Vertical Tube Research Articles

Verification and Validation of Spectral Element Code for Supercritical CO2 Flow in Vertical Heated Tubes

The investigation of heat transfer in supercritical CO2 (sCO2) has garnered considerable attention in recent decades, given sCO2’s potential as a promising working fluid for advanced power conversion cycles. Despite previous research efforts, there are still gaps in our understanding of sCO2 heat transfer, particularly in conditions associated with heat transfer deterioration. To delve into sCO2 heat transfer more comprehensively, we propose employing the high-fidelity computational fluid dynamics code NekRS to simulate sCO2 flow using the large eddy simulation technique. Through graphics processing unit acceleration, NekRS achieves a higher computational speed than traditional CPU-based systems. However, before using NekRS in practical applications involving sCO2, it is imperative to perform verification and validation. This paper presents our efforts to verify and validate the NekRS code’s capability for simulating sCO2 using heated vertical tubes, where heat transfer deterioration usually happens. To accommodate the unique properties of sCO2, we have modified the NekRS code by integrating third-party property modules, such as REFPROP and PROPATH. Our simulations are compared with experimental and numerical data from the literature, instilling confidence in leveraging NekRS for future engineering applications. Our simulations also reveal that the accuracy of the property module significantly impacts the results, with REFPROP outperforming PROPATH for sCO2 properties. Additionally, we observed that, depending on the flow direction, buoyancy can either enhance or suppress turbulence in sCO2 flow. In upward flow, under certain conditions, the suppressed turbulence leads to heat transfer deterioration, resulting in elevated wall temperatures.

Novel Staged Free-Fall Reactor for the (Catalytic) Pyrolysis of Lignocellulosic Biomass and Waste Plastics.

Pyrolysis of lignocellulosic biomass and waste plastics has been intensely studied in the last few decades to obtain renewable fuels and chemicals. Various pyrolysis devices have been developed for use in a laboratory setting, operated either in batch or continuously at scales ranging from milligrams per hour to tenths of g per hour. We report here the design and operation of a novel staged free-fall (catalytic) pyrolysis unit and demonstrate that the concept works very well for the (catalytic) pyrolysis of pinewood sawdust, paper sludge, and polypropylene as representative feeds. The unit consists of a vertical tube with a pretreatment section, a pyrolysis section, a solid residue collection section, a gas-liquid separation/collection section, and a catalytic reaction section to optionally perform ex situ catalytic upgrading of the pyrolysis vapor. The sample is placed in a tube, which is transported by gravity through various sections of the unit. It allows for rapid testing with semicontinuous feeding (e.g., 50 g h-1) and the opportunity to perform reactions under an (inert) gas (e.g., N2) at atmospheric as well as elevated pressure (e.g., 50 bar). Liquid yields for noncatalytic sawdust pyrolysis at optimized conditions (475 °C and atmospheric pressure) were 63 wt % on biomass intake. A lower yield of 51 wt % (on a biomass basis) was obtained for the noncatalytic pyrolysis of paper sludge, likely due to the presence of minerals (e.g., CaCO3) in the feed. The possibility of using the unit for ex situ catalytic pyrolysis (pyrolysis at 475 °C and catalytic upgrading at 550 °C) was also successfully demonstrated using paper sludge as the feed and H-ZSM-5 as the catalyst (21 wt % catalyst on biomass). This resulted in a biphasic liquid product with 25.6 wt % of an aqueous phase and 11 wt % of an oil phase. The yield of benzene, toluene, and xylenes was 1.9 wt % (on a biomass basis). Finally, the concept was also proven for a representative polyolefin (polypropylene), both noncatalytic as well as in situ catalytic pyrolysis using H-ZSM-5 as the catalyst at 500 °C. The liquid yield of thermal, noncatalytic plastic pyrolysis was as high as 77 wt % on plastic intake, while in situ catalytic pyrolysis gave a combined 7.8 wt % yield of benzene, toluene, and xylenes on plastic intake.

