Surface Radiation Heat Transfer Research Articles

Determining energy, exergy and enviroeconomic analysis of stand-alone photovoltaic panel under harsh environment condition: Antarctica Horseshoe-Island cases

It is considered that some of the energy needs of the Turkish Science Base, which will be established on the Antarctic Horseshoe Island, will be supplied from solar energy. In this study, the performance of the PV panel was experimentally tested on the Antarctic Horseshoe Island. Energy, exergy and enviroeconomic analysis of the PV panel were performed. In the experiments carried out for three days, the energy efficiency was determined as 5.40%, 7.39% and 11.35%, respectively. The exergy efficiencies were calculated as 4.53%, 6.58% and 10.25%, respectively. At the end of three experiments, the amount of CO2 blocked to the atmosphere was determined as 3.06 kg on average. The study used meteorological parameters (solar radiation, wind speed, humidity, and ambient air temperature) and other data (current-voltage, panel surface temperature, convection and radiation heat transfer coefficients, sky temperature, total heat transfer coefficient, total heat loss and exergy destruction) that can only be calculated using these variables to estimate the electrical efficiency of the PV module. This was achieved using the multilayer perceptron and decision tree method. The process of approximating the values estimated by statistical evaluations to the values calculated experimentally for energy efficiency was found to have error rates of 5.6% and 12.5%, respectively.

Hierarchical hybrid heat sink material for thermo-electric generators

This short communication focuses on the impact of enhancing heat sink effectiveness of thermoelectric generators (TEG) by attaching covalently bonded carpet-like arrays of carbon nanotubes on prefabricated copper structures. Structural hierarchy of heat sinks are important for synergizing the thermal conductivity of materials with fin architectures tailored for active or passive electronic cooling components. We investigated strategic integration of carefully aligned carpets of carbon nanotubes (CNTs) on heat sink fins to simultaneously enhance conductive, convective, and radiative heat transfer pathways. Carbon nanotubes (CNT) combine ultra-high thermal conductivity along their axial directions with high surface area and radiative heat transfer coefficients. While CNTs are known to enhance nanofluidic coolants and thermal interface composites, their full potential can only be utilized if they are aligned normal to heat sink fins, and strongly anchored to retain the fin/radiator architecture in service conditions. This study focused on measuring the direct impact of CNT carpet on TEG performance under ambient passive cooling conditions without adding any noticeable weight or volume to the heat sink. It was seen that this single step of CNT attachment on the surface could increase the open circuit voltage of TEG modules by over 50% in the passive mode with respect to original untreated heat sink, and over 100% with respect to no heat sink. This improvement may be enhanced even further by introducing forced coolants. These results therefore show significant promise in using the surface nanocarpet approach for development of compact, lightweight, robust and reusable solid-state heat sinks in future.

Pore-scale Numerical Simulations on the Lean Concentration Methane Combustion in Porous Medium with Gyroid Triply Periodic Minimal Surfaces Structures

ABSTRACT In this work, a series of porous medium with Gyroid Triply Periodic Minimal Surfaces (G-TPMS) structures were established for lean concentration methane combustion. Based on pore-scale numerical simulations, the influence of TPMS parameters on porous media combustions, as well as the shape of flame surface in different TPMS structures, heat transfer characteristics between solids and fluids, the radiation characteristics and heat flux on the solid surface were investigated. Results show that, under the same inlet flow rate and equivalence ratio, the value and axial location of the maximum average fluid temperature will be changed due to the difference in porosity and flow resistance of each structure. For the G-TPMS structures’ porous medium, the change of structure period had a greater impact on the combustion compared with the wall thickness. And, in the G-TPMS structures’ porous medium, the flame surface always shows a finger structure. The flow rate that shows the same increase trend with the fluid temperature in the preheating region, which has a periodic change like the porosity in the axial direction. Based on the results, it can be seen that the region of solid-to-solid surface radiative heat transfer and fluid-solid heat flux on the interface was also changed due to the influence of flame front position.

