Radiative Heat Transfer Characteristics Research Articles

Parametric investigation of radiation heat transfer and evaporation characteristics of a liquid droplet radiator

The liquid droplet radiator (LDR) as a frameless heat removal system is considered as a promising solution for the waste heat dissipation of megawatt aerospace applications. The radiation heat transfer and evaporation characteristics of the LDR working fluid are tightly coupled with the operational performance of the liquid droplet generator. In this paper, the effects of operational parameters of droplet generator on the radiative heat transfer and evaporation loss rate are clarified, including the pressure disturbance frequency, pressure difference between the inside and outside of the droplet generator. It is observed that higher coolant mass flow rate does not assure higher heat transfer power for the LDR. For cases with higher pressure difference, where the coolant flow rate is higher consequently, the evaporation loss rate increases continually with pressure difference while the heat transfer power does not increase any more. The disturbance frequency has inconspicuous impact on the evaporation characteristics. Besides, cases with higher pressure differences have wider suitable frequency ranges according to the droplet formation restriction. Generally, a relatively higher pressure difference coupled with a higher disturbance frequency (such as 0.3 MPa and 12 kHz) is suggested to achieve a higher heat transfer power and low evaporation loss rate. This paper may provide favorable reference for determining the operational parameters of the droplet generator in consideration of radiation heat transfer and evaporation characteristics.

Numerical Study of Convective and Radiation Heat Transfer Characteristics in an Upward-Facing Cylindrical Cavity under Back-Side Windy Condition

Under the back-side windy condition, the convection and radiation heat transfer characteristics in an iso-flux upward-facing cylindrical cavity were studied by three-dimensional numerical simulation. The impacts of cavity tilt angle, wind incident angle and wind speed on convection and radiation heat transfer Nusselt number Nuc and Nur were analyzed, and the possible explanations for their impacts were presented. Results show that due to the disturbance of wind, the influence of cavity tilt angle becomes more complicated and is related to wind incident angle and wind speed. The variation of Nuc or Nur with wind incident angle is different for different cavity tilt angles. Despite of the changes of cavity tilt angle or wind incident angle, the Nuc increases with the wind speed while the Nur presents a declination with the increasing of wind speed. Hence, compared with cavity tilt angle and wind incident angle, wind speed may be the dominant factor affecting or controlling the convective and radiation heat transfer of cavity.

Experimental study of radiative heat transfer from laminar non-premixed methane flames diluted with CO2 and N2

This paper presents an experimental study about the effect of fuel dilution with inert gases on the radiative heat transfer characteristics of laminar non-premixed coflow methane flames. The visible flame lengths were determined and flickering is characterized when necessary. Radiative heat flux distributions were measured and used to determine the radiative fraction of each flame. In total, 55 flames were studied with powers up to 0.554 kW and with CO2 and N2 dilutions up to 60%. The flames were grouped in series of constant power and constant flow rate. It is shown that the addition of CO2 may decrease or increase the radiative fraction depending on the dilution level and on the characteristic residence time of the flame. Dilution with N2 always leads to a decrease of the radiative fraction. The results are relevant for practical applications as in biogas combustion and also proved to be useful for validation of radiative heat transfer models.

Numerical simulation of radiation heat transfer characteristics in a cylindrical fluidized bed

In this paper, a 3D two-phase simulation involving the complex heat transfer characteristics within a vertical cylindrical fluidized bed was conducted numerically. The accuracy of mathematical modeling has been validated by comparing with the experimental results. The discrete ordinate model (DOM) was employed to investigate the radiation heat transfer in a fluidized bed. The user-defined function (UDF) was compiled by considering the effect of the particle flow emissivity on radiation heat transfer, and coupled the two-phase flow field by using the gray and non-gray model respectively. The results show that the non-gray model can better depict the selected absorption of radiation wavelength in two-phase flow heat transfer than gray model. Furthermore, under high heat flux density conditions, the proportion of radiation heat transfer increases gradually with the increase of heat flux, which can reach about 40% of the total heat transfer coefficient. Meanwhile, other factors about radiation heat transfer such as initial solid packing height, inlet gas velocity, and other conditions were studied and consistent with the thermal experimental regularity. The findings demonstrated that the radiation heat transfer and it’s modeling play an important role in two-phase heat transfer of fluidized bed.

