Radiative Heat Transfer Coefficients Research Articles

A novel ANN-based approach to estimate heat transfer coefficients in radiant wall heating systems

This paper includes the validation of ANN solutions by reliable experiments to research the heat transfer characteristics in an actual size room in a laboratory. Experimental tests have been done in an experimental chamber at a constant height and a floor area. Heating through three different wall configurations is implemented during experiments. Furthermore, various ANN techniques in Matlab are employed to study the thermal behaviors of the problem with regard to the alterations of heat transfer coefficients. Backpropagation learning methods of Levenberg-Marquardt, Bayesian regularization, resilient backpropagation and scaled conjugate gradient with multilayer perceptron network are used in order to show the artificial intelligence’s predictability area. Reference temperatures for corresponding heat transfer coefficients, heated wall temperatures and supply water temperatures are assigned as input variables, while convective, radiative and total heat transfer coefficients are defined as outputs. In conclusion, developed and detailed ANN model predicted heat transfer coefficients very successfully in tolerable deviation proportions from experimental findings. Also, the influence of supply water temperature on these coefficients was revealed. Moreover, the estimations of the ANN approach have been compared with the radiant heating and cooling data in the literature and a strong consistency has been noticed.

Study on Radiation Properties of Polyurethane/Nano Zirconium Oxide Nanocomposite Coatings

The present research studies on the effect of nanoparticles on polymeric coatings and heat transfer in them. For this purpose, three different weight percentage of nanozirconium oxide (1, 3 and 5%) were added to polyurethane resin and applied on metallic plates. To determine the radiation heat transfer coefficients, emissivity coefficient and thermography of the samples along a region of long wavelengths (IR) were measured. The results showed that by adding zirconium oxide nanoparticles to the polyurethane resin, the absorption and emissivity coefficient of coating in all three samples were improved compared to the coating without nanoparticles.

Casimir Friction and Near-field Radiative Heat Transfer in Graphene Structures

Abstract The dependence of the Casimir friction force between a graphene sheet and a (amorphous) SiO2 substrate on the drift velocity of the electrons in the graphene sheet is studied. It is shown that the Casimir friction is strongly enhanced for the drift velocity above the threshold velocity when the friction is determined by the resonant excitation of the surface phonon–polaritons in the SiO2 substrate and the electron–hole pairs in graphene. The theory agrees well with the experimental data for the current–voltage dependence for unsuspended graphene on the SiO2 substrate. The theories of the Casimir friction and the near-field radiative energy transfer are used to study the heat generation and dissipation in graphene due to the interaction with phonon–polaritons in the (amorphous) SiO2 substrate and acoustic phonons in graphene. For suspended graphene, the energy transfer coefficient at nanoscale gap is ~ three orders of magnitude larger than the radiative heat transfer coefficient of the blackbody radiation limit.

Comparison Study of Solar Flat Plate Collector with Single and Double Glazing Systems

Abstract: Solar energy can be converted into useful energy in the form of thermal energy. One of the most efficient methods is to harvesting heat energy by using solar flat plate collector. The function of solar collector is to heat water from the atmospheric temperature. The heated water can be used for domestic and industrial applications etc. The efficiency of the solar collector is depend with many parameters such as  number of glass cover, wind velocity, space  between absorber plate to the glass cover and overall top loss heat transfer coefficient out of which top loss heat transfer coefficient top loss (U t ) plays an important role for design of solar collector. Taking this point under consideration the present work is to reduce the overall top loss heat transfer coefficient and improve the collector efficiency. A double glaze system was introduced and optimized the space between the absorber plate to glass cover (1) and glass cover (2)were considered to analysis the overall top loss heat transfer coefficient (U t ). The single and double glazing solar flat plate collectors were fabricated with same dimensions and installed at a latitude angle of 12 degree facing towards N-S direction. The experiment has been carried out between 10.00 AM to 4.00 PM with thermosyphon principle. The result shows that the efficiency of double glazing is higher compared to single glazing system with same solar intensity. The higher efficiency has obtained because of the overall top loss heat transfer coefficient was reduced in double glazing system. However, the efficiency of collector is not constant; it varies with wind velocity, convective, radiative heat transfer coefficient and solar intensity.Â