Natural Convection and Radiative Heat Transfer From Constant Surface Area Vertical Helical Coils: Effect of Pitch and Height

Abstract In this work, numerical simulations are carried out to delineate the natural convection and surface radiation heat transfer characteristics of vertically oriented isothermal helical coils having a constant surface area. Numerical computations using the finite-volume method are carried out in the laminar regime for the following non-dimensional parameter ranges: Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity of the coil (0 ≤ ɛ ≤ 1), pitch to the rod-diameter of the coil (3 ≤ p/d ≤ 7.5), and coil-height to the rod-diameter (40 ≤ H/d ≤ 60). Temperature-dependent fluid properties have been implemented to obtain accurate results. The impact of Ra and ɛ on both convective and radiative heat losses is discussed in detail. At a high Ra of 108, when H/d varies from 40 to 60, the mass flowrate inducted through the coil reduces from 40.6% at p/d = 3 to 11.4% at p/d = 7.5. As a result, the relative strength of convection heat loss declines with a rise in H/d. For a higher emissivity of the coil surface of 0.9 and a lower Ra of 104, heat transfer by convection contributes only 12.66% of the total heat transfer. In contrast, the contribution of radiative heat transfer is only 7.46% for a lower emissivity of the coil surface of 0.1 and a higher Ra of 108.

Three-dimensional pore-scale simulation of non-equilibrium in a two-layer porous burner

A pore-scale numerical simulation for combustion of methane-air mixtures in a two-layer porous media burner made of staggered arrangement of connected particles is preformed. Combustion is modeled by a one-step reaction mechanism, and solid surface radiation heat transfer is computed by discrete ordinates (DO) model. The bridge approach is used to deal with the point contact between adjacent particles. The present study is intended to investigate the non-equilibrium characteristics of gas velocity and temperature under two operating parameters such as the inlet velocity and equivalence ratio. A comparison of the numerical values and experimental values was carried out to prove the effectiveness of numerical model, and a good agreement is shown. The results showed considerable non-equilibrium of velocity and temperature along the space coordinate due to the inhomogeneous porous structure. The extent of flow non-equilibrium increases as inlet velocity and equivalence ratio are increased. The extent of gas temperature non-equilibrium decreases slightly with the inlet velocity, but the equivalence ratio effect on thermal non-equilibrium can be neglected.

Buoyancy-Driven Mixed Convection and Radiation Heat Transfer in a Tilted Vented Cavity

Abstract Experimental and numerical studies have been carried out to determine the impact of tilt angle on buoyancy-driven mixed convection coupled to surface radiation heat transfer inside a vented cavity. The present investigation might be rationalized in the thermal design of the electronic equipment housing, solar collector receiver, regenerative heating/cooling system, etc. Keeping the constant ventilation size and its location, the tilt angle measured from horizontal is varied from 0 deg to 90 deg with the increment of 15 deg. The experimental investigation comprising planner symmetry is complemented with the two-dimensional numerical study incorporating the range of Rayleigh number from 7.21 × 108 to 1.75 × 109, Reynolds number from 665 to 9931, Richardson number from 20.1 to 2283.4, and the surface emissivity from 0.05 to 0.85. The results signify that the convective Nusselt number enhances significantly at the optimum tilt angle of 90 deg, whereas the moderate increment in radiative Nusselt number is found at the optimum tilt angle of 0 deg. Based on the obtained experimental data, the empirical correlations of the average convective Nusselt number and dimensionless temperature are developed in terms of Reynolds number, Richardson number, and tilt angle.

Hot filament chemical vapor deposition temperature field optimization for diamond films deposited on silicon nitride substrates

The influence of some key parameters of hot filament chemical vapor deposition (HFCVD) on the temperature distribution during the deposition of diamond coatings on silicon nitride (Si3N4) substrates was assessed with the help of the finite element method. Solid heat transfer, fluid heat transfer and surface radiation heat transfer mechanisms were used to calculate the substrate temperature in the steady state during the deposition process. The accuracy of the model was verified by comparing the simulation model with experimental measurements. The comparison shows that the deviation between the model and the actual substrate temperature measurements is within 3%. Furthermore, a Taguchi orthogonal experiment was designed (3 factors, 3 levels, L9). By changing the number of hot filaments, the distance between the filaments and the substrate, and the separation between two adjacent hot filaments, the influence trend of these parameters on the substrate temperature was assessed, leading to an optimal hot filament arrangement. A deposition experiment was carried out using the optimized parameters, and the results showed that the substrate surface temperature obtained by numerical simulation is highly consistent with the temperature measured by the infrared thermometer. The optimized deposition parameters contributed to a more suitable temperature range and more uniform temperature distribution on the Si3N4 ceramic substrate. The deposited diamond film exhibited uniform crystal quality and grain morphology, thus verifying the validity of the simulation results.

