and 4-electordes.

3.1. CFD Results for the Base Case of Two Electrodes

The computed mean axial velocity profiles at the downstream location of 13/16" from the retention ring for the case with two electrodes along six radial lines are shown in Figure 7. The angle θ is measured in the anti-clockwise direction from

Figure 7. Computed mean axial velocity profiles along six radial lines at the axial location Y = 13/16" for the case of 2 electrode configuration. In the inset of the above figure, θ = 0 is the horizontal radial line from the center towards right direction.

the radial line corresponding to the horizontal arrow pointing to the right in the inset of Figure 7. If the flow velocities were perfectly axisymmetric, all the six velocity profiles would have collapsed on to a single curve. However, as can be seen, Figure 7 shows a significant departure from axisymmetry.

The corresponding computed mean tangential velocity profiles at the same location are shown in Figure 8. A positive value for the tangential velocity denotes a clockwise direction and a negative value denotes a counterclockwise direction. Again, we see a significant departure from axisymmetry. In particular, we notice a sharp discontinuity for the tangential velocity at the center r = 0 and significantly large negative values of the tangential velocity for small values of r (near centerline) along some radial lines. This is clearly only possible if the rotational center of the swirling flow is at some distance away from the centerline of the burner. This can be seen more clearly in Figure 9 which shows the velocity vectors on a cross-sectional plane at the same axial location of Y = 13/16". As can be seen, the center of the swirling flow is shifted significantly to the left side. Thus the swirling flow is not concentric. This shift, s, was measured to be 0.31". When compared to the flame tube radius, R, of 1-11/16", the eccentricity, e, of the swirling airflow = s/R = 0.184.

3.2. CFD Results with No Electrodes

The computed mean axial velocity profiles at the downstream location of 13/16" from the retention ring for the case with no electrodes along six radial lines are shown in Figure 10. Here we observe that all the six profiles have virtually collapsed onto a single curve, indicating axisymmetry of the axial velocity field. The corresponding computed tangential velocity profiles at the same location are

Figure 8. Computed Tangential Velocity profiles along six radial lines at the axial location Y = 13/16" for the case of 2 electrode configuration.

Figure 9. A cross-sectional plot of velocity vectors at the axial location Y = 13/16" for the case of 2 electrode configuration. For better visibility, the intensity of the color code for the magnitude of total velocity has been artificially reduced.

Figure 10. Computed mean axial velocity profiles along six radial lines at the axial location Y = 13/16" for the case with no electrodes.

shown in Figure 11. Here also we note an almost axisymmetric velocity profile that is a dramatic improvement over the results for the 2-electrode case shown in Figure 8. The velocity vectors on a cross-sectional plane at the same axial location for this case are shown in Figure 12. Here we see that the rotational center

Figure 11. Computed mean tangential velocity profiles along six radial lines at the axial location Y = 13/16" for the case with no electrodes.

Figure 12. A cross-sectional plot of velocity vectors at the axial location Y = 13/16" for the case of 2 electrode configuration. For better visibility, the intensity of the color code for the magnitude of total velocity has been artificially reduced.

of the swirling flow coincides with the centerline of the flame tube. Thus it can reasonably be concluded that the two electrodes upstream of the retention ring are a major source of flow field disruption downstream of the retention ring. Nevertheless, electrodes are necessary for combustion initiation, and although the objective of the current paper was not to find an alternative electrode configuration that would not distort the flow field, we decided to investigate improvement, if any, in the flow field if we added two more “dummy” electrodes (Figure 6) on the opposite side. The results for this case are presented in the next section.

3.3. CFD Results for the Four Electrodes Configuration

The axial and tangential velocity profiles for the case with four electrodes (shown in Figure 6) are presented in Figure 13 and Figure 14. When comparing these results to the base case of two electrodes shown in Figure 7 and Figure 8, we see a significant improvement. For example, in Figure 7 for the two electrode case at location r = 0.01 m we note the axial velocity variation of 4 m/s (from 1 to 5 m/s). The corresponding axial velocity variation at the same r for the case with four electrodes in Figure 13 is noted to be 2 m/s (from 2 to 4 m/s), a reduction by a factor of 2. A similar reduction is also noticed for the reduction in tangential velocity variation when comparing the results shown in Figure 8 and Figure 14. Finally, the cross-sectional plot of velocity vectors for this case is shown in Figure 15. When comparing this to the results for the base case of 2 electrodes shown in Figure 9, we clearly notice that the center of the swirling flow is significantly closer to the physical centerline of the flame tube. The shift in the rotational center of the swirling flow, s, was measured to be 0.11", which is lower by almost a factor of 3 when compared to s = 0.31" for the case of two electrodes. Thus it is seen that by the simple introduction of two additional dummy electrodes, the flow field axisymmetry can be improved noticeably.

Figure 13. Computed mean axial velocity profiles along six radial lines at the axial location Y = 13/16" for the case with four electrodes.

Figure 14. Computed mean tangential velocity profiles along six radial lines at the axial location Y = 13/16" for the case with four electrodes.

Figure 15. A cross-sectional plot of velocity vectors at the axial location Y = 13/16" for the 4 electrode configuration. For better visibility, the intensity of the color code for the magnitude of total velocity has been artificially reduced.

4. Conclusion and Suggested Future Work

CFD simulations of the air flow through a residential retention head oil burner were performed to investigate the reasons for large departure from axisymmetry in the flow field at the exit of the retention ring. These simulations were made using SOLIDWORKS with embedded SOLIDWORKS Flow Simulation module.

Table 1. A summary of observations from the CFD Results for the three cases.

s = Distance between rotational center of the swirling flow and the centerline of the burner flame tube. R = Inside radius of the burner flame tube.

The simulation was first carried out for the base case of the normal two electrode configuration. Since it was suspected that the non-axisymmetric nature of the two electrodes just upstream of the retention ring might be responsible for the flow distortion, the simulations were carried out for two additional configurations: no electrodes and 4-electrodes. A summary of the observations from the computed results for the velocity profiles are shown in Table 1. Since the computed velocity profiles were almost perfectly axisymmetric for the case with no electrodes, it is concluded that the presence of two electrodes is indeed responsible for the flow distortion downstream of the retention ring. This distortion results in a swirling axial airflow that is not concentric to the flame tube centerline. The introduction of additional two dummy electrodes on the opposite side seems to reduce the velocity distortion by a factor of 2. Additional CFD simulations with alternative practical electrode configurations are needed to determine an optimal solution for this problem.

Acknowledgements

The first author would like to thank the Alabama Louis Stokes Alliance for Minorities Participation (ALSAMP) for their support and Mr. Bennie Mwiinga for his assistance throughout this project. This project was supported in part by the National Science Foundation HBCU-UP Program through award 1818732.

Cite this paper
Drabo, M. , Tutu, N. , Butcher, T. , Trojanowski, R. and Egarievwe, S. (2019) On the Role of Electrodes in Introducing Airflow Distortion in Residential Oil Burners. Engineering, 11, 260-271. doi: 10.4236/eng.2019.115019.
References

[1]   Tutu, N.K. (2002) Cold Flow Measurements at the Exit of a Beckett Model AF II-85 Burner, Private Communication, Brookhaven National Laboratory (BNL) Internal Departmental Memo.

 
 
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