Constant Flow Rate Conditions Research Articles

Influence of structural properties and connectivity of initial fracture network on incipient karst genesis

A karst system is generally difficult to characterize especially regarding its conduits pattern and morphology. Even though speleology allows to map the geometry of cave systems, a large part of karst conduit networks remains unreachable underground. In this study, we use a reactive transport model that simulates dissolution processes in fracture networks to investigate the effect of discrete fracture network (DFN) properties (i.e., anisotropy, connectivity, fracture intersection types) on incipient karst conduit geometry under different boundary conditions (constant hydraulic head versus constant flow rate conditions, directional versus concentrated recharge conditions). We focused on single fracture (with/without cementation) and ladder like fracture networks which are common on limestone. Results show that fracture network properties, play a major role in the final geometry and topology of the incipient karst conduit patterns. Also, the presence of cement on a single fracture favors tortuous and curvilinear wormholes, while the anisotropy of the fracture network favors the formation of anisotropic incipient karst geometries. Finally, the type of hydraulic BC remains a major factor that governs dissolution processes and thus incipient karst conduit geometry.

Techno-economic and environmental investigation between a low-concentrating photovoltaic thermal collector and a regular photovoltaic flat plate thermal collector

Photovoltaic/thermal (PV/T) systems are highly efficient due to their simultaneous generation of heat and power. To achieve better efficiency of these systems, optimization in design and performance is required. In this research, a thermal flat collector coated with nanoparticles and equipped with concentrators was employed. Flat mirrors were used to reflect more solar radiation and a black graphene nanoparticle coating on the absorber was employed. The primary aim of using this coating is to increase the efficiency of thermal energy production enhancing heat transfer to the harp channels. The harp water channels, welded on the rear side of the absorber, were designed to efficiently pass water flow and absorb heat. To illustrate the performance of this system, it was compared to a regular PV system. A comparison indicated that the low-concentrating PV/T (LCPV/T) system had 41.01% percent more total efficiency than a regular PV/T system. In addition, a 71.90% improvement in the Nusselt number was observed. In this study, both systems were optimized via the Taguchi method in the Minitab software. Moreover, the LCPV/T system showed a 39.15% reduction in emissions and a 2.72-year shorter energy payback time (EPBT) due to the use of effective approaches. The present research involved an experimental performance and optimization comparison between PV/T and LCPV/T systems under constant flow rate conditions at various times of the day.

Optimisation of initial velocity distribution of jets for entrainment and diffusion control using deep reinforcement learning

The control of the entrainment and diffusion of the heat and mass ejected along the jet flow requires the control of fluid motion. In the present study, optimal initial velocity distributions for suppressing and promoting jet entrainment and diffusion were identified using deep reinforcement learning under constant initial jet flow rate conditions. Two-dimensional jet flow simulations using OpenFOAM were combined with a deep Q-network as the learning algorithm. The spatial distribution of the initial jet (velocity and ejection angle) changed. The results show that to suppress the entrainment of the jet, an optimal initial velocity distribution shape in which the velocity in the centre is almost 0, with an inward velocity just outside of it, and a large outward velocity at the outer edge, is desirable. However, to promote entrainment, a shape with a large velocity in the centre portion and small velocity at the outer edge, with an overall inward angle, is desirable. To suppress the diffusion of the jet, an optimal initial velocity distribution shape with a large velocity in the centre portion, a small velocity at the outer edge, and an overall inward angle is desirable. Conversely, to promote diffusion, a shape with a large outward velocity at both the centre and outer edges is desirable. It is also indicated that, when the initial velocity distribution of the jet has a special shape, and the relationship between flow rate, centre velocity, and centre temperature that is normally observed in a jet flow may not hold. In other words, by changing the initial velocity distribution, the entrainment and diffusion of the jet can be separated and controlled, which is usually difficult to achieve using conventional engineering methods.

