Centrifugal compressors are one of the best choices among compressors in supercritical Brayton cycles. A supercritical CO2 centrifugal compressor increases the pressure of the fluid which state is initially very close to the critical point. When the supercritical fluid is compressed near the critical point, wide variations of fluid properties occur. The density of carbon dioxide at its critical point is close to the liquid density which leads to reduction in the compression work. This paper explains a method to overcome the simulation instabilities and challenges near the critical point in which the thermophysical properties change sharply. The investigated compressor is a centrifugal compressor tested in the Sandia supercritical CO2 test loop. In order to get results with the high accuracy and take into account the nonlinear variation of the properties near the critical point, the computational fluid dynamics (CFD) flow solver is coupled with a look-up table of properties of fluid. Behavior of real gas close to its critical point and the effect of the accuracy of the real gas model on the compressor performance are studied in this paper, and the results are compared with the experimental data from the Sandia compression facility.

Rankine and Brayton cycles are common energy conversion cycles and constitute the basis of a significant proportion of global electricity production. Even a seemingly marginal improvement in the efficiency of these cycles can considerably decrease the annual use of primary energy sources and bring a significant gain in power plant output. Recently, supercritical Brayton cycles using CO2 as the working fluid have attracted much attention, chiefly due to their high efficiency. As with conventional cycles, improving the compressor performance in supercritical cycles is major route to increasing the efficiency of the whole process. This paper numerically investigates the flow field and performance of a supercritical CO2 centrifugal compressor. A thermodynamic look-up table is coupled with the flow solver, and the look-up table is systematically refined to take into account the large variation of thermodynamic properties in the vicinity of the critical point. Effects of different boundary and operating conditions are also discussed. It is shown that the compressor performance is highly sensitive to the look-up table resolution as well as the operating and boundary conditions near the critical point. Additionally, a method to overcome the difficulties of simulation close to the critical point is explained.

The decrease in the performance of centrifugal compressors operating at low Reynolds numbers (e.g., unmanned aerial vehicles at high altitudes or small turbomachines) can reach 10% due to increased friction. The purposes of this review are to represent the state-of-the-art of the active and passive flow control methods used to improve performance and/or widen the operating range in numerous engineering applications, and to investigate their applicability in low-Reynolds-number centrifugal compressors. The applicable method should increase performance by reducing drag, increasing blade loading, or reducing tip leakage. Based on the aerodynamic and structural demands, passive methods like riblets, squealers, winglets and grooves could be beneficial; however, the drawbacks of these approaches are that their performance depends on the operating conditions and the effect might be negative at higher Reynolds numbers. The flow control method, which would reduce the boundary layer thickness and reduce wake, could have a beneficial impact on the performance of a low-Reynolds-number compressor in the entire operating range, but none of the methods represented in this review fully fulfil this objective.

The supercritical CO2 (sCO2) Brayton cycle has been attracting much attention to produce the electricity power, chiefly due to its higher thermal efficiency with the relatively lower temperature at the turbine inlet compared to other common energy conversion cycles. Centrifugal compressor operating conditions in the supercritical Brayton cycle are commonly set in vicinity of the critical point, owing to smaller compressibility factor and eventually lower compressor work. This paper investigates and compares different centrifugal compressor design methodologies in close proximity to the critical point and suggests the most accurate design procedure based on the findings. An in-house mean-line design code, which is based on the individual enthalpy loss models, is compared to stage efficiency correlation design methods. Moreover, modifications are introduced to the skin friction loss calculation to establish an accurate one-dimensional design methodology. Moreover, compressor performance is compared to the experimental measurements.

The estimation of boundary layer losses requires the accurate specification of the freestream velocity, which is not straightforward in centrifugal compressor blade passages. This challenge stems from the jet-wake flow structure, where the freestream velocity between the blades cannot be clearly specified. In addition, the relative velocity decreases due to adverse pressure gradient. Therefore, the common assumption of a single freestream velocity over the blade surface might not be valid in centrifugal compressors. Generally in turbomachinery, the losses in the blade cascade boundary layers are estimated, e.g., with different loss coefficients, but they often rely on the assumption of a uniform flow field between the blades. To give guidelines for the estimation of the mentioned losses in highly distorted centrifugal compressor flow fields, this paper discusses the difficulties in the calculation of the boundary layer thickness in the compressor blade passages, compares different freestream velocity definitions, and demonstrates their effect on estimated boundary layer losses. Additionally, a hybrid method is proposed to overcome the challenges of defining a boundary layer in centrifugal compressors.

The effect of tip clearance to the centrifugal compressor diffuser flow fields and stage overall performance are studied experimentally. The relative tip clearance (tip clearance divided by the impeller exit blade height) is increased by shimming the shroud side casing of a high-speed variable speed driven industrial centrifugal compressor. Four different relative tip clearances are studied: 0.027, 0.053, 0.082, and 0.106. The stage efficiency and pressure ratios are measured, as well as the diffuser flow fields. The diffuser flow fields are measured both at the diffuser inlet and outlet. The total pressure and flow angle are measured with a cobra probe, and the total pressure and temperature with three Kiel probes. Static pressures are measured adjacent to the probe measurements. As expected, increasing the tip clearance leads to lower stage efficiency and pressure ratios. The decrement in the efficiency due to the increasing of the tip clearance is higher with higher mass flows, and at lower rotational speeds. Increasing tip clearance increases the size of the secondary flow region present at the impeller outlet near the shroud.