Discrete Ordinates Radiation Model Research Articles

Numerical analysis of the influence of spherical turbulence generators on heat transfer enhancement of flat plate solar air heater

This paper presents the influence of spherical turbulence generators on thermal efficiency and thermohydraulic performance of flat plate solar air heater. The analysis is carried out for the Reynolds number range of 4000–25000. The thermal performance is investigated for various diameter (D) of sphere consisting of 5,10,15,20 and 25 mm and relative roughness pitch (P/D) of 3, 6 and 12. The simulation is carried out using solar insolation as heat input at 12 noon conditions for the global position of Manipal (74.786°E, 13.343°N) obtained through the solar load model, a feature available in the software tool used for the analysis and Discrete Ordinates radiation model is used to compute the radiation heat interactions within the computational domain. The CFD results for the base model are validated against experimental results and are found to have good agreement. The thermal efficiency is found to increase with increasing sphere diameter and reducing relative roughness pitch. The maximum average percentage increase in thermal efficiency is found to be about 23.4% as compared to the base model for D = 25 mm and P/D = 3. The highest increase in the Nusselt number is found to be 2.5 times higher as compared to the base model for D = 25 mm and P/D = 3 at Re = 23560. The analysis shows that the relative roughness pitch and size of the spherical turbulator have significant influence on the thermohydraulic performance of solar air heater.

Finite element analysis of radiant heating systems based on gas-fired infrared heat emitters

The article presents a finite element model for simulating a gas-fired IR radiation system. Simulation of gaseous combustion and discrete ordinates radiation model were used to solve a number of heat-transfer problems in ventilated rooms with radiant heating. We used Ansys Multiphysics software and Fluent CFD solver for implementing finite element analysis. To solve differential equations of heating and gas dynamics, the following boundary conditions were considered. Dry methane was used as the fuel and air with 21% of oxygen, as oxidizer. Fuel consumption was 0.5 m3/hour; the gas pressure before the nozzle was 1270 Pa. The air–fuel ratio was 9.996.

Numerical analysis of solar updraft power plant integrated with external heat source

Many countries started to adopt clean energy sources, where solar energy is the first priority. Solar chimney power plant was selected in this study as a system which converts solar thermal energy to electrical power. In this research, hybrid solar chimney power plant model has been proposed as a day and night functional integrated system using external heat source to enhance the system performance and cover the setback of solar absence at night and cloudy days. With the help of ANSYS Fluent software, three-dimensional steady-state of Navier-Stokes and energy equations have been solved. Realized k–∊ two equation turbulent model equations and discrete ordinates (DO) radiation model equations were solved for the conventional model. The results were validated using experimental measurements. Afterward, the influence of thermal enhancing channels installation on the system performance was predicted and analysed in hybrid mode. The simulation results showed that this system could enhance the plant during the day. Percentage of enhancement was 10% and 14% for velocity and temperature respectively, and the power output percentage enhancement was 18% when the solar intensity was 1000 W/m 2 , and external heat source mass flow rate and the temperature for each channels was 0.015 Kg/s and 100°C respectively, in comparison with the conventional model.

CFD modeling of artificial vortex air generator for green electric power

This paper presents and discusses a Computational Fluid Dynamics (CFD) simulation of artificial vortex air generator as part of the preliminary of Solar Vortex Power Generator for an electrical power generation. A vortex air generator system was built, consisting of concentric cylinders. The inner cylinder was fitted with stationary air guide vanes and covered at the top by a transparent plate to capture the solar radiation and create swirling updraft flow which is able to rotate wind turbine and produces power. The influence of inlet air velocity and temperature on the swirling strength and mass flow generated has been evaluated by validated CFD simulation. ANSYS Fluent software was adopted to solve the 3-D, steady state of Navier-Stokes and energy equations in cylindrical coordinate system integrated with discrete ordinates (DO) radiation model. For the preliminary vortex generator design, the CFD results were validated first with previous experimental measurements. Then the variable operation parameters were carried out on the proposed model. The simulation result demonstrated that inflow velocity is a key parameter for enhancing the system performance. By increasing the inflow velocity from 0.4 m/s to 0.6 m/s and inflow temperature 323°k the enhancement rate of the mass air flow generated reached to 26 compared with 7 when increase the inflow temperature to 328°k and inflow velocity 0.4 m/s. © The authors, published by EDP Sciences, 2017.

