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Design and CFD Analysis of a 150kW 8-Stage ORC-ROT (Organic Rankine Cycle-Radial Outflow Turbine) and Performance Degradation due to Blade Tip Clearance of Labyrinth Seal
Abstract
In this study, blade tip leakages were calculated for a Radial Outflow Turbine (ROT) designed for an Organic Rankine Cycle (ORC) at a 150kW power output. Since the turbine blade sizes are relatively very small for low-capacity systems, the leakages through the blade tip clearances considerably affect the turbine isentropic efficiency. Therefore, labyrinth seals were applied at the blade tips and the ROT’s performance degradation due to blade tip leakages was investigated.
In order to determine the preliminary ROT sizes, an in-house developed 1-D code was utilized. The blade profiles were optimized with CFD analyses to reach high power output and isentropic efficiency. The designed ROT has 8 stages. Toluene is used as the cycle fluid at inlet conditions of 24bar of total pressure, 310°C and outlet conditions of 0.25bar of static pressure. These conditions are chosen for exhaust conditions of a common biogas engine. Thus, the ORC is supposed to operate at a heat source temperature of 460°C and a heat sink temperature of 35°C. The turbine speed of 14000 rpm is determined.
The CFD model for the entire 3-D turbine geometry is built in the FlowVision software. The real gas equation is employed for the compressible flow. The SST turbulent flow model is employed. The CFD model uses transient state and rotating frame approaches.
Four blade tip configurations were analyzed. The CFD results reveal the followings. The turbine isentropic efficiency is calculated to be 87.62% for the unshrouded geometry with no clearance, which is an ideal case. For a manufacturable and manageable blade tip clearance of 0.2 mm, the turbine isentropic efficiency is calculated to be 71.03% for the unshrouded geometry. The shrouded geometry with the same clearance increases the efficiency to 74.03%. When a labyrinth seal is applied to the shrouded geometry, the efficiency reaches to 77.03%. The best practice in terms of turbine power output and efficiency is the shrouded geometry with labyrinth seal applications.
... Figure 2 shows the typical structure of a radial outflow turbine. In the turbine, the working fluid flows in the axial direction and then expands in the radial direction [13]. Because the radius of the blade increases with the expansion of the working fluid, the turbine can be designed such that the blade height remains the same or the difference is not large. ...
... This implies that the enthalpy drop of the downstream stage with a large peripheral velocity must be greater than that of the upstream stage with a small peripheral velocity. Dogu et al. [13] designed a 150 kW-class multi-stage radial outflow turbine for an organic Rankine cycle, optimized it using CFD and analyzed the turbine performance according to the blade tip spacing. ...
Recently, the CO2 power cycle has attracted attention because of tightening environmental regulations. The turbine is a factor that greatly affects the efficiency of the cycle. The radial outflow turbine is a turbomachine with the various advantages of an axial flow turbine and a radial inflow turbine, but the design theory for the turbine is uncertain. In this study, a preliminary design algorithm for a radial outflow turbine with a multi-stage configuration is presented. To verify the preliminary design algorithm, a preliminary design for a two-stage radial outflow turbine for a CO2 power cycle was carried out, and a computational fluid dynamic analysis was performed. Consequently, values close to the target performance were obtained, but blade optimization was performed to obtain more satisfactory results. The final geometry of the radial outflow turbine was obtained through optimization considering the blade exit angle related to the deviation angle, blade maximum thickness-true chord ratio, and incidence angle. In the final geometry, the error rates of power (Ẇ), efficiency (ηts), and pressure ratio (PRts) between target performance and computational fluid dynamic results were improved to 5.0%, 4.8%, and 1.8%, respectively. The performance and flow characteristics of the initial and final geometries were analyzed.
... In study [8], a new labyrinth seal with a gear structure with stepped spiral teeth is proposed; the influence of geometric parameters on its operation was studied by the author using numerical simulation, which indicates the correctness of the use of Computational Fluid Dynamics (CFD) calculation for solving such problems. Additionally, in the works [9,10], attention is paid to labyrinth seals and their significant effect on the operation of turbine units. The works [4,11] are devoted to the study of the influence of geometric and regime parameters on the operation of labyrinth seals. ...
Labyrinth seals are widely used in centrifugal compressors, turbines, and many other pneumatic systems due to their simplicity of design, reliability, and low cost. The calculation scheme for the movement of the working medium in a labyrinth seal is constructed by analogy with the movement of the working medium through holes with a sharp edge. Annular and flat slots, holes, and such a factor as the shaft rotation with a calculated sector of 3 degrees were studied. The purpose of the study is to determine the flow coefficient when the working medium flows through slots of various shapes. To achieve this purpose, modeling of the working medium flow in the FlowVision software was performed. The mass flow and flow coefficients are determined for the studied slot shapes. The convergence of the calculation results was determined by comparing the values of the mass flow rate at the inlet and outlet of the slot. Differences in visualizations of the flow for the studied variants of slots were established. The resulting difference should be taken into account in practical calculations of the working medium mass flow through the slot using a conditional flow rate factor which is determined by the slot design.
Radial-inflow turbine is a core component in supercritical CO 2 (SCO 2 ) Brayton cycle. The leakage from the nozzle outlet towards the impeller back brings a great challenge to the efficiency and security of the power system. In this paper, the labyrinth seal (LS) and dry gas seal (DGS) are arranged on the impeller back of a SCO 2 radial-inflow turbine and the influence on the comprehensive performance is investigated. Results demonstrate that both LS and DGS configurations can significantly reduce leakage of the impeller back and DGS configuration performs better. Compared with the configuration without leakage, the power and efficiency of DGS configuration are only reduced by 0.27% and 0.35% respectively. The seal clearance and the inlet width have a greater effect on LS configuration. The thermo-mechanical seal deformation values of DGS configurations are all less than 8 μm, which verifies the feasibility. Finally, a novel combined seal configuration with both LS and DGS is proposed and excellent performance is achieved, providing a potential approach for the sealing problem of SCO 2 radial-inflow turbine.
A 340 kW ORC system was designed and tested for a Low Pressure Saturated Steam (LPSS) heat source which is an abundant industrial waste heat. The thermal design considered four working fluids with R245fa ultimately selected for its high thermal efficiency. Many thermal simulation conclusions from previous researches were evaluated but found to be improper for the present design. Then, the main system components, including a radial turbine, two shell and tube heat exchangers, and a shield pump were designed with other matching components selected based on the thermal design. The effects of the system operating parameters, including the evaporating temperature, condensing temperature, super heating and subcooling were analyzed with the results showing that the thermal optimization must consider the effects of the operating conditions on the component safeties and efficiencies. Finally, the system was constructed and tested near the design condition with the results showing that the maximum generator output power was 302 kW with a thermal efficiency of 9.7%. For these conditions, the isentropic turbine efficiency was 80.7% at 10,389 RPM. Suggestions for LPSS-ORC system designs are given to improve the system safety and component design.
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