Sep 19, 2012
INTRODUCTION
The aim of atomization is to substantially increase the surface area of the liquid to enhance vaporization, mixing and combustion. The end result is that the liquid jet becomes unstable which leads to the disintegration of the liquid surface into droplets. This surface area increase can be achieved in various ways and shear coaxial jet injector atomization process is one of them. The breakup of the liquid jet is a result of complex interactions between inertial, viscous and surface tension forces. Aerodynamic forces promotes disturbances on the surface while viscous forces have a damping effect. Surface tension tends to pull the liquids together. Turbulence and pressure oscillations in the injected fluids affect atomization. The non dimensional parameters such as Reynolds number, Weber number, Mixture ratio and Ohnesorge number help in characterizing the overall process. Le Visage, D. showed that both the momentum and density ratios determine the breakup length of the liquid core. By plotting the Ohnesorge number vs the Reynolds number, one can distinguish between
a) low-velocity region, where the breakup is due to the action of surface tension forces and
b) high-velocity region, where the influence of aerodynamic forces increases exponentially with Reynolds number at constant Ohnesorge number.
THE PROCESS OF DISINTEGRATION
1) The Primary Atomization zone: In the near field of the injector nozzle, the huge difference in the velocity between the gas and liquid, leads to a surface instability and formation of filaments or drops from the jet surface. This is the primary atomization zone.
2) The Secondary Atomization zone: In the far field of the injector nozzle, the fluid velocity decreases due to mixing with the external atmosphere which leads to instability. The large droplets and ligaments produced in the primary atomization zone and in the jet breakup zone, breakup further into smaller and more stable droplets, depending on the local weber number. The breakup time of the droplets can be expressed as a function of local relative velocity and ratio of gas to liquid density. The breakup time and initial droplet velocity determines the distance from the injector where secondary atomization takes place to the flame front.
CLASSIFICATION OF BREAKUP REGIMES
i) based on Weber number
Extensive experimental study on round liquid jets under conditions of with and without co-flowing gas stream were carried out by Farago, Z. and Chigier, N. They observed
1) a Rayleigh type breakup, which is further divided into two subgroups
a) axisymmetric breakup (We < 15)
b) non axisymmetric breakup (15 < We < 25) and
2) a Membrane type breakup (25 < We < 70), where the round jet develops into a thin sheet, which forms Kelvin-Helmholtz waves and breaks up into drops
3)a fiber type breakup (100 < We < 500).
ii) based on Mixture ratio
Based on previous studies and experimental work carried out by Gomi, N., the breakup regime is classified into three categories depending on the mixture ratio (MR).
1) MR < 0.2, relative velocity determines the drop size
2) 0.2 < MR < 1, relative velocity and mixture ratio determines the drop size
3) MR > 1, many parameters affect the drop size
The basic atomization phenomena that converts primary liquid jets into droplets is not yet fully understood. No unified theory is currently available and experimental investigations are the best way to characterize a given injection element.
Aug 19, 2012
FLUENT is a computational fluid dynamics (CFD) software which consists of modeling capabilities needed to simulate flow, turbulence, heat transfer and chemical reactions for a wide range of applications. It is an integral part in the design and optimization process of any product development.
FLUENT has a wide range of boundary conditions that allows the flow to enter and exit the domain. This article helps you in selecting the appropriate boundary conditions for your specific application.
There are about 10 different types of flow inlet and exit boundary condition options in FLUENT.
1) VELOCITY INLET - It is used to define the velocity and other properties of the flow at the inlet. Intended for in-compressible flows
2) PRESSURE INLET - It is used to define the total pressure and other properties of the flow at the inlet. Flow direction must be defined else non-physical results can occur. Suitable for both compressible and in-compressible flows. Outflow can occur at pressure inlet conditions.
3) MASS FLOW INLET - It is used in compressible flows to define the mass flow rate at the inlet. It is not necessary in in-compressible flows as the velocity inlet itself fixes the mass flow rate.
4) PRESSURE OUTLET - It is used to define the static pressure at the outlet. It often gives better rate of convergence when back-flow occurs. Back-flow can occur at pressure outlet conditions and is assumed to be normal to the boundary. This must be used when problem is set up with pressure inlet.
