Claudio Lettieri

Cavitation Instabilities in an Inducer Using Heated Water as a Cryogenic Simulant

Cavitation instabilities in inducers cause performance and reliability concerns in liquid rocket engines. Attempts to predict the effect of cavitation instabilities have often been limited to subscale testing and simulation with room-temperature water as the working fluid. In this study, heated water is used in experiments and simulations to capture the thermally limited behavior of cavitation exhibited by cryogens. Conducted at the Aerospace Cavitation Test Facility, the experiments measure static performance and dynamic phenomena at temperatures ranging from 297 K to 376 K. The test article is a four bladed inducer designed by MIT and is operated at a flow coefficient of 0.08. Results in 297 K water show rotating cavitation at 1.5 times the shaft speed. Spectrograms of pressure over a range of cavitation numbers show instabilities that exist in room temperature water are never exhibited in water at elevated temperatures. In addition to experiments, computational fluid dynamics simulations were conducted using three different cavitation sub models. The first model is an inertially limited model, but the focus is on the two models that include thermal effects. One thermal model solves the full energy equation and includes a temperature dependent vapor pressure. The other model avoids solving the energy equation by adjusting the vapor pressure based on the energy necessary to vaporize the liquid phase. Transient computations indicate cavitation instabilities in room temperature water that are not present in simulations where elevated temperature water is the working fluid.

Characterization of Rotating Cavitation in a Four Bladed Inducer

United States. National Aeronautics and Space Administration (NASA Marshall Space Flight Center)

Low-Flow-Coefficient Centrifugal Compressor Design for Supercritical CO2

This paper presents a design strategy for very low flow coefficient multi-stage compressors operating with supercritical CO2 for Carbon Capture and Sequestration (CCS) and Enhanced Oil Recovery (EOR). At flow coefficients less than 0.01 the stage efficiency is much reduced due to dissipation in the gas-path and more prominent leakage and windage losses. Instead of using a vaneless diffuser as is standard design practice in such applications, the current design employs a vaned diffuser to decrease the meridional velocity and to widen the gas path. The aim is to achieve a step change in performance.The impeller exit width is increased in a systematic parameter study to explore the limitations of this design strategy and to define the upper limit in efficiency gain. The design strategy is applied to a full-scale re-injection compressor currently in service. Three-dimensional, steady, supercritical CO2 CFD simulations of the full stage with leakage flows are carried out with the NIST real gas model. The design study suggests that a non-dimensional impeller exit width parameter b2* = (b2/R)ϕ of 6 yields a 3.5 point increase in adiabatic efficiency relative to that of a conventional compressor design with vaneless diffuser. Furthermore, it is shown that in such stages the vaned diffuser limits the overall stability and that the onset of rotating stall is likely caused by vortex shedding near the diffuser leading edge. The inverse of the non-dimensional impeller exit width parameter b2* can be interpreted as the Rossby number. The investigation shows that, for very low flow coefficient designs, the Coriolis accelerations dominate the relative flow accelerations, which leads to inverted swirl angle distributions at impeller exit. Combined with the two-orders-of-magnitude higher Reynolds number for supercritical CO2, the leading edge vortex shedding occurs at lower flow coefficients than in air suggesting an improved stall margin.Copyright