Radiative Heat Transfer Characteristics Research Articles

Investigations of heat transfer, entropy generation and pressure build up for upward flow in a vertical channel equipped with a fin array

The optimal thermal systems design criteria by maximizing the amount of heat transfer per pressure losses is a very important topic. In this work, flow and convection and radiation heat transfer characteristics are studied numerically for a flow in a vertical channel equipped with transverse fin array. The influences of fin height on heat transfer characteristics and fluid flow is investigated. Large number of fins is used (40 fins) in order to reach the fully developed conditions after few fins from the entrance. Based on the calculated data of temperature and velocity, the local entropy generation is calculated through the whole channel by solving the entropy generation equation. The results are validated against the available data in the literature and both results are in a good agreement. Optimizations for flow conditions and channel geometry are performed in order to obtain maximum heat transfer per pumping power losses. The results showed that the highest values of total heat transfer per pumping power losses are obtained at fin height to the gap width values of 0.1 and 0.3. The effect of heat transfer by radiation on entropy generation is examined and, the effect of the ratio, Gr/Re2, on the pressure field is also investigated. It was found that a positive pressure gradient appears downstream in the channel when the value of Gr/Re2 exceeds a certain limit. For Gr/Re2 values between 0 and 9, the pressure gradient is negative; however, when the value Gr/Re2 exceeds 9, the pressure starts to build up through the channel axis.

Experimental study on the characteristics of non-steady state radiation heat transfer in the room with concrete ceiling radiant cooling panels

The dominant heat transfer between ceiling radiant cooling panels (CRCP) and indoor environment is the combined heat transfer of thermal radiation and natural convection. Currently, researchers usually discussed indoor radiation heat transfer under steady state conditions rather than non-steady state conditions. In this study, an experimental chamber was built to investigate the indoor parameter variations during the CRCP start-up process in summer, and a mathematical model was also established for further analysis of non-steady state radiation heat transfer. There was a concrete ceiling radiant cooling panel (C-CRCP) equipped in the chamber, and the panel was a concrete slab with pipes embedded in it. The experimental results showed that the temperature of the ceiling surface suddenly dropped after the C-CRCP started up, but the temperatures of the other inner surfaces and the indoor air decreased gradually with a similar trend. The radiant heat flux of the ceiling reached a trough rapidly, and then increased back to steady status step by step. Moreover, it took 7–9 h to nearly get steady status after the C-CRCP started up because of the thermal inertia and the thermal storage of concrete slab. The ratio of radiation heat transfer in the total heat transfer of the ceiling ranged from 40% to 60%. Based on the useful experimental data obtained in this C-CRCP system, a reliable start-up control strategy was also proposed in this paper.

The effect of radiative heat transfer characteristics on vacuum directional solidification process of multicrystalline silicon in the vertical Bridgman system

In the vertical Bridgman system for multicrystalline silicon vacuum directional solidification process, the radiation heat transfer characteristics have a significant impact on the quality of crystal growth. In this paper, the effect of heat transfer characteristics under different pulling-down rates on the vacuum directional solidification process was investigated. Simplified radiation view factor dependent on pulling-down rate in pilot-scale vertical Bridgman system was derived. Combined with the transient simulation results, we obtained the heat transfer characteristics inside the furnace, and proposed the optimization scheme by using a rate of slowly first and then faster to pull down the crucible, which can not only satisfy thermal conditions of crystal growth, but also shorten the solidification time and effectively reduce the energy consumption. In addition, according to the characteristics of the radiative heat transfer under vacuum condition and the change of view factor, it was found that an improved design by widening the cold zone of the furnace can narrow the temperature gradient in the silicon ingot, and greatly decrease the thermal stresses, which in turn can aid in the design of future industrial equipment and process improvements.

Unsteady MHD radiative flow and heat transfer of a dusty nanofluid over an exponentially stretching surface

We analyzed the unsteady magnetohydrodynamic radiative flow and heat transfer characteristics of a dusty nanofluid over an exponentially permeable stretching surface in presence of volume fraction of dust and nano particles. We considered two types of nanofluids namely Cu-water and CuO-water embedded with conducting dust particles. The governing equations are transformed into nonlinear ordinary differential equations by using similarity transformation and solved numerically using Runge–Kutta based shooting technique. The effects of non-dimensional governing parameters namely magneticfield parameter, mass concentration of dust particles, fluid particle interaction parameter, volume fraction of dust particles, volume fraction of nano particles, unsteadiness parameter, exponential parameter, radiation parameter and suction/injection parameter on velocity profiles for fluid phase, dust phase and temperature profiles are discussed and presented through graphs. Also, friction factor and Nusselt numbers are discussed and presented for two dusty nanofluids separately. Comparisons of the present study were made with existing studies under some special assumptions. The present results have an excellent agreement with existing studies. Results indicated that the enhancement in fluid particle interaction increases the heat transfer rate and depreciates the wall friction. Also, radiation parameter has the tendency to increase the temperature profiles of the dusty nanofluid.

