Increase In Fuel Flow Rate Research Articles

Flow Boiling of Liquid n-Heptane in Microtube with Various Fuel Flow Rate: Experimental and Numerical Study.

The evaporation of liquid hydrocarbon n-heptane is discussed in detail with experimentation and numerical techniques. A maximum wall temperature of 1050 K was reported during an experimental process with a two-phase flow that was stable and had a prominent meniscus at a small fuel flow rate (FFR) ≤ 10 µL/min. At medium to high FFR (30-70 µL/min), the flow field was unstable, with nucleating bubbles and liquid droplets inside the microtube and the maximum temperature recorded was 850 K for 70 µL/min. For the numerical model, the temperature of the wall was used as a boundary condition. Using the numerical model, the evaporative flux at the meniscus, pressure drop, pressure oscillation, and heat transfer coefficient (HTC) were investigated. A single peak in HTC was obtained at a low fuel flow rate, while multiple peaks were obtained for high FFR. At low FFR, the pressure peak was observed to be 102.4 KPa, whereas at high FFR, the pressure peak increased to 105.5 KPa. This shows a 2% increase in pressure peak with an increase in FFR. Similarly, when the FFR increased from 5 µL/min to 70 µL/min, the pressure drop increased from 500 Pa to 2800 Pa. The high amplitude of pressure drops and a high peak of HTC were found, which depend on the mass flow rate. The coefficient of variation for pressure drop depends mainly on the fuel flow rate.

Numerical study on pre-film atomization mechanism and characteristics by a coaxial swirl injector

The overall process and mechanism of the centrifugal pre-film atomization with double swirling flow were studied using the methods of large Eddy simulation and volume of fluid. The atomization process includes a centrifugal jet under the primary swirl and a pre-film atomization under the two-stage counter-rotating swirl at the venturi outlet. The fuel is ejected from the outlet of the centrifugal nozzle and undergoes the transient process of reaching the venturi throat. The breaking mechanism of liquid film in this process is the same as that of the formation mechanism of the mushroom-shaped tip of liquid jet. The numerical simulation results are highly consistent with the experimental results. For the formation and development of the liquid film on the venturi wall, collision and wave action promote the expansion of the liquid film. At the outlet position of the venturi tube, the short wave mode and the two-stage reverse swirling structure play major roles in the fragmentation process of the flake liquid film, which coincides with the flow characteristics given by the experiment. It is found that the spray cone angle increases as the fuel flow rate increases, and the numerical results are basically consistent with the predicted values of the empirical formula under different fuel flow rates. The droplet size distribution showed a Poisson distribution during the atomization of centrifugal jets and pre-film, and the peak position and variation trend of the droplet size distribution at the outlet of the venturi tube were basically consistent with experimental results.

Diffusion Combustion of Liquid Heptane in a Small Tube with and without Heat Recirculating

In order to understand diffusion flame characteristics in a small tube, combustion of liquid n-heptane and air was experimentally and numerically studied. A tube of ID 4 mm and OD 6 mm made of quartz was used as the burner. Liquid n-heptane was delivered into a capillary from a syringe pump. Stable flames were established inside the burner with and without heat recirculating. Additionally, numerical simulations were conducted, and effects of equivalence ratio and external heat loss coefficient on diffusion flame were studied. Results show that, for a diffusion flame of liquid n-heptane in a small tube, as fuel flow rate increases, the flammable limits increase. The diffusion flame position moves downstream with increasing air flow, eventually stabilizing at the bottom of the outer tube until extinction. When the flame passes in the tube, the peak temperature would occur on the wall. If there is heat recirculating, the wall temperature of the inner tube is higher than the boiling point of liquid n-heptane. It is conducive to the pre-evaporation of liquid n-heptane. In contrast, if there is no heat recirculating, liquid fuel will be accumulated in the tube. The heat loss coefficient has a great influence on flammable limits of the tube burner without heat recirculating.

