An analysis of the dispersion of bubbles in homogenous and isotropic turbulent liquid flows was performed to study the effects of bubble and flow characteristics on their dispersion. Bubbles were assumed to be spherical and to follow the fluid motion in the mean. No mass transfer occurred between the bubble and liquid; also, there was no interaction between individual bubbles. It was found that for accurate prediction of bubble dispersion requires a simultaneous consideration of the inertia of the added mass of liquid (because the inertia of the bubble itself is small) and the bubble rise velocity. Normalized bubble diffusivity, root-mean-square fluctuating velocity, and Lagrangian integral time scale were related to two non-dimensional parameters: ratio of the added mass response time to the liquid flow integral time scale, and the ratio of the bubble rise velocity to the root-mean-square liquid velocity fluctuation. In general, the bubble Lagrangian velocity auto-correlations decreased as the rise velocity ratio increased. The dependence of the autocorrelations on the time-scale ratio was complex. A surprising result was that the bubble velocity fluctuations could exceed the liquid velocity fluctuations for certain conditions because of their low inertia.

In recent years, biofuels have emerged as an attractive alternative to traditional petroleum fuels due to their renewable and almost carbon-neutral nature. While the fundamental ignition properties of petroleum fuels are well understood, knowledge of ignition properties of biofuels and their blends with petroleum fuels is limited. Studies of the fundamental ignition properties of biofuels are important for the safety of storage and transportation of these fuels. The objective of this study was to compare the relative hot surface ignition properties of biofuel blends at different equivalence ratios in a constant-volume chamber. Properties that were measured in this study are: ignition energy, time interval for ignition, ignition surface temperature, and flame front velocities. The fuels studied were blends of Jet A (petroleum fuel) and canola methyl ester, CME (biofuel) over an equivalence ratio range of 0.75–2.0. A commercially available silicon carbide dryer ignitor was used as the ignition source and was located in the center of the combustion chamber. A high-speed camera recorded the propagation of the flame following ignition, allowing for the calculation of the flame front velocity. K-type thermocouples measured the temperature of the mixture at selected points inside the combustion chamber. It was found that the ignition temperature of the air-fuel mixture was nearly constant for all fuels at about 630°C (903 K). The ignition energy reached a minimum value around an equivalence ratio of 1.1–1.3 for all the blends.

The laminar partially premixed flames of prevaporized Jet-A, canola methyl ester (CME) and blend flames of Jet-A and CME were studied over a range of equivalence ratios of 0.6–0.8 with and without heated coflow. Measurements of blowout velocities and flame temperature profiles were made. The injector was designed to produce a uniform inner flow and had an inner diameter of 12.7 mm. The flames were laminar and blue in color (dominated by homogeneous gas-phase reactions). The temperature profiles in all the flames were similar, with a peak temperature of 1680 K. The blowout velocity increased with equivalence ratio for all the flames; the blowout velocity of pure CME flames was higher than that of pure Jet-A flames with no coflow present. The blowout velocity of the blend flames ranged around the values for the pure fuel flames. A Damkohler number (based on the velocity gradient at the jet boundary and chemical reaction time scale estimated from laminar flame velocity) of unity characterizes the blowout velocity. As the coflow velocity was increased, the blowout velocity was increased, and the differences between the values for the various flames became smaller.

In order to understand the ignition characteristics of liquid biofels such as canola methyl ester (CME), a set-up was built and tested. The rectangular combustion chamber (7 cm by 7 cm by 33cm high) had a viewing window in the front. Liquid fuel was injected into a stream of hot air, vaporized and mixed and the fuel/air mixture was passed through the combustion chamber vertically upward. The combustion chamber was filled with mixtures of various equivalence ratios. A 120V dryer heating element was mounted horizontally at the center of the chamber and served as the hot surface. The power input to the hot surface, the temperatures at different locations inside the chamber and images obtained from a high speed camera were recorded simultaneously. The ignition delay, ignition energy and flame velocities were documented over a range of equivalence ratios with Jet-A and CME as fuels. The ignition delay and ignition energy reached minimum values around equivalence ratios of 1.1–1.3. The ignition delay and ignition energy values for CME were comparable to those of Jet A and the flame velocities were 30% lower. This set-up can be used to measure relative ignition characteristics of liquid biofuels and blends of biofuels and petroleum fuels.

