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\title{\vspace{-1.0cm}Optical Combustion Diagnostics Using UV/Visible Chemiluminescence}
\author{}
\date{}

\begin{document}
\maketitle
\vspace{-0.8cm}

\section*{Purpose}

This note proposes a low-cost optical route for combustion diagnostics using flame
chemiluminescence. The approach is not intended to replace existing oxygen or CO
measurement systems. Instead, it provides an additional diagnostic layer for flame
quality, instability, and air/fuel condition.

Initial tests have already shown that a transimpedance amplifier (TIA) front-end,
connected to UV-sensitive photodiodes, produces clear measurable signals from flames.
This supports the feasibility of extending the existing IR flame-detection PCB concept
towards UV and chemiluminescence sensing.

\section*{Relevant flame chemistry}

Hydrocarbon flames produce excited radical species during combustion. Two particularly
useful emitters are hydroxyl and methylidyne radicals:

\[
\ce{OH^{*} -> OH + h\nu}
\]

\[
\ce{CH^{*} -> CH + h\nu}
\]

The dominant useful bands are approximately:

\[
\ce{OH^{*}} \approx \SI{310}{nm}
\]

\[
\ce{CH^{*}} \approx \SI{430}{nm}
\]

The relative intensity of these bands is linked to combustion state. In particular,
the ratio

\[
R = \frac{I_{\ce{OH^{*}}}}{I_{\ce{CH^{*}}}}
\]

may provide an indication of rich/lean tendency, while time variation in the
UV signal may provide information about flame instability, lift-off, pulsation,
or incipient poor combustion.

\section*{Sensing concept}

The proposed sensing chain is:

\[
\text{Flame emission}
\rightarrow
\text{UV/visible optical filter}
\rightarrow
\text{photodiode}
\rightarrow
\text{TIA}
\rightarrow
\text{gain/filtering}
\rightarrow
\text{ADC/DSP}
\]

A simple photodiode TIA has already been tested with flame sources in the UV region.
The observed response confirms that the required optical signal is detectable using
low-cost analogue electronics. This gives a practical development path:

\[
\text{IR flame PCB}
\rightarrow
\text{UV flame PCB}
\rightarrow
\text{dual-channel OH*/CH* diagnostic sensor}
\]

This is an incremental development, not a new platform from scratch.

\clearpage

\section*{Diagnostic value}

The proposed sensor could provide:

\begin{itemize}
  \item flame presence detection;
  \item flame stability / flicker analysis;
  \item rich/lean indication from the $OH^*/CH^*$ ratio;
  \item early warning of poor combustion before conventional limits are reached;
  \item possible correlation with NOx-forming conditions;
  \item service and commissioning diagnostics;
  \item with the addition of Swan bands ($C_2$), early indication of soot-forming conditions;
  \item ratios and strengths of $OH^*$, $CH^*$ and $C_2$ may provide insight into the instantaneous composition of waste or syngas fuels.
\end{itemize}

It should be stressed that this sensor does not directly measure CO or oxygen
concentration. Its value is as a complementary combustion-quality sensor, especially
where fast optical response provides information that slower gas probes may not.

\section*{Additional advantages}

\begin{itemize}

\item \textbf{Fast response:} The optical signal is generated directly at the reaction zone and is not subject to transport delay.

\item \textbf{Non-intrusive measurement:} No insertion into the flame or flue is required.

\item \textbf{Harsh environment suitability:} Optical sensing may be more robust than conventional probes in high-temperature or contaminated conditions.

\item \textbf{Early fault detection:} Chemiluminescence changes may precede measurable CO or O$_2$ changes.

\item \textbf{Dynamic information:} Temporal behaviour (oscillation, intermittency) becomes observable.

\item \textbf{Independent channel:} Provides plausibility checking against existing sensors.

\item \textbf{Low-cost replication:} Additional channels can be added at low cost.

\item \textbf{Variable fuel suitability:} Particularly relevant for syngas and mixed fuels.

\item \textbf{Degradation monitoring:} Optical fouling may be inferred from signal changes.

\end{itemize}

\clearpage

\section*{Commissioning support}

In industrial burner systems, commissioning involves stepping through firing rates
and storing actuator positions (fuel valve, fan VSD). The aim is stable operation
across the full range.

Currently this relies on:
\begin{itemize}
  \item visual flame observation;
  \item flue gas measurements (O$_2$, CO);
  \item operator judgement.
\end{itemize}

These do not directly observe the reaction zone.

The proposed sensor could:

\begin{itemize}
  \item provide real-time flame quality at each firing point;
  \item identify marginal or poorly mixed conditions;
  \item optimise fuel/air settings based on flame behaviour;
  \item improve repeatability;
  \item enable semi-automated commissioning using metrics such as $OH^*/CH^*$ and signal stability.
\end{itemize}

This extends the concept from monitoring to active commissioning support.

% -------- Research section (conditional) --------
\ifresearch

\clearpage
\section*{Research questions}

\begin{itemize}

\item \textbf{Information content:}  
What combustion state information is recoverable from multi-band chemiluminescence?

\item \textbf{Separability:}  
Can fuel effects be distinguished from air setting, turbulence, and fouling?

\item \textbf{Dynamics:}  
Do temporal statistics indicate instability or blow-off proximity?

\item \textbf{Fuel inference:}  
Can variable fuels (e.g. syngas) be characterised indirectly?

\item \textbf{Robustness:}  
How sensitive are results to burner geometry and viewing conditions?

\item \textbf{Implementation:}  
Is low-cost photodiode hardware sufficient for industrial deployment?

\end{itemize}

\fi

\section*{Proposed next step}

Build a multi-channel demonstrator including:

\[
\SI{310}{nm}\ \ce{OH^{*}} \quad \text{and} \quad \SI{430}{nm}\ \ce{CH^{*}}
\]

Additional channels may include:
\begin{itemize}
  \item broadband visible;
  \item green-filtered ($C_2$ Swan band);
  \item broadband IR.
\end{itemize}

The selected microcontroller provides sufficient ADC channels and DSP capability.
Each IR channel requires one op-amp, while each UV channel requires two, which remains
compatible with the current architecture.

\end{document}