# Design and Fabrication of a Novel Window-Type Convection Device
## 2 Method and Numerical Computation
### Development of an Analysis Model  
Figure 4 depicts the design and structure of the window-type convection device used here. The device comprises an airflow multiplier (composed of a wind-deflector plate and wind-deflector cover) and centrifugal fan, with the bottom of the multiplier attached to the fan (Figure 4a). The multiplier draws air through the rotation of the fan and discharges the air via its outlet slit to entrain outdoor air. Figure 4b presents the runner profile and outlet slit of the airflow multiplier. The present study defined the outlet slit size of the airfoil as the shortest vertical distance between the surfaces of the wind-deflector plate and the wind-deflector cover. In addition, the profile of a wind-deflector plate is based on the 4-digit National Advisory Committee for Aeronautics (NACA) airfoil series, a type of low-speed airfoil with a high lift coefficient and low resistance coefficient, which are extensively applied in the design of airfoil fans [8]. Figure 4c presents the position of the NACA airfoil in the wind-deflector plate, where the cross-section of the NACA airfoil is shown in red. The first NACA airfoil digit denotes the relative camber of the airfoil, or the maximum camber as a percentage of the chord length; the second indicates the distance from the leading edge of the airfoil to the location of the maximum chamber, measured in tenths of the chord length; and the last two represent the relative thickness of the airfoil, or the maximum thickness of the foil as a percentage of the chord length. For example, the NACA 2412 airfoil has a relative camber of 2%, with a maximum camber located 40% (0.4 chords) from the leading edge and a maximum thickness of 12% of the chord length.  
Figure 3: Conceptual design of the proposed window-type convection device.  
Figure 5 depicts the profile of the airfoils we discuss in this paper, in which Figure 5a-e presents the profiles of NACA1412, NACA2412, NACA4412, NACA4418, and NACA4424, respectively. NACA1412, NACA2412, and NACA4412 are used to discuss the influence of the relative camber of the airfoil on the mass flow rate and flow velocity, while NACA4412, NACA4418, and NACA4424 are selected to discuss the influence of the relative thickness of the airfoil on the mass flow rate and flow velocity.  
Figure 4: (**a**) Illustration of the window-type convection device, which comprises an airflow multiplier and centrifugal fan; when the centrifugal fan rotates, the airflow multiplier draws air from outside to create an airflow in its runner. (**b**) Front view and right-side sectional view of the airflow multiplier. (**c**) Position of the NACA airfoil in the wind-deflector plate.  
Figure 6 presents the analysis model used in this study, which comprised three flow regions: (1) the outdoor flow region (438 mm \(\times\) 438 mm \(\times\) 250 mm), (2) the indoor flow region (438 mm \(\times\) 438 mm \(\times\) 500 mm), and (3) the inner flow region of the window-type convection device (a runner composed of a wind-deflector plate and wind-deflector cover). The outdoor and indoor flow regions pertain to the neighboring outdoor and indoor environments where the convection device was installed. The geometric dimensions of the inner flow region were specified according to the airfoil structure and the slit size to optimize the performance of the device in entraining outdoor air. During analysis, the relative camber of the airfoil of the wind-deflector plate ranged from 0% to 4%; the maximum camber from the leading edge measured in tenths of the chord length ranged from 0% to 4%; its relative thickness ranged from 8% to 24%; and its outlet slit size ranged from 0.3 to 1.5 mm. The airflow rate and pressure generated by the centrifugal fan were specified on the basis of the boundary condition at the convergence of the airflow multiplier; thus, the structure of the fan was overlooked during the analysis.