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This study numerically verified the enhanced efficiency of a steel reheating furnace when applying oxy-fuel combustion instead of air-fuel combustion. Only radiation heat transfer was considered to analyze the periodically transient slab heating for an axial-fired furnace. The radiation field was computed without flow field calculation by dividing the entire furnace into ten subzones of which the temperatures were calculated by taking the overall heat balance for all the subzones. A total of five cases, 2 for air-fuels and 3 for oxy-fuels, was analyzed to compare the slab heating behavior between air-fuel and oxy-fuel combustion. The modified 5-gas WSGGM was used for oxy-fuel combustion cases to fulfill the characteristics of CO2 and H2O enriched medium, while ordinary 4-gas WSGGM was used for air-fuel combustion cases. From the efficiency analysis for the total of five cases, it was predicted that oxy-fuel combustion gave an approximately 50% enhancement in efficiency compared to air-fuel combustion.

A transient radiative slab heating analysis was performed to investigate the effect of various fuel mixtures on the performance of an axial-fired reheating furnace. The various fuel mixtures tested were assumed to be attained by mixing COG (coke oven gas) and BFG (blast furnace gas), which are the two main byproduct gases found in the integrated steel mill industry. The numerical prediction of radiative heat transfer was calculated using an FVM radiation solving method, which is a well-known and efficient method for curvilinear coordinates. The WSGGM (weighted sum of gray gas model) was also adopted to calculate the radiative heat transfer in composition dependent media. The entire furnace was divided into fourteen sub-zones to calculate the radiative thermal characteristics of the furnace without flow field calculations. Each sub-zone was assumed to have homogeneous media and wall temperatures. All of the medium and wall temperatures were computed by calculating the overall heat balance using some relevant assumptions. The overall heat balance was satisfied when the net heat input equaled the three sources of heat loss in each sub-zone, wall loss, skid loss, and slab heating loss.

A 3D unsteady numerical simulation of a reheating furnace was performed to obtain the optimal slab residence time. Too long residence time decrease the efficiency of the reheating furnace, whereas too short residence time cannot satisfy the required heating quality of a slab. The total five cases of residence times – 6032 s, 6496 s, 6960 s, 7424 s and 7888 s – were investigated for the optimum residence time analysis with the two slab requirements, those of emission temperature and uniformity. In this study, the slab emission temperature should be in the range between 1373 K and 1573 K. The skid mark severity of an emitted slab should be lower than 50 K to satisfy the uniformity requirement. The numerical analysis was done for the identical geometry and operating condition of the reheating furnace using FLUENT. Slabs were assumed to move very quickly that it took no time for them to move next positions. The quick movements of slabs were processed with the own developed User-Defined Function program. Among the five cases of residence times, the residence time of 7427 s turned out to be most efficient.

Numerical analysis of slab heating characteristics in a reheating furnace has been accomplished using FLUENT, a commercial software. The phenomena in the furnace are periodically transient because the slabs are transported toward a rolling mill with every time interval controlled. Unsteady calculation has been performed to obtain a periodically transient solution. A User-Defined Function (UDF) program has been developed to process the movement of slabs because FLUENT cannot handle the movement using its default functions. When the mean temperature of a slab emitted to the rolling mill does not change, calculation is considered to have converged and is stopped. This convergence criterion is appropriate for achieving an analytical solution. With the boundary and initial conditions given, over 55 new slabs are inserted to get a converged solution. Skid posts and beams are included in the calculation because they disturb radiation heat transfer from hot combustion gas to the slabs. This article examines what the slabs experience in the furnace before they are emitted to the rolling mill and whether a slab emitted to the rolling mill satisfies the required slab conditions, such as target temperature and skid severity.

In this study, the unknown wall temperature profile of a cylinder was predicted by applying the inverse method. The temperature profile of the cylinder wall was predicted from the given temperature data at measurement points near the cylinder wall. The cylinder was assumed to represent a typical pipe in a bundle of heat exchange tubes operating in a high temperature system. Radiative heat transfer was incorporated as one of major heat transfer modes to consider a hot gas flow passing over the cylinder. The corresponding inverse problem was solved by minimizing an objective function by applying the iterative conjugate gradient method. A multi-block grid composed of three different blocks was used for better computational accuracy and convenience in locating the measurement points. A new method, which could be applicable to non-symmetric geometry, was adopted to solve the adjoint equation. In this study, the effects of number and location of the measurement points were numerically investigated. When the measurement points were too close to the cylinder, the predicted temperature profile exhibited larger fluctuations. The results have also shown that an appropriate number of measurement points were required to improve the prediction of the boundary temperature profile.

Transient radiative heating characteristics of slabs in a walking beam type reheating furnace is predicted by the finite-volume method (FVM) for radiation. The FVM can calculate the radiative intensity absorbed and emitted by hot gas as well as emitted by the wall with curvilinear geometry. The non-gray weighted sum of gray gas model (WSGGM) which is more realistic than the gray gas model is used for better accurate prediction of gas radiation. The block-off procedure is applied to the treatment of the slabs inside which intensity has no meaning. Entire domain is divided into eight sub-zones to specify temperature distribution, and each sub-zone has different temperatures and the same species composition. Temperature field of a slab is acquired by solving the transient 3D heat conduction equation. Incident radiation flux into a slab is used for the boundary condition of the heat conduction equation governing the slab temperature. The movement of the slabs is taken into account and calculation is performed during the residence time of a slab in the furnace. The slab heating characteristics is also investigated for the various slab residence times. Main interest of this study is the transient variation of the average temperature and temperature non-uniformity of the slabs.

A numerical program has been developed and applied to investigate the heating characteristics of the slab in a bench scale reheating furnace. Its numerical results were validated by comparison with the experimental data obtained in the test facility in POSCO company