Termodynamic Analysis Of Incoherent Radiative Transfer Process

Thermal radiation is the main manner of heat transfer in many high-temperature systems such as solar collectors, boilers and furnaces. The evaluation of radiative entropy generation and radiative exergy is important when determining the second-law performance of these energy conversion devices.At present, the local entropy generation rate of conduction heat transfer is adopted to calculate radiation entropy generation in engineering application. The processes of radiation heat transfer differ from those of conduction heat transfer. One distinguishing feature between conduction and radiation is the difference in their temperature dependencies. In the view of irreversible thermodynamics, the heat flux of conduction is driven by the local temperature gradient. However, thermal radiation is generally a long-range phenomenon; therefore the local radiative heat flux is dependent on the temperature distribution of the entire enclosure under consideration and is not determined by the local temperature gradient. Hence the conduction entropy generation formula is not appropriate to calculate radiation entropy generation. Therefore it is necessary to develop the new numerical method of radiative entropy generation and exergy losses.A radiation beam carries not only energy but also entropy. Thermal radiation under macroscopic considertaion can be considered as incoherent radiation. By considering the inherent properties of thermal radiation, this paper investigates the basic thermodynamic respects of incoherent radiative transfer process, analyzes the entropy generation in the processes of the combined radiation and conduction heat transfer, high-temperature confined jet flow and combustion, and finally studies the distribution of solar spectral radiation exergy. The scope of present research contains five parts:(1) The thermodynamic respects of incoherent radiative transfer processes are analyzed, it is validated that the traditional entropy generation formula is not appropriate to calculate the entropy generation generated in the processes of radiative heat transfer. The radiative entropy transfer equation in semitransparent media and the new radiation entropy generation formulae are deduced on the basis of the definition of spectral raidative entropy intensity and spectral radiation temperature. The discrete ordinate method is adopted to solve radiative transfer equation and radiative entropy equation. The radiation entropy generation in the system enclosed with semitransparent media is analyzed. It is verified that the new radiation entropy generation formulae and the numerical simulation are correct from the viewpoint of entire thermodynamic analysis of the system. This numerical simulation method can be used in the entropy generation analysis of high temperature systems such as boilers and furnaces, in which radiation is the dominant mode of heat transfer.(2) Analysis of entropy generation for combined radiation and conduction heat transfer processes in one-dimensional steady and unsteady semitransparent media are investigated, respectively. Effect of non-dimensional parameters on entropy generation number is analyzed. And the work losses are analyzed from the view point of entropy generation number. The results show that the work losses due to the irreversibility of the conduction processes increases with the increases of conduction-to-radiation parameter, and that due to the irreversibility of the radiation processes decreases. With the increase of optical thickness, scattering albedo and wall emissivity, the work losses due to the irreversibility of the radiation processes increases and that due to the irreversibility of the conduction processes decreases.(3) The processes of two-dimensional high-temperature confined jet and methane-air diffusion combustion are investigated from the view of thermodynamic analysis, respectively. The results shows that the radiation entropy generation cannot be omitted when second law analysis of thermodynamic is adopted in high-temperature systems such as boilers and furnaces, in which thermal radiation is one of the main modes of heat transfer. Effects of parameters such as Boltzmann number and Reynolds number etc, on entropy generation and entropy generation number are analyzed. The results provide a theoretical reference for improvement of thermodynamic performance of processes. exergy transfer equation is established through radiative transfer equation and radiative entropy transfer equation. Radiative exergy losses in the semitransparent medium and at opaque solid surface are analyzed, respectively. The analytical results show that the relation between radiative exergy loss and entropy generation are consistent with the Gouy–Stodola theorem in classical thermodynamics. By taking the system composed by two infinite flat slabs as an example, the blackbody radiation exergy and gray radiation exergy are studied, respectively. Distribution of spectral radiative exergy intensity with wavelength and emissivity are analyzed. The results show that the peak wavlength of spectral radiative exergy intensity and that of the spectral radiative intensity are not coincident. The spectral radiative exergy intensity under the same wavelength increases with the increase of emissivity.(5) Distribution of solar spectral radiative exergy and energy quality factor with wavelength are investigated. The resuts show that the direct terrestrial solar radiation energy are high-quality energy in the near ultraviolet, the visible light region and the infrared region (0.762–0.931μm) on the practical engineering application, while the global terrestrial solar radiation energy are high-quality energy only in the visible light region (above 0.44μm) and the infrared region (0.764–0.898μm). Effects of atmospheric condition, such as precipitable water, total-column abundance of ozone, tropospheric pollution and air mass, on terrestrial solar radiation exergy for horizontal surface are analyzed, respectively. The results show that the effect of air mass on terrestrial solar radiation exergy is largest. The results of solar spectral radiation exegry analysis provide a theoretical reference for omnicolor utilization of terrestrial solar radiation.