Nanotechnology Integrated Automobile Radiator

During the design and development of vehicle it was a greater challenge to prevent the engine & engine components from excess heat generated inside the engine cylinder. Thus, a cooling system is necessary to prevent failure of the engine from melting and damages. The cooling system of an automobile plays an important role in vehicle’s performance. Currently all automobile industries are using aluminium based Radiator. The Conventional radiator is partial heat dissipation and does not meet the requirement at high engine output. Due to limited space at the front of the engine, the radiator size is restricted and cannot be principally increased, so that, we can increase the heat transfer coefficient from the current radiator with nanotechnology integrated fin material i.e.Al-MWCNT.This study attempts to investigate, convective heat transfer coefficient of automobile radiator using different fin material. Use Nanotechnology integrated fin material instead of aluminium fins on the current radiator design, the average percentage increase of convective heat transfer is more than 10% that of the conventional radiator.

Mass loss and light curve of a large meteoroid. Analytical solution

Abstract The interaction of a large meteoroid with the Earth's atmosphere is considered in the case when it moves as a single body and when it moves as a cloud of fragments with a common shock wave. Using the literature data, an expression has been obtained for the radiative heat transfer coefficient per unit area of the midsection of a meteoroid modelled by an oblate spheroid as a function of its velocity, size, the atmospheric density, and its oblateness coefficient. The regions of predominant influence of the convective and radiative fluxes are estimated. An expression is obtained for the drag coefficient of a spheroid. Assuming that the mass of the meteoroid decreases faster than its velocity, analytical solutions of equations of the physical theory of meteors have been obtained for the mass loss of a meteoroid of spheroidal shape, the profile of the light curve, and the altitude at which the maximum of this curve is reached. The fragmentation model, which considers the meteoroid as a cloud of fragments with the spaces between them filled by a vapour, expanding in the lateral directions and flattening in the flight direction with a rate that depends on the degree of expansion is proposed. Using this model and the analytical solutions obtained, the interaction of the Chelyabinsk meteoroid with the atmosphere is considered. Good agreement between the found solution for the profile of the light curve and the observed data–light curves based on different video recordings–is demonstrated down to an altitude of 27 km.

Heating and Flow Analysis in Hot‐Rolled Stainless Strip Continuous Annealing Furnace Based on CFD Modeling

AbstractA three‐dimensional mathematical model was found for a continuous annealing furnace, the temperature field, and flow field of the furnace and the temperature of the stainless strip could be calculated by using this model. The simulation results were compared with measured data and the accuracy of the model was proved by the predicted temperature distribution. By using this model, the convective heat transfer coefficient and equivalent radiation heat transfer coefficient of the strip surface were also analyzed.

Infrared thermography technique as an in-situ method of assessing heat loss through thermal bridging

A key aspect in assessing the thermal standard of building envelopes is the quantification of the heat loss though thermal bridging, which can be expressed in terms of the linear thermal transmittance Ψ. Values of Ψ may be obtained from tabulated values for standard building details, from numerical modelling or from measurement. Where the internal structure of the building envelope is unknown, which is very often the case, measurement is the only option. This study shows how the infrared thermography technique (ITT) can be used as a non-invasive and easy-to-use method to provide quantitative measures of the actual thermal bridging performance. The novelty of this approach includes evaluation of the actual heat flow rate caused by thermal bridge qTB and Ψ-value by means of the ITT solely, without any supporting methods. Another important aspect of the methodology is that it accounts for the correlation between the surface temperature and the convective and radiative heat transfer coefficients. Values for these coefficients are assessed for the whole range of the surface temperatures recorded on the thermogram resulting in improve accuracy. The qTB and Ψ-value calculated using the presented methodology fully mirrors the real thermal performance of the thermal bridge. The methodology has been tested under laboratory conditions in a steady state in a hot box with excellent agreement.

