In Chapter 2 of "Lord of the Flies", what are two similes to describe the flames?

For yards round the fire the heat was like a blow, and the breeze was a river of sparks. 

The heat from the initial flames is "like a blow" which is to say it is like a wind current. This is a gentle, warming comparison.

The flames, as noted in the previous answer, are also compared in similes to animals. With these symbols, the fire/heat is compared to things in nature. But not the transition of comparisons: from the wind to a squirrel to a jaguar. The sequence suggests a progression beginning with something harmless (even welcomed - a warm breeze) and ends with something predatory. At the end of the chapter, the complete progression of these symbols parallels the boys' progression as well: from civilized to savage, from innocent to warlike. 

A tree exploded in the fire like a bomb. 

This simile does compare the tree (not the flames) to a bomb. But it is the fire that initially brought heat and warmth - now described as something destructive - a bomb, killing something natural and innocent (tree). The sequence of comparisons parallels the boys' descent from innocence to violence: breeze, squirrel, jaguar, bomb.