Performance Analysis on Planar Micro PDMS Proton Exchange Membrane Fuel Cell Stack

The objective of this study is to develop a micro proton exchange membrane fuel cell (PEMFC) stack, in which a flow field plate in each cell is made of polydimethyl-siloxane (PDMS) and manufactured by micro-molding technology. A four-cell micro PEMFC stack in series with a volume of 12 cm × 4.5 cm × 1.2 cm is constructed and tested. The active area of a membrane electrode assembly (MEA) is 2 cm × 2 cm in each individual PEMFC, and the platinum loadings at anode and cathode are 0.3 and 0.5 mg/cm^2, respectively. A series of performance experiments were carried out. Experimental parameters included clamping force and fuel supply condition. Durability tests were carried out as well. Experimental results showed that an appropriate clamping torque range should be considered carefully to enhance performance without narrowing down fuel flow channels. Under a torque of 2.0 kgf•cm, peak power density for this stack is 122.5 mW/cm^2 at 2.0 V. Increase in fuel flow rate is not the best way to improve concentration polarization in this air-breathing stack experiment because of limited air supply, and each cell in the same stack has a different performance under the present arrangement. At present, such stack can maintain a stable power output for a long time (up to 14 hours).

Evaluation of Colloidal Pd and Pd-alloys as Anode Electrocatalysts for Direct Borohydride Fuel Cells Applications

Borohydride oxidation on supported Pd and Pd-alloy nano-electrocatalysts (Pd, Pd-Ir, Pd-Ni, Pd-Au, and Pd-Ag) prepared by a modified Bonneman method has been investigated using cyclic voltammetry (CV), rotating disk electrode (RDE) voltammetry, and single fuel cell test station. Electrochemical parameters, such as Tafel slopes, exchange current densities, oxidation peak potentials, and fuel cell performance, have been determined. The electrochemical parameters impact on the cell performance is considered as important as that of the operating conditions. The influences of temperature, and fuel and oxidant flow rates, on the fuel cell performance were also evaluated. The current density of the borohydride fuel cell increases with increase in temperature for all investigated electrocatalysts. The increase in current density with increase in fuel flow rate was not as high as expected within the investigated flow rate range. This is in a good agreement with RDE results. A fuel cell employing Pd-Ir as the anode catalyst and operated at a cell voltage of 0.5 V, can give a current density of 50 mA cm n2 at 298 K, while the same catalyst and at same cell voltage can give 110 mA cm n2 at 333 K.

Horn Geometry Effects on Atomization Characteristics of a Micro Nozzle Array Type Ultrasonic Fuel Injector

Effects of horn geometry on the atomization characteristics of an ultrasonic fuel injector using a micro nozzle array were investigated experimentally. Micro nozzles whose exit diameter d=3μm are mounted on a thin metal film. The number of the micro nozzles is from 2.0×104 to 1.2×105. Using an ultrasonic oscillator, gasoline is periodically pushed out from the micro nozzles at a frequency from 62 to 65 kHz. A disk type PZT is used as an ultrasonic oscillator, and the oscillation is amplified using a step-type horn. The input voltage to the PZT is varied from 0 to 200 V. To increase the fuel flow rate, the horn small end diameter Ds is increased from 10.5 to 25 mm, while the large end diameter is fixed at 30 mm. To prevent forming a liquid film on the micro nozzle array, gutters are machined on the small end of the horn. It is shown that the SMD (Sauter mean diameter) of the spray is almost uniform around 10 to 14μm, regardless of the horn small end diameter. The fuel flow rate increases, at the maximum, 2.8 times larger when compared to the original horn geometry.

Numerical study of an internal-reforming solid oxide fuel cell and adsorption chiller co-generation system

A study is conducted on a cogeneration system that incorporates a natural gas fed internal-reforming solid oxide fuel cell (IRSOFC) and a zeolite/water adsorption chiller (AC). The main aim is to investigate the performance of this combined system under different operating conditions and design parameters. A mathematical model is developed to simulate the combined system under steady-state conditions. The effects of fuel flow rate, fuel utilization factor, circulation ratio, mass of adsorbent and inlet air temperature on the performance are considered. The results show that the proposed IRSOFC-AC cogeneration system can achieve a total efficiency (combined electrical power and cooling power) of more than 77%. The electrical efficiency is found to decrease as the fuel flow rate increases, while the cooling power increases to a constant value. The total efficiency reaches a maximum value with variation of the fuel utilization factor. Both the circulation ratio and the inlet air temperature exert positive impacts on system efficiency.

