PERMEABILITY TESTING OF CERAMIC FOAM FILTERS

摘要

The construction and validation of an apparatus for measuring the permeability of ceramic foam filters (CFFs) at varying water flow rates is presented here. Commercially available CFFs are specified by pores per inch, which does not uniquely determine the flow characteristics of CFFs. Permeability, the pressure drop per unit filter thickness as a function of velocity, is desired for modeling and quality control purposes. Permeability is typically described with the Forchheimer equation, which can be broken into linear (Darcy) and quadratic (non-Darcy) components. Linear dependence comes from laminar flow at low flow rates, and is scaled by viscosity, while the square dependence dominates at high flow rates, and is scaled by fluid density. CFF measurement systems are not commercially available, and while several examples are found in literature, they are expensive and labor intensive to operate, and are usually limited to specific sizes of filter. The methodology, apparatus, and verification work presented here aims to reduce the cost, time, and complexity of measuring the permeability of commercially available CFFs of different sizes and porosities. Water is used as the working fluid, which is pumped through a CFF held in a modular cartridge. Pressure transducers are mounted close to the filter on either side. For testing different diameters of commercially available CFFs, different inlet and outlet pipe sizes are used to approximately match the diameter of the filter. This approach avoids the need for computational modeling of an effective filter diameter when the pipe and filter diameters do not match. Long, straight rigid pipe runs are used on the inlet to ensure fully developed flow profiles. Flow is measured with a magnetic flow meter. Pressure drop and flow rate are recorded at discrete flow rates after allowing the system to settle to a constant flow rate. A continuously variable methodology was evaluated and rejected. During testing, it was found that the decreased viscosity of water heated by the pump during long testing runs affected the measured permeability. To compensate, water temperature is measured during each run, and the viscosity is calculated for each run. The linear and quadratic permeability coefficients are determined by fitting a quadratic model through pressure drop data as a function of flow rate. In order to verify the accuracy of the device, a validation disc was created. A 49.6 mm disc of aluminum was drilled with 1,467 evenly spaced holes. An analytical formula from literature was used to calculate the theoretical permeability of the array. The measured permeability was below the calculated value, but surface defects in the disk were shown to have a large impact on measured permeability.