Energy for Propulsion

This chapter reviews and discusses the recent development in Lean Direct Injection (LDI) combustion technology. An extended definition of LDI is also provided, followed by a broad description of LDI concepts and examples. Recognizing the needs and opportunities to expand the operability range of LDI, fundamental research has been undertaken to elucidate the effects of air swirler vane angle, pressure drop, air swirler rotation direction, and overall equivalence ratio on LDI flow field and flame behavior. Additional investigation was further conducted to understand fundamental differences between representative LDI and airblast injectors. Results of these fundamental studies are discussed to help identify design changes for improving LDI performance.

Given the increasing focus on climate change and emissions, alongside the motivation to combat these phenomena, it is prudent to consider alternative fuels for gas turbines, a significant source of emissions. Adopting some form of alternative fuels could reduce the carbon footprint as well as the emissions output from gas turbines to manageable levels, provided alternative fuels are coming from overall low life cycle emissions sources. In this chapter, the effects of alternative fuels on the gas turbines performance and their emissions are discussed. With respect to gaseous emissions, it has been found that alternative fuels provide no clear advantage in terms of emissions reduction compared to standard petroleum derived fuels. However, it has been found that the CO₂ emissions of a given fuel is contributed to by the H/C ratio of the fuel. An increase of the H/C ratio could lead to reduction in CO₂ emissions, though energy per unit mass of fuel goes down. The effect of alternative fuels on PM emissions however are more positive if alternative fuels are used, but PM emissions are dependent upon the aromatic content and its species in the fuel. The availability of alternative fuels from F-T processes, as well as bio-derived fuels with very low or no aromatic content, leads to very low PM emissions from alternative fuels. With respect to seal swell in fuel systems, it has been found that some alternative fuels may struggle to maintain good seal swell performance as seal swell has been historically related to aromatic content of the fuel. Therefore, it has been deemed that further research is required to find an alternative. When considering the noise and vibrations from a turbine, there appears to be insufficient data to draw clear correlations between fuel type and amount of noise and vibrations generated, however it has been noted that noise and vibration emitted is a function of the vapour pressure, surface tension and flame velocities used which in turn to a certain extent depend upon the fuel used. In terms of thermal stability, it has been noted that paraffinic fuels are better at absorbing heat and dissipating it without forming carbon deposits on the fuel system components.

Due to the emergence of a new generation of renewable fuels and the need to accurately model the combustion chemistry of multi-component fuels, there is growing interest in examining the effect of fuel molecular structure on fuel reactivity. This book chapter provides an overview of research dealing with the effects of fuel unsaturation on the ignition, combustion, and emission characteristics. Results from both laboratory-scale configurations, such as shock tube, rapid compression machine, and laminar flames, as well as from high-pressure sprays in compression ignition engines are discussed. Experimental and kinetic modeling studies of homogeneous mixtures provide clear evidence that depending upon the number and position of C = C double bonds, and the reactivity of long-chain hydrocarbons is significantly affected by fuel unsaturation, especially at low to intermediate temperatures. Ignition data for saturated and unsaturated components indicate that the presence of double bond inhibits low-temperature reactivity, modifies the NTC behavior, and leads to reduction in CN number in diesel engines. This has important consequences regarding the effect of unsaturation on combustion and emission in practical systems. High-pressure spray simulations under diesel engine conditions indicate longer ignition delays for 1-heptene compared to those for n-heptane. In addition, the n-heptane spray flame contains two reaction zones, namely a rich premixed zone (RPZ) and a nonpremixed reaction zone (NPZ). In contrast, 1-heptene flame is characterized by three reaction zones, i.e., a lean premixed zone (LPZ) in addition to NPZ and RPZ. Simulations of laminar partially premixed flames (PPF) indicate higher amounts of NOx and soot precursor species (C₂H₂, C₆H₆, and C₁₆H₁₀) formed in 1-heptene flames than those in n-heptane flames. Consequently, the soot emission is higher in 1-heptene flames than that in n-heptane flames. Simulations of turbulent n-heptane and 1-heptene spray flames in diesel engines lead to similar conclusions, i.e., higher NOx and soot emissions in 1-heptene flames. The increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to β scission reactions resulting from the presence of double bond in 1-heptene.

