Effect of protrusion on the enhancement of regression rate

It has been reported in literature that the use of protrusion in the combustion chamber of a hybrid rocket motor enhances the regression rate. A detailed study was carried out by using two types of protrusion (a) rubber (which burns along with fuel but at different rate, hence its shape changes with burn time), and (b) graphite (does not burns along with fuel and hence its shape does not changes with burn time). The results showed that the improvement in regression rate with protrusions was only up to a certain fraction of the overall burn time. From the results obtained, it was observed that an X/LX/L of 0.5 was the best location for both types of protrusion. The maximum improvement in regression rate was around 45%. The combustion efficiency was observed to improve from 58% without protrusion to 69% with the use of protrusion, when placed at an X/LX/L of 0.8. It was observed that the use of bluff body and protrusion improves combustion efficiency, when used individually. Hence, combination of the two was used and the combustion efficiency was determined. The combustion efficiency was around 90%, when bluff body was used at the head end and the protrusion at an X/LX/L of 0.8 from head end.