title: Simulation and measurement of condensation and mixing effects in steam ejectors
Creators: name: Ariafar, Kavous
Publisher: name: University of Southern Queensland
Year: 2016
Subjects: subject: 091399 Mechanical Engineering not elsewhere classified

description: Vapour compression via ejectors has become a topic of interest for researchers in the field of air conditioning and refrigeration. Ejectors have the benefit of being extremely reliable with stable operation and no moving parts leading to essentially maintenance free operation. However, these devices typically have very low efficiencies due to the low entrained mass flow rate of the low pressure secondary stream relative to the high pressure primary stream mass flow rate. The entrainment of the secondary stream and mixing between the primary and secondary streams are therefore dominant features which require investigation. Entrainment and mixing typically occurs under conditions of compressible, turbulent flow with strong pressure gradients. Steam ejectors, which are the focus of the present work, have the added complexity of condensation effects which must be accommodated in modelling and simulation work. Condensation in the primary nozzle of steam ejectors alters the steam flow properties relative to properties derived from ideal gas modelling, which is sometimes used for steam ejector analysis work. By performing computational simulations for non-equilibrium wet steam flow in a representative primary nozzle, the altered steam jet properties that arise during the nozzle expansion process are demonstrated, via empirical correlations, to be of sufficient magnitude to affect the mixing rate, and thus the entrainment ratio, of steam ejectors. For the particular primary nozzle and flow conditions considered, it was estimated that these changes in steam properties would cause around 29% increase in the mixing layer growth rate for the wet steam case relative to the ideal gas case. To further explore the influence of wet steam mixing effects, the non-equilibrium wet steam computational simulation approach was then expanded to the case of a complete ejector. Under particular conditions for the choked flow ejector operation, results indicated that the non-equilibrium wet steam model simulates an entrainment ratio that is 10% higher than that for the ideal gas model. The non-equilibrium wet steam model also gives a higher critical back pressure by about 7% relative to the ideal gas model. Enhanced mixing layer growth, which arises due to steam condensation in the primary nozzle, was identified as the main reason for higher entrainment ratio of the ejector simulations using the wet steam model. Higher pitot pressure of the mixture at the diffuser entrance for the wet steam simulation was also identified as the reason for higher critical back pressure for the ejector relative to the case of ideal gas simulation. To estimate the relative significance of pressure-driven effects and mixing-driven effects on the secondary stream entrainment, ideal gas computational simulations were also performed. Under a fixed operating condition for the primary and discharge streams, the ejector entrainment ratio was more strongly influenced by the mixing effects at lower secondary pressure. For a particular ejector and associated operating conditions, about 35% of the ejector entrainment ratio was attributable to mixing effects when the secondary stream pressure lift ratio was 4.5, while this portion was reduced to about 22% when the secondary stream pressure lift ratio was 1.6. Given the significance of ejector mixing effects and the lack of consensus on the most appropriate model for turbulent mixing in steam ejectors, an experimental investigation was performed to provide direct data on the mixing of wet steam jets in steam ejectors for model development and validation of computational simulations. Pitot and cone-static pressures within a high pressure supersonic steam jet that mixed with low pressure co-flowing steam were obtained. Results from the non-equilibrium wet steam simulations were analysed to give values of pitot pressure and cone-static pressure values using both equilibrium and frozen-composition gas dynamic models. The equilibrium analysis appeared reasonable for the pitot pressure, whereas the frozen-composition analysis was a better approximation for the cone-static pressure. Differences between the experimental data and the wet steam computational simulations were in the vicinity of 25% at certain locations. The static pressures downstream of the nozzle exit were lower than the triple point, but energy exchanges associated with the transitions to and from the solid phase were not incorporated in the wet steam model. The development of such a model is required before definitive conclusions can be made regarding the accuracy of the turbulence modelling.