Non-isothermal Flow Research Articles

Numerical study of spread of coughing droplets by human walking and indoor wind environment

Indoor ventilation and human movement are important factors affecting the spread of droplets. In this paper, the effects of indoor wind speed and human movement on the propagation and diffusion of cough ejected droplets are investigated using computational fluid dynamics. The effects of cough non-isothermal jet flow, indoor wind velocity, and human movement on the transmission of exhaled droplets were studied. The droplet size ranges in dynamic indoor environments were redefined based on the risk of droplet spread dispersal. The buoyant flux, gravity, inertial forces, and drag forces that affect the droplet motion were evaluated. The results show that droplet spread above 40 μm is limited by indoor airflow or human-induced wake and settles rapidly under the influence of gravity. The indoor air velocity makes the droplets of less than 40 μm to spread within 4 m in front of the cougher within 18 s. The induced wake allowed the lateral movement of droplets between the coughing person and the moving person, but the diffusion effect was much less than the effect of indoor air velocity. The combined effect of indoor air velocity and human movement makes the indoor flow field very complex. It greatly delays the deposition time of 10–40 μm droplets (2.2 times) and makes droplets below 10 μm to spread rapidly to distant locations by the induced wake, increasing the risk of infection. Further research is required to quantify the influence of parameters, such as the human movement and evaporation of droplets among others in indoor dynamic environment.

Finite Volume Computational Analysis of the Heat Transfer Characteristic in a Double-Cylinder Counter-Flow Heat Exchanger with Viscoelastic Fluids

This work presents a computational analysis of the heat-exchange characteristics in a double-cylinder (also known as a double-pipe) geometrical arrangement. The heat-exchange is from a hotter viscoelastic fluid flowing in the core (inner) cylinder to a cooler Newtonian fluid flowing in the shell (outer) annulus. For optimal heat-exchange characteristics, the core and shell fluid flow in opposite directions, the so-called counter-flow arrangement.The mathematical modelling of the given problem reduces to a system of nonlinear coupled Partial Differential Equations (PDEs). Specifically, the rheological behaviour of the core fluid is governed by the Giesekus viscoelastic constitutive model. The governing system of coupled nonlinear PDEs is intractable to analytic treatment and hence is solved numerically using Finite Volume Methods (FVM). The FVM numerical methodology is implemented via the open-source software package OpenFOAM. The numerical methods are stabilized, specifically to address numerical instabilities arising from the High Weissenberg Number Problem (HWNP), via a combination of the Discrete Elastic Viscous Stress Splitting (DEVSS) technique and the Log-Conformation Reformulation (LCR) methodology. The DEVSS and LCR stabilization techniques are integrated into the relevant viscoelastic fluid solvers. The novelties of the study center around the simulation and analysis of the optimal heat-exchange characteristics between the heated Giesekus fluid and the coolant Newtonian fluid within a double-pipe counter-flow arrangement. Existing studies in the literature have either focused exclusively on Newtonian fluids and/or on rectangular geometries. The existing OpenFOAM solvers have also largely focused on non-isothermal viscoelastic flows. The relevant OpenFOAM solvers are modified for the present purposes by incorporating the energy equation for viscoelastic fluid flow. The flow characteristics are presented qualitatively (graphically) via the fluid pressure, temperature, velocity, and the polymer-stress components as well as the related normal stress differences. The results illustrate the required decrease in the core fluid temperature in the longitudinal direction due to the cooling effects of the shell fluid, whose temperature predictably increases in the counter-flow direction.

Thermographic 3D particle tracking velocimetry for turbulent gas flows

Turbulent flows are characterized by diverse and unsteady three-dimensional (3D) features that require 3D measurements to study. In case of non-isothermal flows, combined 3D measurements of temperature and velocity are necessary. In this paper, a thermographic 3D particle tracking velocimetry (thermographic 3D-PTV) concept is introduced for simultaneous 3D temperature and velocity measurements in turbulent gas flows. It is based on sub-micron thermographic phosphor particles seeded into the flow as flow tracers with low response times of a few microseconds. To obtain each tracer’s position and velocity, the measurement region is illuminated volumetrically using a double-pulse green laser and Mie-scattered light is imaged by four double-frame cameras. Following the pinhole model-based calibration of all cameras, 3D particle positions are computed for both laser pulse-times using a fast minimum line of sight reconstruction code. Three-component velocities are derived from tracking individual particles between these time steps. For simultaneous 3D thermometry, temperature dependent luminescence emissions from the same phosphor particles are exploited. These emissions are excited using a UV laser synchronized with the first green laser pulse and imaged using two cameras equipped with spectral filters for ratiometric phosphor thermometry. As a result, instantaneous 3D fields of discrete temperature and velocity measurements are obtained throughout the volume. The concept is demonstrated in a turbulent heated gas jet emerging from a circular nozzle at a particle image concentration of 0.005 particle per pixel, where the symmetry of the velocity and temperature distributions about the jet axis is successfully reconstructed.