Chemo-hydro-mechanical effects of CO2 injection into a permeable limestone

CO2 storage in carbonate aquifers is a way to mitigate atmospheric CO2 emissions, but it leads to acid-producing reactions that may induce alterations in the rock pore structure and hydromechanical properties. To gain a better understanding of these changes in rock properties, percolation experiments under constant flow-rate conditions with (i) CO2-saturated water (PCO2 = 100 bar and T = 60 °C) and (ii) HCl solutions (P = 1 bar and T = 60 °C) were performed on centimetric cores of the grain-supported Pont du Gard Limestone (permeability about 10−14–10−13 m2). Effluent chemistry analyses, X-ray imaging, and measurements of the hydromechanical properties of intact and altered specimens were used to quantify acid-induced changes in the two acid-rock systems. Experimental results show that the rock dissolution patterns highly depend on the acid type and pore space heterogeneity. Under the flow conditions of these experiments, complete HCl dissociation (strong acid) and rapid HCl-rock reaction result in a compact dissolution pattern that only affects the hydromechanical properties of the core inlet. Conversely, partial dissociation of H2CO3 (weak acid) produces chemical reactions all along the core. Initial structural heterogeneities localize chemical reactions along preferential flow pathways, causing instabilities in the reaction front and wormhole formation, which markedly enhances permeability. A powerlaw relationship with a very large exponent (as high as 24) accounts for the variation in permeability as a function of porosity. Altered cores render both a significant attenuation in mechanical rock properties and ultrasonic velocities, which is reproduced using a Differential Effective Medium (DEM) homogenization approach. The high-pressure percolation experiments of this study using CO2-saturated water represent an extreme scenario of CO2-brine-rock interactions in carbonate reservoirs, which needs to be considered to improve the prediction and monitoring of the storage performance.

Hydrothermal characteristics and irreversibility behavior of microchannel heat sink operated with hybrid nanofluids: A critical assessment

Recently, hybrid nanofluids have been outlined to enhance the cooling performance of microchannel heat sinks (MCHSs) and increase the heat dissipation rate in electronic devices. However, comparative assessments of hybrid nanofluids (HNFs) and water as coolants for MCHSs in terms of their hydrothermal performance and irreversibility characteristics are scarce. The present study aims to numerically assess the hydrothermal performance and entropy generation characteristics of various hybrid nanofluids in an MCHS for cooling a central processing unit (CPU). COMSOL Multiphysics software is used to solve the governing equations of a three-dimensional conjugate heat transfer model using the finite element method. The reduced graphene oxide decorated with cobalt oxide nanocomposite (rGO–Co3O4) and Fe3O4-coated MWCNT (MWCNT–Fe3O4) HNFs with temperature-dependent thermophysical properties and wide range of concentrations (0% ≤ φ ≤ 0.2%) are considered. The simulations are performed under constant Reynolds number (300 ≤ Re ≤ 1500) and constant flow rate (5.42 cm3/s ≤ Q ≤ 27.12 cm3/s) conditions. The results show that increasing the values of φ at a fixed Re significantly enhances the heat transfer coefficient; reduces the CPU temperature, thermal resistance, and total entropy generation; and improves temperature uniformity. However, the pressure drop and pumping power increase significantly. No significant enhancement is achieved when the HNFs are used instead of water at a constant flow rate, and a noticeable pumping power is obtained under all studied conditions. In conclusion, MWCNT–Fe3O4 HNF exhibited better combined thermohydraulic performance than the rGO–Co3O4 HNF.

Numerical study of boundary stresses on Jeffery-Hamel flow subject to Soret/Dufour effects