Experimental and theoretical study on radiative heat transfer characteristics of dimethyl ether jet diffusion flame

Abstract The radiative heat transfer characteristics of the dimethyl ether (DME)/air jet diffusion flame (JDF) were experimentally and theoretically studied in this paper. A series of fuel nozzle diameters ( d f ), fuel jet velocities ( u f ), and air co-flow velocities ( u co ) were investigated for their influences on the thermal radiation behavior individually. The results showed that the DME jet diffusion flame length was independent of u co , but the flame width decreased exponentially with the increase in u co . Besides, new correlations for the DME jet diffusion flame width were developed in this paper. In the laminar regime, the radiation fraction ( χ R ) was nearly constant with the increase in fuel jet Reynolds number ( Re f ). In the transitional regime, it increased with Re f . In the turbulent regime, it began to decrease gradually with Re f . Additionally, χ R increased gradually with the increase in d f . As u co was increased, χ R decreased because of the reductions in flame width and volume. Empirical correlations between χ R and the operational parameters, including d f , Re f , and u co , were developed in this paper to predict the radiative heat flux (RHF) at any position and incident angle outside the DME JDF, in conjunction with the weighted multi-point source model (WMP model). Besides, at any cylindrical surface around the flame, the RHF peaked at about x = 0.7 L f . At a smaller radius, the axial RHF profile was characterized by larger axial gradients and higher global values. Additionally, the RHF distribution in the near-field ( x / L f R / L f x / L f > 2 or R / L f > 2) of the small-scale ones, self-similarity of the local RHF distribution was invalid. In such cases, the RHF was suggested to be calculated by the WMP model with the atmospheric transmissivity.

From Coal to Natural Gas: Its impact on Kiln Production, Clinker Quality and Emissions

In the North American energy market, natural gas (NG) prices have been gradually decreasing during the past several years, primarily due to advances in shale gas extraction techniques. The availability of cheaper NG, while seen as an attractive short-term fuel switching option, is viewed with caution by most cement plants due to long-term procurement concerns. Also, due to traditionally higher NG prices, cement plants have invested heavily into solid fuels, including storage, grinding, handling, and dosing systems—often achieving high thermal substitution rates (TSRs) of solid alternative fuels and raw materials (AFRs). As a result, a wealth of knowledge has been acquired on firing solid fuels, including some of the more difficult ones, e.g., higher sulfur petcoke and bigger size AFRs, where operational issues such as build-ups, emissions, and production losses have been and are being minimized. Switching to gas firing, however, requires readaptation of combustion and process guidelines for a fuel which, although in principal, is easier to burn, but has relatively lower radiative heat transfer and sharper burning characteristics than coal. As such, the plants, which have switched to NG firing, have observed inconsistent trends in production, energy, and emission performance, mainly due to the lack of sufficient information on combustion/process interactions of the two fuel types required for cost-effective optimization. An NG flame ignites earlier, releases intense heat but lacks dissipation of heat as compared with a solid fuel flame, thereby requires plant specific adjustments. This paper presents actual results of NG firing trials at selected cement plants along with mineral interactive computational fluid dynamics (MI-CFD) predictions, subsequent to validation from the plant data, on four kiln and four calciners. Recommendations are also made to improve and optimize NG firing by taking into considerations of the combustion and mineral interactions.

Radiation effects in three-dimensional flow over a bi-directional exponentially stretching sheet

Analysis is performed to explore the characteristics of non-linear radiation heat transfer in the three-dimensional flow above an exponentially stretching sheet. The temperature is also exponentially distributed across the sheet. Local similarity solutions of the arising differential equations are computed through shooting method with fourth-fifth-order Runge–Kutta integration technique. The results corresponding to different values of temperature exponent parameter A reveal some interesting facts about the temperature distribution. The fluid's temperature is largely influenced with the variations in wall to ambient temperature ratio. The solutions are found in excellent agreement with the existing studies in the literature.