Impact of inner-wall reflection on UV reactor performance as evaluated by using computational fluid dynamics: The role of diffuse reflection

Making use of the reflected ultraviolet (UV) radiation with a reflective inner wall is a promising way to improve UV reactor performance. In this study, the impact of inner-wall reflection on UV reactor performance was evaluated in annular single-lamp UV reactors by using computational fluid dynamics, with an emphasis on the role of diffuse reflection. The UV radiation inside the reactor chamber was simulated using a calibrated discrete ordinates radiation model, which has been proven to be a reliable tool for modeling fluence rate (FR) distributions in UV reactors with a reflective inner wall. The results show that UV reactors with a highly reflective inner wall (Reflectivity = 0.80) had obviously higher FRs and reduction equivalent fluences (REFs) than those with an ordinary inner wall (Reflectivity = 0.26). The inner-wall diffuse reflection further increased the reactor REF, as a result of the elevated volume-averaged FR. The FR distribution uniformity had conditioned contributions to UV reactor performance. Specifically, in UV reactors with a plug-like flow the FR distribution uniformity contributed to the REF to some extent, while in UV reactors with a mixed flow it had little influence on the REF. This study has evaluated, for the first time, the impact of inner-wall diffuse reflection on UV reactor performance and has renewed the understanding about the contribution of FR distribution uniformity to UV reactor performance.

Recirculating metallic particles for the efficiency enhancement of concentrated solar receivers

The use of Concentrated Solar Power technology for energy generation relies on the heating of fluids to high temperatures. One of the limitations of this technology is the low working efficiency of current solar receivers which hinders its application and utilization as a renewable clean energy source. This paper presents a novel concept to improve concentrated solar receiver efficiency through the recirculation of solid particles laden flow within the receiver. The recirculating characteristic of the fluid and solid particle mixture is expected to generate higher output fluid temperature than conventional solar receivers. This is expected to lead to significant improvements in the working performance of the concentrated solar receiver. The numerical study investigates this concept, using a commercial CFD package, employing the discrete particle model (DPM), a RNG k-ε flow model and a discrete ordinate (DO) radiation model. It is found that the use of recirculating solid particles improves the energy absorption within the receiver and with it the thermal efficiency of the proposed solar receiver reaches to around 70%. Moreover, comparing this with a conventional solar receiver, this new concept exhibits a more uniform temperature distribution inside the cavity, which is mainly due to the recirculating flow characteristic and the well-controlled mixing of air and particles. The advantages of this novel concept of the solar receiver present its potential benefits in further applications in the process of energy conversion providing on enhanced heat transfer medium.

CFD simulation of the thermal performance of an opaque water wall system for Australian climate

The thermal performance of an opaque water wall system is numerically investigated using the shear-stress transport (SST) k–ω turbulence model and the Discrete Ordinates radiation model for typical winter and summer climate conditions in Sydney, Australia. With a periodic sol-air temperature specified on the outside Perspex panel, the energy performance of the water wall system is examined over a range of water wall thicknesses. The computational fluid dynamics (CFD) model is compared with an experimentally validated transient heat balance model (THBM), and a fair agreement between the CFD model and the THBM results is achieved. The present numerical results indicate that the performance of the opaque water wall system is improved by increasing the thickness of the water column under the winter climate condition in Sydney. It is also found that less supplementary energy is required in winter than that in summer in order to maintain a comfortable interior temperature. Further, a comparison between the present water wall system and a conventional concrete wall system shows that the water wall system performs significantly better than the concrete wall system of the same thickness in the winter climate of Sydney, whereas both the water wall and concrete wall systems have a similar performance in terms of energy savings in the summer climate of Sydney.