This paper presents the redesign and modification of a Ross yoke mechanism driving an alpha Stirling engine with parallel cylinder arrangement. Engine sealing is a crucial factor affecting engine operation, power, and maintenance. Friction and lateral force acting on piston seals induce major wear and finally lead to leakage and losses in both power and operating cost. To reduce these problems, linear reciprocating and balancing motion of both piston and connecting rod are preferred. Mechanical inversion is introduced to invert general motion of connecting rod to rectilinear translation. The original Ross yoke part is altered from pin joints to slot pin joints allowing piston rod to be driven straight. Length of the slot groove was adjusted and optimized. Motion of the modified Ross yoke was investigated theoretically and experimentally. Both analytical model and prototype have the same operating conditions and swept volume of 25 cm³. The ambient air was used as the working gas which was heated by LPG at flow rate of 0.6 kg/h and cooled by water. The maximum speed of 977 rpm was attained. Recorded maximum power and torque were 0.549 W at 486 rpm and 0.014 Nm at 260 rpm, respectively. The modified Ross yoke Stirling engine has operated smoothly when its piston rods were redesigned to slide linearly.

Combustion in physical systems is always affected by dynamic instabilities, most of them being of thermoacoustic origin. The interaction of the acoustics of the flame enclosure and the unsteady heat release from the flame are responsible for such instabilities. During such unstable operation, the flame often changes its dynamic state, with transition across several dynamic states being also quite common. The present chapter presents a brief review on the recent developments of dynamic systems approach applied to laminar ducted flames. The different tools of nonlinear time series analysis that are commonly used for this purpose have been described. Representative case studies of the technique applied to ducted non-premixed, premixed, and inverse diffusion flames have been presented. Finally, the promising nature of the complex networks-based approach for dynamic characterization of combustion systems has been highlighted.

The occurrence of thermoacoustic instability has been a major concern in the combustors used in power plants and propulsive systems such as gas turbine engines, rocket motors. A positive feedback between the inherent processes such as the acoustic field and the unsteady heat release rate of the combustor can result in the occurrence of large-amplitude, self-sustained pressure oscillations. Prior to the state of thermoacoustic instability, intermittent oscillations are observed in turbulent combustors. Such intermittent oscillations are characterized by an apparently random appearance of bursts of large-amplitude periodic oscillations interspersed between epochs of low-amplitude aperiodic oscillations. In most of the earlier studies, the pressure oscillations alone have been analyzed to explore the dynamical transition to thermoacoustic instability. The present chapter focuses on the instantaneous interaction between the acoustic field and the unsteady heat release rate observed during such a transition in a bluff-body-stabilized turbulent combustor. The instantaneous interaction of these oscillations will be discussed using the concepts of synchronization theory. First, we give a brief introduction to the synchronization theory so as to summarize the concepts of locking of phase and frequency of the oscillations. Then, the temporal and spatiotemporal aspects of the interaction will be presented in detail. We find that, during stable operation, aperiodic oscillations of the pressure and the heat release rate remain desynchronized, whereas synchronized periodic oscillations are noticed during the occurrence of thermoacoustic instability. Such a transition happens through intermittent phase-synchronized oscillations, wherein synchronization and desynchronization of the oscillators are observed during the periodic and the aperiodic epochs of the intermittent oscillations, respectively. Further, the spatiotemporal analysis reveals a very interesting pattern in the reaction zone. Phase asynchrony among the local heat release rate oscillators is observed during the stable operation, while they become phase-synchronized during the onset of thermoacoustic instability. Interestingly, the state of intermittent oscillations corresponds to a simultaneous existence of synchronized periodic and desynchronized aperiodic patterns in the reaction zone. Such a coexistence of synchrony and asynchrony in the reactive flow field mimics a chimera state.