Assessment of various tar and soot treatment methods and a priori analysis of the steady laminar flamelet model for use in coal combustion simulation

Soot formed in the fuel-rich regions of coal flames is known to have a significant impact on the radiative heat transfer characteristics of the flame. It is therefore important to account for the effect of soot radiation when simulating coal combustion systems. Several recent coal combustion studies have used acetylene as a surrogate for devolatilized tar and neglect soot formation altogether, while others have utilized empirical models for tar and soot or even chemical mechanisms which include tar and soot as species. In this work, we investigate several methods of treating coal-derived tar and soot and the impact each treatment has on predicted furnace temperature, particle mass loss, and other quantities of interest. Furthermore, we perform a priori comparisons of simulation results to calculations from a steady laminar flamelet model in order to assess the accuracy of a flamelet approach for each tar and soot treatment considered. Data used as a basis for the study is generated through One-Dimensional Turbulence (ODT) simulation of a coal combustion system in which high-fidelity models are used for gas phase chemistry and devolatilization, where char combustion is neglected.

Radiative and evaporative characteristics analysis of a liquid droplet layer for space applications

Liquid droplet radiator is promised to be used for waste heat dissipation of the large power space nuclear reactor. This paper analyzes the radiative heat transfer and evaporative characteristics by treating the droplet layer as a participating medium. The silicon oil is adopted as the working fluid. The mathematical models are established. The effects of the optical thickness κD, initial temperature, and flight time on the droplet radiation and evaporation are investigated. The main works are listed as follows: Firstly, the grid independence analysis of the temperature calculation is conducted, and the grid number on the thickness direction is suggested to be 100 times of κD. Secondly, the calculation results obtained from the one dimensional (1-D) simplified formula and the two dimensional (2-D) simulation have been compared, proving that it is acceptable to obtain the average temperature by 1-D simulation for droplet layers with any κD. Thirdly, detailed temperature distribution is calculated for both thin and thick droplet layers, and higher initial temperature and longer flight time can enhance the radiative heat transfer. Finally, the evaporative loss rate of the working fluid is found to be nearly constant with κD larger than 4. This paper may contribute to the thermal design and optimization of the liquid droplet radiator for space applications.

Development of the Oxygen-Enriched Burner Using Self-Induced Oscillation

We have developed the novel oxygen-enriched burner using the self-induced oscillating phenomenon of jet flow which has excellent radiation heat transfer characteristics. We revealed that the flame property suitable for radiation heat transfer is obtained by extending the oscillation cycle. By optimizing the connecting tube length and diameter, we got the stable oscillating flame in 1 s-oscillation period. At this time, the relationship between the burner structure including the connecting tube and the oscillation period can be easily evaluated by the dimensionless tube length and Strouhal number. In addition, combining multi-staged combustion technology, the burner can improve heat transfer efficiency by 9.1% compared with the conventional oxygen-enriched burner. We confirmed that the burner has a low NOx emission level below 55 ppm.

Spectral Radiation Analysis of Premixed Oxy-Methane Flames

For a hydrocarbon burning with oxygen, the resulting exhaust stream is composed mainly of carbon dioxide and water vapor. This exhaust allows for easier carbon capture and sequestration since the water can be condensed out. Another advantage is the significant reduction of NOx since much of the nitrogen found in air-fired systems is eliminated. Although beneficial, many of the exhaust gas products' radiative heat transfer characteristics are unknown. Motivated by this, this paper focuses on the spectral radiation measurement of premixed oxy-methane combustion flames. This is important for combustion system designers since radiative heat from the flame is significant for oxy-flames. This study is conducted by varying equivalence ratio, firing input, and CO2 recirculation ratio. The spectral radiation of premixed oxy-methane flames is collected from 1.2 μm to 5 μm wavelengths. During the experimental study, it is found that the water vapor emits at 1.4 μm, 1.85 μm, and 2.5 μm wavelengths. A short band of carbon dioxide emission is detected at 1.96 μm. Three other carbon dioxide radiation maxima are observed at the proximity of 2.71 μm, 2.85 μm, and 4.38 μm. The study revealed that the spectral intensity of CO2 and H2O for oxy-methane combustion increases almost five times compared to the air-methane combustion at stochiometric condition. It is also found that the spectral intensity decreases as the equivalence ratio increases. The spectral radiative emission intensity increases as the firing input increases. Another observation includes the fact that spectral intensity increases up to five times when 60% CO2 is recirculated as a diluent in the flame.

