**4.2 Two-phase heat sinks**

These are phase change micro-exchangers, where the cooling fluid (generally a liquid) undergoes phase changes (from liquid to vapor and back to liquid) during the heat transfer process. Two-phase heat sinks are generally constituted of a vessel containing a heat transfer fluid; water in most cases. The lower surface of the vessel is in contact with the heat source (electronic equipment), and the upper surface is in contact with the cooling medium (generally air). **Figure 18** presents the operating principle of this system.

When in contact with the lower surface (heat source), the heat transfer fluid evaporates by absorbing the heat generated at this surface. The vapor then goes

*Exploded view of a chip mounted under a refrigerated heatsink.*

**Figure 18.** *Operating principle of a two-phase heatsink.*

upwards, toward the upper surface where it gets cooled and condensates. Condensed liquid falls back along the walls of the vessel, down to the lower surface, where it absorbs again the heat and evaporates. This cycle is repeated as long as heat is generated by the electronic equipment.

This way, two-phase heat sinks are used to remove heat from the heat source to the ambient environment. The lower surface acts as an evaporator and the upper one plays the role of a condenser.

Note that two-phase heat exchangers involve latent heat instead of sensible heat in energy transfers, thus allowing large amounts of heat to be exchanged over small areas, which leads to great compactness.

Two-phase heat sinks include the following:


#### *4.2.1 Spray coolers*

Spray coolers are a special class of two-phase heat exchangers since the heat transfer fluid undergoes a series of evaporations and condensations [45, 46]. The electronic component is cooled by a jet of fluid, which partially evaporates while absorbing heat (see **Figure 19**). The heat transfer fluid is sprayed directly onto the surface of the power component to be cooled [47–49].

Spraying enhances the vaporization of the fluid even at relatively low temperatures (60–75°C, under 1 Atm.). It is generally realized using a single injector [50] or multiple injectors [51].

However, like refrigerated exchangers, the realization of spray cooling is relatively complex. As shown in **Figure 20**, it requires, in addition to the sprayers, the installation of a condenser to collect the vapors generated and a pump or compressor to feed the injectors.

## *4.2.2 Vapor chambers*

Vapor chambers are the most common two-phase-heat exchangers where a heat transfer fluid (water in most cases) is placed in a sealed envelope: *the chamber* [52]. The lower surface of the chamber is mounted on the electronic component to be

*Heat Exchangers for Electronic Equipment Cooling DOI: http://dx.doi.org/10.5772/intechopen.100732*

cooled. As described above, the liquid absorbs the heat generated by this component to evaporate. The vapors then condense on the cold surface of the chamber, which transfers the ambient environment the energy liberated by the condensation process [53, 54].

**Figure 21.** *Vapor chamber heat exchanger. Source [55].*

**Figure 21** shows this type of heat exchanger. It should be noted that the dimensions are extremely small to meet the requirements of miniaturization of onboard electronics and mobile telephony. The typical thickness of such an exchanger is 2–8 mm.

Current developments focus on the ultra-miniaturization of these exchangers by introducing ultra-thin evaporation chambers: thicknesses from 0.3 to 2 mm using walls made of titanium, stainless steel, or copper alloys. **Table 2** shows the current uses of these types of exchangers, as well as their typical dimensions and the thermal powers conveyed.

#### *4.2.3 Heat pipes*

A heat pipe consists of a sealed tube, containing a heat transfer fluid, without any other gas (see **Figure 22**). In most cases, only a small quantity of water suffices: About 1 cc of water for a 150 mm long, 6-mm heat pipe is typical.

In one of its zones (vaporization zone), the heat pipe tube is in contact with the hot source to be cooled. The heat recovered from this source increases the temperature of the heat transfer fluid causing it to evaporate.

The resulting vapors then accumulate in the condensation zone of the heat pipe where they condense on the internal walls of the tube, releasing their latent heat to the ambient environment (see **Figure 22**). The condensate flows in droplets on


#### **Table 2.**

*Vapor chamber uses and construction materials.*

*Heat Exchangers for Electronic Equipment Cooling DOI: http://dx.doi.org/10.5772/intechopen.100732*

**Figure 22.** *Heat pipe working principle.*

the walls of the tube and returns to the vaporization zone to be again submitted to the heat flow and to evaporate.
