scholarly journals A Novel Model Concerning the Independence of Emissivity and Absorptivity for Enhancing the Sustainability of Radiant Cooling Technology

2021 ◽  
Author(s):  
Fan Zhang ◽  
Guoqiang Zhang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Radiant Heat ◽  
Radiant Heat Transfer ◽  
Radiant Heat Flux ◽  
New Model ◽  
The Past ◽  
Radiant Cooling ◽  
Cooling Technology ◽  
Kirchhoff's Law

Abstract Radiant cooling technology is a sustainable technology for improving built environment. The past research only studied the performance (e.g., radiant heat flux) based on Kirchhoff’s law while the accuracy and its reasons were seldom analyzed. In order to study the mechanism deeply, a new model of radiant heat transfer is derived theoretically which considers emissivity and absorptivity independently. This model is validated by the experimental data then applied in a reference case for further analysis. The analyzing methods of sensitivity and relative deviation are performed to investigate the reasons for the errors. The results of sensitivity analysis show that it is about 20% − 40% more sensitive for the emissivity to the heat flux than the absorptivity. Furthermore, the deviation of the heat flux can reach up to 20% when the absorptivity is in the range from 0.4 to 0.9. This deviation is close to the estimated error range of 21.8% in the past studies. Therefore, the discussion based on the theoretical analysis, shows that the errors in past studies are highly due to the oversimplified preconditions for applying Kirchhoff’s law and they ignored the impact of surface absorption. Additionally, the validation in the previous experiments was highly coincidence, since they neglected the key independent tests of the absorptivity and radiant heat flux. Comprehensively, the new model is valuable to provide a more reliable solution for analyzing the radiant heat transfer and for the future design of an independent test of radiant heat flux.

Radiation Energy Density and Radiation Heat Flux in Small Rectangular Cavities

1972 ◽  
Vol 94 (3) ◽  
pp. 289-294 ◽  
Author(s):  
R. P. Caren
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Energy Density ◽  
Classical Theory ◽  
Radiant Energy ◽  
Radiation Energy ◽  
Radiant Heat ◽  
Radiant Heat Transfer ◽  
Radiation Heat ◽  
Radiation Energy Density

The present paper investigates the impact of one or more small cavity dimensions on the radiation energy density and radiation heat flux in rectangular metallic cavities. The emphasis of the present analysis is the exact treatment of the modal structure of the electromagnetic field in a small cavity in determining the properties of the thermal radiation field in the cavity. The excitation spectrum of the modes is assumed to be given by the Planck distribution function. The Poynting theorem is invoked in order to determine the radiative heat flux absorbed by the walls from the radiation in the cavity. Variation of the dimensions of the rectangular cavity allows the effects of cavity size and shape on the radiant energy density and radiant heat transfer to be assessed, particularly in several interesting limiting cases. It is found that significant deviations from the classical theory occur whenever any of the cavity dimensions satisfy the inequality lT ≤ 1 cm-deg K. It is further found that, when two or more of the cavity dimensions satisfy the above inequality, the radiant energy density and radiant heat transfer are significantly reduced in comparison to the results of classical theory. However, when only one dimension is limited, as in the case of a closely spaced parallel-surface geometry, the radiant energy density and radiant heat transfer are significantly increased compared to the classical theory.

Thermal Radiation From Vertical Turbulent Jet Flame: Line Source Model

2015 ◽  
Vol 138 (4) ◽  
Author(s):  
Kuibin Zhou ◽  
Juncheng Jiang
Keyword(s):  
Heat Flux ◽  
Line Source ◽  
Radiant Heat ◽  
Source Model ◽  
Radiant Heat Flux ◽  
Emissive Power ◽  
Jet Flame ◽  
New Model ◽  
Flame Shape ◽  
Line Source Model

It is often reported that a jet fire occurs in industrial installations or in the transportation of hazardous materials and could amplify the scale of accident by imposing lots of heat on people and nearby facilities. This paper presents a new semi-empirical radiation model, namely, the line source model to predict the radiant heat flux distribution around a vertical turbulent hydrocarbon jet flame. In terms of the fact that the jet flame holds the large ratio of flame length to diameter, the new model assumes that all thermal energy is emitted by a line source located inside the jet flame volume. With three typically different shapes to simulate the jet flame shape, a formula is proposed to characterize the profile of the emissive power per line length (EPPLL), by which the line source model can be closed in theory. In comparison with the point source model, the multipoint source model, and the solid flame model, the new model agrees better with the measurement of the heat flux radiated from a small jet flame. It is found that the line source model can well predict the radiant heat flux of both small and large jet flames, yet with the flame shape simulated by the back-to-back cone and the cone–cylinder combined shape, respectively. By parameter sensitivity and uncertainty analysis, the ranking by importance of input parameters is also given for the new model.

A review of radiant heat flux models used in bushfire applications

2003 ◽  
Vol 12 (1) ◽  
pp. 101 ◽  
Author(s):  
A. L. Sullivan ◽  
P. F. Ellis ◽  
I. K. Knight
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Radiative Heat Transfer ◽  
Radiative Heat ◽  
Wildland Fire ◽  
Radiant Heat ◽  
Complex Nature ◽  
Radiant Heat Flux ◽  
Common Assumption ◽  
Made In

The need to determine the radiant heat flux (RHF) from bushfires for fire behaviour prediction, firefighter safety, or building protection planning purposes has lead to the development and implementation of a number of RHF models, most of which are based on the Stefan-Boltzmann equation of radiative heat transfer. However, because of the complex nature of bushfire flames, a number of assumptions are made in order to make the implementation of the radiative heat transfer equation practical for wildland fire applications. The main assumptions are: bushfire flame characteristics (geometry, temperature), flame radiative qualities (emission type, emissivity), and the view of the flame at the receiving element. The common assumption of a uniform emissivity of unity and an isothermal rectangular emitting surface produces a generic RHF model described here as an 'opaque box'. Because of the broad assumptions inherent in the opaque box model, it predicts the RHF of bushfires poorly. A comparison is made between the generic opaque box RHF model and the measurements of radiant heat flux emitted by a stationary propane-fuelled artificial bushfire flame front. Knowledge about the geometry and an understanding of the flame characteristics of a bushfire front are needed before generic RHF models will adequately describe the RHF emitted from bushfire flames.

Radiative Heat Transfer in Fibrous Insulations—Part I: Analytical Study

1983 ◽  
Vol 105 (1) ◽  
pp. 70-75 ◽  
Author(s):  
T. W. Tong ◽  
C. L. Tien
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Transport Process ◽  
Analytical Study ◽  
Radiative Heat ◽  
Radiant Heat ◽  
Anisotropic Scattering ◽  
Radiative Properties ◽  
Radiant Heat Flux ◽  
The Mean

The purpose of this work is to develop models for predicting the radiant heat flux in lightweight fibrous insulations (LWFI). The radiative transport process is modeled by the two-flux solution and the linear anisotropic scattering solution of the equation of transfer. The radiative properties of LWFI consistent with these solutions have been determined based on extinction of electromagnetic radiation by the fibers. Their dependence on the physical characteristics of fibrous insulations has been investigated. It has been found that the radiant heat flux can be minimized by making the mean radius of the fibers close to that which yields the maximum extinction coefficient. The results obtained in this study are useful to those concerned with the design and application of LWFI.

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