How to make a nice looking flame for an AAS setup

Question

I would like to add a flame like in this picture to what I currently have

I currently have this:

\begin{tikzpicture}
\draw  (-4,-1.5) rectangle (1.5,1.5);
\draw (-4.5,0.5) -- (-1,0.5);
\draw (-0.5,1) .. controls (-1,1) and (-1,1) .. (-1,0.5) .. controls (-1,0) and (-1,0) .. (-0.5,0);
\draw (-4.5,-0.5) -- (-1,-0.5);
\draw[densely dashed] (1.5,0) -- (8.5,0);
\draw (3.5,-0.5) .. controls (3.5,-1.5) and (4,-1.5) .. (4.5,-1.5);
\draw (3.5,-0.5) node (v1) {} -- (6.5,-0.5);
\draw (5.5,-1.5) .. controls (6,-1.5) and (6.5,-1.5) .. (6.5,-0.5);
\draw (4.5,-1.5) -- (4.5,-3.5);
\draw (5.5,-1.5) -- (5.5,-3.5);
\draw (5,-2.5) -- (5,-6);
\draw (4,-4.5) -- (4,-5.5) .. controls (4,-6.5) and (4,-6.5) .. (4.5,-6.5) .. controls (5,-6.5) and (5,-6.5) .. (5.5,-6.5) .. controls (6,-6.5) and (6,-6.5) .. (6,-5.5) -- (6,-4.5);
\draw (4,-5.5) -- (6,-5.5);
\draw (8,-1) -- (9,1) -- (10,-1) -- (8,-1);
\begin{scope}[yshift=5cm,rotate=-30]
\draw  (13,0.5) rectangle (13.5,2);
\node(fail) at (13,1.25) {};
\draw  (13,-2) rectangle (13.5,-0.5);
\draw  (14.5,-1) rectangle (16.5,1);
\node[above,rotate=-30] at (15.5,1) {Detector};
\draw  (17,-0.5) rectangle (18.5,0.5);
\node[above,rotate=-30] at (17.75,0.5) {Recorder};
\node(detector) at (14.5,0) {};
\draw (16.5,0) -- (17,0);
\node[above,rotate=-30] at (13.25,2) {Slit};
\end{scope}
\draw[densely dashed] (9.5,0) -- (fail);
\draw[densely dashed] (9.5,0) -- (detector);
\node[anchor=north,align=center] at (-1.25,-1.5) {Lamp emits wavelengths of required\\energy to excite electron. The cathode\\of the lamp is the metal to be analysed};
\node[anchor=east,align=right] at (4,-6) {Analyte sample\\in beaker};
\draw (5,-4) -- (4,-4) node[left] {Capillary tube};
\node[align=center] at (9,-2) {monochromator\\and slit select\\light of particular\\wavelength};
\draw[-latex] (4,-2) node[left] {fuel} -- (4.5,-2);
\draw[-latex] (4,-2.5) node[left] {air} -- (4.5,-2.5);
\node at (2.5,0.5) {$P_{0}$};
\node at (7.5,0.5) {$P$};
% Flame goes here:
\shadedraw[bottom color=white,top color=cyan!50!blue]  (v1) rectangle (6.5,1.5);
% Terrible Flame:
\shadedraw[bottom color=blue!60!cyan,top color=white] (3.5,-0.5) -- (3.5,1) -- (4,0) -- (4.5,0.5) -- (5,0) -- (5.5,1) -- (6,0) -- (6.5,0.5) -- (6.5,-0.5) -- (3.5,-0.5);
\end{tikzpicture}

Which produces this image:

Is there an easy way or any shortcuts I can take to produce that kind of flame? Note that I am not imitating the rest of the image, it is only really the flame that I would like to reproduce

Flames using Marmot's/Jake's code

The following is an adaption of Marmot's answer below:

which can be reproduced by changing the relevant section to:

\shadedraw[bottom color=blue!60!cyan,top color=white,mysine=20pt] 
    (3.5,-0.5) -- (3.5,0) decorate {(3.5,0) -- (3.7,0.9) -- (3.8,0.1) -- (4,0)} 
    ..controls +(-100:0.3) and +(-80:0.1).. (4,0) [mysine=15pt] decorate {(4,0) -- (4,0.9) -- (4.2,0)} 
    .. controls +(-70:0.3) and +(-90:0.4) .. (4.5,0) [mysine=25pt]  decorate {(4.5,0) -- (4.6,1) -- (4.8,0)}   
    .. controls +(-70:0.2) and +(-80:0.2).. (5,0)[mysine=18pt] decorate {(5,0) -- (5.1,0.9) -- (5.3,0)}   
    .. controls +(-70:0.1) and +(180:0.1) .. +(0.1,-0.2) .. controls +(0:0.1) and +(-90:0.1) .. (5.5,0) decorate {(5.5,0) -- (5.6,0.9) -- (5.8,0)}   
    .. controls +(-70:0.1) and +(-85:0.3) .. (6,0) decorate[mysine=22pt] {(6,0) -- (6.1,0.9) -- (6.3,0)}  
    .. controls +(-70:0.2) and +(90:0.1) .. (6.5,0) -- (6.5,-0.5) --
    (3.5,-0.5) -- (3.5,0);

