Laminar flame speed

Key takeaways

Laminar flame speed belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Laminar flame speed to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Laminar flame speed from memory before moving on to harder problems.

Reference excerpt

Laminar flame speed is an intrinsic characteristic of premixed combustible mixtures. It is the speed at which an un-stretched laminar flame will propagate through a quiescent mixture of unburned reactants. Laminar flame speed is given the symbol sL. According to the thermal flame theory of Ernest-François Mallard and Le Chatelier, the un-stretched laminar flame speed is dependent on only three properties of a chemical mixture: the thermal diffusivity of the mixture, the reaction rate of the mixture and the temperature through the flame zone:

s L ∘ = α ω ˙ ( T b − T i T i − T u ) {\displaystyle s_{\mathrm {L} }^{\circ }={\sqrt {\alpha {\dot {\omega }}\left({\dfrac {T_{\mathrm {b} }-T_{\mathrm {i} }}{T_{\mathrm {i} }-T_{\mathrm {u} }}}\right)}}}

α {\displaystyle \alpha } is thermal diffusivity,

ω ˙ {\displaystyle {\dot {\omega }}} is reaction rate, and the temperature subscript u is for unburned, b is for burned and i is for ignition temperature. Laminar flame speed is a property of the mixture (fuel structure, stoichiometry) and thermodynamic conditions upon mixture ignition (pressure, temperature). Turbulent flame speed is a function of the aforementioned parameters, but also heavily depends on the flow field. As flow velocity increases and turbulence is introduced, a flame will begin to wrinkle, then corrugate and eventually the flame front will be broken and transport properties will be enhanced by turbulent eddies in the flame zone. As a result, the flame front of a turbulent flame will propagate at a speed that is not only a function of the mixture's chemical and transport properties but also properties of the flow and turbulence.

See also Flame speed Chemical kinetics Activation energy asymptotics

References

Worked examples

Example 1 — a first encounter with Laminar flame speed

Start with the simplest possible case. Write down what Laminar flame speed claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Laminar flame speed before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Laminar flame speed ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Laminar flame speed

In research: Laminar flame speed appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Laminar flame speed in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Laminar flame speed is common in secondary-school and first-year university syllabi. It links to neighbouring topics Combustion, so understanding it makes those chapters shorter.
In everyday life: Look for Laminar flame speed outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Laminar flame speed in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Laminar flame speed means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Laminar flame speed out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Laminar flame speed in simple terms?

Laminar flame speed is an intrinsic characteristic of premixed combustible mixtures. It is the speed at which an un-stretched laminar flame will propagate through a quiescent mixture of unburned reactants.

Why does Laminar flame speed matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Laminar flame speed?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Laminar flame speed.