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During the operation of supercritical CO 2 (sCO 2 ) turbomachineries, large pressure ratios, supersonic conditions and non-ideal gas fluid dynamic phenomenon may happen, which will decrease the whole cycle efficiencies. Hence non-standard designs for the turbomachineries geometry are needed. Successfully simulations in capturing non-ideal gas fluid dynamics and coupled with optimization algorithm are hard and currently not available in any commercial CFD software. Therefore, the problem of optimizing sCO 2 turbomachinery is difficult to solve, which has become a major obstacle to obtain compact and high-efficiency sCO 2 turbomachineries. In this study, a modularized geometry optimizer is developed to obtain the non-standard geometric designs for sCO 2 turbomachineries. Multiple techniques are applied to this optimizer, include the Nelder–Mead algorithm, Mahalanobis distance and stochastic algorithm. The newly developed optimizer can successfully find the optimum satisfying the objective function under given weighting factors. The computational cost can be reduced through a stochastic algorithm. To validate the optimizer, a convergent–divergent nozzle for air with a target Mach number equal to 2.4 is optimized. Different starting points and combinations of weighting factors are used to create a Pareto front. Adjust the weighting factors for different terms of the objective function leading the optimizer to go to different directions in n-dimensional spaces. Three optimized cases, one is Mach number optimized, one is outlet flow uniformity optimized and the other is compromised case, are picked out and analyzed. The results show that the optimizer can successfully find optimized geometry than the reference case and potentially save computational cost. Due to the modularized characteristics, the components of this optimizer can be replaced with any available techniques, which mean the optimizer can be applied to solve different types of optimization problems. [ABSTRACT FROM AUTHOR]

Recently, there is an increased interest in supercritical CO2 and organic Rankine cycles (ORC) for their ability to achieve higher thermodynamic efficiency. However, the non-ideal gas thermodynamic in these cycles may affect the flow properties critically, necessitating the research of non-ideal compressible fluid dynamics (NICFD). Thus, simulation tools that can accurately predict fluid flows with NICFD are significant. This study presents a new approximation Riemann solver in OpenFOAM for NICFD. The new solver uses a real-gas (look-up table based) approximate Riemann flux calculator (modified from HLLC ALE flux calculator) through adding a new thermodynamic library tightly coupled with the OpenFOAM®. To validate the solver, three cases are presented, including a NASA transonic nozzle operating with air, to confirm the ability to correctly simulate the transonic flow phenomena and the shock waves; the VKI 2D cascade operated with MDM to assess the ability in simulating non-ideal gas flows typically to industrial applications; and the dense gas flow (MD4M) passing a backward ramp to illustrate the ability of the flux calculator and look-up table mechanism that can work well in the non-ideal region of fluid properties. This study can benefit future engineering applications of computational fluid mechanics of NICFD and the OpenFOAM community. [ABSTRACT FROM AUTHOR]

Heterogeneous hydrophilicity/hydrophobicity surface involves many applications such as dropwise condensation, but droplet dynamics on such surface is not well understood. Here, droplet impacting on heterogeneous surface was described by non-dimensional conservation equations. The volume of fluid (VOF) method tracked the gas-liquid interface. A set of parameters such as impacting velocity, drop size etc. were combined to form three key non-dimensional parameters of We , Oh and β i (size ratio of hydrophilic dot to drop). Numerical simulations agreed with impacting outcomes on uniformly hydrophilic or hydrophobic surface in references. For drop dynamics on heterogeneous surface, the regime maps containing complete-drop, single-drop-pinching-off and multi-drops-pinching-off were demonstrated over a wide range of We = 1–100, Oh = 0.001–1 and β i = 0.5–10. The increased β i enlarges the complete drop regime. The single-drop-pinching-off mode involves combined wall adhesion and surface tension induced short wave mechanism, while the multi-drops-pinching-off mode is caused by the propagation and interference of capillary waves from both ends of an elongated liquid column. The superposition principle was found for the first time: drop patterns include an adhesion part on the wall, similar to that on a hydrophilic surface, plus a rebounding part, similar to that on a super-hydrophobic surface. Spreading diameters are increased by β i at smaller We and moderate or larger β i , but they are not influenced by hydrophilic dot sizes at large We , under which inertia force thoroughly suppress effects of surface tension and wall adhesion. The present findings of this paper are helpful to design hydrophilic/hydrophobic surface and ensure droplet completeness during the impacting process. [ABSTRACT FROM AUTHOR]