Increasing concerns about the role of halons on the depletion of ozone in the stratosphere have led to a search for alternate agents for suppression. Water, sprayed in the form of tiny droplets, has emerged as a potential fire suppressant. The present chapter presents a brief review of the recent studies on flame water spray interaction. The effects of water spray on both premixed and non-premixed flames are discussed. The significance of droplet size in flame suppression is explained in details. This understanding will lead to efficient atomizer design for fire suppression systems.

The use of external enthalpy support (e.g. via heat recirculation) can enable combustion beyond normal flammability limits and lead to significantly reduced emissions and fuel consumption. The present work quantifies the impact of such support on the combustion of lean (\(\varPhi = 0.6\)) turbulent premixed DME/air flames with a Damköhler number around unity. The flames were aerodynamically stabilised against thermally equilibrated hot combustion products (HCP) in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (\(Re \simeq 18{,}400\) and \(Re_t > 370\)). The bulk strain (\(a_b = 750\) s\(^{-1}\)) was of the order of the extinction strain rate (\(a_q = 600\) s\(^{-1}\)) of the corresponding laminar opposed twin flame with the mean turbulent strain (\(a_I = 3200\) s\(^{-1}\)) significantly higher. The HCP temperature (\(1600 \le T_{HCP}\)(K) \( \le 1800\)) was varied from close to the extinction point (\(T_{q} \simeq 1570\) K) of the corresponding laminar twin flame to beyond the unstrained adiabatic flame temperature (\(T_{ad} \simeq 1750\) K). The flames were characterised using simultaneous Mie scattering, OH-PLIF and PIV measurements and subjected to a multi-fluid analysis (i.e. reactants and combustion products, as well as mixing, weakly reacting and strongly reacting fluids). The study quantifies the (i) evolution of fluid state probabilities and (ii) interface statistics, (iii) unconditional and (iv) conditional velocity statistics, (v) conditional strain along fluid interfaces and (vi) scalar fluxes as a function of the external enthalpy support.

An adaptive finite element method (FEM) is used for the solution of turbulent reactive flows in 3-D utilizing parallel methods for fluid dynamic and combustion modeling associated with engines. A dynamic LES method permits transition from laminar to turbulent flow without the assumptions usually required for turbulent sublayers near wall area. This capability is ideal for engine configurations where there is no equilibrium in the turbulent wall layers and the flow is not always turbulent and often in transition. The developed adaptive FEM flow solver uses “h” adaptation to provide for grid refinement. The FEM solver has been optimized for parallel processing employing the message passing interface (MPI) for clusters and high-performance computers.

The Rate-Controlled Constrained-Equilibrium (RCCE) dimension reduction methodology models complex reacting systems within acceptable accuracy with a number of constraints \(N_c\), much smaller than the number of species \(N_s\), in the corresponding Detailed Kinetics Model (DKM). It describes the time evolution of chemical kinetics systems using a sequence of constrained-equilibrium states specified by the chosen constraints. The comprehensive chemical composition at each constrained-equilibrium state is determined by maximizing entropy (or minimizing Gibbs free energies) given the instantaneous values of the constraints. RCCE guarantees final equilibrium concentrations since Lagrange multipliers of all non-elemental constraints will be zero at final state. In this chapter, RCCE fundamentals, constraint and constraint potential representations, methods of initializing constraint potentials (non-dimensional Lagrange multipliers) as well as a brief discussion of RCCE constraint selection are presented. To show its accuracy against DKM, RCCE method is applied to \({\mathrm{H}_{2}}\)/\({\mathrm{O}_{2}}\) and \({\mathrm{CH}_{4}}\)/\({\mathrm{O}_{2}}\) zero-dimensional, constant energy/volume combustion over a wide range of initial conditions. The results show that both mixture results are in excellent agreement with the DKM predictions.

