Well if you ask me, the flame is the fire that burned self, as sacrificed in order to rise above, be selfless, enter heaven. Take that in conjunction with the other symbols and it appears to fit.The Sacred Heart most obviously brings to mind the Passion of Christ on the cross. There is the crown of thorns, the cross, usually atop the heart, and the wound from the spear that pierced His side. But why is the Sacred Heart always shown as if it’s on fire? That certainly did not happen at the crucifixion.
https://aleteia.org/2018/06/08/explaining-the-strange-symbolism-of-the-sacred-heart/