Based on the volume-of-fluid (VOF) method in the FLUENT code, many phase-change models have been proposed and applied to simulate evaporation and condensation problems. To further improve the accuracy, in this article a new phase-change model is built using user-defined functions (UDFs). The accuracy of this new phase-change model is verified by two evaporation problems (a one-dimensional Stefan problem and a two-dimensional film boiling problem) and one condensation problem (single steam bubble condensation in subcooled water). The simulation results of this new model show good agreement with the classical analytical or numerical results, proving its accuracy and feasibility. [ABSTRACT FROM AUTHOR]

Purpose – Based on the numerical research on the relationship between the flow pattern transition and the condensation heat transfer in circular microchannels, the purpose of this paper is to bring forward a concept of external separation circular microchannel to regulate and control the flow pattern for enhancing the condensation heat transfer. Design/methodology/approach – The numerical research is based on the volume of fluid method and the vapor-liquid phase change model proposed by the present authors. Findings – By numerical research on the condensation process of water in a general circular microchannel, it is discovered that, with the increase of the inlet velocity and the reduction of the temperature difference between the saturation temperature and the channel wall temperature, the bubble detachment frequency is raised and the water vapor condensation length is extended, representing an exponential growth. Therefore, for the condensation process with low temperature difference and high mass flow rate, it is in urgent need to regulate and control the flow pattern. Originality/value – To prevent the flow pattern in the general circular microchannel converted from annular flow to slug flow and then to bubble flow, this paper brings forward a concept of external separation circular microchannel, which regulates and controls the flow pattern by discharging partial liquid from the annular wall opening. After regulation and control, the flow pattern is converted from original periodic annular flow/slug flow/bubble flow to current stable annular flow. Accordingly, the heat transfer performance is enhanced and the condensation length is lowered remarkably. [ABSTRACT FROM AUTHOR]

Highlights: [•] Pool boiling heat transfer experiment was performed on seven heater surfaces. [•] The heater surfaces can be divided into three types. [•] The open channel surface and uniform porous coating surface had intercrossed boiling curves. [•] The 2-D and 3-D porous coating surfaces significantly enhance heat transfer. [•] Critical heat flux for 3-D porous heater surface can be 3.7 times of that for plain surface. [Copyright &y& Elsevier]

The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities. [ABSTRACT FROM AUTHOR]

Experiments were performed to study pressure drops in copper foams embedded in a rectangular copper channel. De-ionized water was used as the working fluid with mass fluxes of 30-200 kg/m s, and inlet temperature of 40-80°C. The copper foam has the porosity of 0.88 and the pore densities of 30, 60 and 90 ppi (pores per inch). Both single-phase liquid flow and boiling two-phase flow are studied. Effects of mass fluxes, vapor mass qualities, and average pore diameters of metallic foams are investigated. It is found that friction factors for the single-phase liquid flow are mainly dependent on the Reynolds number and the average pore diameter of metallic foams. The friction factors are decreased with increases in the Reynolds numbers, and will approach 0.22 at high Reynolds numbers. For the boiling two-phase flow, two-phase pressure drops are increased with increases in the outlet vapor mass qualities, mass fluxes, and ppi values. The two-phase multiplier is increased with increases in the outlet vapor mass qualities and mass fluxes, and it is decreased with increases in the Martinelli parameter and will attain a constant value depending on the mass fluxes. The larger the mass fluxes, the larger the constant value is. An experimental correlation considering the effects of vapor mass qualities, mass fluxes, and average pore diameters of metallic foams is recommended, showing good accuracy to predict the two-phase pressure drops in metallic foams. [ABSTRACT FROM AUTHOR]

Abstract: We provide three-dimensional numerical simulations of conjugate heat transfer in conventional and the newly proposed interrupted microchannel heat sinks. The new microchannel heat sink consists of a set of separated zones adjoining shortened parallel microchannels and transverse microchambers. Multi-channel effect, physical property variations, and axial thermal conduction are considered. It is found that flow rate variations in different channels can be neglected, while heat received by different channels accounts for 2% deviations from the averaged value when the heat flux at the back surface of the silicon chip reaches 100W/cm2. The computed hydraulic and thermal boundary layers are redeveloping in each separated zone due to shortened flow length for the interrupted microchannel heat sink. The periodic thermal developing flow is responsible for the significant heat transfer enhancement. Two effects influence pressure drops across the newly proposed microchannel heat sink. The first one is the pressure recovery effect in the microchamber, while the second one is the head loss when liquid leaves the microchamber and enters the next zone. The first effect compensates or suppresses the second one, leading to similar or decreased pressure drop than that for the conventional microchannel heat sink, with the fluid Prandtl number larger than unity. [Copyright &y& Elsevier]

