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Computer simulation of the push-type slab reheating furnace

University of British Columbia

A mathematical heat-transfer model for the slab reheating furnace has been developed. Radiation in the furnace chamber was calculated using the zone method, with the gas temperature distribution being assumed, and heat transfer in the slab was determined using a finite-difference approximation of two-dimensional transient conduction. These individual calculations were coupled to allow prediction of the temperature profiles in, and heat flux to, refractory walls and slabs at any point inside the furnace. The emissive/absorptive characteristics of the gas mixture within the furnace chamber were simulated with a clear-plus-two-gray-gas model which simulated the real gas behaviour to within 5%. For the calculation of radiative exchanges, the furnace chamber was subdivided into 432 isothermal zones, and radiative exchange factors to slab surfaces were evaluated rather than relying on empirical or experimental estimations as in previous studies. An iterative technique was devised in order to combine the radiative and slab heat conduction calculations. For the purpose of identifying the mechanism of skidmark formation, the region of skidrail/slab contact was examined in detail by introducing a radiation shielding factor to account for the presence of the skid structure. The gas temperature distribution inside the furnace chamber was found to have a significant influence on the heat flux to the slab surface. Nonuniform gas temperature transverse to the push direction causes an uneven transverse slab temperature distribution and subsequent rolling problems. Higher gas temperatures near the sidewall refractory were shown to cause serious distortion of the transverse heat-flux distribution. The heating practice for the hot charging of slabs was simulated by the model in order to improve the process from the standpoint of energy conservation and slab temperature uniformity. Model predictions have shown that the fuel input could be reduced substantially near the slab entrance where the port to the chimney is located, thus maximizing the residence time of the combustion products. Alternatively the throughput of the furnace can be increased if the fuel input remains the same as for charging cold slabs. The extent of increase in production rate can be determined by the off-line computer model. The model was used to predict the thermal behaviour of slabs for various thicknesses, steel grades and push rates. The results consistently indicated that the selection of an appropriate push rate is crucial to the final temperature distribution. The study of the mechanism of skidmark formation showed' that the radiation shielding effect of the skidrail was the dominant factor, accounting for 90% of the heating deficit around the slab/skidrail contact region. Computer simulation of the possible measures that could be taken to alleviate the skidmark formation has indicated that reducing the height and width of the skidrail improved radiative heat transfer in the contact region. Coating highly reflective materials on the exterior surface of the skidrail to increase reflectivity from 0.3 to 0.8, could enhance heat transfer locally around the the skidrail by about 25% - 30% when the skidrail temperature is lower than the slab bottom temperature.

Text

2010-07-10

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http://hdl.handle.net/2429/26308

Materials Engineering

University of British Columbia

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