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Organic Rankine cycles have been identified as a suitable technological option for converting low-grade heat into electricity with relatively high efficiency, and the organic Rankine cycle technology has been successfully implemented in different power production systems and in recovering heat in industrial processes. This paper studies the greenhouse gas emission reduction potential by using organic Rankine cycles for recovering exhaust gas heat of biogas engines. The study concentrates especially on the biogas engine power plants in Europe. Life cycle assessment methods are used and various waste heat utilization scenarios are compared. According to the results, greenhouse gas emissions can be reduced significantly if the thermal energy of the exhaust gases, otherwise lost in the process as waste heat, is utilized for additional electricity production by means of organic Rankine cycle. However, there may already be use for the exhaust gas heat in biogas plants in the form of heat power. In these cases, the use of organic Rankine cycle does not necessarily lead to greenhouse gas emission reductions. The results also indicate, that the working fluid leakages and production as well as the organic Rankine cycle construction materials and production have only marginal effects on the results from greenhouse gas perspective.

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Seven different vaneless diffuser designs varying in diffuser width are studied experimentally. One design is the basic design with the diffuser width and impeller exit and tip clearance width ratio of 1.0. The other six diffusers have width ratios of 0.903, 0.854, and 0.806. Three of the narrower diffusers have the width reduced from the shroud, and three from the hub and the shroud, divided evenly. The total-to-total efficiency and pressure ratio over the whole compressor are studied. It is possible to increase the efficiency of the compressor stage by reducing the diffuser width. The efficiency is increased over a wide operating range area at three different rotational speeds. The pressure ratios are increased at the design and low rotational speeds, but decreased at the high rotational speed. The shroud pinch seems to be more beneficial to the performance, while the hub pinch seems to have only a minor effect. The best design is the one with the width ratio of 0.854, with width reduction only at the shroud. A major finding is the experimental confirmation that the pinch influences the performance of the impeller, reducing the work input at the higher rotational speed.

The efficiency is reduced in very small centrifugal compressors due to low Reynolds numbers. In the past, the effect of the Reynolds number on centrifugal compressor performance has been studied experimentally, and empirical correction equations for the efficiency have been derived based on those results. There is a lack of numerical investigations into the effect of the Reynolds number on centrifugal compressor performance and losses. This paper aims to compare the numerical results to the efficiencies predicted by the correction equations found in the literature. The loss generation in the impeller blade passages is also studied in order to find out which loss production mechanism has the most potential to be reduced or eliminated. The effect of the Reynolds number on compressor performance is investigated in the chord Reynolds number range varying from 0.8 · 105 to 17 · 105 by simulating numerically the original compressors and downscaled ones. The numerical results are validated against experimental data and the results are compared with the efficiency correction equations used in the literature. The results indicate that the performance of the downscaled compressors follow quite precisely the most recently published correction equation. The results also show that the increased losses in low-Reynolds-number compressors are caused both by the relatively increased boundary layer thickness and by the shear stress resulting from the increased vorticity.

The effects of changing the stator–rotor axial gap on the performance and flow field of a low-reaction supersonic axial turbine are studied at design and off-design conditions. The objectives are gaining a better understanding of the effects and giving information and recommendations that a designer can use in his work. Three different axial gaps are modelled numerically with computational fluid dynamics at design and low off-design conditions. The geometry with the smallest axial gap is also measured at intermediate off-design conditions. The efficiency of the turbine decreases when the axial gap increases. The efficiency decrement is steeper at the off-design than the design conditions. It is concluded that the efficiency drop is mainly caused by the increased total pressure losses at the axial gap. It is recommended that the axial gap should be as small as possible. The smaller axial gap makes the rotor inlet flow angle distribution less curved, which is recommended to be taken in account when designing the turbine rotor for the studied turbine type. At the rotor outlet, the changing axial gap changes the absolute flow angle.

Seven different vaneless diffuser designs for a centrifugal compressor, varying only in diffuser width, were studied experimentally. The studied diffuser widths versus impeller exit width were 1.0, 0.903, 0.854, and 0.806. Three of the narrowed diffusers had the width reduced from the hub and shroud divided evenly, and the three others had the width reduced only from the shroud. The total and static pressures, the total temperature and the flow angles at the diffuser inlet and outlet were measured at the design rotational speed with three different mass flows. The impeller and diffuser performance was studied along with the axial distributions of flow angles and velocities in the diffuser. The results revealed that the pinch improved the compressor stage and impeller performance but deteriorated the diffuser performance. The pinch clearly decreased the secondary flow region present near the shroud.The pinch implemented in the shroud is more beneficial than pinch divided between the hub and the shroud. In order to obtain the beneficial effects of pinch, the pinch should be sufficient. However, excessive pinch deteriorates the compressor performance.

Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor overall performance and diffuser flow fields were studied for each of the three diffusers at four different tip clearances. For the diffuser flow fields, the total pressures were measured with probes traversed over the diffuser width both at the diffuser inlet and outlet. Along with the total pressures, the static pressures were measured adjacent to the probes. This enabled the axial flow angle and velocity distributions to be studied. The pinches tested improved the stage efficiency mainly by suppressing the secondary flow region present near the shroud at the impeller outlet. This leads to a lower strain rate, resulting in lower losses. The efficiency decrease due to the increased tip clearance was similar with and without pinch present. This indicates that in the diffuser, the main source of the tip clearance associated losses is the tip jet, and the tip clearance vortices mix out already in the impeller.