heatwave effect

[Rob Wilton]

Dear Cinematography.com,
I've been tasked with creating a heatwave effect in front of a macro high speed shot of a coffee bean falling through the air.
I've used a flame bar before, to create the effect, but it's been so many years I've forgotten the ins and outs. Unfortunately I don't have any time to test this.

Does anyone know - do some focal lengths work better than others for this heatwave effect? Does a lighter or darker background make a difference? How does lighting affect the effect?

Thanks again for your time and help!
yours gratefully,

Rob Wilton
DoP