Heat transfer pattern judgment and thermal performance enhancement of insulation air layers in building envelopes

Building envelopes act as the thermal interfaces between the indoor and outdoor environments, thus can greatly influence the indoor thermal condition and the energy consumption of air-conditioning systems. The development of high-performance exterior envelopes is anticipated to be the most effective way to guarantee both low energy consumption and high indoor thermal comfort for a building. Recently, designing and structuring intermediate enclosed air layers have become a popular way to improve the thermal insulation property of building envelopes. Based on the establishment of a dimensionless model, this study numerically investigates the flow and heat transfer characteristics of the insulation air layers with different geometrical sizes and temperature boundary conditions. By analyzing the variation tendencies of the streamlines, isotherms and temperature profiles, a simplified Rayleigh number (Ra) based judgment basis is summarized for the heat transfer pattern of the insulation air layers. Simultaneously, the critical thicknesses of the heat transfer pattern are determined under different temperature boundary conditions. Furthermore, the coupled convective and radiative heat transfer characteristics and the influencing factors of the heat transfer through the air layer are examined. Finally, two measures are proposed to enhance the air layer’s thermal insulation performance. The optimal air layer thickness is determined to be 20–30 mm depending on the temperature boundary conditions. Reducing the surface emissivity enjoys a great potential for the thermal performance improvement of insulation air layers. When the emissivity decreases from 0.95 to 0.2, the thermal resistance of the air layer can be improved by 87.15–172.73%. A case study indicates that using the air layer as insulation helps to reduce the annual heat transfer through the building envelopes by 10.54–39.23% depending on the climate condition.

Computational Investigation on Radiative Heat Transfer Characteristics of an Automotive LED Tail Light

A recent survey by National Transportation Highway Traffic Safety Administration (NHTSA) stated that the rear-end crash of passenger vehicles is the most accounted cause of vehicle crash (more than 21 %) compared among all other vehicle crashes. Tail light malfunctions or failure is often the cause for such accidents. There is a pressing need for design and optimization of an automotive tail light system to increase the road safety. The service life for the conventional incandescent light bulb is comparatively lesser than the Light Emitting Diode (LED) tail light. During the physical testing process and its validation in the later development stage, the computational methods can be effectively used for the design optimization of LED tail light system in the development based on the thermal-radiative performance. The results could be incorporated in automotive design industries to produce nearly zero defect and failure-free design with better thermal performances. Therefore, it is highly significant to simulate and study the thermal management through simulation analysis. The present study focuses on a computational analysis of a complex automotive tail light system with a near net shape using Computational Fluid Dynamics (CFD) methodology. The simulation results compared three different materials for housing of tail light assembly along with re-arranging the LED arrays. Among the selected housing materials, Acrylonitrile Butadiene Styrene (ABS) exhibited better thermal performance with the maximum temperature of around 365.74 K compared with other housing materials. The computational results were validated using an experimental set up and the temperature measurements followed the same trend as CFD. The maximum percentage of deviation was not more than 2.6%. DOI: http://dx.doi.org/10.5755/j01.mech.25.2.22116

Improved spectral absorption-coefficient grouping strategies in radiation heat transfer calculations for combustion gases with pressure and temperature inhomogeneity

The grouping strategy for the spectral absorption coefficient of gases used in multigroup full spectrum k distribution and multiscale multigroup full spectrum k distribution models was improved to enable radiation-heat transfer calculations of combustion gas with temperature inhomogeneity under various pressures or large pressure gradients. The improvements in calculation accuracy resulting from different grouping strategies were evaluated using a series of semi-1D problems in which strong temperature and pressure inhomogeneity were present. Finally, the evaluations are given for the calculation of radiation heat transfer characteristics and thermal emission imaging of the exhaust system of a parallel turbine-based combined cycle (TBCC) engine.

Radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina RPC structures as integrated solar thermochemical reactor

This paper investigated radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina reticulated porous ceramic structures as integrated solar thermochemical reactor. High-flux solar radiation absorption and axial temperature distribution in the ceramic foams reactor were analyzed by adopting surface-to-surface radiation model coupled to the P1 approximation for radiation heat transfer. The radiative heat transfer and thermal characteristics of different foam-type RPC structures, including SiC, CeO2, FeAl2O4, NiFeAlO4, Fe3O4/SiC, and NiFe2O4/SiC were evaluated. The mass flow rate and foam structural parameters, including the permeability, pore mean cell size, and extinction coefficients have significantly affected the axial temperature distribution, pressure drop, heat transfer, and fluid flow. Integrated porous structure to the solar receiver could maximize the incorporation of redox powder in the reacting medium, lower the pressure drop, and enhance the thermal performance of the thermochemical reacting system. SiC structure was the candidate materials in the case where more heat flux and high axial temperature distribution is needed. However, Fe-based oxide coated Al2O3 structure could be considered regarding the heat transfer enhancement along with the catalyst activity of oxygen carriers for solar thermochemical reacting system performance.