Turbulence-chemistry interaction is known to play a vital role in changing the characteristics of a flame surface. It changes evolution, propagation, annihilation, local extinction characteristics of the flame front. This study seeks to understand how turbulence interaction affects flame surface geometry and propagation of turbulent premixed H₂/Air flames in a three-dimensional configuration. 3D Direct Numerical Simulation (DNS) study of premixed turbulent H₂/Air flames has been carried out using an inflow–outflow configuration at moderate Reynolds number (Re) with a fairly detailed chemistry. The simulations are conducted at different parametric conditions in conjunction with differential diffusion (non-uniform Lewis number) effects. The topology of the flame surface is interpreted in terms of its propagation and statistics. Statistics related to the flame surface area and the correlations between the curvature and the gradient of temperature are obtained from the computed fields. It is found that the displacement speed increases with the negative mean curvature, while it correlates well for high turbulent cases and scattered for low turbulent cases. It is also observed that the diffusion effects become more dominant for deciding the flame structure when the mean flow is lower (low Re case). Further, the unsteady effects of tangential strain rate, curvature on flame propagation, and heat release rate are also investigated. Later, effects of prominent species and radicals are described to correlate the production of the maximum heat release rate in the lower temperature regions.

The study reports on the numerical investigation of lifted turbulent jet flames with H₂/N₂ fuel issuing into a vitiated coflow. The hot vitiated co-flow containing oxygen as well as combustion products stabilize the lifted turbulent flame by providing an autoignition source. A 2D axisymmetric formulation has been used for the predictions of the flow field, while multidimensional Flamelet Generated Manifold (multi-FGM) approach has been used for turbulence-chemistry interactions in conjunction with RANS approach. The chemical kinetics in H₂-O₂ combustion is followed by (Mueller et al, Int J Chem Kinet 31: 113–125, 1999 [1]) and (Li et al, Int J Chem Kinet 36(10): 566–575, 2004 [2]) mechanisms and the difference in chemical kinetics is analyzed (in terms of auto-ignition distance) using one-dimensional calculations. The major difference between the two mechanisms is the value of rate constants contributing towards the source of the autoignition and the corresponding enthalpy of formation of OH radicals. The lift-off height is determined from the axial distance (from the burner exit) at which the auto-ignition occurs, and is located through local concentration of OH radical equivalent to 2 × 10⁻⁴. In order to understand the impact of chemical kinetics on the autoignition, speeding up (Set A) and delaying (Set B) auto-ignition controlled reaction rates are augmented and corresponding changes in lift-off height are observed. Hereafter, the comprehensive chemical kinetics sensitivity analysis is carried out in understanding the underlying behavior of HO₂ radicals as autoignition precursor and OH radicals as reaction rate determinant. It is found that some specific reaction is most sensitive to auto-ignition and plays a vital role in lift-off height predictions. The results obtained in the current study elucidates that the flame is largely controlled by chemical kinetics.

Mixing dynamics arising from the interaction of two convecting line vortices of different strengths and time delays have been investigated in this chapter. Experimentally obtained planar laser induced fluorescence images of acetone vortices mixing in air are thoroughly substantiated with computational modeling and analysis. Same and opposite direction of rotation, generating vortex pairs and couples were found to augment and dissipate species mixing respectively as indicated by global scalar statistics like mean and variance. Local mixing characteristics are analyzed using joint probability density function of vorticity and species concentration. This quantitatively showed how mixing is dissipated or augmented for individual vortices and at different vorticity magnitudes, even by the decaying and apparently indiscernible presence of a favorable or unfavorable direction of rotation of the preceding vortex.

