Experimental Investigation of Critical Flow of Supercritical Carbon Dioxide

by Mark Anderson, Guiliume Mignot, & Michael Corradini
University of Wisconsin - Madison

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New power high density power cycle designs have recently focused on using supercritical carbon dioxide (SCO2) as a working fluid. Among the areas of interest are heat transfer and fluid flow phenomena. This work focuses on the characterization of S-CO2 during a rapid depressurization process and its impact on the environment. In an effort to study these phenomena, a (0.123 m3) facility has been constructed and used to perform measurements of the critical flow rate of carbon dioxide over a wide range of conditions (pressures up to 240 bars and temperatures of 260°C). The design and parameters of the facility will be discussed along with the results of over 30 experiments. A set of exit nozzles with various entrance shapes, roughness and length to diameter ratios ranging from 3.7 to 168 was used. Stagnation pressure, stagnation temperature and fluid mass time traces were collected for each test. For temperatures higher than 140°C, a single-phase flow was observed whereas, for lower temperatures, a second liquid phase or even a third solid phase appeared, as the jet temperature approached the triple point of the fluid.

The data was scaled successfully using the initial mass and the initial mass flow rate for most conditions except for high density, low temperature cases where non-equilibrium effects occurred within the vessel. Under conditions of large length to diameter ratio, the fluid behavior was found to be predicted accurately using a homogeneous equilibrium model with friction, even under conditions with two phases. At a pressure of 10 MPa for length to diameter ratios smaller than 14.7, the critical mass flux results exhibited a plateau, indicating that the critical mass flow rate was governed by the vena contracta. Under all conditions, multi-dimensional and repetitive shock structures both within the exit nozzle and outside of the vessel in the jet free stream were induced. Initial work has begun to optically determine the fluid state within the tube using an IR absorption technique. Initial testing has indicated that this new technique can correctly measure pressure trends within the jet. However, in order to measure the spatial distribution of pressure within the jet structure, further calibration of this technique using accurate high pressure carbon dioxide data is needed.