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Dynamics >>23
Model
of a thermal gun for abrasive blasting
.A.
Gorlach, Department of Industrial Engineering, Port Elizabeth Technikon,
South Africa
Thermo-abrasive blasting is a new method of
surface preparation. In thermo-abrasive blasting, combustion is
the major source of energy for the propulsion of abrasives. Combustion
of fuel releases heat and generates a high temperature pressurized
gas, which is directed through a nozzle to form a high velocity
flow stream. Modifications of the ram-jet engine, whereby abrasive
particles are introduced into the gas flow and accelerated by it,
recently led to the design of an apparatus called the thermal gun
(Fig. 1).
The combustion process generated by a thermal gun is classified as a High Velocity Air Fuel (HVAF) process. Modeling of the thermal gun was done with a purpose of obtaining the optimal nozzle geometry for thermo-abrasive blasting.
The following modeling strategy was used:
-
cylindrical coordinate system with hexahedral cells,
-
inlet, outlet, cyclic and wall boundary conditions, 1/6th of the
thermal gun was modeled,
- steady, compressible
and non-isothermal flow options,
- combustion models
of propane (C3H8) with the PPDF analysis and the 3-step reaction,
- the standard k-_ turbulence and RNG models for
the turbulence characteristics,
- walls with the
heat flux, resistance, radiation and material properties,
-
two-phase Lagrangian model for particles.
Figure 1. Cross section of the thermal gun
Figure 2 shows the resulting temperature distribution inside the thermo-abrasive gun with the PPDF analysis and the 3-step reaction. In the PPDF analysis, it appears that the combustion chamber housing is subjected to the highest temperature in the middle section, whereas in the 3-step reaction, the high temperature zone is spread over the entire length. Figure 3 shows the combustion chamber housing with the color changes due to heat in the middle section, which corresponds with the PPDF combustion analysis and the RNG turbulence model.
Figure 2. Modeled temperature distributions inside the thermal gun
Figure 3. Combustion chamber housing showing a high temperature zone
The modeling results also showed that the gas temperatures in the nozzle diverging section are considerably higher than in the converging section. That was observed during trials. Figure 4 shows the abrasive blasting nozzle with the signs of material deterioration due to overheating in the diverging section (See also Figure 6).
Figure
4. Abrasive blasting nozzle after overheating
Particles of Al2O3 with a 0,5 mm fraction size was chosen for modeling of the two-phase flow. The particle tracks are shown in Figure 5. The biggest acceleration is achieved in the diverging section of the nozzle where the high velocity flow increases the drag force. The prediction of the particle velocity agrees well with the theory of multiphase flows. The length of the nozzle diverging section is an important factor in determining particle velocity and blasting productivity. The lengths of the nozzle diverging section were chosen as 50, 70, 100, 120 and 150 mm. These values were alternatively used for the simulation. Different nozzle geometries (straight and Laval-type) were also modeled in order to compare the velocities of the gas flow and the abrasives. Although the particle velocity increases with nozzle length, the rate of velocity increase does not justify the length of the diverging section of thermo-abrasive nozzles above 70 – 100 mm due to overheating of the nozzle. In practice, high blasting rates were obtained with the 70 mm long nozzle (Fig. 6).
Figure 5. Modelled particle tracks and velocities
In this research project, modeling of the thermal gun with STAR-CD allowed to achieve the optimization of the nozzle geometry for thermo-abrasive blasting as well as the temperature distribution in the nozzle that was further applied in the stress analysis of nozzle materials in order to assess their ability to withstand thermal stresses.
Figure 6. High velocity flow generated by the thermal gun
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