We have mathematically modeled the problem of convective Jeffery-Hamel flow in the presence of thermo-diffusion and diffusion-thermo effects. The flow is considered to be the steady, incompressible and unidirectional by assuming the velocity only in radial direction. We have also modeled the boundary conditions representing the boundary stresses and assumed that these stresses apply only radially. Then we analyzed the effects of these stresses on the problem. The boundary stresses are defined through traction boundary conditions which are of Robin type. The flow is assumed in the domain [[Formula: see text]] and the constant flow rate condition is applied instead of taking symmetry of the channel. The governing equations are nonlinear coupled partial differential equations. They become a system of nonlinear coupled ordinary differential equations by adopting suitable transformations. Therefore, they are handled numerically. For this purpose, we have developed a scheme using shooting method to solve nonlinear coupled differential equations representing the flow problem and shown the solution procedure via a flow chart. In order to obtain the numerical results, we have taken different values of parameters. Since we have considered the problem of Newtonian fluid, therefore, we considered the values [Formula: see text] for air, carbon disulfide and water respectively at 25°C. Whereas the values of Schmidt number are considered as [Formula: see text] for hydrogen, water vapor, and ammonia respectively at 25°C. The numerical solution is validated by comparing the results with those published in literature. The numerical results of present work show that there occurs no-slip for wedge angle of [Formula: see text] by fixing the other parameters whereas slip is achieved in some other situations. It is noticed that the flow reversal occurs for convergent as well as divergent flow. All boundary layers are affected greatly due to presence of the boundary stresses and the hydrodynamic boundary layer that disappears in the absence of boundary stresses. The freedom in constraint of channel symmetry leads to the occurrence of symmetric as well as asymmetric flow. An interesting theoretical result of opposite behaviors of temperature and concentration profiles for convergent and divergent flows is observed in case of variation of all of the governing parameters. It is also noticed that the simultaneous increase of Soret number and decrease of Dufour number results in occurrence of opposite behavior of temperature and concentration profiles for divergent flow. However, it results in similar behavior of these profiles. The results show that the boundary layer completely disappears in the absence of boundary stresses.

The effect of different CO2 phases on wormhole development in carbonate rocks

Acid stimulation is commonly used for carbonate reservoirs to enhance wells’ productivity by creating highly conductive channels called wormholes. The success of the stimulation depends on how deep these channels penetrate the formation. Hydrochloric acid (HCl) is commonly used for the carbonate stimulation process with carbon dioxide (CO2) as a byproduct of the reaction between HCl and calcium carbonate (CaCO3). Depending on the operating temperature and pressure, CO2 can form a gaseous phase (bubbles) or be dissolved completely in the fluid. To achieve an understanding of the effect of CO2 bubble formation on wormhole development, we used a low acid concentration (not more than 1 wt% HCl) at a range of flow rates. In this study, an elevated back pressure of 8.2 MPa is applied to keep the CO2 dissolved in the solution and then compared with another set of experiments where no back pressure is applied. Sensitivity runs on various back pressures (while keeping all other parameters constant) are conducted to acquire a detailed understanding of the wormhole behaviour at a range of back pressures (0.1, 2.7, 5.5 and 8.2 MPa). We test the results in the dissolution phase space of Peclet and Damköhler dimensionless numbers. Although we show that for constant flow rate conditions, the existence of gaseous CO2 significantly increases the pressure prior to the wormhole breakthrough, surprisingly no noticeable effect on the wormhole initiation process itself was found.

High Efficiency Compound Vacuum Oil Gas Separation Technology Based on Integrated Oil Filter of Ultra-High Voltage Transformer

In order to improve the filtering effect of vacuum oil purifier, the author proposes the research and application of high-efficiency compound vacuum oil-gas separation technology based on electric ultra-high voltage transformer integrated oil purifier. The author studies under the condition of constant oil temperature and flow rate, the effect of vacuum degree on the filtering effect of transformer oil vacuum oil filter. Using the research results, a frequency conversion automatic vacuum oil filter is designed, which makes the processing process have the characteristics of simple operation, high degree of automation, and high work efficiency. The test results show that if the transformer oil processed by the variable frequency automatic vacuum oil purifier is new oil, its water content is less than 45 mg/kg and the gas content is less than 12%; when the breakdown voltage is greater than 25 kV, the transformer oil index can meet the following requirements after the equipment is filtered for one cycle: breakdown voltage ≥75 kV; water content ≤5 mg/kg; and gas content ≤0.1% (volume ratio). After one cycle (after 100 min), the variable frequency automatic (three-stage) vacuum oil filter can greatly shorten the time of transformer oil circulating and filtering. Conclusion. The designed frequency conversion automatic vacuum oil purifier is obviously better than other transformer oil vacuum oil purifiers.