Numerical study of chemical kinetics and radiation heat transfer characteristics on the temperature distribution in the oxy-fuel combustion

In this study, The IFRF methane-oxygen combustion furnace is used to investigate the effect of the radiation model, the method of evaluated absorption and emission coefficients and the different chemical mechanisms. The simulations are performed with OpenFOAM open source software by using of PaSR (partially stirred reactor) combustion model. The numerical investigations are carried out without radiative heat transfer and with the modeling of the radiation source term using the P1 (spherical harmonic radiation model) and DO (discrete ordinate) models. To evaluate the absorption and emission coefficients used from the constant coefficients, the grey mean gases model in terms of temperature polynomial, the WSGGM (weighted sum of grey gases model) and refined WSGGM models. The investigation of the global mechanism effect on the temperature distribution in the oxy-fuel combustion is performed by the modified 2-step Westbrook-Dryer mechanism by Yin and modified 4-step Jones-Lindstedt mechanisms by Yin and Andersen. The results indicate that the lack of consideration of radiation heat transfer in the oxy-fuel combustion leads to a large error in the prediction of the maximum and average temperature distributions inside the furnace. Also, the DO model has less error than P1 model due to more heat loss prediction by P1 model in the low optical thickness. The WSGGM model for calculating of absorption and emission coefficients provide the best result in comparison with other methods. The refined 4-step Jones-Lindstedt mechanism by Andersen has best prediction of on the basis of numerical simulations.

Optimal thickness determination of insulating air layers in building envelopes

In building envelopes, using enclosed air layer becomes a popular way of thermal insulation, since the air has a much lower conductivity coefficient, and is much cheaper than other building materials. The motivation of this research is to investigate the coupled convective and radiative heat transfer in the air layer of building envelopes, in order to find the optimal geometrical parameters of the insulating air layers. Based on CFD technology, a Ra number judgment basis is summarized for flow pattern judge in insulation air layers, and the coupled convective and radiative heat transfer characteristics across the air layer is analyzed. The results indicated that a larger layer height results in a weaker convective heat transfer; when the thickness is below 20mm, an increasing thickness leads to a considerable decrease in the heat transfer; but when thickness exceeds 20mm, the heat transfer is slightly influenced by the thickness. The optimal thickness of the insulation air layer is 20-30mm depending on the climate condition.

Improved Spectral Absorption Coefficient Grouping Strategies in Radiation Heat Transfer Calculations for H2O–CO2-Soot Mixtures

In the present work, strategies for the grouping of the spectral absorption coefficients used in multiscale (MS) multigroup (MG) full-spectrum k-distribution models were improved by considering the effects of variations in both temperature and species molar ratio on the correlated-k characteristics of the spectra of H2O–CO2-soot mixtures. The improvements in the accuracy of predictions of radiation heat transfer characteristics resulting from these new grouping strategies were evaluated using a series of semi-one-dimensional (1D) cases with significant temperature, participating species molar ratio, and pressure inhomogeneities. Finally, evaluations of grouping strategies were presented on calculation of the full-spectrum thermal images of an actual aeroengine combustor.

New weighted-sum-of-gray-gases model for typical pressurized oxy-fuel conditions

Pressurized oxy-fuel combustion technology has received considerable attention due to its ability to improve the overall system efficiency and to control CO2 emissions. The characteristics of radiation heat transfer are significant for pressurized oxy-fuel gas mixture and different from those under atmospheric conditions. Therefore, to calculate the radiation characteristics of pressurized oxy-fuel gas mixture quickly and accurately, new weighted-sum-of-gray-gases (WSGG) model for pressurized oxy-fuel conditions was first presented in this paper, which was applied in 3 typical high pressure conditions: 5, 10, and 15 bar. The new WSGG model correlations were suitable for pressurized conditions with a molar ratio range of 0.125-2, temperature range of 400-2500 K, and path length range of 0.1-20 m. Calculations for a variety of typical pressurized oxy-fuel combustion cases showed that the new WSGG model can accurately predict the radiation characteristics and heat transfer characteristics of the gas mixtures compared with the SNB model benchmark. In addition, the application of the previous atmospheric WSGG models yielded non-ideal results under pressurized conditions. Consequently, the new model can provide efficient and accurate radiation heat transfer results for pressurized oxy-fuel conditions and can be used to design pressurized oxy-fuel combustion furnaces or boilers.

