Computational study of bubble, thin-film dynamics and heat transfer during flow boiling in non-circular microchannels

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to ℎ = 0.229 mm and the range of width-to-height aspect ratios = 0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range = 50 − 200 kW∕m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with = 0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for = 1 (square channels) and 10% higher than that for = 4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for = 0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on in the range = 0.5 − 2

Numerical simulation of crust freezing in processed meat: A fully coupled solid–fluid approach

We present a numerical model for the simulation of continuous impinge ment freezing of processed food products. This model is capable of fully describing the fluid dynamics of the non-isothermal flow field, including turbulence with conjugate heat transfer (CHT). The motion of the solid region is captured by advecting the solid rathe than employing a moving mesh algorithm, resulting in a model that is more computationally efficient. This methodology is implemented as a numerical solver using the well-known open-source library OpenFOAM. Our results confirm that the proposed model can provide detailed insight on the freezing process at a minimum computational cost.The authors would like to thank University of Nottingham Hermes fund for sponsoring the research

Dynamics of long bubbles propagating through cylindrical micro-pin fin arrays

The dynamics of two-phase flows confined within complex and non-straight geometries is of interest for a variety of applications such as micro-pin fin evaporators and flow in unsaturated porous media. Despite the propagation of bubbles in straight channels of circular and noncircular cross-sections has been studied extensively, very little is known about the fluid dynamics features of bubbles and liquid films deposited upon the inner walls of complex geometries. In this work, we investigate the dynamics of long gas bubbles and thin films as bubbles propagate through arrays of in-line cylindrical pins of circular shape in cross-flow, for a range of capillary and Reynolds numbers relevant to heat transfer applications and flow in porous media, different pitch of the cylinders and bubble lengths. Three-dimensional numerical simulations of the two-phase flow are performed using the open-source finite-volume library OpenFOAM v.1812, using a geometric Volume of Fluid (VOF) method to capture the interface dynamics. Systematic analyses are conducted for a range of capillary numbers Ca = 0. 04 --2R, Reynolds numbers Re = 1 -- 1000, streamwise pitch of the cylinders sx = 0. 125R, with R being the radius of the pin fins, and initial bubble length Lb = 2. 5R --12R. The simulations reveal that when bubbles propagate through pin fin arrays, they tend to partially coat the cylinders with a thin liquid film and to expand in the cross-stream direction within the gap left between adjacent cylinders. The liquid film deposited on the cylinders is significantly thinner than that reported for straight channels and similar geometrical constraints. As the streamwise distance between the cylinders is decreased, the flow configuration tends towards that for a straight channel, whereas larger distances cause the bubble to expand excessively in the cross-stream direction, and to eventually arrest when sx > 2R. Inertial effects have a strong impact on the bubble shape and dynamics when Re > 500, triggering time-dependent patterns that lead to bubble fragmentation and much thicker liquid films

Conjugate heat transfer effects on flow boiling in microchannels

This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of Dh=229 µm, a uniform base heat flux of qb=100 kW/m2, and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ=0.25–4 and Wf=Dh/8−Dh, respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu≃4 and Tb−Tsat≃9K when ϵ=4, to Nu≃6 and Tb−Tsat≃2K when ϵ=0.25, for same fin thickness Wf=Dh/8. In the two-phase flow regime, Nusselt numbers in the range Nu=12−36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, Wf=Dh/8 and Wf=Dh/4, an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ=0.5–2 the Nusselt number appears weakly dependent on ϵ. For thicker fins, Wf=Dh/2 and Wf=Dh, the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ=0.5–2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ<1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits

Retrospective evaluation of metformin and/or metformin plus a new polysaccharide complex in treating severe hyperinsulinism and insulin resistance in obese children and adolescents with metabolic syndrome