Experimental and numerical study of mixed convection with surface radiation heat transfer in an air-filled ventilated cavity

The present study deals with the experimental and numerical investigation of steady combined laminar mixed convection and surface radiation heat transfer in the air-filled enclosure subjected to a constant heat flux supplied to the vent side of the cavity. Experimental results are presented for heat inputs, Q = 3.19 W–16.71 W, Reynolds number, Re = 4814–166385, and Richardson number, Ri = 0.1–25. In the present study, the numerical simulation is also performed in the two–dimensional vented cavity to enrich the experimental work for aspect ratio, A = 1–5, and surface emissivity, ε = 0.05–0.85. The momentum and energy equations under Boussinesq approximations are solved using a well-known finite-volume method, and numerical code is developed in Fortan 90 for getting all numerical results. Based on the experimental data, correlations are developed for the Nusselt number and maximum temperature of the heated surface.

Non-linear boundary conditions for the convection-diffusion equation in lattice Boltzmann framework

Implementation of nonlinear boundary conditions like nth-order surface reactions or surface radiation heat transfer has not been investigated in lattice Boltzmann (LB) framework. This study presents a novel kinetic level method for their implementation. The method couples Taylor expansion of the conditions with counter-slip approach to find unknown distribution functions at boundary nodes. The proposed scheme guarantees the locality and orientation independency of formulations. To evaluate the proposed scheme performance, several 1D and 2D test cases were simulated by D2Q9 LBM and the outcomes were compared with analytical and numerical solutions. The geometry evolution by nth-order surface reaction was investigated for dissolutions in a simple fracture and a spherical carbonate particle in a channel. The results demonstrated the method performs promisingly in terms of accuracy. The convergence rate of scheme based on the results from l2-norm analyses showed first or second-order rates of convergence, depending on constraint’s degree of nonlinearity.

Natural Convection and Surface Radiation Heat Transfer in a Square Cavity with an Inner Wavy Body

An in-house CFD code has been developed to address the coupling between natural convection and surface radiation in irregular geometries encountered in cooling electronic components. In the current numerical study, the discrete ordinate methods with the concept of blocked-off region are used in order to investigate the conjugate natural convection and surface radiation in an enclosure containing a wavy inner circular cylinder. The validity of the numerical code is ascertained by comparing our results with previously published results dealing with the limiting case of a smooth circular cylinder enclosed in a cavity. Extensive computations have been performed to study in detail the effect of the Rayleigh number ( $$10^{3} \le {\text{Ra}} \le 10^{7}$$ ), the inner and outer surface emissivities ( $$0 \le \upvarepsilon_{{{\text{E}}\,{\text{or I}}}} \le 1$$ ), the undulations number ( $$4 \le {\text{N}} \le 1 2$$ , their amplitude ( $$0 \le {\upalpha} \le 1$$ ) and the average radius of the wavy circular cylinder ( $$0.05 \le {\text{R}}_{ 0} \le 0.25$$ ) on the flow structure as well as on the convective and radiative heat transfer characteristics. The obtained results showed that the amplitude and the average radius have more significant effect on the total heat transfer rate than the undulations number of the wavy wall.

Experimental and numerical studies for applying hybrid solar chimney and photovoltaic system to the solar-assisted air cleaning system

Under the background of global energy shortage and environment deterioration, researchers of the world pay high attention to develop renewable energy. This paper proposes a hybrid solar chimney and photovoltaic system for the novel solar-assisted air cleaning system. Small-scale laboratory setups are designed and fabricated. Experimental results reveal that replacing 50.60% acrylic glass of the collector top with photovoltaic panels will reduce the thermal air flow rate only by 14%, but generate significant electric power output. A three-dimensional numerical simulation model is established and validated by experimental results (air flow rate, temperature distribution). The model includes solar ray tracing model, surface to surface radiation model, buoyancy-driven flow and heat transfer model and power generation model. Then, this model is used to predict a large-scale system based on the Manzanares pilot power plant in Spain. For the large-scale system, the electrical energy generated by the photovoltaic panels can be used to drive suction fans to increase air input. Covering the entire top surface of the collector by photovoltaic panels (113-meter-wide), the total air flow rate would increase to 2.21 times compared with the system without photovoltaic panels. And setting photovoltaic panels on the collector bottom with 113-meter-wide, the total air flow rate would increase to 2.42 times. Thus, adding photovoltaic panels for the collector can greatly improve the utilization of solar energy. It can increase the amount of air purification, or reduce land requirement for the same flow rate.

