# CFD Analysis on the Heat Dissipation of a Dry-Lubricated Gear Stage
## 3 Results
### Analysis of the Heat Transfer Coefficient
#### 3.2.2 Influence of Flow Characteristics  
Section 3.1 describes the flow characteristics of the different gears. The inflow of air to the gears takes place predominantly in the axial direction. In the following, the influence of the axial inflow of air to the gears on the heat dissipation from the gears is analyzed in detail. Therefore, the axial flow component in the z-direction is analyzed. Figure 8 visualizes the axial flow velocity (z-direction) on a plane congruent to the side surface of gear-C (a) and gear-LL (b) exemplarily for the circumferential speed \(\mathrm{v_{t}}=10.0\) m/s.  
For gear-C, around the gears, a flow component directed to the center of the gears is present on the visualization plane, for example, in the surrounding yellow-colored regions. An axial flow out of the plane and away from the gears can be seen above the meshing area where the pinion and gear airflow encounter each other. This finding serves a deeper understanding of the flow field caused by gear-C described in Section 3.1. For gear-LL, a more distinct axial flow component near the teeth is present. Note that, in contrast to gear-C, no axial flow out of the plane is predominant over the meshing area. Instead, it is more pronounced near the housing walls of the gearbox, especially on the wheel side. These findings correlate with the conveying effect of gear-LL described in Section 3.1.  
A more focused view on the axial flow velocity (z-direction) on a plane congruent to the side surface and the HTC between the gear surface and the air is presented in Figure 9 for the wheels gear-C (a) and gear-LL (b). Corresponding to the rotational direction, a leading and a trailing tooth flank are exemplarily marked in Figure 9. For gear-C, a distinct axial air flow with relatively high velocity directed into the plane is predominant near the leading tooth flanks. In contrast, at the trailing tooth flanks, no distinct axial air flow is present. It appears that the trailing tooth flanks are rotating in the slipstream of the leading flank. Focusing on the HTC, a direct correlation of the local axial air inflow velocity and the HTC values is observed. Due to the distinct axial inflow of air to the leading tooth flanks, the side areas of the teeth show relatively high HTC values. The relatively low HTC values  
Figure 8: Axial flow velocity (z-direction) on a plane congruent to the side surface for gear-C (**a**) and for gear-LL (**b**) at circumferential speed \(\mathrm{v_{t}}=10.0\) m/s.  
in the middle of the tooth flanks can be explained by a weak axial inflow of air in these areas.  
For helical gear-LL, a distinct axial air flow into the plane is present near the teeth that extends over most areas of the tooth gaps. Comparably to gear-C, the leading tooth flanks show higher HTC values than the trailing tooth flanks. In contrast, relatively high HTC values are present in the middle of the trailing tooth flanks. This can be explained by the impingement of the axial air flow in these areas, as visualized in the complemented top view. Hereby, the axial flow is schematically visualized by black arrows. The results show that for gear-LL, more gear surface interacts with inflowing air compared to gear-C, especially at the tooth tips at the front side of the gears. Therefore, gear-LL has higher HTC values than gear-C.