Understanding the self-sustained oscillating two-phase flow motion in a closed loop pulsating heat pipe

In the framework of efficient thermal management schemes, pulsating heat pipes (PHPs) represent a breakthrough solution for passive on-chip two-phase flow cooling of micro-electronics. Unfortunately, the unique coupling of thermodynamics, hydrodynamics and heat transfer, responsible for the self-sustained pulsating two-phase flow in such devices, presents many challenges to the understanding of the underlying physical phenomena which have so far eluded accurate prediction. In this experimental study, the novel time-strip image processing technique was used to investigate the thermo-flow dynamics of a single-turn channel CLPHP (closed loop pulsating heat pipe) charged with R245fa and tested under different operating conditions. The resulting frequency data confirmed the effect of flow pattern, and thus operating conditions, on the oscillating behavior. Dominant frequencies from 1.2 Hz for the oscillating regime to 0.6 Hz for the unidirectional flow circulation regime were measured, whilst wide spectral bands were observed for the unstable circulation regime. In order to analytically assess the observed trends in the spectral behavior, a spring-mass-damper system model was developed for the two-phase flow motion. As well as showing that system stiffness and mass have an effect on the two-phase flow dynamics, further insights into the flow pattern transition mechanism were also gained. (C) 2015 Elsevier Ltd. All rights reserved.

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

Time-strip visualization and thermo-hydrodynamics in a Closed Loop Pulsating Heat Pipe

Navid Borhani, Giulia Spinato, John Richard Thome

New trends in the microprocessor industry are leading not only to miniaturization and integration but also to increases in power dissipation rates which now require advanced cooling solutions to prevent thermal damage to the devices. Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of such 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. This paper reports on flow visualization experiments in a Closed Loop Pulsating Heat Pipe (CLPHP)-charged with R245fa-operating over a range of test conditions. A novel time-strip image processing technique has been applied to the flow videos to extract qualitative details of flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. The latter can be coupled with thermal data to reveal new details regarding flow characteristics, such as two-phase flow pattern and its oscillation. Four distinct flow regimes and their steady thermal oscillation characteristics have been identified and discussed. (C) 2014 Elsevier Ltd. All rights reserved.

Elsevier2015

Operational regimes in a closed loop pulsating heat pipe

Navid Borhani, Giulia Spinato, John Richard Thome

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.

Elsevier France-Editions Scientifiques Medicales Elsevier2016

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

Navid Borhani, Sylwia Szczukiewicz, John Richard Thome

Elsevier2013