Thermal Turbomachinery Design for Closed Thermal Cycles and Multiple Fluids

Abstract

Closed-cycle gas turbines can complement conventional power conversion systems due to their potential for improved efficiencies, compact system layouts, and the ability to exploit non-fossil fuel energy sources which leads to low carbon emissions. Moreover, they can be adopted for distributed power generation applications. This thesis provides an understanding of the true essence of closed-cycle gas turbines with a focus on the development of turbomachinery design methodologies. The methodologies have been applied for the radial turbomachinery design (small-scale power range) for supercritical Brayton cycles and other thermal cycles such as air cycle, organic Rankine cycle, cryogenic cycles, and steam Rankine cycle. The thesis begins with an elaborate review of the potential of closed-cycle gas turbines. Thermodynamic analysis has been carried out for the recuperated closed Brayton cycle with and without intercooling employing different working fluids. It has shown that the supercritical carbon dioxide gives considerably higher efficiencies at mild turbine inlet temperatures of 400-700°C and helium can be considered at higher temperatures of above 800°C. The closed Brayton cycle turbomachinery designs with multiple fluids have been brought together uniquely on two charts, one in absolute scale (∆H-M-D) and other in non-dimensional scale (NS-DS) by carrying out a detailed survey of the closed-cycle gas turbine plants and concept designs, which can aid in the design of turbines and compressors for different applications. Turbomachinery design can have two approaches. The first one is scaling a benchmark design for different fluids catering to a particular application. The second one is a thorough step-by-step meanline design methodology for both turbines and compressors for any new application. Turbomachinery development through scaling a good benchmark design using the power of similitude to adapt it to the different working fluids employed in various thermal cycles can save considerable amount of time and provide a quick solution with good performance. The scaling methodology has been used for the development of radial turbomachinery for a 50 kW supercritical carbon dioxide (SCO2) power plant. The CFD simulations along with experimental results have confirmed that the scaling technique is quite good. The radial inflow turbine had an isentropic efficiency of 77 % from the CFD simulation at the design point with SCO2 fluid. The torque coefficients, flow coefficients, and the efficiencies determined from the CFD simulations for the turbine, when superimposed on the benchmark turbine experimental curves, showed good agreement. The efficiencies for the compressor from CFD with SCO2 fluid were lower compared to the experimental efficiencies of the benchmark compressor, but the curves showed the same trend. The highest CFD efficiency of 77.2 % was obtained at a flow coefficient of around 0.46. Experiments were carried out for the developed turbine and compressor assembly using air as the fluid (aeroloop) to mainly observe the vibrations at high speeds. The assembly had run-up to a maximum speed of 70000 rpm. The turbine flow coefficients from the aeroloop experiment were not far away from the simulation and the NASA benchmark data. The compressor flow coefficient zone from the experiment coincided with the flow coefficient zone of open-type boundary condition simulation results of the compressor with air as the fluid. Also, the absolute plot of speed vs. mass flow rate for the compressor in the aeroloop test matched well with the open boundary type CFD simulation results. The thesis has also presented a scaling procedure to develop turboexpanders from a benchmark air turbine having experimental characteristics. It has been applied for the design of turboexpanders for supercritical carbon dioxide (SCO2) Brayton cycle, R143a organic Rankine cycle, and helium cryogenic cycle. The dimensionless iso-Mach number characteristic curves and efficiency curves of the SCO2 turbine, helium turbine, and R143a turbine determined from the CFD simulations were superimposed on the experimental characteristics of the benchmark turbine. There was good conformance between both, which proved that the proposed scaling methodology can be adopted for the turboexpander design. Detailed meanline design procedures were proposed in this thesis for radial inflow turbine and centrifugal compressor using specific speed and specific diameter parameters so that the design can be started from scratch for a new application. These were used for the development of a turbo-compressor for the 2 kW air cycle machine for DARE (DRDO) which is employed in aircraft cooling. The radial inflow turbine had an isentropic total efficiency of 85.6 %. The centrifugal compressor designed based on the proposed methodology had an isentropic total efficiency of 76 %. Another design of centrifugal compressor which was carried out in a different approach had attained an efficiency of 78.6 %. Radial turbomachinery design for a 1 MW supercritical carbon dioxide power plant has been presented in which the radial turbine is designed using the proposed 1-D meanline design methodology and the centrifugal compressor is designed using the scaling school of thought. Numerical simulations showed isentropic total efficiencies of 91% for the radial inflow turbine and 77.25 % for the centrifugal compressor at the design point. The last part of the thesis has discussed the design procedure of an unconventional turbine type called ‘radial outflow turbine’ for a 200 kW steam Rankine cycle and 1 MW supercritical carbon dioxide Brayton cycle. The design point CFD simulation of the 18-stage steam radial outflow turbine showed efficiency close to 75 %. A new zone was identified for the radial outflow turbine in Balje’s specific speed-specific diameter chart. The design point CFD efficiency of the supercritical carbon dioxide radial outflow turbine was 84.6 %.