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The Laws of Nuclear Heat Transfer
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
The thermal resistance of a material can be thought of as a measure of its resistance to the flow of heat. This resistance is related to a material’s internal structure and, in particular, it depends on its molecular bonds. Large thermal resistances are associated with insulators, and small thermal resistances are associated with conductors. When two surfaces are in physical contact, the interface between the materials can also offer some additional resistance to this flow. This resistance per unit area is called the thermal contact resistance RC. The easiest way to understand the thermal resistance is to compare Ohm’s law, which governs the flow of current across a voltage difference ΔV, to Fourier’s law, which governs the flow of heat across a temperature difference ΔT. As we saw earlier, Fourier’s law can be written as
Heat Transfer in Food Processing
Published in Susanta Kumar Das, Madhusweta Das, Fundamentals and Operations in Food Process Engineering, 2019
Susanta Kumar Das, Madhusweta Das
Low internal thermal resistance occurs for bodies with high thermal conductivity values (e.g. metals). This low NBi value signifies uniform temperature inside the whole body at any given time as stated previously. An analysis of transient heat transfer for any system with uniform temperature in the whole body at any moment of time is carried out following lump heat capacity method (also called Newtonian heating or cooling). Heat gain in the body by convection is balanced with an increase in internal energy of the body. Thus, energy balance for the system gives q=hA(Tf−T)=ρCpVdTdt
Applications of Diamond in Computers
Published in Mark A. Prelas, Galina Popovici, Louis K. Bigelow, Handbook of Industrial Diamonds and Diamond Films, 2018
The foregoing discussion clearly illustrates the benefits of, and critical need for, lowering the thermal resistance of IC packaging for computers (as well as other applications, one should add). The obvious first step toward reducing thermal resistance is to increase the thermal conductivity of the materials through which the heat must flow. In many ways diamond offers virtually an ideal material for electronic packaging applications, including both single-chip packages and multi-chip module (MCM) substrates. Of prime interest, of course, is its extremely high thermal conductivity, about k=2000 W/m°K in natural diamond, with values up to 70% to 80% of that measured in CVD synthetic diamond [Lu et al. 1993]. Table I compares the thermal conductivity of natural diamond with those of various other electronic packaging materials. Also of interest for electronic packaging applications, and included in Table I, are the coefficient of thermal expansion (CTE) values, as well as note of the electrically conducting or insulating nature of the materials. In many electronic packaging applications, a high density of insulated signal interconnections are required. With an insulating packaging material it is relatively easy to implement these isolated lines, even if a high density of interconnects must pass through the substrate itself (as required, for example, in packages with area array contacts such as ball-grid array [BGAs], as well as for 3-D packaging).
Evaluation of machine learning approach for base and subgrade layer temperature prediction at various depths in the presence of insulation layers
Published in International Journal of Pavement Engineering, 2023
Yunyan Huang, Mohamad Molavi Nojumi, Shadi Ansari, Leila Hashemian, Alireza Bayat
Table 1 shows the properties of the materials used in the test sections insulated with BA and PS layers. The thermal conductivity (k) of a material measures its ability to conduct heat, and is influenced by whether the material is frozen or not; in Table 1, the minimum value of k is included, for a period in which the material is not frozen (Côté and Konrad 2005, Bai et al.2015). According to available literature (Klein et al.2003, Côté and Konrad 2005, Haghi et al.2014, Bai et al.2015), of the materials considered, k is the lowest for polystyrene board. Thermal resistance represents the resistance of materials to the conduction of heat; and is calculated by the thickness of a material divided by its thermal conductivity. A higher thermal resistance value indicates a better-performing insulation layer. The thermal resistance of the bottom ash layer is 1.43 m2·°C/W, while the thermal resistance value of the 0.10 m polystyrene board is much higher (16.67 m2·°C/W). The thermal resistance of the insulation layers is higher than that of the GBC and subgrade layers. Therefore, the insulation layers protect the subgrade from frost and influence the range of pavement temperatures in the subgrade. Also, the higher thermal resistance of the 10-cm polystyrene layer blocks heat exchange and results in more significant temperature variation in the base layer (above the polystyrene layer) (Huang et al.2021).
Experimental and theoretical analysis of thermoelectric energy generating system collecting concentrated solar energy
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Ali Murat Mahmat, Yavuz Köysal, Yusuf Yakut, Tahsin Atalay, Seyda Özbektaş
Thermal resistance is a measure of a material’s ability to resist heat transfer. It can be said that the more the heat transfer is blocked along the surfaces of the TE module, the higher the thermal resistance. In the experimental system, the temperature values on the surfaces of the TE module placed in the head of heat pipe changed constantly in response to the solar irradiance values (Figure 7). For this reason, the thermal resistance values also changed constantly. In Figure 7, it is seen that the thermal resistance increases rapidly with solar irradiance at the beginning of the measurement and then reaches a maximum. However, as the time passes, an opposite thermal resistance behavior was observed against solar radiation values. In the experimental measurements, the highest thermal resistance value was calculated as 0.82for the one axis solar tracking system where the working fluid is pure water.
Effects of wall thickness and material property on inverse heat conduction analysis of a hollow cylindrical tube
Published in Inverse Problems in Science and Engineering, 2018
Jung-Hun Noh, Dong-Bin Kwak, Se-Jin Yook
In this study, a thermal resistance network was used for heat transfer modelling. Thermal resistance networks apply electrical resistance concepts to heat transfer, based on similarities between thermal and electrical conductivities. The thermal resistance network for the tube was configured using the resistance equations of conduction () and convection () between material points that are expressed as follows: