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We will begin by using a quick Gasoline Explosion preset. 

Create a Sphere with a Radius of 24.0 meters. This sphere will be the source of the explosion.

Here is a GPU Preview of the simulation.

Let’s take a look at setting up a gasoline explosion simulation manually.

If you are continuing from the preset steps above, select the Simulator and press the Clear Simulator Cache button from the Phoenix Shelf and then delete the Simulator and the Source objects from your scene.

To add variation, select the Fire/Smoke Source and increase the Noise value to 10.

Notice the variation that is added to the simulation.

The graph in the Color and Opacity rollout controls only the opacity of the fire. Double-click to add a new point to the curve and drag it down to reduce the opacity of the fire, revealing more of the internal fire content and making it look brighter. Drag the point up to increase the opacity and hide more of the internal structure. Add points in the middle for a gentle curve.

Click the Expand button for a larger graph to work with.

You can further adjust the opacity with the Opacity Multiplier above the graph. For this example, we will lower this value to 0.5, and notice how the fire increases in brightness.

The Physically Based setting blends between artistic and physically accurate results. Change Physically Based to 0.75 and observe the more "fiery" look in the simulation. This parameter balances between how much of the fire intensity comes from the color gradient plus the fire multiplier versus how much is calculated using the physically-based black body shader.

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To adjust the explosion to look less contained, navigate to the Dynamics rollout and under Conservation, adjust the Quality setting. This setting allows the simulation to spread more and gives it a swirling motion. Increase Quality to 80. Note that increasing this value will increase simulation time. Start another simulation.

The explosion is breaking up a bit more, but it's not quite there yet. Open the Rendering rollout → Fire again. Disable the Fire for now. We can see there is a lot of smoke as a result of the fuel burning so much. Stop the simulation to address this.

In the Fuel settings of the simulation, reduce the Smoke Amount to 0.4 to allow for more burning and a brighter fire, and also increase the Smoke Threshold to 1.0. The Smoke Threshold will lessen how much of the fuel is created as smoke. Reduce Propagation of the fuel to 2 to reduce how fast it propagates through the simulation.

In the Fire rollout, remember to set the Fire's Based on parameter back to Temperature before restarting the simulation.

The fire now looks too hot. Go to the Fuel rollout and reduce the Fuel's Energy to 5 and restart the simulation.

The explosion is starting to look good, but it stays hot for too long. We need to increase the cooling of the fire by going to Dynamics rollout and increasing Cooling to 0.3.

To prepare your scene for rendering a simulation make sure V-Ray is set as your Renderer in the Render Setup window.

We need a ground plane, so create a floor by navigating to the V-Ray Menu → V-Ray Plane, and place it in the scene. Set it a little lower than the simulation and give it a gray color.

Do another render. 

Notice some areas of the floor are too bright. You are able to control this in Rendering → Fire Lights rollout.

Set the Light Power on Scene value to 0.5 and re-render the frame. By reducing this value, the light is more evenly distributed.

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To get finer detail for a final render, increase the Resolution of the Grid. In the Attribute Editor, go to the Grid rollout and click Increase Resolution a couple of times to have about 6.1 million Total cells and re-run the simulation.

Now let's apply what we've learned to a real-world example and make these barrels explode!

The following scene file can be downloaded from the Overview section above.

Select the blue barrel, pCylinder3, and apply the Gasoline Explosion Preset.

Select the Source and set a keyframe on the Discharge to 3100 at Frame 1.

Keep the keyframe on the Discharge set to 0.0 at Frame 2.

Start a simulation and you'll see that the blue barrel explodes. Now we will focus on the other barrels.

Stop the simulation.

Select the Fire/Smoke Source and press Ctrl+D to Duplicate it and move it over a little so it's not overlapping the original.

Notice that the Discharge is set to 0.0, because we duplicated the Source, and the keys don’t transfer over with a simple Duplicate in Maya.

Select the red barrel pCylinder2, shift-select the duplicated Source and click Add Selected Objects.

Notice that this barrel laying on its side is slightly outside of the Simulator. Select the Simulator box and move it over to hold all three barrels.

When you start the simulation now, you will see that only the blue barrel explodes.

Stop the simulation.

Now when you start the simulation again you will see two explosions, the second being delayed by five frames.

Stop the simulation.

For the third barrel, press Ctrl+D on the second Fire/Smoke Source to make a third copy and add pCylinder1 to it.

For this last barrel we are going to let the explosion cool off some before ignition.

Select the third Source and set a keyframe on the Discharge to 0.0 at Frame 41.

At Frame 42, set Discharge to 6000 and keyframe.

At Frame 43, key the Discharge back to 0.0.

Start the simulation and let it run. The third explosion starts as the first two are cooling off.

Do a test render to see the results. If the fire is too bright, go into the Fire rollout and decrease the Fire Multiplier value.

Go to the Rendering → Fire Lights tab and change the Light Power on Scene to 0.1.

To make the final simulation, go into the Grid rollout and increase the Cell Size to 1.5. This will increase the Total Cells to about 4.5 million.

Set the Adaptive Grid to Smoke and the Threshold to 0.002.