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If nothing appears in the render, this is the first parameter you should change. Generally, the idea is to avoid seeing individual particles and instead have a continuous looking smoke.
From left to right, changing the radius multiplier: Value of 2 (perhaps a bit too much), 1 (seems a good value in this case), 0.3 (too small)
Density multiplier
You can increase or decrease the amount of density that each particle will contribute with this parameter. Note that the final density will still be limited by the "Max final density" parameter.
Values of 2, 5, 100 ("Max final density" parameter was kept at 35 for all renders)
Max final density