# Design of a Mass Air Flow Sensor Employing Additive Manufacturing and Standard Airfoil Geometry
## 2 Materials and Methods  
The required air flow during the operation of an Internal Combustion (IC) engine can vary depending on its operating conditions (i.e., the load applied); a typical air flow intake for a low power industrial engine is 0 to 615 kg/h [14], which is equal to about 0 to 8806 SLPM (25 \({}^{\circ}\)C at 1 bar), a noticeably large range. In reference [15], the authors successfully demonstrated a flow sensor based on PCB low-cost technology that was able to measure the air intake of a working IC engine up to 180 kg/h under the hot-wire principle of operation for constant current (CC) and constant resistance (CR) mode. However, saturation in the sensor's signal was noticed for flows above the specific value of air intake, limiting the measuring range of the device. Furthermore, during IC engine operation, the intake flow was not constant but pulsating, due to the engine's principle of operation [16]; this effect was not taken into consideration in the specific PCB-based sensing device. In reference [12], an engine air intake sensor based on PCB and 3D printing technology was presented by the authors. Briefly, the sensor's sensing element was fabricated utilizing standard PCB fabrication techniques and commercially available components, while 3D printing technology was employed for fabricating an appropriate air flow shaping housing with an integrated flow-bypass pathway as a first attempt to address the measuring challenges present on the intake flow (Figure 1). The functionality of the device has been well demonstrated for a range of 0 to 260 kg/h (0 to 3723 SLPM), which is increased by 44% compared to the increase initially obtained in [15]. However, we should note that for flow rate values approximately above 260 kg/h (3723 SLPM), saturation in the sensor's signal was still present. A series of appropriate CFD simulations concerning the developedflow profile inside the bypass pathway revealed a significant circulating flow at the end of the pathway, near the sensing element's mounting location; furthermore, the sudden expansion at the inlet of the employed bypass design can lead to boundary layer separation with a reattachment point that can be situated near the sensing element area as the flow rate increases [17]. As mentioned by the authors, both of these geometry-induced effects restricted the device's measuring range. The current work focuses on further optimizing the engine air intake sensor presented in [12] and drastically increasing its measuring range. In more detail, an optimal bypass pathway design, employing inlet and outlet designs, which are based on airfoil geometries (i.e., National Advisory Committee for Aeronautics airfoils (NACA) [18], Joukowski airfoils [19]), was derived through suitable CFD simulations. As we present in the later sections, by appropriately shaping the flow profile at both inlet and outlet ports of the bypass, boundary layer separation and circulating flow were minimized significantly, expanding the sensor's measuring range. The initial housing design (Figure 2) was re-designed regarding both the inlet and outlet of the bypass section geometries, along with the necessary orifice for appropriately decreasing the bypass inflow and the outflow topology for constraining the back flow induced during IC engine operation. Three test design concepts (i.e., test designs 1, 2, and 3) were investigated through CFD simulations. In more detail, a series of simulations were performed, assuming a maximum engine air intake of 1000 kg/h (14076 SLPM), which corresponds to an air velocity of 29.87 m/s at the main inflow pathway of the housing, taking into consideration its cross-section area, equal to 78.54 cm\({}^{2}\). For the specific value of air velocity entering the bypass' inlet, with a much smaller cross-section area of 3.14 cm\({}^{2}\), the corresponding Reynolds number for the flow inside the Bypass pathway can be easily estimated at 38,248 pointing to turbulent flow.