Background: Pharmacological treatment of obesity and glucose-insulin metabolism disorders in children may be more difficult than in adults. Thus, we evaluate the effects of metformin in comparison with metformin plus a polysaccharide complex (Policaptil Gel Retard®, PGR) on body weight and metabolic parameters in obese children and adolescents with metabolic syndrome (MetS). Patients and methods: We retrospectively collected 129 children and adolescents (67 girls, 62 boys; median age 12.6 years) treated for a minimum of two years with metformin and low glycemic index (LGI) diet. Of these, 71 patients were treated with metformin plus PGR after at least 12 months of metformin alone. To minimize the confounding effect of the LGI on auxological and metabolic parameters, the patients were compared with age-, sex-, and BMI-matched control group with obesity and MetS (51 subjects; 24 males, 27 females) treated only with a LGI diet. Assessments included lipids, glucose and insulin (fasting and after oral glucose tolerance test) concentrations. The Homeostatic Model Assessment of Insulin Resistance (HOMA-IR), Matsuda, insulinogenic and disposition indices were calculated. Results: Metformin treatment led to a significant reduction in BMI SDS (p &lt; 0.0001), with a significant difference in ΔBMI SDS between patients and controls (p &lt; 0.0001). Moreover, metformin treated patients showed a reduction in HOMA-IR (p &lt; 0.0001), HbA1c levels (p &lt; 0.0001) and a significant increase in Matsuda index (p &lt; 0.0001) in respect to the reduction discovered in controls (p &lt; 0.05). Moreover, in contrast to the group treated with metformin alone and controls, patients treated with metformin plus PGR showed a further reduction in BMI SDS (p &lt; 0.0001), HOMA-IR (p &lt; 0.0001), HbA1c (p &lt; 0.0001), total, HDL and LDL cholesterol (p &lt; 0.0001), as well as an increase in Matsuda (p &lt; 0.0001), disposition (p &lt; 0.005) and insulinogenic (respectively, p &lt; 0.05 and p &lt; 0.0001) indices. Conclusions: Metformin appears to show short-term efficacy in reducing BMI, adiposity and glucose and insulin parameters in obese children and adolescents with MetS. However, PGR added to metformin may be useful to potentiate weight loss and to improve glucose-insulin metabolism and adiposity parameters in these patients

Description of chemical transport in laboratory rock cores using the continuous random walk formalism

We investigate chemical transport in laboratory rock cores using unidirectional pulse tracer experiments. Breakthrough curves (BTCs) measured at various flow rates in one sandstone and two carbonate samples are interpreted using the one-dimensional Continuous Time Random Walk (CTRW) formulation with a truncated power law (TPL) model. Within the same framework, we evaluate additional memory functions to consider the Advection-Dispersion Equation (ADE) and its extension to describe mass exchange between mobile and immobile solute phases (Single-Rate Mass Transfer model, SRMT). To provide physical constraints to the models, parameters are identified that do not depend on the flow rate. While the ADE fails systematically at describing the effluent profiles for the carbonates, the SRMT and TPL formulations provide excellent fits to the measurements. They both yield a linear correlation between the dispersion coefficient and the Péclet number (DL Pe for 10 < (Pe) < 100), and the longitudinal dispersivity is found to be significantly larger than the equivalent grain diameter, De. The BTCs of the carbonate rocks show clear signs of nonequilibrium effects. While the SRMT model explicitly accounts for the presence of microporous regions (up to 30% of the total pore space), in the TPL formulation the time scales of both advective and diffusive processes (t1 (Pe) and t2) are associated with two characteristic heterogeneity length scales (d and l, respectively). We observed that l 2.5 × De and that anomalous transport arises when ld (1). In this context, the SRMT and TPL formulations provide consistent, yet complementary, insight into the nature of anomalous transport in laboratory rock cores

Highly efficient spatial data filtering in parallel using the opensource library CPPPO

This program has been imported from the CPC Program Library held at Queen's University Belfast (1969-2018) Abstract CPPPO is a compilation of parallel data processing routines developed with the aim to create a library for “scale bridging” (i.e. connecting different scales by mean of closure models) in a multi-scale approach. CPPPO features a number of parallel filtering algorithms designed for use with structured and unstructured Eulerian meshes, as well as Lagrangian data sets. In addition, data can be processed on the fly, allowing the collection of relevant statistics without saving individual snapshot... Title of program: CPPPO Catalogue Id: AFAQ_v1_0 Nature of problem Development of closure models for momentum, species transport and heat transfer in fluid and fluid-particle systems using purely Eulerian or Euler-Lagrange simulators. Versions of this program held in the CPC repository in Mendeley Data AFAQ_v1_0; CPPPO; 10.1016/j.cpc.2016.05.02