Coherent regime and far-to-near-field transition for radiative heat transfer

Radiative heat transfer between two semi-infinite parallel media is analyzed in the transition zone between the near-field and the classical macroscopic, i.e. incoherent far-field, regimes of thermal radiation, first for model gray materials and then for real metallic (Al) and dielectric (SiC) materials. The presence of a minimum in the flux-distance curve is observed for the propagative component of the radiative heat transfer coefficient, and in some cases for the total coefficient, i.e. the sum of the propagative and evanescent components. At best this reduction can reach 15% below the far-field limit in the case of aluminum. The far-to-near-field regime taking place for the distance range between the near-field and the classical macroscopic regime involves a coherent far-field regime. One of its limits can be practically defined by the distance at which the incoherent far-field regime breaks down. This separation distance below which the standard theory of incoherent thermal radiation cannot be applied anymore is found to be larger than the usual estimate based on Wien׳s law and varies as a function of temperature. The aforementioned effects are due to coherence, which is present despite the broadband spectral nature of thermal radiation, and has a stronger impact for reflective materials.

Experimental investigation of heat transfer and dehumidifying performance of novel condensing panel

In this study, we propose a novel cooling and dehumidification strategy, in which a condensing panel can be hydronically connected in series with the radiant cooling system to overcome condensation risks. According to the concept, the sensible load is primarily treated by radiant cooling panels and the latent load and some amount of sensible load can be accomplished by the condensing panel. This paper evaluates the heat transfer and dehumidifying performance of the proposed condensing panel. For these purposes, a new experimental set-up was developed and condensation heat transfer of water vapor in the presence of air, on a vertical surface with different sub-cooling temperatures studied experimentally. However, the new test set-up allowed the measurement of the heat transfer coefficient from the measurement of the total sensible, radiation and condensation heat flux densities and the characteristic temperature differences. To investigate the heat transfer coefficient, tests were conducted using surface temperatures of 10.6, 10.9, 11.4, 12.2, 13 and 14°C with the same initial conditions of mixture (mixture temperature 25°C and relative humidity 75%). The average heat transfer coefficients were determined using the characteristic temperatures. The approximate average values 2.21, 4.45, 3.24 and 6.69Wm−2K−1 were found respectively for the radiation, convection, condensation and total sensible heat transfer coefficients. The present experimental data provide a new database for the natural convective steam condensation in the presence of non-condensable gas over the vertical plate for the full scale atmospheric condition in a closed room.

Thermal modelling and experimental validation of a semi-transparent water wall system for Sydney climate

A transient heat balance model (THBM) based on energy conservation is developed for predicting the thermal performance of a semi-transparent water wall system. In order to validate the THBM against field measurements, real climate conditions in Sydney, Australia are adopted and the time variations of both internal and external convective and radiative heat transfer coefficients are calculated using empirical correlations from the literature. An alternative concrete wall model is also developed for further validation of the THBM. Good agreements between the THBM simulation and field data are achieved for both the water wall and concrete wall systems. The validated THBM is then adopted in a sensitivity analysis in order to assess the thermal performance of the semi-transparent water wall system under different configurations. It is found that the daily maximum water and air temperatures decrease significantly with the decrease of the transmissivity of the semi-transparent panels, whereas an increase of the attenuation coefficient of solar radiation in water has an insignificant impact on the water and air temperatures. The present results also indicated that increasing the thickness of the water column can reduce the fluctuations of the water temperature and the diurnal peak air temperature. From the practical application point of view, reducing the transmissivity of the external panel provides the most effective way to mitigate over-heating in the semi-transparent water wall system.

Minimum radiative heat transfer between two metallic half-spaces due to propagating waves

The gap dependence of radiative energy transfer due to propagating waves between two identical metallic half-spaces separated by vacuum is investigated. The dielectric function of the metallic half-spaces is described by the Drude model. Analytical expressions for the minimum radiative heat transfer coefficient, hmin, and the gap, dmin, at which the minimum value of radiative transfer is attained are determined in terms of the parameters of the dielectric function and the absolute temperature T. We show that hmin∝T2 in the high temperature limit and hmin∝T7/2 in the low temperature limit.

