Konstantina Koutita | University College London (original) (raw)
Papers by Konstantina Koutita
In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial... more In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the pilot 1 MW th CFB boiler of the Technical University of Darmstadt. Experiments have been performed with the same unit, the data of which are utilized for the model validation. The examined conditions correspond to the steady-state operation of the boiler at 100, 80, and 60% heat loads, as well as for transient conditions for the load changes from 80 to 60% and back to 80%. Fair agreement is observed between the simulations and the experiments regarding the temperature profiles in the riser, the heat extracted by the cooling lances, as well as the concentration of the main species in the flue gases; a small deviation is observed for the pressure drop, which, however, is close to the results of a CFD simulation run. The validated model is extended with the use of a thermal energy storage (TES) system, which utilizes a bubbling fluidized bed to store/return the particles during ramp up/down operation. Simulations are performed both with and without the use of TES for the load path 100-80-60-80-100%, and the results showed that the TES concept proved to be superior in terms of changing load flexibility, since the ramp up and down times proved to be much faster, and lower temperature drops between the loads are observed in this case.
The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled com... more The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled combustor consisting of an array of five planar rectangular channels was found to be influenced by the equivalence ratio and flow-rate of the continuously and steadily injected premixed fuel charge. Three distinct flame stages were observed-planar, chaotic and trident, which were strongly correlated to the flow dynamics. The effect of the flow on the flame behaviour was investigated by characterizing the cold flow in a scaled-up model channel with the same aspect ratio as the combustion micro-channel. Direct flow visualization using flow tracers and quantitative velocity-field data from PIV measurements showed both an increase in the bottom recirculation zone reattachment length (along the floor of the channel) and a decrease in the lateral recirculation zone reattach-ment length (along the sides of the channel) with increasing flow Reynolds number. Comparison of the flow and flame transition locations downstream of the injection point suggested that the location of trident flame onset coincides with the flow bottom recirculation zone reattachment length. The planar-chaotic flame transition location was observed to be influenced by the homogeneity of the mixture downstream of the injection plane.
There are several options for biomass valorization, either for value-added products manufacturing... more There are several options for biomass valorization, either for value-added products manufacturing or for heat and power generation. Among the various concepts, gasification-based pathways attract a lot of interest, mainly because all the biomass components that consist of C and H can be converted into syngas, which in turn can be valorized for various applications. Many numerical studies on steady state process modeling have been performed to simulate the complex processes and to propose novel concepts for the effective design of a gasification-based route. However, dynamic process modeling is an important numerical tool providing a deeper understanding of the behavior of each process step under unsteady conditions and the potential to develop an appropriate control strategy. A review of the dynamic modeling approaches and the control strategies for the main process sections of biomass thermochemical conversion, either via gasification or through direct combustion, was undertaken. Methodologies towards the proper control of the main units, the different tools and platforms that are used for the dynamic modeling and the control system design are presented and analyzed. Finally, the challenges that need to be overcome for the plantwide control of the thermochemically based systems are discussed .
This article demonstrates a new simplified mathematical model developed for an external loop airl... more This article demonstrates a new simplified mathematical model developed for an external loop airlift bioreactor, derived from recognised chemical engineering formulae, with the minimum possible reliance on empirical correlations with adjustable parameters. Bubble slip velocity, liquid circulation velocity and gas holdup are simply estimated based on bubble diameter, gas flow rate, riser diameter and riser height. The model reveals the contribution of bubble diameter to gas holdup and liquid circulation velocity, filling a gap in the literature. Bubble size is known as an important variable for optimising gas absorption and energy input. Validation of the model is conducted using our own and other experimental data. The current model was found to provide a better estimate of gas holdup than the literature model compared with, but liquid velocity was overestimated. The impact of using various drag coefficient correlations was also revealed.
In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial... more In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the pilot 1 MW th CFB boiler of the Technical University of Darmstadt. Experiments have been performed with the same unit, the data of which are utilized for the model validation. The examined conditions correspond to the steady-state operation of the boiler at 100, 80, and 60% heat loads, as well as for transient conditions for the load changes from 80 to 60% and back to 80%. Fair agreement is observed between the simulations and the experiments regarding the temperature profiles in the riser, the heat extracted by the cooling lances, as well as the concentration of the main species in the flue gases; a small deviation is observed for the pressure drop, which, however, is close to the results of a CFD simulation run. The validated model is extended with the use of a thermal energy storage (TES) system, which utilizes a bubbling fluidized bed to store/return the particles during ramp up/down operation. Simulations are performed both with and without the use of TES for the load path 100-80-60-80-100%, and the results showed that the TES concept proved to be superior in terms of changing load flexibility, since the ramp up and down times proved to be much faster, and lower temperature drops between the loads are observed in this case.
The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled com... more The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled combustor consisting of an array of five planar rectangular channels was found to be influenced by the equivalence ratio and flow-rate of the continuously and steadily injected premixed fuel charge. Three distinct flame stages were observed-planar, chaotic and trident, which were strongly correlated to the flow dynamics. The effect of the flow on the flame behaviour was investigated by characterizing the cold flow in a scaled-up model channel with the same aspect ratio as the combustion micro-channel. Direct flow visualization using flow tracers and quantitative velocity-field data from PIV measurements showed both an increase in the bottom recirculation zone reattachment length (along the floor of the channel) and a decrease in the lateral recirculation zone reattach-ment length (along the sides of the channel) with increasing flow Reynolds number. Comparison of the flow and flame transition locations downstream of the injection point suggested that the location of trident flame onset coincides with the flow bottom recirculation zone reattachment length. The planar-chaotic flame transition location was observed to be influenced by the homogeneity of the mixture downstream of the injection plane.
There are several options for biomass valorization, either for value-added products manufacturing... more There are several options for biomass valorization, either for value-added products manufacturing or for heat and power generation. Among the various concepts, gasification-based pathways attract a lot of interest, mainly because all the biomass components that consist of C and H can be converted into syngas, which in turn can be valorized for various applications. Many numerical studies on steady state process modeling have been performed to simulate the complex processes and to propose novel concepts for the effective design of a gasification-based route. However, dynamic process modeling is an important numerical tool providing a deeper understanding of the behavior of each process step under unsteady conditions and the potential to develop an appropriate control strategy. A review of the dynamic modeling approaches and the control strategies for the main process sections of biomass thermochemical conversion, either via gasification or through direct combustion, was undertaken. Methodologies towards the proper control of the main units, the different tools and platforms that are used for the dynamic modeling and the control system design are presented and analyzed. Finally, the challenges that need to be overcome for the plantwide control of the thermochemically based systems are discussed .
This article demonstrates a new simplified mathematical model developed for an external loop airl... more This article demonstrates a new simplified mathematical model developed for an external loop airlift bioreactor, derived from recognised chemical engineering formulae, with the minimum possible reliance on empirical correlations with adjustable parameters. Bubble slip velocity, liquid circulation velocity and gas holdup are simply estimated based on bubble diameter, gas flow rate, riser diameter and riser height. The model reveals the contribution of bubble diameter to gas holdup and liquid circulation velocity, filling a gap in the literature. Bubble size is known as an important variable for optimising gas absorption and energy input. Validation of the model is conducted using our own and other experimental data. The current model was found to provide a better estimate of gas holdup than the literature model compared with, but liquid velocity was overestimated. The impact of using various drag coefficient correlations was also revealed.