How to Design a 3D Ordered Microstructure for Redox Flow Batteries: A Pore Network Modeling Study

Amid growing energy demand and the progressive contribution of intermittent renewable energy, the need for large-scale energy storage systems have become critical. Redox flow batteries can be a legitimate choice to store a huge amount of energy, subject to further improvement in their components, specifically the porous electrode. 3D printing technology is a viable method to manufacture porous electrodes with an engineered design and controllable properties. Attaining this goal requires an understanding of the effect of electrode microstructure which this study strove to provide. The effect of different parameters, including pore size, throat size, frequency of lattice units, and permeability anisotropy in the through-plane and in-plane directions were investigated in the flow-through as well as interdigitated batteries.Pore network modeling was utilized to investigate the effect of microstructure on the performance of Hydrogen-Bromine (H2-Br2) flow batteries. Simulations were carried out under steady-state conditions for electrolyte transport, species transport, and charge transport. To isolate the role of microstructure on the performance of the battery, simulations were performed at the constant flow condition in which the advective force was constant.The simulations were validated against the experimental study using an unstructured pore network, extracted from a tomography scan of a commercial carbon paper. A 3D printed ordered cubic lattice with the same permeability of the carbon paper showed a better performance. This was mainly achieved by better mass transport as a result of higher advection, diffusion, and lower tortuosity of the ordered lattice.It was found that a combination of bigger pores and smaller throats at constant permeability, facilitated the diffusion rate especially in the in-plane direction, as well as, increased the specific surface area of the network. It was also shown that a larger number of lattice units, resulted in a higher surface area, consequently a higher current generation.For both of the flow-through and interdigitated flow fields, permeability change in the through-plane direction (x-direction and y-direction) at the constant flow rate condition, almost did not affect the performance of the battery. However, the latter drastically changed the required electrolyte supplying pressure that could translate into a more economically feasible process. The permeability variation in in-plane direction reinforced or hampered the diffusion and also the current generation.Finally, an improved design of microstructure for the flow-through and interdigitated batteries were proposed and investigated. The specific surface area, permeability, and flow rate were kept constant to solely observe the role of microstructure on the battery performance. Results presented the superior performance of the improved microstructure in both flow fields. It was achieved by deviating the electrolyte main flow path, to the proximity of the membrane, where more reactions were taking place. This study took an initial step and provided insight into designing and manufacturing a porous electrode using 3D printing technologies. However, it can be continued in various paths including using complex 3D pore structures rather than simple cylindrical lattice to optimize the hydrodynamic behavior of the electrolyte and the specific surface area of the electrode.

Experimental Investigation on the Cooling Performance of a Counterflow Dew Point Evaporative Cooler

In this study, an innovative indirect evaporative cooler is presented and experimentally analysed. A prototype was assembled for the experimental investigation of the system and tested in the laboratory environment under the conditions of constant air flow rate of 350 m3/h, circulating water temperatures of 15°C and 20°C, inlet air temperatures of 25°C and 30°C and lastly inlet air humidity of 9g/kg and 13g/kg, respectively. Based on the data obtained during the experiments, calculations were performed for the cooling capacity, cooling efficiency, energy efficiency and exergy efficiency of the system for each case. The findings showed that the highest wet bulb efficiency, dew point efficiency and EER of evaporative cooler were found to be 0.91, 0.62, and 0.77, respectively.

