Resumo:
The cooling technique using impinging laminar jets is one of the most widely used in steel mills, especially in continuous hot rolling lines for steel plates. This method produces cooling rates of ~85ºC/s, with heat fluxes of around 10 MW/m². To evaluate the phenomena involved, an experimental apparatus was adopted containing a heating furnace, a water tank, a U-tube type distributor tube, and 304 steel plates instrumented with K-type thermocouples. The initial cooling temperature varied from 750ºC to 1000ºC, with water flow measurement. Temperature transients were collected and stored by a datalogger following heating and cooling cycles. The different test conditions allowed detailed analysis of the cooling curves and extraction of thermal characteristics. A mathematical model based on the finite element method, developed in Python 3.11 algorithm, was used. By solving the inverse heat conduction problem, the surface temperatures, heat fluxes and heat transfer coefficients were calculated. The regularization parameter by the “sequential method” and optimization with the BOBYQA algorithm were developed and compared to the results obtained experimentally. The impacts of the variation of the initial temperatures of the samples, behavior of the heat flux curves and heat transfer coefficient according to the time variation and in relation to the numerically calculated surface temperature were evaluated. The 220-node mesh used presented an excellent compromise between computational effort and results. The temperature variations were less than 1.24%. The variations of the heat fluxes were less than 10.7%. The study concluded that the heat transfer coefficient is highly influenced by the initial temperature of the samples and physical characteristics of the jet, presenting a maximum value of 17300 W/m².K. The heat transfer coefficient was evaluated at the stagnation point, presenting a sharp decline along the heat flux from a temperature close to 250ºC in the three samples. The maximum heat flux reached a value of 3.26 MW/m² for a temperature of 1000ºC. It decreased with decreasing temperature, as concluded by Lee.
