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Based on the first-law and second-law of thermodynamics, we investigate thermal performance and entropy generation of water–alumina nanofluid flows in porous media embedded in a microchannel under local thermal non-equilibrium condition. Analytical closed-form solutions of two-dimensional temperature distributions are obtained for the cases with and without the viscous dissipation term in the energy equation. The thermal non-equilibrium entropy generation function is derived using the differential method. Due to the embedment of the porous medium in the microchannel and the suspension of the nanoparticle in the working fluid, the viscous dissipation effect is magnified significantly, altering thermal characteristics and entropy generation of the system. For the case where the viscous dissipation effect is neglected, total entropy generation and fluid friction irreversibility are overrated while heat transfer irreversibility is remarkably underestimated. In a low-aspect-ratio microchannel, the suspension of nanoparticles in the fluid decreases the thermodynamic efficiency from the second-law point of view. Utilization of nanofluids in a high-aspect-ratio microchannel enhances exergetic effectiveness in low-Reynolds-number flow regime. By reducing the nanoparticle size, entropy generation can be decreased by as much as 73%. The optimum Reynolds number associated with minimum entropy generation for nanofluid flow in a porous microchannel is identified. The optimum range of porous medium permeability is characterized by Da ⩾ 10 - 1 . It is observed that effectiveness of the interstitial heat transfer between the solid and fluid phases of the porous medium induces a pronounced effect on the entropy generation, signifying the importance to consider the thermal non-equilibrium condition in the second-law performance analysis of porous-medium flow.

Field-synergy analysis is performed on the viscous dissipative water–alumina nanofluid flow in circular microchannel heat sinks to scrutinize the synergetic relation between the flow and temperature fields for both heating and cooling processes. By varying the Reynolds number and the nanoparticle volume fraction, the effect of viscous dissipation in the convective heat transfer of nanofluid is investigated based on the field synergy principles. For the heating process, under the effect of viscous dissipation, the degree of synergy between the velocity and temperature fields of nanofluid flow deteriorates, leading to a dwindled heat transfer performance of the nanofluid. Due to the presence of two difference heat sources, the synergy angle and field synergy number for viscous dissipative flow are defined to characterize the synergetic behavior. The field synergy in the nanofluid is greater at low-Reynolds-number flow due to the reduced effect of viscous dissipation. By reducing the size of the nanoparticle and increasing the diameter of the microchannel, the degree of synergy between the velocity and temperature fields of the nanofluid flow in microchannel can be intensified, yielding convection heat transfer enhancement. On the other hand, for nanofluid cooling process, the presence of viscous dissipation would augment the field synergy of the flow.

An analytical study on the viscous dissipation effect on entropy generation in laminar fully developed forced convection of water–alumina nanofluid in circular microchannels is reported. In the first-law analysis, closed form solutions of the temperature distributions in the radial direction for the models with and without viscous dissipation term in the energy equation are obtained. The results show that the heat transfer coefficient decreases with nanoparticle volume fraction largely in the laminar regime of nanofluid flow in microchannel when the viscous dissipation effect is taken into account. In the second-law analysis, the two models are compared by analyzing their relative deviations in entropy generation for different Reynolds number and nanoparticle volume fraction. When the viscous dissipation is taken into account, the temperature distribution is prominently affected and consequently the entropy generation ascribable to the heat transfer irreversibility is significantly increased. The increase of entropy generation induced by the increase of nanoparticle volume fraction is attributed to the increase of both the thermal conductivity and viscosity of nanofluid which causes augmentation in the heat transfer and fluid friction irreversibilities, respectively. By incorporating the viscous dissipation effect, both thermal performance and exergetic effectiveness for forced convection of nanofluid in microchannels dwindle with nanoparticle volume fraction, contrary to the widespread conjecture that nanofluids possess advantage over pure fluid associated with higher overall effectiveness from the aspects of first-law and second-law of thermodynamics.