What is a heat dissipater and how does it work?
The heat dissipaters  are used to reduce the infra-red signature of the helicopter engine exhaust. They are also used in other helicopters, like the 'black hole ocarina' (BHO) infra-red suppression device used in AH-64 Apache.



Source: b-domke.de

The main purpose of the IR suppressors in use today is twofold:


Reduce the IR signature by reducing the exhaust temperature
'Shield' the hot engine from view.


The infra-red suppression systems in service today usually work by


'Bending' the exhaust, usually upwards, and shielding the exhaust from view by an insulating cowl. In these IR suppressors in Mi- 35 (export version of Mi-24), the exhaust is turned almost 90°.




Source: mamboccv.com


Mixing the hot engine exhausts with the cool air, effectively reducing the exhaust temperature. In the Black Hole IR suppression system, the exhaust temperature is reduced from ~600 °C to ~300 °C .
The RAH-66 Comanche had the infra-red suppression system integrated into the airframe (the exhaust was routed through the tail boom), though the principle is the same- mixing of engine exhaust with ambient cool air.




Source: sikorskyarchives.com


In some cases, the special high emissivity coatings are applied to the engine exhausts for reducing the IR signature. 


Most of the present IR suppressors focus on achieving a thorough mix of the cool ambient air and the exhaust to reduce temperature and uneven mixing, in order to prevent 'hot spots'. Cooling of the exhaust pipe itself is achieved in some cases by using film cooling. In most helicopters, the infra red suppression system (IRSS) is optional and can be added separately.

The figure shows the schematic of the infrared suppression system used in AH- 64 Apache and the associated reduction in IR signature.



Image from Progress in helicopter infrared signature suppression by Jingzhou Zhang et al., Chinese Journal of Aeronautics, Volume 27, Issue 2