266 lines
4.2 KiB
Markdown
266 lines
4.2 KiB
Markdown
|
||
# Chemiluminescence in Hydro-Carbon 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:
|
||
|
||
- CH* (≈ 431 nm, blue)
|
||
|
||
- 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:
|
||
|
||
- Translational motion (heat)
|
||
|
||
- Vibrational energy
|
||
|
||
- Rotational energy
|
||
|
||
- **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:
|
||
|
||
- High temperature
|
||
|
||
- Fast radical–radical collisions
|
||
|
||
- 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:
|
||
|
||
- Sufficient reaction energy (~2–5 eV)
|
||
|
||
- Good overlap between reaction geometry and excited state (Franck–Condon principle)
|
||
|
||
- 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
|
||
|
||
- CH* peaks in the **reaction front** (fuel breakdown)
|
||
|
||
- 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
|
||
|
||
- Fuel-rich:
|
||
|
||
- High CH*
|
||
|
||
- Lower OH*
|
||
|
||
- $$R \text{ small}$$
|
||
|
||
- Lean:
|
||
|
||
- High OH*
|
||
|
||
- Lower CH*
|
||
|
||
- $$R \text{ large}$$
|
||
|
||
- Stoichiometric:
|
||
|
||
- Balanced
|
||
|
||
- $$R \text{ intermediate}$$
|
||
|
||
|
||
---
|
||
|
||
## Why the Ratio is Useful
|
||
|
||
Absolute intensity depends on:
|
||
|
||
- Optical alignment
|
||
|
||
- Flame size
|
||
|
||
- Window fouling
|
||
|
||
- Sensor gain
|
||
|
||
|
||
But the ratio:
|
||
|
||
$$
|
||
\frac{OH^*}{CH^*}
|
||
$$
|
||
|
||
cancels many of these effects and provides a **robust indicator of combustion state**.
|
||
|
||
---
|
||
|
||
## Physical Interpretation
|
||
|
||
- CH* → “fuel is still breaking apart”
|
||
|
||
- 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:
|
||
|
||
- CH* is fed by **hydrocarbon radical pathways**
|
||
|
||
- OH* is fed by **oxidation pathways**
|
||
|
||
|
||
Thus the ratio is not just energy-related — it reflects:
|
||
|
||
> **Which chemical pathways dominate locally**
|
||
|
||
---
|
||
|
||
## Summary
|
||
|
||
- Chemiluminescence arises from **excited radicals formed during reactions**
|
||
|
||
- CH* and OH* originate from **different parts of the combustion network**
|
||
|
||
- Their ratio provides a **real-time optical measure of stoichiometry and flame structure**
|
||
|
||
|
||
--- |