Results are presented of a theoretical and experimental study dealing with critical and choking Mach numbers of organic vapor flows through turbine cascades. A correlation was derived for predicting choking Mach numbers for organic vapor flows using an asymptotic series expansion valid for isentropic exponents close to unity. The theoretical prediction was tested employing a linear turbine cascade and a circular cylinder in a closed-loop organic vapor wind tunnel. The cascade was based on a classical transonic turbine airfoil for which perfect gas literature data were available. The cascade was manufactured by Selective Laser Melting (SLM), and a comparable low surface roughness level was established by subsequent surface finishing. Because the return of the closed-loop wind tunnel was equipped with an independent mass flow sensor and the test facility enabled stable long-term operation behavior, it was possible to obtain the choking Mach number with high accuracy. It was observed that non-perfect gas dynamics affect the critical Mach number locally, but the observed choking behavior of the turbine cascade was in good agreement with the asymptotic result for the considered dilute gas flow regime.

There is relatively little literature concerning the effect of Reynolds number on multi-hole aerodynamic probe performance. In particular, there is almost no discussion in the literature of the underlying mechanisms of Reynolds number (Re) sensitivity for such probes. In order to close this gap, detailed investigations of the effect of Re on a five-hole probe have been performed using both PIV techniques and oil flow visualizations. Wind- and water-tunnels were used to cover a wide range of Re. The open-access Oxford Probe was used for these studies because of the readily available data-sets and processing routines, and to allow future comparisons by other authors. Complex flow dynamics including flow separation and re-attachment were identified, which cause Re-sensitivity of the calibration map at low Re even for low yaw or pitch angles. By comparing calibration maps across a wide range of Re, we demonstrate that the Oxford Probe can be employed without much loss of accuracy at lower Re levels than initially (conservatively) suggested, and quantify the errors in the extreme low-Re regime. Overall we demonstrate the robustness of the Oxford Probe concept across a wide range of Re conditions, we more clearly defined the low-Re limit for the probe design and quantify errors below this limit, and we illustrate the fundamental mechanisms for Re-sensitivity of multi-hole probes.

Flexible plant operation and rapid load changes become major issues for steam turbine operation. In thermal power plants, the steam turbine performance is closely related to the condenser, and an accurate prediction of coolant temperature as function of changing weather conditions is necessary in order to optimize power plant fleet operation. In this contribution, a one-dimensional model for simulating the performance of large natural draft wet cooling towers is presented. The evaporation zone model rests on the evaporative cooling theory developed by Merkel and Poppe. The off-design behavior of the cooling tower, that is relevant to part load performance, is modeled by an empirical power-law approach. A user-friendly method is presented in order to identify required model parameters by means of already available power plant data. The simulation tool can be employed easily for existing power plants for which the original cooling tower design and construction data lost their validity. The outcome of the present calculation method is successfully compared with field data from representative cooling towers at Middle-European sites.

For fans without cooling it is possible to determine the hydraulic efficiency measuring the pressure and the temperature rise through the fan. The shaft work can be determined according applying the first law of thermodynamics for an open system. Without any losses the change of state would be isotropic and the work done equal to the specific heat at constant pressure of the fluid times the isentropic temperature rise in the impeller. Due to the losses, however, the real temperature at the exit of the impeller will be higher than the isentropic temperature since the real process is polytropic. The isentropic temperature at the exit of the impeller can be computed by the isentropic relations with the inlet temperature and the pressure rise. The hydraulic efficiency can be computed as the ratio of the isentropic temperature rise divided by the real temperature rise. In order to verify this thermodynamic approach for the determination of the hydraulic efficiency CFD simulations of a radial fan were performed. In the CFD simulation the hydraulic power, the shaft power, the pressure rise and the temperature rise can be read out and computed directly. In such a way the hydraulic efficiency computed by the ratio of the hydraulic power by the shaft power can be compared by the thermodynamically computed efficiency. In this work this comparison has been performed and the results and the precision of the thermodynamically predicted efficiency are presented and discussed in detail.

This paper presents a method for a cost- and time-effective calibration procedure for five-hole probes for the transonic flow regime based on additive manufacturing and a numerical calibration routine. The computational setup and calibration routine are described in detail. The calibration procedure is tested on a custom-built L-shaped conical probe of 30° half-angle with a flat tip and an outer diameter of 2.4mm. The probe tip is manufactured in stainless steel using DMLS. Numerical calibration is carried out over a Mach number range of 0.2 to 1.4 and pitch and yaw angles of ±45°. The numerical calibration charts are validated with wind tunnel tests and the expected accuracy of the numerical calibration method is quantified. Exemplary results of area traverses up- and downstream of a linear transonic turbine cascade with tip clearance are presented and discussed briefly.

