Periodic Channel Geometries

In the articles cited above, the studies were restricted to steady-state flows, and steady-state solutions could be determined for the range of Reynolds numbers considered. Experimental work on flow and heat transfer in sinusoidally curved channels was conducted by Rush et al. [121]. Their results indicate heat-transfer enhancement and do not show evidence of a Nusselt number reduction in any range 

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Similarly to the structure of the flow fleld, heat transfer has also been studied in curved channel geometries. The complicated branch structure with competing patterns of two and four counter-rotating vortices in channels of square cross section is reflected in the Nusselt number [34]. When plotting the Nusselt number as a function of Dean number, different branches are found corresponding to symmetric and asymmetric secondary flow patterns with two and four vortices. However, the relative difference between the different branches is not very pronounced and should be hard to measure experimentally. For a Dean number of 210 and a Prandtl number of 0.7 a heat-transfer-enhancement factor of about 2.8 was determined, thus showing that curved channels as well as other channels with specific periodically varying cross sections may be used for applications where rapid heat transfer is desired. 

Endurance Burn Under certain cou(itious, a successfully arrested flame may stabilize on the unprotected side of an arrester element. Should this condition not be corrected, the flame will eventually penetrate the arrester as the channels become hot. An endurance burn time can be determined by testing, which specifies that the arrester has withstood a stabilized flame without penetration for a given period. The test should address either the actual or worst-case geometry, since heat transfer to the element will depend on whether the flame stabilizes on the top, bottom, or horizontal face. In general, the endurance burn time identified by test should not be regarded as an accurate measure of the time available to take remedial action, since test conditions will not necessarily approximate the worst possible practical case. Temperature sensors may be incorporated at the arrester to indicate a stabilized flame condition and either alarm or initiate appropriate action, such as valve closure. 

The overal1 cavlty geometry and channel length were similar to those found In the third test, and burning rates were also similar. Over most of the burn period (11 hrs) stable conditions prevailed, and a gas with over 110 BTU/scf was produced (C02 11.3, CO-15.H2-I3.3 , CHi -1.6, CnHm-0.3, 02+Ar-1.2, N2-56.9 ). 

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

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