Analysis of combined solar photovoltaic-nanostructured thermoelectric generator system

ABSTRACTIn this paper, a combined solar photovoltaic (PV) and thermoelectric (TE) generator system is investigated. The TE generator converts the temperature gradient into electricity eventually improving the overall performance of the system. A nanostructured TE material is used in this investigation and its power generation performance is compared with the traditional TE material. Using the analytical solutions, different performance parameters (e.g. heat input, power output, efficiency) are calculated and expressed graphically as a function of solar radiation and convection heat transfer coefficient. In addition to this, different performance parameters were also compared between traditional and nanostructured TE materials. Moreover, the nanostructured TE material leads to improvement in performance of TE generator due to the reduction in thermal conductivity and improvement in electrical conductivity. The TE generators have a large impact on the overall efficiency of the combined system at higher radiation.

Determination of convective and radiative heat transfer coefficients using 34-zones thermal manikin: Uncertainty and reproducibility evaluation

A lot of research has been done in order to investigate heat transfer coefficients of a human body in various postures, wind speeds and wind directions. However, there has not been any reference to measurement reproducibility and measurement confidence intervals. The purpose of this study is to determine heat transfer coefficients of a thermal manikin experimentally, while focusing on the repeated determination of the coefficients and statistical data evaluation. The manikin imitates human metabolic heat production; it measures combined dry heat flux from its surface and also its surface temperature.The major part of the radiative heat flux was eliminated by a low-emissivity coating applied to the surface of the nude manikin. The tests were performed across 34 zones that correspond to parts of a human body. Both standing and seated postures were investigated. The tests were conducted at constant air temperature (24°C) and constant wind speed (0.05ms−1). Based on three repetitions of each case, the mean values of heat transfer coefficients, with their uncertainty intervals, were calculated. Next, the results of this paper were compared to the results of similar experimental work of de Dear et al. (1997) and Quintela et al. (2004). A mismatch of the values is up to 1Wm−2K−1, while an extreme was found on the manikin’s seat with a difference of over 1Wm−2K−1. The outcomes of this study provide essential information in form of separated values of the convective and radiative heat transfer coefficients that enable us to create detailed computational models of a thermal comfort.

The effects of transparent long-wave radiation through glass on time lag and decrement factor of hollow double glazing

Time lag (TL) and decrement factor (DF) for a hollow double glazing have been investigated numerically with considering the long-wave radiation transmittance of glass that is small and is usually ignored according to “greenhouse effect”. The long-wave radiation transmittance of ordinary soda-lime silicate glass is selected as 0.1, the combined surface heat transfer coefficients (including convection and radiation heat transfer) of the outside and inside glazing surfaces are included in the boundary conditions of the numerical model, in which the increment of the surface radiation heat transfer coefficient of the inside and outside surfaces of the glazing ranges from 1.1 to 3.7W/(m2K). By this way the transmittance effects on TL and DF shift to the effects of the combined heat transfer coefficients on TL and DF. The results show that when the transmittance is 0.1, TL and DF decrease about 3–17% comparing TL and DF without considering the transmittance; and the transmittance effects are stronger in summer than in winter. Furthermore, compared to TL and DF of thick solid walls TL of hollow double glazing is far smaller, and DF is relatively larger. Increasing air layer thickness and glass thickness can effectively increase TL and decrease DF.

Study of Parameters Influencing Fluid Flow and Wall Hot Spots in Rotary Kilns using CFD

AbstractIn this paper, 3‐D‐CFD simulation using an ANSYS‐Fluent package in a 4 m diameter kiln with 40 m length firing methane (CH4) gas is applied to avoid undesirable thermal behaviour (wall hot spots; peak wall temperatures) in industrial rotary kilns. New influencing parameters are introduced, including primary air ratio, burner configuration (two configurations with different fuel jet momentums), and burner power. The influence of these parameters on the peak kiln flame and on wall temperatures, flame radiation heat flux, radiative heat transfer coefficient, temperature contours, and pathlines are investigated and discussed. Preliminary comparison of jet flames with available experimental data is carried out to select and validate the proper turbulence model for the present simulations. Results reveal that the peak flame temperature, flame radiation heat flux, and radiative heat transfer coefficient increase with higher fuel jet momentum, lower primary air ratio, and higher burner power. On the other hand, the wall hot spots emerge when operating the kiln at higher burner power or by lowering the jet momentum (larger fuel inlet diameter). Stable flames and a higher recirculation size can be obtained by operating the kiln under a higher primary air ratio and higher jet momentum (narrower fuel inlet diameter). Based on these results, operators are shown a way to adjust controllable kiln parameters to reduce wall hot spots and to improve product quality, in addition to controlling the ringing problems.

