Mixing ventilation flow in an enclosure driven by a transitional wall jet

Confined Flows

Underlying Flow Regime 4-20

Test Case Studies

Brief description of the study test case

The geometry of the reduced-scale enclosure studied in this UFR was cubical with edge L = 0.3 m. A linear ventilation inlet slot was present at the top with a height h_inlet/L = 0.1 and a width w_inlet/L = 1, and a linear ventilation outlet was located at the bottom of the opposing wall (h_outlet/L = 0.0167) (see Fig. 1). The walls had a thickness d (d/L = 8/30). The slot Reynolds number was defined as Re = U₀h_inlet/ν, with U₀ the bulk inlet velocity, h_inlet the slot height (see Fig. 1a) and ν the kinematic viscosity at room temperature (≈ 20°C). For this study two Re-values were included: Re = 1,000 and Re = 2,500, which resulted in mixing ventilation driven by a transitional wall jet, including the typical large recirculation cell, jet detachment from the top surface and Kelvin-Helmholtz-type instabilities in the shear layer of the wall jet (van Hooff et al. 2012a). No temperature differences were included (isothermal case); i.e. no buoyancy effects were present.

Test case experiments

Experimental setup

A reduced-scale water-filled model was built to perform flow visualizations and PIV measurements of forced mixing ventilation (Fig. 2). The experimental setup consisted of: (1) a water column to drive the flow by hydrostatic pressure; (2) a flow conditioning section; (3) a cubic test section with dimensions 0.3 x 0.3 x 0.3 m³ (L₃); and (4) an overflow. The conditioning section in front of the inlet consisted of one honeycomb, three screens and a contraction. The diameter of the hexagonal honeycombs (type ECM 4.8/77) was d_h = 4.8 mm and the length to diameter ratio was 10.4 (L_h = 50 mm). Three screens were included downstream of the honeycombs to reduce the longitudinal components of turbulence and the mean-velocity gradients, with porosities of 72%, 65% and 60%, and thread diameters of 1.6 mm, 1.0 mm and 0.4 mm, respectively (porosity reduces in flow direction). Finally, a contraction further reduced the velocity gradients and turbulence intensities and accelerated the flow. The shape of the contraction was based on the fifth-order polynomial of Bell and Mehta (1988), which has been optimized by Brassard and Ferchichi (2005) by adding the coefficient α:

{\xi ={\frac {x}{L_{c}}}\qquad (1)}

{h_{g}=\left[(-10\xi ^{3}+15\xi ^{4}-6\xi ^{5})(H_{i}^{1/\alpha }-H_{e}^{1/\alpha })+H_{i}^{1/\alpha }\right]^{\alpha }}\qquad (2)

with ξ the normalized length of the contraction, h_g the normalized height of the contraction, L_c the length of the contraction and H_i and H_e the heights of the contraction inlet and the contraction outlet, respectively. The length L_c of the contraction was 0.09 m, He is 0.03 m, Hi is 0.09 m. Defraeye (2006) found an optimum value for α of 0.5, which was used for the contraction in this setup as well.

The test section was constructed from glass plates with a thickness of 8 mm (Fig. 1a). The maximum local velocity U_M (Fig. 1b) was used to make the velocities non-dimensional (U/U_M). Note that U_M was defined as the local maximum time-averaged x-velocity, and thus varies with both x/L and Re. It was chosen to use U_M since the exact inlet velocity was not measured and thus not known and could therefore not be used to make the velocities inside the enclosure dimensionless.

Contributed by: T. van Hooff — Eindhoven University of Technology