Flow and aerosol transport and dynamics in a reaction chamber, part of an aerosol generation system, is analyzed by coupling Computational Fluid Dynamics (CFD) and Aerosol Dynamic Equation. A predictable parametric aerosol output from reaction chamber is desirable for different contexts. The effect of residence time of the aerosol particles and mixing characteristics of the flow on the aerosol size distribution is analyzed using the ANSWER finite volume CFD code. The ANSWER uses the steady state turbulent flow field to solve the General Dynamics Equations (GDE) for the aerosol particles. The GDE includes mechanisms such as coagulation, gravitational settling and thermophoretic drift etc. A volume (and mass) preserving nodal method is implemented to model particle distribution changes due to coagulation. The modules modeling coagulation and gravitational settling were validated with respect to analytical solutions taken from the literature. The size distribution in reaction chamber design is seen to be robust for various flow scenarios at inlet number concentration of 1 × 10¹²/m³. This is explained by flow time scale being much smaller than coagulation time scale. At higher inlet concentration of 1 × 10¹⁵/m³ the average size distribution and outlet size distribution are significantly shifted from the inlet distribution, due to the much lower coagulation time scale. A noticeable difference between no or low swirl and high swirl flow is seen. A secondary ring inlet within the reaction chamber was seen to lead to identical aerosol distribution for different flow scenarios even at higher inlet concentration.

Many experimental/numerical studies on single compartment or single room fires have been reported in the literature but few studies are available for multiple compartments. There is a need for fire studies in multiple compartments as it may help in predicting fire impact in buildup environments with multiple compartments. In the present work, fire experiments were conducted in a real life two-storey compartmentalized concrete building. Aviation Turbine Fuel (ATF) was used as the liquid fuel to generate pool fire. The work, focused on the thermal hydraulic aspects like heat distribution to the corresponding walls, temperature near walls, vertical distribution of temperature close to fire and comparison of heat-flux and temperature at specified locations in the interconnected rooms. The study elucidates the impact of ventilation on various thermal and hydraulic aspects of fire.

Aluminum-based solid fuel is widely used in solid rocket propulsion system. During combustion, the solid fuel transforms into liquid status oxidant and agglomerate into droplets which impinge to the inner wall of solid rocket motor (SRM) nozzle and result in erosion problem. When a droplet breaks up into smaller size droplet results in less inertia which has a higher chance to following the exhaust gas stream instead of impinging to the inner wall of the nozzle. In this study, the liquid breakup is achieved by changing the fluid property of surface tension. The result of a liquid breakup is obtained using computational fluid dynamics (CFD) simulation of large eddy simulations (LES) and compared with experiment. The result presents the reduction of droplet size by changing the liquid surface tension. The mechanism of droplet breakup is discussed which is found to be due to the lower Laplace pressure or droplet bounding pressure lead to the lower surface tension of the liquid.

Growing population and consequential rise in energy demand are contributors to overdependence of carbon based fossil fuels combustion for various applications which continues to increase the atmospheric CO₂ to unrecyclable levels leading to anthropogenic global warming from insufficient CO₂ capture and sequestration, and thus incomplete carbon cycle. Depleting fossil fuel reserves and concerns of CO₂ emissions from fossil fuel combustion, along with the concerns of improper waste disposal poses great global challenges that need to be addressed for energy and environment sustainability. Many techniques for energy and fuel production from biomass and solid wastes have been examined in the recent past of which thermochemical reformation of wastes are dominant, compared to biochemical processes such as anaerobic digestion, as they provide high reaction rates from their high operational temperatures. This chapter serves the purpose of providing detailed scenario of thermochemical processes starting with classification to include pyrolysis, gasification, and hydrothermal conversion techniques. Gasification techniques offer efficient and effective transformation of solid biomass and wastes into gas/liquid fuels and value added materials. This technique offers clean energy production at high efficiency compared to other transformation techniques via syngas which can be used for combined heat and power generation, production of fuels for transportation using Fischer Tropsch synthesis, and production of value added chemicals. Challenges of gasification, which include tar residuals and low-grade feedstocks are explained in detail in the chapter including catalytic and sorption based tar removal techniques. Low grade or high moisture content feedstocks may need drying before gasification which can significantly lower the economic value and efficiency of gasification that depends on net energy density of the feedstock. Hydrothermal processing is beneficial for the conversion of high moisture content feedstock such as wet grass, algal biomass, municipal waste, and sludge to bio-oils, which can further be refined to produce liquid biofuels that helps to reduce fossil fuel requirement of gasoline, diesel, and other fuels used for transportation, energy, power purposes. Other thermochemical methods such as fast pyrolysis have also been examined during the past couple of decades for the production of bio-oils for biofuel synthesis. Catalytic conversion techniques for refining of the bio-crude and bio-oils produced from liquefaction and fast pyrolysis are also discussed with focus on hetero-atom removal such as hydrodeoxygenation and the challenges associated with it. This chapter provides a review on the various thermochemical reformation techniques, their advantages and drawbacks. It emphasizes on informing various advancements in terms of the reactors used, the operational parameters that control the reactions and the proposed reaction pathways for these techniques. A focus in this chapter on state of the art global scenario to develop these processes includes catalytic reforming of their products to achieve enhanced quality products and their corresponding challenges to produce clean and sustainable energy, fuels and value added products.

