Gas velocity in a tube under isothermal heating

Hi,
I'm working on modelling a gas flow inside a quartz tube. the gas flows from room temperature zone (25 cm long) and enters a hot zone (75 cm at 1000 C) . according to calculation the mean velocity of gas suppose to be some thing like 15 m/s but cmosol shows 1cm/s, I thing the effect of temperature on velocity doesn't calculated, so pleae can you help?

Regards


Samir