Heat pipes are capillary-pumped two-phase devices that transport heat from localized sources in electronics to a heat sink through continuous evaporation and condensation of an internal working fluid. Given the reliance of a heat pipe on capillary transport of liquid through internal wicking structures to the evaporator section where heat is applied, operating it at a power exceeding the so-called capillary limit can lead to dryout at the evaporator and subsequent device failure. However, in response to highly dynamic workloads in the electronics being cooled, heat pipes may more typically be exposed to heat loads above the capillary limit only intermittently over brief time intervals. Understanding the heat pipe response to these transient workloads is critical, as designing heat pipes for steady-state operation at the peak transient heat load would represent an expensive overdesign. Our previous work has experimentally characterized the transient heat pipe response to power pulses exceeding the capillary limit. It was demonstrated that a pulse must be sustained for a minimum duration termed the timeto-dryout before dryout is initiated. Once a pulse-induced transient dryout does occur in a heat pipe, its thermal resistance does not necessarily recover back to the pre-dryout performance even after the power input drops below the capillary limit. This behavior, termed thermal hysteresis, can be circumvented if the power is lowered (or throttled) to a sufficient threshold below the capillary limit for an extended time interval. In the current work, a first-of-its-kind transient heat pipe model is developed to predict the salient features of heat pipe response to pulse-load-induced dryout as well as recovery from dryout. The model uniquely considers spatiotemporal variations in local liquid saturation in the wick (i.e., the fraction of pore volume occupied by the liquid). Experiments are performed using commercial heat pipe samples that span a range of sizes and wick types to validate the model predictions. It is shown that the model can predict the transient thermal response, including thermal hysteresis, of the heat pipe during pre-dryout, dryout (pulse load), and post-dryout (recovery) stages with good accuracy. The model results are also validated against experiments for heat pipes spanning a range of wick types, heat pipe lengths, and heat pipe thicknesses. The capability to accurately predict crucial temporal events during dryout and recovery is key to establishing power expenditure strategies in electronics and designing heat pipes with improved dryout and recovery performance.

Heat pipes and vapor chambers are passive heat spreaders driven by capillary pumping of an internal working fluid via a porous wick. The capillary limit is the maximum steady-state heat input at which the fluid pressure drop can be supported by the capillary pressure head generated in the wick. However, heat pipes and vapor chambers often find application in devices where the heat input is highly transient and can exceed the capillary limit for brief time intervals. Operating heat pipes briefly above the capillary limit will not result in a dryout if the operating time interval does not exceed a characteristic time-to-dryout. Operation over a duration that exceeds this time-to-dryout can induce transient dryout and may lead to thermal hysteresis, that is, the original heat pipe thermal resistance may not be recovered even after the heat input is lowered back below the capillary limit. To fully recover the heat pipe performance after a transient dryout event, our recent experiments have shown that the heat input must be lowered (or throttled) significantly below the capillary limit. Due to the highly transient nature of power dissipation from electronic devices, it becomes imperative to characterize the throttling power level and duration required to ensure full recovery of a heat pipe from dryout under transient operations. This work experimentally characterizes recovery of heat pipes from dryout by power throttling under transient conditions, where 'power throttling' is the act of reducing the operating power level significantly below the capillary limit to eliminate post-dryout thermal hysteresis. We deduce from the experiments that the power must be throttled for longer than a minimum throttling time interval, defined as the time-to-rewet, in order to eliminate dryout-induced thermal hysteresis. The dependence of this time-to-rewet on the throttling power level is explored, and guidelines are presented on the need for throttling and the choice of throttling power under transient conditions.

