Experimental investigation of local heat transfer in multiple free surface triangular jet impingement and generalised thermal–hydraulic comparison of orifice shapes

Free surface jet impingement is an effective technique for high heat flux cooling. The flow field and heat transfer characteristics are strongly affected by the orifice geometry. The present study experimentally examines a symmetric 3 × 3 array of free surface triangular water jets using infrared (IR) thermography. The experiments are conducted with a hydraulic diameter of 3 mm, a uniform pitch of 4 d , Reynolds numbers from 1200 to 6000 and z / d from 2 to 10. Local surface temperature distributions are measured over a 37 mm × 37 mm region to determine the local and overall average Nusselt numbers. The triangular jets exhibit a distinct 180° inversion of the impingement thermal footprint at low z / d , as identified from the local Nusselt number contours. To the best of the authors' knowledge, this behaviour is experimentally reported for the first time in multiple free surface triangular jet impingement. The local Nusselt number reaches its maximum near the stagnation region and decreases by approximately 35 – 45% towards the outer regions. A cosine-function correlation predicts the local Nusselt number distribution, with approximately 95% of the experimental data lying within 20% deviation. The thermal and hydraulic performance of triangular, elliptical, rectangular, square and circular orifice shapes is also compared. At identical Reynolds numbers, the investigated non-circular orifice shapes provide up to 30% higher overall average Nusselt numbers than the circular orifice. However, this improvement is accompanied by higher hydraulic losses. The discharge coefficients of the non-circular orifices approach approximately 0.7 at higher Reynolds numbers, whereas the circular orifice approaches approximately 0.87. The results establish general thermal and hydraulic trends for non-circular orifice shapes. These trends may provide useful guidance for estimating the performance of other non-circular orifice shapes with similar geometric characteristics. The study also shows that comparison at identical Reynolds numbers is more appropriate than comparison at identical total mass flow rates when all orifice shapes have the same hydraulic diameter.