PCM energy storage considering nanofluid volumetric radiation and natural convection in an inclined non-uniformly heated enclosure: LBM simulation

In the present study, the natural convective heat transfer in the turbulent flow of water/CuO nanofluid with volumetric radiation and magnetic field inside a tall enclosure has been numerically investigated. The thermophysical properties of nanofluid have been considered variable with temperature and the effects of Brownian motion of nanoparticles have been considered. The main objective of this work is an investigation of the effect of using water/CuO nanofluid and presence of magnetic field on turbulent natural convection in three types of enclosures (vertical, inclined, and horizontal) by considering the volumetric radiation. The governing equations on turbulent flow domain under the influence of the magnetic field and by considering the combination of volumetric radiation and natural convection have been solved by a coupled algorithm. For validating the present research, a comparison has been carried out with the laminar natural convection flow under the influence of the magnetic field and radiation effects and also, the natural turbulent convection flow of previous studies and a proper coincidence has been achieved. The results indicated that by increasing volume fraction and Hartmann number the average Nusselt number enhances and reduces, respectively. By adding 1% CuO nanoparticles to the base fluid, heat transfer improves from 10.59% to 17.05%. However, by increasing the volume fraction from 1% to 4%, heat transfer improves from 1.35% to 4.90%. By increasing Hartmann number from 0 to 600, heat transfer reduces from 9.29% to 22.07%. Also, the results show that the ratio of deviation angle of the enclosure to the horizontal surface has considerable effects on heat transfer performance. Therefore, in similar conditions, the inclined enclosure with a deviation angle of 45° compared to the vertical and horizontal enclosure has better thermal performance.

The investigation of thermal radiation and free convection heat transfer mechanisms of nanofluid inside a shallow cavity by lattice Boltzmann method

Understanding the thermal comfort and safety of diverse populations within indoor settings requires a quantitative understanding of the primary heat exchange pathways between occupants and their surroundings: radiation and free convection. Thus far, however, free convective heat transfer coefficients have only been determined for the average Western adult. To this end, we investigated how variation in body shape impacts free convection heat transfer using an experimentally validated numerical model. The multiphysics model was compared against experiments conducted using the thermal manikin ANDI ("Advanced Newton Dynamic Instrument") in a climate-controlled enclosure across five air-to-skin temperature differences ranging from 4.9 to 13.9°C. The difference between measured and simulated heat fluxes for the whole body, and per anatomical region, was typically <5%, occasionally reaching 15-20%, for some body regions due to physical features not modeled in the virtual ANDI model. Using the validated model, we simulated free convection around a family, or diverse group, of virtual manikins representing the 1st to 99th percentile body mass index (BMI) and height variation in the United States adult population. Our results show that the free convection heat transfer coefficient is independent of human sex and height but decreases slightly with increased BMI. However, the variation from the average manikin in the whole body and regional free convection coefficients with BMI was small, not exceeding 8% and 16%, respectively. Furthermore, our regression coefficients and exponents can be derived from the theorical correlation for free turbulent convection from a vertical plate, which also explains the observed independence of the heat transfer coefficient from the manikins' height. Overall, these findings demonstrate the general applicability of using an average body shape in indoor thermal audits and/or overheating risk assessments to understand thermal comfort and heat stress. The results and valid application of the model support critical insights for human health, productivity, and well-being connected to heat and cooling in buildings.

Supravital energy production in early post-mortem phase – Estimate based on heat loss due to radiation and natural convection

An empirical solution to turbulent natural convection and radiation heat transfer in square and rectangular enclosures

A numerical study is performed to analyse the entropy generation through combined non-grey gas radiation and natural convection in vertical circular tubes. The flow field and the energy equations are solved simultaneously and the radiative properties of water vapour and its spectral nature are modelled by using the 'Ray tracing' method through S4 directions, associated with the Statistical Narrow Band Correlated-K (SNBCK) model. The effects of radiation and natural convection on Nusselt number and flow rate are presented. The development of velocity, temperature and entropy generation are also analysed in this study.