Palm methyl ester (PME) is an attractive alternate fuel that is becoming popular in Asia and has the potential for a wider application. In this study, the near-burner flame characteristics in the laminar partially premixed flames of Jet A, PME and their blends (25%, 50% and 75% by volume) were investigated. Planar laser-induced fluorescence (PLIF) technique was used to determine the relative concentration of OH and CH radicals in the near-burner region of the flames. The fuel was prevaporized by injecting into a hot air stream and passed through a 9.5 mm stainless steel tube which served as the injector. The burner exit equivalence ratios of 2, 3 and 7, were selected to simulate a wide range of (high to low) partially premixed modes of combustion that exist in practical combustors. The PLIF measurements indicated that the OH and CH concentrations closely followed the temperature profiles at the equivalence ratio of 2. The blend-fuel flames had higher OH and CH concentrations than the neat fuel flames at equivalence ratios of 2 and 3. At the equivalence ratio of 7, a significant reduction in OH concentration was observed in the near-burner region of the flames having biofuel content, and the CH concentration was comparatively higher than OH concentration of corresponding flames. The presence of high CH concentration in the region of peak NO concentration indicated that the NO formation at this condition was likely through the Fenimore mechanism. The concentration measurements of OH and CH radicals revealed a non-monotonic variation with the proportion of biofuel in the fuel blend and suggested a complex competition between the growth and oxidation of poly aromatic hydrocarbons (PAH) and soot particles, and the formation of combustion products.

Biofuels, such as palm methyl ester (PME), are attractive alternates to petroleum fuels. In order to isolate the effects of fuel chemistry on the combustion properties, laminar partially premixed pre-vaporized flames of blends of Jet-A and PME (volume concentrations of 25%, 50%, 75% PME) were studied. A stainless steel circular tube (ID of 9.5 mm) served as the burner. The liquid fuel was supplied with a syringe pump into a high temperature (390°C) air flow to vaporize it completely without coking. The fuel flow rate was maintained constant and the air flow rate adjusted to obtain burner-exit equivalence ratios of 2, 3 and 7. The global flame properties including flame length, CO and NO emission indices, radiative heat fraction and in-flame properties including gas concentration (CO, CO 2 , NO, O 2 ), temperature and soot volume fraction were measured. The near-burner homogeneous gas-phase reaction zone increased in length with the addition of PME at all equivalence ratios. The concentration and global emission measurements highlight the non-monotonic variation of properties with the volume concentration of PME in the fuel. The fuel-bound oxygen of PME affected the combustion properties significantly.

Palm Methyl Ester (PME) is an attractive alternate fuel to petroleum diesel because it can be produced from a renewable source, is close to being carbon-neutral in the environment, and has many properties similar to those of petroleum fuels. Although a few engine studies have been completed using PME, basic studies on the combustion of PME sprays are limited. Hence, the objective of this investigation was to study the combustion characteristics of spray flames of PME and diesel at three equivalence ratios: 0.4, 0.6 and 0.8. The liquid fuel was atomized and combusted with air in a heated environment; the coflow air temperature was maintained at 65% of the midpoint of the boiling point range of the fuels. The equivalence ratio was changed by altering the fuel flow rate, while maintaining the atomizing and coflow air flow rates constant, thus maintaining the gas velocity field invariant. The PME flames were shorter and less luminous than the diesel flames and had significant near-burner homogeneous-gas-phase reaction zones. The global CO emission index of the PME flames was higher than that of diesel flames at equivalence ratios of 0.4 and 0.6. The NO emission index of the PME flames was almost twice that of the diesel flames. The in-flame concentration profiles highlighted the effects of the significant fuel-bound oxygen content in PME.

The combustion characteristics of spray flames of canola methyl ester (CME) and blends with diesel fuel within a re-radiating environment were studied. The combustion chamber was lined with refractory bricks that were preheated to about 725 K (1305 °R). The flow rates of the fuels provided a constant heat release rate of about 7.33 kW (25,000 BTU/hr) at atmospheric pressure. Measurements of flame temperature, in-flame concentrations, global emissions, flame radiation and soot volume fraction were taken. The global CO emission index was significantly lower in the biofuel blend spray flames compared to that of the diesel spray flame. The global NO emission index was comparable for all spray flames, which agreed with peak flame temperature and in-flame NO concentration measurements. The radiative fraction of heat release was also comparable for all spray flames.

