Gravity-Driven Two-Phase Cooling for the 21st Century

Two-phase cooling has been utilized in the electronics cooling industry for many decades, with possibly the most well-known adaptation being the heat pipe. Heat pipes are capillary-driven, two-phase devices that rely on the boiling and condensation of a working fluid to transfer heat significant distances with minimal temperature gradient. The flow of the working fluid inside of a heat pipe is facilitated by a capillary wick structure that relies on surface tension to return the condensed liquid to the heat generating components. Heat pipes have found their way into a large number of industries and applications because of their high performance, high reliability, and low cost. Unfortunately, as the electronics industry’s insatiable quest for smaller, higher-powered devices soldiers on, the discrete cooling power of the heat pipe approaches obsolescence.

Cue the heat pipe’s lesser-known cousin; the loop thermosyphon (or thermosiphon). Loop thermosyphons (LTS) are gravity-driven, two-phase devices that operate in a similar manner to a heat pipe in so far as a working fluid is evaporated and condensed in a closed loop to transfer heat over a given distance. Some readers may be more familiar with a traditional thermosyphon, shown in Figure 1a, where the liquid and vapor occupy a single tube. Loop thermosyphons, as shown in Figure 1b (and as the name suggests), operate in more of a loop fashion where the liquid and vapor travel more independently.

Figure 1. Schematic representation of a (a) traditional thermosyphon and (b) loop thermosyphon.

Contrary to the capillary pumping of the working fluid in a heat pipe, an LTS relies on gravity head to circulate the fluid around the loop. This, of course, means that loop thermsyphons can only operate in a vertical orientation, but if this condition can be met, an LTS can offer a wide range of benefits that most other cooling systems cannot. This article will provide an overview of how an LTS operates, how system integrators could incorporate a technology like this to advance their products, and the benefits of using an LTS over most existing cooling technologies; passive or active.

Loop Thermosyphon Operation

A typical LTS consists of an evaporator, a condenser, and plumbing between the two for the liquid and vapor to travel. The liquid return line (or downcomer) is connected to the evaporator cavity to facilitate the flow of the working fluid. In a similar fashion, the vapor line (or riser) is connected to the condenser completing the loop. The system is hermetically sealed and filled with a particular inventory of working fluid. Working fluids are typically dielectric refrigerants with high liquid-to-vapor density ratios and high latent heat. The reason for selecting fluids with these properties is because flow in the loop is driven by the density difference between the downcomer and riser. Larger differences between liquid and vapor states results in a larger driving force and more fluid flow rate.

Figure 2. Example of a loop thermosyphon for power electronics cooling applications.

Heat is applied to the loop through the evaporator. The evaporator could take on any number of forms to cool the system in question. The most common configuration for the evaporator is a traditional-looking liquid cold plate in which heat generating components are mounted and the heat is conducted into the system. An example of such a configuration is shown in Figure 2 for a traditional power electronics cooling application. The functionality of an LTS is mostly agnostic to the form of the evaporator, so many variations of an evaporator are possible. Some of these configurations are discussed in more detail in the following section.

In the “off state” the loop sits idle with an equal height of liquid filling the downcomer (h2) and evaporator cavity (h1). As heat is applied to the evaporator region of the loop, vapor bubbles are generated in the flow as the latent heat of the working fluid absorbs the applied energy. These bubbles (or voids) serve to reduce the effective density of the liquid column inside of the evaporator resulting in a net pressure head difference between the downcomer and the evaporator. As more heat is applied to the system, more of the liquid in the evaporator is converted into vapor further reducing the effective density and driving more fluid flow. The maximum amount of fluid flow, and corresponding power input, is determined by the available height difference between the evaporator and condenser (h2 – h1).

Figure 3. (left) Void fraction can be highly non-linear with increasing power while quality remains mostly linear. (right) Flow rate in the system is mostly tied to void fraction and behaves similarly with respect to increasing power.

It is useful to define a term, void ratio, to refer to the ratio of void space in the evaporator to the volume still occupied by liquid. As more heat is applied to the LTS, the void ratio approaches 1 (or 100%). At this point the maximum height gradient between the condenser and evaporator is achieved because there is no more liquid head inside of the evaporator (i.e. h1 = 0). As shown in Figure 3, this point near maximum void fraction is not necessarily a point of dryout (or maximum vapor quality) like would be seen in other two-phase systems. Since void fraction is a density-driven term, fluids with low vapor densities and relatively high latent heat will reach a state near maximum voiding before all of the latent heat is consumed (i.e. quality = 1).

In practical terms what this means is that an LTS will always operate in an excess liquid flow regime. As shown in Figure 3, the flow rate around the loop could approach its maximum level at a vapor quality of around 0.5. In contrast, heat pipes operate in a binary boiling and condensation process where the evaporator sends 100% quality vapor to the condenser and only saturated liquid (i.e. quality = 0) is returned to the evaporator. In this case the maximum power that a heat pipe can carry is directly proportional to the latent heat of the working fluid. Since excess liquid is virtually guaranteed in an LTS, the maximum power handling capability can far exceed that of a heat pipe provided that sufficient vertical height is available.

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