Gamma Meccanica’s R&D department is constantly researching new solutions to improve the overall performance of equipment, production capacity and reliability while developing environmentally sustainable processes and applications to benefit its customers.

The most recent example is the study of a new electric melter (see
Fig. 1a). The company purchased the ANSYS Fluent software to
perform a computational fluid dynamics (CFD) simulation of an
electric melter for the fusion of basalt and dolomite rocks (see
Fig. 1b).

The use of a mathematical model allows an in-depth understanding
of the operation, optimizing the geometric characteristics such as
the center-to-center spacing of electrodes or of their depth of
immersion in the melt.

Gamma Meccanica conducted a CFD simulation on a 7 ton/h electric melter (see Fig. 2a). The actual geometry was imported via a STEP in SPACECLAIM. Reasonable simplifications were applied to reduce the mesh size. The resulting geometry (Fig. 2b) was parameterized. Several geometric parameters are the subject of these studies;

- The electrode diameter
- The electrode depth
- The distance between t he electrode axes

The parameters were configured in ANSYS Workbench and a
complete tetrahedral mesh of about 3.9M, which is well-suited to
a parametric geometry, was created. The mesh was converted into
a polymesh (of 0.8M) in ANSYS Fluent. Only one fluid domain,
called the melt domain, was used (see Fig. 3). The Air domain was
considered as a solid with the thermal properties of air.

A single Fluent case is able to include the electrical physics, the
thermal physics and the melt fluid dynamics, if an appropriate
configuration, as described in the following paragraph, is used.

One of the most important properties is the electrical conductivity of the melt. Electrical resistance is a function of:

- the distance between the electrode tips and the iron domain (d))
- the electrical properties of the melt (kele))
- the area through which the current passes (S)

A specific combination of these three parameters defines the electric resistance (Rele) (see Fig. 4a). The Joule effect generated by one electrode is:

The electrical model considers that the three electrodes are at a
steady state and have a constant I_{current} value (their inflow to the
melt domain), which is equal for all the electrodes. K_{ele} is constant
and temperature and zero voltage are fixed throughout the solid
domain (except in the melt and in the air). The resulting electrical
potential is shown in Fig. 4b. These hypotheses derive from
some preliminary studies, in which different boundary conditions
were tested. Once the electrode distance (d) is set, the K_{ele} was
corrected via the preliminary studies to obtain the expected P_{ele}.
The electrical potential is shown in Fig. 5.

In the real furnace, solid material is added from seven different inlets (Fig. 6a) at different times during the process (controlled by local measurement of the melt temperature). Quantities of the melt are then removed (Fig. 6b) via a single outlet to maintain a prescribed melt level. A prescribed constant mass flow rate was set in the numerical model in order to use a steady-state simulation. The solidification/melting model in Fluent was used to include the latent heat absorbed by the inflow from these inlets as a result of the phase change from solid to liquid in the melt. These hypotheses allow the modelling of a continuous flow, and the evaluation of the average velocity field (see Fig. 7) from the inlet to the outlet of the melt domain.

After setting a base case (DP0), three different design points (DP1-DP3) were tested. Fig. 8 shows the distribution of the temperature fields in these four different cases:

- base case (DP0),
- increasing the axle spacing (DP1)
- increasing the depth (DP2)
- decreasing the diameter of the electrodes (DP3)

It is possible to see that a reduction of the distance between
the electrode tips and the iron domain induces a reduction of
the temperature near the electrodes (due to the lower electrical
resistance).

The relative position between the inlet and the electrodes also
appears to be important to achieve a homogenous distribution
of the temperature inside the melt. Fig. 9 shows the temperature
ranges of the melts on the z-x plane. The flow lines from the
inlet, colored according to the intensity of the joule effect, show
that the DP1 configuration was able to generate a more uniform
temperature in the highlighted area, compared to the DP0.

This paper presents a numerical model for a melting furnace for
stone wool. The numerical model was designed using ANSYS
FLUENT. The model includes all the main aspects of the real
process (the electrical field, the heat transfer process and the fluid
dynamics of the melt).

The CFD simulation provided a lot of information about the
operating conditions of the electric melter while considering the
hypothesis of the dissipation terms. A proper validation on site
in the field will enable us to obtain a more reliable setup of the
model, to reflect reality as closely as possible.

Right - Detail of the fusion of basalt and dolomite rocks.

Gamma Meccanica is one of the world leaders in mineral wool production lines, both of individual machines and of complete lines for the production of mineral wools, namely stone wool and glass wool. The company also manufactures special lines to produce pipe sections and stitched mattresses, lamellar production lines, ceramic fiber machinery, and stone wool and glass wool hydroponic production lines. Gamma Meccanica’s machinery offers a combination of high performance and advanced technology. It meets and exceeds customer requests by constantly improving quality and energy efficiency, by means of technological evolution and high levels of technical support, in compliance with the strictest environmental standards..

CASE STUDY

The challenge was to find the optimal aerodynamic design while substantially reducing the costs associated with traditional CFD modeling.

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