The result obtained around the maximum FDE point was used to verify the flow trend inside the fan unit and consequently to modify the shapes and geometries to minimize the recirculation and backflow zones. FDE depends on the volumetric air flow (QBEP), backpressure level (PBEP) and energy consumption of the motor (WBEP), as shown in the equation below:
The following analyses were conducted for the same engine performance in order to maintain the same product cost. It was therefore important to monitor the engine torque (which influences the consumption) for all impeller designs. Considering the above equation, it was necessary to improve the mass flow rate or the backpressure of the centrifugal fan unit.
In the first part of the optimization, the fan of the present centrifugal fan unit underwent minimal changes to the inlet area and the grid located in the inlet areas to ensure safety. In the second part, the geometric parameters of the impeller were modified. The correct way to obtain higher performance is to operate the electric motor close to the point of maximum efficiency. To achieve this, a dozen or so configurations were tested by changing the number, angle and pitch of the blades. In some configurations, the numerical results showed that even if smoother flow fields were obtained, the FDE was not improved due to the decrease in backpressure. For this reason, a trade-off was chosen between the smooth flow field, the pressure increase and the torque value.
The most relevant element in terms of FDE was a seal located at the top of the blades to reduce the mass flow rate of the by-pass flow between the rotational parts.
The best configuration obtained works at RPM values close to the point of maximum motor efficiency and a higher back pressure, as shown in Fig. 3. The back pressure value is about 10% higher than the previous one and, considering that the engine consumption remains almost unchanged, the FDE improvement is about 10%.
The positive result was analyzed with experimental tests in line with European standards; these tests included the volumetric airflow test (to obtain the P-Q curve) and the airborne acoustic noise test (to calculate the noise level). These activities were quick due to the rapid internal 3D prototyping. The figure below shows the comparison for the noise level and the volumetric air flow between the previous centrifugal fan unit and the optimized fan.
At the same volumetric air flow value, the noise level decreased by about 6%. This is due to the smoother flow inside the fan, decreased by-pass flow (backflow) at the top of the impeller, and lower RPM.
Fig. 4 - Noise-volumetric air flow comparison