Parametric Study of Solid-Phase Axial Heat Conduction in Thermally Integrated Microchannel Networks

Abstract

A parametric study is presented to highlight design challenges of thermally integrated microchannel networks for portable chemistry and/or fuels reforming. One-dimensional modeling analysis of heat transfer in a twofluid system is presented for the case of (i) two nonreacting fluids (heat exchanger), (ii) a single exothermic reacting fluid and a second nonreacting fluid (regenerative combustor), and (iii) one exothermic reacting fluid and a second endothermic reacting fluid (heat exchanger reactor). In each case, the influence of solid-phase thermal conductivity and thermal packaging upon thermal efficiency, reaction conversion, and steady-state multiplicity is investigated. Results demonstrate the importance of both packaging and solid-phase axial thermal conduction upon system performance, with optimal performance obtained using low thermal conductivity substrates. Modeling analysis predicts steady-state multiplicity when employing low thermal conductivity materials, illustrating the need for future detailed stability analysis. Lastly, simplified mechanical analysis is presented to illustrate the value of coupled thermomechanical analysis.

Autothermal hydrogen generation from methanol in a ceramic microchannel network

“…Due to the limited axial heat conduction associated with low thermal conductivity (2 W m −1 K −1 ) cordierite, the process is thermally selfsustaining with a minimum temperature increase in the brass distributors (around 40-50 • C), making the device safe and easy to handle. These phenomena had been previously investigated theoretically by Moreno et al [29], who predicted that low thermal conductivity substrates would enable placement of a hot-spot in the middle of the reactor, in turn achieving high thermal efficiency.…”

“…Recent one-dimensional analysis of several heat-exchanger configurations performed by Moreno et al [29] illustrated how high thermal conductivity materials (e.g. silicon, stainless steel) may significantly limit thermal efficiencies of high temperature microreactors via rapid thermal equilibration of the solid phase.…”

Axial Back Conduction through Channel Walls During Internal Convective Microchannel Flows

“…Axial wall conduction also affects the performance of microreactors. Therefore, selection of substrate material is an important issue in the design of microreactor systems [45]. Depending on the constraints on large thermal gradients, localized hot-spot formation, isothermal operation, etc., substrate thermal conductivity may be selected to facilitate by limiting the rate of axial heat conduction.…”

“…Depending on the constraints on large thermal gradients, localized hot-spot formation, isothermal operation, etc., substrate thermal conductivity may be selected to facilitate by limiting the rate of axial heat conduction. Moreno et al [45] presented a parametric study to highlight design challenges of thermally integrated microchannel networks for chemical and/or fuels reforming and investigated the influence of solid-phase thermal conductivity and thermal packaging on (1) thermal efficiency (2) reaction conversion and (3) steady-state multiplicity. Their results indicated that low thermal conductive substrates provide optimal reactor performance.…”

Development of a single stage heat integrated water–gas shift reactor for fuel processing

“…A high thermal conductivity of the solid phase would lead to an unwanted flattening of the desired temperature gradient in the direction of the flows. To reach the optimal declining temperature profile, careful tuning of the thermal conductivity of the solid phase is required [8]. Opposed to micro-structured reactors, Groppi showed that radial isothermal operation of a cooled honeycomb monolith reactor is enhanced by increasing the thermal conductivity of the monolith [9,10].…”