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Fig. 6.3. (a) The temperature profile that would be expected when the bed-to-air heat transfer coefficients (a and ab) at the top and bottom of the tray are identical. (b) The temperature profile that would be expected when the bed-to-air heat transfer coefficient at the bottom of the tray (ab) is smaller than that at the top of the tray (a). (c) The effect of the bed-to-air heat transfer coefficient at the upper tray surface on the surface temperature, for three different heat generation rates (from bottom to top the curves represent 20, 40, and 60 W kg"1, for a 6-cm-high bed of the type shown in Fig. 6.3(a) that is incubated in a 100% relative humidity atmosphere at 30°C. (d) How the temperatures at the center of the bed (-)

and the bed surface (---) might typically be expected to vary over time, for the case where a = 10 W m-2 °C-1. This figure is based on data provided by Szewczyk (1993)

shown in Fig. 6.3(c). The temperature profile within the bed will depend on the relative values of the heat transfer coefficients at the top and bottom of the bed. If they are equal, then the profile will be symmetrical about the center plane of the bed (Fig. 6.3(a)). If not, then the profile will be asymmetrical (Fig. 6.3(b)). The surface temperature of the bed is greatly affected by the heat transfer coefficient a at values below 10 W m-2 °C-1. Above this value, further increases in the heat transfer coefficient have little effect (Fig. 6.3(c)). The value of a will depend on the velocity at which air is circulated past the tray surface. Szewczyk (1993) simulated the growth of Aspergillus niger on wheat bran in a tray, with a value of a of 10 W m-2 °C-1. At the time of peak heat generation, the center of the bed was 10°C hotter than the surface (Fig. 6.3(d)).

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