Modeling of thermal stresses on the slits-and-holes burner mantel

Contribution: Faculty of Mechanical Engineering, Belgrade (MFB)

Summary

The first step in obtaining the mantle thermal stress-strain field is simulation of the temperature field in the mantle structure under combustion conditions. The temperature field, obtained using the commercial CFD software, was extracted and used as input data for FEM calculations. Stress and strain calculations were performed using the commercial software. Simulations identified the area where the maximum stresses occur was, as the area on the mantle slot curvatures. Suggestions were given for optimizing the burner mantel design.

1. Temperature field over the burner mantel

In order to determine the temperature field over the burner mantel, 3D-simulation of the mantle with combustion and heat transfer was made. Simulation did not include heat transfer by radiation, due to the complexity and consequent high hardware demands. Heat transfer included transfer through the gas phase, from the gas to the metal surface, and through the metal.

The simulations were done for the lower power range, according to the project specifications. The excess air ratio is fixed to l = 1,3. At this low power, the flame is situated very close to the mantle surface, and the expected metal temperature is the highest for the whole modulation range. Under this condition, induced thermal stresses reach maximum.

Figure 1 shows the temperature distribution over the mantle outer surface. Temperature is maximum in the region, where the flame (reaction of the primary heat realization) is "connected" to the mantle surface. Globally, the maximum temperature on the burner mantle is in the zone of slots. Because of the non-uniform temperature field over the mantle, the thermal stresses are induced.

Figure 1. Temperature field (in K) over the outer side of the mantle perforation (MFB).

2. Stress-strain conditions of the burner mantel

Temperature field, obtained by CFD simulations, was extracted and used as input data for FEM calculations. Stress and strain calculations were done by commercial software. The calculations are in the linear domain, i.e. the material have completely linear behavior (the domain of Hook's law). Since the cross-section of the burner mantel does not change with the length and is symmetric, the half cross-section was used for structural calculations.

3. Stress over the burner mantel

Results of the stress-strain numerical simulation are presented in Figure 2. Because the stress is a complex value, the Von Mises stress are assumed as equivalent. The maximum stress value is located on the mantle surface, and particularly on the mantle slot curvatures. The maximum value of the Von Mises stress is 6,07 1e8 Pa.

Figure 2. Dominant Von Mises Stress around perforation of the burner mantle (MFB).

4. Burner mantel deformation

The stresses, occurring at the mantle, are accompanied with the strain, which leads ,to mantle deformation. Simulations of displacements and deformations are performed in the scope of the FlexHEAT project, giving an insight into the thermal deformation of the burner mantel under combustion conditions.

Conclusions

Based on the performed numerical simulations, the following conclusions can be drawn:

  1. The maximum stress occurs on "ideally" sharp edges of the slots. In reality, those edges are irregular to some extent, as a consequence of the production technology. If the average stress on the surface between two sots is adopted as adequate, the safety factor can be calculated. In order to obtain safety factor larger than 1, the yield stress, Rp0.2 of the chosen material must exceed the critical value. If not, the behavior of the material on the most stressed region will have nonlinear (plastic) behavior. This would produce the permanent deformations, which would lead to the stress decrease.
  2. The simulations suggest that the upper mantle plate (with the slits-and-holes) pattern and the lower mantel part should be produced from different materials. The upper mantel plate is subjected to the higher thermal stress comparing to the lower mantel plate, thus it should be manufactured from the material with higher Rp0.2 (more expensive materials). After punching the perforation, the upper mantel part might be welded from the both sides to the lower mantel part, and then bended.
  3. Numerical simulations showed that the deformation of the burner shape are not big enough to have influence of the burner functionality.