add vertical space between equations

You can manually add a vertical distance to each line-break. I hope, this is what you want.

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\setcounter{equation}{2}
\begin{equation}
    \begin{aligned}
        T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\[1pt]
        Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\[10pt]
        N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\[100pt]
        K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
    \end{aligned}
\end{equation}
\end{document}

You can also modify the length \jot if you don't want to manually specify the skips for each line. For example

\setlength{\jot}{10pt}

If you want the change to be localised to that equation, insert the line inside the equation environment, as in the following example

\documentclass{article}
\usepackage{mathtools}

\begin{document}

an equation with more spacing

\begin{equation}
\setlength{\jot}{10pt}
    \begin{aligned}
        T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\
        Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\
        N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\
        K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
    \end{aligned}
\end{equation}

and a normal one

\begin{equation}
    \begin{aligned}
        T_{P} &= K_{T}. \rho . n^{2}_{p} . D^{4}_{p} \\
        Q_{P} &= K_{Q}. \rho . n^{2}_{p} . D^{5}_{p} \\
        N_{P} &= K_{N}. \rho . n^{2}_{p} . D^{6}_{p} \\
        K_{P} &= K_{K}. \rho . n^{2}_{p} . D^{7}_{p}
    \end{aligned}
\end{equation}

\end{document} 

Output