The growth and characterization of Au-catalyzed gallium oxide nanowires

AbstractGallium oxide is a powerful and versatile wide band gap semiconductor, which has been gaining scientific interest in recent years due to its applications in power electronics. In this work, the growth of β-gallium oxide (Ga2O3) nanowires has been achieved by the vapor–solid–liquid method on silicon wafer, fused quartz, and gallium oxide thin-film substrates. In a chamber purged of ambient atmosphere, pure gallium was heated to produce gallium vapor, which was then oxidized by minute oxygen impurities in the backing gas to form gallium oxide nanowires on a substrate, inside a vertical tube furnace. On the silicon wafer substrates, β-Ga2O3 nanowires were observed to grow in a specific crystallographic direction via transmission electron microscopy. The resulting nanowires were highly faceted and had a unique polyhedral structure on the gold catalyst ‘cap’ of the nanowire. Time of growth and oxygen-level studies were conducted on nanowires grown on various substrates. Graphical abstract

Experimental and theoretical study of single bubble formation at the nozzle in a liquid oxygen crossflow

In this paper, the condensation bubbles generated by the mixing and condensation of gaseous and liquid oxygen crossflow experimentally and theoretically. The mixing and condensation experiment of the bubbles was carried out, employing gaseous and liquid oxygen as working fluids in vertical tubes. Three successive flow patterns were observed: initial gas plume, condensation oscillation, and separation condensation. The corresponding characteristics include gas mass aggregation, bubble surface smoothness and wrinkling alternately, and bubble disappearance after separation and condensation. Then, a Nusselt number (Nu) heat transfer correlation formula is fitted with a corresponding fitting error in the range of −31% ∼ 35%. In addition, bubble force balance and conservation equations were employed to establish a mixing condensation bubble model suitable for gaseous and liquid oxygen in vertical tubes, and the error between theoretical and experimental results was in the range of −25% ∼ 20%. Furthermore, the effects of working conditions and forces on bubble condensation length has been analyzed. The results showed that increasing the liquid oxygen flow rate, reducing the gas oxygen volume flow rate, or appropriately reducing the gas inlet diameter led to reducing the bubble condensation length. It was shown that the bubble drag and shear lift forces affect the condensation length mostly.

Improving pressure drop predictions for R134a evaporation in corrugated vertical tubes using a machine learning technique trained with the Levenberg–Marquardt method

The present investigation utilized a machine learning structure to ascertain the pressure drop in vertically positioned, corrugated copper tubes during the evaporation process of R134a. The evaporator was a counter-flow heat exchanger, in which R134a flowed in the inner corrugated tube and hot water flowed in the smooth annulus. Different evaporation mass fluxes (195–406 kg m-2 s-1) and heat fluxes (10.16–66.61 kW m-2) were used with artificial neural networks at different corrugation depths. A multilayer perceptron artificial neural network model with 13 neurons in the hidden layer was proposed. Tan-Sig and Purelin transfer functions were used in the network model developed with the Levenberg–Marquardt training algorithm. The dataset, which consisted of 252 data points, related to the evaporation process, was divided into training (70%), validation (15%), and testing (15%) groups in an arbitrary manner. The artificial neural network model has been demonstrated to effectively forecast the pressure drop that occurs during evaporation. The mean squared error was computed for the ΔP values observed during the evaporation processes, yielding a value of 1.96E-03. The artificial neural network exhibited a high correlation coefficient value of 0.94479. The estimation fluctuations exhibited a range of ± 10%, whereas the experimental and anticipated ΔP data demonstrated a divergence of ± 10.3%.

Thermal analysis of electroosmotic flow in a vertical ciliated tube with viscous dissipation and heat source effects: implications for endoscopic applications

Thermal analysis of electroosmotic flow in a vertical ciliated tube with viscous dissipation and heat source effects: implications for endoscopic applications

Burnt Gas Flow Stability of Limit Flames in Vertical Tubes

Burnt Gas Flow Stability of Limit Flames in Vertical Tubes

Blast wave pressure measurement and analysis in air and granular media inside a shock tube using a fiber Bragg grating sensor.

