Study of Thermofluid Dynamics of an Ultra-Micro Scale Gas "Turbine"
This thesis aims to study the thermofluid dynamic behaviour through a component of an ultra-micro scale gas turbine (UMGT), the turbine. Here an experimental investigation is firstly conducted to serve as a basis for numerical validation and to prove the feasibility of a 3 dimensional (3D) UMGT scale radial inflow turbine. A numerical investigation of the experimental turbine is then carried out, where the effects of heat loss, which are enhanced at small (and therefore UMGT) scales, on the turbine performance are studied through adiabatic and isothermal boundary conditions. Lastly, four different turbines from microscale gas turbines (MGT) (100kW and 20kW) to UMGT scales (1kW and 250W) that are geometricallyand dynamically similar, are numerically investigated. The performance of the turbine and thermofluid dynamic losses due to lowered Reynolds ( ) numbers when scaling down to UMGT scale are quantified.The experimental investigation successfully tested the UMGT turbine, with a 20mm rotor diameter, 3Dprinted from Ti64. A maximum electrical power of 5.4W was attained at a rotational speed of 57,000 rpm for the highest temperature and flow rate of 80°C and 150 standard litres per minute. Here an overall efficiency as large as 28% was attained, and has therefore successfully proven the feasibility of a UMGT turbine. Experimental findings however, were limited, as the turbine was not tested at design point due to component and material inabilities. The experimental turbine also served as a basis to validate the numerical procedure, and here a difference of less than 10 and 2% was attained for the turbine power and pressure ratio plots, as a function of the rotational speed. A numerical investigation on the experimental turbine showed the effects of heat loss reduced the performance of the turbine by 2% from the adiabatic wall boundary condition, where an efficiency of 76% was attained, at the design specific speed ( ) of 0.51. The drop in efficiency is not as significant as would be expected, and this is due to the relaxation of the turbine parameters for experimental feasibility. If conventional gas turbine (CGT) parameters were imposed, the losses due to heat transfer would be expected to be significantly greater. The losses brought upon due to heat loss were classified to be 3 fold in nature; the loss of kinetic energy within the flow, the increment in viscous losses and the enhancement of secondary flow. Scaling down of the turbine from the MGT to the UMGT class showed isentropic efficiencies of 84.4, 82.8, 80.8 and 77.6% for the 100kW, 20kW, 1kW and 250W turbines respectively. Therefore an approximate 7% loss is brought upon due to scaling a MGT to a UMGT, and this is attributed to the reduced numbers, placing the flow from the turbulent to the laminar/transitional regime. Also, the observed off-design operation of the MGT is far superior to the UMGT, where a 73.8 and 60% efficiency was observed at the extreme off-design of 0.64, for the MGT (100kW) and UMGT (250W) respectively. An approximate 1% drop in the efficiency is noted between a 2D and 3D blade profile UMGT turbine when compared to literature. Although this can be considered insignificant, as it might be in the numerical error range, it is still suggested a 3D blade profile UMGT turbine is superior to a 2D blade profile. And this would result in better off-design operation, due to its better ability to account for 3D variations within the flow; as the effects of heat losses are enhanced at UMGT scales. The present study has therefore revealed the thermofluid dynamics through a UMGT turbine, and has thus quantified the losses associated with it due to heat transfer, and has also quantified and classified the loss sources due to geometrical scaling. It has also successfully presented non-dimensional maps on the performance of a UMGT turbine, and this is hoped to serve as a basis for UMGT design and optimisation.
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