The current investigation analyzes experimentally and numerically the hydrothermal performance of a water-cooled pin-fin heat sink by changing pin-length and longitudinal-pitch. The new designs of heat sinks are named as variable pin-length with low to high arrangement (PL-LH), variable pin-length with high to low arrangement (PL-HL), variable longitudinal-pitch with low to high arrangement (LP-LH), and variable longitudinal-pitch with high to low arrangement (LP-HL). The comparison results show that the numerical results gives reasonable agreement with the experimental data with the maximum percent of error within ±20%. The average augmentation percentages of the friction factor-Nusselt number for the PL-LH, PL-HL, LP-LH, and LP-HL compared with the smooth case are 224%-184%, 246%-214%, 305%-195%, and 349%-203%, respectively. As mentioned, the highest heat transfer enhancements are recorded for the PL-HL, and the LP-HL comes in the second. This implies that adopting the pin-fins with larger pin-length at the entrance region and shorter longitudinal-pitch at the exit region is an effective way to enhance the heat transfer in the heat sink. Finally, the PL-HL presents the best overall hydrothermal performance when compared with the other models. The maximum value of the performance index can be 1.65 when the mass flow rate is 0.00585 kg/s.

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