Numerical investigation of velocity and temperature distributions in thermoacoustic stacks
2021, Schittl, Florian
Energy conversion based on the interaction of acoustic fluctuations in thermal and viscous boundary layers offers a promising possibility for reducing fossil energy resources. Machines based on this so-called thermoacoustic effect can be used as heat engines as well as heat pumps or refrigerators. However, the problem with the implementation of such systems is the insufficient understanding of the various phenomena (e. g. turbulence) of thermally excited flows. Numerical methods, such as CFD, are established for the optimization and further development of thermoacoustic systems due to increasing computational capacities. The foundation for optimizations is a reliable and realistic prediction of the flow and heat transfer process in thermoacoustic systems. Therefore, in this study, different approaches based on the RANS equations are investigated with the use of the CFD code ANSYS Fluent®. In addition to two standard eddy-viscosity models, two model extensions are implemented in the CFD code. Extensive measurement data based on time-resolved laser-assisted flow measurements are used for the validation of the investigated turbulence models. As the studies show, the standard eddy-viscosity models provide a good prediction of the velocity and temperature profiles. However, it can be pointed out that both the elliptic relaxation model (V2F) and the local formulation of the Transition SST model bring an improvement, especially in the interpretation of non-linear effects.
Untersuchung eines Elektroabscheiderkonzepts zur Reduktion von Staubemissionen
2020-11-26, Schittl, Florian, Jauschnik, Gabriela, Pöttler, Martin, Krail, Jürgen
Particulate emissions are formed during the combustion of biogenic fuels depending on the type of furnace, the operating conditions in terms of the combustion quality and the different fuel properties. The release of especially small particles often leads to health problems such as the development or worsening of lung diseases. Downstream electrostatic precipitators (ESP) represent a state of the art separation technology in medium and large biomass plants. However, these precipitators are often difficult to implement in smaller furnaces due to economic aspects and space constraints. This study deals with the integration and experimental investigation of an ESP system into the boiler body of a small scaled biomass furnace (< 100 kW). In Addition to the full load behaviour of the firing system, further test arrangements with different part load conditions of the boiler are being considered in order to analyse the particle precipitation under realistic plant operation with regard to flue gas properties and flow conditions. Furthermore, different fuels are considered. Both, discontinuous as well as time-resolved aerosol measuring methods are used to determine particulate matter emissions. The results of the discontinuous dust measurements show that with the integrated ESP, at least 50 % of the particles in the fine dust range are separated, both at full and partial load operation of the boiler, irrespective of the fuel used. Furthermore, it is shown that partial load conditions favour the separation efficiency due to low velocities and low temperatures of the gas flow over the discharge electrode, which is situated in the reversing chamber. Accordingly, the separation efficiency in part load is between 65 and 85 %, depending on fuel used. In order to enable a more precise observation of the separation behaviour with regard to particle size, additional continuous ELPI (electrical low pressure impactor) measurements are carried out for a selected fuel (wood chips). These measurements show that for small particle collectives (dP < 1 μm) separation efficiencies of over 55 % (full load) and over 80 % (part load) are achieved.
Comparative thermodynamic analysis of an improved ORC process with integrated injection of process fluid
2022-12, Krail, Jürgen, Beckmann, Georg, Schittl, Florian, Piringer, Gerhard
In contrast to water-steam Rankine cycles, the ORC process uses organic working fluids. For working fluids of the dry class, a recuperator heat exchanger is frequently installed to increase the cycle efficiency. This paper analyses an improved ORC process with these features: A liquid working fluid stream is injected into the vapour flow between the high-pressure and the medium-pressure stage of the turbine. Furthermore, the recuperator is replaced by a spray condenser. The main objective is to increase efficiency with moderate changes in the process layout. A thermodynamic comparison of the improved process with a state-of-the-art ORC process is carried out by simulations and optimisations. A significant efficiency gain for the improved ORC process is obtained by a combination of the aforementioned features, mainly because of an increase of the mass flow in the economiser of the vapour generator (better heat utilization) and a corresponding mass flow in the medium stage of the turbine (additional power production). As a use case, waste heat utilization from clinker cooler at a temperature level of 275 °C was simulated. The improved process would lead to a significant increase in the overall net efficiency by up to 14%, compared to a state-of-the-art ORC process.
Improved ORC process for power production by using low temperature heat
2021-10-13, Krail, Jürgen, Beckmann, Georg, Schittl, Florian
Organic Rankine Cycles (ORC) are a modification of the classical water-steam process and are particularly suitable for electricity generation from low and medium temperature heat sources, e.g., industrial waste heat or geothermal energy. In contrast to the water-steam process, the ORC process uses organic fluids as working fluids. When using working fluids of the dry class (e.g. n-pentane), a recuperator is frequently installed in state-of-the-art ORC processes to increase the cycle efficiency. This paper analyses an improved ORC process design: A liquid working fluid stream is mixed with the vapour flow between the high-pressure stage and the medium-pressure stage of the turbine. Furthermore, the recuperator is replaced by a spray condenser. These two improvements were analysed by thermodynamic process simulations. As a use case, electricity production from clinker cooler waste heat at a temperature level of 275°C was simulated. The improved process as described would lead to an increase in the overall net efficiency up to 14%, compared to a state-of-the-art ORC process.
Emission limited model predictive control of a small-scale biomass furnace
2021-03-01, Böhler, Lukas, Fallmann, Markus, Görtler, Gregor, Krail, Jürgen, Schittl, Florian, Kozek, Martin
This paper presents the application of an emission limiting model-based predictive controller for a small-scale biomass grate furnace. The furnace has a nominal power of 100 kW with wood pellets as fuel, but it can be operated with different fuels as well. The model predictive approach extends the existing static feedforward controller of the investigated furnace with a dynamic feedback controller that is able to improve the combustion performance. Simultaneously, the formation of carbon monoxide emissions is minimized within the prediction horizon based on an available emission estimation model for pellets. The results obtained from closed-loop measurements show that the control concept is able to reduce carbon monoxide emissions in partial load operation up to four times while the control error of the supply water temperature for heating is nearly halved during transient operation. This is achieved by incorporating the emission estimation model into the constrained optimization of the predictive controller. Additional results obtained from closed-loop experiments for different fuel types with varying water contents demonstrate the advantages of the proposed model-based approach and its robustness with respect to typical uncertainties of the combustion process.