One of the main challenges currently hindering the transition to energy systems based on renewable power generation is grid stability. To compensate for the volatility of wind- and solar-based power generation, storage facilities able to adapt to seasonal and short term differences in energy production and demand are required. Liquid Organic Hydrogen Carriers (LOHCs) represent a viable method of chemically binding elemental hydrogen, offering opportunities for largescale and safe energy storage. In times of energy shortage, flexible and dispatchable power generation technologies such as gas turbines can be fueled by hydrogen stored in this manner. Hydrogen can be released from its liquid carrier via an endothermic dehydrogenation reaction using waste heat provided by the gas turbine. This gaseous hydrogen can be supplied to the gas turbine combustion chamber using a hydrogen compressor. In the present study a steady state model is developed in order to analyse the heat-integrated combination of a 7.7 MW hydrogen-fired gas turbine and a H18-DBT/H0-DBT LOHC system. For the best-performing parameter set the effective storage density of the LOHC oil comes to 1.5 kWh/L. This value is situated in-between that of compressed hydrogen at 350 bar (1.01 kWh/L) and liquid hydrogen (2.33 kWh/L). Concurrently, the corresponding energy required for hydrogen compression reduces the overall system efficiency to 22.00 % (η_GT = 30.15%). The resulting optimal electricity yield, being a product of these two values, amounts to 0.33 kWhel/L.

In turbomachinery design, the accurate prediction of the life cycle is one of the most challenging issues. Traditionally, life cycle calculations for radial turbine wheels of turbochargers focus on mechanical loads such as centrifugal and vibration forces. Due to the increase of exhaust gas temperatures in the last years, thermomechanical fatigue in the turbine wheel came more into focus. In order to account for the thermally induced stresses in the turbine wheel as a part of the standard design process, a fast method is required for predicting metal temperatures. In order to develop a suitable method, the mechanisms that cause the thermal stresses have to be understood. Thus, in a first step, a detailed analysis of the temperature fields is conducted in the present paper. Extensive numerical simulations of a thermal shock process are carried out and validated by experimental data from a test rig. Based on the results, the main heat transfer mechanisms are identified, which are crucial for the critical thermal stresses in transient operation. It is shown that these critical stresses mainly depend on local 3D flow structures. With this understanding, two fast methods to calculate the transient temperatures in a radial turbine were developed. The first method is based on a standard method for transient fluid/solid heat transfer. In this standard method, heat transfer coefficients are derived from steady-state computational fluid dynamics (CFD)/conjugate heat transfer (CHT) calculations and are linearly interpolated over the duration of the transient heating or cooling process. In the new method, this interpolation procedure was modified to achieve an exponential behavior of the heat transfer coefficients over the transient process in order to enable a sufficient accuracy. Additionally, a second method was developed. In this method, the specific heat capacity of the solid state is reduced by a “speed up factor” to shorten the duration of the transient heating or cooling process. With the shortened processes, the computing times can be reduced significantly. After the calculations, the resulting times are transferred into realistic heating or cooling times by multiplying them with the speed up factor. The results of both methods are evaluated against experimental data and against the results of a numerical method known from literature. The methods show a good agreement with those data.

Synchronous vibrations, which are caused by periodic excitations, can have a severe impact on the service life of impellers. Blade Tip Timing (BTT) is a promising technique for monitoring synchronous vibrations due to its non-intrusive nature and ability to monitor all blades at once. BTT generally employs a Once-per-Revolution (OPR) sensor that is mounted on the shaft for blade identification and deflection calculation. Nevertheless, OPR sensors can be unreliable, as they may be affected by shaft vibrations, and their implementation can be restricted by space constraints. Moreover, the low number of BTT sensors typically leads to under-sampled deflection signals, which consequently hinders the estimation of the vibration frequencies due to aliasing problems. For this reason, BTT is commonly accompanied by strain gauge (SG) measurements on some blades. In this paper, improved BTT techniques are presented, which enable the determination of vibration properties of synchronous vibrations without the need for an OPR sensor and ensure a reliable frequency assessment. Specifically, the blades are identified by unique characteristics resulting from manufacturing tolerances, while the blade deflections are calculated through a novel method, which relies on the impeller’s circumferential position. The proposed method enables accurate OPR-free calculation of blade deflections, by accounting for speed variations within a revolution and considering the actual blade positions on the impeller. By completely eliminating the need for an OPR sensor, the accuracy of BTT is enhanced, as the blade deflections are no longer affected by shaft vibrations, while speed variations within a revolution can be accounted for. Moreover, the implementation possibilities of BTT are improved, allowing its application in systems, where an OPR sensor cannot be instrumented due to space constraints. Subsequently, the vibration frequencies are accurately estimated, by employing an improved Multi-Sampling method based on Non-Uniform Fast Fourier Transform. This approach enables the blind analysis of BTT measurements and can identify multiple vibration frequencies. The proposed method expands the capabilities of BTT through a reliable assessment of vibration frequencies from under-sampled BTT signals. Therefore, it is no longer necessary to accompany BTT measurements with SG measurements for frequency identification. Finally, the vibration properties are determined using regression models. The proposed BTT techniques are validated through comparison with SG measurements as well as a commercial BTT system, using experimental data from a test bench of a turbocharger used for marine applications. The vibrations were recorded under real operating conditions, thus demonstrating the industrial applicability of the proposed BTT evaluation procedure.

