Aspects of ethanol laminar, turbulent and dynamics combustion
Virginia Fratalocchi is a PhD-Student in the Research Group Thermal Engineering. Her supervisors are Theo van der Meer and Jim Kok from the Faculty Engineering Technology.
The combustion process is currently the main heat, power and propulsion supplying system in a wide range of sectors, such as: thermal and power generation, and aero and ground transportation.
In order to meet the growing energy demand from one side, and to fulfill the more restrictive emissions regulations on the other side, a more efficient and cleaner combustion system design is required. When designing a combustor, the engineering concerns depend on the specific application, however, common key aspects are: reduction of the toxic emissions, fuel flexibility and flame stability. In this scenario, the industry has been going and is going towards the development of new complex technologies and the usage of new fuels. Since experimental measurements are often too expensive or time consuming, the numerical modeling represents an attractive tool to investigate reactive flows in industrial applications. A key aspect in capturing the physics of the combustion processes in practical designs, is the accurate representation of the interaction between the chemistry and the turbulence. This represents a great challenge, especially because of the large range of time and length scales of the phenomena involved. In the last few decades, as a response to the need of low-cost computational tools, combustion models based on tabulated chemistry were developed.
The main concept on which this approach is based is that a turbulent flame can be represented by a 1D flamelet.
The interaction between turbulence and chemical reactions is then taken into account for example by means of a presumed shape Probability Density Function (PDF).In order to retain the accuracy of the detailed chemistry and limit the computational costs of the simulations, a turbu-chemical database is built prior to the simulation, and the fluid properties are retrieved during run-time.
The storage of the data is a parametrized function of specific controlling variables, such that only the transport equations of these variables are solved, rather than transporting the total number of species. The flamelet assumption is generally valid in the combustion regimes which establish in gas turbines engines. The Computational Fluid Dynamics has received a growing interest not only to explore new combustor designs, but also to explore the performances of new bio-fuels.
In this context, the question if the numerical models adopted to study the burning characteristic of traditional fossil fuels can also well represent new fuels, has to be addressed.The present PhD thesis focuses on exploring a variety of combustion aspects of one of the most attractive liquid bio-fuels: ethanol.
The two areas studied are the combustion characteristics of the ethanol as prevaporised gas fuel in turbulent flames, and the forced ethanol spray flame response to fluctuations of the gas velocity.
The first part of the dissertation is concerned with the combustion of prevaporised ethanol and the combustion is treated with a tabulated chemistry approach based on an optimized choice of the reaction progress variable.
The approach is developed for premixed combustion, in which case the single controlling scalar is the reaction progress variable. A Computational Singular Perturbation algorithm is used, along with a sensitivity analysis of the chemical time scales performed in a Perfectly Stirred Reactor, to determine the optimal combination of the species mass fractions defining the reaction progress variable.
This approach is first validated against laminar flames and it is found that the choice of the reaction progress variable has a relevant effect in the solution of the reacting field.
Following, the same formulation is applied in a confined turbulent jet flame, where the turbulence-chemistry interaction is predicted by means of a presumed-shape PDF. Blends of ethanol/water/iso-octane were used to test the capability of the method and it is found that the adopted framework simulation can be successfully extended to complex fuel mixtures. In such simulation framework, the database is implemented in the commercial software Ansys CFX, where Reynolds averaged Navier-Stokes equations are solved in steady state regime. In the second part of the thesis, numerical simulations of a piloted turbulent ethanol spray flame are presented. Both steady and transient simulations are performed in the Eulerian-Lagrangian framework. Due to an intrinsic limitation of the commercial code, the database could not be coupled with the Lagrangian solver. For this reason the available models in CFX were used to perform the spray flame simulations. The reference test-case is the Sidney spray flame, on which a large set of measurement data is available. Simulations were validated against two flames in particular, the EtF6 and the EtF7. A study on the effect of the boundary conditions assigned to the gas phase is performed. The spray is modeled under the main assumption of the dilute spray regime such that all phenomena of atomization, break-up and collisions are neglected. The mixture entering the computational domain is a mixture of air and gaseous ethanol and liquid ethanol. The flame is stabilized by a pilot flame, modeled as hot burnt gas. The combustion model used in these simulations is the Burning Velocity Model, so-called because the closure of the chemical source term occurs by means of the burning velocity, multiplied by the gradient of the reaction progress variable. A good agreement is found between the experimental data and the droplet velocity. The solution of the temperature indicate, however, that an investigation of other combustion models should be considered.
Finally, the forced spray flame response is studied with URANS simulations and presented in the last chapter. Two frequencies signals are chosen as upstream perturbation of the gas flow. The reference test-case, EtF6, is simplified and the liquid ethanol is injected as mono-dispersed spray.
The behaviour of the droplets under gas velocity oscillations is investigated in terms of spatial distribution and evaporation rate, and compared to the acoustic-free case. The spray flame response is also compared to the response of prevaporised ethanol flames, at constant global equivalence ratio.
The analysis is made both in the time and frequency domain, and a comparison of the discrete flame transfer function is performed between spray and gaseous flames. The methodology proposed for ethanol and its blends is successfull in laminar and turbulent regimes. Moreover, the features of heat loss and non-premixed combustion can easily be implemented in the proposed formulation.
The response of the ethanol spray flame to gas velocity oscillations is an important investigation towards the study of flame stability. In conclusion, the aspects of ethanol combustion presented in this dissertation provide insights in the modeling techniques used to simulate reactions of complex ethanol blends and its burning characteristics in off-design transient regimes. The tools developed will assist in the design of gas turbine combustors fired on blends of ethanol and other species.