Simulation and measurement of condensation and mixing effects in steam ejectors

Ariafar, Kavous (2016) Simulation and measurement of condensation and mixing effects in steam ejectors. [Thesis (PhD/Research)]

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Abstract

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.


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Item Type: Thesis (PhD/Research)
Item Status: Live Archive
Additional Information: Doctor of Philosophy (PhD) thesis.
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Mechanical and Electrical Engineering
Supervisors: Buttsworth, David
Date Deposited: 28 Jul 2017 00:20
Last Modified: 07 Mar 2018 01:55
Uncontrolled Keywords: vapour compression; steam ejectors
Fields of Research : 09 Engineering > 0913 Mechanical Engineering > 091399 Mechanical Engineering not elsewhere classified
URI: http://eprints.usq.edu.au/id/eprint/32860

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