Internal thermal resistance

Experimental uptake data for sorption0of i-octane on 13X zeolite and n-pentane on 5A zeolite were quantitatively described by the model. The results show that internal thermal resistance of the adsorbent mass plays a significant role during the uptake for these systems even though the adsorbent temperature changes are small. 

The proposed model for non-isothermal sorption kinetics can quantitatively describe uptake0data for adsorption of i-octane on 13X and n-pentane on 5A zeolites. The study indicates that the principal resistance to mass transfer for these systems may be confined at the surface of the zeolite crystals. It is also found that the internal thermal resistance of the assemblage of the micron size zeolite crystals used in the kinetic test is significant which produces a substantial thermal gradient within the assemblage and slows down the heat dissipation from it. 

Thus, an apparently slower mass uptake is observed as compared with the isothermal uptake or the case when heat transfer is controlled by the external film resistance. Consequently, ignoring the internal thermal resistance may lead to erroneously low mass transfer coefficient. 

Equation (4.8) indicates that the one-dimensional transient temperature distribution inside a solid sphere without internal heat generation varies with Fo and Bi. On the basis of the equation, the temperature distribution in the solid can be considered uniform with an error of less than 5 percent when Bi < 0.1, which is the condition for most gas-solid flow systems. In transient heat transfer processes where the gas-solid contact time is very short, it also requires Fo > 0.1 [Gel Perin and Einstein, 1971] for the internal thermal resistance within the particles to be neglected. In the following, unless otherwise noted, it is assumed that the temperature inside a solid particle is uniform. 

For suspension-to-gas (or bed-to-gas) heat transfer in a well-mixed bed of particles, the heat balance over the bed under low Biot number (i.e., negligible internal thermal resistance) and, if the gas flow is assumed to be a plug flow, steady temperature conditions can be expressed as... 

The entire internal thermal resistance of the heat input zone is... 

Ratio of internal thermal resistance of solid to fluid thermal resistance Heat transfer between fluid and solid... 

As reported in the introduction, the development of an accurate thermal model for lithium-ion technology is quite a difficult task due to the many phenomena that occur. However, in [23] it is documented that a thermal model can be proposed by its main parameters such as thermal heat capacitance Qh and internal thermal resistance Rthi- However, in [22], the authors found that the convective thermal resistance i con cannot be neglected in the development of thermal models. Based on these two works, a novel thermal model is presented in Figure 11.5. The parameters of this model can be defined as follows ... 

Internal thermal resistance 0int, largely bulk conduction from the circuits to the case/package surface, thus also containing thermal spreading and contact resistances... 

Internal thermal resistance A term used to represent thermal resistance from the junction or any heat generating element of a device, inside an electronic package, to a convenient point on the outside surface of the package. 

The temperature gradient is not to be confused with thermal lag, which is another physical property that should also be minimized in DSC experiments. Thermal lag is the difference between the average sample temperature and the sensor temperature and is caused by so-called thermal resistance, which characterizes the ability of the material to hinder the flow of heat. Thermal lag is smaller in DSC than in DTA because of smaller sample size (milligrams in DSCs), but more types of thermal resistance develop in DSC than in DTA. These effects are caused by introduction of the sample and reference pans into the DSC sample and reference holders. Thus, in DTA thermal resistance develops between the sample holder (in some instruments called the sample pod) and the sample (analogously, between the reference holder and the reference material), and within the sample and the reference materials. On the other hand, in DSC thermal resistance will develop between the sample holder and the bottom of the sample pan and the bottom of the sample pan and the sample (these are called external thermal resistances), and within the sample itself (this is called internal thermal resistance). These thermal resistances should be taken into account since they determine the thermal lag. Let us suppose that the cell is symmetric with regard to the sample and reference pods or holders, the instrumental thermal resistances are identical for the sample and reference holders, the contact between the pans and the pods are intimate, no crosstalk exists between the sample and reference sensors (i.e.. 

Pan Crimping. It is critical to decrease both the external and internal thermal resistance (see Section 2.3) during the purity runs. For this reason it is important to ensure the maximum area of contact between the bottom of the DSC pan and the cell, and also between the sample and the bottom of the DSC pan. Therefore, it is not recommended to run chunky samples. Also, when running powdery samples, the particle size of the powder should be as small as possible. Whenever possible, the sample... 

As predicted by the Arrhenius equation (Sec. 4), a plot of microbial death rate versus the reciprocal or the temperature is usually linear with a slope that is a measure of the susceptibility of microorganisms to heat. Correlations other than the Arrhenius equation are used, particularly in the food processing industry. A common temperature relationship of the thermal resistance is decimal reduction time (DRT), defined as the time required to reduce the microbial population by one-tenth. Over short temperature internals (e.g., 5.5°C) DRT is useful, but extrapolation over a wide temperature internal gives serious errors. 

R thermal resistance x specific entropy S entropy t time T temperature u specific internal energy U internal energy v specific volume velocity V volume W shaft work x coordinate distance... 

In reality we cannot usually neglect the internal thermal time constant r of the sample and the contact resistance Rc. Let us consider the example of Fig. 4.6 that shows a cylindrical sample connected to a thermal bath by a thermal resistance of negligible value. 

The lowest temperature is reached in the mixing chamber (MC) where the experiments are placed (sometimes inside, see Section 6.5) and where there is the interface between the concentrated and the diluted phase. The MC is in most cases made of Cu. The internal wall of the MC are covered with a sintered metallic powder (Ag or Cu) to reduce the thermal resistance Rk (see Section 4.4) between the liquid mixture of He and the walls. 