Numerical Simulation of Nonlinear and Non-Isothermal Liquid Chromatography for Studying Thermal Variations in Columns Packed with Core-Shell Particles

A high-resolution flux-limiting semi-discrete finite volume scheme (HR-FVS) is applied in this study to numerically approximate the nonlinear and non-isothermal flow of one-dimensional lumped kinetic model (1D-LKM), for a fixed-bed column loaded with core-shell particles. The developed model comprise a system of convection-dominated partial differential for mass and energy balances in the mobile phases coupled with differential equation and algebraic equation in the stationary phase. The solution of the model equations is obtained by utilizing a HR-FVS, the scheme has second-order accuracy even on the grid coarse and its explicit nature has the potential to resolve the arisen sharp discontinuities in the solution profiles. A second-order total variation diminishing (TVD) Runge-Kutta technique is used to solve the system of ODEs in time. Several forms of a single-solute mixture are produced to investigate the influences of the fractions of core radius on thermal waves and concentration fronts. Moreover, a particular criterion is introduced for analyzing the performance of the underlying process and to identify the optimal parameter values of the fraction of core radius.

Exact Solutions for Non-Isothermal Flows of Second Grade Fluid between Parallel Plates.

In this paper, we obtain new exact solutions for the unidirectional non-isothermal flow of a second grade fluid in a plane channel with impermeable solid walls, taking into account the fluid energy dissipation (mechanical-to-thermal energy conversion) in the heat transfer equation. It is assumed that the flow is time-independent and driven by the pressure gradient. On the channel walls, various boundary conditions are stated. Namely, we consider the no-slip conditions, the threshold slip conditions, which include Navier's slip condition (free slip) as a limit case, as well as mixed boundary conditions, assuming that the upper and lower walls of the channel differ in their physical properties. The dependence of solutions on the boundary conditions is discussed in some detail. Moreover, we establish explicit relationships for the model parameters that guarantee the slip (or no-slip) regime on the boundaries.

Nonisothermal optimal power and gas flow considering energy storage

The isothermal assumption that treats the gas temperature as a constant is widely adopted in the traditional research. However, the gas temperature has a noticeable effect on the optimal power and gas flow (OPGF) calculation. Moreover, the rational allocation of energy storage can enhance the flexibility and economy of the power and gas system. This paper presents a nonisothermal OPGF considering energy storage, which effectively avoids the constant assumptions on the gas temperature. A simplified partial differential equation which only has bilinear constraints is introduced to depict the temperature change of natural gas. Then, the partial differential equation is discretized by the finite difference scheme and an algebraic pipeline thermal model is developed to avoid the mathematical complexity of the energy conservation equation. The energy storage model is embedded into the OPGF model to enhance the system economy and flexibility. Coupling the DC power flow model and the nonisothermal gas flow model, the nonisothermal OPGF model considering energy storage is formulated. Comprehensive simulations illustrates that the proposed nonisothermal OPGF model considering energy storage can better evaluate the operation state of the integrated energy system. In addition, the energy storage can improve the operation flexibility of systems, thereby significantly reducing the operation costs of the integrated energy systems.

Non-isothermal simulation of a corner vortex within entry flow for a viscoelastic fluid

Abstract A modified White-Metzner viscoelastic constitutive equation is incorporated into the state-of-the-art Three Dimensional Computational Fluid Dynamics (3D-CFD) framework for performing isothermal and non-isothermal entry flow simulations of a polymer melt. As a result, the corner vortex becomes smaller with increased isothermal temperature, namely, the so-called temperature-induced vortex reduction. In addition, the vortex grows with raising wall temperatures, whereas the vortex reduction is found under high inlet temperatures. Through the visualized flow patterns, it is significant to investigate the dramatic variations of vortex size in relation to fluid temperature, weighted viscosity and extension rate, as well as Weissenberg number and Trouton ratio.

