Heat Transfer Coefficient During Water Jet Cooling of High-Temperature Steel

Abstract

Impinging water jets promote high heat flux extraction rate. Steel industry widely employs the process for accurate temperature control to improve the microstructure and to ensure adequate mechanical properties. The range of surface temperatures, heat fluxes and cooling rates are very large, which makes it important to obtain an accurate value of the heat transfer coefficient. This paper presents an experimental and numerical study of the heat transfer behavior of a high temperature (450°C - 900°C) steel plate cooled by a water jet at 20°C to 70°C. High-speed imaging (up to 20,000 fps) within water jet impingement zone allowed the characterization of the boiling regimes in the early stages of cooling. The effects of initial temperature, water jet temperature and velocity and on the heat transfer coefficient were analyzed by inverse heat conduction method that predicts the heat flux and temperature on the top surface from temperatures measured with thermocouples inserted in test plate. Heat transfer is strongly affected by the initial temperature of the hot steel, water jet temperature and, less intensely, by jet velocity. High cooling rates start when liquid water is in direct contact with surface temperatures above 700°C.The results will contribute to the enhancement of the temperature cooling control on the runout table and cooling model employed at Usiminas Hot Strip Mill.

Inaccuracies in the inverse heat conduction problem solution and their effect on the estimation of heat fluxes during quenching

“…The results reported in literature show another common characteristic in the negative slope side of the boiling curve: the slope in the initial stages of the experiment seems to be independent of the initial temperature [3,10] . However, stagnation zone recordings show significantly different boiling regimes in the first instants of quenching, for different initial plate temperatures.…”

“…The effect of initial plate temperature on the boiling curve has been studied in several transient quenching experiments and the results are consistent [2,3,10] . The general trend is illustrated in Fig.…”

“…Studies reported in literature [10,21] agree that quenching with subcooled water jets results in either immediate rewetting or very short film boiling periods in the order of milliseconds, at least at initial temperatures below 900 • C. As a consequence, a sharp increase of heat flux comparable to a step function is expected. Such a step function rise of the boiling curve has been claimed to be the case for industrial quenching technologies that impede the formation of a stable film boiling regime [22] .…”

“…Subdomain F1 is based on the typical size of the drill tip used to make the thermocouple holes (0.2 mm depth). The dimensions of subdomain F2 are based on the typical distance between 2 thermocouples in quenching experiments (approximately 1 cm, [10] ). Subdomain F3 is based on the typical thermocouple dimensions (1 mm diameter).…”

“…As a result, it is impossible to corroborate if the results are representative of the whole surface or only of the phenomena occurring on the sensor. Consequently, the most used temperature measurement technique is by thermocouples that are welded or fixed inside the steel plate with some thermally conduc- tive paste [1][2][3][10][11][12] . In such experiments, the experimental data consists of the internal temperature histories at various locations inside the plate.…”