The Boiling Advantage
Cooling effectiveness, which is defined in terms of rate of heat removed per unit area per unit device-to-coolant temperature difference, is governed by both cooling configuration and type of coolant used. Three cooling configurations are commonly used, natural convection, where mild coolant motion is achieved by temperature-induced density gradients, forced convection, where coolant motion is achieved by a mechanical source such as a pump or fan, and phase change (boiling). Forced-convection is superior to natural convection, but the most demanding applications are managed with boiling.
Boiling a cooling fluid on the surface of a heat-dissipating device occurs when the surface temperature exceeds the fluid’s boiling point. Growth and departure of vapor bubbles at the surface draws bulk liquid towards the surface at high frequency, which, along with the ensuing latent heat exchange, greatly increases cooling effectiveness, allowing for dissipation of a broad range of heat fluxes corresponding to only modest increases in device temperature. These unique cooling attributes are realized in the nucleate boiling regime. The vapor-liquid exchange process that is responsible for much of the cooling effectiveness within this regime requires uninterrupted liquid access to the surface. Higher heat flux fluxes are dissipated by production of an increasing number of vapor bubbles per unit surface area. Increased bubble coverage of the surface will eventually lead to vapor coalescence and begins restricting liquid access to the surface. Once the vapor-liquid exchange process is interrupted, the power dissipated in the device itself will no longer be rejected, and the device temperature begins to escalate uncontrollably. This condition is the upper-most heat flux limit for the nucleate boiling regime termed critical heat flux (CHF). Once CHF is exceeded, the surface transitions into the film boiling regime, which corresponds to very high surface temperatures. In film boiling, the surface is completely isolated from liquid by a continuous vapor blanket, and heat is transferred to the liquid by conduction across this virtually insulating vapor blanket, as well as by radiation. Poor heat transfer coefficient associated with combined conduction/radiation explains the high surface temperatures encountered in film boiling. This is why CHF constitutes the upper heat flux design limit for any heat-flux-controlled boiling system.
Overall, boiling offers two key advantages over competing cooling configurations:
- Greatly decreased surface temperature
- Fairly constant surface temperature during large fluctuations in device heat flux.