In this work, we have demonstrated the use of a fiber Bragg grating (FBG) sensor to measure the pressure profile of blast waves generated inside a vertical shock tube (VST). An FBG pressure sensor probe has been designed and developed that can be incorporated into the wall of the VST. The VST facility is used to generate blast waves with decay times of the order of a few milliseconds to simulate explosive events. Pressure measurement experiments have been carried out at different incident blast wave peak pressures inside the VST. The FBG pressure sensor measurements are validated against a standard piezoelectric pressure transducer at an acquisition rate of 1MHz. The pressure signals of both sensors are found to match well with similar rise times and decay profiles. The validated FBG pressure sensor is then incorporated into a sand column mounted in the test section of the VST to measure the pressure profile of blast wave-induced stress waves in granular media. The FBG and piezoelectric pressure sensor data are compared using fast Fourier transform analysis and continuous wavelet transform. The feasibility of FBG sensors for blast pressure measurement under harsh conditions imposed inside shock tube environments is established.

The influence of borehole lengths on a numerical model of a double-tube vertical ground heat exchanger

GHEs are underground pipes that transmit heat between the earth and pipes. ANSYS Fluent was used to numerically investigate the thermal and hydraulic performance of a vertical twin tube ground-linked heat exchanger in a pile foundation in cooling mode. The CFD analysis uses three borehole length models—M1-10 m, M2-15 m, and M3-20 m—with flow rates from 1-100 lit/min and Reynolds numbers from 162 to 1625. The numerical results show that the average heat transfer rate of the Model M1 was 13% higher than that of the Model M2, 32% higher than that of the Model M3, and 95% higher than that of the Model M2. The pressure drop was minimal for 1 to 5 lit/min (laminar flow condition) but very high for 10 to 100 lit/min. The bore cavity length increased the pressure drop and decreased the average output temperature. These two-tube vertical GHEs with optimized length and selection of pipe materials can improve natural convection heat transfer by contacting ground waters.

Flow stability and distribution in horizontal serpentine tube bundles with vertical headers

Particle-based systems have immense potential for combining thermal energy storage (TES) with renewable energy sources. The so-called sandTES system, which is an active TES system, utilizes sand or other small particles as a storage material and consists of a hot tank, a cold tank, and a reversible fluidized bed heat exchanger. In the preferred design, the tubes are arranged in horizontal serpentine tube bundles; thus, the headers are positioned vertically, for one phase subcritical, two-phase and supercritical water/steam conditions. So far, no design principles have been published for macro-sized vertical headers and horizontal tube bundles. This paper investigates the unbalance in flow distribution and other flow instabilities of such an unusual heat exchanger and its scale-up using APROS simulations.The steady-state simulations of the tallest investigated heat exchanger show a strong unbalance in the mass flows between individual heat exchanger tubes: the smallest mass flow was only 55% of the largest one when generating steam. Smaller heat exchangers experience less pronounced unbalance factors in the order of 88%. When steam is condensed, the unbalance is insignificant. Dynamic simulations of start-up and shutdown revealed instabilities and that the flow maldistributions even lead to flow reversals in most investigated cases although neither last for stationary operation. Part-load cases show higher unbalances than their respective full-load case by 9 to 17 percentage points. Header design modifications – secondary headers with submodules and orifices – improve the unbalance factor from 55% to 88%.

