كيف أوسّط الرسم على LaTeX؟

في هذه الوثيقة، لدي مشكلتين اثنتين:

1. الرسم لا يتوسط الوثيقة-كما يمكنك ملاحظته من خلال الصورة في الأسفل-.
2. بخصوص تموضع ${lambda_\mathrm{acc\$، حيث أن الاحداثيات (0.5,0.5)، فلمَ تتغير إلى (2,0.5)؟

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}                
\usepackage[utf8]{inputenc}             
\usepackage[english,italian]{babel}
\usepackage{graphicx}
\usepackage{caption}                    
\usepackage{amsmath,amssymb,amsthm}
\usepackage{tikz}
    \usetikzlibrary{shapes.arrows}
\usepackage[%bindingoffset=1.5cm, 
left=3cm, right=3cm, top=3cm, bottom=3cm]{geometry}

\begin{document}

\title{Title}
\author{Author} 
\date{22 dicembre 2015}

\maketitle

\begin{figure}
  \centering
    \begin{tikzpicture}
    \draw [help lines] (0,0) grid (10,10);
    \draw [] (0,0) rectangle (6,1); %acciaio
    \draw [] (0,4.5) rectangle (6,5.5); %piastra impingement
    \node [text width=3cm,font=\footnotesize] at (2,0.5) {$\lambda_\mathrm{acc}$};
    \node [text width=3cm,font=\footnotesize] at (7.6,5) {piastra\\impingement};
    \draw [latex-latex]  (-0.2,0) -- (-0.2,1) node [black,midway,xshift=-0.4cm]{s\ped{acc}};;
    \end{tikzpicture}
    \caption{Distribuzione puntuale di Nusselt per $Re=5000$}\label{fig:Nu_Re5000}
\end{figure}
\end{document}

كود الرسم بالكامل:

\documentclass[11pt,a4paper]{article}
\usepackage[T1]{fontenc}                
\usepackage[utf8]{inputenc}             
\usepackage[english,italian]{babel}
\usepackage{graphicx}
\usepackage{caption}                    
\usepackage{amsmath,amssymb,amsthm}
\usepackage{xcolor}
    \colorlet{ColorPink}{red!10}
\usepackage{tikz}
    \usetikzlibrary{shapes.arrows}
    \usetikzlibrary{decorations.pathreplacing} %curly bracket
\usepackage[%bindingoffset=1.5cm, 
left=3cm, right=3cm, top=3cm, bottom=3cm]{geometry}

\begin{document}

\title{Title}
\author{Author} 
\date{22 dicembre 2015}

\maketitle

\begin{figure}
  \centering
    \begin{tikzpicture}
    \draw [help lines] (0,0) grid (10,10);
    \draw [fill=blue!30] (0,0) rectangle (6,1); %acciaio
    \draw [fill=blue!15] (0,1) rectangle (6,1.8); %biadesivo
    \draw [fill=gray!10] (0,1.8) rectangle (6,2.3); %inconel
    \draw [fill=blue!30] (0,4.5) rectangle (6,5.5); %piastra impingement
    \draw [fill=white]   (2.4,4.5) rectangle (3.6,5.5); %hole
    \draw [fill=blue!30] (2.75,-0.4) rectangle (3.15,-0.5); %lens
    \draw [fill=blue!30] (2.55,-0.5) rectangle (3.35,-1.5); %camera
    \node [text width=3cm,font=\footnotesize] at (7.6,5) {piastra\\impingement};
    \node [text width=3cm,font=\footnotesize] at (6,0.5) {acciaio};
    \node [text width=3cm,font=\footnotesize] at (6,1.4) {biadesivo};
    \node [text width=3cm,font=\footnotesize] at (6,2.05) {inconel};
    \node [text width=3cm,font=\footnotesize] at (2,2.05) {$\lambda_\mathrm{inc}$};
    \node [text width=3cm,font=\footnotesize] at (2,1.4) {$\lambda_\mathrm{biad}$};
    \node [text width=3cm,font=\footnotesize] at (2,0.5) {$\lambda_\mathrm{acc}$};
    \node [text width=3cm,font=\footnotesize] at (5,-1) {IR camera};
    \node [text width=3cm,font=\small] at (6.8,-1) {$\varepsilon_\mathrm{amb}$};
    \node [text width=3cm,font=\small] at (6.5,-0.4) {$\varepsilon_\mathrm{ext}$};
    \node [text width=3cm,font=\footnotesize] at (8.1,1.2) {piastra\\target};
    \node [text width=3cm,font=\footnotesize] at (5.4,4.2) {getto};
    \node [text width=3cm,font=\small] at (6.5,2.5) {$\varepsilon_\mathrm{int}$};
    \node [text width=3cm,font=\small] at (6.5,4.2) {$\varepsilon_\mathrm{acc}$};
    \node [text width=3cm,font=\small] at (2.6,2.6) {$\dot{q}_\mathrm{joule}$};
    \draw [-latex,ultra thick]  (1,2.3) -- +(1.1,0);
    \node[single arrow,draw=black,fill=blue,minimum height=1cm,minimum width=1cm,shape border rotate=270] at (3,4.5) {}; %freccia blu
    \draw [decorate,decoration={brace,amplitude=10pt,mirror,raise=1pt},yshift=10pt] %parentesi graffa
    (6.1,-0.3) -- (6.1,1.95) node [black,midway,xshift=0.8cm]{};
    \draw [] (0,0) -- (-0.4,0);
    \draw [] (0,1) -- (-0.4,1);
    \draw [] (0,1.8) -- (-0.4,1.8);
    \draw [] (0,2.3) -- (-0.4,2.3);
    \draw [latex-latex]  (-0.2,0) -- (-0.2,1) node [black,midway,xshift=-0.4cm]{s\ped{acc}};;
    \draw [latex-latex]  (-0.2,1) -- (-0.2,1.8) node [black,midway,xshift=-0.4cm]{s\ped{biad}};;
    \draw [latex-latex]  (-0.2,1.8) -- (-0.2,2.3) node [black,midway,xshift=-0.4cm]{s\ped{inc}};;
    \end{tikzpicture}
    \caption{Distribuzione puntuale di Nusselt per $Re=5000$}\label{fig:Nu_Re5000}
\end{figure}

\end{document}