Half cycle

As the Reynolds number rises above about 40, the wake begins to display periodic instabiUties, and the standing eddies themselves begin to oscillate laterally and to shed some rotating fluid every half cycle. These still laminar vortices are convected downstream as a vortex street. The frequency at which they are shed is normally expressed as a dimensionless Strouhal number which, for Reynolds numbers in excess of 300, is roughly constant ... 

Adsorbent drying systems are typicaHy operated in a regenerative mode with an adsorption half-cycle to remove water from the process stream and a desorption half-cycle to remove water from the adsorbent and to prepare it for another adsorption half-cycle (8,30,31). UsuaHy, two beds are employed to aHow for continuous processing. In most cases, some residual water remains on the adsorbent after the desorption half-cycle because complete removal is not economically practical. The difference between the amount of water removed during the adsorption and desorption half-cycle is termed the differential loading, which is the working capacity available for dehydration. 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

This can be achieved at any current zero which occurs every half cycle (Figure 24.30). 

Phase controlled A single pulse of variable time is produced per half-cycle. 

Pulse width modulation Each half-cycle of output is made up of pulses of varying duration but equal magnitude. 

There are two points of the cycle at which velocity decreases very rapidly from a high value to zero. To give a.familiar analogy, the van der Pol oscillator for this value of y. behaves not as a decent oscillator but rather as a pneumatic hammer, which idles for some time while the air pressure builds up, delivers a hammer blow, losing its kinetic energy, and then begins a similar half-cycle. 

Fig. 3. Time variation of the catalyst bed temperature and the relative S03 signal in the stream leaving the cycled bed for composition forcing of the final stage of a S02 converter with an air stream and effluent from the previous stage (a) half cycle with air feed, (b) half cycle with S03/S02 feed. Feed to the system contains 12.4 vol% S02, conversion in the first stage = 90% and t = 26 min, s = 0.5. (Figure adapted from Briggs etal., 1977, with permission, 1977 Elsevier Science Publishers.)...

If the S02 and 02 concentrations are switched 180° out of phase so that S02 is absent from the reactor feed during one half cycle and 02 is absent in the other half cycle, Fig. 6 shows that is less than 1 regardless of the cycle period. Forcing just the S02 concentration at a constant 02 concentration also fails to enhance the rate of S02 oxidation in a back-mixed reactor. Even though the experiments of Unni et al. (1973), discussed earlier, were performed under isothermal conditions and differentially so that they could have been simulated by Strots model, the strategy used by Unni was different from those investigated. Nevertheless, one of the experiments undertaken by Unni switched between a reactant mixture and a feed that did not contain S02. This experiment exhibited < 1. Strots model predicts this observation. 

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