24 Questions
What occurs in the Atomization Zone of a liquid propulsion system?
Mixing of oxidizer and fuel vapors primarily occurs
What is a characteristic of premixed flames?
They do not require time for reactant mixing
What is a consequence of diffusive flames in propulsion systems?
They reduce the risk of explosive reactions
What is true about the temperature gradient in combustion systems?
It weakens the heat feedback over time
What is a characteristic of the Rapid Combustion Zone in liquid propulsion systems?
It is a highly turbulent environment
What type of flames are commonly found in standard liquid rocket engines?
Diffusive flames
What is a consequence of premixed combustion in confined volumes?
It leads to supersonic flame speeds
What is a key difference between premixed and diffusive flames?
Premixed flames do not require time for reactant mixing
What is the primary concern with nitrous oxide fuel blends in propulsion systems?
Potential for flame back propagation
Why do faster flames tend to stretch the reaction zone?
Because the flame's velocity affects its propagation
What occurs when the reaction zone is excessively stretched?
Incomplete combustion occurs, leading to flame extinction
Where does the highest temperature gradient occur in a premixed flame?
In the reaction zone
What is the purpose of metallic systems in propulsion systems?
To quench back-propagating flames
How do reactants entering the reaction zone experience reduced reaction rates?
Due to reduced heat feedback
What is the primary location of energy loss in a premixed flame?
In the reaction zone
Why do diffusive flames tend to be slower than premixed flames?
Due to the time needed to diffuse and mix
What sustains the flame in combustion chambers?
Heat feedback
What happens when heat feedback is disrupted in combustion chambers?
The reaction halts
What type of flame involves a well-mixed combination of oxidizer and fuel?
Premixed flames
What determines the burning speed in premixed flames?
Global reaction rate
What is the characteristic of diffusive flames?
Separate injection points or ducts for oxidizer and fuel
Why does the flame not propagate downwards beneath the cooking plane in kitchen stoves?
Due to the speed of diffusion of heat
What is the role of the metal plate atop the stove in kitchen stoves?
To act as a flame-stopper
What is the difference between premixed and diffusive flames?
Diffusive flames are slower than premixed flames
Study Notes
Flame Generation
- Heat feedback plays a crucial role in flame generation, as it propagates backward, heating incoming reactants and sustaining the flame.
- Disrupting heat feedback halts the reaction, similar to blowing out a candle.
Types of Flames
- There are two primary types of flames: premixed and diffusive.
- Premixed flames involve a well-mixed combination of oxidizer and fuel, where chemical kinetics dictate the reaction speed.
- Examples of premixed flames include Otto cycle engines and Bunsen burners.
- Diffusive flames entail separate injection points or ducts for oxidizer and fuel, where diffusion governs the reaction.
- Examples of diffusive flames include kitchen stoves.
Characteristics of Premixed and Diffusive Flames
- Premixed flames have higher reactivity and are prone to explosions, deflagration, or detonation.
- Diffusive flames are slower due to the time required for reactant mixing, making them less prone to explosive reactions.
- Premixed conditions allow instant reaction without fluid-dynamic slowdown, potentially causing violent combustion.
- Diffusive flames are commonly found in standard liquid rocket engines, microscale combustion in composite solid propellants, and hybrid rocket propulsion systems.
Combustion Process in Liquid Propulsion Systems
- The combustion process unfolds in three distinct regions: atomization, rapid combustion, and vaporization.
- The atomization zone is where liquid fuel is atomized into droplets and mixed with oxidizer vapor.
- The rapid combustion zone is where rapid combustion takes place in a highly turbulent environment.
Characteristics of Diffusive Flames in Propulsion Systems
- Diffusive flames are slower than premixed flames due to the time required for diffusion and mixing.
- Nitrous oxide fuel blends (NOFB) offer promising performance and simplified management in propulsion systems.
- Metallic systems are utilized to quench back-propagating flames and ensure safety and efficiency in propulsion systems.
Premixed Flame Profile
- The temperature profile of a premixed flame shows a strong gradient in the reaction zone.
- Fresh gases enter from the left, while combusted gases exit from the right.
- The heat flux moving from right to left is a key factor in the energy balance equation.
Understanding the role of heat feedback in combustion chambers, where oxidizer and fuel mix, and how it sustains flame generation. Explore the process of continuous reaction and heat propagation.
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