The current global energy crisis is as a result of human population growth and technological advancements in developing countries. Beside the potential depletion of conventional fossil fuel reserves, fossil fuel is related with some environmental problems, i.e. global warming, high toxicity and non biodegradability, hence it is considered as non sustainable source of energy. Biofuels, particularly biodiesel is considered as one of the most promising and long-term energy source that can replace the fossil fuel. This is due to its biodegradability, non toxicity, safe and recyclable nature. Nevertheless, biodiesel production through the utilization of homogeneous catalytic system from food grade vegetable oil is no longer justifiable by industries in the near future, principally as a result of food versus fuel rivalry and other environmental issues associated to the conventional homogeneous catalytic system. Employing the waste cooking oil as feedstock to produce biodiesel using heterogeneous solid catalyst would help in making biodiesel fuel affordable and sustainable. This chapter review the general concepts of catalytic synthesis of biodiesel from waste cooking oil using heterogeneous catalysts and the problems associated with conventional catalysts usage for biodiesel production. The chapter also discussed the biodiesel quality assessment parameters.

Use of biofuels in aviation and ground transportation is increasing. The advantages of biofuels include their broad availability, carbon neutrality, environmental friendliness with potential economic and social benefits for the local communities. Bioethanol and biodiesel are by far the most prominent because of their ease of substitution of the conventional fossil fuels. Due to stringent quality standards in the aviation industry, rigorous testing of the biofuels is crucial. A number of feedstocks and technological routes are being explored for biofuel production including hydrotreated vegetable oils, Fischer-Tropsch fuels, synthesized iso-paraffinic fuels and alcohol-to-jet fuels. Some commercial flights have started using biofuels. Even though bioethanol and biodiesel are being blended with conventional fuel, there are still technological and economical challenges that prevent this fuel type to completely replace fossil fuels. The concept of bio-refinery which will utilize all parts of the biomass and transform all the co-products to value-added chemicals will give the biofuel industry a more competitive economic edge. Biofuels are promising alternatives and their use will continue to increase in the near future.

Each year, millions of tons of corn are harvested from the farm fields. Their residues, e.g. stems/leaves and husks, are left in the field, while the empty cobs are processed at the mills and the byproduct is inefficiently used as low-grade fuel. The stems/leaves and husks are not used as fuel because of their poor fuel quality and because they are difficult to handle. Innovative bio-char briquetting from corn residues to improve their fuel properties has been proposed using the torrefaction process. The temperature and retention time for this process affects properties of the empty cobs, stems/leaves, and husks from the residual corn, which helps to improve their heating value and changes the volatile matter and fixed carbon proportion in the fuel. All of the torrefied corn residues is hydrophobic, which has less ability to absorb water due to the change in pore structure. The biochar from the corn residual can be used as premium feedstock for heating purposes with high heating value and low smoke.