Wall Surface Emissivity Research Articles

LBM simulation for combined thermal radiation and natural convection in 2D enclosure with multiple solid blocks

A numerical investigation of conjugate natural convection in a 2D square enclosure with multiple solid blocks, considering the impact of volumetric thermal radiation, was conducted using the lattice Boltzmann method (LBM). The effect of Planck number wall surface emissivity extinction coefficient solid-to-fluid volume fraction and the number of solid blocks on the fluid-structure and heat transfer within the enclosure were analyzed. The numerical model was validated using benchmark problems from the literature, showing good agreement. The results revealed that at and increasing wall surface emissivity significantly increased the Nusselt number, ranging from approximately 51-116% for different wall emissivity values, as compared to scenarios without surface radiation (). On the other hand, reducing χ, Pl, and β improved the heat transfer rate. The results also demonstrated that subdividing the initial block reduced the heat transfer rate depending on the number of blocks and the value of the Planck number. For instance, when 64 solid blocks were present in the cavity and the Nusselt number decreased by 59.36% compared to the case with a lower solid-to-fluid volume fraction (χ = 15%). The parametric analysis provided useful insights for engineering design in the modern construction industry, with possible applications in building block design and energy efficiency enhancements.

Simulation and performance improvement of an industrial steam methane reformer: depreciation and ceramic coating effects

In this work, Computational Fluid Dynamic (CFD) is applied to compare the performance of an industrial reformer furnace in four cases. The first and tenth years of operation are two cases with different emissivity factors and fuel components. The results are validated with industrial data and with other CFD simulation typical plants reported in the SMR literature. The results show that a 10% increase in fuel consumption in the tenth year cannot compensate for all temperature drop in skin tubes, and there is still a 14 K temperature drop, leading to a 5% decrease in hydrogen production in tubes. This is due to the different fuel components of the tenth year compared to the first year. To examine the effect of fuel change more closely, the third case is defined with the fuel components of the tenth year and the emissivity factor of the first year. The comparison of this case with others shows that fuel components have a high effect on system performance. The major reason for efficiency reduction between the first and tenth years correlates to a 50% decline in the wall surface emissivity factor. Finally, in the fourth case, applying a ceramic coating with a high emissivity factor is considered via the CFD model for the reformer in the tenth year. This change leads to an increase of about 19 K in tube temperature in the tenth year, which is 3 K more than that in the first year. It can be concluded that the ceramic coating application in the wall of the refractory of the reformer can reduce 14% fuel consumption and enhance hydrogen production.

Thermal radiation and natural convection in a large-scale enclosure heated from below: Building application

A computational research of radiative-convective energy transport in large-scale enclosure with a heat-generating heater under normal room conditions has been conducted. The heater (underfloor heating system) is located at the bottom of the room. Employing the Boussinesq assumption, the control equations have been solved contemporaneously to receive both the velocity fields and temperature patterns. To generate the systems of linear equations using vorticity and stream function, the finite difference technique has been employed. The developed convective-radiative model has been validated through a comparison with several problems. The influence of heater size and location, internal surfaces emissivity from 0 to 1, Ostrogradsky number for a wide range from 0 to 5 on Nusselt numbers and both stream function and temperature distributions has been investigated. The results demonstrate that the influence of the thermal radiation on total heat transfer increases with surface emissivity of walls and heater surfaces.

Mixed convection and radiation studies on thermally developing laminar flow in a horizontal square channel with variable side heated wall

In this work presents the interaction of mixed convection and surface radiation heat transfer studies on thermally developing laminar airflow in a differentially heated horizontal channel with isothermal vertical walls and adiabatic top and bottom walls. The side heated isothermal wall incorporated a heater, and the opposite wall acts as a constant temperature heat source. The major thermal and geometric parameter includes variable wall temperatures, wall surface emissivity, Reynolds number, heat flux and air chosen as the working medium. Due to the wall surface emissivity of the channel, the internal wall surface radiation will significantly change the rate of total heat transfer inside the channel. Therefore, the current research focuses on the mixed convective and surface radiation heat transfer process happening in laminar flow over a horizontal channel. Radiative heat transfer is determined with the help of experimental results and the use of the numerical scheme. The results presented here show the rate of convection heat transfer and the distribution of surface temperature along the walls inside the channel. The flow configuration the channel inside is pictured by passing smoke at the bottom of the entrance section. The quantitative and qualitative results showed that the emissivity of walls and other thermal and geometric parameters significantly affected the overall heat transfer improvement from the square channel.

Heat transfer and fluid flow analysis of porous medium solar thermochemical reactor with quartz glass cover

In this paper, heat transfer and fluid flow of porous media solar thermochemical receiver with quartz glass cover were investigated. The Surface-to-surface radiation model and Rosseland approximation for radiation heat transfer were adopted for the transport of diffused solar irradiance and radiative transfer in the fluid phase and porous medium. An experimental test was conducted on a laboratory-scale solar thermochemical reactor. The effects of structural parameters in term of diffused irradiance intensity, the mass flow rate, heat transfer coefficient, quartz glass and inner cavity wall surface emissivity, the porosity and extinction coefficient that could affect heat transfer and fluid flow performance of the proposed solar cavity receiver were sufficiently investigated. It was found that the substantial drops in temperature were mainly attributed to the thermal losses by radiative, convective and conductive heat transfer. The numerical results are compared with the experimental data for the model validation. The thermal loss at the solar flux inlet of the receiver was obviously inevitable due to the stronger effect of heat transfer coefficient that altered the over increasing temperature and heat flux at the surface of diffuse irradiance. However, the use of optimum pore size and higher porosity material could significantly enhance the thermal performance of porous media solar thermochemical reactor.

Numerical study of surface radiation and combined natural convection heat transfer in a solar cavity receiver

PurposeThe purpose of this paper is to present the numerical solutions of surface radiation and combined natural convection heat transfer in a solar cavity receiver. The paper aims to discuss sundry issues that take place in the said model.Design/methodology/approachThe numerical solutions are developed by means of second-order upwind scheme using the SIMPLE algorithm.FindingsThe effects of physical factors such as Rayleigh number (104 ≤ Ra ≤ 106), inclination angels of insulated walls (0º ≤ θ ≤ 10º) and the wall surface emissivity (0 ≤ ε ≤ 1) on natural convection-surface radiation heat transfer rate are analyzed. Impact of sundry parameters on flow quantities are discussed and displayed via graphs and tables. Stream lines and isothermal lines have also been drawn in the region of cavity. The numerical results reveal that increasing the Rayleigh number, wall surface emissivity and inclination angels of insulated walls in an open cavity enhances the mean total Nusselt number. The variations of the surface radiation and natural convection heat transfer mean Nusselt numbers are very small to the inclination angle of θ, while a significant change is noted for the case of Rayleigh number and emissivity.Originality/valueTo the best of authors’ knowledge, this model is reported for the first time.

Numerical and experimental investigation of multi-mode heat transfer in a square cavity with and without triangular fins

Numerical and experimental studies on laminar natural convection and radiation in a differentially heated cavity with/without triangular fins are presented. The square cavity is filled with air (Pr = 0.71). The top and bottom walls are adiabatic, while the active (hot) and cold walls are isothermal. Triangular conductive fins made of highly conductive material are placed on the heated and cooled vertical surfaces of a square cavity. The effects of pertinent parameters on fluid flow and heat transfer characteristics are studied. The governing differential equations, assuming laminar flow, are solved using the commercial software FLUENT. Experiments have been carried out for three different configurations viz square cavity without fins, square cavity with two fins on vertical walls and square cavity with continuous fins on vertical walls. Rayleigh number varies from 1.18 × 105 to 2.15 × 105 for the first experiment (square cavity without fins). The other two experiments with fins have been conducted for six different heat inputs. The numerical results have been validated against the experimental results obtained. It is concluded that the triangular fins on isothermal walls enhance the heat transfer in the cavity. Natural convection is influenced by wall surface emissivity. The square cavity with continuous fins is more effective than square cavity with two fins on vertical isothermal walls.

Numerical investigation and sensitivity analysis of effective parameters on combined heat transfer performance in a porous solar cavity receiver by response surface methodology