5) PRESSURE FAR-FIELD - It is used to model free stream compressible flow at infinity, with free stream mach number and static conditions specified. This is available only for compressible flows when density is calculated from ideal gas law.
6) OUTFLOW - It is used to model flow exits where flow velocity and pressure are not known prior to solution of the flow. It cannot be used for compressible flows, with pressure inlet boundary condition and in unsteady flows with variable density. Can be used with velocity inlet.
7) INLET VENT - It is used to model inlet vents with specified loss coefficient, flow direction and inlet pressure and temperature.
8) INLET FAN - It is used to model an external intake fan with specified pressure jump, flow direction and intake pressure and temperature.
9) OUTLET VENT - It is used to model an outlet vent with specified loss coefficient and discharge static pressure and temperature.
10) EXHAUST FAN - It is used to model an external exhaust fan with a specified pressure jump and discharge static pressure.
These boundary conditions help design and analyze the domain and model the flow through it.
Jun 20, 2012
Stealth or Low Observability is one of the most misunderstood concepts by the common man. Stealth aircraft are considered to be the invisible aircrafts that dominate the skies. But, in simple terms, stealth technology allows an aircraft to be partially invisible to Radar or any other means of detection. This is similar to the camouflage used by soldiers in jungle warfare. Unless he comes close to you, you cant see him. Before getting into the stealth technology, we must first know how a radar works. The radar sends out radio waves which is reflected back by any object it happens to encounter. The radar antenna measures the time taken by the wave to return and with that information it can tell how far the object is. The metal body of the aircraft is a very good reflector of radar signals, and this makes it easy to find and track planes with radar equipment.
The goal of stealth technology is to make an aircraft invisible to radar. It is a combination of technologies.
1) The aircraft should be shaped so that any radar signals it reflects are reflected away from the radar equipment.
2) The aircraft should be covered in materials that absorb radar signals.
3) Reducing visibility and infrared signature.
Usually conventional aircraft have rounded shape which makes them aerodynamic but it also creates a very efficient radar reflector. The round shape means that wherever the radar signal hits the plane, some of the signal gets reflected back. A stealth aircraft on the other hand has completely flat surfaces and very sharp edges which reflects the radar signals away from the radar antenna. The most efficient way to reflect radar waves back to the emitting radar is with orthogonal metal plates, forming a corner reflector consisting of either dihedral (two plates) or a trihedral (three orthogonal plates). This configuration is used in the tail of conventional aircrafts, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical method is to eliminate the tail completely. In addition to altering the tail, the engines must be buried within the wing or fuselage, install baffles in the air intakes so that turbine blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions such as weapons, fuel tanks etc. and they must not be carried externally. These shaping requirements have strong negative effects on the aircraft's aerodynamic properties and hence they are inherently unstable and cannot be flown without a fly-by-wire control system.
In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy and convert it into heat rather than deflecting them in other directions. Commonly used radar absorbent materials are iron ball paint and foam absorber. The simplest stealth technology is simply camouflage by using paint or other materials to blend with the environment or by resembling something else. Most stealth aircraft use matte paint and dark colors and operate only at night. With interest in daylight stealth, emphasis is on the use of gray paint in disruptive schemes. Usually planes are visible in thermal imaging systems because of the high temperature exhaust they give out. The exhaust plume contributes a significant infrared signature. This is a great disadvantage to aircrafts as they are vulnerable to missiles with IR guidance system. By minimizing the exhaust cross sectional volume and maximizing the mixing of hot exhaust with cool ambient air, IR signature can be reduced. Another way to reduce the exhaust temperature is to circulate coolants such as fuel inside the exhaust pipe.
The stealth aircrafts cannot fly as fast or are not maneuverable like conventional aircrafts. The reduced amount of payload it can carry and its sheer cost are the major factors that sharply reduced their research and development. As air defense systems are becoming more and more accurate and deadly, stealth technology can be a decisive factor for any country over the other. Nowadays the stealth technology is being incorporated in ships, helicopters and tanks as well and in the coming years we can see many more advancements in the field of military aviation.
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Jun 19, 2012
OVERVIEW
Everyone gets fascinated when it comes to Space, many of us couldn't afford to go to into space because it is just that expensive. But what if a commercial, cost effective spacecraft can take off from the ground on its own, travel into space and return? Well, this is exactly what XCOR Aerospace has managed to do with its EZ-Rocket. It is the first rocket plane to be built and flown by a private organization. It is modified from Burt Rutan's Long-EZ home-built fixed wing, canard aircraft manufactured by Rutan's Aircraft Factory. It has good gliding characteristics which makes it ideal for a rocket plane.
