Fluid Dynamics in Microchannel Reactors. The possible benefits of using microchannel reactors in the chemical process industries have been under discussion for several years. Outstanding heat transfer properties, short residence times and short diffusion lengths are considered key advantages, while the cost of the approach and the pending question of long‐term stability are still major sources of skepticism. A good starting point for a feasibility study is to investigate some major fluid dynamic properties. For a unit consisting of hundreds of parallel microchannels, the pressure drop for the laminar flow is just a few millibar. However, at high gas flows the distribution over the numerous channels may become non‐uniform. It is shown how the resulting residence time distribution affects both the yield of intermediates in consecutive reactions, and step functions of the concentration during periodic operation, respectively. All in all, the results of the estimations are encouraging.

Zusammenfassung: Der Cauchy-Green-Tensor einer beliebig vorgegebenen Grundstr6mung wird ffir kleine St6rungen linearisiert und das Ergebnis in geschlossener Form angegeben. AnschlieBend werden das Stabilit~itsverhalten einer viskoelastischen Fltissigkeit hinsichtlich spezieller St6rungen untersucht und verschiedene Grenzfalle (kurze und lange Wellen) diskutiert. Zum SchluB wird ein durch die elastischen Eigenschaften der Fltissigkeit bestimmter Instabilit~itsmechanismus ftir Maxwell-und Doi-Edwards-Fluide aufgezeigt.Abstract: The Cauchy-Green tensor of a given basic flow is linearized for small perturbations and the result is given in explicit form. The stability behaviour of an elastic fluid is then investigated for a special type of perturbation, and several limiting cases (short and long waves) are discussed. Finally, an instability mechanism of Maxwell and Doi-Edwards fluids caused by the elastic properties of these fluids is demonstrated.

Zusammenfassung: Die Stabilit~it der ebenen Scherstr6mung eines einfachen Fluids wird im Rahmen der Kurzwellenapproximation ftir St6rungen in der viskosimetrischen Ebene untersucht. Ffir kurze Wellen ist eine Stabilit~itsanalyse unabhiingig von der speziellen Form der Stoffgleichung m6glich. Die vorliegende Analyse stellt einen ersten Schritt in diese Richtung dar und ftihrt zu einem hinreichenden Stabilit~itskriterium. Ffir kurze Wellen sind die Maxwell-Flfissigkeiten A und B bezifglich ebener St6rungen stabil. Abstract:The stability of a plane shear flow of simple fluids is investigated for perturbations in the viscometric plane within the framework of the short wave approximation. For short waves it is possible to carry out a stability analysis, which is independent of the type of constitutive equation. The analysis presented is the first step in this direction and leads to a sufficient stability criterion. For short waves Maxwell fluids A and B are stable for perturbations in the viscometric plane.

Zusammenfassung: Es wird das Stabilit~itsverhalten eines Maxwell-Fluids in einer einfachen ebenen Scherstr6mung ftir eine spezielle St6rungsklasse untersucht. Notwendige und hinreichende Stabilitatskriterien sowie eine kritische Weissenbergzahl (Wek ~ 4) werden angegeben. Die Ergebnisse der Analyse stehen mit experimentellen Befunden in qualitativer Obereinstimmung.Abstract." The stability behaviour of a Maxwell fluid in a simple plane shear flow for a class of special perturbations is investigated. Necessary and sufficient stability criteria, especially a critical Weissenberg number for the stability (Wek ~ 4) are given. The results of the analysis are in qualitative agreement with experimental observations.