This thesis describes an experimental investigation of the flame transfer function between flow disturbances and heat release oscillations in lean, premixed combustors. This research effort was motivated by the fact that modern gas turbines, operating fuel-lean to minimize exhaust emissions, are susceptible to self-excited combustion oscillations. These instabilities generally occur when the unsteady combustion process couples with the acoustic modes of the combustion chamber. The resultant flow and structural vibrations can substantially reduce hot section part life. As such, avoiding operating regimes where high dynamics occur often requires operating at lower power outputs and/or higher pollutant emissions than the turbine is otherwise capable. This work demonstrated nonlinearities in the chemiluminescence response at large amplitude velocity oscillations in a turbulent, swirling flame. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed depending on the operating conditions of the combustor. The phase between the flow oscillations and heat release is also seen to have substantial amplitude dependence. In addition, the interactions between the fundamental frequency and the higher and subharmonics of the measured signals can significantly influence the flame as well as the frequency response of the system. The nonlinear flame dynamics are governed by different mechanisms in different frequency and flowrate regimes. Three mechanisms, vortex rollup, unsteady flame liftoff, and parametric instability, are identified to influence the nonlinear flame response in these combustors. Analysis of the results shows that the mechanisms responsible for nonlinearity in the flame response are influenced by the Strouhal number, the mean velocity at the combustor dump plane, and the ratio of the oscillating velocity amplitude to the laminar flame speed.

No description available.

The objectives of this study were to 1) determine the effects of cycle-to-cycle variations (ccv) on nitric oxide (NO) emissions, and 2) determine if the consideration of ccv affects the average NO emission as compared to the mean cycle NO emission. To carry out the proposed study, an engine simulation model was used. The simulation determines engine performance and NO emissions as functions of engine operating conditions, engine design parameters, and combustion parameters. An automotive, spark-ignition engine at part load and 1400 rpm was examined in this study. The engine cycle simulation employed three zones for the combustion process: (1) unburned gas, (2) adiabatic core region, and (3) boundary-layer gas. The use of the adiabatic core region has been shown to be especially necessary to capture the production of nitric oxides which are highly temperature dependent. Past research has shown that cyclic variations in combustion cause ccv of burn duration, ignition delay and equivalence ratio. Furthermore, literature has shown that variations of these three input parameters may be approximated by a normal frequency distribution. Using the mean and standard deviation, and a random number generator, input values were tabulated for the ignition delay, burn duration and equivalence ratio. These three input parameters were then used to simulate cyclic variations in the combustion process. Calculated results show that cyclic variations of the input parameters cause the cycle-by-cycle NO emissions to increase and decrease by as much as 59% from the mean cycle NO of 3,247 ppm. The average NO emission resulting from ccv was 4.9% less than the mean cycle NO emission. This result indicates that cyclic variations must be considered when calculating the overall NO emissions.

The quasi-steady, spherically symmetric combustion of multicomponent isolated fuel droplets has been modeled using modified Shvab-Zeldovich variable mechanism. Newly developed modified Shvab-Zeldovich equations have been used to describe the gas phase reactions. Vapor-liquid equilibrium model has been applied to describe the phase change at the droplet surface. Constant gas phase specific heats are assumed. The liquid phase is assumed to be of uniform composition and temperature. Radiative heat transfer between the droplet and surroundings is neglected. The results of evaporation of gasoline with discrete composition of hydrocarbons have been presented. The evaporation rates seem to follow the pattern of volatility differentials. The evaporation rate constant was obtained as 0.344mm2/sec which compared well with the unsteady results of Reitz et al. The total evaporation time of the droplet at an ambience of 1000K was estimated to be around 0.63 seconds. Next, the results of evaporation of representative diesel fuels have been compared with previously reported experimental data. The previous experiments showed sufficient liquid phase diffusional resistance in the droplet. Numerical results are consistent with the qualitative behavior of the experiments. The quantitative deviation during the vaporization process can be attributed to the diffusion time inside the droplet which is unaccounted for in the model. Transient evaporation results have also been presented for the representative diesel droplets. The droplet temperature profile indicates that the droplet temperature does not reach an instantaneous steady state as in the case of single-component evaporation. To perform similar combustion calculations for multicomponent fuel droplets, no simple model existed prior to this work. Accordingly, a new simplified approximate mechanism for multicomponent combustion of fuel droplets has been developed and validated against several independent data sets. The new mechanism is simple enough to be used for computational studies of multicomponent droplets. The new modified Shvab-Zeldovich mechanism for multicomponent droplet combustion has been used to model the combustion characteristics of a binary alcohol-alkane droplet and validated against experimental data. Burn rate for the binary droplet of octanol-undecane was estimated to be 1.17mm2/sec in good concurrence with the experimental value of 0.952mm2/sec obtained by Law and Law. The model has then been used to evaluate the combustion characteristics of diesel fuels assuming only gas phase reactions. Flame sheet approximation has been invoked in the formulation of the model.