Technique Detects Well Interference Using Long-Term Pressure Data

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206354, “Well-Interference Detection From Long-Term Pressure Data Using Machine Learning and Multiresolution Analysis,” by Dante Orta Aleman, SPE, and Roland Horne, SPE, Stanford University. The paper has not been peer reviewed. Methods such as interference tests and tracers have been developed to obtain knowledge of reservoir heterogeneity and connectivity from dynamic data. However, detecting well connectivity using interference tests requires long periods with stable reservoir pressure and constant flow-rate conditions. Conversely, the long duration and high frequency of well-production data have high value for detecting connectivity if noise, abrupt changes in flow rate, and missing data are considered. In this work, a methodology to detect interference from long-term pressure and flow-rate data was developed using multiresolution analysis in combination with machine-learning algorithms. Introduction Multiple methodologies have been proposed to detect well connectivity using production data. Most such methods apply a statistical or physical model to relate production and injection rates. In previous works, pressure data rarely has been used and data were fitted to a predefined mathematical model with strong physical assumptions. The approach of the current work is to infer connectivity by estimating the pressure response of a well to a second “interfering” well. Because the pressure response at a given well is the convolution of all the connected wells’ flow rates with the reservoir response, the methodology attempts to deconvolve the flow rates and assign a pressure response to each. Previous work showed that, by using wavelet decomposition and deep learning, capture of the pressure response of a single well using only flow-rate data as input is possible. The complete paper expands on the idea of using the multiresolution properties of wavelets to simplify the flow-rate/pressure response of more than one well and to capture that response by using a machine-learning model with a flexible function representation.

Influence of wet compression on aerodynamic performance and stability boundary of transonic compressor

The gas–liquid two-phase flow in a transonic compressor stage, Stage 35, under wet compression was numerically simulated by Computational Fluid Dynamics (CFD) technique along with Euler–Lagrange approach. The evaporation and cooling effect of water droplets on compressor performance and stable flow range was investigated by changing the inlet spraying conditions (droplet sizes and injected water flow rates). Further, the influencing mechanism was explained by the changes of rotor velocity triangle, rotor specific work, blade loading, and tip leakage flow before and after spraying. The results indicate that wet compression can improve the pressure ratio and efficiency of the compressor, but, under different comparison conditions before and after spraying, the influence on compressor power consumption has different trends. Wet compression improves the compressor specific power consumption under constant flow rate conditions, while it reduces compressor specific power consumption under equal pressure ratio conditions. Additionally, the stalling flow rate of compressor increases by wet compression effect, and the increasing degree of stalling flow rate is positively correlated with the increasing level of pressure ratio. The evaporative and cooling effect of droplets reduces the airflow temperature at rotor outlet, leading to the increase of airflow density, the decrease of axial velocity, which results in the promotion of rotor specific work—this is the main reason for the improvement of pressure ratio under constant flow rate conditions. Under wet compression condition, the enhanced blade loading near tip strengthens the tip leakage flow, making the compressor achieve stall criteria at larger flow rate, which is the cause for wet compression influencing the compressor stall boundary.

Influence of Particle Mass Fraction over the Turbulent Behaviour of an Incompressible Particle-Laden Flow

The presence of spherical solid particles immersed in an incompressible turbulent flow was numerically investigated from the perspective of the particle mass fraction (PMF or ϕm), a measure of the particle-to-fluid mass ratio. Although a number of different changes have been reported to be obtained by the presence of solid particles in incompressible turbulent flows, the present study reports the findings of varying ϕm in the the turbulent behaviour of the flow, including aspects such as: turbulent statistics, skin-friction coefficient, and the general dynamics of a particle-laden flow. For this purpose, a particle-laden turbulent channel flow transporting solid particles at three different friction Reynolds numbers, namely Reτ=180, 365, and 950, with a fixed particle volume fraction of ϕv=10−3, was employed as conceptual flow model and simulated using large eddy simulations. The value adopted for ϕv allowed the use of a two-way coupling approach between the particles and the flow or carrier phase. Three different values of ϕm were explored in this work ϕm≈1,2.96, and 12.4. Assessment of the effect of ϕm was performed by examining changes of mean velocity profiles, velocity fluctuation profiles, and a number of other relevant turbulence statistics. Our results show that attenuation of turbulence activity of the carrier phase is attained, and that such attenuation increases with ϕm at fixed Reynolds numbers and ϕv. For the smallest Reynolds number case considered, flows carrying particles with higher ϕm exhibited lower energy requirements to sustain constant fluid mass flow rate conditions. By examining the flow velocity field, as well as instantaneous velocity components contours, it is shown that the attenuation acts even on the largest scales of the flow dynamics, and not only at the smaller levels. These findings reinforce the concept of a selective stabilising effect induced by the solid particles, particularly enhanced by high values of ϕm, which could eventually be exploited for improvement of energetic efficiency of piping or equivalent particles transport systems.

