Experiments were conducted to investigate the interactions between blade vibrations and self-excited flow oscillations in a high performance centrifugal compressor system. Unsteady pressure fluctuations and dynamic stress levels were measured during compressor operation near choke, in self-excited oscillating flow conditions and near surge for four different speed lines. The unsteady pressure field for every operating condition was determined from the simultaneous recording of the output of twelve dynamic pressure transducers which were successively positioned in two peripheral and one meridional planes. Corresponding blade vibration data were collected using an eight channel telemetry system transmitting the outputs of semi-conductor strain gages located on different blades. Analysis of the measurements showed that the unsteady pressure field due to self-excited flow oscillations can be characterized by multiples of rotating and non-rotating pressure patterns at different frequencies. The blade vibration signals clearly demonstrated the excitation of the blade by each of the different unsteady pressure patterns and the frequencies of the unsteady flow field demonstrates the complexity of this flow phenomena and the need to understand the mechanism of its occurrence in order to avoid blade resonance excitation and failure during compressor operation.
The aim of this article is twofold: first, to compare the performance of R410A with that of R32, which is considered a possible HFC substitute with lower global warming potential (675 instead of 2088); second, to exploit the effect of the circuit length in the finned coil of a packaged air-to-water reversible unit with given water plate heat exchanger and scroll compressor. Both scopes are pursued through the analysis of a case study, a system with nominal cooling capacity of about 74 kW at 35◦C dry-bulb outdoor air temperature and a nominal heating capacity of about 70 kW at 2◦C dry-bulb and 1◦C wet-bulb ambient temperature. The performance of the two refrigerants inside a “real” machine is simulated by means of an advanced numerical model of a packaged reversible refrigeration unit. The system consists of a single refrigerating circuit with two identical scroll compressors. The compressor was characterized by its experimental performance curve according to Standard EN 12900 (EN 2013a) for both R410A and R32. Off-the-shelf copper tubes and louvered aluminum fins were considered for the condenser and typical brazed-plate heat exchanger for the evaporator of the chiller configuration. The finned coil heat exchanger was first thermodynamically optimized for R410A and R32 for both condenser and evaporator operation with regard to the number of internal circuits according to the total temperature penalization performance evaluation criteria (Cavallini et al. 2010; Brown et al. 2013), without changing the overall heat exchanger dimensions. The effect of finned coil circuit length on the performance of the investigated reversible unit with the two refrigerants was then analyzed at nominal design conditions and under a seasonal perspective. Based on the modeling work, it is possible to conclude that R32 system efficiency performance is acceptable as alternative to R410A for packaged air-to-water reversible unit
The feasibility of using surface intensity measurements for the determination of sound power as a tool for noise source identification was studied with an experiment on a circular cylinder. This cylinder was an idealized model of a muffler shell of a heavy diesel truck. Two experiments were performed with the cylinder excited by an electromagnetic shaker. In the first one, the shaker was driven with a one-third octave band of white noise centered at 6300 Hz and in the second experiment it was driven with broadband white noise between 1000 and 5000 Hz. The sound power measurements were made in a reverberant room and the reverberant room method was used to provide a comparison with the sound power levels obtained from the surface intensity measurement. The acoustic surface intensity was computed from the cross-spectral density between the acoustic pressure and the normal surface velocity of the cylinder. An equation was developed to allow for inclusion of a correction for phase shifts that were caused by the instrumentation and by the finite distance between the microphone and the vibrating surface. The phase shift resulting from this distance was found to have a dominant effect in the higher frequency range and it was important to make these phase-shift corrections in order to obtain accurate measurements of sound power. After including these phase-shift corrections, reasonably good agreement was achieved between the sound power levels obtained from the surface intensity measurements and those from the reverberant room method.
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