Perfection of an Energy-Economic and Environmental Parameters of the Ground Source Heat Pump Systems with Preventing Freezing of the Soil around Ground Pipes

This article is devoted to the methods for perfection of energy, economic factors, environ-mental factors and work modes of the ground source heat pump systems with the account of climate conditions. The purpose of the work is to predict and prevent freezing of the soil during long term of exploitation of the low-power and high-power ground source heat pump systems for justifying rational conditions of implementation GSHP in Ukraine and South-East Regions of Europe. This goal is achieved by solving the following problems: determining the energy-economic and environmental factors based on considering the work modes of the ground source heat pumps; evaluation of the operating efficiency of ground source heat pump systems based on minimizing fuel consumption and green-house gas emissions; perfection of solutions enable implementation of ground source heat pumps friendly to the environment and justifying rational conditions of implementation the ground source heat pumps for residential sector. The important results of the work are the obtained and analyzed data on the influence of energy and economic factors both environmental criteria on the work modes and scheme-constructive solutions of ground source heat pumps. Modeling of heat exchange processes in the soil around the soil tube shows that in order to avoid freezing of the soil, the minimum permissible specific number of vertical tubes should be at least 0.12–0.15 pieces/m2 of heated area. The significance of the results consists in the possibility of using the results of numerical simulation for preventing freezing of the soil around ground pipes of the ground source heat pump systems.

Effect of Al2O3-Cu/Water Hybrid Nanofluid on Natural Convection in an Inverted T-Shaped Enclosure

In the present study, the effect of Al2O3-Cu/water hybrid nanofluid (HNF) flow, governed by modified Navier-Stokes equations coupled with energy conservation on heat and mass transfer due to the natural convection in an inverted T shape domain is investigated numerically by solving coupled non-linear PDE model using finite element method. The effect of the governing parameters like source length, Rayleigh number (Ra), 1 ≤ Ra ≤ 104, breadth of the vertical tube, volume fraction of nanoparticle (Φ), 0 ≤ Φ ≤ 0.01, 1 ≤ Pr ≤ 50, and aspect ratio (AR), 0.5 ≤ AR ≤ 2, etc. on Nusselt number has been analyzed and presented through tabulated values and Matlab plotting. Also, the flow and heat transfer sensitivity to the the governing parameters are depicted through the streamline and isotherm contours.

An efficient unconditional energy stable scheme for the simulation of droplet formation

We have developed an efficient and unconditionally energy-stable method for simulating droplet formation dynamics. Our approach involves a novel time-marching scheme based on the scalar auxiliary variable technique, specifically designed for solving the Cahn-Hilliard-Navier-Stokes phase field model with variable density and viscosity. We have successfully applied this method to simulate droplet formation in scenarios where a Newtonian fluid is injected through a vertical tube into another immiscible Newtonian fluid. To tackle the challenges posed by nonhomogeneous Dirichlet boundary conditions at the tube entrance, we have introduced additional nonlocal auxiliary variables and associated ordinary differential equations. These additions effectively eliminate the influence of boundary terms. Moreover, we have incorporated stabilization terms into the scheme to enhance its numerical effectiveness. Notably, our resulting scheme is fully decoupled, requiring the solution of only linear systems at each time step. We have also demonstrated the energy decaying property of the scheme, with suitable modifications. To assess the accuracy and stability of our algorithm, we have conducted extensive numerical simulations. Additionally, we have examined the dynamics of droplet formation and explored the impact of dimensionless parameters on the process. Overall, our work presents a refined method for simulating droplet formation dynamics, offering improved efficiency, energy stability, and accuracy.

Gas-particle flow induced erosion of economizer in low-low temperature electrostatic precipitators: Numerical prediction and engineering application

In this work, the gas-particle flow and the induced erosion in the economizer region in low-low temperature electrostatic precipitator were investigated by CFD-DPM-EPM coupling approach. The results show that positioning the economizer on the vertical flue or employing a vertical tubes arrangement for the heat exchange tubes, utilizing a small expansion angle (less than 35°) in the expansion section, ensuring a more uniform distribution of inlet gas velocity and dust concentration, and incorporating short and densely positioned guide plates in the expansion section can effectively reduce erosion in the low-temperature economizer. The above results were applied to the economizer equipped with the low-low temperature electrostatic precipitator of a 1000 MW unit in a certain power plant, and the wear situation was significantly improved. The research findings can serve as a reference for diagnosing and analyzing erosion phenomena in low-temperature economizer, and can guide the improvement design to prevent erosion.

Effects of Clay Contamination on the Stability of Aqueous Foams at High Pressure

Summary Because of its low density, high viscosity, and good hole-cleaning performance, foam is used in the industry as a drilling and completion fluid. Due to these unique properties, foam can be applied in underbalanced drilling. After an extended period, however, the degradation of its thermodynamically unstable structure leads to the gradual loss of these valuable properties. While a number of research studies have been performed to investigate foam flow behavior, more is needed to know about their drainage characteristics and stability at high pressure. The primary goal of this investigation is to examine the effects of clay contaminants on the drainage of foam at high pressure. Moreover, the results and findings of this study not only show the effect of clay contaminants on foam stability but also help develop clay-based stable foam formulations without using chemicals that have the potential to contaminate groundwater or seawater. This paper shows the findings of an investigation on the aqueous foam stability in the presence of clay (bentonite and kaolinite) contaminants. Experiments were conducted at ambient temperature while varying foam quality and clay concentration at a constant pressure of 6.8 MPa. During the study, the foam was created in a flow loop. After generation, its rheology and stability were measured using a pipe viscometer and a vertical test tube (column). The hydrostatic pressure profile in the column was measured with time to assess foam drainage. The results show that clay type and concentration affect aqueous foam drainage and flow behavior. The impact of clay on these foam properties is controlled by foam quality. Adding more than 2.5% bentonite considerably enhanced foam stability and viscosity. In contrast, the impacts of kaolinite on these foam properties were minimal at the same concentration. The drainage became negligible when 5% bentonite was added to the foam. However, at a reduced concentration (2.5%), bentonite addition was only an effective stabilizer for low-quality foam (40%). Microscopic examination of the foams prepared under ambient conditions demonstrated the accumulation of colloidal particles at the plateau borders and nodes that block the drained liquid flow and reduce drainage.

Fe2O3 nanoparticles encapsulated in the inner walls of integrated 3D carbon tube grid as high-performance anode for lithium-ion batteries

Ferric oxide (Fe2O3) holds huge potential as a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity and low cost, however, its battery performance is still limited by the intrinsic poor conductivity and huge volume variation during lithiation and de-lithiation processes. Herein, we developed a self-standing 3D interconnected vertical and lateral carbon tube (3D-CT) grid with Fe2O3 nanoparticles (NPs) encapsulated in the wall of CT (denoted as 3D-CT@Fe2O3-NPs@C), as structurally-integrated anode without any inactive components. The 3D-interconnected vertical and lateral CTs with open channels not only provide plenty of attachment sites for the Fe2O3 NPs and accommodate the strain generated by the volume variation of Fe2O3, but also facilitate the fast ions and electrons transfer. Therefore, the self-supported 3D-CT@Fe2O3-NPs@C electrode delivers superior cycling and high-rate performance with a high stable specific capacity of 896.2 mAh g−1 at the current density of 1.0 A g−1 after 700 cycles. Even at the high current density of 4.0 A g−1, a reversible capacity of 488.1 mAh g−1 can be reached based on the mass of the electrode. This study provides a facile and efficient strategy to improve the electrochemical performance of LIBs through the design of structurally-integrated grid electrode.

Effects of Darcy and Viscous Dissipation on Natural Convection Flow in a Vertical Tube Partially Filled with Porous Material under Convective Boundary Condition

In this paper, a study is presented considering the effects of Darcy and Viscous dissipation in a steady natural convection incompressible fluid through a vertical composite tube partially filled with a porous material. The fluid motion in the vertical composite tube is caused by a temperature gradient due to heat applied to the wall of the tube. The Brinkman Darcy extended model was used to stimulate the fluid flow under the working fluids of water and air. The Homotopy pertubation method was used to solve the temperature and velocity equations in both the clear and porous fluids. An increase in the Brinkman number increases the temperature and velocity profiles of both working fluids. An increase in the thickness of the porous material can be used to increase the velocity of the fluid in the composite vertical tube. An increase in the internal conductive resistance leads to a decrease in the buoyancy force and, as a result, the fluid thickness, which leads to a decrease in the velocity of the fluid.