Investigation of boundary stresses on MHD flow in a convergent/divergent channel: An analytical and numerical study

This study investigates the boundary stresses prescribed by the traction boundary condition on an incompressible magnetohydrodynamic Jeffery-Hamel flow of viscous fluid. The governing partial differential equations are transformed to a nonlinear ordinary differential equation (ODE) by the transformation obtained from continuity equation. We have modeled traction boundary conditions to solve the problem. We have not imposed flow symmetry condition and applied prescribed flow rate condition. We have computed analytical and numerical solutions of the problem. We have developed modified Adomian decomposition method based on Daun-Rach Approach (DRA) for general nonlinear third order ODE subject to Robin’s and integral boundary conditions. The governing equation is solved using the developed scheme. The Mathematica routine, NDSolve, is used to obtain numerical solution by first approximating the constant flow rate condition by Gauss-Lobatto integration formula of four and five points as well as by trapezoidal rule. Analytical and numerical results are compared and found in good agreement. The slip and no-slip scenarios are observed both for inertial and non-inertial flows. The back flow occurs both for convergent and divergent channel geometries. Flow bifurcations and boundary layer get strengthened due to presence of boundary stresses. The symmetric and asymmetric natures of flow are also captured due to presence of boundary stresses.

Numerical simulation of clogging in a microchannel with planar contraction

Clogging is the mechanism that interrupts the flow in confined geometries due to the complete blockage of the channel cross section. It represents a critical issue in the processing of particle suspensions for both industrial and biological applications, and it is particularly relevant in microfluidics and membrane technology due to the high particle confinement and the difficult device cleaning. Although numerous experimental and numerical studies have been carried out to understand the mechanism governing this complex multiscale phenomenon, the picture is not yet clear and many questions still remain, especially at the particle level. In this regard, the numerical simulations may represent a useful investigation tool since they provide a direct insight to quantities not easily accessible from experiments. In this work, a detailed computational fluid dynamics-discrete element method simulation study on the clogging mechanism in a microchannel with planar contraction is carried out. Both constant flow rate and constant pressure drop conditions are investigated, highlighting the effect of flow conditions, particle volume fraction, cohesion forces, and contraction angle. The onset of clogging conditions is discussed.

Hybrid nanofluids with temperature-dependent properties for use in double-layered microchannel heat sink; hydrothermal investigation

The forced convection heat transfer through a double-layered microchannel heat sink is simulated by the finite volume method. The simulations are conducted under the conditions of constant Reynolds number and constant flow rate. The Al2O3-Cu/water and Al2O3-SiO2/water hybrid nanofluids with temperature-dependent properties and different volumetric concentrations are considered. The cooling capacities of these nanofluids are compared with that for pure water. The results showed that the base surface temperature is decreased by using both nanofluids compared to the base fluid at constant Reynolds numbers. The Al2O3-SiO2/water nanofluid has a better cooling capacity compared to Al2O3-Cu/water nanofluid as the Reynolds number is constant. At a constant flow rate, nanofluids with different volumetric concentrations do not affect the base surface temperature compared to the base fluid. The Al2O3-SiO2/water nanofluid gives lower thermal resistances compared to the Al2O3-Cu/water nanofluid but it has higher pumping power consumption than Al2O3-Cu/water nanofluid. The Al2O3-SiO2/water nanofluid has better Nusselt numbers compared to the pure water and Al2O3-Cu/water nanofluid. In general, increasing the volumetric concentration of both nanofluids reduces the thermal resistance and raises the pumping power consumption.

Numerical modelling of the flow in a swelling preform during LCM mould filling

Composite industry increasingly uses natural fibres because of their environment-friendly advantages. These natural fibres may swell during the mould filling process when they absorb resin, and this swelling reduces the porosity and permeability of the preform. Hence, computational modelling of the flow in swelling porous media would be useful to model the different mould filling processes with the swelling effect. This paper demonstrates the possibility of using computational fluid dynamics to study the effect of swelling on liquid composite moulding mould filling in isotropic and orthotropic porous media. An empirical relation for local permeability changes is used to model the flow of resin under constant volume flow rate and constant injection pressure conditions. The flow front locations and inlet pressure predicted by the computational fluid dynamics simulations are in good agreement with the experimental data for 1D rectilinear flow case. Further, to capture the flow patterns, two different arrangements employing point injection are considered. It was observed that the volume fraction of resin in swelling porous medium is 6% less than rigid porous medium at any given time. It was also observed that the location of the inlet and outlet has a considerable effect on the flow front advancement.

