Implementation
Starting with the default particle system object (menu: GameObject > Create General > Particle System), go to the Shapemodule and set the emitter shape to a small Sphere, say about 0.5 units in radius. The particles in the standard assets include a material called Fire Add which is very suitable for explosions (menu: Assets > Import Package > Particles). You can set this material for the system using the Renderer module. With the Renderer open, you should also disable Cast Shadows and Receive Shadows since the explosion flames are supposed to give out light rather than receive it.
At this stage, the system looks like lots of little fireballs being thrown out from a central point. The explosion should, of course, create a burst with lots of particles all at once. In the Emission module, you can set the Rate value to zero and add a single Burst of particles at time zero. The number of particles in the burst will depend on the size and intensity you want your explosion to have but a good starting point is about fifty particles. With the burst set up, the system is now starting to look much more like and explosion but it is rather slow and the flames seem to hang around for a long time. In the Particle System module (which will have the same name as the GameObject, eg, “Explosion”), set both the Duration of the system and the Start Lifetime of the particles to two seconds.
You can also use the Size Over Lifetime module to create the effect of the flames using up their fuel. Set the size curve using the “ramp down” preset (ie, the size starts off at 100% and reduces to zero. To make the flames darken and fade, enable the Color Over Lifetime module and set the gradient to start with white at the left and finish with black at the right. Since the Fire Addmaterial uses and additive shader for rendering, the darkness of the color property also controls the transparency of the particle; the flame’s will become fully transparent as the color fades to black. Also, the additive material allows the brightness of particles to “add” together as they are drawn on top of each other. This helps to further enhance the impression of a bright flash at the start of the explosion when the particles are all close together.
As it stands, the explosion is taking shape but it looks as though it is happening out in space. The particles get thrown out and travel a long distance at constant speed before fading. If your game is set in space then this might be the exact effect you want. However, an explosion that happens in the atmosphere will be slowed and dampened by the surrounding air. Enable the Limit Velocity Over Lifetime module and set the Speed to about 3.0 and the Dampen fraction to about 0.4 and you should see the explosion lose a little strength as it progresses.
A final thing to note is that as the particles move away from the centre of the explosion, their individual shapes become more recognisable. In particular, seeing the particles all at the same size and with the same rotation makes it obvious that the same graphic is being reused for each particle. A simple way to avoid this is to add a bit of random variation to the size and rotation of the particles as they are generated. In the Particle System module at the top of the inspector, click the small arrow to the right of the Start Size and Start Rotation properties and set them both to Random Between Two Constants. For the rotation, set the two values to 0 and 360 (ie, completely random rotation). For the size, set the values to 0.5 and 1.5 to give some variation without the risk of having too many huge or tiny particles. You should now see that the repetition of particle graphics is now much less noticeable.