The conjugate effect, also known as the coupling effect, involves the thermal profiles of the solid and the fluid domain due to heat interactions between the solid and fluid domains. A detailed three-dimensional numerical study is conducted to understand the effect of solid substrate heat conduction on the fluid convection in diamond (diverging-converging) microchannels. Experiments have also been conducted to validate the mathematical model employed in the present study. The first part of the study covers the impact of solid-fluid thermal conductivity and solid-fluid thickness ratio on thermal conditions at the solid-fluid interface with the help of peripheral heat flux and solid-substrate isotherms. The isotherms show multi-directional thermal gradients for low solid-fluid thermal conductivity ratios, whereas only axial thermal gradient is seen for a higher solid-fluid thermal conductivity ratio. The second part covers the influence of solid-fluid thermal conductivity ratio and solid-fluid thickness ratio on Nusselt number characteristics of a diamond microchannel with different divergence-convergence angles and width ratios. The analysis also reveals that at a particular intermediate solid-fluid thermal conductivity ratio, the Nusselt number becomes maximum. The last part covers the quantification of conjugate effects in diamond microchannels by employing a non-dimensional wall conduction number. The analysis reveals that the wall conduction effect is inconsequential in diamond microchannels when the non-dimensional wall conduction number is <0.01.

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