Two-Phase Flow Simulations within Plate Heat Exchangers

Plate heat exchangers are widely adopted in many industrial applications, such as ventilation, air conditioning, heat transfer processes, heat pumps and cooling of hydrodynamic circuits in engines. They provide higher heat transfer performance, compactness and flexibility as well as lower pressure drops compared to conventional tubular heat exchangers and are thus also used in various electronics cooling systems. Thus, they are also excellent units to use, for instance, as water-cooled condensers in electronics thermosyphon two-phase cooling systems. The current paper presents a novel inhouse simulation code for accurately rating and designing, under steady-state conditions, of single-and multi-passage plate heat exchangers. The basis of the code is a local one dimensional effectiveness-NTU approach coupled with the best proven methods for heat transfer coefficients and frictional pressure drops from literature; it has been implemented to model single-phase, evaporating and condensing flows over a wide range of operating test conditions, plate geometries and working fluids. The first part of the paper presents an overview of the modeling scheme and the associated flow chart. After that, the simulation results are validated against a diversified experimental database demonstrating a good agreement in terms of two-phase refrigerant side heat transfer coefficients and frictional pressure drops. Next, a simulation case study is provided to compare the local thermal-hydraulic performance between parallel-and counter-flow configurations. Finally, a sensitivity analysis is performed to analyze in detail the effects of the chevron angle and corrugation aspect ratio through the definition of the plate heat exchanger performance index that includes both overall heat duty and total pressure drop.

Two-Phase Heat Transfer Mechanisms within Plate Heat Exchangers

Raffaele Luca Amalfi

Plate heat exchangers are adopted in many domestic and industrial applications, such as ventilation, air conditioning, evaporation or condensation process, heat pumps and cooling of hydrodynamic circuits in engines. In the present thesis, an extensive experimental study to characterize thermal and hydraulic performance of a compact plate heat exchanger has been carried out and then follow up with proposition of new prediction methods for single-phase and evaporating flows in the plate¿s channels, in the form of general correlations that include large additional databases culled from the literature. Specifically, upward single-phase flow and flow boiling heat transfer of low pressure liquid refrigerants were investigated within a plate heat exchanger prototype fabricated with 1 mm pressing depth and chevron angle of 65°. High spatial and temporal resolution infrared measurements were implemented to obtain local (pixel-by-pixel) heat transfer coefficients and frictional pressure drops. Single-phase experiments were carried out for Reynolds numbers ranging from 34 to 1615 and Prandtl numbers from 4.9 until 6.5, and the associated effect of the main involved parameters on the thermal and hydraulic performance was analyzed in detail. Several of the most quoted prediction methods available in the literature were statistically evaluated against the present heat transfer and pressure drop database and a new models were proposed to predict mean and local thermal and hydraulic performance of the present compact plate heat exchanger. Two-phase experiments were carried out for mass fluxes ranging from 10 kgm-2s-1 to 85 kgm-2s-1, imposed heat fluxes from 225 Wm-2 to 4100 Wm-2, saturation temperatures from 19°C to 35°C and vapor qualities from 0.05 to 0.90. The heat transfer coefficient increased with mass flux, heat flux and saturation temperature (system pressure) whilst the frictional pressure drop rose with mass flux and vapor quality but decreased with saturation temperature. Based on local infrared measurements, a new correlation for predicting local frictional pressure gradient through the test section was proposed. Next, a wide experimental databank was culled from thirteen literature research studies, which investigated flow boiling heat transfer and two-phase frictional pressure drop within chevron plate heat exchangers. The database was first adopted to validate the predicted capabilities of 29 literature models, and then utilized to provide the new prediction methods to evaluate local heat transfer coefficients and frictional pressure gradient. These new models were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total) and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The general flow boiling prediction methods have been programmed into a simulation code to analyze local thermal and hydraulic performance of the plate heat exchangers over a large range of test conditions. The simulation results were then validated against independent experimental database from literature, resulting in very good agreement between each other. The present simulation code represents a powerful tool to be used for practical applications by the thermal engineers in order to design and rate commercial plate heat exchangers.

EPFL2016

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

State of the Art of High Heat Flux Cooling Technologies

Bruno Agostini, Bruno Michel, Jung Eung Park, John Richard Thome, Leszek Wojtan

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm2) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm2 of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.