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The phenomenon of thermal channelling in countercurrent gas‐solid heat exchangers
Author(s) -
Wonchala E. P.,
Wynnyckyj J. R.
Publication year - 1987
Publication title -
the canadian journal of chemical engineering
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.404
H-Index - 67
eISSN - 1939-019X
pISSN - 0008-4034
DOI - 10.1002/cjce.5450650505
Subject(s) - countercurrent exchange , heat exchanger , channelling , mechanics , thermodynamics , thermal , reynolds number , dimensionless quantity , chemistry , physics , ion , organic chemistry , turbulence
Maldistributed flow in moving‐bed reactors and heat exchangers is recognized to be a serious problem. Heat exchanger equations do not provide a clue to the cause. In this paper it is shown that maldistributed flow can occur even in perfectly homogeneous packed beds and that this arises because of inherent temperature non‐uniformities. The term thermal channelling is introduced to describe this effect. It is shown that simultaneous consideration of both the heat and the mechanical energy (the Ergun equation) balances predicts that the gas, in cooling a hot bed, divides into a multiplicity of stable channels with significantly differing mass velocities. This does not, however, occur for the inverse case of heating the bed. Further, it is shown that the system conditions where a multiplicity of states is stable can be defined in terms of an integral mean temperature and two dimensionless variables, NTU and the bed Reynolds number. In general tall beds with large temperature differences are most prone to thermal channelling.