In turbocharger design, the accurate determination of thermally induced stresses is of particular importance for life cycle predictions. An accurate, transient, thermal finite element analysis (FEA) of turbocharger components requires transient conjugate heat transfer (CHT) analysis. However, due to the vastly different timescales of the heat transfer mechanisms in fluid and in solid states, unsteady CHT simulations are burdened by high computational costs. Hence, for design iterations, uncoupled CFD and FEA approaches are needed. The quality of the uncoupled thermal analysis depends on the local heat transfer coefficients (HTC) and reference fluid temperatures. In this paper, multiple CFD-FEA methods known from literature are implemented in a numerical model of a turbocharger. In order to describe the heat transfer and thermal boundary layer of the fluid, different definitions of heat transfer coefficients and reference fluid temperatures are investigated with regard to calculation time and accuracy. For the transient simulation of a long heating process, the combination of the CFD-FEA methods with the interpolation FEA approach is examined. Additionally, a structural-mechanical analysis is conducted. The results of the developed methods are evaluated against experimental data and the results of the extensive unsteady CHT numerical method.

Highly efficient and concurrent flexible operation are heavy demands on today's centrifugal compressor units. Diffuser end wall contouring is a measure to delay the incipience of instability and therefore to extend the compressor's operating range while maintaining efficient performance. In the presented paper, a hub-side wall contouring, applied in the vaneless space upstream the diffuser vane's leading edge and within the diffuser passage of a state-of-the-art centrifugal compressor stage with an open impeller, is examined. Computational fluid dynamics computations are performed for both a baseline diffuser design with parallel channel walls and the hub-side wall contoured diffuser design. Comparisons of characteristic and diffuser stability decisive flow variables are made in perpendicular sections along an extrapolated camber line of the diffuser vane for full span, near shroud and near hub wall. In operating points near the stability limit at two different stage Mach numbers, the stabilizing effect of the hub-side wall contouring on the diffuser flow is clearly shown. In a scale-model test rig, experimental data including pneumatic five-hole probe data for a full diffuser blade-to-blade passage, static wall pressures at various planes as well as total temperatures were measured. The experimental data are utilized for the validation of the presented numerical calculations. The flow stabilizing effect of the hub-wall contouring is clearly visible in the measurements, which showed 8% range extension at highest stage Mach number Mu₂ = 1.16 and a range extension of 2% at design stage Mach number Mu₂ = 1.

Many energy supply systems around the world are currently undergoing a phase of transition characterized by a continuing increase in installed renewable power generation capacities. The inherent volatility and limited predictability of renewable power generation pose various challenges for an efficient system integration of these capacities. One approach to manage these challenges is the deployment of small-scale dispatchable power generation and storage units on a local level. In this context, gas turbine cogeneration units, which are primarily tasked with the provision of power and heat for industrial consumers, can play a significant role, if they are equipped with a sufficient energy storage capacity allowing for a more flexible operation. The present study investigates a system configuration, which incorporates a heat-driven industrial gas turbine interacting with a wind farm providing volatile renewable power generation. The required energy storage capacity is represented by an electrolyzer and a pressure vessel for intermediate hydrogen storage. The generated hydrogen can be reconverted to electricity and process heat by the gas turbine. The corresponding operational strategy for the overall system aims at an optimal integration of the volatile wind farm power generation on a local level. The study quantifies the impact of selected system design parameters on the quality of local wind power system integration, that can be achieved with a specific set of parameters. In addition, the impact of these parameters on the reduction of CO2 emissions due to the use of hydrogen as gas turbine fuel is quantified. In order to conduct these investigations, detailed steady-state models of all required system components were developed. These models enable accurate simulations of the operation of each component in the complete load range. The calculation of the optimal operational strategy is based on an application of the dynamic programming algorithm. Based on this model setup, the operation of the overall system configuration is simulated for each investigated set of design parameters for a one-year period. The simulation results show that the investigated system configuration has the ability to significantly increase the level of local wind power integration. The parameter variation reveals distinct correlations between the main design parameters of the storage system and the achievable level of local wind power integration. Regarding the installed electrolyzer power consumption capacity, smaller additional benefits of capacity increases can be identified at higher levels of power consumption capacity. Regarding the geometrical volume of the hydrogen storage, it can be determined that the storage volume loses its limiting character on the operation of the electrolyzer at a characteristic level. The additional investigation of the CO2 emission reduction reveals a direct correlation between the level of local wind power integration and the achievable level of CO2 emission reduction.