Thermoneutral zone

A different example is the thermoneutral zone within the thermoregulatory systems of humans and other warm-blooded animals. Sweating to remove heat and thermogenesis to produce extra heat are both energy consuming. There is a temperature range where neither of these mechanisms operates, the thermoneutral zone. Temperature control within the thermoneutral zone is accomplished by vasodilation or vasoconstriction of cutaneous blood vessels. The amount of heat lost by this means is determined by controlling the skin surface temperature (see Section 2.7). 

If this pig were subjected to an increasing temperature, it would have difficulty in losing heat by sensible losses and woidd need to increase its evaporative loss. Eventually, a temperature woidd be reached at which the pig would need to reduce its heat production, which it might do by restricting its muscular activity and also by reducing its food intake. The temperature above which animals must reduce their heat production is known as the upper critical temperature. The range between the lower and upper critical temperatures is known as the thermoneutral zone. 

We can now move from this simple situation of the pig exposed to oidy one climatic variable, temperature, to other animals and other climates. The lower critical temperatures of animals kept in different environments are presented in Table 14.4. Ruminants have a wider thermoneutral zone and a lower critical temperature compared... 

Environmental temperature influences the intake of ruminants as it does of mono-gastric species. At temperatures below the thermoneutral zone (see p. 350), intake is increased, and at temperatures above the thermoneutral zone, intake is reduced. Well-fed ruminants have a broad thermoneutral zone, extending to quite low critical temperatures at the lower end (see Table 14.4 in Chapter 14). However, at the upper end (i.e. in hot climates), temperature can exert a strong influence on intake, especially in more productive animals with a high nutrient demand. For example, for temperate (i.e. Bos taurus) breeds of cattle, it has been estimated that intake falls by 2 per cent for every 1 °C rise in average daily temperature above 25 °C. 

From a practical point of view and to avoid bias in the ealeulation of NE for different feeds, it is necessary to carry out energy balance measurements in similar animals (i.e. same sex, same breed and in the same body-weight range), to keep these animals within their thermoneutral zone, to minimize variation in behaviour, and to feed the animals at about the same feed intake level with balanced diets so that the animals can express their growth potential. Under these circumstances, an erroneous estimate of FHP will affect the absolute NE value, but not the ranking between feeds. This also means that NE should not be measured in animals fed ingredients for which the chemical characteristics are very different from those of a complete balanced diet. 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

For liquid fuels, reformers are used that incorporate a fast oxidation zone and a separate slow SR zone, where the O/C and the steam/C ratios are optimized to yield thermoneutral ATR. Unsurprisingly ATR is a complex combination of homogeneous partial oxidation, thermal cracking, dehydrogenation, SR, WGS reaction, methanation, and others. Owing to this complexity, the reaction mechanism occurring in liquid fuel ATR is not well understood. Catalysts used in the ATR are... 

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