4.2 KiB
Chemiluminescence in Hydro-Flames — CH* and OH*
Overview
Chemiluminescence in flames arises when chemical reactions produce excited radicals that emit photons as they relax.
The two most important species for combustion diagnostics are:
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CH* (≈ 431 nm, blue)
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OH* (≈ 310 nm, UV)
These are not just “present radicals” — they are selectively excited products of specific reactions, and their relative intensity encodes combustion chemistry.
Key Reactions
CH* Formation (fuel breakdown zone)
CH* is associated with hydrocarbon fragmentation and early flame chemistry.
Typical pathways:
\mathrm{C_2H + O \rightarrow CH^* + CO}
\mathrm{CH_2 + O \rightarrow CH^* + OH}
\mathrm{C + H_2 \rightarrow CH^* + H}
These reactions dominate in fuel-rich or near-front regions of the flame.
OH* Formation (oxidation zone)
OH* is associated with high-temperature oxidation chemistry.
Typical pathways:
\mathrm{H + O_2 \rightarrow OH^* + O}
\mathrm{O + H \rightarrow OH^*}
\mathrm{H_2 + O \rightarrow OH^* + H}
These reactions dominate in lean or well-oxidised regions.
Why Excited States Are Produced
A chemical reaction releases energy:
\mathrm{A + B \rightarrow C + D + energy}
This energy can be distributed into:
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Translational motion (heat)
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Vibrational energy
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Rotational energy
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Electronic excitation
If enough energy is deposited into the electronic degree of freedom:
\mathrm{OH^* \rightarrow OH + h\nu}
\mathrm{CH^* \rightarrow CH + h\nu}
The emitted photon is what we detect.
Non-Equilibrium Nature
These reactions occur under:
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High temperature
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Fast radical–radical collisions
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Non-equilibrium conditions
Therefore:
Products are often formed directly in excited states, rather than being thermally excited.
Why Only Certain Species Emit Strongly
Not all reactions produce visible/UV light. Strong chemiluminescence requires:
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Sufficient reaction energy (~2–5 eV)
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Good overlap between reaction geometry and excited state (Franck–Condon principle)
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Fast radiative decay
CH* and OH* satisfy these conditions.
Energy Scale
Typical photon energies:
E = h\nu
CH* emission (~431 nm):
E \approx 2.9 \text{ eV}
OH* emission (~310 nm):
E \approx 4.0 \text{ eV}
Spatial Structure of the Flame
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CH* peaks in the reaction front (fuel breakdown)
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OH* peaks slightly downstream in the oxidation zone
Thus, they probe different regions of the flame.
The OH*/CH* Ratio
Define:
R = \frac{OH^*}{CH^*}
This ratio reflects:
R \propto \frac{\text{oxidation rate}}{\text{fuel fragmentation rate}}
Interpretation
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Fuel-rich:
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High CH*
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Lower OH*
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R \text{ small}
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Lean:
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High OH*
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Lower CH*
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R \text{ large}
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Stoichiometric:
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Balanced
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R \text{ intermediate}
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Why the Ratio is Useful
Absolute intensity depends on:
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Optical alignment
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Flame size
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Window fouling
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Sensor gain
But the ratio:
\frac{OH^*}{CH^*}
cancels many of these effects and provides a robust indicator of combustion state.
Physical Interpretation
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CH* → “fuel is still breaking apart”
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OH* → “oxidation is occurring”
So:
The ratio indicates where the flame sits between fuel-dominated and oxidation-dominated chemistry.
Deeper Insight
Even though both species arise from excited states:
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CH* is fed by hydrocarbon radical pathways
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OH* is fed by oxidation pathways
Thus the ratio is not just energy-related — it reflects:
Which chemical pathways dominate locally
Summary
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Chemiluminescence arises from excited radicals formed during reactions
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CH* and OH* originate from different parts of the combustion network
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Their ratio provides a real-time optical measure of stoichiometry and flame structure