Porous Manifold Research Articles

Fluid–Structure Interaction in the Flexible Porous Stratification Manifold

A model of the flexible porous manifold that captures the interaction between the flow field and the deformation of the manifold is developed and applied to understand how the fabric manifold works for conditions expected in solar water heaters. Contrary to the widely held hypothesis that the change of cross-sectional area induced by the fluid–structure interaction is beneficial, the numerical results demonstrate the change of cross-sectional area has no significant impact on the effectiveness of the manifold. In comparison to a rigid porous manifold, the performance of the flexible manifold is slightly worse because the collapse of the manifold encourages entrainment. The dimensionless permeability plays a crucial role in determining the performance and can be selected to limit entrainment and release fluid near the vertical level of neutral buoyancy.

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

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.

Performance of Rigid Porous Stratification Manifolds With Interpretation for Off-Design Operation

Thermal stratification of solar water storage tanks improves collector efficiency and provides higher quality energy to the user. A crucial aspect of maintaining stratification is preventing mixing in the tank, particularly during solar charging and hot water draws. An effective and simple approach to flow control is an internal stratification manifold. In this paper, the performance of the rigid porous manifold, which consists of a series of vertical hydraulic resistance elements placed within a perforated tube, is considered for charging operation. A 1D model of the governing mass, momentum, and energy conservation equations is used to illustrate the procedure for designing a manifold and to explore its performance over a broad range of operating conditions expected in solar water storage tanks. A manifold performance indicator (MPI) is used to evaluate the effectiveness of the manifold relative to an inlet pipe positioned at the top of the tank. The rigid porous manifold improves the stratification in the tank over a wide range of operating conditions unless the inlet flow rate is significantly reduced from the design point.

Enhanced thermal stratification in a liquid storage tank with a porous manifold

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 L and a height-to-radius ratio of 4 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.

Effects of a porous manifold on thermal stratification in a liquid storage tank

A numerical model for a porous manifold has been developed to study its effects on thermal stratification in a typical storage tank. The tank considered has a capacity of 300 l and a height-to-diameter ratio of 2. The numerical model employs a transient stream function-vorticity formulation to predict the development of flow and temperature fields in a charging process. The Reynolds number of the inlet flow is fixed at 200 and a wide range of the Richardson number (0.01≤ Ri≤100) is considered. Other parameters considered include the Biot number (i.e. heat loss from the tank), the Darcy number (permeability of the porous manifold), the location of baffle, and the thickness of the porous tube. The results show that all these factors should be taken into account for a better design of a thermal storage tank. In addition, the results provide an envelope for these design parameters.

Fabric Stratification Manifolds for Solar Water Heating

The level of thermal stratification that can be maintained in forced-flow, direct solar water-heating systems using a fabric manifold is studied in a 372-liter tank with an inlet flow rate of 0.07 1/s. A rib-knit, lightweight, spun-orlon acrylic is the most effective manifold material in a comparative study of 13 synthetic and natural fabrics. Thermal stratification (or more appropriately mixing) in the tank equipped with this acrylic manifold is compared to the level of stratification achieved using a rigid, porous manifold and a conventional drop-tube inlet. Initial tank temperature profile, temperature of the water entering the tank, and test duration are varied in three testing schemes. Comparison of vertical temperature profiles and height-weighted energy stored in the tank indicate that under realistic operating conditions, the fabric manifold is 4 percent more effective than the rigid manifold, and 48 percent more effective than the conventional drop-tube inlet.

A Model for Flow Distribution in Manifolds

An analytical investigation of the performance of flow distribution systems was conducted for both intake and exhaust manifolds. Primary emphasis was placed on configurations in which the lateral tubes formed sharp-edged junctions at right angles to the manifold axis. A mathematical model describing the flow behavior at a discreet branch point was formulated in terms of a momentum balance along the manifold. The model was extended to the case of continuous discharge or intake for a uniformly porous manifold. Numerical solutions of the governing flow distribution equation were obtained and compared with experimental data. Dimensionless parameters characterizing the performance of manifolds were formulated from the analytical model.