For an aspect ratio (transport length-to-width) of 5, Pr=1.13, Le=1.91, Pe=4.3, Cv=1.01, $P_B=20;Torr$, the effects of addition of inert gas Ne on thermally buoyancy-driven convection ($Gr=2.44{ imes}10^3$) are numerically investigated for further understanding and insight into essence of transport phenomena in two dimensional horizontal enclosures. For $10K{leq}{Delta}T{leq}50;K$, the crystal growth rate increases from 10 K up to 20 K, and then is slowly decreased until ${Delat}T=50;K$, which is likely to be due to the effects of thermo-physical properties stronger than the temperature gradient corresponding to driving force for thermal convection. The dimensional maximum velocity gratitude reflecting the intensity of thermal convection is directly and linearly proportional to the temperature difference between the source and crystal regions. The rate is first order-exponentially decreased for $2{leq}Ar{leq}5$. This is related to the finding that the effects of side walls tend to stabilize convection in the growth reactor. In addition, the rate is first order exponentially decayed for $10{leq}P_B{leq}200;Torr$.