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Thermal Environment Design Strategies
Published in Chitrarekha Kabre, Synergistic Design of Sustainable Built Environments, 2020
Surface resistance to heat flow is a function of the combined radiant and convective components of heat transfer (Equation 3.11). The convection heat transfer coefficient is dependent on airspeed and direction of heat flow. The radiation heat transfer coefficient is dependent on the view factor and the emittance of the radiation and absorbing surfaces. In the analysis of heat transfer from air into a body such as a wall, roof, or vice versa, it is convenient to use the published standard values (Table 3.2). RsoorRsi=1fr+1fc
Heat Transfer
Published in C. Anandharamakrishnan, S. Padma Ishwarya, Essentials and Applications of Food Engineering, 2019
C. Anandharamakrishnan, S. Padma Ishwarya
The radiation heat transfer between surfaces depends on the on the properties of the radiating surface such as the temperature, emissivity, and absorptivity, and orientation of the surfaces relative to each other. View factor, also termed as shape factor, is a geometrical parameter that accounts for the effect of orientation on the net heat exchange between two radiating surfaces. It can be defined as the fraction of radiant energy leaving a surface which strikes another surface directly (reflected and re-radiated energy is not taken into account). In such cases, where it is not possible for the materials to absorb all the emitted energy, view factor is used to parameterize the fraction of radiant energy leaving the first body and reaching the second body. During the calculation of view factor, the space/medium between the radiating surfaces is assumed to be devoid of bodies that absorb, emit, or scatter radiation. The value of view factor ranges between 0 and 1. While a view factor of zero indicates that two surfaces do not see each other directly, the value of one indicates that the first surface completely surrounds the second surface.
Radiation Heat Transfer between Surfaces
Published in William S. Janna, Engineering Heat Transfer, 2018
The view factor (also called configuration- or geometry factor) is a geometry term for a system in which two surfaces exchange energy by radiation. Radiation waves travel in straight lines, and if one surface cannot “see” another, then there is no direct radiation from the first surface to the second. Consider a fire in a fireplace. Anyone sitting where a view of the fire is totally obstructed receives no energy from the fire directly. Conversely, anyone sitting in view of the fire receives heat directly from the fire by radiation. Furthermore, someone sitting close to the fire receives more heat than someone sitting far away. Someone sitting just in front of the fire receives more heat than someone sitting off to either side. As indicated by this example, position (or the geometry) between the source (fire) and receiver is important in determining the radiative energy-transfer rate. The view factor expresses the geometry of position that exists between source and receiver.
Numerical analysis of non-uniform Cu(In, Ga)Se2 growth in a selenization process on large-area substrates for mass production
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Taejong Yu, Daegeun Yoon, Donghyun You
Thermal conduction is the propagation of internal energy in a solid. Following the Fourier's law, the heat flux equation for thermal conduction is given as follows: where k is the thermal conductivity. Thermal radiation is produced from emission or absorption of electromagnetic waves on a solid surface as it undergoes internal energy-state transition. Since a selenization process is a high-temperature process, the thermal radiation is occurred by the heaters attached to the reactor walls. The heat flux of the radiation is the sum of emitted energy and absorbed energy on each surface and can be expressed as follows: where ε is the emissivity, α is the absorptivity of thermal radiation, and σ is the Stefan–Boltzmann constant. Thermal radiation is calculated with a view factor to consider a geometry-factor of the reactor. The view factor is a value related to the proportion of the radiation which leaves the surface i that strikes the surface j. The view factor is expressed by a twofold integral method as follows: where i and j are the surface element indices at the distance . and are the element area and the angle between the surface normal and a ray between the two surface elements, respectively.
Regression modelling of radiant fluxes on different view factors under shading in a densely built environment
Published in Architectural Science Review, 2018
Alan Lai, Yu Ting Kwok, Minjung Maing, Edward Ng
The MRT can be obtained by several means with the aid of computer software and field measurements of radiant fluxes and globe temperature. The most accurate method to determine MRT, integral radiation measurement, requires the measurement of short-wave and long-wave radiant fluxes that affect the human body in three dimensions (Thorsson et al. 2007). The three-dimensional radiant fluxes, both short-wave and long-wave, are represented by fluxes from the four cardinal directions, as well as the upper and lower hemispheres. Therefore, to simulate the reception of radiative energy fluxes on the human body, the measuring instrument should consist of six net radiometer sensors facing all six directions. The transfer of radiative energy between the human body (i.e. surfaces of the sensors) and the outdoor environment can then be described based on the enclosure theory, or the net radiation method (Mbiock and Weber 2000; Howell, Menguc, and Siegel 2010), for each direction. Such energy transfer depends on the relative positions and orientations of different surfaces, for example the human skin (i.e. surfaces of the sensors) and the building façades in the outdoor environment. The view factor is a parameter describing the geometric relationships between different surfaces with respect to radiative energy transfer. Therefore, the effects of view factors on determining MRT are fundamentally important.