Cooling stress transfer

The non-electrochemical techniques include direct immersion of materials samples in the test fluid in either the laboratory or plant. These s lmples sometimes have an artificial crevice generated with a serrated washer. They may be welded to determine the effects of welds and weld heat affected zones. Real-time information can be obtained using electrical resistance probes. Heat transfer effects can be evaluated by having a test sample that is exposed to the corrodent on one side and the other side heated or cooled. Stressed samples are used to evaluate stress corrosion cracking tendencies [33]. 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Some hot (370°C) pipework was supported by spring hangers to minimize stress as it was heated and cooled. The atmosphere was corrosive, and the spring hangers became impaired. They were removed, and the pipework was left solidly supported. It could not withstand the stress, and a condenser fractured hot heat-transfer oil was released and caught fire. 

This work was motivated by cracking of a thermoformed part while cooling on the mold, the complexity of the problem could be immediately appreciated since the effect was sensitive to very delicate changes in material composition. Due to coupling between the heat transfer and stress evolution, both problems were solved simultaneously ... 

No slip Is used as the velocity boundary conditions at all walls. Actually there Is a finite normal velocity at the deposition surface, but It Is Insignificant In the case of dilute reactants. The Inlet flow Is assumed to be Polseullle flow while zero stresses are specified at the reactor exit. The boundary conditions for the temperature play a central role in CVD reactor behavior. Here we employ Idealized boundary conditions In the absence of detailed heat transfer modelling of an actual reactor. Two wall conditions will be considered (1) adiabatic side walls, l.e. dT/dn = 0, and (11) fixed side wall temperatures corresponding to cooled reactor walls. For the reactive species, no net normal flux Is specified on nonreacting surfaces. At substrate surface, the flux of the Tth species equals the rate of reaction of 1 In n surface reactions, l.e. 

For material initially undamaged, the appropriate parameter expressing the tendency for cracks to be developed, and therefore strength to be lost, can be considered to be that for crack initiation. This has been expressed in terms of thermal stress resistance parameters.25,30,52,86-88 Kingery used the infinite slab symmetrically heated or cooled with a constant heat transfer coefficient to derive thermal shock fracture resistance parameters R, R and fusing the equations ... 

The parameter R is applicable for the case of instantaneous change in surface temperature (infinite h) for conditions of rapid heat transfer R is for a relatively low Biot modulus ( jl< 2) for conditions of slow heat transfer R" is for a constant heating or cooling rate.88 defines the minimum temperature difference to produce fracture under conditions of infinite heat-transfer coefficient, i.e. A = 1. The parameter Ris inversely proportional to a. Alow value of a is therefore essential for good thermal stress resistance. The coefficient of thermal expansion normally increases with increasing temperature however, thermal conductivity decreases. 

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