Coupled simulation of convection section with dual stage steam feed mixing of an industrial ethylene cracking furnace

A complete coupled simulation of the convection chamber and tubes with dual stage steam feed mixing of an industrial ethylene cracking furnace has been carried out with the computational fluid dynamics (CFD) method for the first time. In the convection chamber, the standard k–ε model and discrete ordinates (DO) radiation model were respectively used in the descriptions of turbulence characteristics and radiative heat transfer. In the tubes, renormalization group (RNG) k–ε model and volume of fluid (VOF) model were respectively applied to the turbulence flow and the liquid–vapor two phases flow. Simulation results agree well with the industrial data. Based on the coupled result, a dynamic simulation was calculated in the feedstock preheater (FPH). Simulation results show that the velocity and temperature fields are inhomogeneous distributions along the width direction due to the asymmetrical structure of convection chamber. Two recirculation zones occur at the corner both near and away from the entrance to the convection chamber, which will cause a longer residence time of flue gas and local overheating in furnace wall of convection chamber. The process gas temperature, tube skin temperature and heat flux profiles are respectively different along the axial and radial direction of the high temperature coil (HTC-I). The changes of flow pattern from bubble flow to spray flow are effected by gravity and centrifugal force during evaporation. The results will be helpful for the design and operation in cracking furnace.

Numerical Simulation of Polysilicon Chemical Vapor Deposition in Siemens Reactor

The Siemens CVD reactor should be refined when addressing the low energy consumption requirement for the production of solar grade polysilicon. The object of this paper is to investigate the heat transfer in Siemens reactor, which will provide the theoretical guidance for saving energy of polysilicon scale production. Based on Discrete Ordinates (DO) radiation model, an analysis is presented which studies the radiation exchange between the hot polysilicon rods and the cold reactor wall of four typical reactor configurations, especially the variation of the average power emitted per rod under various conditions of different rod diameter and wall emissivity. The results show that the energy consumption can be reduced by decreasing the emissivity of the reactor wall and enlarging the reactor capacity.

Nanoparticle scattering characterization and mechanistic modelling of UV–TiO2 photocatalytic reactors using computational fluid dynamics

A computational fluid dynamic (CFD) model was developed to describe the process performance of a semi-batch annular TiO2–UV photoreactor in an Eulerian framework. The model accounted for the optical behaviour of titanium dioxide (TiO2) suspensions, the flow distribution and the oxalic acid degradation in the reactor. The scattering component of the optical model, explicitly included in the CFD simulations using a TiO2-specific scattering phase function integrated in the radiative transfer equation, was calibrated using an optical goniometer by comparing simulated scattering light profiles against irradiance measurements collected for various TiO2 concentrations and UV wavelengths and subsequently solved by the discrete ordinate (DO) radiation model. Several scattering phase functions were tested against the goniometric measurements confirming that the Henyey-Greenstein (HG) equation was the most appropriate angular distribution function at 254 and 355 nm, irrespective of the TiO2 concentration. Using the calibrated HG function, a new approach for quantifying the absolute values of absorption and scattering coefficients in TiO2 suspensions was proposed. It consists of iteratively solving, using the DO model, the radiative transfer equation for various combinations of absorption and scattering coefficients until the error between observed and predicted angular irradiance measurements is minimized. The accuracy of the optical parameters was verified with independent CFD simulations carried out for an annular photoreactor and already available in the literature. Predicted and simulated irradiance and oxalic acid degradation data were found to be in excellent agreement, confirming the considerable potential of the integrated modelling approach presented in this paper for the design, optimization and scale-up of photocatalytic technologies for water and wastewater treatment applications.

On numerical modelling of conjugate turbulent natural convection and radiation in a differentially heated cavity

Turbulent natural convection with and without radiation transfer in two-dimensional (2D) and three-dimensional (3D) air-filled differentially heated cavities is numerically investigated using various RANS (Reynolds Averaged Navier–Stokes) turbulence models and the Discrete Ordinates radiation model. Five different two-equation eddy-viscosity models including the standard k–ε model, the renormalization group (RNG) k–ε model, the realisable k–ε model, the standard k–ω model and the shear-stress transport (SST) k–ω model are selected for comparison. Qualitative and quantitative data are presented to demonstrate the effects of three-dimensionality, radiation transfer and the thermal boundary conditions on the horizontal surfaces on the numerical solution of the convective flow in the cavity. The present numerical results are compared against published experimental and direct numerical simulation data. It is found that the predicted thermal stratification in the interior of the cavity is improved when the simulation is extended from 2D to 3D and when the effect of radiation transfer is accounted for. The discrepancy with regard to the interior stratification between the experiment and numerical simulation is mainly caused by the negligence of radiation transfer. The thermal boundary conditions on the horizontal surfaces also have a significant impact on the numerical solution, especially when the radiation transfer is not accounted for. Further, the present results show that all the RANS models are capable of capturing the main features of the flow and the overall performance of these turbulence models in terms of predicting time-averaged quantities is acceptable. It is found that the variation of the numerical results obtained with the three k–ε models is very small, whereas the discrepancy between the two k–ω models is significant. The SST k–ω model has the best overall performance and the standard k–ω model has the worst overall performance.

