I want to know what is the purpose of this silvery foil situated above the bell in Merlin Vacuum Engines, and what purpose does it fulfill exactly?
The nozzle radiates a lot of heat, the plumbing leading to the Merlin engine contains liquid oxygen pretty close to boiling. Local hot spots on the plumbing could cause the oxidizer to flash, which would be quite problematic. The foil works by creating a radiative buffer and radiating away some of the absorbed heat. You can see how this works here https://en.wikipedia.org/wiki/Multi-layer_insulation (although, they may only be using one layer here).