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This work investigates heat transfer and entropy generation of a turbulent flow of an Al2O3–Cu/water hybrid nanofluid in a plain tube (PT) with classical (TPT) and elliptical‑cut twisted tape (TECT) inserts. The heat transfer and pressure drop are investigated numerically at Re (7000–15,000), mass concentration (1–4%), and the inlet temperature of the fluid (300 K). Further, the total entropy generation and Bejan number are examined at Re = 7000 and a mass concentration of 4%. The obtained results indicate that heat transfer can be intensified when inserting classical and elliptical‑cut twisted tape. In addition, an increase in the thermal conductivity of the fluid may cause a slight increase in the heat transfer coefficient. Moreover, heat transfer and thermal performance factors increase when the mass concentration of nanoparticles increases. The Nusselt numbers for TECT and TPT are 1.7 and 1.57 times higher than those for PT, respectively. The Nusselt number and thermal performance factor of hybrid nanofluid are greatest at roughly 195 and 1.9, respectively, showing 3.9% and 7.73% improvement compared to CuO/water nanofluid at Re = 7000. The analysis of the generation of entropy is expressed as a function of thermal and frictional contributions. The results indicate the existence of a minimum entropy generation for each type of tubes for Al2O3–Cu/water hybrid nanofluid. Total entropy generation analysis demonstrates that thermal entropy generation dominates at high heat flux. Moreover, increasing the nanoparticles decreases the generation of total entropy, which is ascribed to the thermal conductivity increment. In addition, the rate of total entropy generation declines as the vortex flow increases.

Microreactors for chemical synthesis and combustion have attracted increased attention in recent years. Due to the high degree of thermal control exhibited by microreactors, exothermic catalytic activity features heavily in these devices and thus advective-diffusive transport is of key importance in their analyses. An analytical study of the transport phenomena in a single microchannel microreactor filled with a porous medium is presented, first as a one-dimensional, then expanded to a two-dimensional model with catalytic activity. The systems under investigation include fluid and porous solid phases inside the microchannel under local thermal non-equilibrium (LTNE), in addition to the enclosing structure of thick walls subject to distinct thermal loads. The thermal diffusion of mass, viscous dissipation of the fluid flow is examined for both a heterogeneous catalyst placed on the channel wall and for a homogeneous reaction process. The axial and transverse variations of heat and mass transfer processes are considered to provide two-dimensional solutions of both the temperature and concentration fields. These are then used to calculate the local and total entropy generation within the system. A novel extension of an existing LTNE model capable of taking into account the enclosing structure as well as the porous solid and fluid phases is presented. The thickness of this enclosing structure is shown to have a major influence on heat and mass transport within the system, particularly the Nusselt number. Irreversibilities in the system are found to be dominated by the mass transfer contributions and the influence of the Soret effect as well as the Damköhler number. A numerical investigation is undertaken to examine the effect of hydrodynamics upon the activity of a heterogeneous catalyst. Three corrugated wall channel configurations with varying phase difference between upper and lower walls were generated. Low Reynolds number flows of fuel lean methane in air were catalytically combusted over platinum in the numerical model. Hydrodynamic reflux features were observed in the out of phase cases. Coinciding with these zones, the site surface concentration of carbon dioxide was observed to vary significantly as compared to the base case. The extension of the LTNE model significantly increases the capability of modelling the interface between thick walls and a porous medium under thermal load, permitting more accurate modelling of microreactors. The effect of the reflux feature on the surface site fraction of product for the corrugated channels reveals the complex interaction between hydrodynamics and catalytic activity.