Thermal optima

Adaptation of the nervous system to temperature is likely to play a major role in governing the thermal tolerance ranges and thermal optima of animals (Cossins and Bowler, 1987). Conduction along axons and, particularly, transmission of signals at synapses are strongly affected by temperature. It is probable, therefore, that alterations in the properties of neural membranes will be found to correlate closely with changes in the thermal sensitivity of behavior. The nearly perfect homeoviscous adaptation of brain synaptosomes from differently... 

Thermal optima Can we validly define thermal optima for physiological systems If so, what are these thermal optima, and how much change in temperature is adequate to create a sub optimal state of function Do all physiological systems of a species display similar thermal optima ... 

Bar-Cohen, A. and Rohsenow, W.M.. "Thermally Optimum Spacing of Vertical Natural Convection Cooled, Parallel Plates , J. Heat Transfer, Vol. 106. p. 116. 1984. 

Brandon (1967) postulated that diurnally changing thermoperiods must be responsible for the control of CAM. This hypothesis is based on in vitro experiments which showed that the malate-synthesizing enzymes (PEP carboxylase plus MDH) reach their optimum at 35° C, the thermal optimum of malate enzyme which may initiate malic acid consumption during CAM, was not reached even at 53° C. Thus, with in vitro combinations of PEP-C, MDH, and malate enzyme, net malate synthesis was observed only at temperatures clearly below 20° C. Above that temperature, depletion of malate dominated. Brandon (1967) concluded from these results that functioning of CAM predominates at low night and high day temperatures. 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Aqueous Dispersions. The dispersion is made by the polymerization process used to produce fine powders of different average particle sizes (58). The most common dispersion has an average particle size of about 0.2 p.m, probably the optimum particle size for most appHcations. The raw dispersion is stabilized with a nonionic or anionic surfactant and concentrated to 60—65 wt % soHds by electrodecantation, evaporation, or thermal concentration (59). The concentrated dispersion can be modified further with chemical additives. The fabrication characteristics of these dispersions depend on polymerization conditions and additives. 

Optimum heat-transfer results when the thermal capacity rates of the two fluid streams are balanced, ie, where or = 1.0. 

Often, maximum thermal operating efficiency is incompatible with optimum lime quahty. Usually this problem is resolved by operating under compromise conditions between these two extremes. 

In aerospace appHcations, low density coupled with other desirable features, such as tailored thermal expansion and conductivity, high stiffness and strength, etc, ate the main drivers. Performance rather than cost is an important item. Inasmuch as continuous fiber-reinforced MMCs deUver superior performance to particle-reinforced composites, the former are ftequendy used in aerospace appHcations. In nonaerospace appHcations, cost and performance are important, ie, an optimum combination of these items is requited. It is thus understandable that particle-reinforced MMCs are increa singly finding appHcations in nonaerospace appHcations. 

Cooling is most effectively accompHshed with a tandem arrangement of two extmders, as shown ia Figure 12, whereia the first extmder ensures complete dissolution of the blowiag ageat, and the second extmder is operated at slow speed for optimum cooling. Additional information on extmsion of foams is contaiaed ia Reference 28 (see FoAAffiD plastics Insulation, thermal). 

Other minor raw materials are used for specific needs. Eumaric acid [110-17-8] the geometric isomer of maleic acid, is selected to maximize thermal or corrosion performance and is the sole acid esterified with bisphenol A diol derivatives to obtain optimum polymer performance. CycloaUphatics such as hydrogenated bisphenol A (HBPA) and cyclohexanedimethanol (CHDM) are used in selective formulations for electrical apphcations. TetrahydrophthaUc anhydride [85-43-8] (THPA) can be used to improve resiUence and impart useful air-drying properties to polyester resins intended for coating or lining apphcations. 

Oxane bonds, M—O—Si, are hydroly2ed during prolonged exposure to water but reform when dried. Adhesion in composites is maintained by controlling conditions favorable for equiUbrium oxane formation, ie, maximum initial oxane bonding, minimum penetration of water to the interface, and optimum morphology for retention of silanols at the interface. The inclusion of a hydrophobic silane, such as phenyltrimethoxysilane [2996-92-17, with the organofunctional silane increases thermal stabiUty of the silane and make the bond more water resistant (42). 

There is the possibiUty of a chemical reaction between a plastic and a colorant at processing temperatures. Thermal stabiUty of both the polymer and colorant plays an important role. Furthermore, the performance additives that may have been added to the resin such as antioxidants, stabilizers, flame retardants, ultraviolet light absorbers, and fillers must be considered. The suitabiUty of a colorant in a particular resin must be evaluated and tested in the final apphcation after all processing steps to ensure optimum performance. 

The optimization of heat-transfer surfaces also plays a role. At the optimum, the lifetime cost of a surface is approximately equal in value to the lifetime cost of power used to overcome the temperature differential in the condenser and evaporator. Additionally, condensation on insulation is a sign of questionable insulation (see Insulation, thermal). Frost is a certain signal that insulation can be improved. 

Multilayer Insulation Miiltilayer insulation consists of alternating layers of highly reflec ting material, such as aluminum foil or aluminized Mylar, and a low-conduc tivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net, all under high vacuum. When properly applied at the optimum density, this type of insulation can have an apparent thermal conduc tivity as low as 10 to 50 jlW/m-K between 20 and 300 K. 

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