A Gradient-Based Economic Model Predictive Controller with Zone Control Scheme Applied to Natural Gas Pipeline Networks

This paper addresses a one-layer model predictive control (MPC) strategy that simultaneously deals with safety and economic issues for natural gas networks (NGN). The simulations consider a nonlinear pipeline model based on the non-isothermal flow and non-deal gas behavior. The proposed NGN-oriented MPC strategy uses an adaptive scheme that relies upon the successive linearization of the nonlinear NGN model and the surge prevention constraints of the compression stations, incorporated into the control law to avoid unsafe operating conditions. The controller has the guarantee of feasibility by incorporating a suitable set of slack variables into its formulation, mainly in the surge avoidance constraints. At the same time, the resulting control law is more flexible by adopting output zone tracking cases rather than setpoint tracking. The simulated study, aiming at minimizing the power consumption of the centrifugal compressors, sought to control the pressures in the consumer nodes of NGN into a predefined zone while meeting the process constraints. In all scenarios of zone changes, the controller could lead the controlled outputs in their respective zones, accommodating the operation in steady states with a minimal power consumption of three compression stations considered in NGN. By respecting the surge prevention constraints flexibly, and using the slack variables when necessary, immediately after perturbation, the proposed NGN-oriented adaptive zone MPC controller has proved to be a suitable tool to manage the NGN with control performance, operational safe and economic competitivity.

Robust actuator based on photothermally responsive hydrogel with fast recovery capability

Propulsion by deformation and oscillation is a familiar law of motion in nature. Often, these motions are signaled and driven by constant external conditions, such as the phototropism of plants. Here, we have prepared a soft photosensitive material utilizing ice templates and cryo-polymerization, which combines light absorbers and thermally responsive hydrogels and thus possesses superb photothermal conversion, mechanical deformation, and fast recovery capabilities. The soft actuator based on this material generates a dynamically balanced oscillation with intermittent shadows creating a continuous self-feedback loop, allowing the actuator to operate continuously and durably under constant illumination. Combining the modules of conjugate heat transfer, non-isothermal flow, and fluid-solid coupling, we have gained a deeper understanding of the interaction of the soft actuator with the temperature and flow fields. In the future, the soft actuator can be widely developed and combined with other conventional drive strategies in an attempt to apply in soft robotics, underwater operations, and other fields.

Joule heating and Soret effects on an electro-osmotic viscoelastic fluid flow considering the generalized Phan-Thien–Tanner model

This work analyzes the non-isothermal electro-osmotic fluid flow in a microchannel considering the Soret effect and temperature-dependent properties. The constitutive equation that models the fluid rheology corresponds to the generalized Phan-Thien–Tanner (gPTT) model. Temperature and pressure gradients are induced due to the interaction between an ionized fluid and the electrical field imposed at the microchannel's ends, resulting in Joule heating. The temperature-dependent physical properties of the fluid modify the ionic distribution in the electric double layer and its thickness change along the microchannel walls. The generalized Phan-Thien–Tanner (gPTT) model is used as a constitutive equation that describes the fluid rheology, where the trace-stress tensor is based on the Mittag–Leffler function, which represents the destruction of physical junctions and entanglements in the Lodge–Yamamoto network of viscoelastic fluids, through the inclusion of two fitting parameters: α and β. The gPTT model allows better fitting and flexibility to experimental data and a wider range of variation in the description of rheological responses of complex fluids. The hydrodynamics and thermodiffusion obtained through the gPTT model are compared against that using the linear form of the Phan-Thien–Tanner model (lPTT).

Non-isothermal thin-film flow of a viscoplastic material over topography: critical Bingham number for a partial slump

This paper considers the non-isothermal flow of a viscoplastic fluid on a horizontal or an inclined surface with a flat, a step-up and a step-down topography. A particular application of interest is the spread of a fixed mass—a block—of material under its own weight. The rheology of the fluid is described by the Bingham model which includes the effect of yield stress, i.e. a threshold stress which must be exceeded before flow can occur. Both the plastic viscosity and the yield stress are modelled with temperature-dependent parameters. The flow is described by a reduced model with a thin-film equation for the height of the block and a depth-averaged energy conservation equation for the heat transfer. Results show that for large values of the yield stress, only the outer fraction of the fluid spreads outward, the inner fraction remaining unyielded, hence the block only partially slumps. Conversely, for small values of the yield stress, the entire block of fluid becomes yielded and therefore slumps. We present an analysis which predicts the critical value of the yield stress for which partial slump occurs and how it depends on temperature.Graphical abstract

Fast prediction of indoor airflow distribution inspired by synthetic image generation artificial intelligence.