Experiment and simulation of non-catalytic thermal decomposition of CH4 for CO2-free hydrogen production in a vertical tube

To address climate change issues, there has been growing interest in low-carbon H2 production technologies with the potential for zero-CO2 emissions. Non-oxidative CH4 pyrolysis is a promising method for producing H2 and solid carbon without generating CO2 emissions. This study presents experimental and numerical studies on non-catalytic CH4 pyrolysis using a vertical tube reactor (28 mm in diameter and 1800 mm in height). A three-dimensional (3D) Eulerian computational fluid dynamics (CFD) model coupled with chemical reaction and heat transfer was developed and validated against the experimentally determined axial temperature profiles and methane conversion (XCH4). The hydrodynamics, reaction kinetics, and heat transfer characteristics were investigated using the validated CFD model. The highest XCH4 of 99% was achieved at 1200 °C with a gas flow rate of 0.5 LPM. Heat transfer combined with natural and forced convection was confirmed by the CFD simulation of the tubular reactor. The heat transfer coefficient in the reaction zone ranged from 43 to 46 W/m2/°C. The CFD simulation served as a viable tool for examining the influence of the hydrodynamics, heat transfer, and reaction kinetics on the performance of the reactor with a given geometry and under specified operating conditions in non-catalytic CH4 pyrolysis.

Efficiency enhancement in solar energy storage: Impact of oval inner tube geometries on phase change material units

This comprehensive study systematically investigates the melting and solidification processes in phase change material (PCM) units, specifically comparing different inner tube geometries, including horizontal oval, inclined oval (45°), and vertical oval, against the circular inner tube. Through meticulous analysis, the study reveals that the vertical oval inner tube demonstrates superior efficiency, achieving complete melting and solidification in significantly shorter times compared to other geometries. In the melting process, the unit with a circular inner tube completes melting at 7560 s, the horizontal oval and inclined oval units complete at 7500 s, and the vertical oval unit completes at 7260 s. Notably, the reduction in melting time for horizontal oval and inclined oval units is approximately 0.8%, while for vertical oval unit, it is notably greater at 3.96%. Similarly, in the solidification process, the unit with the circular inner tube completes solidification at 17760 s; the horizontal oval completes at 17160 s, the inclined oval unit completes at 16080 s, whereas the vertical oval unit completes at 14400 s. The reduction in solidification time for the horizontal oval and inclined oval is 3.37% and 9.46%, respectively, whereas for the vertical oval, it is notably greater at 18.91%. These findings underscore the profound influence of inner tube design on heat absorption, temperature profiles, and overall PCM unit performance. The substantial reductions in both melting and solidification times, particularly for the unit with a vertical oval inner tube, highlight the critical role of inner tube geometry in optimizing PCM efficiency and responsiveness to thermal changes.

Impact of variable properties in natural convection airflows within simple and adiabatically-extended vertical tubes

Influence of variable thermophysical properties is investigated in airflows established by natural convection inside vertical tubes. Systematic numerical results are obtained for covering the entire range laminar to turbulent (modified Rayleigh number ranging 10−3–109), for both isothermal and isoflux heating conditions. Particular attention is devoted to the effects of variable properties of air. Own functions are used to take into account the variation of thermophysical properties as a function of temperature. Results are compared to those computed assuming the Boussinesq approach. For isoflux heating, it is proven that the disruptive effect consisting of the emerging of a critical heat flux and the consequent thermal crisis phenomenon, appear in the regarded cylindrical geometry. An analytical estimation of the critical heat flux is developed for vertical tubes, valid for the laminar regime, and an overall correlation is proposed for the laminar-turbulent range. The impact of the height and the widening of a funnel-shaped adiabatic extension is explored for selected values of the modified Rayleigh number, and the conditions under which its inclusion is beneficial to postpone the appearance of the crisis phenomenon, are determined.