V.I. Luzgin, A.Yu. Petrov, V.A. Prakht, F.N. Sarapulov, V.E. Frizen
(The Ural State Technical University-UPI, CJSC “RELTEC”)

Abstract

Composite billet heating up to 600-1050 C is one of the technological operations used to produce composite billets out of special alloys. Preliminary investigations in the field of elaborating the technology for producing such composite billets have provided the experimental data on composite billet heating in a resistor furnace. The heating rate, however, appeared to be too low to allow for integrating the resistor furnace into the production line. Besides, the area provided for the heating site was rather small. To speed up the heating process, it has been suggested to use an induction heating unit. This gave positive results. The heating rate of the composite billet increased several times.

A mathematical model has been worked out and research has been conducted in the field of the composite billet heating in a continuous induction heating furnace. In the process of modeling, the inductor length was measured.

The problem of defining the billet heating time and designing an inductor has been split into several stages: the first stage was aimed at defining the heat engineering parameters of the billet heated as this object displayed a complex nature of the heat transfer from the billet surface to its centre; at the second stage it was necessary to define the heating time of the billet with the specified diameter and structure, the inductor power for the billet heating thermal zone while the heat flow was constant, the length and power of the temperature equalization zone depending upon the billet cross-section; at the third stage it was required to evaluate the heat loss of the steady-state heating unit, the inductor required power, the number of the inductor turns and the inductor current.

1. Defining the thermophysical parameters of the billet.

To define the thermophysical parameters of the billet, the experimental data obtained during the process of heating the tested billet of the given parameters were used. The all-purpose finite element package Elcut 5.1 was used for modeling the heating process.

In modeling, only the internal space of the composite billet is considered, i.e. the area where the heat exchange processes are less obvious.

To conduct a computational experiment, the data received experimentally while heating the tested billet of diameter 250 mm up to 600 C in a resistor furnace were used.

In modeling the following allowance were made:

1) The temperature difference in the casing is negligible.

2) The heat transfer between the billet shell and its other parts is by radiation only; the contact heat exchange is negligible.

3) The internal part of the billet is solid in spite of the fact that there are heterogeneous areas the properties of which differ from one another, i.e. the heat propagation inside the billet is by contact, heat transfer by convection and radiation is left out.

The preliminary analysis of the data received experimentally showed the clearly marked solidity of the billet, i.e. a predominance of the internal thermal resistance over the external one. Nevertheless, while defining Biot number for a solid body with the specified material properties of the composite billet parts, it was found out that the billet was to be a thin body in terms of heat engineering. That was evinced by the computational experiment.

Thus, the problem of selecting the thermophysical parameters of the composite billet has been confined to selecting the radiation factor in the process of heat transfer from the internal shell surface to the heating parts of the composite billet (the radiation factor determines the external thermal resistance) and the composite billet thermal conduction factor (it determines the internal thermal resistance). The criterion for selecting the above-mentioned factors was the agreement between the data received experimentally with those obtained in the course of modeling.

2. Modeling induction heating of the composite billet in a two-zone continuous induction furnace

If only two furnace zones of the temperature control are used, it is necessary to adjust the capacity level so that at the end of the heating cycle the shell temperature should reach a specified level at the admissible temperature difference between the billet surface and centre. Modeling this heating process almost always involves the difficulty of heat losses evaluation at the beginning and end of the second zone (that of thermostating), as the shell temperature in this zone, which, in its turn, is much longer than the acceleration one, will be continuously changing with time. For this reason, to solve this problem a method of equivalent heating circuits (networks) has been chosen. Due to its constraints, Elcut program can not be applied in this case. The results of modeling are represented in Fig. 1.

        Fig. 1

REFERENCES

1. Thermostability of thermal technical spaces in aspect of saving energy / M.M. Janczarek. Proceedings of Ninth International Symposium on Heat Transfer and Renewable Sources of Energy, Szczecin, Poland, 2002. P. 613-618

2. Heat models of linear induction motors / F.N. Sarapulov, S.V. Karas, P. Szymczak. Proceedings of Sixth International Conference on Unconventional Electromechanical and Electrical Systems. Alushta, The Crimea, Ukraine, 2004.P. 127-136.