266 lines
4.2 KiB
Markdown
266 lines
4.2 KiB
Markdown
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# Chemiluminescence in Hydro-Flames — CH* and OH*
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## Overview
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Chemiluminescence in flames arises when chemical reactions produce **excited radicals** that emit photons as they relax.
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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)
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These are not just “present radicals” — they are **selectively excited products of specific reactions**, and their relative intensity encodes combustion chemistry.
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---
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## Key Reactions
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### CH* Formation (fuel breakdown zone)
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CH* is associated with hydrocarbon fragmentation and early flame chemistry.
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Typical pathways:
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$$
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\mathrm{C_2H + O \rightarrow CH^* + CO}
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$$
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$$
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\mathrm{CH_2 + O \rightarrow CH^* + OH}
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$$
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$$
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\mathrm{C + H_2 \rightarrow CH^* + H}
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$$
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These reactions dominate in **fuel-rich or near-front regions** of the flame.
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---
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### OH* Formation (oxidation zone)
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OH* is associated with high-temperature oxidation chemistry.
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Typical pathways:
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$$
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\mathrm{H + O_2 \rightarrow OH^* + O}
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$$
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$$
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\mathrm{O + H \rightarrow OH^*}
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$$
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$$
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\mathrm{H_2 + O \rightarrow OH^* + H}
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$$
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These reactions dominate in **lean or well-oxidised regions**.
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---
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## Why Excited States Are Produced
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A chemical reaction releases energy:
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$$
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\mathrm{A + B \rightarrow C + D + energy}
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$$
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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**
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If enough energy is deposited into the electronic degree of freedom:
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$$
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\mathrm{OH^* \rightarrow OH + h\nu}
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$$
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$$
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\mathrm{CH^* \rightarrow CH + h\nu}
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$$
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The emitted photon is what we detect.
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---
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## Non-Equilibrium Nature
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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
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Therefore:
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> Products are often formed **directly in excited states**, rather than being thermally excited.
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---
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## Why Only Certain Species Emit Strongly
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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
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CH* and OH* satisfy these conditions.
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---
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## Energy Scale
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Typical photon energies:
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$$
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E = h\nu
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$$
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CH* emission (~431 nm):
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$$
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E \approx 2.9 \text{ eV}
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$$
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OH* emission (~310 nm):
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$$
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E \approx 4.0 \text{ eV}
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$$
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---
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## 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**
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Thus, they probe different regions of the flame.
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---
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## The OH*/CH* Ratio
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Define:
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$$
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R = \frac{OH^*}{CH^*}
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$$
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This ratio reflects:
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$$
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R \propto \frac{\text{oxidation rate}}{\text{fuel fragmentation rate}}
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$$
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---
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### 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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---
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## Why the Ratio is Useful
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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
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But the ratio:
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$$
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\frac{OH^*}{CH^*}
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$$
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cancels many of these effects and provides a **robust indicator of combustion state**.
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---
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## Physical Interpretation
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- CH* → “fuel is still breaking apart”
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- OH* → “oxidation is occurring”
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So:
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> The ratio indicates where the flame sits between **fuel-dominated** and **oxidation-dominated** chemistry.
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---
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## Deeper Insight
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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**
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Thus the ratio is not just energy-related — it reflects:
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> **Which chemical pathways dominate locally**
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---
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## 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**
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--- |