Experimental study on enhanced heat transfer during rapid cooling of modified and oxidized rods

Highlights: • Experimental quenching study performed for three surface features (smooth, threaded, and knurled). • $T_{\min }$, quenching time, HTC, and vapor film thickness were measured and analyzed. • Investigate the effect of surface oxidation on pool boiling performance by performing ten sequential experiments for the three samples. • Experimental results were compared to related correlations in literature. This research investigates the effects of surface modification and surface oxidation on pool boiling heat transfer performance. An experimental facility was designed and built to perform vertical quenching testings at atmospheric pressure and close to the saturation water pool. Three surface structures (smooth, threaded, and knurled) were fabricated to study the effect of surface area variation and surface oxidation on minimum film boiling temperature ($T_{\min }$). A visualization study was performed using a high-speed camera to capture the thermohydraulic behavior of the generated vapor around the heated sample. The results showed that surface features have a significant impact on the enhancement in heat transfer as the knurled surface exhibited a strong impact on increasing $T_{\min }$, quenching time, and heat transfer coefficient (HTC). The visualization study displayed different behaviors of the vapor layer breakage where the smooth and the threaded samples experienced larger bubbles and smoother movement of the quench front. Whereas smaller and intense bubbles were generated around the knurled sample at $T_{\min }$ point. The successive experiments resulted in oxidation layers on the test samples, which proved that the oxidation mechanism depends on the surface finish that caused the surface to experience different heat fluxes. The comparison between the experimental results and available correlations in literature showed the importance of incorporating the surface characteristics effect in $T_{\min }$ correlations due to its significant impact on pool boiling heat transfer performance.