Discrete phase approach for nanofluids flow in pipe

Nanofluid is known as a new generation of fluid and it has been introduced almost several decades ago. But its effectiveness in practical thermal engineering applications has started to diminish with time due to the several factors such as physical instability, complex procedure for production of nanofluids and its cost, instability of suspension of nanoparticles into a base fluid, choice of thermophysical properties and reliability of nanofluids. To overcome these problems, two different phases such as a base fluid (water) and nanoparticles can be considered instead of a typical nanofluid which actually acts like a fluid-solid mixture. However, the interaction between the fluid and particles needs to be investigated to assess its performance. In the present work, Eulerian- Lagrangian discrete phase model has been used with temperature dependent thermophysical properties of the base fluid (water) and nanoparticles to study the thermal performance behaviour of Al2O3 and TiO2 nanoparticles inside a horizontal pipe within the transition to turbulent flow regimes. SST and Realizable models are considered for the modelling of transition and turbulent flow fields respectively with an enhanced near wall treatment. Results reveal that the different phases for water and nanoparticles can be used instead of a nanofluid and no thermophysical properties of nanofluid are needed to explain such behaviour. Also, it is found that the enhancement of heat transfer rate is feasible and such enhancement is fully dependent of the thermal conductivity of nanoparticles as well as nanoparticles size diameters and volume concentrations

Analysis of heat transfer and entropy generation of TiO2-water nanofluid flow in a pipe under transition

Single and multi-phase numerical simulations are carried out to investigate the heat transfer and entropy generation behaviour of transitional flow of TiO2H2O nanofluid in a circular pipe. Results reveal that the small diameter of nanoparticles has the highest heat transfer rate for χ = 6% and the TiO2-water nanofluid shows higher heat transfer rate using multi-phase model compared to that of the single phase model. Also no optimal Reynolds has been observed which could minimise the total entropy generation. New correlations are proposed to calculate the average Nusselt number using a nonlinear regression analysis with a standard deviation of error of less than 0.5%

Numerical simulation of turbulent heat transfer and fluid flow in different tube designs

Numerical simulation was carried out to study the heat transfer, friction factor and thermal performance of water inside different tubes induced with different twisted tapes. The purpose is to ascertain which of the tube designs gives the best performance when compared with the plain tube. The tubes were under uniform wall heat flux condition and Reynolds number in the range 5000 ≤ Re ≤ 20000 were considered. RNG κ − ε equation model was selected for the numerical simulations and RANS equations were employed to render the Navier-Stokes equations tractable. The best performance was obtained when the tube was fitted with alternate-axis triangular cut twisted tape. Its Nusselt number and friction factor are respectively 2.07 – 3.33 and 10.65 – 13.1 times better than that of a plain tube and its thermal performance factor is 1.35 – 1.43 times better than that of the tube with plain twisted tape

LES modelling of nitric oxide (NO) formation in a propane-air turbulent reacting flame

Large Eddy Simulation (LES) technique is applied to investigate the nitric oxide (NO) formation in the propane-air flame inside a cylindrical combustor. In LES a spatial filtering is applied to the governing equations to separate the flow field into large scale eddies and small scale eddies. The large scale eddies which carry most of the turbulent energy are resolved explicitly while the unresolved small scale eddies are modelled. A Smagorinsky model with model constant Cs = 0.1 as well as a dynamic model has been employed for modelling of the sub-grid scale eddies, while the nonpremixed combustion process is modelled through the conserved scalar approach with laminar flamelet model. In NO formation model, the extended Zeldovich (thermal) reaction mechanism is taken into account through a transport equation for NO mass fraction. The computational results are compared with those of the experimental results investigated by Nishida and Mukohara [1] in co-flowing turbulent flame

Large-eddy simulation of physiological pulsatile flow through a channel with double constriction