Palm methyl ester (PME) is a renewable biofuel that is produced by the transesterification of palm oil; it is a popular alternative fuel used in the transportation sector. The objective of this investigation was to study the combustion characteristics of flames of pre-vaporized diesel and PME in a laminar flame environment at initial equivalence ratios of 2, 3 and 7 and to isolate the factors attributable to chemical structure of the fuel. The equivalence ratio was changed by altering the fuel flow rate, while maintaining the air flow rate constant. The global CO emission index of the PME flames was significantly lower than that of the diesel flames; however, the global NO emission index was comparable. The radiative fraction of heat release and the soot volume fraction were lower for the PME flames compared to the diesel flames. The peak temperatures were comparable at an equivalence ratio of 2, but at higher equivalence ratios, the peak temperatures in the PME flames were higher. The measurements highlight the differences in the combustion properties of biofuels and petroleum fuels and the coupling effects of equivalence ratio.

Flame extinction experiments were conducted using propane (with and without additives) and Jet A (in partially premixed and diffusion flame modes). A single-flame counter-flow burner configuration was used to measure the extinction limits. The set-up consisted of two contoured nozzles (exit diameter of 6.4 mm), designed to produce top-hat velocity profiles at the exit with a separation distance of 12.7 mm between them. A cooling system, which consisted of a submersible water pump, water reservoir and a split aluminum cooling jacket, was used to cool the top burner and avoid heating of the top burner by the hot buoyant flame products. A nitrogen shield was used to protect the flame from air entrainment and perturbations from the surroundings. A heating system was used to vaporize the liquid Jet A fuel in air or nitrogen maintained at a temperature up to 325 °C. For premixed propane flames, as the stretch rate increased from 364 s −1 to 462 s −1 , the volumetric percentage of propane varied from 2.23% to 2.83% at lean extinction. These results, when modified suitably to take into account that only air was fed from the top burner, agreed well with previous results obtained from twin-flame configurations. For a given stretch rate, the propane flame extinguished at a richer condition when nitrogen or carbon dioxide was added to the air stream. Also, carbon dioxide was more effective in extinguishing the flat flame than nitrogen. The flame extinction limits were measured for Jet A for stretch rates varying between 218.7 s −1 and 412.9 s −1 . It was found that the Jet A flames in diffusion mode were more resistant to extinction than those in the premixed mode.

Butanol is an attractive alternate fuel because it can be produced from renewable sources and has properties similar to those of petroleum fuels. Thus, blending butanol with petroleum fuels is a promising solution to reduce the dependence on petroleum fuels. In a previous study, we investigated the differences in the structure and emissions of Jet A and Butanol flames. The objective of this investigation was to study the emission and in-flame temperature characteristics of spray flames of Jet A/butanol blends at two equivalence ratios: 0.75 and 0.95. In addition to pure Jet A and pure butanol, blends of Jet A with 25%, 50% and 75% volumetric concentrations of butanol were used as fuel. The liquid fuel was atomized and combusted with air in a heated environment (479 K). The equivalence ratio was changed by altering the fuel flow rate, while maintaining the atomizing and coflow air flow rates constant, thus maintaining gas velocity field invariant. The global emission index of CO varied non-monotonically with the volume concentration of butanol in the blend at the lower equivalence ratio whereas the variation was gradual at the higher equivalence ratio. The global NOx emission index decreased monotonically as the butnaol content was increased at both equivalence ratios. The global NOx emission index level in the flames at equivalence ratio of 0.95 was higher than that at equivalence ratio of 0.75. At 25% flame height, the peak reaction zone was located off-axis; this radial location moved further away from the centerline as the equivalence ratio was increased. The peak temperatures were comparable in all the flames. The flames with butanol highlighted the effects of preferential vaporization of butanol.

