Understanding friction factor behavior in liquid annular seals with deliberately roughened surfaces, roughness geometric characteristics defining flow resistance
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Multistage centrifugal pumps and compressors are among the most widely used pieces of rotating machinery in industry. A typical application demands the arrangement of several impellers mounted on a shaft that spin within a stationary case. Annular seals are the most common sealing devices used in this type of machinery. The annular seal design affects both (i) machinery performance in terms of energy conversion efficiency, and (ii) stability due to the interaction within the rotor and the stator through the fluid flow within the seals. Extensive numerical research of several liquid annular seal experiments is in progress, focusing on the patterns that experimentally and numerically provide larger resistance to flow, hexagonal cells (honeycomb) and equilateral triangular cells (isogrid), both constant depths. In the present paper, the geometric characteristics that define the leakage resistance of roughened seals are explored. First, the results of a parametric analysis are discussed, which include previous 2-D predictions and new simulations of multiple rectangular grooved seal geometries. The analysis is limited to the original clearance in the experiments, 0.175 mm. The results indicate that roughened surface to total area ratio, and the depth-to-length ratio are the primary parameters defining the leakage characteristics of these surfaces. Secondly, the effect of the roughness orientation on the friction factor characteristics of these surfaces is addressed. Previous numerical tests have shown that solving the flow up to the laminar sub-layer better captures the behavior of the friction-factor-to-clearance proportionality. Resolving this layer is demanding. Recent numerical studies show promise in reducing computational efforts by employing a simplified empirical model with extended wall functions, indeed avoiding including each roughness in the numerical domain. These studies evaluated the flow behavior in round-hole seals, a perfectly symmetric pattern. Contrary to round holes, nonsymmetric patterns; knurls, honeycombs, or isogrids, consistently provide larger friction factor values. The numerical evaluation of the leakage characteristics in the last two patterns indicates a non-negligible sensitivity to the pattern orientation in one of them.
