Numerical Simulations for Studies of Unsteady Combustion in Practical Combustion Devices

A major problem in the design of combustion equipment is to prevent the occurrence of combustion instabilities. Combustion instabilities appear as periodic oscillations in pressure and heat release through a feedback mechanism between the fluid dynamics and the combustion process. These oscillations may induce e.g. mechanical vibrations and increased heat transfer, leading to poor performance or even total loss of the system. The characteristic features of the unsteady combustion process observed for combustion instabilities can also be utilized advantageously in certain applications, e.g. pulse combustors. In pulsating combustion, the oscillations are utilized to enhance the efficiency and reduce emission of pollutants. The interaction between fluid dynamics and the chemical reactions play an important role in unsteady combustion processes. The mechanisms in the feedback process in terms of injection, mixing and ignition are of particular importance. The objective of this thesis is to utilize numerical simulations for studies of unsteady combustion. A physical or theoretical model for chemically reacting mixtures, based upon the theory of mixtures within continuum mechanics, is presented. A reduced physical model consisting of equations for conservation of mass for the mixture, balance of mass for the constituents and conservation of momentum and energy for the mixture is derived. With suitable constitutive relations a closed system of non-linear partial differential equations is achieved. A simulation model is derived from the reduced physical model. For numerical treatment of turbulent flow, the conservation and balance equations are spatially filtered according to the Large Eddy Simulation (LES) technique. The interaction from the small-scale fluctuations on the large-scale flow field is modelled by Sub Grid Scale models. The equations are discretized according to the Finite Volume Method. The model presented has been used for simulation of test rig assimilating the jet engine after burner, featuring unsteady flow in terms of a fluctuating vortex street. Additionally, the unsteady combustion process found in pulse combustors of Helmholtz type has been simulated. Results from the simulations have been validated against a number of experimental data, such as time resolved velocity fields, chemiluminescence from OH and CH, OH-LIF, as well as pressure and flue gas composition. The simulation model is found able to mimic most of the characteristic features of the unsteady combustion processes. Numerical simulations can serve as a useful tool for studies of unsteady combustion

Prismodell på solceller : En vägledning för dig som vill installera solceller i Sverige.

This paper is a tutorial for homeowners in Sweden that is about to install solar cells and it gives apicture of the current economic conditions. The pricing is produced by analyzing invoices from thecompany Nyedal solar energy. This is done for three sizes of installed peak effect and for five differentrooftop materials and for integrated solar cells. The analysis is made by Excel. Charts and diagrams areproduced out of this material. The tutorial is a literature study of what is applicable for solar cells intoday’s Sweden. As a homeowner, it is good to be as well informed as possible before the offers aretaken in. In this thesis the homeowner is informed and prepared for an efficient first meeting with theentrepreneur. The homeowner can identify problems and get a reasonable picture of what it will costand how profitable it will be. From an economic perspective it is still the standard module that is mostprofitable, the alternatives on the market are still too expensive

Teknik - ämnet som bygger broar : Ämnesövergripande undervisning med teknikämnet i fokus

Hur ser det ämnesövergripande arbetet ut hos fyra tekniklärare på fyra olika skolor i sydvästra Sverige? Hur har det arbetet förändrats över tid? Vilka visioner har dessa lärare om ämnesintegrering i teknikämnet? Detta är frågor som undersökningen avser att ge svar på. Det inhämtade materialet från fyra semistrukturerade intervjuer visar att det finns en positiv syn på ämnesövergripande arbete men att det arbetet har minskat de senaste åren. Ett ökat bedömningsfokus har enligt intervjuerna bidragit till denna minskning av ämnesintegration och forskning stöder detta. Teknikläraren har en viktig position i ett ämnesövergripande arbete då teknikämnet det knyter an till många andra ämnen och initiativ till ämnessamarbete kommer främst från lärarna själva. Detta initiativ tar de intervjuade lärarna gärna, men de vill ha mer tid till samplanering i schemat. Troligtvis kommer de reviderade kursplanerna som träder i kraft sommaren 2021 att ge detta utrymme.What does the interdisciplinary work by four technology teachers at four different schools insouthwest Sweden look like? How has it changed over time? What visions do these teachers haveabout subject integration? These are questions that the survey refers to answer. With semistructured interviews, a material has been produced, this was interpreted as a positive view ofinterdisciplinary work by the interviews, but that that work has decreased in recent years. Anincreased focus on assessment has contributed to this reduction according to the interviews andthere is research that supports this. The technology teacher has an important position in a crossdisciplinary work as the technology subject it relates to many other subjects and initiatives forcollaboration between subjects come mostly from the teachers themselves. The interviewees gladlytakes this initiative, but they want more time for co-planning in the schedule. It may be that therevised syllabi that come into force in the summer of 2021 will provide this space

Large Eddy Simulation of Reacting Flows Applied to Bluff Body Stabilized Flames

The objective of this paper is to present a large eddy simulation model for chemically reacting flows. The large eddy simulation model, founded on a physical model based on modern continuum mechanics, includes a complete treatment of the subgrid stresses and fluxes including both backscatter and diffusion. To investigate the predictive capabilities of the large eddy simulation model, numerical simulations of a configuration corresponding to a rig consisting of a rectilinear channel with a triangular-shaped bluff body have been performed. Both nonreacting and reacting flows have been examined under a variety of operating conditions. This paper focuses on the reacting case, which is characterized as lean and premixed. The simulation results are compared to experimental measurements of temperature, constituent mass fraction, and velocity fields in the test rig. The results indicate that the large eddy simulation technique works well and mimics most of the significant flow features, including the typical unsteady flow structures. The results from the large eddy simulations are furthermore used to investigate the mechanisms responsible for the typical flowfield in a bluff body stabilized flame

Theoretical and experimental investigation of the operating characteristics of a Helmholtz type pulse combustor due to changes in the inlet geometry

The inlet geometry of the combustion chamber of a Helmholtz type pulse combustor was changed by altering the position of a stagnation plate, or flame holder, near the inlet. Three different flame fielder positions were studied. The operating characteristics in terms of time resolved pressure and heat release, time averaged tail pipe temperature and concentrations of O2 and NOx in the outlet decoupler were investigated. These quantities were measured and also simulated in a model based upon the LES concept. The effect of different flame holder positions on the operating characteristics is discussed. The simulation model was found able to satisfactorily describe the combustion processes for all three cases, although the results indicate that some sub models need further improvement

The enhancement of heat transfer in the tail pipe of a pulse combustor

For pulse combustors of the Helmholtz type, a heat transfer of the order two to five times higher than expected has been reported. Experiments, where the temperature profile in the tail pipe of a pulse combustor has been measured, give Ilo indication why the heat transfer should be enhanced. The interaction between the oscillating velocity field and the oscillating temperature field might explain the observed enhanced heat conduction. In order to examine this interaction, a thermomechanical pulsating flow between two parallel plates has been considered. The governing equations were coupled with the classical constitutive assumptions of linearly viscous fluid and Fourier's law for heat conduction. The pressure and temperature gradients in the axial direction were approximated as harmonically oscillating functions in time, where the coefficients were estimated from experimental data. By neglecting the viscous net power term in the energy balance equation, the postulated general equations of motion were solved analytically in the transversal direction. The solutions obtained for the velocity and temperature fields were found to be in good agreement with experimental results. The analytical expression for the temperature field was then used to determine the heal transfer at the plates. The heat transfer at the plates was found to be dependent on the phase difference between the pressure and temperature gradients