Answer 1

This uses Jake's complete sines as well as some tricks Mark Wibrow taught me in the comments: one can use decorate in a path, see the application below. I am sure Mark can do much better. I also indented your code because nowadays I get downvoted for not doing that.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{decorations}
% https://tex.stackexchange.com/a/25689/121799
\pgfdeclaredecoration{complete sines}{initial}
{
    \state{initial}[
        width=+0pt,
        next state=sine,
        persistent precomputation={\pgfmathsetmacro\matchinglength{
            \pgfdecoratedinputsegmentlength / int(\pgfdecoratedinputsegmentlength/\pgfdecorationsegmentlength)}
            \setlength{\pgfdecorationsegmentlength}{\matchinglength pt}
        }] {}
    \state{sine}[width=\pgfdecorationsegmentlength]{
        \pgfpathsine{\pgfpoint{0.25\pgfdecorationsegmentlength}{0.5\pgfdecorationsegmentamplitude}}
        \pgfpathcosine{\pgfpoint{0.25\pgfdecorationsegmentlength}{-0.5\pgfdecorationsegmentamplitude}}
        \pgfpathsine{\pgfpoint{0.25\pgfdecorationsegmentlength}{-0.5\pgfdecorationsegmentamplitude}}
        \pgfpathcosine{\pgfpoint{0.25\pgfdecorationsegmentlength}{0.5\pgfdecorationsegmentamplitude}}
}
    \state{final}{}
}

\begin{document}
\begin{tikzpicture}[mysine/.style={decoration={complete sines,segment
length=#1,amplitude=1.23pt,path has corners}},mysine/.default=9.5pt]
 \draw  (-4,-1.5) rectangle (1.5,1.5);
 \draw (-4.5,0.5) -- (-1,0.5);
 \draw (-0.5,1) .. controls (-1,1) and (-1,1) .. (-1,0.5) .. controls (-1,0) and (-1,0) .. (-0.5,0);
 \draw (-4.5,-0.5) -- (-1,-0.5);
 \draw[densely dashed] (1.5,0) -- (8.5,0);
 \draw (3.5,-0.5) .. controls (3.5,-1.5) and (4,-1.5) .. (4.5,-1.5);
 \draw (3.5,-0.5) node (v1) {} -- (6.5,-0.5);
 \draw (5.5,-1.5) .. controls (6,-1.5) and (6.5,-1.5) .. (6.5,-0.5);
 \draw (4.5,-1.5) -- (4.5,-3.5);
 \draw (5.5,-1.5) -- (5.5,-3.5);
 \draw (5,-2.5) -- (5,-6);
 \draw (4,-4.5) -- (4,-5.5) .. controls (4,-6.5) and (4,-6.5) .. (4.5,-6.5) .. controls (5,-6.5) and (5,-6.5) .. (5.5,-6.5) .. controls (6,-6.5) and (6,-6.5) .. (6,-5.5) -- (6,-4.5);
 \draw (4,-5.5) -- (6,-5.5);
 \draw (8,-1) -- (9,1) -- (10,-1) -- (8,-1);
 \begin{scope}[yshift=5cm,rotate=-30]
 \draw  (13,0.5) rectangle (13.5,2);
 \node(fail) at (13,1.25) {};
 \draw  (13,-2) rectangle (13.5,-0.5);
 \draw  (14.5,-1) rectangle (16.5,1);
 \node[above,rotate=-30] at (15.5,1) {Detector};
 \draw  (17,-0.5) rectangle (18.5,0.5);
 \node[above,rotate=-30] at (17.75,0.5) {Recorder};
 \node(detector) at (14.5,0) {};
 \draw (16.5,0) -- (17,0);
 \node[above,rotate=-30] at (13.25,2) {Slit};
 \end{scope}
 \draw[densely dashed] (9.5,0) -- (fail);
 \draw[densely dashed] (9.5,0) -- (detector);
 \node[anchor=north,align=center] at (-1.25,-1.5) {Lamp emits wavelengths of required\\energy to excite electron. The cathode\\of the lamp is the metal to be analysed};
 \node[anchor=east,align=right] at (4,-6) {Analyte sample\\in beaker};
 \draw (5,-4) -- (4,-4) node[left] {Capillary tube};
 \node[align=center] at (9,-2) {monochromator\\and slit select\\light of particular\\wavelength};
 \draw[-latex] (4,-2) node[left] {fuel} -- (4.5,-2);
 \draw[-latex] (4,-2.5) node[left] {air} -- (4.5,-2.5);
 \node at (2.5,0.5) {$P_{0}$};
 \node at (7.5,0.5) {$P$};
 % Flame goes here:
 \shadedraw[bottom color=white,top color=cyan!50!blue]  (v1) rectangle (6.5,1.5);
 % Terrible Flame:
 \shadedraw[bottom color=blue!60!cyan,top color=white,mysine=8pt] 
  (3.5,-0.5) -- (3.5,0) decorate {(3.5,0) -- (3.7,0.9) -- (3.8,0.1)} 
  -- (4,0) [mysine=7pt] decorate {(4,0) -- (4,0.9) -- (4.2,0)} 
   -- (4.5,0) [mysine=9pt]  decorate {(4.5,0) -- (4.6,1) -- (4.8,0)}   
   -- (5,0)[mysine=8pt] decorate {(5,0) -- (5.1,0.9) -- (5.3,0)}   
   -- (5.5,0) decorate {(5.5,0) -- (5.6,0.9) -- (5.8,0)}   
   -- (6,0) decorate {(6,0) -- (6.1,0.9) -- (6.3,0)}  
   -- (6.5,0) -- (6.5,-0.5) --
    (3.5,-0.5) -- (3.5,0);
\end{tikzpicture}
\end{document}