ENGINE
The four cylinder air cooled, piston aircraft engine Lycoming O-320 with constant speed propeller of the Long-EZ is replaced by a pair of 400 lbf thrust, pressure fed regeneratively cooled, non throttle-able, restart-able liquid fueled rocket engines. A pressurized fuel tank filled with isopropyl alcohol and two Styrofoam insulated aluminium tanks that hold liquid oxygen are placed at the bottom and top of the plane respectively.
PERFORMANCE
Thrust :- 800 lbf (both engines together)
Take-off roll :- 1650 ft (500 m) in 20 seconds
Max. climb rate :- 10000 ft/min (52 m/s)
Max. altitude attained :- 11500 ft
Never exceed speed :- 195 knots
Sound level :- 128 dB at 10 meters
SAFETY FEATURES
Just like any other plane, the XCOR EZ-Rocket has a lot of safety features installed for the safety of the pilot. The canopy is quick to open and the pilot has a parachute in case of emergency exit. The engine has its own Kevlar blast shield. An ultraviolet fire sensor illuminates a light on the instrument panel in the event of engine fire. The plane is equipped with large bottles of pressurized helium which are used as fire extinguishers when engine catches fire. The pilot can manually shut off both fuel and oxidizer supply to the engines if a fire is detected or engine fails to shut down. A burn through sensor signals the pilot when the fuel tank is empty.
After 26 successful flights of the EZ-Rocket, XCOR Aerospace is now focusing on the development of rocket racers and a suborbital spacecraft Xerus for space tourism and to launch micro satellites.
Jun 10, 2012
A Cycloidal Rotor consists of several blades that rotate about a horizontal axis that is perpendicular to the direction of flight. Blade span is parallel to the axis of rotation and the pitch angle of each of the blades is changed periodically as the blade moves around the azimuth of the rotor.
Blades at the top and bottom produce a vertical lifting force while those at the left and right produce very little force because of their small angle of attack. When resolved into vertical and horizontal directions, the sum of horizontal components is zero, resulting in a vertical thrust. A unique and desirable characteristic of cycloidal blade system is its ability to change direction of thrust enabling any vehicle utilizing this system to take-off and land vertically, hover and to fly forward or reverse by changing the direction of thrust. For implementation on a vehicle, two cycloidal rotors would be necessary, one on each side of the fuselage.
Lateral motion and roll control is achieved through differential control of the magnitudes of the two vectors.
Yawing motion is accomplished through directional control of thrust vectors.
ADVANTAGES:
It provides the same hover capability as a conventional rotor. However unlike a conventional rotor, the blades on a cycloidal rotor operate at constant speed along the entire blade span, allowing all the elements operate at their peak efficiencies.
Cycloidal rotors operate at much lower rotational speeds than conventional rotors, and as such the acoustic signature should be significantly lower.
The greatest advantage of this design is the possibility of greater thrust to power ratios than can be achieved by a conventional rotor.
DISADVANTAGES:
The mechanism required to achieve the periodic pitch changes for each of the blades is by nature more complex than what is required for a conventional rotor.
The complex flow surrounding the rotor makes analysis of cycloidal propulsion difficult.
Weight of the rotor is another problem. The huge no. of components necessary for operation, i.e. multiple blades, bearings and linkages incur more weight penalty compared to a conventional rotor.
REFERENCES:
http://www.inderjitchopra.umd.edu/projects/proj10.html
http://drum.lib.umd.edu/bitstream/1903/3068/1/umi-umd-2875.pdf
May 22, 2012
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Mar 30, 2012
The pintle injectors enjoy several advantages over other types of liquid bipropellant injectors. The design is inherently simpler than the face type injectors in the sense that there is only one injector element, but the single element can have multiple holes. In any case the pintle injectors have lesser number of injection sites than the face type injectors. The second advantage is it's inherent combustion stability. The pintle engines have never reported any cases of combustion instability which reduces risk and the need for stability aids such as baffles. The third attractive feature of the pintle injector is its throttleability. Throttling ratios of 10-20 : 1 have been demonstrated with hypergolic propellants.