Bifurcations and structure of surface interacting methane–air diffusion flames

Methane–air diffusion flames near surfaces are modeled using numerical bifurcation theory. Ignition, extinction, and stability boundaries are studied as functions of fuel flow rate and oxidizer strain rate, using three reaction mechanisms of varying complexity (a one-step reaction, a C 1 reaction mechanism, and the Gas Research Institute [GRI 1.2] mechanism). It is found that as the fuel flow rate increases, the ignition temperature increases, whereas the extinction temperature decreases. For sufficiently high fuel flow rates and low oxidizer strain rates, the flame separates from the surface, and extinction cannot be caused by thermal quenching. In the absence of surface heat loss, two distinct extinction branches are found, namely, a blowoff or stretch limit branch at high fuel flow rates and a thermal quenching limit branch at low fuel flow rates. The complexity of the reaction mechanism affects primarily the blowoff branch at high fuel flow rates but not the thermal quenching branch due to the O 2 leaking through the reaction zone at low strain rates. Surface radiation alters the bifurcation behavior, beyond an absolute stability curve. The theoretically predicted flame structure and stability limits compare well with experimental results. Implications for modeling of extinction of complex condensed fuels are also discussed.

The effect of hydrogen and propane addition on the operation of a natural gas-fired pulsed combustor

A commercialy available nonpremixed pulsed combustion room heater designed to burn natural gas has been used to study the effect of gas composition on the operational characteristics of the combustor. Three parameters have been monitored as fuel flowrates and gas composition were varied, viz., the amplitude of the pressure oscillations, the operating frequency and the phase difference between the pressure and heat release oscillations. When burning methane only, an increase in fuel flowrate caused a decrease in the phase difference which led to an increase in chamber pressure. This is attributed to an increase in the ignition delay time as the fuel flowrate increases. Addition of hydrogen, however, caused a sharp drop in pressure, and increases in the operating frequency and in the phase difference. This is due to a decrease in the ignition delay time arising chiefly from the far higher burning velocity of hydrogen compared with methane. For hydrogen contents of over 20 vol%, the phase difference approached 90°, which means that Rayleigh's criterion is no longer satisfied. Under such conditions the combustor emitted large quantities of carbon monoxide and had clear difficulties in sustaining combustion. This explains previous reports that pulsed combustors are intolerant of hydrogen in the fuel gas. Addition of propane to the fuel gas has a much less marked effect; slight increases were recorded in phase difference, pressure and operating frequency, which were attributed to the marginally higher burning velocity of propane compared with methane.

Influence of ambient pressure on drop-size and velocity distributions in dense sprays

The primary aim of the research is to determine the capatibilities of modern noninvasive diagnostics for characterising the sprays produced by a practical gas turbine atomizer when operating at realistic engine conditions of pressure, fuel type, and fuel-air throughput. A single-velocity-component Phase Doppler Particle Analyzer is used to measure local variations of drop-size distributions and drop velocities along three spray radii at downstream distances from the atomizer of 50 and 70 mm. In the 50-mm plane, excessive signal rejection rates limit measurements to a maximum air pressure of 9 bar and a maximum kerosine flow rate of 18.6 g/s. At the 70-mm measurement plane, satisfactory results are obtained at air pressures up to 12 bar and fuel-flow rates up to 24.8 g/s. The results show that increases in ambient air pressure lead to larger mean drop sizes and lower mean drop velocities in the spray. This is attributed to the fact that the beneficial effect of an increase in air pressure in raising Weber number is more than offset by several adverse factors, all of which are related to the increase in fuel-flow rate that accompanies an increase in air pressure at constant fuel/air ratio.