Dissipative heat release

It is interesting to realize that, eventually, all exergy in the sun s radiation to the earth is dissipated. Heat released to the environment, without exergy,... 

A significant heating-through of the material begins after it comes from the transportation zone into the plastification zone, because of contact with the hot surface of the gate channel, as a result of compaction and the appearance of dissipative heat release [38], The process of conversion into the melt (plastification stage) of the composition must be carried out in such a manner that chemical transformation at this stage are practically absent. 

If the fluid temperature at the entry cross-section is equal to the wall temperature Ts, then, along some initial part of the tube, the fluid is gradually heated by internal friction until a balance is achieved between the heat withdrawal through the walls and the dissipative heat release. In the region where such an equilibrium is established, the fluid temperature does not vary along the channel, that is, the temperature field is stabilized (provided that the velocity profile is also stabilized). In what follows we just study this thermally and hydrodynamically stabilized flow. 

Let i = 2.405 be the first zero of the Bessel function Jo(a i). It follows from (6.6.17) that by increasing the pressure gradient AP/L according to the law cra(3n+i)/(2n) Xu one can obtain arbitrarily large temperature on the flow axis. For cra(3n+1)A2n) > x, there is no bounded solution of problem (6.6.3), (6.5.8), (6.6.16) at all. In this case, the dissipative heat released cannot be completely withdrawn through the walls, and hence, rapid transient heating of the medium is initiated. 

In [300-302], thermohydrodynamic problems for non-Newtonian fluids were studied under the assumption that temperature varies along the walls of the tube (or channel) in this case, convective heat transfer plays an important role. It was assumed that the dependence of the apparent viscosity of the medium on temperature is exponential or power-law dissipative heat release was neglected. In one-dimensional steady-state flows of this type, the pressure gradient varies along the tube. It was shown that in some cases a situation typical of thermal explosion may arise. In this situation, heat supply due to fluid convection exceeds heat withdrawal through the walls. It was also discovered that there exists another mechanism for crisis phenomena to arise. If there is a constant heat withdrawal from the tube walls and the fluid velocity is sufficiently small, then the intensive cooling of the fluid may result in an accelerated increase of the fluid viscosity, which, in turn, results in flow choking. 

Problems of dissipating heat released during recharge... 

The total heat released is the sum of the entropy contribution plus the irreversible contribution. This heat is released inside the battery at the reaction site. Heat release is not a problem for low rate appHcations however, high rate batteries must make provisions for heat dissipation. Failure to accommodate heat can lead to thermal mnaway and other catastrophic situations. 

An experimental study of the laminar-turbulent transition in water flow in long circular micro-tubes, with diameter and length in the range of 16.6-32.2 pm and 1-30 mm, respectively, was carried out by Rands et al. (2006). The measurements allowed to estimate the effect of heat released by energy dissipation on fluid viscosity under conditions of laminar and turbulent flow in long micro-tubes. 

Equation (4.12) indicates the effect of viscous dissipation on heat transfer in micro-channels. In the case when the inlet fluid temperature, To, exceeds the wall temperature, viscous dissipation leads to an increase in the Nusselt number. In contrast, when To < Tv, viscous dissipation leads to a decrease in the temperature gradient on the wall. Equation (4.12) corresponds to a relatively small amount of heat released due to viscous dissipation. Taking this into account, we estimate the lower boundary of the Brinkman number at which the effect of viscous dissipation may be observed experimentally. Assuming that (Nu-Nuo)/Nuo > 10 the follow-... 

For the heating regime at small X+, the heat transferred from the wall to the cold fluid and the heat released due to viscous dissipation lead to an increase in... 

Processes which generate heat in organic materials are reviewed. At ordinary temperatures, respiration of living cells and particularly the metabolism of microorganisms may cause self-heating, while at elevated temperatures pyrolysis, abiotic oxidation, and adsorption of various gases by charred materials drive temperatures up whenever the released heat is unable to dissipate out of the material. The crucial rate of pyrolytic heat release depends on exothermicity and rates of the pyrolysis process. 

