The sizing of surge protection devices for both compressor and surrounding system may require the knowledge of performance curves in 2nd quadrant with a certain level of accuracy. In particular two performance curves are usually important: the pressure ratio trend versus flow rate inside the compressor and the work coefficient or power absorption law. The first curve allows estimating mass flow in the compressor given a certain pressure level imposed by system, while the second is important to estimate the time required to system blow down during ESD (emergency shutdown). Experimental data are routinely not available in the early phase of anti-surge protection devices and prediction methods are needed to provide performance curves in 2nd quadrant starting from the geometry of both compressor and system. In this paper two different approaches are presented to estimate performance curves in 2nd quadrant: the first is a simple 1D approach based on velocity triangle and the second is a full unsteady CFD computation. The two different approaches are applied to the experimental data more deeply investigated in part I by Belardini E.[3]. The measurement of compressor behavior in 2nd quadrant was possible thanks to a dedicated test arrangement in which a booster compressor is used forcing stable reverse flow. The 1D method showed good agreement with experiments at design speed. In off-design condition a correlation for deviation angle was tuned on experimental data to maintain an acceptable level of accuracy. With very low reverse flow rates some discrepancies are still present but this region plays a secondary role during the dynamic simulations of ESD or surge events. The unsteady CFD computation allowed a deeper insight into the fluid structures, especially close to very low flow rates when the deviation of the 1D method and the experimental data is higher. An important power absorption mechanism was identified in the pre-rotation effect of impeller as also the higher impact of secondary flows. These two methods are complementary in terms of level of complexity and accuracy and to a certain extent both necessary. 1D methods are fast to be executed and more easily calibrated to match the available experiments, but they have limited capability to help understanding the underlying physics. CFD is a more powerful tool for understanding fluid structures and energy transfer mechanisms but requires computational times not always suitable for a production environment. 1D method can be used for standard compressor and applications for which consolidated experience have been already gathered while CFD is more suitable during the development of new products or up to front projects in general.

The usage of abradable seals in the secondary flow path of centrifugal compressors is very high whenever higher efficiency is the main requirement. This is due to the fact that when compared with other sealing technologies the rubbing of teeth into the abradable material allows for the formation of grooves that result in very tight clearances between static and rotating components. Flow physics and mass flow leakage through abradable seals are strongly dependent on the presence of these grooves and their respective shapes. For cases where no grooves are formed, the seal leakage is mainly a function of inlet pressure, pressure ratio across the seal and tooth radial clearance. Once grooves are formed, the flow physics and seal leakage are also a function of groove dimensions and tooth axial position inside the groove. The scope of this paper is to present results obtained from an extended numerical campaign that was performed to understand the effect of groove formation and teeth position on seal leakages. Computation Fluid Dynamics (CFD) was used to study these effects. To achieve higher accuracy of numerical results, all key parameters in CFD such as mesh topology, grid independence, y+ criterion and turbulence model were studied in great detail. In cases where no grooves are present, the vena contracta (the narrowest point of the carry-over jet) is observed across the first tooth and its effect on flow physics at higher tooth gap is highlighted in this paper. For cases where grooves are present, it is seen that the flow field strongly depends on the following parameters: 1. Groove dimensions (e.g. depth and width), 2. Tooth radial clearance and 3. Tooth axial position inside groove. The impact of all the above parameters on seal leakage have been discussed and described in this paper. Finally, seal leakages are compared for grooved geometry with respective non-grooved geometry cases (in terms of tooth radial clearance). In the second part of the paper, the accuracy of CFD results is verified with available experimental data. For judicious comparison, geometrical specifications of all components and boundary conditions in CFD model and experimental setup were maintained very closely.

In the secondary flow path of centrifugal compressors, abradable seals are in high demand when higher efficiency is the main requirement. This is because abradable seals can maintain very tight clearances between static and rotating components compared to other sealing technology. However, due to the rubbing of the teeth into the abradable material, some grooves can form. The flow physics and mass leakage of abradable seals are strongly dependent on the presence of these grooves and their shape. For cases where no grooves are present, seal leakage is mainly a function of inlet pressure, pressure ratio across the seal and tooth radial clearance. Once grooves are formed, the flow physics and seal leakage also are a function of groove dimensions, tooth clearance and tooth axial position inside the groove. The scope of the present paper is to describe the experimental campaign that has been performed to validate the numerical analysis of part 1 of the paper [1]. The experimental test matrix investigates the groove and teeth positioning effects on seal leakages. To achieve higher accuracy, the key geometric parameters, such as radial and axial gaps, were controlled in the test sample during the tests. For cases where grooves are present the experimental measurements reveal that flow field strongly depends on groove dimensions, tooth radial clearance and tooth axial position. The authors, finally, found generally a good agreement between numerical predictions and measured data, both in terms of leakage and pressure drops across the teeth.

Stump appendicitis is a rare delayed complication of incomplete appendectomy. The clinical symptoms and signs are like acute appendicitis. The possibility of stump appendicitis is not clear to all clinicians. It represents a diagnostic dilemma with incidence of one in 50,000 cases. We report how we encountered diagnostic dilemma during the management of a 30 years old male patient of acute abdomen with history of appendectomy done for acute appendicitis even years back.

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