Discharge duration limits of contemporary tokamaks and stellarators

The realization of long-time discharges in magnetic confinement devices (MCDs), namely, in tokamaks and stellarators, is a key issue in the development of controlled fusion energy. Experiments demonstrate that the discharge duration achieved in contemporary MCDs is limited and the normal operation is terminated after 0.3 to 3600 s depending on plasma, discharge and device parameters. This paper is devoted to the analysis of physical mechanisms which may be responsible for the discharge duration limit in MCD operation. The impact of heat transfers to plasma facing components from the plasma, coolants and thermal radiation is evaluated and compared with the available experimental database. The critical temperature Tcr ≅ 2300 K of plasma facing components is considered as the key parameter that limits the discharge duration. The regimes of the first wall temperature growth governed by both the heat conductivity and heat capacity are identified experimentally and analytically. Heat removal from wetted areas by means of thermal surface radiation and linear heat transfer to the coolant is identified as the key physical mechanism that determines the boundary of time-limited discharges in MCDs. The principal role of localized wetted areas with a size of ≅0.2–0.6 m2 is revealed for operation of contemporary devices. This means that in further development of fusion reactors major attention should be devoted to the organization of a more uniform and distributed heat exhaust. It is shown that the proposed semi-analytical approach explains the experimentally discovered trends in the MCD operation and may be used for the evaluation of the discharge duration limit of new facilities designed to obtain steady-state discharges.

Natural convection and surface radiation heat transfer in a cavity with vertically oriented fins

In this paper, the finite volume method is combined with the discrete ordinate method to study the natural convection coupled to surface radiation in a cooled square finned cavity. The fin array under consideration is made of N thin fins arranged in a single horizontal row. For better heat transfer performances, all these fins are oriented vertically inside the cavity and heated isothermally at 299K≤TH≤341K. The cavity is filled with air (Pr=0.71) and cooled by the side vertical walls maintained at a cold temperature TC=293K. The study investigates the effect of the Rayleigh number ranging from 103 to 107, the fins number 1≤N≤15 and the emissivity 0≤ε≤1. The results are reported in terms of streamlines, isotherms and the local and the average convective and radiative Nusselt numbers. The numerical work is carried out using an in-house CFD code written in FORTRAN. The obtained results show a considerable effect of the surface radiation on the flow structure in the cavity and the maximum rate of heat transfer is reached when the fins number exceeds 9.

Numerical Investigation of Coupled Surface Radiation and Natural Convection in a Triangular Shaped Roof (Gabel Roof) under Winter Conditions

In the building, roof is a major element contributing to the space thermal load. Due to its importance, this component has been widely studies in the literature and under various climatic conditions. In this paper, a numerical study was carried out for the coupling of natural convection and surface radiation heat transfer in a triangular shaped roof with eave (Gabel roof) for cold climates. The numerical solution is obtained using a finite volume method based on the SIMPLER algorithm for the treatment of velocity-pressure coupling. Concerning the radiation exchange, the working fluid (air) is assumed to be transparent, so only the solid surfaces (assumed diffuse-grey) give a contribute to such exchange. Governing parameters on heat transfer and flow fields are Rayleigh number (Ra), aspect ratio (A) and eave lengths (e*). Numerical results are obtained to display the isotherms, streamlines and the heat transfer rate in terms of local and average Nusselt numbers. We found that the production of several circular cells is proportional to the decrease of aspect ratio and the increase of Rayleigh number. In addition, the heat transfer is much more pronounced in the presence of thermal radiation.

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.

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.

