Laminar Flame Characteristics of Premixed Methanol–Water–Air Mixture

Methanol is hygroscopic in a gaseous state and is a promising alternative fuel for internal combustion engines. It is understood that adding water can improve the antiknock performance for spark ignition engines, but this will also affect the flame speed and stability. In this work, laminar flame characteristics of methanol/water/air mixtures were experimentally investigated at a temperature range of 380–450 K, a pressure range of 1–4 bar, and water fractions (vaporous water molar fraction in the water–methanol fuel gas) of 0–40%. The results show that laminar burning velocity increases with temperature but decreases with pressure. The burning velocity decreases linearly with water fraction at a stoichiometric ratio. For rich mixtures and high pressures, the laminar flames tend to be more sensitive to stretch and, thus, more prone to being unstable. Increasing the water fraction can slightly increase the Markstein length. Increasing the initial pressure enhances the general flame instability, while increasing the initial temperature suppresses the general flame instability. Increasing the water fraction can lead to a decreasing thermal expansion ratio and an elevated flame thickness, both of which can lead to a suppression of hydrodynamic instability. An increase in the water fraction decreases the Lewis number, resulting in preferential diffusion instability. There is no direct relationship between the onset of cellularity and general flame instability.

Laser-Induced Ignition and Combustion Behavior of Individual Graphite Microparticles in a Micro-Combustor

Microscale combustion has potential application in a micro power generator. This paper studied the ignition and combustion behavior of individual graphite microparticles in a micro-combustor to explore the utilization of carbon-based fuels at the microscale system. The individual graphite microparticles inside the micro-combustor were ignited by a highly focused laser in an air flow with natural convection at atmospheric temperature and pressure. The results show that the ignition of graphite microparticles was heterogeneous. The particle diameter had a small weak effect on ignition delay time and threshold ignition energy. The micro-combustor wall heat losses had significant effects on the ignition and combustion. During combustion, flame instability, photophoresis, repetitive extinction and reignition were identified. The flame structure was asymmetric, and the fluctuation of flame front and radiation intensity showed combustion instability. Photophoretic force pushed the graphite away from the focal point and resulted in extinction. Owing to large wall heat loss, the flame quickly extinguished. However, the graphite was inductively reignited by laser.

Effect of Thermal Boundary Conditions on Propagation of Non-Equidiffusive Flames

Boundary conditions constitute one of the key factors influencing combustion in chambers with large aspect ratios such as narrow channels or pipes. Specifically, the flame shape and propagation velocity are impacted by wall friction and wall heat transfer. Both factors continuously influence the shape of the flame front, thereby resulting in its larger surface area as compared to a planar flame front. Such a corrugated flame consumes more fuel per unit time and thereby propagates faster than the planar flame at the same thermal-chemical conditions. Consequently, a flame accelerates due to the boundary conditions. In the recent years, there have been many studies scrutinizing the role of boundary conditions in flame acceleration scenario by means of analytical formulations, numerical studies or experimental measurements. However, the majority of these works was limited to equidiffusive flames, where the thermal-to-mass diffusivity ratio (the Lewis number; Le) is unity. In this respect, the present work removes this limitation by analyzing non-equidiffusive (Le < 1 or Le > 1) flames propagating in pipes of various widths. Specifically, a parametric study has been conducted by means of simulations of the basic hydrodynamic and combustion equations. In this particular study, two-dimensional channels with smooth walls and different thermal conditions such as isothermal and adiabatic walls, have been employed for various Lewis numbers in the range 0.2 ≤ Le ≤ 2.0, and for various Reynolds number associated with the flame propagation in the range 5 ≤ Re ≤ 30. As a result, a strong coupling between the wall conditions and the variations of the Lewis and Reynolds numbers is demonstrated. Specifically, it is observed that the increase in the Lewis number results in moderation of flame tip acceleration. It is also found that there is a change in the burning rate and surface area of the flame front at the lower Lewis numbers, where flames appear unstable against the diffusional-thermal flame instability. Moreover, a substantial difference between the cases of isothermal and adiabatic wall conditions is demonstrated.

Numerical Investigation of Inlet Turbulence Intensity Effect on a Bluff-Body Stabilized Flame at Near Flame Blow-Off Conditions

This paper investigates the effects of the inlet turbulence intensity (ITI) on the dynamics of a bluff-body stabilized flame operating very close to its blow-off condition. This work is motivated by the understanding that more stringent regulations on combustion-generated emission have forced the industry to design combustion systems that operate at very fuel-lean conditions. Combustion at very lean conditions, however, induces flame instability that can ultimately lead to flame extinction. The dynamics of the flame at lean conditions can therefore be very sensitive to boundary conditions. Here, a numerical investigation is conducted using Large Eddy Simulation method to understand the flame sensitivity to inlet turbulence intensity. Combustion is accounted for through the transport of chemical species. The sensitivity to inlet turbulence is assessed by carrying out simulations in which the inlet turbulence is varied in increments of 5%. It is observed that while the inlet intensity of 5% causes blow-off, further increased to 10% preserves a healthy flame on account of greater heat release arising from greater and balanced entrainment of combustible mixtures into the flame zone just behind the bluff-body. This balanced stabilization is again lost as the inlet turbulence intensity is further increased to 15%. Since experimental data pertaining to the topic of this paper are rare, the reasonableness of the combination of models is first checked by validating Volvo propane bluff-body flame, whereby reasonable agreement is observed. This study will advance our understanding of the sensitivity of bluff-body flames to boundary conditions specifically to the inlet turbulent boundary condition at near critical blow-off flame conditions.

Influence of Plate Orifice in the Pre-Mixing of Gas-Powered Water Heaters

Amongst the various alternatives for hot water production for domestic use, instantaneous heaters are still widely used in many markets such as the Portuguese market. In this system, a gas boiler converts the chemical energy of the gas (Liquefied Petroleum Gas, Natural Gas) to a water stream, as it is used. The complexity of such devices ranges from those with a natural convection to those with full pre-mixing of the air-fuel. The tightening of the legislation targeting these appliances is promoting an increase in efficiency, pollutant emission reduction and an increase in the safety features. The purpose of this work was to test the thermal performance of a water heater prototype with 22 kW of nominal heat output, running on Propane. Changes were made to the plate with orifices that limit the air supply to the burner flutes, where the pre-mixture with the fuel is partially made. Four different plates with different orifice diameters were built and tested in real case scenarios, taking into special consideration the pollutant emission and the fuel consumption verified. From the results, it was concluded that the best configuration in terms of efficiency is the original one, followed by the “−0.5 mm” and “−1 mm” plates. On the other hand, the best plate in terms of CO emission was the “−1 mm” plate. Concerning the plates with larger diameters to the manufacturer’s original configuration, flame instability was verified as a result of the greater primary airflow. Under the same test conditions, it was noted that the introduction of a nozzle into the fan inlet led to the suction of a larger amount of air. Finally, it was also concluded that the reduction in the orifice diameters of the plates reduces the split of primary air, resulting in an increased pressure drop in the flutes and in the overall pressure drop of the system.