Design Optimization of a Novel Receiving Cavity for Concentrated Solar Power Applications by Means of 3D CFD Simulations

Abstract The aim of this work was the design optimization, by means of accurate steady state 3D CFD simulations, of a novel receiving cavity for Concentrated Solar Power (CSP) applications. The receiving cavity has been developed by Airlight Energy Manufacturing SA in collaboration with ETH Zurich and SUPSI-DTI-ICIMSIwithin the framework of the SolAir-2 project. It is made of a helically coiled steel tube, and resulted highly effective in converting the radiative energy coming from the sun into thermal energy gathered by air which was selected as heat transfer fluid (HTF) being cheap, environmentally friendly and suitable for high temperatures applications. The main geometrical parameters considered for the optimization were: cavity height, varied by increasing or decreasing the number of coils, cavity external diameter, and the presence of a spiral coiled tube closing the cavity top. For each configuration the air flow rate, with an inlet temperature of 120 °C, was tuned in order to reach an outlet temperature close to the target value of 650 °C. Cavity performance were evaluated in terms of thermal efficiency and pressure drop under two different skew angle conditions for the incoming solar radiation, 18° and 40°. CFD simulations were performed with Ansys Fluent. Navier-Stokes and energy equations were numerically solved using the finite-volume method approach; radiation heat transfer inside the cavity was taken into account by means of the Discrete Ordinates (DO) radiation model.

CFD analysis of solar tower Hybrid Pressurized Air Receiver (HPAR) using a dual-banded radiation model

Solar receivers used for central tower Concentrated Solar Power (CSP) plants use either a surface-based or volumetric heat transfer region. Volumetric receivers are more efficient but require more sophisticated and expensive materials that can withstand the elevated temperatures (in excess of 1000°C). For a solarized gas-turbine application, pressurized air from the compressor is used as heat transfer fluid (HTF) in order to increase both the density and heat capacity of the HTF. If a volumetric receiver is used, it needs to be located in a pressure vessel with a pressurized quartz window located at the aperture of the concentrator. An alternative approach to utilize pressurized air in a pseudo-volumetric fashion is to populate a volumetric region with piped pressurized air. A tubular-type volumetric receiver (named a Hybrid Pressurized Air Receiver (HPAR)) is studied here. The HPAR provides the challenge of enabling maximum heat transfer without causing hot spots on the side of the solar irradiation source. Heat transfer in the HPAR would not be as effective as for a true volumetric receiver because the heat transfer area is limited by the tube wall. The heat transfer to the HTF is however enhanced through mixing generated by external forced convection caused by suction due to a downstream fan in the receiver cavity. In addition, the aperture of the cavity contains a glass windowed louver system, to limit re-radiation losses. A Computational Fluid Dynamics (CFD) model is generated of the solar receiver cavity. The commercial CFD code ANSYS Fluent v14.5 is used to evaluate the heat transfer between the incoming solar flux and the HTF. A numerical validation is performed to illustrate mesh dependency and the choice of turbulence model. The incoming solar irradiation and its absorption, reflection and transmission are modeled using the Discrete Ordinates (DO) radiation model in ANSYS Fluent. An idealized and a solar flux map based on the PS10 field are used as source. For comparison, both a gray (without a glass louver) and a semi-gray two-banded DO approach are used when modeling the absorption of high-wavelength re-radiation by the glass in front of the aperture. This two-banded approach is also applied to investigate the influence of a band-selective absorber tube and cavity surfaces. The DO model also predicts the emission of thermal re-radiation from all surfaces. The geometry is parameterized in order to allow for various cavity layouts to be automatically generated. Results include typical CFD results of a candidate geometry to illustrate the solar irradiation input, the effect of tubular layout as well as temperature and heat flux distributions for single and dual-band radiation.