A novel case study for thermal radiation through a nanofluid as a semitransparent medium via discrete ordinates method to consider the absorption and scattering of nanoparticles along the radiation beams coupled with natural convection

A numerical model is developed to simulate combined natural convection and radiation heat transfer of various anisotropic absorbing-emitting-scattering media in a 2D square cavity based on the discrete ordinate (DO) method and Boussinesq assumption. The effects of Rayleigh number, optical thickness, scattering ratio, scattering phase function, and aspect ratio of square cavity on the behaviors of heat transfer are studied. The results show that the heat transfer of absorbing-emitting-scattering media is the combined results of radiation and natural convection, which depends on the physical properties and the aspect ratio of the cavity. When the natural convection becomes significant, the convection heat transfer is enhanced, and the distributions of Nu R and Nu c along the walls are obviously distorted. As the optical thickness increases, Nu R along the hot wall decreases. As the scattering ratio decreases, the Nu R along the walls decreases. At the higher aspect ratio, the more intensive thermal radiation and natural convection are formed, which increase the radiation and convection heat fluxes. This paper provides the theoretical research for the optimal thermal design and practical operation of the high temperature industrial equipments.

Numerical study of natural convection effects on effective thermal conductivity in a pebble bed

Investigation of the Influence of Free Thermal Convection on Heat Transfer through Granular Material

Chapter 10 - Heat Transfer

Attenuation of radiative heat transfer in natural convection from a heated plate by scattering properties of Al2O3 nanofluid: LBM simulation

Polymer optical fibers are drawn from a preform rod in a manufacturing process that requires heating. A polymer preform in a tubular furnace enclosure receives energy from the furnace wall via natural convection and thermal radiation. Natural convection, in comparison to radiation, contributes a smaller fraction of the total energy required during the transient heating of the preform and also during the fiber drawing. Based on numerical predictions, natural convection and radiation contribute approximately equal heating to the preform when it is initially introduced at room temperature into a preheated furnace. As the preform temperature rises, the fraction of convective heating decreases as a result of weakening of gas‐phase circulation cell(s) between the furnace wall and preform. These findings are supported by the measured temperature histories in the interior of the preforms that, for all of the cases studied, differed by less than 1.3°C from numerical predictions. Although radiation contributes a larger fraction of the total required energy, natural convection can nevertheless have a strong and detrimental effect on the fiber quality. Specifically, convective instabilities caused air temperature oscillations of 0.3 to 2°C with frequencies from 0.01 to 0.30 Hz. Experimental observations show that these gas phase temperature oscillations promote unwanted diameter variations during fiber drawing.

This paper focuses on an approach to predict the temperature of a Photovoltaic panel under varying irradiation conditions in Cordoba, Colombia. The thermal model developed considers a heat transfer analysis in order to estimate the performance of a photovoltaic solar system due to local temperature variation. The heat transfer model analyzes the photovoltaic cell as a system exposed to radiation and natural convection by carrying out a first law energy balance which takes into account the radiation energy from the sun that hits the panel and the energy lost from the photovoltaic cell through natural convection and radiation. To determine the natural convection heat transfer coefficient, the Grashof number was employed along with Nusselt and Rayleigh number in a dimensionless form. The model has been implemented in the Matlab-Simulink platform that allows to establish a specific empirical correlation among the Nusselt number and Rayleigh for PV statics panels operating under natural convection condition. This experimental process consists in an iterative adjust of the theoretical equations of natural convection with experimental data gathered from a real PV module operation. The variables measured were the surface temperature, the environmental temperature and the solar irradiation provided by a pyranometer. It is found a good agreement between the radiation behavior and the predicted temperature. The higher values of the irradiation and environmental temperature coincides with predicted and observed PV surface temperatures and the thermal performance of the panel. The mean absolute error of the model was 3.09 K and the root mean square deviation 3.47 K.

In this paper, a numerical investigation of the two modes of heat transfer, natural convection and surface thermal radiation, in a tilted slender cavity such a collector is presented. The 2-D conservation of mass, momentum and energy are coupled with a radiative model through the boundaries and solved by the finite volume method. The studied parameters are: aspect ratios (8≤A≤16), inclination angles (15°≤λ≤35°) and Rayleigh numbers (104≤Ra≤106). The results indicated that the radiative surface radiation coupled with the natural convection modifies the flow patterns and the average heat transfer in the slender cavity between the absorber plate and the glass in the collector. The convective heat transfer coefficient and the radiative heat transfer coefficient as a function of the aspect ratio and the inclination angles are shown. It was found that the radiative heat transfer contributes more than 40% of the total heat transfer. A comparison between the present Nusselt numbers against the ones used for the design of solar collectors reported in the literature is presented.