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.
A simile is a figure of speech where two unlike things are compared using "like" or "as". There are some notable similes used in Chapter 2 to describe the flames.
In one especially descriptive simile, the author compares the flames to a squirrel. He says,
"Small flames stirred at the trunk of a tree and crawled away through leaves and brushwood...one patch touched a tree trunk and scrambled up like a bright squirrel...the squirrel leapt on the wings of the wind and clung to another standing tree, eating downwards".
A few lines later in the same paragraph, the flames are compared to another animal, the jaguar -
"The flames, as though they were a kind of wild life, crept as a jaguar creeps on its belly toward a line of birch-like saplings that fledged an outcrop of the pink rock" (Chapter 2).