Low speed and high speed flow phenomena in pillow plate channels are considered. High speed flows were investigated by means of analytical methods and fully three-dimensional computational fluid dynamics (CFD) simulations. The theoretical analysis indicated that a Fanno-type flow model described high speed flow behavior in pillow plate channels reasonably well. Since only wavy walls with smooth profiles were involved, linearized gas dynamics was applied in order to derive similarity laws for the high speed flows. The detailed CFD analysis was used to support the assumption of a Fanno-type flow. The effects of the wavy wall structures on pressure drop and Mach number distribution within the flow path were investigated in detail. The present analysis demonstrates that pillow plate heat exchangers represent promising candidates for high speed turbo machinery applications.

The stagnation flow and heat transfer from the blunt surface of a finite circular disk subjected to a uniform stream of an incompressible fluid is revisited in this paper. A laminar boundary layer analyses was carried out employing the method developed by Frössling. The involved auxiliary functions were calculated for several Prandtl numbers. It was found that the exact knowledge of the velocity at the outer edge of the boundary layer was essential to achieve an accurate velocity solution. In addition to the analytical work, computational fluid dynamics (CFD) simulations and an experimental study were carried out. The analytical treatment enabled a clear discussion of the effect of the Prandtl number on convective heat transfer from a blunt disk. A primary and a secondary effect were distinguished based on the rigorous analytical treatment. The boundary layer theory offered a rather efficient calculation method, and its results were in an excellent agreement with available literature data.

In this contribution, an assessment of compressible Reynolds Averaged Navier Stokes equations (RANS) and Large Eddy Simulation (LES) is presented using transonic organic vapor flow past a NACA4412 airfoil as a case study. The NACA4412 represents a canonical geometry, which, in case of air, has been well investigated numerically and experimentally. The results of the real gas simulations are compared with those of air simulations. For the real gas, the organic vapor Novec 649 ® is chosen as a representative fluid. The thermodynamic behavior of Novec 649 ® is modeled with the Peng-Robinson equation of state. Different inlet Mach numbers are applied, namely, a sub-critical, the critical, and a super-critical Mach number. It turns out, that the critical Mach number of the NACA4412 airfoil increases when Novec 649 ® is used as working fluid. Furthermore, it is shown that real gas flow simulations cause additional difficulties for the computational fluid dynamics (CFD) analysis. Although the speed of sound of Novec 649 ® is lower than the speed of sound of air, a finer grid resolution is required for the real gas simulations due to its high density. Based on an extensive simulation study, an assessment of different numerical modelling strategies and methods is given.

A novel thermal measurement approach is presented for the determination of fan efficiencies. This approach is not resting on a simple direct measurement of the occurring temperature difference but on a system identification based on the transient time signal of the involved temperature measurement device. The values of the system parameters can be identified after recording the transient response of the system to a step input, i. e. the fan is suddenly turned on. By means of a representative case study considering a centrifugal fan, the system identification and its relation to fan efficiency are presented. The outcome of the new thermal approach is compared with results of standardized fan performance tests and with results obtained by the direct temperature measurement method without system identification. It is shown that the direct temperature measurement provides typically poor data whereas the novel thermal approach leads to results which are well comparable with these obtained by standard performance test at the design point.

Efficient processes with organic fluids are becoming increasingly important. The high tech fluid Novec™ is such an organic fluid and is used, for example, as a coolant for highperformance electronics, low-temperature heat transfer applications, cooling of automotive batteries, just to mention a few. Thus, efficient designed fans for the transport of organic fluids are becoming more and more important in the process engineering. CFD-simulations are nowadays integral part of the design and optimization process of fans. For air at the most usual application conditions, i.e. no extreme temperatures or pressures, the ideal gas model is in good agreement with the real gas approach. In the present study, this real gas approach for organic fluids have been investigated with CFD methods and, the deviation from the ideal gas model has been analyzed. For this purpose, a simulation model of a centrifugal fan with volute has been designed as a test case. First, the ideal gas model approach has been compared with the real gas approach model of Peng-Robinson for air using the commercial solver ANSYS CFX. Thereafter, the same comparison has been performed using the organic fluid Novec™. After a detailed grid study, the entire fan characteristics, i.e. the design point and the off-design points, have been simulated and evaluated for each fluid (air and Novec™) and gas model (ideal gas and Peng-Robinson real gas). The steady state simulations of the centrifugal fan have been performed using the Frozen Rotor model. The simulation results have been compared, discussed and presented in detail.