We take the heat transfer coefficient a to be independent of the jet velocity and of the residence time in the vessel. Physically, this assumption together with the assumption of complete mixing of the substance in the reaction vessel and of a constant mean temperature throughout the vessel corresponds to the idea that for heat transfer the governing factor is the thermal resistance from the internal wall of the vessel to the outside space in which the temperature is kept at T0. In other words, our assumption corresponds to the concept of a vessel which is thermally insulated from outside. 

Abrasive wear is a complex combination of a number of factors, including resilience, stiffness, thermal resistance, thermal stability, resistance to cutting, and tearing (Smith, 1993). There are a number of laboratory tests, both international standards and commercial tests, for the evaluation of abrasive wear. The results from these tests normally represent only an indication of the actual wear that can be found in practice. The test equipment generally has a loaded sample against course abradant or, in the case of a Taber abrader, a loaded abrasive wheel against a flat sample (see Chapter 8). 

The monofunctional epoxy diluents are essentially chain stoppers since they inhibit crosslinks from forming. The extent to which the cured properties are affected is directly dependent on the concentration of the diluent added to the epoxy resin. The general effect is to reduce viscosity and improve the impact and thermal shock resistance while slightly reducing the thermal resistance. The thermal expansion of the cured resin is increased, as it is also with nonreactive diluents. This can lead to internal stress on the bond line depending on the thermal expansion of the substrate material. 

Equipment designs based on indirect conduction usually transfer the heat from the primary heat transfer fluid to the intermediate wall within some kind of internal duct or channel. Transfer coefficients for these cases depend on the nature of the flow (laminar or turbulent) and the geometry of the duct or channel (short or long). Expressions for evaluating the transfer coefficients for these cases are available in standard texts. An expression for the convective thermal resistance can be generated similar to that derived for the conductive resistance ... 

By contrast, in finite time, thermodynamics is usually considered an endoreversible Curzon-Ahlborn cycle, but in nature, there is no endoreversible engine. Thus, some authors have analyzed the non-endoreversible Curzon and Ahlborn cycle. Particularly in [16] has been analyzed the effect of thermal resistances, heat leakage, and internal irreversibility by a non-endoreversibility parameter, advanced in [14],... 

Since we are attempting to reconcile measurements made by different techniques, the artifacts, limits and constraints that accompany each technique must be identified. These include the challenges inherent in the methods of sample-sensor coupling. For example, fhermal femperafure or heat flow sensors are influenced by factors such as the thermal conductivity of the cell-sensor construct, the thermal resistance of fhe sample-cell interface, and the internal thermal properties of the sample. The geometry of the heat flow pathways is also important. Mechanical sensors (force or... 

This example also shows the effects of mass- and enei y-transfer resistances within the catalyst pellet. The temperature increases toward the center of the pellet and increases the rate, but the oxygen concentration goes down, tending to reduce the rate. The global value of 49.8 x 10" is the resultant balance of both factors. Hence the net error in using the bulk conditions to evaluate the rate would be [(49.8 — 43.6)/49.8] (100), 12.5%. In this case the rate increase due to external and internal thermal effects more than balances the adverse effect of internal mass-transfer resistance. The procedure for calculating the effects of internal gradients on the rate is presented in Chap. 11. 

Absorptive glass mat (AGM) Very high porosity of separator High purity and stability of separator Low internal electrical resistance Relatively large separator pores acid stratification -electrolyte drainage risk of internal shorts -risk of thermal runaway -risk of PCL-3... 

Chiewchan, N., Pakdee, W., and Devahastin, S. 2007. Effect of water activity on thermal resistance of Salmonella Krefeld in liquid medium and on rawhide surface. International Journal of Food Microbiology 114 43 9. 

Quintavalla, S., Larini, S., Mutti, P, and Barbuti, S. 2001. Evaluation of the thermal resistance of different Salmonella serotypes in pork meat containing curing additives. International Journal of Food Microbiology 67 107-114. 

Carnavos [155] reported typical overall improvements that can be realized with a variety of commercially available enhanced horizontal condenser tubes. The heat flux for single 130-mm-long tubes, in most cases with outside diameters of 19 mm, is plotted in Fig. 11.22 against AT/m for 12 tubes qualitatively described in the accompanying table. The overall heat transfer performance gain of the enhanced tubes over the smooth tube is as high as 175 percent. Internal enhancement is a substantial contributor to the overall performance, since the more effective external enhancements produce a large decrease in the shell-side thermal resistance. 

If there is any contact or bond resistance present between the fin and tube or plate on the hot or cold fluid side, it is included as an added thermal resistance on the right side of Eq. 17.5 or 17.6. For a heat pipe heat exchanger, additional thermal resistances associated with the heat pipe should be included on the right side of Eq. 17.5 or 17.6 these resistances are evaporator resistance at the evaporator section of the heat pipe, viscous vapor flow resistance inside heat pipe (very small), internal wick resistance at the condenser section of the heat pipe, and condensation resistance at the condenser section. 

ISO 8302, Thermal insulation - determination of steady-state thermal resistance and related properties - guarded hot plate apparatus, International Organization for Standardization. 1991. 

ISO 5085-1, 1989. Textiles - Determination of Thermal Resistance Part 1 Low Thermal Resistance. International Organization for Standardization, Geneva, Switzerland. 

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