A comprehensive evaluation of the non gray gas thermal radiation using the line-by-line model in one- and two-dimensional enclosures

In oxy-fuel combustion with CO2 recycle, the non-gray gas radiative heat transfer characteristics of gaseous participating media are different from those in air-fuel combustion. Water and carbon dioxide are the most important gas products of combustion under O2/CO2 atmosphere. A line-by-line model with HITEMP2010 database was comprehensively investigated in one- and two-dimensional enclosures containing H2O/CO2/N2 in this study. The radiative transfer equation was solved using the discrete ordinates method. The thermal radiation characteristics of water and carbon dioxide were determined numerically per their non gray radiative characteristics. The line-by-line model, statistical narrow band model, and statistical narrow band correlation-k model were used to run the calculations. The effect of four factors on radiation were accounted for in the line-by-line model: Spectral line resolution, spatial discretization scheme, quadrature, and total pressure. The calculation results showed that spectral line resolution, which varied from 0.02 to 0.08cm−1, has a significant impact on computing time. The total pressure was the most influential factor overall, however. Increase in total pressure from 1 to 10atm significantly enhanced thermal radiation in regards to the magnitude of the local radiative source term.

Comprehensive evaluation and analysis of the weighted-sum-of-gray-gases radiation model

In oxy-fuel combustion with CO2 recycle, the non-gray gas radiative heat transfer characteristics of gaseous participating media are different from those in air-fuel combustion. Therefore, the choice of a non-gray gas radiation model should be carefully made since it plays an important role in modeling the oxy-fuel combustion system. Using the statistical narrow-band model as a benchmark, in this paper we provide a comprehensive assessment of the development of the weighted-sum-of-gray-gase (WSGG) model, which has been achieved in recent years. The results show that the predicted values obtained by the WSGG model are generally reasonably accurate, though some significant differences still exist. For the total emissivity, the WSGG models by Dorigon et al. (2013 Int. J. Heat Mass Transfer 64 863) and Bordbar et al. (2014 Combust. Flame 161 2435) are consistent well with the benchmark model, within a relative error of less than about 20%. Under the conditions of PH2O/PCO2=1 and 2, the magnitudes of radiative heat transfer between two planar plates are calculated using the discrete-ordinate method and WSGG model. It is found that the radiative source and radiative net heat flux obtained using the WSGG model parameters of Dorigon et al. and Bordbar et al. are more accurate than using other parameters developed in the literature (about 10% relative errors). It is worth noting that the WSGG model parameters of Jonhansson et al. (2011 Combust. Flame 158 893) and Bordbar et al. have a wider range of applications.

Improved MSMGFSK Models Apply to Gas Radiation Heat Transfer Calculation of Exhaust System of TBCC

The multiscale multigroup full-spectrum k-distribution (MSMGFSK) model was improved to adapt to radiation heat transfer calculations of combustion gas flow field with large temperature and pressure gradient. The improvements in calculation accuracy resulting from new sorting strategy of the spectral absorption coefficients were validated using a series of semi-1D problem in which strong temperature, pressure, and mole fraction inhomogeneities were present. A simpler method to attain compatibility between the MSMGFSK model and the gray-wall radiation emission has been established and validated. Finally, estimates are given for the calculation of wall radiation heat transfer characteristics and thermal emission imaging of the exhaust system of the parallel turbine-based combined cycle (TBCC) engine, using finite volume method (FVM) and ray trace method (RT), respectively.

115 10kW試験炉におけるNH_3/N_2/O_2非予混合火炎のふく射特性に関する研究(OS1 燃焼,エネルギー変換,熱利用(5),研究討論セッション)

The total radiative heat flux and the radiation spectra under the condition of the ammonia fuel combusted in a 10 kW test furnace were measured to understand the radiative heat transfer characteristics under the condition of the ammonia fuel combusted in this study. The concentration of O_2 in the oxidizer was varied for increasing the flame temperature and radiative heat flux. The results showed that radiative heat flux under the condition of the oxygen enriched ammonia non-premixed flame formed in the 10kW test furnace was lower than that under the condition of the methane/air non-premixed flame.