Investigation of boiling behaviors in vapor chambers in response to transient heat input profiles

Vapor chambers are being increasingly utilized as passive heat spreaders in various high-power density thermal management architectures. Particularly, in applications involving power electronics packaging, there is a need to understand and predict thermal performance of vapor chambers under different transient heat loads. It is known that boiling could occur in the wick structure of a vapor chamber at the location of heat input if sufficiently high heat fluxes are directly imposed on the chamber wall. Prior experimental studies that characterize the steady state thermal resistance of vapor chambers versus input power can only identify the behavior before and after the transition to boiling. However, because the initiation of boiling is a discrete event that occurs during the transient powering up, it is critical to understand how boiling behaviors will affect the performance of the vapor chamber in response to the transient heat input profile characteristics. Furthermore, currently available transient vapor chamber modeling efforts do not consider the occurrence of nucleate boiling, precluding their usage for applications where boiling is likely to occur. In this paper, we experimentally characterize the occurrence and effect of nucleate boiling in a vapor chamber coupled with an air-cooled heat sink, subject to various transient heat input profiles. The long-time step response behaviors are first studied for ten on-off cycles, at different on-powers, to illustrate the complete temperature response to a steady state in each cycle. For a range of intermediate powers, an interesting phenomenon is observed, where boiling occurs in every alternate cycle; we identify this range as the transition regime of boiling incipience in the vapor chamber. We further investigate the transient response to a pulsed heat input for various on-times ranging three orders of magnitude from 0.1 s to 400 s, and with two duty cycles (0.25 and 0.5), while keeping either the on-power or the average power constant. For a fixed on-power, lower on-times (0.1 s – 2 s) do not cause any boiling, but for longer on-times (≥ 20 s), cases with lower duty cycles led to occurrences of boiling incipience. Furthermore, for a fixed average power, a higher on-power at a lower duty cycle is more likely to cause boiling, which interestingly leads to an even lower mean steady temperature compared to cases with lower on-power and larger duty cycle. These studies provide insights into the nature of boiling incipience in vapor chambers for their usage under realistic transient input powers mimicking power electronics