Hello Seung,
The Ansys fluent actually predicts a gauge pressure in the below equation and adds it to ideal gas law (user guide -8.3.7)and considers the air as compressible flow when you choose ideal gas option for density whereas the constant option just allocates the value and the Ansys fluent does not add the gauge pressure term and treats the air as incompressible. The air although practically is compresssible by nature it can be theoretically considered incompressible for mach umber less than 0.3.So this can be the reason why you experience the difference in temperature profile and also you cannot observe conduction for density as ideal gas option. I recommend you to check with incompressible ideal gas option for density you should observe the changes. This might solve your problem.I am also attaching the tutorial for thermal conduction in heat sink.
Into to Heat Transfer in Fluids Quiz | Ansys Courses
Thanks,
Chaitanya Natraj
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