Prediction of indoor airflow distribution often relies on high-fidelity, computationally intensive computational fluid dynamics (CFD) simulations. Artificial intelligence (AI) models trained by CFD data can be used for fast and accurate prediction of indoor airflow, but current methods have limitations, such as only predicting limited outputs rather than the entire flow field. Furthermore, conventional AI models are not always designed to predict different outputs based on a continuous input range, and instead make predictions for one or a few discrete inputs. This work addresses these gaps using a conditional generative adversarial network (CGAN) model approach, which is inspired by current state-of-the-art AI for synthetic image generation. We create a new Boundary Condition CGAN (BC-CGAN) model by extending the original CGAN model to generate 2D airflow distribution images based on a continuous input parameter, such as a boundary condition. Additionally, we design a novel feature-driven algorithm to strategically generate training data, with the goal of minimizing the amount of computationally expensive data while ensuring training quality of the AI model. The BC-CGAN model is evaluated for two benchmark airflow cases: an isothermal lid-driven cavity flow and a non-isothermal mixed convection flow with a heated box. We also investigate the performance of the BC-CGAN models when training is stopped based on different levels of validation error criteria. The results show that the trained BC-CGAN model can predict the 2D distribution of velocity and temperature with less than 5% relative error and up to about 75,000 times faster when compared to reference CFD simulations. The proposed feature-driven algorithm shows potential for reducing the amount of data and epochs required to train the AI models while maintaining prediction accuracy, particularly when the flow changes non-linearly with respect to an input.

Numerical Analysis of Fluid Flow and Temperature Distributions of O<sub>2</sub>/N<sub>2</sub> Gas Mixtures in AOD Nozzles

A two-dimensional CFD model was developed to simulate the fluid flow and temperature distribution in an AOD nozzle using a mixture of oxygen and nitrogen as the fluid phase, aiming at predicting how the outlet gas properties are influenced by the inlet pressure, inner nozzle length/diameter and process gas composition. The proposed mathematical model assumes a steady, non-isothermal flow condition, using the realizable k-ε turbulence model to describe the gas phase. A mesh sensitivity analysis was performed where predictions were compared to experimental data. The results show that the gas properties are mainly dependent on the inlet pressure, nozzle length/diameter and heating condition but less dependent on the composition of the gas mixture. This fundamental model can be applied to provide a process specified boundary condition for gas blowing when simulating a multiphase flow in a full-scale AOD converter.

Numerical solution of non-isothermal flow in oil reservoirs using OpenACC

In this work, we performed a numerical simulation of the non-isothermal flow in an oil reservoir. Also, we used the OpenACC API to parallelize specific parts of the original code, which allowed the simultaneous execution of different tasks on a NVIDIA GTX 970 G1 video card in a shared memory architecture. The problem was studied using a vertical producer well and static heating wells in a two-dimensional domain. We used the Control Volume Finite Difference (CVFD) method to discretize the governing equations and the Conjugate Gradients method to obtain the solutions (pressure and temperature) of the systems of algebraic equations. As a result, concerning computational performance, a significant reduction in execution time was obtained with the parallelized version.

A comprehensive study of Fin-Assisted horizontal ground heat exchanger for enhancing the heat transfer performance

A horizontal ground heat exchanger installation is less expensive than a vertical ground heat exchanger, but more land is required. The system’s required pipe length and land area can be decreased by improving the system. On the other side, adding fins is one strategy to increase heat transfer. In this paper, cylindrical fins are explored in horizontal ground heat exchangers for the first time to improve heat transfer and, the overall efficiency of the ground-source heat pump. These fins were examined by varying parameters such as length, diameter, position, and material. The heat transfer rate changes with and without fins were also investigated as soil properties changed. The heat transfer simulations in cooling mode for a 1D-3D model using COMSOL Multiphysics revealed that changing the fin diameter directly affects the outlet temperature and, the non-isothermal pipe flow (niofl) is used for the pipe. There is minimaldifference between improving and increasing the heat transfer rate when the fin length is increased to more than 1 m. Moreover, the distance between the ground heat exchanger and the fin is critical; if it exceeds 5 cm, it loses some effectiveness, although it is still useful for soil recovery. In general, the fin increases the soil contact area, and a fin of length one meter can increase heat transfer per unit of pipe length up to 20.7 %. Comparing horizontal and vertical ground heat exchangers in finned and non-finned modes revealed that both modes increase performance with the fin. Still, the horizontal pipe increases performance by about 3 % more than the vertical spiral with the same number of fins.

