Heat and mass transfer in a square microchannel with asymmetric heating

Abstract

This paper describes the heat and mass transfer in a square microchannel that is heated from one side. This microchannel represents a reaction channel in a microreactor that is used to study the kinetics of the catalytic partial oxidation of methane. The microchannel is contained in a silicon wafer and is covered by a thin silicon sheet. At the top side of this sheet, heating elements are present which mimic the heat that is produced as a result of the exothermic chemical reaction. Correlations for Nusselt and Sherwood numbers as a function of the Graetz number are derived for laminar and plug flow conditions. These correlations describe the heat and mass transport at the covering top sheet of the microchannel as well as at its side and bottom walls. By means of computational fluid dynamic simulations, the laminar flow is studied. To determine an approximate laminar flow Nusselt correlation, the heat transport was solved analytically for plug flow conditions to describe the influence of changes in the thermal boundaries of the system. The laminar flow case is experimentally validated by measuring the actual temperature distribution in a 500 lm square, 3 cm long, microchannel that is covered by a 1 lm and by a 1.9 lm thick silicon sheet with heating elements and temperature sensors on top. The Nusselt and Sherwood correlations can be used to readily quantify the heat and mass transport to support kinetic studies of catalytic reactions in this type of microreactor.

Chemistry in Microstructured Reactors

“…Since the assemblies are made of materials with different conductivities, heat management in narrow spaces and at interfaces is rather complex, and requires special methods for heat addition and removal, such as, for example, an asymmetric introduction of cooling fluid flows. [58][59][60] The small reaction volume of microreactors is also responsible for advantages in their technical safety. [61] The small inner dimensions not only serve to aid heat removal but can also mechanistically control a reaction where, for example, radical chains are terminated such that the nonexplosive region is significantly broader as compared to conventional reactors.…”

Enhanced liquid–solid mass transfer in microchannels by a layer of carbon nanofibers

“…Van Male et al already reported that the condition given in Eq. (5) is not always valid for microchannels [29]. For a gas phase reaction in square microchannels with a hydraulic diameter of 500 m, they observed the following Sh correlation:…”

Chemie in Mikrostrukturreaktoren

“…B. das unsymmetrische Einbringen von Kühlfluidströ-men. [58][59][60] Die sicherheitstechnischen Vorteile von Mikroreaktoren beruhen allerdings nicht nur auf der besseren Wärmeabfuhr in den kleinen Reaktionsvolumina, sondern auch auf ¾nderungen des Reaktionsmechanimus: Beispielsweise werden Kettenreaktionen durch eine Oberflächenreak-tion der Radikale an den Wänden des Mikrokanals unterbrochen (Quenchen), sodass der Nichtexplosionsbereich deutlich größer ist als beim Arbeiten in konventionellen Reaktoren. [40,61,62] Als Folge der laminaren Strömung stellen sich im Kanal typischerweise parabole Geschwindigkeitsprofile ein, die zu breiten Verweilzeitverteilungen führen können.…”