China
Africa
Explore chapters and articles related to this topic
Basic Heat Transfer
Published in Neha Gupta, Gopal Nath Tiwari, Photovoltaic Thermal Passive House System, 2022
Any material will have high thermal diffusivity if it has a high thermal conductivity or a low heat capacity. High thermal diffusivity of the material means faster propagation of heat into the medium, whereas if the value is small, it means that most of the heat is absorbed by the material and only a small amount of heat will be further conducted. Appendix E gives thermal diffusivity (α) of various materials.
Characterization Techniques of Phase Change Materials: Methods and Equipment
Published in Amritanshu Shukla, Atul Sharma, Pascal Henry Biwolé, Latent Heat-Based Thermal Energy Storage Systems, 2020
Karunesh Kant, Amritanshu Shukla, Atul Sharma
Characterization of PCM usually includes quantifying the thermo-physical properties such as thermal conductivities and heat capacities of the solid and liquid phases, as well as transition temperatures and latent heat. Furthermore, for PCM experiencing melting over a range of temperature, enthalpy-temperature function is required too. Characterization is, thus, carried out using different samples and experimental devices. For instance, thermal conductivity and thermal diffusivity can be measured by the hot plate method and the flash method, respectively. Dynamic hot probes methods allow simultaneous determination of thermal conductivity and capacity. As for specific heat and latent heat, separate and specific differential scanning calorimetric (DSC) tests are usually used. Transition temperatures are better determined using differential thermal analysis (DTA) methods and enthalpy-temperature function estimation requires DSC tests in isothermal step mode (Richardson, 1997; Rudtsch, 2002) of the number of apparatus/tests required for complete characterization, one notices that the tests based on DTA/DSC devices generally require very small samples (some few millilitres), so that they become inappropriate for testing heterogeneous materials with large-size representative volumes. Such a problem could be partially overcome using the T-History method (Zhang and Jiang, 1999; Hong et al., 2004), a cheap and easy way for the determination of latent heats and specific heats. Unfortunately, a T-History method is unable to reliable estimation of transition temperatures and enthalpy-temperature functions (Gunther et al., 2009).
Radiation Sterilization
Published in Sandeep Nema, John D. Ludwig, Parenteral Medications, 2019
Barry P. Fairand, Dusan Razem, Karl Hemmerich
The unit of thermal diffusivity is cm2/s, which is a measure of the rate at which a heat front moves through a material. Because the thermal diffusivities of typical pharmaceutical products and medical devices are relatively low (i.e., they are not good conductors of heat such as metals), the rate of diffusion of thermal energy from the region being irradiated is normally quite low. Although little can be done to remedy this condition, it may be possible to enhance heat flow by appropriate selection of packaging materials and other materials that may surround the product unit. In this regard, removal of packing material such as Styrofoam that may encase the product or replacement with a more efficient heat-conducting material may be beneficial.
Numerical investigation of a non-linear moving boundary problem describing solidification of a phase change material with temperature dependent thermal conductivity and convection
Published in Journal of Thermal Stresses, 2023
Vikas Chaurasiya, Jitendra Singh
Thermal diffusivity expresses the rate of propagation of the isothermal surface front in a given material. Globally, it is the ratio of heat conductivity to specific heat capacity and density at a uniform pressure. Thermal diffusivity measures the ability of a material to transfer thermal energy relative to its ability to store thermal energy. With large diffusivity, rapid heat transfers occur in the material, resulting in fast freezing. Figure 7 illustrates the nature of moving front propagation as a function of time for different values of thermal diffusivity at and As time increases, the solid–liquid interface propagates faster and faster, as expected. This is happening due to higher heat transfer into the liquid phase. Clearly, for a large value of thermal conductivity, the rate of heat transfer becomes fast, and thus, the material gets solidified accordingly.
Smoke stratification in a mine drift with a burning mining vehicle
Published in Mining Technology, 2022
With a temperature difference within a body, between a fluid and a body etc., energy will be transferred from the region, fluid, or body with higher temperature to lower temperature regions, fluids etc. The thermal diffusivity – – of a material will describe the heat transfer rate through the material and thus also the heat losses of adjacent fluids, bodies etc. The thermal diffusivity is defined as: where is the density of the material [kg m−3] and is the specific heat capacity of the material [J kg−1·K−1]. With a higher thermal diffusivity, larger amounts of heat will diffuse through the material. A high thermal diffusivity due to a high thermal conductivity would imply a fast heat transfer rate. A high thermal diffusivity due to a low volumetric heat capacity – – would imply that a smaller amount of the transferred energy would be absorbed by the material and more energy would therefore transfer further. Table 1 contains thermal diffusivity data for various rock materials and shotcrete. As seen in Table 1, shotcrete presents a material with distinctly lower thermal diffusivity compared with the rock surfaces.
Experimental study on thermophysical properties of HMA during compaction
Published in International Journal of Pavement Engineering, 2021
Huanan Yu, Changyun Shi, Guoping Qian, Xiangbing Gong
The impact of temperature on the thermal diffusivity of asphalt mixture was shown in Figure 10. The thermal diffusivity is used to illustrate the characteristics of the ability of different parts of the material to reach a uniform temperature during the cooling process. Therefore, the higher the thermal diffusivity of the material, the faster the heat transferred and the quicker the objective to reach an uniform temperature. The results indicated that the thermal diffusivity increased slightly with the decrease in temperature, and the heat transfer speed increased. It can be seen from the above analysis that the initial surface temperature of asphalt mixture specimens was very high and begun to cool down after contact with air. The surface temperature dropped and decreased faster than that of the middle and lower layers. The temperature difference between the middle and lower layers and the surface layer increased, so the upward heat transfer speed of the middle and lower layers was increasing, that is, the thermal diffusivity of specimen increased with the decrease of temperature in the rolling temperature range.