The injector flows and combustion have been much less studied than those of flat face type injectors and all the designs and analysis are within the industry and not available for general public. The major issues of concern that tend to complicate the development of these injectors are as follows. Manufacturing issues related to maintaining the required gap between injector holes. As the pintle tip lies in the re-circulation zone, it is subjected to high heat flux. In engines using hypergolic propellants, because of local combustion, the pintle tips can get damaged. To alleviate this, size of re-circulation zones should be minimized.
Very little research work has been carried out on these injectors and much information related to them is not available in open literature. By integrating the design of the pintle injector with that of the combustion chamber, better results can be achieved and it even helps reduce the size of the combustion chamber.
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Jan 3, 2012
INTRODUCTION
The cost of launching spacecrafts using expendable vehicles is very high and efforts are being made to reduce these costs significantly. This reduction in costs can be achieved either by cutting down the overall weight and cost of the vehicle by selecting suitable low cost materials and optimize the design of the components or by opting for reusable vehicles so that the initial high development costs can be recovered over number of flights.
The main issues pertaining to reusable aerospace vehicles are a) recovery and reflight, b) maintainability, c) reusable materials, d) thermal management etc. The intense aero-thermal loads to which the vehicle is subjected during its flight, reentry and stringent mass budget make selection of materials for reusable aerospace vehicles a challenging task.
The main considerations for the development of Reusable Launch Vehicle's (RLV) are
1. they can bring down the launch cost substantially as compared to that of expendable vehicles.
2. their design having in-built abort and emergency landing capabilities would fructify mission success probability and overall safety.
3. retrieval of payloads for overhaul and reuse, in-orbit servicing of space systems and re-fueling of satellites are possible.
4. they can avoid debris in orbit and loss of pricey materials, and can cut down environmental pollution.
MATERIALS FOR REUSABLE AEROSPACE VEHICLES
The materials for reusable aerospace vehicles can be classified as
1. Air-frame materials
2. Thermal protection materials
AIR FRAME MATERIALS
Sizeable cost savings can be obtained by minimizing the overall weight and part count. Air frame weight reduction can be normally achieved by the use of lighter materials and using efficient structural designs.
Concisely, materials for constructing air-frame should have high
1. stiffness
2. stress corrosion resistance
3. fracture toughness
4. fatigue strength
5. creep resistance
6. ease of fabrication and repair.
Aluminium Alloys
Most commonly used Aluminium alloys are AA2024 and AA7075. Modifications to the base alloy composition resulted in higher fracture toughness alloys such as AA7175 and AA7475. AA7150, AA7055 and AA2524 have higher compression yield strength, corrosion resistance and fatigue crack growth resistance.
Composite Materials
They have very high strength and resistance to corrosion and fatigue. Their properties can be tailored to meet the specific needs and they can be formed to complex shapes. High strength composites such as Carbon Fibre Reinforced Polymer (CFRP) and Graphite-Epoxy are used for making Air-frame structures.
THERMAL PROTECTION SYSTEMS
The factors which lead to heating of the external surface of the reusable vehicle are aerodynamic heating and the thermal properties of the materials used in making the air-frames. The aerodynamic heating is dependent on flight profile of the vehicle, i.e. the angle of attack, mach number, body geometry, pressure etc. The thermal properties such as emissivity, absorptivity, catalycity and conductivity of the external surface decides the heat load acting on the reusable vehicle.
Reinforced Carbon-Carbon (C-C) composites have operating range of -150K to about 2000K
Carbon/Silicon Carbide Ceramic Matrix Composites (C/SiC) can operate up to 1800K
Silicon Carbide/Silicon Carbide Ceramic Matrix Composites (SiC/SiC) are resistant up to 1600K
Alumina borosilicate (ABS)-silica ceramic tiles can withstand temperatures up to 1600K
Toughened Unified Fibre Insulation (TUFI) tiles can operate up to 3000K
Carbon aerogels up to 3200K and tiles coated with carbides of hafnium, zirconium and titanium can withstand temperatures as high as 3250K
Since the prime requisite of the materials selected for fabrication is re-usability, the testing and quality requirements have to be very rigorous and precise. The development of reusable aerospace vehicles can result in lower launch costs of satellites as compared to the use of expendable launch vehicles.
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