Eductors for active vapor transport and condensation in desalination with low grade thermal energy
Creators: name: Koirala, Ravi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401205
subject: Experimental methods in fluid flow, heat and mass transfer
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: This PhD thesis has been prepared based on the study on the application of eductors for freshwater production when combined with a membrane-based distillation process. In membrane-based desalination technology, water vapor is separated from saline solution using a hydrophobic membrane. The driving force for separation and vapor trapping depends on the type of process selected. The use of eductors both for driving vapor across the trans-membrane boundary and for condensation is a novel application of the technology. Although eductor and ejectors are synonymously used but this study uses eductor for water jet system and ejectors for all other systems. This work has been performed in five different stages which cover assessment of the technology, application, and design modification. 

Research and Development Status: The available literature has limited evidence of the use of eductors with primary liquid jet two phase single species flow. Most of the works reviewed are focused on air or steam jet technology for compression purpose. Other applications of the technology involve pumping, mixing, etc. The eductor as a condenser has never been explored. Some work presents direct contact as a drawback due to the resulting limitation in mixture isolation at the outlet. Since it is a static technology, the performance is very sensitive to internal geometry. The geometry of the primary nozzle, secondary inlet, suction chamber, mixing chamber, throat and diffuser are the primary influencing factors. This study explores heat exchanging mechanism and capacity of eductors, together with pumping, degassing, and mixing.

Maximum operational limit of Eductor: The upper limit of the mechanical work of an eductor is best described by its maximum suction capacity. After a review of current research and developmental status, the capacity of commercial design was computationally and experimentally studied. For the considered designs, the nature of flow (axial or whirling) and geometric parameters both influence the output. The case with spear valve allows for flexible operation in ‘off-design’ conditions. This work will aid in marking the operational limit by identifying the cooling capacity of the eductor. 

Multi-phase flow analysis: For this analysis, the eductor operates with liquid water as the primary flow and vapor as the secondary flow. The mechanism of condensation in two phase eductors was reported through experimentally verified unsteady computational analysis. The study used a Fluent based Thermal Phase Change Model with two resistance heat transfer to predict the mechanism of condensation. It presents multiple new findings on the mechanism of the eductor along with the influence of condensation on entrainment. Confirmation that complete condensation can occur within the length of an eductor and the identification of the oscillating behavior of the zone of complete condensation are among the major outcomes from this work. The occurrence of condensation increases entrainment due to the voids which are created after the collapse of bubbles during condensation which are then refilled by incoming vapor. The findings were aided by three different sets of experiments: The measurement of entrainment ratio, axial pressure distribution and visualization of mechanism. Both commercial and 3D printed eductors were used for this purpose. These studies establish the eductor as a reliable device that has potential to replace existing vacuum pumps and condensers in industrial applications. 

Application of Eductor: The eductor can be used for any application requiring combined sub-atmospheric vapor generation and condensation. Here the primary focus is on membrane-based distillation. A modified eductor-based membrane module was developed at the lab and experimental and empirical study was performed on it. The developed system is in principle like Vacuum Membrane Distillation (VMD) but differs with the use of direct contact condensation for mass transfer. The use of an eductor drastically reduces the existing footprint of the membrane distillation unit, along with enhancement in the mass flux. 

Design modification: Since eductors were never designed for combined vacuum generation and condensation, the existing designs limit the thermal performance of the system. Particularly, the computational and experimental study concluded that condensation occurs due to inter-phase interaction at the interface between primary and secondary flows. Further, with a single jet, this interaction occurs only around the circumference of the jet, with no thermal interaction with the jet core. Therefore, most of the cooling capacity remains unutilized. The 2-phase mixing zone is the region of maximum exergy destruction within the eductor. Hence it was felt that to enhance the thermal interaction, the inter-phase interaction area should be increased by increasing number of nozzles, The results of the computational analysis show improvement in exergetic efficiency with reduced specific exergy destruction. 

In summary, a new application of eductor technology has been explored, and answers to most of the application issues identified have been provided. The status of research and development, limits of commercial products, possibility and mechanism of 2-phase operation, and design modifications for desalination applications have been presented. This work includes six manuscripts addressing these major outcomes.