105c is pretty high. And it says its reducing performance, probably to cool down. It does depend on your exact CPU though.
The first gen Core i3s can go to 90c for PGA or 105c for BGA before throttling. The first gen Core i5s can go to 105c.
The second gen Core i3s can go to 85c for PGA and 100c for BGA. The second gen Core i5s can go to 100c.
Unfortunatley Dell has multiple 15Rs, ones with Core i3 and i5s from both first and second generation.
Your laptop is throttling the CPU to cool it down then shutting down to keep it from being damaged. Its a feature in all modern CPUs.
Still thats very high. All laptops have whats called a fan assembly. It is normally heatpipes that go to a fin heatsink with a small fan in the middle. The fan spins and pulls air in one way and pushes it out the other way. If its caked with dust, it wont have good air flow. If the fan does not work at all, it would not work properly at all.
You can buy cans of compressed air pretty much anywhere, Walgreens, CVS, Target etc for about $5 bucks. The bottom, side and back should have holes/slots that you can blow the air into to help remove dust. Do that then when your laptop is on, check to see if the fan is working.