A numerical study on natural convection along with surface radiation heat transfer in an inclined porous solar cavity receiver by means of response surface methodology (RSM) is investigated. Discretization of the governing equations is first performed using a finite volume method and then solved with SIMPLE algorithm. Effects of physical parameters such as Rayleigh number (104⩽Ra⩽106), Darcy number (10−5⩽Da⩽103), inclination angles (0°⩽θ⩽90°), dimensionless porous substrate thickness (1/3⩽δ⩽1), wall surface emissivity (0⩽ε⩽1) and surface radiation heat transfer rate are examined. The sensitivity analysis is also presented. It is found that the sensitivity of mean Nusselt number increases by increasing Rayleigh number, Darcy number and inclination angle θ whereas the sensitivity of mean Nusselt number of natural convection heat transfer to porous substrate thickness δ declines. Consequently increase in the thickness of porous substrate reduces mean Nusselt number of natural convection heat transfer. Also, the sensitivity of mean Nusselt number of natural convection heat transfer to Rayleigh number Ra, porous substrate thickness δ, Darcy number Da and the inclination angle θ is found to be higher than the mean Nusselt number of radiation heat transfer. However, the sensitivity of mean Nusselt number of radiation heat transfer to the surface emissivity ε is higher than the mean Nusselt number of natural convection.

Two phase simulation and sensitivity analysis of effective parameters on combined heat transfer and pressure drop in a solar heat exchanger filled with nanofluid by RSM

In this paper, a 2-D numerical investigation and a sensitivity analysis have been done utilizing two phase mixture model on the combined mixed convection and surface radiation heat transfer in a solar heat exchanger. To serve the purpose, the effects of four parameters, the Richardson number (10−2≤Ri≤102), the volume fraction of nanoparticles (0.00≤ϕ≤0.04), nanoparticles diameter (40nm≤dp≤60nm) and wall surface emissivity (0≤ε≤1), are taken into account. Discretization of the governing equations is performed using a finite volume method and are solved with SIMPLE algorithm. The effective parameters analysis is processed utilizing the Response Surface Methodology (RSM). It is found that the mean total Nusselt number increases by decreasing and increasing of Ri number and ϕ, dp and ε respectively. Also, decreasing of Ri number and ϕ and increasing of dp and ε cause an increase in the pressure drop ratio. The sensitivity of the mean total Nusselt number to ϕ, dp and ε is observed positive whereas negative sensitivity of the pressure drop ratio to ϕ and ε. The sensitivity of the pressure drop ratio to dp is also found positive. The effects of physical parameters are illustrated graphically.

Two phase simulation and sensitivity analysis of effective parameters on turbulent combined heat transfer and pressure drop in a solar heat exchanger filled with nanofluid by Response Surface Methodology

In this paper, a 2-D numerical investigation and a sensitivity analysis of combined turbulent mixed convection and radiation heat transfer have been done using two phase mixture model in a solar heat exchanger. Numerical simulations are carried out to study the effects of the four parameters, the Richardson number (10−2 ≤ Ri ≤ 102), volume fraction of nanoparticles (0.00 ≤ ϕ ≤ 0.04), nanoparticles diameter (40 nm ≤ dp ≤ 60 nm) and the wall surface emissivity (0 ≤ ε ≤ 1). The effective parameters analysis is processed utilizing the RSM (Response Surface Methodology). It is found that decreasing the Ri number and ϕ and also increasing in ε and dp increases the mean total Nusselt number. Due to a decrease in Ri number and increase in ϕ, ε and dp the pressure drop ratio enhances. The sensitivity of the mean total Nusselt number and pressure drop ratio to Ri number is negative. The sensitivity of the mean total Nusselt number to dp and ε is positive but to ϕ is negative. Moreover, the sensitivity of the mean Nusselt number to Ri number, ϕ, dp and ε is more than the sensitivity of the pressure drop ratio to these parameters.

Role of Prandtl number in the interaction phenomenon of surface radiation with an opposing mixed convection within a differential heated cavity

AbstractA numerical analysis has been done for opposing mixed convection resulting due to wall movement and buoyancy induced by a clockwise fluid motion in a differentially heated cavity. The effect of Prandtl number (Pr) and wall surface emissivity (ϵ) has been investigated for different values of Richardson number. The net radiation method has been employed to simulate the effect of surface radiation. The energy equation along with its nonlinear boundary condition is treated with the Newton‐Raphson scheme. As momentum and energy equations are coupled with each other through their source terms, an iterative solution procedure is employed. A vorticity‐stream function formulation of the momentum equation has been adopted and solved by using an underrelaxation parameter of 0.45. The effect of Prandtl number with respect to the transformation of a multi‐cellular structure of streamline into a unicellular structure has been analyzed. For the same Richardson number (Ri) with an increase in Prandtl number, the flow and heat transfer phenomena changes from a buoyancy‐induced dominated flow to a shear‐induced dominated flow, which leads to some exciting results with respect to wall movement as well. Furthermore, the role of surface radiation in this respect has been emphasized. Nusselt number variation with the Prandtl number and surface emissivity has also been presented. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21003