Investigations on Thermal Radiative Characteristics of LPG Combustion: Effect of Alumina Nanoparticles Addition

Use of nanoparticles in thermal applications has attained large interest in recent years because of their improved heat transfer characteristics. The effect of nanoparticle addition on radiative heat transfer characteristics in gaseous fuel combustion has been studied in this research. Radiative heat fluxes (RHFs) are considered to play a very important role in improving the thermal efficiency in furnaces by controlling the heat transfer from the flame to the furnace wall. The current research work is focused on investigating the effect of addition of noncombustible metal oxide (Al2O3) nanoparticles on radiative heat transfer capabilities in liquefied petroleum gas (LPG) combustion. Concentration of nanoparticles in water suspensions was varied up to 3.40 wt% to avoid choking of the feeding capillary. Temperature data inside the furnace was obtained and significantly lower peak temperatures were observed with lower concentration of nanoparticles, and with an increase in concentration, peak temperature also increases. Heat flux data was recorded at the furnace wall, and heat fluxes were significantly influenced by varying the nanoparticle concentration. An overall increase in RHF was observed, with increased contribution of RHF to total heat flux (THF). A higher contribution of RHF led to a substantial increase in the THF.

Assessment of radiative heat transfer characteristics of a combustion mixture in a three-dimensional enclosure using RAD-NETT (with application to a fire resistance test furnace)

Using RAD-NETT, a neural network correlation of the non-gray radiative absorption properties of combustion gases (CO2 and H2O) and soot, the emissivity and hemispherical absorptivity of a combustion mixture to a boundary in a rectangular enclosure is determined. Results show that the both the emissivity and hemispherical absorptivity have a strong dependence on the mixture properties, as well as the medium temperature and wall temperature. The gray assumption with emissivity equal to absorptivity is generally inaccurate. The numerical model is used to analyze temperature and heat transfer data generated from a fire resistance test furnace. Results show that emission and reflection from the wall boundaries have a major effect of the radiative heat flux measurement in a test sample in a fire resistance test. Numerical results also demonstrate that the furnace was operating essentially in an isothermal condition. From the perspective of a compartment fire, numerical data show that soot emission and emission from the wall are essential in the initiation of flashover in a compartment fire.

Combustion of methane/air mixtures in a two-layer porous burner: A comparison of alumina foams, beads, and honeycombs

This study investigates the premixed combustion of methane/air mixtures in different alumina (Al2O3) packings (foams, beads, or honeycombs) based on the almost identical pressure drops for cold flow. A burner was packed with alumina beads with diameter of 3mm in the preheating zone and 10ppi (pore per inch) alumina foams, alumina beads with diameter of 13mm, or 200cpsi (channel per square inch) alumina honeycombs in the combustion zone. The 10ppi foams, 13mm diameter beads, or 200cpsi honeycombs had porosities of 82%, 52% and 80%, respectively. The flame stability limits, flame temperature profiles, flame temperature, pressure drop, and pollution emissions for carbon monoxide (CO), hydrocarbon (HC), and nitric oxide (NOx) for the studied structures are discussed. The flame stability limits are decreased in the order of 10ppi foams, 13mm diameter beads, and 200cpsi honeycombs. The flame temperature was significantly affected by heat release at lower flame speed and by heat loss at higher flame velocity under various flame speeds. At the same flame speed, the flame temperature of the foams was significantly lower compared with those of the packed beads and honeycombs because of the significant radiative heat transfer characteristics of the foams. The pressure drop of the reaction flow was significantly higher than that of the corresponding cold flow because of the significant density change. The CO emission was mainly determined by the flame temperature, whereas the HC emission was mainly controlled by the mixing uniformity of fuel/air. The NOx was very low (below 4ppm) in the three structures because the flame temperature was relatively low (below 1250°C).

CO2 radiation heat loss effects on NOx emissions and combustion instabilities in lean premixed flames

The effects of CO2 dilution on NOx emissions and combustion instability are studied in an effort to use biogas as a fuel in dual lean premixed gas turbines. More specifically, the influence of the radiation heat transfer characteristics of CO2, which is a major component of biogas, on the combustion characteristics of a dual lean premixed flame of biogas is investigated. OH* chemiluminescence images are used to observe the flame structure for various CO2 dilution rates. The results show little difference in the flame structure for CO2 dilution rates of up to 40%, while higher dilution rates lower the flame intensity. Also, dual lean premixed flames show very different flame structures and temperatures depending on the pilot fuel mass fraction. The results show a decrease in the flame temperature when the dilution rate is increased, resulting in a reduction in the thermal NOx emissions. The present paper also shows that the radiation heat release, which is due to the high heat release rate of CO2, promotes a further drop in the flame temperature, in addition to the thermal effects of CO2. These findings are numerically modeled, empirically verified, and added to the NOx prediction model. CO2 dilution also changes the combustion oscillation frequency. To estimate this, the flame temperature is calculated as a function of the CO2 dilution rate and the pilot fuel mass ratio. The accuracy of the flame temperature calculation can be improved with a more accurate estimation of the radiation heat loss. In calculating the radiation heat loss for an unstable flame, considering the heat release fluctuation (i.e. the temperature fluctuation) improves accuracy, whereas taking the average temperature results in an underestimation. With an improved estimation of the temperature, the combustion oscillation frequency is more accurately predicted.

