Selection of permeability for optimum performance of a porous tube thermal stratification manifold

• The impact of the permeability of a porous manifold on thermal stratification is modeled.
• For intermediate charging, the manifold enhances stratification regardless of permeability.
• For top charging, the permeability should be high.
• Recommended permeability is provided over a wide range of Richardson number.

In this study, we investigate the ability of porous tubes to maintain thermal stratification in a water storage tank in comparison to a top-mounted inlet pipe. Transient distributions of temperature and velocity are presented for operating conditions and a tank geometry representative of solar domestic hot water systems using a transient two-dimensional computational fluid dynamic model. The stratification in the tank is quantified by a dimensionless exergy efficiency. The results demonstrate the importance of selection of the permeability of the manifold on performance. A porous manifold improves thermal stratification for intermediate charging, regardless of the choice of permeability. Over a large range of Richardson number, the manifold is most effective for a dimensionless permeability, defined by the ratio of the pressure drop across the porous wall to the axial pressure gradient, equal to 0.02. Such a manifold will deliver the fluid to the tank in a narrow band at the vertical position of neutral buoyancy and will prevent suction of warmer fluid stored at the top of the tank into the manifold. For top charging the porous manifold is comparable in performance to a well-designed inlet pipe. Recommendations of selection of porous material based on dimensionless permeability are provided.