Abstract
Abstract A thermal power plant for the East African Rift countries is under study for combined energy and freshwater generation using geothermal water, available at above 500 kPa pressure and temperature exceeding 150°C. This article presents the computational fluid dynamics (CFD) model and analysis of the two-phase turbine used for power generation in this total flow thermal plant. Flash boiling was implemented using a two-fluid multiphase model with the thermal phase-change criteria for heat, mass, and momentum transfer in the CFD solver ANSYS CFX. Initially, flashing flow in a converging–diverging nozzle was validated. This stationary nozzle model was then extended to a curved rotating nozzle reaction turbine and the results of flow and power were evaluated against available test data at 400 kPa feed water pressure under subcooled condition of 117°C and a very low backpressure of 6 kPa. Flow through this turbine was predicted within 8% deviation. An overestimate in thermodynamic power by 30–50% was predicted at speeds below 4000 rpm, while at the design speed of 4623 rpm the deviation was less than 5%. Rotor torque and hence power estimate was found to be dependent on the bubble size, bubble number density, and heat transfer parameters prescribed in the CFD model. The vapour dryness fraction at turbine exit was close to an isentropic expansion vapour quality. The isentropic efficiency was 7.5–17% for the analysed speed range.
Highlights
Geothermal hot brine deposits are a natural source of hightemperature and high-pressure fluids
Rotor torque and power estimate was found to be dependent on the bubble size, bubble number density, and heat transfer parameters prescribed in the computational fluid dynamics (CFD) model
A two-phase CFD model for calculation of flash boiling flows was applied using the thermal phase-change formulation in ANSYS CFX solver to nozzles
Summary
Geothermal hot brine deposits are a natural source of hightemperature and high-pressure fluids. Fabris (1993, 2005) identified that the high inlet pressure to the two-phase nozzle entry, resulting from the centrifugal acceleration of the liquid flowing radially outwards, thereby subcooling it and delaying the flashing, was a deficiency of House’s (1978) radial outflow reaction turbine configuration. Grid refinement reported in this study showed an optimum node count of 744 800 for the nozzle geometry and the numerical solver used was ANSYS CFX in steady state Water data for both liquid and vapour phases were specified using IAPWS-IF97 (Wagner et al, 2000). A phase-change validation study has been conducted on published experimental data on a CD nozzle flow at various operating conditions (Abuaf et al, 1981; Liao & Lucas, 2017). The model was extended to the test turbine design reported in Date et al (2015) and the results have been evaluated
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