Heterogeneous Multi-Rate mass transfer models in OpenFOAM®

We implement the Multi-Rate Mass Transfer (MRMT) model for mobile–immobile transport in porous media (Haggerty and Gorelick, 1995; Municchi and Icardi, 2019 [1]) within the open-source finite volume library OpenFOAM® (Foundation, 2014). Unlike other codes available in the literature (Geiger et al., 2011 [2]; Silva et al., 2009), we propose an implementation that can be applied to complex three-dimensional geometries and highly heterogeneous fields, where the parameters of the MRMT can arbitrarily vary in space. Furthermore, being built over the widely diffused OpenFOAM® library, it can be easily extended and included in other models, and run in parallel. We briefly describe the structure of the multiContinuumModels library that includes the formulation of the MRMT based on the works of Haggerty and Gorelick (1995) and Municchi and Icardi (2020a). The implementation is verified against benchmark solutions and tested on two- and three-dimensional random permeability fields. The role of various physical and numerical parameters, including the transfer rates, the heterogeneities, and the number of terms in the MRMT expansions, is investigated. Finally, we illustrate the significant role played by heterogeneity in the mass transfer when permeability and porosity are represented using Gaussian random fields

Heterogeneous Multi-Rate mass transfer models in OPENFOAM®

We implement the Multi-Rate Mass Transfer (MRMT) model for mobile–immobile transport in porous media (Haggerty and Gorelick, 1995; Municchi and Icardi, 2019 [1]) within the open-source finite volume library OpenFOAM® (Foundation, 2014). Unlike other codes available in the literature (Geiger et al., 2011 [2]; Silva et al., 2009), we propose an implementation that can be applied to complex three-dimensional geometries and highly heterogeneous fields, where the parameters of the MRMT can arbitrarily vary in space. Furthermore, being built over the widely diffused OpenFOAM® library, it can be easily extended and included in other models, and run in parallel. We briefly describe the structure of the multiContinuumModels library that includes the formulation of the MRMT based on the works of Haggerty and Gorelick (1995) and Municchi and Icardi (2020a). The implementation is verified against benchmark solutions and tested on two- and three-dimensional random permeability fields. The role of various physical and numerical parameters, including the transfer rates, the heterogeneities, and the number of terms in the MRMT expansions, is investigated. Finally, we illustrate the significant role played by heterogeneity in the mass transfer when permeability and porosity are represented using Gaussian random fields

Heterogeneous Multi-Rate mass transfer models in OPENFOAM®

We implement the Multi-Rate Mass Transfer (MRMT) model for mobile–immobile transport in porous media (Haggerty and Gorelick, 1995; Municchi and Icardi, 2019 [1]) within the open-source finite volume library OpenFOAM® (Foundation, 2014). Unlike other codes available in the literature (Geiger et al., 2011 [2]; Silva et al., 2009), we propose an implementation that can be applied to complex three-dimensional geometries and highly heterogeneous fields, where the parameters of the MRMT can arbitrarily vary in space. Furthermore, being built over the widely diffused OpenFOAM® library, it can be easily extended and included in other models, and run in parallel. We briefly describe the structure of the multiContinuumModels library that includes the formulation of the MRMT based on the works of Haggerty and Gorelick (1995) and Municchi and Icardi (2020a). The implementation is verified against benchmark solutions and tested on two- and three-dimensional random permeability fields. The role of various physical and numerical parameters, including the transfer rates, the heterogeneities, and the number of terms in the MRMT expansions, is investigated. Finally, we illustrate the significant role played by heterogeneity in the mass transfer when permeability and porosity are represented using Gaussian random fields