Radiative properties and heat transfer characteristics of fiber-loaded silica aerogel composites for thermal insulation

The radiative properties and heat transfer in fiber-loaded silica aerogel composites were investigated using modified anomalous diffraction theory in a combined heat conduction and radiation model. The randomly parameterized 2-D fiber distribution was generated to simulate a very realistic material structure. The finite volume method was then used to solve a two flux radiation model and the steady-state energy equation to calculate the effective thermal conductivity of the composite. The numerical results provide theoretic guidelines for material designs with optimum parameters, such as the inclination angle, diameter and length-to-diameter ratio of the fibers. The results show that the fiber extinction coefficient increases as the fiber length-to-diameter ratio is reduced or the fiber inclination angle is increased. The effective thermal conductivity of the fiber-loaded aerogel can be reduced by reducing the fiber length-to-diameter ratio and the inclination angle and by moderately increasing the fiber volume fraction. The 4–6μm diameter silicon fibers are optimum for high-temperature thermal insulation.

Computational Fluid Dynamics Modeling of Oxy-Fuel Flames: The Role of Soot and Gas Radiation

This work applies a computational fluid dynamics (CFD) approach to examine gas and soot-related radiation mechanisms in air and oxy-fuel flames operated with propane as fuel. In oxy-fuel combustion, CO2 and H2O replace the N-2 in air combustion. As a result, the radiative heat transfer characteristics differ between the combustion atmospheres. Moreover, changes in soot formation have been observed in oxy-fuel compared to air-fired flames. Both gas- and soot-related radiation can be essential for the design of oxy-fuel furnaces and need to be accounted for when temperature and heat transfer conditions are modeled. The aim of the work is to determine the respective impact of combustion gases and soot in heat transfer modeling of the flames. Both gray and nongray approaches are used to account for the gas radiation and the results are compared to measured data from a 100 kW oxy-fuel unit to investigate if a gray model is sufficient to generate a reliable solution when applied in CFD simulations of oxy-fuel combustion. In addition, calculations of the radiative source term are performed for a domain between two infinite plates, with temperature and concentration profiles from the CFD simulations of the present work. It is shown that the nongray approach accurately predicts the source term in both combustion environments, whereas the gray model fails in predicting the source term. The source term has a direct influence on the temperature field in CFD calculations. However, this work also shows that the inclusion of soot radiation is more critical in sooty air and oxy-fuel flames than the use of a more rigorous description of the radiative properties of the gas.

Comparison of effects on heat transfer and burnout under wet and dry recycle oxyfuel combustion conditions

In this study, the effect of using both wet and dry recycled flue gases on oxyfuel combustion has been investigated using an International Flame Research Foundation (IFRF) burner installed on a 0·5 MWt combustion test facility firing Russian coal and Russian coal/biomass blend under wet and dry recycle conditions. Radiative and convective heat transfer characteristics and burnout measurements were the primary focus of the work. Recycle ratios were varied between 65 and 75%. The peak radiative heat flux decreases with increasing recycle ratio under both wet and dry recycle conditions for both the parent coal and the coal/biomass blend. The opposite trend is observed with the convective heat flux. The peak radiative heat fluxes are higher and the convective heat fluxes are lower for the dry recycle compared to the wet recycle. Low residual carbon in ash levels is measured with oxyfuel under both wet and dry recycle ratios.

Characteristics of radiation and convection heat transfer in indirect near-infrared-ray heating chamber

Numerical study was performed to evaluate the characteristics of combined heat transfer of radiation, conduction and convection in indirect near infrared ray (NIR) heating chamber. The effects of important design parameters such as the shape of heat absorbing cylinder and heat releasing fin on the pressure drop and heat transfer coefficient were analyzed with different Reynolds numbers. The Reynolds numbers were varied from 103 to 3×106, which was defined based on the hydraulic diameter of the heat absorbing cylinder. Analyses were performed to obtain the inner and outer flow and the temperature distributions in the heat absorbing cylinder and the rates of radiation heat transfer and convection heat transfer. As the Reynolds number increases, the convection heat transfer rate is increased while the radiation heat transfer rate is decreased. The average convection heat transfer rate follows a power rule of the Reynolds number. Addition of three-dimensional heat releasing fin to the outside of the heat absorbing cylinder enhances the convection heat transfer.