Combined Effect of Mixed Convection and Surface Radiation Heat Transfer for Thermally Developing Flow in Vertical Channels

ABSTRACTCombined effect of laminar flow mixed convection and surface radiation heat transfer for thermally developing airflow in a vertical channel heated from a side has been experimentally examined with different thermal and geometric parameters. The channel boundary is made of two isothermal walls and two adiabatic walls, the isothermal parallel wall is heated uniformly and the opposite cold wall temperature is maintained equal to the inlet conditions. The heated wall temperature ranged from 55 to 100°C, Reynolds number ranged from 800 to 2900 and the heat flux was varied from 250 to 870 W/m2. To cover the wide range of Reynolds numbers, two aspect ratios of square and rectangular section were used. Surface radiation from the internal walls is considered through two emissivities i.e. 0.05 and 0.85, to represent weak and strong radiation effects, respectively. From the experiments, surface temperature and Nusselt number distributions of convection and radiation heat transfer are obtained for different heat flux values. Flow structure inside the channel is visualized to observe the flow pattern. The results show the combined effect of laminar flow mixed convection and surface radiation on the total heat transfer rate within the channel. The accumulating buoyancy force and airflow moves together vertically in the upward direction to give significant heat transfer enhancement in the vertical orientation of the channel.

Numerical simulation of conjugate heat transfer and surface radiative heat transfer using the [formula omitted] thermal radiation model: Parametric study in benchmark cases.

A parametric investigation of radiative heat transfer is carried out, including the effects of conjugate heat transfer between fluid and solid media. The thermal radiation is simulated using the P1-model. The numerical model and the thermal coupling strategy, suitable for a transient solver, is described. Such numerical coupling requires that the radiative equation is solved several times at each iteration; hence, the computational cost of the radiative model is a crucial issue. The P1-model is adopted because of its particularly fast computation. First, a collection of benchmark cases is presented and used to carefully validate the radiation model against literature results and to analyse the model prediction limits. Despite the simplicity of the model, it satisfactorily reproduces the thermal radiation effects. Some lack of accuracy is identified in particular cases. Second, a number of benchmark cases are described and adopted to investigate fluid–solid thermal interaction in the presence of radiation. Three cases are designed, to couple radiation with: pure conduction, conduction and forced convection, conduction and natural convection. In all the cases, the surface radiative heat transfer strongly influences the system thermodynamics, leading to a significant increase of the fluid–solid interface temperature. The main non-dimensional numbers, related to the mutual influence of the different heat transfer modes, are introduced and employed in the analyses. A new conduction-radiation parameter is derived in order to study the conductive boundary layer in absence of convective heat transfer.

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.

Combustion stability and hetero-/homogeneous chemistry interactions for fuel-lean hydrogen/air mixtures in platinum-coated microchannels

The hetero-/homogeneous combustion and stability limits of fuel-lean hydrogen/air mixtures (equivalence ratio φ=0.40) were investigated numerically in a platinum-coated planar microchannel with a length of 10mm and a height of 1mm. A two-dimensional numerical model was used for both the gas and the solid, which included elementary heterogeneous and homogeneous reaction mechanisms, detailed transport, heat conduction in the solid, surface radiation heat transfer, and external losses via a heat transfer coefficient h. Pressures of 1 and 5bar and solid thermal conductivities ks = 1 and 16Wm-1K−1 were analyzed, while stability maps were constructed in terms of the critical extinction heat transfer coefficient hcr versus inlet velocity UIN (or mass throughput). For a given solid thermal conductivity, there existed a crossover mass throughput above (below) which the stability envelope was broader at 5bar (1bar). Simulations with a surface perfectly stirred reactor (SPSR) model qualitatively reproduced the crossover points, which originated from a shift in the pressure dependence of the catalytic reactivity of hydrogen. For the low solid thermal conductivity ks = 1 Wm-1K−1, a non-monotonic dependence of the stability limits on the mass throughput was shown, with local minima created below the crossover point. The stability limits of hydrogen were solely determined by catalytic chemistry, as it sustained combustion at temperatures down to 320–380K, at which gas-phase chemistry was frozen. Away from the critical extinction points, both catalytic and gas-phase reaction pathways were controlling. The diffusional imbalance of hydrogen, which led to catalytically-induced superadiabatic surface temperatures, and the suppression of the surface superadiabaticity by gaseous chemistry resulted in rich combustion phenomena, such as increasing peak wall temperatures with increasing heat transfer coefficients h. Critical extinction heat transfer coefficients for hydrogen were three to four orders of magnitude higher than those reported for methane and propane fuels in a similar channel geometry.