Dynamic Numerical Microchannel Evaporator Model to Investigate Parallel Channel Instabilities

A fully dynamic model of a microchannel evaporator is presented. The aim of the model is to study the highly dynamic parallel channel instabilities that occur in these evaporators in more detail. The numerical solver for the model is custom-built and the majority of the paper is focused on detailing the various aspects of this solver. The one-dimensional homogeneous two-phase flow conservation equations are solved to simulate the flow. The full three-dimensional (3D) conduction domain of the evaporator is also dynamically resolved. This allows for the correct simulation of the complex hydraulic and thermal interactions between the microchannels that give rise to the parallel channel instabilities. The model uses state-of-the-art correlations to calculate the frictional pressure losses and heat transfer in the microchannels. In addition, a model for inlet restrictions is also included to simulate the stabilizing effect of these components. In the final part of the paper, validation results of the model are presented, in which the stability results of the model are compared with the existing experimental data from the literature. Next, a parametric study is performed focusing on the stabilizing effects of the solid substrate properties. It is found that increasing the thermal conductivity and thickness of the solid substrate has a strong stabilizing effect, while increasing the number of microchannels has a small destabilizing effect. Finally, representative dynamic results are also given to demonstrate some of the unique capabilities of the model.

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

Thermal and Visual Operational Characteristics of Multi-Microchannel Evaporators using Refrigerants

Sylwia Szczukiewicz

Two-phase flow operational stability of refrigerants R245fa, R236fa, and R1234ze(E) in 100 x 100 μm2 multi-microchannels for cooling of future high-performance 3D stacked architectures with interlayer cooling has been addressed in the current experimental investigation. Without any inlet restrictions in the micro-evaporator, significant flow instabilities, back flow, and flow maldistribution led to high-amplitude and high-frequency temperature and pressure oscillations. Such undesired phenomena were inhibited by placing rectangular restrictions (micro-orifices) at the inlet of each channel, thus ensuring a wide range of stable two-phase flow operating conditions. The effects of different orifice expansion ratios and fluids on the performance of the evaporator were studied by using suitably designed modular test sections. Simultaneous high-speed video and infra-red camera visualizations of the two-phase flow and heat transfer dynamics across the micro-evaporator area allowed the various different operating regimes to be identified and then represented by the two-phase flow operational maps. Two-phase flow flashed by the micro-orifices was identified as the optimal operating condition, while dissipating high heat fluxes and keeping the junction chip temperature below a typical CPU operating condition. In the present study, a novel in-situ pixel by pixel technique was developed to calibrate the raw infra-red images, thus converting them into two-dimensional temperature fields of 10’000 pixels over the test section surface. A comprehensive analysis of those temperature maps supported by the flow visualization videos confirmed that the two-phase flow patterns appearing in the channel and the transitions between them have a remarkable influence on the heat transfer coefficients, which were determined taking into account 3D heat spreading. The inlet and the outlet restriction pressure losses were quantified in order to accurately simulate the hydraulic performance of microchannel evaporators and provide more reliable heat transfer data. The heat transfer coefficients were determined with a very fine resolution that enabled the trends in the heat transfer coefficients along the channel length in the neighborhood of the flow transition from coalescing bubble to the annular flow to be studied in detail. It was shown that the heat transfer coefficient does not change sharply at the transition zone, but rather has a smooth change in trend. The characteristic U-shape of the heat transfer coefficient trend was observed, where the descending branch of the curve corresponds to the coalescing elongated bubble flow regime, while ascending one represents the increasing heat transfer coefficient in the annular flow regime. A good agreement of the experimental heat transfer coefficients and one existing flow pattern-based prediction method was found and a new vapor quality buffer is proposed as an update of this model.

EPFL2012

Flow boiling heat transfer and two-phase flow phenomena of CO2 in macro-and micro-channel evaporators: Fundamentals, applications and engineering design

John Richard Thome

The paper presents a comprehensive review of fundamentals and engineering applications of CO2 flow boiling heat transfer, flow patterns and two-phase pressure drops in macro- and micro-channel evaporators. First, distinctions of macro- and micro-channels are discussed. Second, the review addresses the extensive experimental studies on CO2 flow boiling heat transfer and two-phase flow in macro- and micro-channels. The effects of the physical properties on flow boiling heat transfer, flow patterns and pressure drops are analysed by simulations using various physical property packages. Furthermore, analysis of the existing experimental studies of flow boiling heat transfer is presented and the physical mechanisms are discussed. Next, generalized CO2 flow pattern map and flow pattern based mechanistic flow boiling heat transfer and two phase pressure drop models specially developed for CO2 are discussed. New experimental database of flow boiling heat transfer and diabatic two phase frictional pressure drop have been set up to evaluate the models. Comparative results of the flow pattern map, heat transfer and pressure drop models to the experimental database are analysed. In addition, studies of CO2 flow boiling in enhanced channels are summarized. The oil effects on the flow boiling heat transfer and two phase frictional pressure drop are analyzed and simulated as well. According to the extensive analysis, the physical mechanisms and prediction models of the CO2 flow boiling heat transfer and two phase flow phenomena in evaportaors have been well understood. In the aspect of engineering application, comparisons of simulation results and the experimental data in the real thermal systems are discuseed. Furthermore, research need of CO2 evaporators is discussed. Some practical design methods of CO2 evaporators are recommended according to the analysis. Finally, the potential application of CO2 for high heat flux cooling, thermal and power systems are also discussed. Future research needs in flow boiling heat transfer and two phase flow of CO2 in evaporators and engineering applications are discussed and recommended according to this comprehensive review.

PERGAMON-ELSEVIER SCIENCE LTD2021

Simulation and experimental validation of pulsating heat pipes

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

PERGAMON-ELSEVIER SCIENCE LTD2021

Thermal and Visual Operational Characteristics of Multi-Microchannel Evaporators using Refrigerants

Sylwia Szczukiewicz

EPFL2012

Flow boiling heat transfer and two-phase flow phenomena of CO2 in macro-and micro-channel evaporators: Fundamentals, applications and engineering design

John Richard Thome

PERGAMON-ELSEVIER SCIENCE LTD2021