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Boiling water heat transfer during quenching of steel plates and tubes Li, Dianfeng

Abstract

The design, analysis and control of many metallurgical and materials operations hinges, in part, on being able to quantify the heat transfer occurring at the boundary of the components being processed. Critical to the accurate determination of the surface heat flux is a precise inverse heat conduction model as well as correct temperature measurement during the quench test. This research has explored the factors that can affect the accuracy of a boiling curve prediction including parameters related to both the inverse heat conduction model as well as the temperature measurement technique. During the research, extensive analysis as well as some experiments were done to determine a "best practices method" to measure the temperature-history experienced by the sample during a quench operation. Specifically, two different temperature measurement techniques including surface, where the thermocouple is located on the quench surface, and sub-surface, where the thermocouple is located at an interior position, were analyzed in detail to identify and quantify errors that can be induced in the measured data. Results from the research have highlighted the need to include the thermocouple hole in cases where the thermocouples are instrumented at 90° and a severe water quench occurs at the surface of the sample. Although the work in this study has been conducted on steel alloys, the analysis has been extended to other material and quench conditions and has identified, in a very simple manner using the Biot number, under which quench conditions the thermocouple hole needs to be included in the Inverse Heat Conduction (IHC) analysis. The optimized IHC model and measurement techniques were then used to assess the influence of material start temperature and sample thickness during a water quench test on samples of AISI 316 stainless steel plate. Start temperature is an important parameter during transient quench conditions as it can influence the overall shape and magnitude of the boiling curve. It was determined that, unlike steady state boiling conditions, under transient boiling conditions, a unique boiling curve does not exist for the different quench conditions. Instead the boiling curve becomes a function of the thermal history experienced by the material during the quench and can vary both in magnitude and shape. The work has also identified a new region in boiling water heat transfer during transient quench conditions that identifies the initial interaction of the water and the hot surface and has been called the initial cooling region. In cases where the start temperature of the sample is above the Leidenfrost point, after the initial cooling region is complete, the sample will experience the full boiling curve including, film boiling, transition boiling, nucleate boiling and convective cooling. However if the sample start temperature is below the Leidenfrost point, after the initial cooling region, the sample will only experience nucleate boiling and convective cooling. The work has proposed a relatively simple method to incorporate the influence of sample start temperature during transient quench conditions on the boiling curve and the method involves knowing what the full boiling curve is for the quench condition being studied as well as the change in the heat flux as a function of surface temperature during the initial boiling region.

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