Heat pipes and vapor chambers are passive thermal management devices used for efficient heat trans-port by phase change. Their passive operation is enabled by capillary pumping of the working fluid in a porous wick, which is operationally limited by the maximum pressure head it can provide. This cap-illary limit marks the maximum heat input at which the capillary pressure generated can overcome the pressure drop in the wick; operating above the capillary limit at steady state leads to dryout. Heat pipes and vapor chambers are increasingly being used in electronics systems where end-user activity dictates the transient power input which can therefore be highly variable and time-dependent. It was recently shown that heat pipes can withstand a power pulse exceeding the capillary limit for brief time intervals. Under such operating conditions, the heat pipe will experience dryout only if the duration of the pulse load is longer than a certain characteristic time interval. The pulse-load-induced dryout may result in an increased thermal resistance when the power is reduced back down to pre-dryout levels, thus exhibiting a hysteresis in heat pipe thermal performance. In this work, we experimentally characterize the recovery from pulsed-load-induced dryout. We further propose that the observed change in steady-state thermal performance before and after dryout results from contact angle hysteresis at the three-phase contact line of the wick-liquid interface. A model is developed based on this proposed mechanism to predict the na-ture of recovery from dryout-induced thermal hysteresis, as well as to identify that a given heat pipe has a maximum possible hysteresis. The experiments illustrate the trends inferred from the model for the re-covery process and confirm the existence of a "maximum hysteresis line," which identifies the worst-case scenario for thermal hysteresis after heat pipe dryout. Based on these mechanistic learnings, a new test -ing protocol is proposed for experimentally characterizing this post-dryout maximum hysteresis signature for a heat pipe.(c) 2023 Elsevier Ltd. All rights reserved.

An easy-to-use representation of vapor chambers is developed in terms of effective anisotropic properties. This approach enables accurate simulation of the vapor chamber represented as a solid conduction block by assigning appropriate values to its effective density, specific heat, in-plane thermal conductivity, and through-plane thermal conductance. These effective properties are formulated such that the vapor chamber operation in terms of steady-state and transient thermal responses matches a full, physical simulation of phase change and energy transport in the vapor core; they are intrinsic properties that can be applied independently of the boundary conditions and heat input.

The thermal resistance of a vapor chamber is primarily governed by conduction across the evaporator wick and the saturation temperature gradient in the vapor core. The relative contributions of these two predominant resistances can vary dramatically with vapor chamber operating conditions and geometry. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. It therefore becomes critical to simultaneously consider the contributions of the wick and vapor core thermal resistances in the development of a generalized methodology for vapor chamber working fluid selection. The current work uses a simplified thermal-resistance-network-based vapor chamber model to explore selection of working fluids and wick structures that offer the minimum overall thermal resistance as a function of the vapor chamber thickness and heat input. An illustrative example of working fluid selection, for cases with and without the contribution of wick thermal resistance, is first used to demonstrate the potential significance of the wick thermal resistance on fluid choice. This influence of the wick on working fluid selection is further explained based on the wick properties (effective pore radius, permeability, and effective thermal conductivity). The ratio of effective pore radius to wick permeability is found to be the most critical wick parameter governing the overall vapor chamber resistance at thin form factors where minimizing the wick thickness is paramount; the wick conductivity becomes an equally important parameter only at thicker form factors. Based on this insight, a new approach for vapor chamber design is demonstrated, which allows simultaneous selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition. (C) 2019 Elsevier Ltd. All rights reserved.

The maximum heat load that a heat pipe can sustain at steady state is governed by the balance between the capillary pressure provided by the wick and the flow resistance to liquid resupply at the evaporator. At heat loads beyond the capillary limit, the wick will dry out at the evaporator. However, the nature of the imposed heat load may be highly transient in various applications ranging from consumer electronic devices to server processors, depending on the end user needs and workloads. For such scenarios, it becomes critical to assess the operation of heat pipes in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to demonstrate that, under transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. The time-to-dryout during which the heat pipe can sustain a higher heat load is characterized for a step input higher than the steady-state capillary limit. The time-to-dryout is found to decrease as the magnitude of the step heat input increases.

The balance between the capillary pressure provided by the wick in a heat pipe or vapor chamber and the flow resistance to liquid resupply at the evaporator determines the maximum heat load that can be sustained at steady state. This maximum heat load is termed as the capillary limit; operation at steady heat loads above the capillary limit will result in dryout at the evaporator wick. However, different user needs and device workloads can lead to highly transient heat loads in applications ranging from consumer electronic devices to server processors. In these instances, the operation of heat pipes must be assessed in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is demonstrated that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady-state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. This characteristic temperature hysteresis following transient dryout has significant implications for the use of heat pipes for pulsed power dissipation. Further experiments are performed to bound the range of heat loads over which the temperature hysteresis is present following a transient dryout event. (C) 2019 Elsevier Ltd. All rights reserved.