21st Australasian Fluid Mechanics Conference Adelaide, Australia 10-13 December 2018 Measurement of Water Vapour in Axisymmetric Jet Development Using TDLAS A. Al-Manea 1,2 , D. Buttsworth 1 , J. Leis 1 , R. Choudhury 1 and K. Saleh 1 1 School of Mechanical and Electrical Engineering University of Southern Queensland, Toowoomba, Queensland, 4350, Australia 2 Al-Samawah Technical Institute Al-Furat Al-Awsat Technical University, Al-Samawah, 66001, Iraq Abstract Turbulent mixing of multi-phase compressible jets formed within steam ejectors requires further investigation so that re- liable models can be developed to aid in the ejector design process. As a step towards the non-intrusive measurement of the flow conditions in such a jet, tunable diode laser absorp- tion spectroscopy (TDLAS) is used to investigate the develop- ment of an axisymmetric subsonic jet of air containing a high concentration of water vapour (with a relative humidity around 70%), and a co-flowing stream of dry nitrogen. The radius of the jet nozzle was 14 mm and the measurements were made at distances of 5, 10, and 15 mm downstream of the nozzle exit. At each downstream distance, TDLAS measurements of water vapour features at around 1392 nm were made using a discrete mode laser. At each measurement location, the absorbance was recorded and the water vapour concentration was determined by using an Abel inversion and by fitting spectral results from the HITRAN 2012 database to the measured spectra using custom MATLAB scripts. Simulation of the experimental jet conditions was used to assess the accuracy of analysis method, and results indicate an accuracy of better than ± 5 % in the water vapour concentration can be achieved using the method proposed in this paper. Introduction The development of jet mixing plays a critical role in steam ejectors which are used in a wide variety of industrial applica- tions to induce pumping and compression effects. For typical steam ejector operating conditions, the primary flow of steam experiences a non-equilibrium condensation process within the jet nozzle prior to the development of compressible, turbulent mixing with the surrounding flow. For the development of im- proved ejector devices, reliable simulation tools are required, but the modelling of the compressible turbulent mixing of wet steam flows is challenging because of the number and complex- ity of interacting physical processes: non-equilibrium multi- phase turbulent compressible flow with strong pressure gradi- ents, including shock waves [3]. Over the past decades, significant work has been performed in the area of TDLAS techniques [11]. Measurements of the temperature and water mole fraction using this technique have been extensively studied and implemented for flame measurement [4, 5]. This study seeks to apply TDLAS (tunable diode laser absorption spectroscopy) to the measurement of steam ejector mixing flows, in order to obtain experimental data that can be used for model development and validation. However, because of the significant complexities associated with such ejector flow conditions, we have chosen to tackle a simplified steam jet mixing problem to assess and optimize the technique, and to develop methods to reconstruct the water-vapour concentration distribution at different locations downstream from the nozzle. TDLAS theoretical background The TDLAS technique is well established [11]. Transmission of light at a particular wavelength, through a particular medium, can be described by the Beer-Lambert relation: T = I t I o = exp(-k v .L) (1) where I o and I t are the strengths of the beams before and after passing through the medium respectively, k v is the spectral ab- sorbance coefficient (cm -1 ), and L is the beam path length (cm). The value of k v for an individual spectral line can be defined as [10]: k v = S(T ).φ(ν).N d (2) where, S(T ) is the line strength at a certain wavenumber ν (cm -1 /molecules.cm -2 ), φ (ν) is the line shape function (1/cm -1 ), N d is the number of the absorbing molecules per unit volume, and can be calculated as N d = N L .P T .X abs .  296 T  (3) where T is the temperature (K), P T is the total pressure (atm). X abs is the mole fraction of absorbing medium, and N L is the Loschmidts’ number, N L =2.447 × 10 19 (molecules/cm 3 /atm at 296 K). Experimental setup Figure 1 presents a photograph of the experimental arrangement which was used to study the axisymmetric subsonic jet flow. A tunable diode laser (EP1392-DM-DX1) with salient character- istics similar to that described in [6] and [7], was modulated across the wavelength range between 1392 and 1393 nm by controlling the laser current with a frequency of 10 Hz. A jet nozzle with a nominal radius of 14 mm, was used to provide the apparatus with a flow of moist air. A plate with uniformly dis- tributed 2 mm holes distributed around the nozzle was used to deliver a dry nitrogen co-flow for the jet of moist air. Nitrogen was also used to purge the water-vapour from the optical path, other than in the jet flow. The detected signal is received by an Auto-Balanced Photoreceiver model (Nirvana2017), which is connected into an oscilloscope and DAQ system (LabVIEW) to monitor and record the data. The test section has been de- signed to move in two axes, to allow the laser to traverse the jet at various radial and streamwise locations. The conditions are monitored by using a hygrometer (± 2 %), thermocouple (K- type ± 1 ◦ C), and Vernier mercury barometer (±0.05 kPa). A volumetric flow meter (± 0.001 LPM) was also used to main- tain the mixing ratio (mass of nitrogen/mass of air) constant at MR = 1.5.