Canola methyl ester (CME) is a biofuel that is produced by the transesterification of canola oil; it is renewable, carbon-neutral and low in sulfur content. The objective of this study was to document the combustion characteristics of spray flames of CME and No 2 diesel (petroleum fuel) blends. Three blends with 25%, 50% and 75% volume concentration of CME were studied. The fuel was atomized and mixed with air in a heated environment at a supply equivalence ratio of 0.62. Measurements of global CO and NO emissions, inflame temperature and in-flame concentrations of combustion products were made. The near-injector homogeneous gas-phase reaction zone increased in size with the addition of CME. The global CO and NO emissions decreased with the increase in CME content in the fuel blend. The in-flame NO concentration profiles and flame temperature profiles followed similar trends, suggesting that the thermal mechanism of NO formation was dominant in these flames.

Alcohols serve as an alternate energy resource to the conventional petroleum-based fuels. The objective of this study was to document the performance and emission characteristics of blends of n-propanol and Jet A fuel in a small-scale gas turbine engine. The experiments were conducted in a 30kW gas turbine engine with a single-stage centrifugal flow compressor, annular combustion chamber and a single-stage axial flow turbine. In addition to neat propanol and Jet A fuel, three blends, with 25%, 50% and 75% of propanol by volume, were used as the fuels. The thrust, thrust-specific fuel consumption, and the concentrations of CO and NOx in the exhaust were measured and compared with those measured with Jet A fuel. The engine was operated at the same throttle settings with all the fuels. The operational range of engine rotational speed was shifted downwards with the addition of propanol due to its lower heating value. The thrust specific fuel consumption increased with the addition of propanol, while the CO emission index increased and NOx emission index decreased.

Soy methyl ester (SME) is a biofuel that is a renewable alternative energy resource and is produced by the transesterification of soy oil; it is carbon-neutral and low in sulphur content. The objective of this study was to document the combustion characteristics of blends of SME and No 2 diesel fuel in a partially-premixed turbulent flame environment. The experiments were conducted at an initial equivalence ratio of 7 and three Reynolds numbers (based on the injector diameter and the bulk burner-exit velocity of the air/fuel mixture): 2700, 3600 and 4500. Three blends, B25, B50 and B75 corresponding to 25, 50 and 75% volume concentration of SME were studied. The liquid fuel was completely vaporized and mixed with air before exiting the burner. The radiative heat fraction measured in the SME flames was lower than the corresponding value in pure diesel flames and increased with Reynolds number. The global emission measurements indicated that the NO x emissions from the SME-diesel blend flames were lower than those from the pure diesel flame. At quarter and half flame height the temperature peaked at the edge of the flame where as for three quarters the temperature peaked at the centerline of the flame. In-flame NO x concentrations decreased with an increase in Reynolds number. The CO emission index decreased with the increase in the SME concentration in the fuel blend and decreased with the increase in Reynolds number.

The use of porous-media burners in air-heating systems, gas turbine combustors, and steam generators is a potential method to reduce pollutant emission levels. Biofuels, such as canola methyl ester (CME), are an attractive alternate energy resource; however, pure biofuels have lower energy content than petroleum-based fuels. Therefore, the combustion characteristics of blends of Jet A and CME were studied in a porous-media burner. Two silicon carbide coated carbon-carbon matrix porous media of square section were used. The upstream porous medium with a pore size of 8 pores per centimeter (20 pores per inch) served as the evaporation porous medium; the downstream porous medium with a pore size of 31 ppcm (80 ppi) was used as the combustion porous medium. The CME-Jet A fuel blends were injected from an air-blast atomizer into a coflow of hot air, which entered the evaporation porous medium. The combustion characteristics of three blends (volume percentages of CME equal to 25%, 50% and 75%) were studied at four different initial equivalence ratios. The global pollutant emissions, axial temperature profiles and the radiative heat fraction of the flame downstream of the combustion porous medium were measured. The results indicated that for lean air-fuel mixtures, the addition of CME to Jet A resulted in a reduction of the CO emission index. However, the NO x emission index was increased with the CME content in the blend for a given equivalence ratio. Also, the maximum flame temperature increased with equivalence ratio. In general, it was found that the porous-media burner was useful in reducing emissions and controlling flame temperatures.

