Thermoelectric generators (TEGs) have the ability to convert waste heat into electrical energy under unfavorable conditions and are becoming increasingly popular in academia, but have not yet achieved a broad commercial success, due to the still comparably low efficiency. To increase the efficiency and economic viability of TEGs, research is performed on the materials on one hand and on the system connection on the other. In the latter case, the net output power of the cooling system plays a key role. At first glance, passive cooling seems preferable to active cooling because it does not affect the net electrical output power. However, as shown in the present review, the active cooling is to be preferred for net output power. The situation is similar in air and water-cooling. Even though air-cooling is easier to set up, the water-cooling should be preferred to achieve higher net output power. It is shown that microchannel cooling has similar hydraulic performance to conventional cooling and inserts increase the net output power of TEG. As the review reveals that active water-cooling should be the method of choice to achieve high net output power, it also shows that a careful optimization is necessary to exploit the potential.

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around ${\mathrm{M}}_{2,is}$ = 0.8.

High-fogging is widely used to rapidly increase the power outputs of stationary gas turbines. Therefore, water droplets are injected into the inflow air, and a considerable number enter the compressor. Within this paper, the primary process of droplet evaporation is investigated closely. A short discussion about the influential parameters ascribes a major significance to the slip velocity between ambient gas flow and droplets. Hence, experimental results from a transonic compressor cascade are shown to evaluate the conditions in real high-fogging applications. The measured parameter range is used for direct numerical simulations to extract evaporation rates depending on inflow conditions and relative humidity of the air flow. Finally, an applicable correlation for the Sherwood number in the form of $Sh(R{e}^{1/2}S{c}^{1/3})$ is suggested.

The purpose of the current work is to develop an appropriate control system for a solid oxide fuel cell/gas turbine hybrid system. The main focus lies on the usage of linear control theory for the parameterization of the controller structure. The studies are carried out with a control oriented simplified dynamic model of a $25MWe$ hybrid system based on a conceptual design previously presented in literature. As a specific feature additional firing of the gas turbine combustor is used as an extra actuating variable offering several advantages. Foregoing necessary investigations deal with calculating the on- and off-design behavior of the sample system and with defining a reasonable part-load operating curve taking into account several constraints like efficiency, stack temperature, surge margin, etc. Also, preliminary studies of the transient open loop behavior are performed. They reveal that the input variables should be changed with specific care to avoid critical situations during load change. As a precondition for linear control theory a low-order linear model is deduced and validated. With consideration of the transient simulation results and of the properties of the linear model a proper control strategy is suggested, which consists of a proportional output feedback taking into account the multi-input-multi-output character of the system and three distributed proportional-integral controllers that define the requested load point onto the operating curve. Lastly, the controllers are parameterized based on linear control theory and verified.