Abstract: Multi-channel effect is important to understand transport phenomenon in phase change systems with parallel channels. In this paper, visualization studies were performed to study the multi-channel effect in a silicon triple-channel condenser with an aspect ratio of 0.04. Saturated water vapor was pumped into the microcondenser, which was horizontally positioned. The condenser was cooled by the air natural convention heat transfer in the air environment. Flow patterns are either the annular flow at high inlet vapor pressures, or a quasi-stable elongated bubble at the microchannel upstream followed by a detaching or detached miniature bubble at smaller inlet vapor pressures. The downstream miniature bubble was detached from the elongated bubble tip induced by the maximum Weber number there. It is observed that either a single vapor thread or dual vapor threads are at the front of the elongated bubble. A miniature bubble is fully formed by breaking up the vapor thread or threads. The transient vapor thread formation and breakup process is exactly symmetry against the centerline of the center channel. In side channels, the Marangoni effect induced by the small temperature variation over the channel width direction causes the vapor thread formation and breakup process deviating from the side channel centerline and approaching the center channel. The Marangoni effect further forces the detached bubble to rotate and approach the center channel, because the center channel always has higher temperatures, indicating the multi-channel effect. [Copyright &y& Elsevier]

Abstract: High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0mm by 12.0mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0mm, and porosity of 0.88 and 0.95. The surface superheats are from −20 to 190K, and the heat fluxes reach 140W/cm2. The 30 and 60ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed. [Copyright &y& Elsevier]

Remarkably different behaviors are found when comparing micro-scale flow boiling heat transfer data by distinct authors, even under similar experimental conditions. Such differences are almost certainly related to the complexity of confined forced-flow boiling. Certain aspects of the phenomenon, which are negligible in the macro-scale, become surprisingly relevant when the system size becomes small. From the results reported in the literature on the thermal-fluid features of evaporating flows in small channels, the following study presents a discussion concerning convective boiling heat transfer, highlighting the aspects that are characteristic to confined two-phase flows. [ABSTRACT FROM AUTHOR]

Xu et al. have recently published a set of results for boiling heat transfer measurements in a multi-channel micro-scale evaporator for flow boiling of acetone in triangular cross-section channels (hydraulic diameter of 155.4 mm). In the present collaboration, we assess our current capability to predict this independent flow boiling data set with a fluid not in the original database and also much smaller in size using the phenomenological three-zone model of Thome, Dupont, and Jacobi. The method models boiling in small diameter channels in the elongated bubble/slug flow regime. The boiling data falling in this regime are identified here using a new micro-scale flow pattern map proposed by Revellin in order to utilize only test data corresponding to the elongated bubble flow mode. The decrease of the measured wall temperature due to the heat spread by longitudinal conduction through the heat sink was investigated through a finite differences analysis. In addition, a data reduction procedure different than that one used by Xu et al. was used and, consequently, some differences in the heat transfer behavior were found. Based on the present database, a new set of empirical parameters for the three-zone model was proposed. The conjugated effect of flow pattern and bubble/slug frequency on the heat transfer coefficient was also investigated. [ABSTRACT FROM AUTHOR]

Abstract: The steady two-dimensional Navier–Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent–divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle. [Copyright &y& Elsevier]

Abstract: Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O2) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended. [Copyright &y& Elsevier]

Abstract: Manifold microchannel (MMC) heat sinks have many advantages on low thermal resistance, compact structure, small amount of coolant, low flow rate, uniform temperature distribution along the flow direction, etc. However, a high pressure drop is needed due to the small channel size used thus the pumping power is increased. Here we study the effect of surfactant on friction pressure drop in a specifically designed manifold microchannel heat sink. The microchannel has a cross section of 100 microns by 300 microns with a length of 10.0 mm. An anionic surfactant of 100 ppm Sodium Dodecyl Sulphate (SDS with 95% purity grade) aqueous solution and a new type of 300 ppm green non-ionic surfactant of Alkyl Polyglycoside (APG1214 with 98% purity grade) aqueous solution are used as the working fluids. It is found that the drag reductions are dependent on the flow velocities and the fluid temperatures. The drag reduction is not significant for laminar flow, but it is increased in the transition flow regime. The temperature rise can promote the drag reductions in the presence of SDS and APG. It is shown that the APG is better than the SDS regarding the drag reductions at high temperatures. [Copyright &y& Elsevier]