas for methanol or Fischer-Tropsch purposes has been modelled for steady state conditions. The model is in two dimensions and is made up of a fixed bed reactor model, representing one of the reactor tubes in the GHR, and an annulus model, representing the annular space on the shell side of the GHR where hot, fully converted syngas emits heat to the reactor tube. The annulus model is described and evaluated in this article. This is a plug flow model which involves heat transfer in radial direction caused by radiation and by turbulence. The gas radiation is modelled by the use of the discrete ordinates method and the effect on heat transfer from turbulence is modelled as an effective radial thermal conductivity. Both heat transfer mechanisms vary with the radial position. An additional annulus model, made in the commercial CFD code FLUENT and based on the k-? turbulence model and the discrete ordinates radiation model, is used to estimate effective radial thermal conductivities. These were implemented in the annulus model of the GHR model. The reactor and the annulus models were combined and linked by the wall temperature profile and the heat flux profile on the outer reactor tube wall. The simulation results were used to study the heat flux and temperature profiles along the reactor length and the radial temperature profiles in the annulus.

A Critical Evaluation of NOx Modeling in a Model Combustor

Reliable NOx modeling depends on the accurate prediction of both velocity and temperature fields. The velocity and temperature fields of a propane diffusion flame combustor, with interior and exterior conjugate heat transfers, were first numerically studied. The results from three combustion models, together with the renormalization group (RNG) k-ε turbulence model and the discrete ordinates radiation model are discussed, and compared with comprehensive experimental measurements. The flow patterns and the recirculation zone length in the combustion chamber are excellently predicted, and the mean axial velocities are in fairly good agreement with the experimental data for all three combustion models. The mean temperature profiles are fairly well captured by the probability density function (PDF) and eddy dissipation (EDS) combustion models. However, the EDS-finite-rate combustion model fails to provide an acceptable temperature field. Based on the acceptable velocity and temperature fields, a number of NO modeling approaches were evaluated in a postprocessing mode. The partial-equilibrium approach of O and OH radical concentrations shows a significant effect on the thermal NO formation rate. In contrast, the prompt NO, the NO reburn mechanism and the third reaction of the extended Zeldovich mechanism have negligible effects on the overall NO formation in the present study. This study indicates that the semiempirical, postprocessing NO model can provide valuable NO simulations as long as the velocity and temperature fields are adequately predicted.

Application of Three Combustion Models to a Model Combustor

The entire flow field of a propane diffusion flame combustor, including interior and exterior conjugate heat transfers, was studied numerically. The results obtained from three combustion models, together with the re-normalization group k–ε turbulence model, discrete ordinates radiation model, and enhanced wall functions are discussed and compared with a comprehensive data base obtained from a series of experimental measurements. The predictions of the flow patterns and recirculation zone length in the combustion chamber are excellent, and the mean axial velocity profiles are in fairly good agreement with the experimental data, particularly at downstream sections for all three combustion models. The mean temperature profiles are also fairly well captured by the probability density function (PDF) and eddy dissipation (EDS) combustion models; the PDF results show a slight improvement over those of the EDS model. However, the EDS-finite-rate combustion model fails to provide an acceptable temperature field. The comparisons between the numerical and experimental results suggest that the EDS and PDF combustion models combined with other selected physical models can provide adequate simulations for axisymmetric gas diffusion flame combustors stabilized with a bluff-body flame holder.