scholarly journals Effect of radiative heat transfer in porous comet nuclei: case study of 67P/Churyumov-Gerasimenko

2019 ◽  
Vol 630 ◽  
pp. A5 ◽  
Author(s):  
Xuanyu Hu ◽  
Bastian Gundlach ◽  
Ingo von Borstel ◽  
Jürgen Blum ◽  
Xian Shi
Keyword(s):  
Heat Transfer ◽  
Pore Size ◽  
Production Rate ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Strong Temperature Dependence ◽  
Water Production ◽  
Water Production Rate ◽  
Nicolson Scheme ◽  
The Impact

Context. Radiative heat transfer occurs in a porous medium, such as regolith on planetary bodies. Radiation enhances the efficiency of heat transport through the subsurface, effecting a strong temperature dependence of thermal conductivity. However, this effect has been omitted in many studies of comet 67P/Churyumov-Gerasimenko (67P). Aims. We concisely review the method for characterizing radiative heat transfer and present a generic treatment in thermal modeling. In particular, we study the impact of radiative heat transfer on 67P subject to both diurnal and seasonal variations of insolation. Methods. We adapted a numerical model based on the Crank–Nicolson scheme to estimate the subsurface temperatures and water production rate of 67P, where conductivity may vary with depth. Results. Radiative heat transfer is efficient during the day near the surface but it dicreases at night, which means that more energy is deposited underneath the diurnal thermal skin. The effect increases with pore size and accordingly, with the size of the constituent aggregates of the nucleus. It also intensifies with decreasing heliocentric distance. Close to perihelion, within 2 au, for example, radiation may raise the temperature by more than 20 K at a depth of 5 cm, compared with a purely conductive nucleus. If the nucleus is desiccated and composed of centimeter-sized aggregates, the subsurface at 0.5 m may be warmed to above 180 K. Conclusions. Radiative heat transfer is not negligible if the nucleus of 67P consists of aggregates that measure millimeters or larger. To distinguish its role and ascertain the pore size of the subsurface, measurements of temperatures from a depth of ~1 cm down to several decimeters are most diagnostic. The water production rate of the nucleus, on the other hand, does not provide a useful constraint.

Influence of Radiative Heat Transfer on Variation of Cell Voltage Within a Stack

2003 ◽  
Author(s):  
A. C. Burt ◽  
I. B. Celik ◽  
R. S. Gemmen ◽  
A. V. Smirnov
Keyword(s):  
Heat Transfer ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Flow Distribution ◽  
Cell Voltage ◽  
Uniform Flow ◽  
Transfer Model ◽  
Computational Performance ◽  
Uniform Flow Distribution ◽  
The Impact

In this study, a numerical investigation of cell-to-cell voltage variation by considering the impact of flow distribution and heat transfer on a stack of cells has been performed. A SOFC stack model has been previously developed to study the influence of flow distribution on stack performance (Burt, et al., 2003). In the present study the heat transfer model has been expanded to include the influence of radiative heat transfer between the PEN (positive electrode, electrolyte, negative electrode) and the neighboring separator plates. Variations in cell voltage are attributed to asymmetries in stack geometry and nonuniformity in flow rates. Simulations were done in a parallel computing environment with each cell computed in a separate (CPU) process. This natural decomposition of the fuel cell stack reduced the number of communicated variables thereby improving computational performance. The parallelization scheme implemented utilized a message passing interface (MPI) protocol where cell-to-cell communication is achieved via exchange of temperature and thermal fluxes between neighboring cells. Inclusion of radiative heat transfer resulted in more uniform temperature and voltage distribution for cases of uniform flow distribution. Non-uniform flow distribution still resulted in significant cell-to-cell voltage variations.

