Experimental study in steady-state for determining the coefficient of external heat transfer on the outer wall of rotating tubes during the quenching process by immersion in water tank

Abstract

The quenching process by immersion is one of the steel tube industry's most commonly used heat treatment processes. For mathematical models to be reliably used as tools capable of simulating different geometries and cooling mechanisms, knowledge of the heat transfer coefficient between the surface of the tube and the water is essential. The present work's primary objective is the experimental determination of the heat transfer coefficient between the external surface of the tube and the water. For this purpose, an experimental apparatus capable of representing the conditions of a real process was built. The predominant heat transfer regime in the single-phase region was mixed convection for the conditions tested. Two correlations were proposed to represent the data obtained for this region. Based on existing correlations in the literature, the first one returned a mean absolute error of 13.97 %. The second one was proposed based on an optimization tool, returning a mean absolute error of 6.29 %. Concerning the two-phase region, the obtained data characterized a transition regime and the beginning of the nucleated boiling. In this region, it was possible to observe that the effect of increasing rotation on the external heat transfer coefficient decreases with increasing tube surface temperature, which follows studies in the literature. The repeatability test shows that the apparatus could replicate the heat transfer coefficient values for the free convection region, staying within the range of uncertainties. However, for the boiling region, the data showed more significant disagreement. This behavior is mainly due to the difficulty of keeping the surface under the same conditions, which have greatly influenced the formation of bubbles and heat transfer.