The recent pollution norms have brought a strong emphasis on the reduction of diesel engine emissions. Low temperature combustion technology such as premixed compression ignition (PCI) has the capability to significantly and simultaneously reduce nitric oxides (NOx) and particulate matter (PM), thus meeting these specific pollution norms. There has been, however, observed loss in fuel conversion efficiency in some cases. This study analyzes how energy transfer and brake fuel conversion efficiency alter with (or are affected by) injection timings and exhaust gas recirculation (EGR) rate. The study is conducted for PCI combustion for four injection timings of 9°, 12°, 15° and 18° before top dead center (BTDC) and for four exhaust gas recirculation (EGR) rates of 39%, 40%, 41% and 42%. The data is collected from the experimental apparatus located in General Motors Collaborative Research Laboratory at the University of Michigan. The heat release is calculated to obtain various in-cylinder energy transfers. The brake fuel conversion efficiency decreases with an increase in EGR. The decrease in the brake fuel conversion efficiency is due to the decrease in work output. This decrease is due to an increase in the pumping work and an increase in friction and decrease in gross indicated work. The decrease in the combustion efficiency is because of the increased formation of unburnt products due to increased ignition delay caused by the application of EGR and decreasing air-fuel (A/F) ratio. A definite trend is not obtained for the contribution of heat transfer to the total energy distribution. However the total heat transfer decreases with retardation of injection timing because of decreasing combustion temperature. As the injection timing is retarded, the brake fuel conversion efficiency is found to decrease. This decrease is because of a decrease in net work output. This is because the time available for utilization of the energy released is less because of late combustion. The total heat transfer decreases with retardation of injection timing because of decreasing combustion temperature. The contribution of heat transfer to the total energy distribution decreases with increase in EGR.

The exhaust gas recirculation (EGR) system has been widely used to reduce nitrogen oxide (NOx) emission, improve fuel economy and suppress knock by using the characteristics of charge dilution. However, previous studies have shown that as the EGR rate at a given engine operating condition increases, the combustion instability increases. The combustion instability increases cyclic variations resulting in the deterioration of engine performance and increasing hydrocarbon emissions. Therefore, the optimum EGR rate should be carefully determined in order to obtain the better engine performance and emissions. A thermodynamic cycle simulation of the four-stroke spark-ignition engine was used to determine the effects of EGR on engine performance, emission characteristics and second law parameters, considering combustion instability issues as EGR level increases. A parameter, called 'Fuel Fraction Burned,' was introduced as a function of the EGR percentage and used in the simulation to incorporate the combustion instability effects. A comprehensive parametric investigation was conducted to examine the effects of variations in EGR, load and speed for a 5.7 liter spark-ignition automotive engine. Variations in the thermal efficiencies, brake specific NOx emissions, average combustion temperature, mean exhaust temperature, maximum temperature and relative heat transfer as functions of exhaust gas recycle were determined for both cooled and adiabatic EGR configurations. Also effects of variations in the load and speed on thermal efficiencies, relative heat transfers and destruction of availability due to combustion were determined for 0% EGR and 20% EGR cases with both cooled and adiabatic configurations. For both EGR configurations, thermal efficiencies first increase, reach a maximum at about 16% EGR and then decrease as the EGR level increases. Thermal efficiencies are slightly higher for cooled EGR configuration than that for adiabatic configuration. Concentration of nitric oxide emissions decreases from about 2950 ppm to 200 ppm as EGR level increases from 0% to 20% for cooled EGR configuration. The cooled EGR configuration results in lower nitric oxide emissions relative to the adiabatic EGR configuration. Also second law parameters show the expected trends as functions of EGR. Brake thermal efficiency is higher for the 20% EGR case than that for the no EGR case over the range of load (0 to WOT) and speed (600 rpm to 6000 rpm). Predictions made from the simulation were compared with some of the available experimental results. Predicted thermal efficiencies showed a similar trend when compared to the available experimental data. Also, percentage of unused fuel availability increases as the EGR level increases, and it can be seen as one of the effects of deteriorating combustion quality as the EGR level increases.

A full Strouhal number thermo-acoustic model is purposed for the feedback control of self excited noise driven combustors. The inclusion of time delays in the volumetric heat release perturbation models create unique behavioral characteristics which are not properly reproduced within current low Strouhal number thermo acoustic models. New analysis tools using probability density functions are introduced which enable exact expressions for the statistics of a time delayed system. Additionally, preexisting tools from applied mathematics and control theory for spectral analysis of time delay systems are introduced to the combustion community. These new analysis tools can be used to extend sensitivity function analysis used in control theory to explain limits to control effectiveness in self-excited combustors. The control effectiveness of self-excited combustors with actuator constraints are found to be most sensitive to the location of non-minimum phase zeros. Modeling the non-minimum phase zeros correctly require accurate volumetric heat release perturbation models. Designs that removes non-minimum phase zeros are more likely to have poles in the right hand complex plane. As a result, unstable combustors are inherently more responsive to feedback control.

Akademisk avhandling--oorganisk Kemi--Göteborgs universitet, 2002. / Bibliogr. p. 86-92.

Reproduction de : Thèse de doctorat : Dynamique des fluides : Toulouse, INPT : 2004. / Titre provenant de l'écran-titre. Bibliogr. 53 réf.

Thesis (M.S.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains xv, 110 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 43).