Experimental performance investigation of two-phase flow stationary nozzle for organic trilateral flash cycle heat engines

The demand for sustainable power generation technologies has remarkably increased following rapid growth in the human population and consequently energy demand, global warming, and environmental concerns. The abundant availabilities of low-grade thermal resources including solar thermal, biomass, geothermal, and industrial waste heat over the world have motivated the development of heat conversion to power technologies from low-grade thermal resources.

To date, several operational organic cycle thermal engines have been made throughout the world which is mainly organic Rankine cycle, and even trilateral flash cycle consisting of screw expander and reaction turbine for electricity generation. However, it is yet to reveal the application of converging-diverging stationary nozzle in the trilateral flash cycle in which impulse turbine along with single element working fluid is employed. Furthermore, according to previous studies and literature, the converging-diverging stationary nozzle plays a crucial role in the total efficiency of power generation systems. Although the force required to rotate the rotor of an impulse turbine (e.g. Windmills and waterwheels) is provided by the high-speed two-phase fluid leaving a stationary nozzle, a proper expander is still required to convert the large heat energy to power in the TFC system. A stationary two-phase converging-diverging nozzle along with an impulse turbine is applied as an expander in this research work. In fact, the impulse turbine converts the kinetic energy generated by the working fluid after expansion in the converging-diverging nozzle to mechanical power. The change of the fluid momentum in the stationary nozzle enhances the force, which rotates the rotor. The system designed in this thesis can apply to any low-grade thermal resources, such as industrial waste heat, solar thermal resources, etc.

The present research focuses on experimental study and efficiency improvement of a stationary converging-diverging nozzle in a trilateral flash cycle which can be applied to an impulse turbine. For this purpose, a converging-diverging nozzle is employed in a TFC system and the performance of the proposed system to recover and convert the thermal energy from low-grade thermal resources to mechanical energy is experimentally investigated. In the proposed system, Isopentane is utilised as a working fluid because of its low boiling temperature. The working fluid heated by low-temperature hot water is pumped through a two-phase stationary nozzle, in which the energy conversion occurs. The liquid partially evapourates in the nozzle, and as a high-velocity mixture leaves the nozzle and impinges a Pelton turbine bucket shape target. The high potential of the proposed system in the conversion of low-grade thermal resources is exhibited through theoretical analysis. A lab-scale test rig is designed and manufactured to experimentally measure the force generated by the high-velocity flow out of the two-phase stationary nozzle. Moreover, the nozzle isentropic efficiency is calculated on the basis of the speed of two-phase fluid leaving the nozzle. Eventually, the findings obtained from these experiments are applied to a geothermal power plant in Birdsville, Australia as a case study.

The later focus of this study is to elucidate the effect of divergent geometry of nozzle on its performance by measuring the force generated by high-velocity two-phase flow at the nozzle exit. The experiments are carried out by using the nozzles with two basic diverging profiles of conical and bell and various divergent angles of 6°, 18°, and 30° corresponding to the divergent lengths of 45 mm, 17 mm and 12 mm respectively. The nozzles are examined at inlet temperatures of 50 °C and 60 °C for Isopentane as working fluid and the change in the pressure of working fluid over the nozzles are measured and compared at the corresponding temperatures.

Results of this study findings indicated an increase in the thrust force by increasing the inlet temperature for all examined nozzles. Besides, the bell and conical nozzles obtained relatively higher performances compared to other nozzles and reached to isentropic efficiency. The highest value of isentropic efficiency was obtained by the bell shape nozzle with the corresponding diverging angle of 6°.

The pressure measurements along conical and bell shape nozzles are indicative of a greater pressure drop after the nozzle throat for bell shape nozzle compared to the conical nozzle.

The empirical data obtained from this study would lay the foundation for designing high-performance nozzles and consequently more efficient heat-to-power recovery technologies from low-grade thermal resources that are currently wasted, and hence contribute to the national and international attempts to mitigate greenhouse gas emissions.