Thermal diffusion length, effect

The deterioration of the grating (shown in Fig. 91) with successive pulses cannot be attributed to thermal effects, since in an experiment using a mask and projection technique (1 pm slit pattern and multiple pulses with 200 mj cm-2) sharp contours remained. Additionally, the thermal diffusion length for the laser pulse corresponds only to 30-40 nm for commercial polymers such as poly(ethylene terephthalate). 

Another type of purely neutronic effect which is important in thermal reactors arises from the heating of the moderator and consequent rise in neutron temperature. This results in a change in both the thermal diffusion length and the neutron age to thermal, thus a change in migration length and hence in neutron leakage. 

The worth of a control rod is a direct function of the thermal neutron flux to which it is exposed and to the thermal diffusion length. It is inversely proportional to effective core size. 

B. Density Coefficient A decrease in density decreases the macroscopic cross sections, which results in an increase of the mean free paths for absorption and scattering. The result is that the thermal diffusion length (L) and neutron age (t) increase. Because of the increase in L and T, the thermal and fast nonleakage probabilities are reduced. From Eq. (1) note that the reduced nonleakage probabilities decrease kg , which means that the reactivity effect is negative. A partial compensation for the effect of an increased L and t on the nonleakage if the core volume exp[Pg.193]

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

In turbulent flow, the intense iqjxing during random fluctuations usually overshadows the effects of molecular diffusion, and therefore the hydrodynamic and thermal entry lengths ate of about the same size and independent of the PflLndil number. The hydrodynamic entry length for turbulent flow can be detennined from [see Bbatti and Shah (1987) and 7.hi-qing (1982)]... 

Brownian motion refers to the random thermal diffusion of a particle immersed in a fluid of interest due to continual collisions between the particle and the fluid molecules that are in continual, random motion. Such an effect is increasingly prevalent as the characteristic length scale... 

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