Supersonic condensing steam jet measurements using TDLAS

Supersonic condensing steam jet measurements using TDLAS

Ejectors have no moving parts and are preferable to mechanical compressors in many applications, but ejectors typically have a relatively low efficiency. Efforts to understand the dominant factors which control the mixing-induced entrainment of the low pressure stream inside ejectors are on-going. In the case of steam ejectors, additional complexity in the mixing process arises because the steam expansion in the supersonic nozzle typically causes condensation. The mixing between the high speed and low speed flows in a steam ejector involves the supersonic turbulent mixing of two phase flow in the presence of pressure gradients which provides substantial challenges for modelling and simulation. This thesis demonstrates the application of a tunable diode laser absorption spectroscopy (TDLAS) technique in the study of a supersonic steam jet within the mixing chamber of an ejector-like apparatus. It is expected that the TDLAS data can provide useful information for future modelling efforts.

The TDLAS method was first assessed within a non-flowing experiment using a gas cell with a 100 mm optical path length containing saturated water vapour at a pressure of 2.2 kPa. The TDLAS measurements for the temperature and pressure were then compared with reference values measured using independent instrumentation. The results demonstrated that temperature could be measured from the TDLAS data with an error between 0.34 and 0.44 %, and the pressure within error between 2 and 12% at the flow pressure saturation conditions in the gas cell. Before attempting the supersonic steam jet measurements, the TDLAS method was further tested through application in a simplified flow configuration involving a low-speed (subsonic) jet of moist air with a co-flow of dry nitrogen. The flow was nominally axisymmetric, so an Abel inversion method was developed to obtained the radial distribution of the absorption coefficient from the TDLAS data. Based on an assessment of the methods using simulated jet concentration data, the method should be capable of yielding concentration to within 6% for the conditions considered in this work.

TDLAS measurements were obtained within an axisymmetric supersonic steam jet apparatus that was developed for this work. The supersonic steam jet nozzle exit diameter was 13.6 mm, and a low-speed flow of dry nitrogen surrounding jet. The nitrogen was used in the apparatus to provide a non-absorbing co-flowing stream that enabled the supersonic wet steam jet absorption to dominate the line-of-sight TDLAS measurements. The TDLAS was traversed through the
flow at three planes downstream of the supersonic nozzle exit: 15, 20, and 30 mm. At each of the three planes, the line-of-sight TDLAS measurements were made with the laser passing through locations between 0 and 20 mm from the jet centreline. Through the analysis of the TDLAS data and application of the Abel inversion method, the radial distribution of the pressure, temperature, and the concentration of the water-vapour were obtained. Results were compared to CFD simulations using a non-equilibrium wet-steam model and the average error between the CFD simulations and the TDLAS measurements was between 4 and 24% in the case of the pressure and between 1.5 and 5% in the case of the temperatures. Suggestions for improvement to the TDLAS technique and computational simulation methods are offered.