\documentclass{article}
\usepackage[stdsubgroups,nocfg]{nomencl}
\usepackage{setspace}
    \makenomenclature
\begin{document}
\section*{Main equations}
Here an equation
    \begin{equation}\label{eq:heatflux}
        \dot{Q} = k \cdot A \cdot \Delta T
    \end{equation}%
    \nomenclature[aQ]{$\dot{Q}$}{heat flux}%
    \nomenclature[ak]{$k$}{overall heat transfer
      coefficient}%
    \nomenclature[aA]{$A$}{area}%
    \nomenclature[aL]{$L$}{length}%
    \nomenclature[aT]{$T$}{temperature}%
    \nomenclature[aT]{$\Delta T$}{temperature difference}%
or another one
    \begin{equation}\label{eq:ohtc}
        \frac{1}{k} = \left[\frac{1}{\alpha _{\mathrm{i}}\,r_{\mathrm{i}}} +
        \sum^n_{j=1}\frac{1}{\lambda _j}\,
        \ln \frac{r_{\mathrm{a},j}}{r_{\mathrm{i},j}} +
        \frac{1}{\alpha _{\mathrm{a}}\,
          r_{\mathrm{a}}}\right] \cdot r_{\mathrm{reference}}
    \end{equation}%
    \nomenclature[ga]{$\alpha$}{convection heat transfer coefficient}%
    \nomenclature[zi]{i}{in}%
    \nomenclature[gl]{$\lambda$}{thermal conductivity}%
    \nomenclature[za]{a}{out}%
    \nomenclature[zn]{$n$}{number of walls}%
    \nomenclature[zj]{$j$}{running parameter}%
\onehalfspacing
\printnomenclature
\end{document}