Figure 4 shows the results. Above 200"C, the dashed line reflects heats of pyrolysis according to Equation 2 and Figure 3, while the more realistic, full line is based on heats which approach zero at 300 C. According to the figure, heat release peaks between 130 and 230 C. Since heat dissipates out of self-heating materials in proportion to temperatures above ambient, and at 230"C the dissipation should exceed the dissipation at 130 C by a factor of two, the most rapid pyrolytic self-heating can be expected between 120 and 170 C. 

Runaway reactions can be triggered by a number of causes, but, in most cases., their resultant features after initiation are similar [31]. Whenever the heat production rate exceeds the heat removal rate in a reaction system, the temperature begins to rise and can get out of control. The runaway starts slowly but the rate of reaction accelerates, and the rate of heat release is very high at the end. Most runaways occur because of self-heating with the reaction rate (and reaction heat output) increasing exponentially with temperature, while the heat dissipation is increasing only as a linear function of the temperature. 

The observed temperature change of the calorimeter proper during the main period, 7> - 7j, is not exclusively determined by the amount of heat released in the bomb process. It is also due to the heat exchanged with the surroundings, the heat of stirring, and the heat dissipated by the temperature sensor. The observed temperature change must therefore be corrected for these contributions by an amount represented by A7COrr in equation 7.2 to obtain the adiabatic temperature rise ... 

The ratio of the oxide formed to the metal consumed is called the Piling and Bedworth number. When the number is over 1, the metal rusts. Aluminum and magnesium are the best examples of metals that do not rust because a protective oxide coat forms that is, they have a Piling to Bedworth number of 1. Scratch an aluminum ladder and notice a bright fissure forms and quickly self-coats. The heat release in the sealing aluminum oxide is dissipated to the ladder structure. 

The equilibria of all reactions under such conditions are displaced toward exothermic processes, even those that lead to the formation of highly ordered systems. Furthermore, one should bear in mind the possibility of a kind of autoregulation of the predominant direction of such spontaneous reactions processes with a relatively small heat release (closer to resonance processes ) could proceed with higher probability and, as the complexity of the molecules formed increases, the probability of the dissipation of the evolved energy among the intramolecular degrees of freedom becomes more pronounced. Therefore it seems possible that at very low temperatures under the conditions of initiation by cosmic rays, even most complex molecules can be formed with a small, but still measurable, rate, and that slow exothermic low-temperature reactions can play some part in the processes of chemical and biological evolution. 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

It can be observed, in one line, that under severe heat accumulation conditions, there is no difference in the time-scale that corresponds to the time to maximum rate under adiabatic conditions (TMRld). Thus, severe heat accumulation conditions are close to adiabatic conditions. At the highest temperature, even the small container experienced a runaway situation. Even at this scale, only a small fraction of the heat release rate could be dissipated across the solid the final temperature was only 191 °C instead of 200 °C. For small masses, the heat released is only partly dissipated to the surroundings, which leads to a stable temperature profile with time. Finally, it must be noted that for large volumes, the time-scale on which the heat balance must be considered is also large. This is especially critical during storage and transport. 

Figure 1. A representation of the change in internal energy, AU of a closed system, when a 0.1 M solution of sodium bicarbonate is allowed to mix with a 0.1 M solution of hydrochloric acid by breaking the infinitesimally thin partition in a "Battley-Kemp" rubber water bottle using virtual "foot" power. As detailed in the text, carbon dioxide is evolved in the reaction to expand the bottle and thus work, W, is performed on the environment. At the same time heat, Q, is dissipated to the environment. When the reaction is complete, the internal energy at the final state of the system U2, will change from the initial state, Uu by exactly the sum of the pressure-volume work, pAV, and the heat released by the system (see equation (1)).

For highly exothermic reactions, the preferential accumulation of coarse particles near the bottom, with their relatively lower specific surface, ameliorates rapid heat release, and the high degree of turbulence due to the higher fluid velocity near the apex of the cone helps heat dissipation. 

Normally the rate of this secondary reaction is slow, and UCle is not contaminated with UF4, but in larger-scale preparations, if heat is not adequately dissipated, the rate of the secondary reaction can become considerable. In these circumstances the UCle will be contaminated with UF4. For larger-scale preparations apparatus must therefore be designed to dissipate rapidly the heat released. It should also be noted that with this preparative method it is often convenient to prepare the UCle in situ, e.g., within a spectroscopic cell. This can completely eliminate handling and consequent contamination of the UCle-... 

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