In this paper, a (three) batten-reinforced fixed wing membrane micro air vehicle is used to determine the effect of membrane pre-strain on flutter and limit cycle behavior of fixed wing membrane Micro Air Vehicles. For each configuration tested, flutter and subsequent limit cycle oscillations are measured in wind tunnel tests and predicted using an aeroelastic computational model consisting of a nonlinear finite element model coupled to a vortex lattice solution of the Laplace equation and boundary conditions. Agreement between the predicted and measured onset of limit cycle oscillation is good as is the prediction of the amplitude of the limit cycle at the trailing edge of the lower membrane. A direct correlation between levels of strain and the phase of the membranes during the limit cycle is found in the computation and thought to also occur in the experiment.

The fluid and structural response of two different membrane wing Micro Air Vehicles is studied through computation and experiment. A (three) batten-reinforced fixed wing membrane micro air vehicle is used to determine the effect of membrane prestrain and fixed angle of attack on flutter and limit cycle behavior of fixed wing membrane Micro Air Vehicles. For each configuration tested, flutter and subsequent limit cycle oscillations are measured in wind tunnel tests and predicted using an aeroelastic computational model consisting of a nonlinear finite element model coupled to a vortex lattice solution of the Laplace equation and boundary conditions. Correlation between the predicted and measured onset of limit cycle oscillation is good as is the prediction of the amplitude of the limit cycle at the trailing edge of the lower membrane. A direct correlation between levels of strain and the phase of the membranes during the limit cycle is found in the computation and thought to also occur in the experiment. The second membrane wing micro air vehicle configuration is that of a plunging membrane airfoil model. This model is studied computationally using a sixth-order finite difference solution of the Navier-Stokes equations coupled to a nonlinear string finite element model. The effect, on the structural and fluid response, of plunging Strouhal number, reduced frequency and static angle of attack is examined. At two degree angle of attack, and Strouhal number of 0.2, the effect of increasing the plunging reduced frequency is to decrease the sectional lift coefficient and increase the sectional drag coefficient. At this angle of attack, minimal change in the sectional lift coefficient is found when increasing from a Strouhal number of 0.2 to 0.5 at reduced frequencies of 0.5 and 5.903, the lowest and highest values of this parameter which are studied in this work. For this angle of attack the maximum change which occurs when increasing the Strouhal number from 0.2 to 0.5 is at a reduced frequency of 1.5. When the effect of angle of attack is studied, it is found that at a Strouhal number of 0.5 and reduced frequency of 1.5 the plunging flexible model demonstrates improved lift characteristics over the fixed flexible airfoil case. The greatest improvement occurs at an angle of attack of 2 degrees followed by 10 degrees and then 6 degrees. Finally the effect on the flow characteristics of airfoil flexibility is investigated by increasing the membrane pre-strain from a nominal value of 5 percent to that of 20 percent. This increase in pre-strain results in a reduced value of sectional lift coefficient as compared the 5 percent pre-strain case at the same fixed angle of attack, Strouhal number and reduced frequency.

A theoretical and experimental investigation of slip flow over an ultrahydrophobic surface in a rectangular cross-sectional channel was conducted. The experiments consisted of the flow of water over a LOTUS-paint coated surface in a rectangular channel. The tests included flows at three different Reynolds numbers, Re = 129.6, Re = 103.6, Re = 69.1. Velocity measurements were made using a Laser Doppler Velocimeter (LDV). Velocity measurements for the no-slip conditions were also made for a flow rate of Re = 129.6 and compared with the slip profile. An uncoated surface was used for these measurements. The velocity profile and the flow rate for fully-developed channel flow were derived as a function of the slip length and slip velocity. The experimental data was used to determine the slip velocities and slip lengths for different Reynolds numbers from the theoretical equation. These quantities, used in the theoretical equation generated the necessary theoretical profiles for comparison with experiments. The slip velocities and slip lengths were evaluated by considering two locations, y = h and y = h/3. The profiles drawn from the theoretical equation evaluated from the slip velocities and lengths using the locations y = h and y = h/3 compared reasonably well with the experimental measurements. The volumetric flow rates computed theoretically using the slip lengths and slip velocities also compared well with the experimental values. The measurements highlight the effects of slip on one surface of the channel.