An analysis of the second law of thermodynamics was performed for Al2O3/H2O nanofluid in different plate-pin fin-splitter heat sinks in the present study. The square pin fins with rectangular, arched, and wavy splitters behind them were inserted inside the plate-fin heat sink. We investigated the effects of nanofluid, volume fraction of nanoparticles, Reynolds number, splitter shape, pin width, and splitter length on the heat transfer, fluid flow, thermal, frictional entropy generation rates or irreversibilities, the sensitivity analysis, and Bejan number inside the plate-pin fin-splitter heat sink. The nanofluid was simulated using a two-phase mixture model. The numerical results were validated with both empirical correlations and experimental data. The results indicate that the thermal performance is improved by about 15% for pure water, and the f factor is reduced by about 0.6% using rectangular splitters behind pin fins. Also, Nu is increased by about 25% with the simultaneous use of 2% vol. Al2O3/H2O nanofluid and a rectangular splitter. It is increased more by using arched splitters compared to rectangular and wavy splitters. The rectangular, arched, and wavy splitters generate up to 39.34%, 42.36%, and 38.61% lower thermal entropy generation than the smooth heat sink, respectively. Among splitter geometries, the arched splitters have the highest amount of frictional entropy generation. The thermal irreversibility is increased by about 71% and 43.2%, and the frictional irreversibility is reduced by about 4.1% and 0.3% with reduced pin width and splitter length in the plate-pin fin-rectangular splitter heat sink, respectively. The use of nanofluid reduces thermal irreversibility and increases frictional irreversibility. By increasing the volume fraction of nanoparticles from 0.5 to 3%, the thermal irreversibility is reduced by about 63.3%. Thus, the frictional irreversibility is increased by about 6.8% in the plate-pin fin-arched splitter heat sink. Moreover, the results show that thermal performance and thermal irreversibility are sensitive to changes in the pin width, splitter length, Re, and nanofluid volume fraction.

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