Forced Convection of Al2O3–Cu, TiO2–SiO2, FWCNT–Fe3O4, and ND–Fe3O4 Hybrid Nanofluid in Porous Media

Adding nanoparticles to fluid has led to a new class of fluids named as nanofluids. Different concentrations and its effective cooling have attracted many engineering applications to test this new fluid. Lately, important heat enhancement has been observed by dispersing two distinct nanoparticles in the regular fluid. This type of hybrid nanofluid has led researchers to study its effectiveness in the cooling process. Here, we experimentally studied the forced convection of Al2O3–Cu hybrid nanofluid in porous media at a constant flow rate and heating condition. The numerical code after being calibrated with the experimental results is used to predict the effectiveness in cooling by using a set of hybrid fluid of TiO2–SiO2, MWCNT–Fe3O4, and ND–Fe3O4 at different concentrations. In the experiment, we used water and a water–ethylene glycol mixture as base fluids. The results revealed that the hybrid fluid contributed to heat enhancement levied increased pumping power. However, the index of efficiency, obtained by combining the Nusselt number and pressure drop, indicated that the best hybrid fluid for such an application is ND–Fe3O4 in the water–ethylene glycol mixture.

Determination of velocity profiles of Bird-Carreau fluids in curvilinear microchannels using random sample consensus

Flow features of rheologically complex fluids inside curved microchannels should be meaningfully scrutinized for effective mixing, sorting, and manipulation of nano- and micro-sized colloids or particles. In this study, a particle streak velocimetry method with coordinate transformation is incorporated to depict experimentally the axial velocity profiles of Newtonian and non-Newtonian (Bird-Carreau, BC) fluids in a curvilinear microchannel under constant flow rate conditions. Theoretical velocity distributions for both fluids are favorably substantiated from experimental observations that employ a random sample consensus (RanSAC) algorithm under various channel geometric conditions, demonstrating the good agreement between experiments and simulations previously developed. It is confirmed that the BC fluid showed blunt and non-parabolic profiles in comparison to the Newtonian case at a low Dean number. The suggested algorithm and method for accurately observing microscale flow fields provide useful insights into the elaborate manipulation and processing of non-Newtonian fluids in curved channel devices.

Experimental study on heat transfer characteristics of helix ground heat exchanger coil under dynamic load

In order to study the thermal behavior of helix ground heat exchanger coil when subjected to different thermal load distributions, a full scale helix ground heat exchanger coil system was constructed in this paper. On basis of the system, two kinds of simulated thermal loads were subjected to the full scale helix ground heat exchanger coil. To obtain the actual thermal performance, the temperature response and transient effective heat transfer coefficient were analyzed under the two kinds of simulated thermal loads. The results indicate that under the cyclic dynamic load, the temperature of the inlet and outlet of the helix ground heat exchanger coil also changes periodically. And from the overall trend, the inlet and outlet water temperature of the exchanger increase gradually. In the constant flow rate condition, when the load rate of the helix ground heat exchanger coil is low, the effective heat transfer coefficient is small, thus the load rate of the exchanger should be as high as possible. When the system is under part load, the number of sections of the running helix ground heat exchanger coil should be reduced in order to guarantee a high load value. The accumulated heat rejection from the heat exchanger to ground will affect the effective heat transfer coefficient to a certain extent. The effective heat transfer coefficient under dynamic load condition can be up to 67.4% higher than fixed load condition under the effect of accumulated heat rejection. It is also preliminarily found that rainfall infiltration can effectively strengthen the heat transfer effect of the helix ground heat exchanger coil by 18.4%, which is almost equal to the increase of the effective heat transfer coefficient after the night-time recovery period during intermittent operation.