Study of internal radiation with solute inclusions during Czochralski sapphire crystal growth

Internal radiation is investigated using discrete ordinates (DO) radiation model for Czochralski growth of sapphire single crystal. Its effects on the solid/liquid (S/L) interface, melt flow and temperature distribution are presented. Using the internal radiation model, the influences of melt inclusions on the S/L interface are further examined with special attention directed to optical properties of inclusions, opaque or transparent, and locations of inclusions in the crystal. The results show that the predicted convexity of the interface increases after taking into account internal radiation. A local surface with accumulation of opaque inclusions bends towards the crystal. On the contrary, transparent inclusions result in a more convex interface into the melt. These phenomena are both revealed in two- and three-dimensional calculations.

Characteristics of Oxyfuel and Air–Fuel Combustion in an Industrial Water Tube Boiler

The characteristics of oxyfuel combustion and air–fuel combustion in the furnace of a typical industrial water tube boiler using methane as the operating fuel are investigated. Two oxyfuel cases are considered. The analysis is conducted for two oxyfuel cases that correspond to 21% O2 and 29% O2 in the oxidizer mixture (O2 + CO2). A renormalized group (RNG) turbulence model and the eddy dissipation model are utilized in the present work to provide the turbulence characteristics and the production rate of species. The solution of the radiative transfer equation was obtained using the discrete ordinates radiation model. The set of governing equations and the boundary conditions are solved numerically using Fluent computational fluid dynamics code considering a single-step reaction kinetics model for methane–oxyfuel combustion. Comparison of both oxyfuel combustion and air–fuel combustion indicates that the temperature levels are reduced in oxyfuel combustion. The results show that the temperature levels are greatly reduced as the percentage of recirculated CO2 is increased. It is concluded that the flame propagation speed in the CO2 environment is lower than that in N2. It is found that the natural gas and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion. Heat transfer from the burnt gases to the water jacket along the different surfaces of the furnace is calculated. It is shown that the energy absorbed is much lower in the case of oxyfuel combustion along all surfaces except for the end part of the furnace close to the furnace rear wall. However, the same performance of the methane-oxy-flames is expected by increasing the oxygen concentration slightly above 29%.

Analysis of Flame Characteristic of Kerosene Pool Fire

Based on the computational fluid dynamics (CFD) simulation software and its parallel techniques, the standard k-ε turbulent flow equations with additional buoyancy-modified, the PDF(Probability Density Function) non-premixed combustion model, and the discrete ordinates (DO) radiation model were used to simulate the kerosene pool fire scenarios of various pool diameters in stagnant air and in a cross-wind. Based on these simulations, the relationships between the pool size and the flame height and flame temperature were obtained, as well as the effective law of the wind speed change on the fire tilt angle. The results presented in this paper are of great significance in the hazard evaluation of the thermal radiation of the pool fire.

Heat Transfer Modeling and Analysis of Solar Thermo-chemical Reactor for Hydrogen Production from Water

A solar thermo-chemical reactor is modeled and analyzed for the solar thermal dissociation of zinc oxide into zinc and oxygen involved in the thermo-chemical cycle for hydrogen production. The reactor consists of a cavity surrounded by a rotating insulation layer made of alumina. The granular zinc oxide particles are fed into the cavity and are directly exposed to the solar radiation entering the cavity through a quartz window. A three dimensional numerical model coupling the multiphase particle dynamics in gravitational field, multiphase heat transfer, k-ɛ turbulence model, discrete ordinates radiation model, Arrhenius reaction rate model is developed. The cavity temperature and oxygen molar flow rate at the outlet of the reactor which is the indicator of the reaction rate has been validated with a 10kW reactor prototype. An energy balance study of thermal performance parameters including the various losses occurring from the reactor and efficiency is also done. The major losses were contributed by re-radiation (46%) and sensible heating of reactor components (35.5%), while the minor losses were contributed by convection by argon (1%) and conduction through insulation (2%).The thermal efficiency of the reactor is calculated to be 15.5%.

