112
M. Feng et al.
Fig. 9.7 that the relationship curve shape between the ST or CT and resident time
are both similar to “S” shape. In the preheating zone, the heating rate of the slab is
low at the start, then, gradually increases. After the slab moving to the heating zone,
both of the surface and core temperature are rapidly increasing. In the soaking zone,
the surface temperature is almost not increasing with resident time, and the core
temperature still increases although the heating rate is somewhat lower. These cases
are concert with the temperature distribution in the furnace. It can also be known
from Fig. 9.7 that during the preheating zone and the first part of heating zone, the
temperature difference between the ST and CT increases gradually with the slab
gradually moving forward. And, it increases more quickly in the heating zone than
that of in the preheating zone. When the slab moves to near the middle of the heating
zone, the temperature difference reaches the peak value of 230.8 K. In the next zone,
the temperature difference gradually decreases to about 21.9 K. That means the nonuniformity of temperature field in the slab increases firstly then decreases in the
whole reheating process. As shown previously, the process of reheating slab in the
furnace is a typical 3-D transient heat transfer process. In the process, the heating
rate is mainly caused by the heat transfer flux on the surfaces of the slab, which
mainly depends on the temperature difference between the gas and the surfaces. The
higher temperature difference between the gas and the surfaces is favorable to higher
heat transfer flux to increase the heating rate, but unfavorable to the uniformity of
temperature field in the slab. This is exactly the reason why setting the soaking zone
in the furnace. Generally, under the condition of keeping the size of the slab constant,
the temperature difference in the cross section will be principally dependent on Bolt
number (Bi) and Fourier number (Fo). The relationship can be expressed as Eq. 9.13:
T = f (Bi, Fo).
(9.13)
To decrease Bi or increase Fo is helpful for decreasing the temperature difference
in the cross section of the slab and improving the quality of produce. By comparison,
it can also be seen that the temperature of the surfaces with oxide scale is lower
than that of without oxide scale during the reheating process, particularly in the high
temperature zone. Because, the heat conductively of the oxide scale is very small. It
can cause a great thermal resistance between the surroundings and slab surfaces to
decrease the heat transfer flux.
9.5 Conclusions
In this work, a 3D transient mathematical heat transfer model for a pusher type
reheating furnace was established. The heat transfer behavior of the slab was studied
during the whole reheating process and the effect of oxide scale on the surfaces on
heat transfer characteristics was full considered. Get the following conclusions:
M. Feng et al.
Fig. 9.7 that the relationship curve shape between the ST or CT and resident time
are both similar to “S” shape. In the preheating zone, the heating rate of the slab is
low at the start, then, gradually increases. After the slab moving to the heating zone,
both of the surface and core temperature are rapidly increasing. In the soaking zone,
the surface temperature is almost not increasing with resident time, and the core
temperature still increases although the heating rate is somewhat lower. These cases
are concert with the temperature distribution in the furnace. It can also be known
from Fig. 9.7 that during the preheating zone and the first part of heating zone, the
temperature difference between the ST and CT increases gradually with the slab
gradually moving forward. And, it increases more quickly in the heating zone than
that of in the preheating zone. When the slab moves to near the middle of the heating
zone, the temperature difference reaches the peak value of 230.8 K. In the next zone,
the temperature difference gradually decreases to about 21.9 K. That means the nonuniformity of temperature field in the slab increases firstly then decreases in the
whole reheating process. As shown previously, the process of reheating slab in the
furnace is a typical 3-D transient heat transfer process. In the process, the heating
rate is mainly caused by the heat transfer flux on the surfaces of the slab, which
mainly depends on the temperature difference between the gas and the surfaces. The
higher temperature difference between the gas and the surfaces is favorable to higher
heat transfer flux to increase the heating rate, but unfavorable to the uniformity of
temperature field in the slab. This is exactly the reason why setting the soaking zone
in the furnace. Generally, under the condition of keeping the size of the slab constant,
the temperature difference in the cross section will be principally dependent on Bolt
number (Bi) and Fourier number (Fo). The relationship can be expressed as Eq. 9.13:
T = f (Bi, Fo).
(9.13)
To decrease Bi or increase Fo is helpful for decreasing the temperature difference
in the cross section of the slab and improving the quality of produce. By comparison,
it can also be seen that the temperature of the surfaces with oxide scale is lower
than that of without oxide scale during the reheating process, particularly in the high
temperature zone. Because, the heat conductively of the oxide scale is very small. It
can cause a great thermal resistance between the surroundings and slab surfaces to
decrease the heat transfer flux.
9.5 Conclusions
In this work, a 3D transient mathematical heat transfer model for a pusher type
reheating furnace was established. The heat transfer behavior of the slab was studied
during the whole reheating process and the effect of oxide scale on the surfaces on
heat transfer characteristics was full considered. Get the following conclusions: