Operational regimes in a closed loop pulsating heat pipe

Synchronized thermal and visual investigation was carried out on a single-turn channel CLPHP using R245fa as the worldng 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 orientations. A systematic analysis of the flow patterns, their transitions and thermal resistance measurements suggests a strong coupling between the two-phase flow pattern and the system thermal behavior. The effect of the flow dynamics on the system thermal performance was also qualitatively and quantitatively assessed and presented as 'operational maps'. Local time-averaged heat transfer coefficients were extracted by applying a-state-of-the-art mechanistic model for the evaporation of elongated bubbles in the CLPHP microchannels using the flow measurements. The obtained local and averaged results were then used to qualitatively assess and account for the heat transfer characteristics in the CLPHP evaporator U-turn for the different flow patterns. Based on this analysis, thin film evaporation was found to be the dominant thermal mechanism, while heat transfer into the oscillating liquid slug and localized nucleate boiling were of secondary importance. (C) 2015 Elsevier Masson SAS. All rights reserved.

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

Fine-resolution two-phase flow heat transfer coefficient measurements of refrigerants in multi-microchannel evaporators

Navid Borhani, Sylwia Szczukiewicz, John Richard Thome

The time-averaged two-phase flow heat transfer coefficients were determined with a very fine resolution based on infra-red (IR) temperature measurements conducted over a 1 cm(2) heated area of a multi-micro-, channel evaporator. This experimental investigation included two low-pressure refrigerants, R245fa and R236fa, as well as one medium-pressure new refrigerant R1234ze(E) flowing in four different test sections. The number of microchannels and their cross-sectional areas were the same for all the tested micro-evaporators, respectively, 67 and 100 x 100 mu m(2). Channel entrances with and without inlet micro-orifices were tested for establishing stable flow. Using 90 x 90 pixel grid (and then averaged width-wise), 90 local heat transfer coefficients were measured from inlet to outlet, yielding a U-shape of the heat transfer coefficient trend, where the descending branch of the curve corresponds to the coalescing elongated bubble flow regime (CB), while ascending one represents the increasing heat transfer coefficient in the annular flow regime (AF). The effects of channel mass flux, wall heat flux, orifice expansion ratio, and fluid properties on the wall heat transfer coefficient were identified for a selected number of data sets. In terms of predicted heat transfer coefficient values, in both the CB and AF flow regimes, the experimental results were found to be in a good agreement with respective existing flow pattern-based heat transfer prediction methods. Whereas, in order to better reflect the obtained experimental U-shaped trend of the heat transfer coefficient close to the CB-AF flow transition (which is a churn flow), the joined model of these two was modified by proposing a new vapor quality buffer for the width of the transition. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013

Simulation and experimental validation of pulsating heat pipes

Philippe Aubin, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The one-dimensional transient numerical code of JJ Cooling Innovation has been significantly improved to simulate the thermal/hydraulic performance of pulsating heat pipes (PHPs). Using a mechanistic approach, the liquid slug and vapor plug flows and their interaction with the surrounding wall are modeled. The heat transfer is predicted using the Three-Zone Model of flow boiling in microchannels, which includes liquid film evaporation and liquid film condensation of vapor plugs and convection of liquid slugs. Discretizing the entire PHP serpentine into one continuous grid, the code solves all variables based on local conditions. The code allows the user to define the working fluid, the channel's shape, material and wall roughness, the serpentine's pitch, the geometrical parameters of evaporator, adiabatic region and condenser zones, and the operating parameters. The boundary conditions in the evaporator zone can either be constant heat flux or constant temperature, while on the condenser side, the coolant temperature is constant as well as the heat transfer coefficient. The code works without any fitting factor and predicts all local variables in the 1D grid as a function of time, including the liquid film thickness and the onset of boiling for the formation of new vapor plugs within the evaporating and adiabatic zones, if local nucleation conditions are reached. Growth, collapse and coalescence of generated bubbles are taken into account. The film thermal resistance is corrected to a radial conduction model. The implementation of the initial liquid film thickness laid behind a liquid slug is updated to include the effect of the bubble velocity and acceleration as well as the length of the leading slug. The nucleation of new vapor plug has also been updated. The onset of nucleation is only dependent on the local thermodynamic state. If the threshold is reached, a new method is used to simulate micro-bubbles formation and coalescence to a channel-size vapor plug. These modifications greatly improve the code accuracy and stability allowing simulations with high pressure fluid rather than ethanol only. Furthermore, in the on-going EU project Pulsating Heat Pipes for Hybrid Propulsion Systems (PHP2), a new PHP test bench has been designed and built at Provides Metalmeccanica. A copper tube PHP with a 19-turn of 2.0 mm internal diameter and 12 mm tube pitch was tested over a wide range of imposed heat loads. A microchannel water-cooling plate was used at the condenser side with an inlet temperature of 20 degrees C. Then, tests were done for vertical and horizontal orientations with R1233zd(E) as the working fluid. The code, which was previously validated for ethanol PHP test data from KAIST, is now successfully validated for the present experimental data points. Accurate results were obtained from low heat loads up to 250 W for a set of 49 data points, predicting 97% within an error bandwidth of 30%.

PERGAMON-ELSEVIER SCIENCE LTD2021