Experimental investigation into radiative heat transfer characteristics for mould fluxes containing transition oxides

Infrared transmittance of glassy and crystalline mould fluxes was measured using a Fourier transform infrared spectrometer at room temperature. Radiation heat transfer from the steel shell to the mould was calculated by a model. The results indicate that transition metal oxides MnO, FeO and TiO2 have a marked negative effect on infrared transmittance and radiation heat flux of glassy samples. With MnO, FeO and TiO2 added, the reductions of radiation heat flux in glassy samples are 19–25%, 34–36%, 6–29% respectively. X-ray diffraction results indicated that the crystalline phase in transition oxides free samples was mainly Ca4Si2O7F2. After transition oxides MnO, FeO and TiO2 added, Mn2SiO4, Fe2SiO4, CaTiO3, Ca2SiO4 and other minor phases were also precipitated in mould fluxes. On account of strong refraction and scattering, the negative effect on radiation heat flux in crystalline samples was much larger than that in the glassy ones.

Investigations on the Characteristics of Radiative Heat Transfer in Liquid Pool Fires

This work numerically investigates the radiative flux and flame shape from heptane liquid pool fires with small to medium sized pool fires (14–38 cm), grid sizes, spectral bands and solid angles. Experimental results were originated for comparison. The results show that calculated flame shapes and radiative fluxes are in good agreement with the measurements for suitable grid resolution. Besides, the effect of the intermittency of a flame on thermal radiation was performed. Experimentally, a shield was used to shade the radiant power from a persistent flame, and the radiative flux was measured from an intermittent flame on 30 cm diameter pool fires. The results show that 36% of the radiative flux measured is emitted from the intermittent flame when the radiometer is located at the base of the pool fire. As the radiometer is moved upwards, the radiative fluxes measured from the intermittent flame increase gradually, even to 50%.

A Numerical Study on Nonequilibrium Heat Transfer and Crater Formation in Thin Metal Films Irradiated by Femtosecond Pulse Laser

The ultimate goals of this study are to investigate numerically nonequilibrium energy transfer between electrons and phonons, and to predict the crater formation shapes of gold thin film structures irradiated by femtosecond pulse lasers. In particular, the present article expands the onedimensional two-temperature model (1DTTM) to the two-dimensional model (2DTTM) considering wave interference, and it involves the quantum effect to predict thermal and optical properties. The predictions by using 2DTTM are extensively compared with those of 1DTTM, and the influence of film thickness on radiation heat transfer and optical characteristics are also examined. From the results, it is found that the predictions of 2DTTM are in good agreement with those of 1DTTM. As the gold film thickness decreases, the reflectivity decreases dramatically and the absorbed laser intensities at the top surface increases substantially because of wave interference in thin films.

In-Line Oxygen Sensors for the Glass Melt and the Float Bath

In-line oxygen sensors have been developed for the glass melt and for the float bath. Glass melt oxygen sensors are used for the continuous monitoring of the oxidation state (or redox) of the glass melt and are very important for the control of many glass melt and glass product properties such as radiative heat transfer, fining, foaming, forming and optical characteristics. Too high oxygen levels in the float bath can be prevented by using both oxygen sensors in the tin melt and the atmosphere above it. Oxygen related defects on the glass sheet surface such as dross, tin pick-up, bloom and tin drips are reduced or even prevented. Moreover, expensive hydrogen gas can be saved by a more effective dosage.

Wave Interference Effect in Thin Film Structures under Pulsed Laser Irradiation

The present article conducts extensive numerical simulations to investigate conduction and radiation heat transfer characteristics in thin silicon layers irradiated by pulsed lasers. The two temperature model is used for estimating carrier and lattice temperatures during laser irradiation. The energy absorption in thin films is predicted by the electromagnetic theory, including wave interference effects through thin film optics. The present study predicts the carrier and lattice temperatures during laser irradiation and examines the influence of film thickness and laser pulse duration on characteristics of energy transport. It is observed that, unlike bulk materials, the variation of the film thickness causes significant changes in the reflectivity of silicon film due to wave interference effects in thin film structures. The maximum value of the reflectivity is estimated to be about seven times larger than the minimum value. For the spatial distributions of carrier and lattice temperatures, it is found that a periodic tendency appears for picosecond pulse because of the difference between the pulse duration and the time for energy diffusion. It indicates that the traditional usage of Beer’s law is not appropriate for prediction of radiation heat transfer in thin film structures.