A novel turbo expander based on the Tesla turbine is proposed to be applied to a heat pump or refrigeration cycle to improve the overall cycle efficiency. Initial numerical modelling of this turbo expander at representative conditions was carried out using the homogeneous relaxation model (HRM) to assess the influence of phase change on performance. The presence of a dense cloud of liquid droplets within the rotor was predicted to produce a significant back pressure on the turbine nozzle postponing the phase change. This was expected to occur in the vicinity at the outlet of the nozzle, but high volume fractions of liquid was predicted to penetrate deeper inside the rotor, especially at higher RPM. The resulting lower velocities of the liquid flow at the inlet of the rotor was predicted to significantly degrades the performance of the turbine. It is thus important for a successful implementation of this concept to remove as much liquid droplets as possible before the flow enters the rotor in order to minimise the back pressure.
Present day land-based gas turbine combustors, operating on oil, must meet strict requirements for emissions (CO, unburned hydrocarbons, particulates, smoke and NOx) and burn stabily without pulsations over a wide range of operating conditions. In addition many engines, such as those produced by ABB, operate with both oil and natural gas fuels either together or independently. This paper concentrates on the development of an oil injection system which is optimised for ABB’s double cone burner (Figure 1) and which does not affect the operation of this burner on natural gas. The development procedure, which involved a coupling of numerical and experimental techniques, is described. The results of the application of this procedure indicate that a simple plain jet atomiser in conjunction with a small quantity of unswirling air admitted at the head of the burner is the best option for this burner.
A numerical analysis based on Computational Fluid Dynamics (CFD) is carried out to investigate the influence of the fuselage transition and axial offset, on the inlet distortion and performance of a tail mounted fan, for a short-haul commuter aircraft. For the examined configurations, it is predicted that varying the angle of the fuselage transition does not have a significant impact on the radial distortion, for a typical fan mounted at the centreline of the fuselage. The wake behind the fuselage is predicted to increase in size as the slope of the fuselage is increased, however, the positive suction from the fan is sufficient for the flow to fully recover before the fan duct inlet. Nevertheless, it is predicted that offsetting the fan from the fuselage centreline produces a more significant increase in distortion in the circumferential direction. The propulsive power of the fan is predicted to increase slightly with increasing offset mainly due to the side of the fan which is less obstructed by the fuselage. However, the wake on the opposite side is predicted to increase significantly persisting almost to the inlet of the fan duct. A vortex formed upstream of the fan increases in strength with increasing offset. This vortex helps to offset the increase in circumferential distortion by re-energizing the flow in the wake of the fuselage. This causes the circumferential distortion to remain roughly constant between offsets of 25% and 50% of the fuselage radius. It is likely though that this vortex will deteriorate the performance of the fan.
The design and development of an innovative Tesla style turbo expander for two-phase fluids is proposed, as a substitute for the lamination valve of a traditional Heat Pump cycle. Thereby enhancing the overall performance of the Heat Pump, by recovering mechanical work to offset the compressor requirements. The major challenge in such configurations is the reliable operation of the expander, when phase change occurs across it, from a purely liquid flow to a mainly vapour flow by volume with a dense cloud of liquid droplets. To investigate the phase change, a modelling approach is adopted which is routinely applied to modelling fuel-flashing in direct injection diesel engines, where the phase change deviates strongly from equilibrium. The Homogeneous Relaxation Model (HRM) is employed, which utilizes an Eulerian approach. The proposed computational model is firstly validated against experimental results available in the literature. A sensitivity analysis of the phase change model relaxation parameter is performed. It was found that a value 10 times lower than the published value gave closer agreement to the measured results. It is believed that this result is due to the roughened walls of the experiment, which would produce more nucleation sites for vapour bubble formation. This suggests that this model maybe is sensitive to the geometry of the turbine. Following this validation, the detailed flow profile in the proposed Tesla turbo-expander is investigated. Two different expander designs are considered in this project, one working with water [4,20] and the other with butane (R600). This study focuses particularly on the butane expander design. The expander performance is evaluated for rotational speeds up to 32’000 RPM. Results on the turbo-expander under investigation, showed that the presence of a dense cloud of liquid droplets produces a significant pressure drop across the turbine rotor, which increases with RPM, postponing the phase change. High volume-fraction of liquid was predicted to penetrate deeper inside the rotor above 16’000 RPM for the butane expander. The resulting lower liquid flow velocity relative to the rotor disk speed at the inlet of the rotor is predicted to significantly degrade the performance of the turbine at high rotational speeds. Decreasing the nozzle throat area improves the situation, by initiating the phase change further upstream and increasing the RPM operational range by 50%. Angling the nozzle radially inward by 10° was found to not have a great impact on the performance of the turbine. It was determined from this study that it is critical to predict correctly where the phase change starts, in order to accurately predict the performance of the turbine. Important is to remove as much liquid as possible from the flow, before it enters the rotor, to minimize the impact of the phase change on the turbine performance.
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