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The Effects of Porosity and Inclination on the Pressure Drop across Porous Screens and Honeycombs Used for Heat Exchanger Simulations in Wind Tunnel Studies
Abstract:
The simulation of heat exchanger air flow characteristics in a sub-scale wind tunnel test requires an accurate representation of the full-scale pressure drop across the element. In practice this is normally achieved using laminations of various porous materials and honeycombs on the basis of experience and ad hoc data. In view of this, a series of measurements of the pressure drop, in both the near and far field, across screens with porosity (β) in the range 0.41 ≺ β ≺ 0.76 are reported. The aim being to establish a
relationship between the porosity and the pressure drop characteristics of a given material at various angles of inclination to the free-stream flow. Furthermore, the effect of screen depth was investigated using honeycombs. This data will facilitate detailed design and accurate representation of the flow characteristics at sub scale.
The measurements were made in a simple open return blower wind tunnel within a range of Reynolds number (Re), based on screen thickness of 200 ≺ Re ≺ 2100 and screen inclination to the free stream (α) in the range 0 ≺ α ≺ 45 deg.
At screen porosities of β = 0.5 the pressure drop coefficient (Δp /q) was found to be independent of Reynolds number above Re = 500. Pressure drop coefficients for screens orthogonal to the free stream (α = 0) were found to agree well with existing experimental data and empirical predictions. The pressure drop coefficient is shown to reduce with increasing screen porosity and inclination to the free stream. The screen permeability was found to increase with increasing screen porosity and inclination. Empirical equations are developed giving the predicted pressure drop and permeability for
screens with a given porosity and inclination.