Gas Flow Regimes in the Headspaces of Rotating Drums

The necessity of knowing the headspace flow patterns in order to calculate bed-to-headspace exchange can be seen by using convective heat exchange as an example, although the argument also applies to exchange of O2 and water. Convective heat removal to the headspace gases (Rconv, W) is described as follows:

where h is the heat transfer coefficient (W m-2 °C-1), A is the contact area between the bed and the headspace, Tbed is the bed temperature, and Thead is the headspace gas temperature. It is assumed that the bed is well mixed. As shown in Fig. 8.13, if the headspace is well mixed, then the driving force for heat transfer is constant, and the rate of heat transfer is the same at each location on the bed surface. On the other hand, if the flow through the headspace follows the plug-flow regime, then the driving force for heat transfer decreases as the gas heats up as it flows through the drum. In this case the rate of heat exchange between the bed and the headspace is greater near the inlet end of the drum than near the outlet end.

Some studies of headspace flow patterns have been undertaken. Stuart (1996) used a drum of 19 cm internal diameter by 85 cm length that was initially aerated with air and then a 5-minute pulse of pure N2 was introduced. The outlet O2 concentration was monitored with a paramagnetic O2 analyzer and the shape of the response curve was compared with curves that would be expected for several theoretical flow regimes. She studied the effects of two flow rates (2.7 and 5.0 L min-1) three substrate loadings (0, 1, and 2 kg of wheat bran substrate) and 4 rotational speeds (0, 5, 10, and 50 rpm).

well-mixed headspace

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plug-flow headspace

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