# Design of a Mass Air Flow Sensor Employing Additive Manufacturing and Standard Airfoil Geometry
## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Design Concept** & **Main Simulation Results** & **Overall Outcome** \\ \hline \multirow{3}{*}{Test Design concept (1) and (2)} & Significant velocity reduction & \\ \cline{2-3}  & Unsteady velocity profile across the entire bypass pathway & Rejected \\ \cline{2-3}  & High circulating flow present & \\ \hline \multirow{3}{*}{Test Design concept (3)} & Significant velocity reduction & \\ \cline{2-3}  & Stable velocity profile obtained & Accepted \\ \cline{2-3}  & Minimized circulating flow & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test design concepts. Figure 5: (a) Test design concept (3) 3D, mid-section; (b) test design concept (3) simulation result. Determining the Optimum Geometry of the Sensor's Housing  
Having determined the most appropriate bypass topology, we proceeded with the necessary study for finalizing the sensor's complete housing geometry. A maximum engine air intake of 1000 kg/h (14076 SLPM) was assumed, exceeding by over 50% the maximum air intake for a typical low power industrial IC engine [14]. We should note that, in order to expand the measuring range of the final sensor as much as possible, the employed housing design featured two bypass sections, each with a separate sensing element. That is, one bypass (\"High Velocity Bypass\") with an optimized geometry for a moderate incoming flow range (0-350 kg/h) and a second bypass (\"Low Velocity Bypass\") for a considerably higher flow range (350-1000 kg/h); note that the names of each bypass refer to the magnitude of the developed velocity inside the pathway for the same incoming flow and not to the magnitude of the incoming flow itself. In addition, a third sensing element was mounted directly on the main inflow pathway of the housing for reference. The final topology is presented in Figure 7.  
As in the previous study steps, the exact geometry of the overall design (i.e., orifice diameters, bypass diameters, outlet areas, etc.) was obtained through a series of CFD simulations. For the maximum engine air intake (1000 kg/h or 14076 SLPM), an inflow velocity of 50.82 m/s was obtained, considering the main area at the housing's inlet. For the specific velocity, present at each of the bypass inlets (with a much smaller cross-section area of 3.14 cm\({}^{2}\)), the corresponding Reynolds number was estimated at 65,064, pointing to turbulent flow for both bypass pathways. Figure 6: Detailed view of Figure 5a; (Hatch and Orifice, cross section). Figure 7: Design geometry schematic (dimensions in mm). Considering the above remark, the turbulence model k-omega (k\(-\omega\)) SST was employed, as in the previous series of simulations performed. Characteristic simulation results (for fully developed flows, corresponding to intakes of 1000 kg/h and 500 kg/h) are presented at Figure 8 for the deduced optimum geometry. As we can see in the specific figure, a drastic drop of over 50% for the Low Velocity bypass and 80% for the High Velocity Bypass of the initial value for the air velocity entering the housing inlet was predicted, resulting in reduced air velocity at the sensing element, as presented in Table 2; an approximately three-fold increase was noticed at the orifice areas, as expected. Having finalized the geometry details, an appropriate model was built, employing additive manufacturing (i.e., 3D printing technology). In more detail, Fused Deposition Modelling (FDM) [25] was employed through a typical commercial 3D printer (Ultimaker S5), while the building material selected was standard PLA in combination with supportive material PVA; the constructed housing is presented at Figure 9.