High-precision nonisothermal transient wellbore drift flow model suitable for the full flow pattern domain and full dip range

A reliable multiphase flow simulator is an important tool to improve wellbore integrity and production decision-making. To develop a multiphase flow model with high adaptability and high accuracy, we first build a multiphase flow database with 3561 groups of data and developed a drift closure relationship with stable continuity and high adaptability. Second, a high-order numerical scheme with strong fault capture ability is constructed by effectively combining MUSCL technology, van Albada slope limiter and AUSMV numerical scheme. Finally, the energy equation is coupled into the AUSMV numerical scheme of the drift flow model in the form of finite difference. A transient non-isothermal wellbore multiphase flow model with wide applicability is formed by integrating the three technologies, and the effects of various factors on the calculation accuracy are studied. The accuracy of the simulator is verified by comparing the measurement results with the blowout experiment of a full-scale experimental well.

On the non-isothermal, non-Newtonian Hele-Shaw flows in a domain with rough boundary

In this paper, we investigate the roughness-induced effects on the nonlinear system describing the non-isothermal Hele-Shaw flow of a non-Newtonian fluid. Allowing different orders between the amplitude and the period of the boundary roughness, we rigorously determine the limit problems and also provide the corresponding rates of convergence in the critical case.

Thermoelectric instabilities in a circular Couette flow.

The stability of a non-isothermal circular Couette flow is analyzed when subjected to a dielectrophoretic force field. Outward and inward heating configurations are considered when the inner cylinder is rotating and the outer cylinder is at rest. In addition, an alternating voltage is applied between the two cylinders to induce a radial electric buoyancy that acts on the dielectric fluid. The linear stability analysis provides the threshold for the first transition to instability, as well as the corresponding wavenumber and frequency of the modes. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Three-dimensional numerical study of acoustic streaming phenomenon in rectangular resonator

The article presents a three-dimensional numerical study of the large-amplitude, acoustically driven streaming flow in rectangular resonator for different frequencies of the acoustic wave and different temperature regime, isothermal and 60 K temperature difference between the top and bottom walls. The utilized numerical model was based on the Navier–Stokes compressible equations, the ideal gas model, and finite volume discretization. The oscillating wall of the resonator was modeled as a dynamically moving boundary of the numerical domain. The size of the resonators was adjusted to fit one period of the acoustic wave. The research revealed a stationary pair of streaming vortices in the resonator with a characteristic three-dimensional structure. Their intensity was much greater in the case of nonisothermal flow. The study of the impact of side walls on the intensity of streaming revealed its gradual decrease with approaching the walls, creating a quasiparabolic profile in the resonator. Interestingly, the relationship between the intensity of streaming and the frequency of the acoustic wave turned out to be not trivial and two maxima for different frequencies could be observed.

Numerical Analysis of the Free-Falling Process of a Water Droplet at Different Temperatures

The collision behavior and ice formation of a water droplet are affected by its falling process. In this paper, the two-phase flow of air and a water droplet at a specific temperature is adopted to investigate the processes of falling and freezing of a single water droplet. To track the air–water droplet interface and the temperature distribution, the level-set method and the non-isothermal flow coupling method are used, and the freezing model is added into the water’s control equations. The numerical results indicated that with the initial temperature at 283.15 K and the spherical shape, the water droplet changes to the shape of a straw hat at 293.15 K and a drum at 293.15 K but an oval face in freezing temperatures at 0.10 s. There is an obvious drop in the downward velocity when the water droplet falls in mild temperatures at 0.09 s. The downward velocity of the water droplet in air at sub-zero temperatures has a continuous increase during the time span from 0 s to 0.10 s. There is also an obvious difference when the water droplet impinges on the solid bottom. Lastly, the freezing of sessile water droplets attached on the horizontal surface is helpful to reveal the unique phase change process of water droplets in cold air.