Abstract—The effect of choke ring position on the thermal and fluid flow in a SRU thermal reactor is investigated numerically. It is found that zone 1 is a higher temperature region while zone 2 is a lower temperature region. The average temperature and peak temperature in the SRU thermal reactor for a rich oxygen supply is higher than those of a normal oxygen supply. The temperature difference between zone 1 and zone 2 is smaller for a rich oxygen supply. The peak temperature for a smaller zone 1 is higher. The optimal location of the choke ring for the lowest peak temperature is 6m away from zone 1 corner. The highest sulfur concentration at exit occurs when zone 1 is the smallest.