Heat Transfer at spray-wall inpingement

SPRAY-WALL INTERACTION MECHANISMS

There is a significant number of applications, such as miniaturization of electronics in microprocessor developments, power electronics, high power amplifiers for radar systems, and dermatologic surgery, to name but a few, which demand for new cooling techniques capable of removing large heat dissipation rates. Among these new techniques, spray cooling appears to be one of the most notable methods with its ability for removing heat relying on phase-change, additionally to the convective heat transfer linked with fluid motion.

The two-phase flow generated in the spray cooling event has a highly complex nature and depends not only on several spray parameters, such as droplet size, velocity and density, but also on surface properties and morphology (e.g. roughness), and even on the initial surface temperature upon spray impact, therefore, it is difficult to clearly state which parameter is actually governing heat transfer.

Intermittent Spray-Cooling:

A new technology for controlling surface temperature

PULSED SPRAYS

An intermittent spray has the ability to provide a short time scale response in the control of heat fluxes by proper matching the frequency of injection with the pulse duration in the nucleate boiling heat transfer regime, according to the classical boiling the ory.

 

At low injection frequencies, nucleate boiling heat transfer is not limited by the delivery of liquid to the surface, or by the removal of vapor. At higher frequencies, an enhanced vaporization is induced by piercing and mixing the liquid film due to multiple drop impacts and a greater interaction between consecutive injections. This ability for transient cooling is useful in microelectronic devices, for example, where power dissipation rates depend on electronic current variations.

 

In some cases, the heat dissipation rates may vary by orders of magnitude, which means that cooling time scales can be equal to or less than 2s. Therefore, the variable control of the cooling potential is an advantage of thermal management systems using intermittent sprays, providing an adequate response to eventual overall temperature variations by proper adjusting its cooling timescale.

Interfacial plus Micro-scale Transport Phenomena

Center for Innovation, Technology and Policy Research

Instituto Superior Técnico

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