Thermal Performance of a Weir-Type Cascade Solar Still: an Experimental Study

Water is essential for all living creatures. It is abundantly available on earth surface but quantity of potable water is very less. Solar distillation is environmental friendly, economical and cheap water purification techniques. This technique is not commercialized yet because of its lower distillate output. In this communication weir-type cascade solar still with phase change material (PCM) are designed for increasing the productivity of still. Effect of three different phase change material (Bees wax, stearic acid and palmitic acid) of different melting point (61 oC, 55 oC and 54.3 oC respectively) have been studied on the performance of cascade weir type solar still. Maximum productivity of 2.35 litre/m2/day has been obtained with the use of palmitic acid as phase change material with the still. Mean values of convective, evaporative, radiative heat transfer coefficient with thermal efficiency of still has also been calculated and found to be 1.193, 13.13, 6.913 W/m2 oC and 24.58% respectively. The productivity of weir-type solar still with PCM is observed 33.69% higher than conventional still. Thermal efficiency of still with PCM is 35.46 % more than still without PCM.

Thermal conductivity of silica nanoparticle powder: Measurement and theoretical analysis

The hot-wire method was applied to experimentally determine the thermal conductivity (TC) of a silica nanoparticle powder. A fitting model was further employed to analyze the experimental results and to predict the TC over a wider porosity scale. Results show that the effective TC of the silica-nanoparticle powder can be less than that of free air because of the low TC of both the silica nanoparticles and the air confined in the pore spaces; the relative contribution of the nanoparticle TC, the confined air TC, and the radiation heat transfer coefficient to the effective TC will significantly affect at which porosity the extreme value of the effective TC occurs; the porosity obtained when the contribution to the effective TC of the confined air equals that of the nanoparticles is the most favorable for constructing thermal insulation materials.

Repeated determination of convective and radiative heat transfer coefficients using 32 zones thermal manikin

Average European citizens spend 80 % to 90 % of their workday time indoors, in buildings or vehicles [1]. Owing to this fact, building design in terms of thermal comfort, air quality, and low energy demands is important. In addition, many independent studies provide evidence of improper thermal environment and the negative influence of this on the human body, e.g. [2], [3]. However, such situations can be tackled, in the future, using modern computational methods. To do so there is a need for anatomically detailed heat transfer coefficients that quantify heat flux from a human body. A lot of research has been done in order to investigate heat transfer coefficients in various body postures, wind speeds and wind directions, e.g. [4], [5], [6], [7]. On the other hand, there has not been any emphasis given to measurement reproducibility. A thermal manikin was involved to determine convective and radiative heat transfer coefficients in a sitting and standing posture repeatedly.

Experimental and numerical investigation of the correlation between radiative and convective heat-transfer coefficients at the cooled wall of a real-sized room

Radiant heating and cooling systems can be utilized in association with renewable energy sources and are currently being used in newly constructed homes, as well as in buildings where heating and cooling systems have been converted from conventional systems to these systems. Thus, for a radiant system, the fundamental characteristic parameters, such as radiative, convective and total heat-transfer coefficients, carry the utmost importance. In the present study, these coefficients were determined through measurements from a test facility as well as numerical case studies.This article provides the heat-transfer coefficients from experimental research on cooled hydronic radiant panels placed on a chamber's walls. The experimental study utilized 4 different flow rates between 0.08kg/s and 0.15kg/s, and water supply temperatures between 14°C and 19°C.Furthermore, computational and theoretical case studies found that the ratio of radiative to convective heat transfer coefficients (hr/hc) varied in the range of (0.88–1.79), throughout all of the experimental and numerical case study results. Also, correlations for the proportions of the coefficients and the radiative heat-transfer coefficient were derived.