Heat transfer_heating a nanoparticle

Hello guys

I'm trying to get thermal distribution of nanorod(gold) which are placed inside the sphere(water).
I designated nanorod as a heat source and simulate the heat distribution near nanoparticle over time.
I got the thermal profile without an error.

However, the heat distribution seems weird.

1. Thermal distribution doesn't change a lot in radial direction as I expected.(I think temperature decreases a lot as it moves away from the particle) Does anyone knows what the problem is?

2. I want to know why the 'mean temperature' decreases, when the size of the sphere increases. (It seems thermal energy is confined inside the sphere)
How to achieve consistent thermal profile despite the change in the size of sphere.

Thank you!

Have a nice day:D