Analysis and testing of refrigerant lubricated bearings for centrifugal compressors

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Numerical Investigations of Near Surge Operating Conditions in a Two-Stage Radial Compressor With Refrigerant Gas

Modern compressor design targets require high performance and a wide operating range in order to reduce the environmental impact. To understand the fluid dynamics mechanisms that trigger instability, studying the system at the stability limit is required. In this work, a two-stage back-to-back centrifugal compressor for refrigerant applications has been simulated with computational fluid dynamics (CFD) techniques using unsteady calculations in different operating points close to surge. These models have been validated by comparing numerical performance with experimental data. An in-depth fluid dynamics analysis combined with the monitoring of several pressure signals, postprocessed with FFT, identified different flow phenomena in the two stages toward the surge limit. The key role of the volute that induces a stronger upstream counterpressure in the first stage has been highlighted. This effect causes the formation of high entropy (low momentum) rotating cells in the diffuser that involve a higher channel portion with respect to the flow structure in the second diffuser. This phenomenon affects the upstream flow conditions at the impeller. In addition, the interaction between the inlet guide vane (IGV) and the inducer has been analyzed, observing that in the second stage, due to the flow nonuniformity after the intermediate compressor pipe, non-negligible separations occur. Starting from the peaks detected in the FFT analysis of the pressure signals, all the above flow mechanisms have been detected and discussed

Centrifugal Compressor Surge In Innovative Heat Pump - Part 1: Fluid Dynamic And Vibrational Analysis

Abstract In the current energy scenario, it is necessary to reduce fossil fuel consumption to achieve the far-sighted and stringent decarbonization goals. To date, heat is mainly produced through fossil fuels. Alternatively, electrically driven heat pumps can exploit renewable power to recover environmental and waste heat, offering energy efficient and environmentally friendly heating and cooling for applications ranging from domestic and commercial buildings to process industries. Centrifugal compressors are already used as prime movers of the working fluid in heat pumps, thanks to their industrial replicability, compact size, affordable costs, and good performance in terms of efficiency and low noise. However, they are subject to instabilities such as surge and stall like any other dynamic compressor and these phenomena develop quite differently than in classic open-loop systems such as gas turbines. In fact, such peculiarity is mainly due to the closed loop configuration with real gases in two-phase conditions, occurring in typical heat pump cycles. The aim of this paper is to experimentally investigate the behavior of a centrifugal compressor installed into an innovative close loop heat pump system under stable and unstable conditions from both vibrational and fluid-dynamic points of view. The impact of the main process parameters on the evolution of the instability is shown, highlighting how surge cycles change by varying system operating conditions. The experimental results shown in this paper can be a basis for the future development of validated mathematical models of closed loop heat pumps systems equipped with dynamic compressors