Accounting for Circumferential Flow Nonuniformity in a Multi-Stage Axial Compressor

The flow field in a compressor is circumferentially non-uniform due to geometric imperfections, inlet flow nonuniformities, and blade row interactions. Therefore, the flow field, as represented by measurements from discrete stationary instrumentation, can be skewed and contribute to uncertainties in both calculated one-dimensional performance parameters and aerodynamic forcing functions needed for aeromechanics analyses. Considering this challenge, this paper documents a continued effort to account for compressor circumferential flow nonuniformities based on discrete, under-sampled measurements. First, the total pressure field downstream of the first two stators in a three-stage axial compressor was measured across half of the annulus. The circumferential nonuniformities in the stator exit flow, including vane wake variability, were characterized. In addition, the influence of wake variation on stage performance calculations and aerodynamic forcing functions were investigated. In the present study for the compressor with an approximate pressure ratio of 1.3 at design point, the circumferential nonuniformity in total pressure yields an approximate 2.4-point variation in isentropic efficiency and 54% variation in spectral magnitudes of the fundamental forcing frequency for the embedded stage. Furthermore, the stator exit circumferential flow nonuniformity is accounted for by reconstructing the full-annulus flow using a novel multi-wavelet approximation method. Strong agreement was achieved between experiment and the reconstructed total pressure field from a small segment of measurements representing 20% coverage of the annulus. Analysis shows the wake-wake interactions from the upstream vane rows dominate the circumferentially nonuniform distributions in the total pressure field downstream of stators. The features associated with wake-wake interactions accounting for passage-to-passage variations are resolved in the reconstructed total pressure profile, yielding representative mean flow properties and aerodynamic forcing functions.

Design, Analysis, and Development of a Wave-Current Laboratory

In real ocean environments, offshore structures are exposed to a combination of wave and current loading conditions. This scenario presents the need to study fluid-structure interactions in the presence of both conditions, achievable through experimentation in a recirculating flume coupled with a wavemaker. The Ocean Resources and Renewable Energy (ORRE) group set out to design a recirculating wave-current flume at the University of Massachusetts Amherst to enable the study of technologies such as scale floating platforms and marine energy converters. In this paper, we present the methods used to arrive at an optimal flume design under strict spatial constraints posed by the available lab space. Limitations on the length, width, and height of flume are overcome via innovative flow designs and compact structures. The final design is approximately 11.5 m (37.7 ft) in length and 1.2 m (3.9 ft) wide with a nominal water depth of 1 m (3.3 ft). The 2 m long test section begins 6 m beyond the inlet of the flume to maximize flow uniformity. A 24” thruster driven by 75 hp electric motor maintains a current velocity of 0.5 m/s throughout the section while a wedge-shape plunger is implemented at the inlet to generate 0.6–2.8 s period waves with a maximum height of 0.2 m. During the design process, 2D computational fluid dynamics (CFD) simulations are employed to maximize flow uniformity over a range of inlet angles and guide vane configurations. In the optimal scenario, a flow nonuniformity of 8.7 % was obtained across a 0.7 m water column measured from the free surface. Results from the 3D simulation around the tight corner section showed significant increase in flow nonuniformity. The implementation of the screens along the flow path might be necessary in the future.

Numerical Investigation on Maldistribution of Supercritical CO2 Flow Inside Printed Circuit Heat Exchanger

Supercritical CO2 (S-CO2) Brayton cycle has been identified as a promising power conversion method for the next generation of nuclear reactors due to its high efficiency and compactness. The heat exchanger is one of the most important components for S-CO2 Brayton cycle, and the printed circuit heat exchanger (PCHE) is supposed to be one of the promising candidates for the heat exchangers in S-CO2 Brayton cycle. It should be noted that the fluid maldistribution would induce heat transfer deterioration, especially for heat exchangers with micro- or mini-scale channels like PCHE. The thermal-physical properties of S-CO2 change violently during the heat transfer process, which makes the flow inside PCHE more complex. In this paper, the distribution of S-CO2 flow inside PCHE would be studied by 2-D CFD simulations. For the working fluids with constant properties, the flow nonuniformity increases with the mass flow rate. For the working fluid with S-CO2, the thermal-physical properties change significantly with temperature, and there exist a minimum value in the flow nonuniformity-mass flow rate curves (1.64 × 105 ≤ Rein ≤ 1.31 × 106). Insertion of baffles at manifolds could significantly improve the flow distribution uniformity and reduce the pressure drop. And it has been found that insertion of baffles at the collecting manifold has better performance compared with that at the distributing manifold or both.