Projeto e validação de um banco de testes para determinação do coeficiente de troca de calor entre a superfície interna de um tubo de aço e a água de resfriamento durante o processo de têmpera

Abstract

Quenching is a heat treatment process of paramount importance in the production of steel tubes. This treatment aims to give the steel the desired mechanical properties. Among the various existing quenching processes, one of the most used consists of immersing the tube in a water tank equipped with a system that promotes the rotation of the tube during cooling. The present work has as main objective the design, the dimensioning and the construction of a test bench for experimental determination of the heat transfer coefficient between the inner surface of the tube and the water under conditions similar to the quenching process. To validate the experimental procedures, almost all tests were conducted with the static tube with internal single-phase water flow of liquid and steam. The results were compared with theoretical values obtained through correlations available in the literature. In the case of steam flow, the experimental data showed good agreement, with 95 \% of the points showing a maximum error of $ \pm $ 30 \% in relation to theoretical values with an mean absolute relative difference (MARD) of 14 \% . For liquid water, 75 \% of the experimental points showed a maximum error of $ \pm $ 30 \% compared to theoretical values and the absolute average was 28 \%. In addition, the values obtained with liquid water were systematically lower than the theoretical ones. Tests to assess the effects of rotation were conducted using water in the vapor state for rotations of 25, 50 and 100 RPM. At the highest rotational speed, for all tested situations, the heat exchange coefficient was lower than that for the stationary tube, suggesting that higher speeds inhibit heat exchange. The heat transfer coefficients for the rotating tube were compared with values obtained by a correlation of SEGHIR-OUALI et al. Despite the test conditions being outside the range of validity of this correlation, the evolution curve of the experimental results showed a coherent behavior in relation to the referred correlation.