Experimental investigation of axis switching and local heat transfer in multiple elliptical free surface water jet impingement

Abstract

The present study experimentally investigates the local and overall heat transfer from multiple elliptical free surface water jets impinging on a thin stainless-steel foil under constant heat flux. Local surface temperatures are measured using infrared thermography to obtain detailed Nusselt number distributions. A symmetric 3 $\times$ 3 jet array with a hydraulic diameter of 3 mm and a pitch of $4d$ is examined for two elliptical orientations, namely streamwise and spanwise. The jets issue from a 2 mm thick orifice plate and impinge on a 0.06 mm thick foil. Experiments are conducted for Reynolds numbers from 1700 to 8500 and $z/d$ ratios from 2 to 10.

To the authors’ knowledge, axis switching is experimentally demonstrated for the first time in multiple elliptical free surface jets. The switching direction depends on jet orientation and shows a 90° phase difference between the two configurations. Local Nusselt numbers decrease with spanwise distance, but this decay weakens as Reynolds number increases. The influence of $z/d$ on stagnation and spanwise average heat transfer remains within 10–15%, indicating a stronger effect on cooling distribution than on magnitude. Stagnation, spanwise average and overall average Nusselt numbers nearly double over the tested Reynolds number range.

The overall average Nusselt number is nearly identical for both orientations, showing that axis switching mainly affects local heat transfer patterns. Streamwise oriented elliptical jets outperform circular jets reported in the literature, providing 30–45 % higher local Nusselt numbers and 70–100 % higher overall averages at low Reynolds numbers and 45–50 % at higher Reynolds numbers. The proposed cosine-based correlations predict approximately 97 % of the data within 20% accuracy.