Optimization of Solar Tower Hybrid Pressurized Air Receiver Using CFD and Mathematical Optimization

Solar receivers used for central tower Concentrated Solar Plants (CSPs) use either a surface-based or volumetric heat transfer region. Volumetric receivers are more efficient but require more sophisticated and expensive materials that can withstand the elevated temperatures (in excess of 1000°C). If pressurized air is used as heat transfer fluid (HTF) in order to increase both the density and heat capacity of the HTF, the volumetric receiver needs to be located in a pressure vessel with a pressurized quartz window located at the aperture of the concentrator. An alternative approach to utilize pressurized air in a pseudo-volumetric fashion is to populate a volumetric region with piped pressurized air (Kretzschmar & Gauché, 2012). This tubular-type volumetric receiver provides the challenge of enabling maximum heat transfer without causing hot spots on the side of the solar irradiation source. Heat transfer would not be as effective as for a true volumetric receiver because the heat transfer area is limited by the tube wall. A Computational Fluid Dynamics (CFD) model is generated of the solar receiver cavity. The commercial CFD code ANSYS Fluent v14.5 is used to evaluate the heat transfer between the incoming solar flux and the HTF. The incoming solar irradiation is modeled using the Discrete Ordinates (DO) radiation model ANSYS Fluent. The DO model also predicts the resulting thermal radiation in the conjugate heat transfer problem. The geometry is parameterized in order to allow for the selection of design variables in a formal design optimization formulation. Results include typical CFD results of a candidate geometry to illustrate the solar irradiation input, the effect of tubular layout as well as temperature and heat flux distributions that provide candidate objective functions for optimization. Recommendations are made for the implementation of the optimization process.

Three-dimensional CFD analysis for simulating the greenhouse effect in solar chimney power plants using a two-band radiation model

The greenhouse effect in the solar collector has a fundamental role to produce the upward buoyancy force in solar chimney power plant systems. This study underlines the importance of the greenhouse effect on the buoyancy-driven flow and heat transfer characteristics through the system. For this purpose, a three-dimensional unsteady model with the RNG k–ε turbulence closure was developed, using computational fluid dynamics techniques. In this model, to solve the radiative transfer equation the discrete ordinates (DO) radiation model was implemented, using a two-band radiation model. To simulate radiation effects from the sun's rays, the solar ray tracing algorithm was coupled to the calculation via a source term in the energy equation. Simulations were carried out for a system with the geometry parameters of the Manzanares power plant. The effects of the solar insolation and pressure drop across the turbine on the flow and heat transfer of the system were considered. Based on the numerical results, temperature profile of the ground surface, thermal collector efficiency and power output were calculated and the results were validated by comparing with experimental data of this prototype power plant. Furthermore, enthalpy rise through the collector and energy loss from the chimney outlet between 1-band and two-band radiation model were compared. The analysis showed that simulating the greenhouse effect has an important role to accurately predict the characteristics of the flow and heat transfer in solar chimney power plant systems.

Investigation of radiation models in entrained-flow coal gasification simulation

Adequate modeling of radiation heat transfer is important in CFD simulation of coal gasification process. In an entrained-flow gasifer, the non-participating effect of coal particles, soot, ashes, and reactive gases could significantly affect the temperature distribution in the gasifier and hence affects the local reaction rate and life expectancy of wall materials. For slagging type gasifiers, radiation further affects the forming process of corrosive slag on the wall which can expedite degradation of the refractory lining in the gasifier. For these reasons, this paper focuses on investigating applications of five different radiation models to coal gasification process, including Discrete Transfer Radiation Model (DTRM), P-1 Radiation Model, Rosseland Radiation Model, Surface-to-Surface (S2S) Radiation Model, and Discrete Ordinates (DO) Radiation Model. The objective is to identify the pros and cons of each model’s applicability to the gasification process and determine which radiation model is most appropriate for simulating the process in entrained-flow gasifiers.The Eulerian–Lagrangian approach is applied to solve the Navier–Stokes equations, nine species transport equations, and seven global reactions consisting of three heterogeneous reactions and four homogeneous reactions. The coal particles are tracked with the Lagrangian method. Six cases are studied—one without the radiation model and the other five with different radiation models. The result reveals that the various radiation models yield uncomfortably large uncertainties in predicting syngas composition, syngas temperature, and wall temperature. The Rosseland model does not yield reasonable and realistic results for gasification process. The DTRM model predicts very high syngas and wall temperatures in the dry coal feed case. In the one-stage coal slurry case, DTRM result is close to the S2S result. The P1 method seems to behave stably and is robust in predicting the syngas temperature and composition; it yields the result most close to the mean, but it seems to underpredict the gasifier’s inner wall temperature.