Radiative Heat Transfer Effects on Transient Operation of a Planar Solid Oxide Fuel Cell

2010 ◽  
Author(s):  
J. Chris Ford ◽  
Comas L. Haynes
Keyword(s):  
Heat Transfer ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Solid Oxide ◽  
Oxide Fuel ◽  
Transient Operation ◽  
Transfer Effects ◽  
Start Up ◽  
Heat Transfer Effects ◽  
The Impact

A considerable design and operational challenge for solid oxide fuel cells (SOFCs) is the material degradation caused by high temperature (600–1000°C) operation. At such high temperatures, it is reasonable to assume that radiative heat transfer effects may be significant; however, few studies have rigorously investigated the impact of radiative heat transfer on the operation of SOFCs. Accordingly, modeling efforts have been made to characterize the temperature profile evolutions that occur within planar SOFCs during transient operation. Therefore, initial results are presented to elucidate the impact of radiation upon SOFCs during start up, load changes, and shut down.

Near-Field Radiative Heat Transfer under Temperature Gradients and Conductive Transfer

2017 ◽  
Vol 72 (2) ◽  
pp. 141-149
Author(s):  
Weiliang Jin ◽  
Riccardo Messina ◽  
Alejandro W. Rodriguez
Keyword(s):  
Heat Transfer ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Near Field ◽  
Current Method ◽  
Temperature Gradients ◽  
Large Temperature ◽  
Analytical Formulas ◽  
Vacuum Gaps ◽  
The Impact

AbstractWe describe a recently developed formulation of coupled conductive and radiative heat transfer (RHT) between objects separated by nanometric, vacuum gaps. Our results rely on analytical formulas of RHT between planar slabs (based on the scattering-matrix method) as well as a general formulation of RHT between arbitrarily shaped bodies (based on the fluctuating–volume current method), which fully captures the existence of temperature inhomogeneities. In particular, the impact of RHT on conduction, and vice versa, is obtained via self-consistent solutions of the Fourier heat equation and Maxwell’s equations. We show that in materials with low thermal conductivities (e.g. zinc oxides and glasses), the interplay of conduction and RHT can strongly modify heat exchange, exemplified for instance by the presence of large temperature gradients and saturating flux rates at short (nanometric) distances. More generally, we show that the ability to tailor the temperature distribution of an object can modify the behaviour of RHT with respect to gap separations, e.g. qualitatively changing the asymptotic scaling at short separations from quadratic to linear or logarithmic. Our results could be relevant to the interpretation of both past and future experimental measurements of RHT at nanometric distances.

Modelling Radiative Heat Transfer in Oxycoal Combustion

2009 ◽  
Author(s):  
Rachael Porter ◽  
Mohamed Pourkashanian ◽  
Alan Williams ◽  
David Smith
Keyword(s):  
Heat Transfer ◽  
Carbon Dioxide ◽  
Radiative Heat Transfer ◽  
Flue Gas ◽  
Fossil Fuels ◽  
Radiative Heat ◽  
Radiative Properties ◽  
Pure Oxygen ◽  
Distribution Method ◽  
The Impact

The reduction of greenhouse gas emissions is essential to mitigate the impact of energy production from fossil fuels on the environment. Oxyfuel technology is a process developed to reduce emissions from power stations by removing nitrogen from air and burning the fossil fuels in a stream of pure oxygen. The remaining oxidiser is composed of recycled flue gas from the furnace to reduce temperatures. The product of this system is a flue gas with very high carbon dioxide concentration enabling more efficient capture and storage. Accurate modelling of oxyfuel is essential to gain better understanding of the combustion fundamentals and obtain accurate predictions of properties within the furnace that cannot be measured. Heat transfer to the furnace walls will be affected due to the different composition of the gases in the furnace. Carbon dioxide has higher heat capacity than nitrogen. Water vapour and carbon dioxide also exhibit absorption spectra of radiation in the infra-red region of the spectrum relating to wavelengths observed in combustion. Accurate CFD modelling of radiative heat transfer in oxyfuel combustion will require improvements to the radiative properties model to account for the spectral nature of radiation. In addition the impact of the solid fuel particles, soot and ash are considered. Several different radiative properties models have been tested to assess the impact on the predicted radiation and temperatures under air and oxy firing conditions. The results for radiation transferred to the walls are highly dependent upon the model chosen and the need for an accurate radiative properties model for oxyfuel firing, such as the full-spectrum k-distribution method is demonstrated.