Conjugate Thermal Analysis of Air-Cooled Discrete Flush-Mounted Heat Sources in a Horizontal Channel

Thermal analysis with comprehensive treatment of conjugate heat transfer is performed in this study for discrete flush-mounted heat sources in a horizontal channel cooled by air. The numerical model accounts for mixed convection, radiative exchange and two-dimensional conduction in the substrate. The model is first used to simulate available experimental work to demonstrate its accuracy and practical utility. A parametric study is then undertaken to assess the effects of Reynolds number, surface emissivity of walls and heat sources, as well as thickness and thermal conductivity of substrate, on flow field and heat transfer characteristics. It is shown that due to radiative heat transfer, the wall temperatures are brought closer, and the trend of temperature variation along the top wall is significantly altered. Such effects are more pronounced for higher surface emissivity and/or lower Reynolds numbers. The influence of substrate conductivity and thickness is related in that a large value of either substrate conductivity or thickness facilitates redistribution of heat and tends to yield a uniform temperature field in the substrate. For highly conductive or thick substrate, the “hot spot” cools down and may occur in upstream sources. Radiation loss to the ambient increases with substrate conductivity and thickness due to the elevated temperature near the openings, yet the total heat transfer over the bottom surface by convection and radiation remains essentially unaltered.

The effect of surface roughness and emissivity on radiator output

The effect of altering the emissivity and the roughness of a wall behind a radiator on the radiator heat output has been studied experimentally and by using computational fluid dynamics. The results of a 3D RNG k– ɛ turbulent model agree well with, and have the same trend as, the experimental results. The results indicate that the presence of large scale surface roughness and a high emissivity surface increases both the heat flow rate and the air velocity behind the radiator compared to a smooth shiny surface. The former increases the wall surface emissivity which causes the surface temperature of the wall to increase, effectively creating additional convective heat transfer surface. The surface roughness will increase both the surface area for heat transfer and the turbulent intensity which increase the mass transfer and free convective heat flux through the air gap. The results indicate that the heat transfer can be increased by about 26% through the use of a high emissivity saw-tooth surface compared to a smooth shiny one. This means that using a wall surface with high roughness and emissivity behind the radiator will increase the heat output from the radiator.

High Heat-Flux Sensor Calibration: A Monte Carlo Modeling

Conventional calibration of heat-flux sensors uses high-temperature blackbody radiation and places the sensors away from the blackbody aperture. This approach limits the achievable calibration heat flux to about 50 kW/m 2 . Recent interest in extension of the calibration to higher heat-flux levels requires placement the sensors inside the heated cavity under nearly hemispherical irradiation environment. The incident flux at the sensor location depends on the effective emissivity, which is a function of the combined cavity and sensor geometry, and the properties of the radiating surfaces. A scheme is presented to compute the effective emissivity for such measurement schemes by the use of the Monte Carlo technique. Typical results presented demonstrate the influence of the cavity wall surface emissivity and diffusity, nonuniform temperature distributions, and the sensor location on the calculated effective emissivity

A simplified design tool for composite Trombe-Michel wall solar collectors

A simplified design tool for composite Trombe-Michel wall solar collectors has been developed. The relations have been derived using the hourly thermal model PASOPT 2.0 and the hourly climatic data for Victoria (British Columbia), London (Ontario) and Moncton (New Brunswick). Various parameters were: load to collector ratio LCR=1.2, 3.5, 6.0, 10.0, 16.0, 50.0 W/m 2 K; massive wall thickness L =0.05, 0.15, 0.30, 0.46 m; massive wall conductivity k w =0.433, 0.865, 1.730 W/m K; massive wall surface emissivity ϵ =0.1, 0.9; and an insulating wall thickness of 0.05 m polyurethane with a thermal conductivity of 0.023 W/m K. As a result, 24 correlations in the form of SSF monthly =f(SLR monthly ) have been derived. In order to compare the thermal performance of the composite systems with those of the vented and unvented Trombe-Michel wall solar collectors, other parameters have been taken the same as in the Los Alamos Passive Solar Design Handbook. The results indicate that: (1) the accuracy of the correlations derived is satisfactory; (2) the design tool for the composite systems can be used together with the others developed for the vented and unvented wall systems; (3) a performance comparison indicates that the composite system performs better than vented and unvented systems in cold climates.