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A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat. The liquid then returns to the hot interface through capillary action, centrifugal force, or gravity and the cycle repeats. Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors. The effective thermal conductivity varies with heat pipe length and can approach 100kW/(m⋅K) for long heat pipes, in comparison with approximately 0.4kW/(m⋅K) for copper. Structure, design and construction A typical heat pipe consists of a sealed pipe or tube made of a material that is compat

Thermo-Hydrodynamics in a Closed Loop Pulsating Heat Pipe

Giulia Spinato

Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of micro-electronics, providing advantages such as a simple construction and operation in any gravitational orientation. Unfortunately, the unique coupling of thermodynamics, hydrodynamics and heat transfer responsible for their operation has so far eluded comprehensive description or accurate prediction. The complexity of the self-sustained two-phase flow in PHPs presents many challenges to the understanding on the physical phenomena taking place. It is important to evaluate the heat and mass transfer mechanisms occurring during their operation in order to better describe their performance as a function of the operating conditions. In the present study, a new facility at the Laboratory of Heat and Mass Transfer (LTCM) was built to allow the synchronized thermal and visual investigation of a Closed Loop Pulsating Heat Pipe (CLPHP). A single-turn channel CLPHP was investigated using R245fa as the working fluid. The tests were carried out at filling ratios from 10 to 90 % and heat inputs from 2 to 60 W, for vertical and inclined orientation. Flow visualization was attained via the transparent front side of the test section which provided full optical access to the flow inside of the CLPHP channels. A novel time-strip image processing technique was applied to the high speed videos to extract qualitative details of the flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. Local temperature oscillations were also measured and their frequency spectra further helped in characterizing the self-sustained two-phase flow. Thermal resistance measurements were used to qualitatively and quantitatively assess the effect of the flow dynamics on the system thermal performance, which are presented as operational maps. The dynamics governing the PHP operation during oscillating and circulating flows and their effects on the system thermal performance were investigated and insight gained. Four distinct flow regimes and their thermal and flow-dynamics characteristics were identified, suggesting the strong coupling between the two-phase flow pattern and the system thermal behavior. Thin film evaporation was observed to be the most dominant thermal mechanism while heat transfer into the oscillating liquid slug was of second importance, together with localized nucleate boiling. Moreover, the net forces acting on the system could be identified through the novel time-strip technique, revealing new details on the mechanisms producing self-sustained two-phase flow oscillation and circulation, and the two-phase flow pattern transition. The role of gravity for the operation of this single-turn CLPHP was also assessed.

EPFL2014

Numerical Simulations of Pulsating Heat Pipes, Part 1: Modeling

Raffaele Luca Amalfi, Philippe Aubin, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The pulsating nature of a closed loop pulsating heat pipe (CLPHP) makes the prediction of its thermal-hydraulic performance extremely challenging. Unfortunately, the design of CLPHPs is still quite an empirical process, requiring testing of many prototypes by cold plate fabricators and thus inhibiting their possibilities for application. This means that a reliable and robust CLPHP simulation code is needed to do the design and prediction of its thermal performance. In this study, an updated code based on a 1-D model that traces out the entire loop is presented, starting from its first version developed in 2015. The present code is based on the principles highlighted in the Three-Zone Model for flow boiling in a micro-channel (i.e. thin liquid film zone, liquid slug zone and dry-out zone), but applied to the oscillating flow of CLPHPs in both evaporating and condensing processes. The heat transfer processes between the wall and the fluid are modelled for both the liquid slugs (convective) and vapor plugs (film evaporation and condensation). The wall receives heat from the electronics to be cooled based on the boundary conditions applied locally and the wall conducts heat axially through a 1-D conduction scheme in addition to the two-phase pulsating mechanism. The present code can be classified as an engineering tool, being pragmatic and thus achieving accurate predictions with reasonably low computational times, as opposed to a research tool. More specifically, the kinetics of the flow are solved exclusively on the liquid slugs as the dynamics of the vapor and liquid film were assumed negligible, except for their changing lengths. The code also includes mechanisms related to bubble creation (new vapor plug formation by nucleation) and collapse (when two liquid slugs coalesce due to the vapor plug in between collapses).

IEEE2019

Thermo-Hydrodynamics in a Closed Loop Pulsating Heat Pipe

Giulia Spinato

EPFL2014

Numerical Simulations of Pulsating Heat Pipes, Part 1: Modeling

Raffaele Luca Amalfi, Philippe Aubin, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

IEEE2019