Control of Net Radiative Heat Transfer With a Variable-Emissivity Accordion Tessellation

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Rydge B. Mulford ◽  
Vivek H. Dwivedi ◽  
Matthew R. Jones ◽  
Brian D. Iverson
Keyword(s):  
Heat Transfer ◽  
Surface Area ◽  
Analytical Model ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Radiative Properties ◽  
Apparent Temperature ◽  
The Impact ◽  
Specular Surfaces ◽  
Cavity Effect

Origami tessellations have been proposed as a mechanism for control of radiative heat transfer through the use of the cavity effect. This work explores the impact of a changing projected surface area and varying apparent radiative properties on the net radiative heat transfer of an accordion fold comprised of V-grooves. The net radiative heat transfer of an accordion tessellation is obtained by a thermal energy balance at the cavity openings with radiative properties of the cavities given as a function of various cavity parameters. Results of the analytical model are experimentally confirmed. An accordion tessellation, constructed of stainless-steel shim stock, is positioned to achieve a specified fold angle and placed in a vacuum environment while heated by Joule heating. A thermal camera records the apparent temperature of the cavity openings for various fold angles. Results are compared to apparent temperatures predicted with the analytical model. Analytically and experimentally obtained temperatures agree within 5% and all measurements fall within experimental uncertainty. For diffusely irradiated surfaces, the decrease in projected surface area dominates, causing a continuous decrease in net radiative heat transfer for a collapsing accordion fold. Highly reflective specular surfaces exposed to diffuse irradiation experience large turn-down ratios (7.5× reduction in heat transfer) in the small angle ranges. Specular surfaces exposed to collimated irradiation achieve a turn down ratio of 3.35 between V-groove angles of 120 deg and 150 deg. The approach outlined here may be extended to modeling the net radiative heat transfer for other origami tessellations.

scholarly journals The impact of radiative heat transfer in combustion processes and its modeling – with a focus on turbulent flames

Fuel ◽  
2020 ◽  
Vol 281 ◽  
pp. 118555
Author(s):  
Fengshan Liu ◽  
Jean-Louis Consalvi ◽  
Pedro J. Coelho ◽  
Frédéric Andre ◽  
Mingyan Gu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Turbulent Flames ◽  
Combustion Processes ◽  
The Impact

scholarly journals Positive and negative role of negative ions in cosmic exploration

2021 ◽  
Vol 69 (3) ◽  
pp. 607-637
Author(s):  
Leonid Gretchikhin
Keyword(s):  
Heat Transfer ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Negative Ions ◽  
Dust Particles ◽  
Compression Waves ◽  
A Value ◽  
Negative Role ◽  
Small Parts ◽  
The Impact

Introduction/purpose: At altitudes of 80 to 40 km, while the spacecraft made of duralumin without a thermal-protective coating was descending from the flight orbit at the first and second cosmic velocities, data were obtained on the increase in density, pressure, and temperature behind the shock front, as well as on the backout of the shock wave from the surface of the descending spacecraft. Methods: Calculations were made of the energy fluxes on the surface of the spacecraft for every 10 km, for convective and radiative heat transfer, as well as for the impact of electrons produced due to ionization of negative ions. Results: At the first cosmic velocity, the greatest energy flux is realized under the influence of an electron flux, and at the second cosmic velocity, radiative heat transfer occurs. In the shock-compressed gas at all the considered altitudes, pressure increases instantly to a value of 109 ÷ 1011 Pa, which leads to a sequential explosion with increasing power resulting in shock waves in the surrounding atmosphere and compression waves in the entire aircraft structure. The last most powerful explosion occurs at an altitude of approx. 40 km. Conclusion: The descending aircraft is destroyed into separate small parts to the size of small dust particles.

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