Transient heating and cooling

FIGURE 15.33 Pool-boiling curves obtained under transient heating and cooling compared with pool-boiling curve obtained for steady state (boiling of Fluorinert FC-72) (from Blum et al. [72], with permission). 

Should the glass be allowed to cool before the softening temperature is reached all the stresses will disappear. The strain set up during such heating and cooling is known as transient thermal strain, as it is clearly temporary. 

The relationships for steady heat flow can also be applied to the solution of a transient heat transfer problem, namely to the calculation of the temperature change with time during the heating and cooling of a thin walled vessel filled with a liquid. Two simplifications have to be made ... 

Overview. This section deals with the transient heating or cooling of a fluid in an enclosure, such as a tank. Applications include storage of cryogenic fluids and transients in startup or shutdown of operating systems. Literature reviews by Clark [59] and Hess and Miller [135] are available. 

Because of the relatively slow graphite heating and cooling periods, one or more couples on the graphite temperature recorder may increase or decrease over a period of time before emerging from the stripchart "field as a potential threat to flux stability. This occurs on all multiple type recorders and results in a tendency to ignore small changes until the transient becomes more or less firmly established. 

Of these receptor sites, it is the distinct subsets of thermoreceptors in combination with nociceptors that give the sensations of heat and cooling by chemical stimulus in the mouth. In mammals it is proposed that a set of ion channels, called transient receptor potential (TRP) channels, are the primary molecular transducers of thermal stimuli. One such molecular transducer is the vanilliod receptor (VRl) channel, which is an ion-gated channel that is activated by temperatures above 43°C and by chemical irritants, such as, capsaicin and acidic pH [34]. Vanilliod receptor. 

Maintenance of isothermal conditions requires special care. Temperature differences should be minimised and heat-transfer coefficients and surface areas maximized. Electric heaters, steam jackets, or molten salt baths are often used for such purposes. Separate heating or cooling circuits and controls are used with inlet and oudet lines to minimize end effects. Pressure or thermal transients can result in longer Hved transients in the individual catalyst pellets, because concentration and temperature gradients within catalyst pores adjust slowly. 

Complicated problems of transient heat flow can be resolved by computer. Typical time-temperature curves for non-steady cooling are shown in Figures 16.1 and 16.2, and the subject is met again in Section 26.2. 

In the case of reagent 1 the background turned yellow transiently on heating and then returned to a white color on cooling. 

Figure 2.32 Dependence of the dimensionless critical heat flux on the contact angle during steady-state heating and transient cooling. (From Liaw and Dhir, 1986. Copyright 1986 by Hemisphere Publishing Corp., Washington, DC. Reprinted with permission.)...

With the above-described heat transfer model and rapid solidification kinetic model, along with the related process parameters and thermophysical properties of atomization gases (Tables 2.6 and 2.7) and metals/alloys (Tables 2.8,2.9,2.10 and 2.11), the 2-D distributions of transient droplet temperatures, cooling rates, achievable undercoolings, and solid fractions in the spray can be calculated, once the initial droplet sizes, temperatures, and velocities are established by the modeling of the atomization stage, as discussed in the previous subsection. For the implementation of the heat transfer model and the rapid solidification kinetic model, finite difference methods or finite element methods may be used. To characterize the entire size distribution of droplets, some specific droplet sizes (forexample,.D0 16,Z>05, andZ)0 84) are to be considered in the calculations of the 2-D motion, cooling and solidification histories. 

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