Organic vapor flows are met in a wide range of technical applications (e.g., energy conversion, chemical processes, and refrigeration). Typically, organic fluids contain complex molecules, and their thermodynamic behavior deviates significantly from the ideal or perfect gas laws. The applicability of scaling laws to organic vapor flows is very limited, and there is a need for detailed experimental investigations under relevant process conditions. Furthermore, such investigations can provide a validation basis for the simulations performed with Computational Fluid Dynamics (CFD) tools. On the other hand, there exists a serious lack in experimental organic vapor flow test facilities. In this contribution, a novel Closed Loop Organic vapor Wind Tunnel (CLOWT) is presented. The concept of CLOWT is based on a closed-loop continuously running wind tunnel cycle. Its main components are a blower, a diffuser, a settling chamber, a contraction zone, a test section module, and a return, including a throttle valve and a mass flow meter. The test facility CLOWT applies the modular design approach which enables analysis of various flow configurations and components like blowers, small axial test turbines, nozzle flows or transonic flows past test objects. Thanks to an auxiliary heating system, organic vapor flows can be investigated at elevated pressure and temperature levels. The operation of CLOWT is based on closed gas turbine cycle control methods (e.g., inventory control). In addition to the general test facility concept, the paper gives a detailed discussion of the CLOWT special design features.

In this contribution, an optimization study based on computational fluid dynamics (CFD) in combination with Stratford’s analytical separation criterion was developed for the design of piece-wise conical contraction zones. The occurrence of flow separation can be formally described by a newly introduced dimensionless separation number. In the optimization process, the risk of flow separation is reduced by minimizing this separation number. It was found that the optimized piece-wise conical nozzle shape did not correspond to a simple geometric approximation of the ideal polynomial shape. In fact, it was beneficial to reduce the deflection in the outlet region for a piece-wise conical nozzle stronger than for a conventional one. In order to validate the new design method, large-scale tests for different nozzle designs were conducted. The measured velocity profiles and wall pressure distributions agreed well with the CFD predictions. The new method was applied for designing the contraction zone of a new closed-loop organic vapor wind tunnel (CLOWT) working at elevated pressure levels.

This paper presents results of a detailed investigation of turbine tip-leakage flows at high Mach numbers. The experimental work was carried out using a small blow-down wind tunnel. An idealized blade test section was used to study blade tip-clearance effects in transonic conditions. Unshrouded blade tips are considered and different tip gap heights are investigated. A high blade exit Mach number of M e = 2 was selected deliberately. While conventional transonic turbine stages generally operate at lower supersonic exit Mach numbers, the conditions are representative for ORC turbines. Both experimental and numerical results are presented in this contribution. The results indicate, that tip leakage flow under transonic conditions leads to a complex three-dimensional flow field. A strong interaction between tip gap vortex and trailing edge shocks was observed, that also had a profound effect on the base region. While no final statement on losses could be made in the present configuration, the results indicate a weakened shock system.

A semi-empirical steam excitation force model is presented for freestanding unshrouded blades. The model is resting on a superposition of the classical Thomas-Alford cross forces and additional fluid derived forces. The additional fluid derived forces are caused by static pressure modifications in the blade tip region and large-scale redistributions of the inflow velocity caused by the varying tip gap width due to eccentricity. The empirical model parameters are obtained by means of computational fluid dynamics (CFD) and experimental turbine cascade results. The Thomas-Alford cross force contribution is formulated by means of an analytical relation for the effective discharge coefficient. It is found, that the Thomas-Alford coefficient for the excitation cross-force increases if the blade tip gap decreases. That effect is caused by taking viscous flow effects into account, and it is in good agreement with available literature data. The direct force contribution results mainly from local pressure variation in the blade tip region, and it depends also on the mean tip gap width. A further effect is caused by an upstream redistribution of the velocity due to the varying tip gap width. This effect was discovered by Song and Martinez-Sanchez first, and it is confirmed experimentally by cascade measurements in the present paper.

Tip clearance losses represent a major source of efficiency losses in turbomachinery. A novel method based on passive injection for reducing tip leakage losses in axial turbine cascades has been proposed in 2007 by Willinger and co-workers, and first experimental demonstrations of the potential of this approach were reported recently. However, these experimental tests were limited to linear cascade experiments w6ith stationary endwalls, and they were not able to explain the underlying physical mechanism of the observed loss reduction. In the present contribution, results of detailed experimental and numerical analyses of the tip gap flow, the interaction of the passive tip injection jet with the gap and the main flow, and the effect of the moving endwall are presented. Both numerical and experimental results indicated that the beneficial effect of the passive injection mainly results from a weaker tip gap vortex and not so much from a simple blockage of the tip gap flow itself.

An experimental and numerical study of the flow in the tip gap of a linear turbine blade cascade has been performed. Besides the plain tip gap flow, a passive injection with a single injection hole at the blade tip near the pressure side has been considered. Detailed measurements of the static pressure distribution on the cascade end wall and on the blade surface are conducted. The pressure measurements are supported by surface oil flow visualizations of the blade tip and suction side surface to get qualitative insights into the underlying flow structures. The experimental data is compared to highly resolved unsteady RANS simulations. Previous investigations on the same cascade have shown, that flow separation phenomena on the suction side surface play an important role on this blade profile, hence the transition model by Menter was used for all simulations. It was found, that the influence of a single passive injection hole is limited to a narrow region downstream. The additional injection jet leads to a changed pressure distribution, indicating a lower velocity level and a weakened tip gap vortex.