Pulsatile flow in a 3D model of arterial double stenoses is investigated using a large eddy simulation (LES) technique. The computational domain that has been chosen is a simple channel with a biological-type stenosis formed eccentrically on the top wall. The pulsation was generated at the inlet using the first four harmonics of the Fourier series of the pressure pulse. The flow Reynolds numbers, which are typically suitable for a large human artery, are chosen in the present work. In LES, a top-hat spatial grid-filter is applied to the Navier–Stokes equations of motion to separate the large-scale flows from the sub-grid scale (SGS). The large-scale flows are then resolved fully while the unresolved SGS motions are modelled using a localized dynamic model. It is found that the narrowing of the channel causes the pulsatile flow to undergo a transition to a turbulent condition in the downstream region; as a consequence, a severe level of turbulent fluctuations is achieved in these zones. Transitions to turbulent of the pulsatile flow in the post stenosis are examined through the various numerical results, such as velocity, streamlines, wall pressure, shear stresses and root mean square turbulent fluctuations

Numerical investigation of the heterogeneous combustion processes of solid fuels

Two-phase computational modelling based on the Euler–Euler was developed to investigate the heterogeneous combustion processes of biomass, in the solid carbon phase, inside a newly designed combustion chamber (Model 1). A transient simulation was carried out for a small amount of carbon powder situated in a cup which was located at the centre of the combustion chamber. A heat source was provided to initiate the combustion with the air supplied by three injection nozzles. The results show that the combustion is sustained in the chamber, as evidenced by the flame temperature. An axisymmetric combustion model (Model 2) based on the Euler–Lagrange approach was formulated to model the combustion of pulverized coal. Three cases with three different char oxidation models are presented. The predicted results have good agreement with the available experimental data and showed that the combustion inside the reactor was affected by the particulate size. A number of simulations were carried out to find the best values of parameters suitable for predicting NOx pollutants

Numerical Modelling for Process Investigation of a Single Coal Particle Combustion and Gasification

Combustion and Gasification are commercial processes of coal utilization, and therefore continuous improvement is needed for these applications. The difference between these processes is the reaction mechanism, in the case of combustion the reaction products are CO2 and H2O, whereas in the case of gasification the products are CO, H2 and CH4. In order to investigate these processes further, a single coal particle model has been developed. The definition of the chemical reactions for each process is key for model development. The developed numerical model simulation uses CFD (Computational Fluid Dynamic) techniques with an Eddy Break Up (EBU) model and a kinetics parameter for controlling the process reaction. The combustion model has been validated and extended to model the gasification process by inclusion of an additional chemical reaction. Finally, it is shown that the single coal particle model could describe single coal particle combustion and gasification. From the result, the difference between single coal particle combustion and gasification can clearly be seen. This simulation model can be considered for further investigation of coal combustion and gasification application processes

Liquid cooling of non-uniform heat flux of chip circuit by submicrochannels

Sumbmicrochannels have been placed on the hotspots in a non-uniform heat generated chip circuit to increase the liquid/solid interaction area and then to enhance the heat dissipation. Main microchannels width is 185µm, which is twice the width of the submicrochannels and also includes the wall thickness of 35µm, and wall height is 500µm. The chip dimension is 10mm×10mm and the hotspot is 4mm×10m. Different positions of the hotspot have been investigated e.g. upstream, middle and downstream. Uniform heat flux is 100W/cm2 while for the hot spot is 150 W/cm2. Single channel simulation reveals that the downstream hotspot gives a lower temperature of the chip circuit surface; however the upstream hotspot has more uniform temperature distribution. A special design of manifold was adopted to ensure an equal mass distribution through the microchannels

Rôle of contrast media viscosity in altering vessel wall shear stress and relation to the risk of contrast extravasations

Iodinated contrast media (CM) are the most commonly used injectables in radiology today. A range of different media are commercially available, combining various physical and chemical characteristics (ionic state, osmolality, viscosity) and thus exhibiting distinct in vivo behaviour and safety profiles. In this paper, numerical simulations of blood flow with contrast media were conducted to investigate the effects of contrast viscosity on generated vessel wall shear stress and vessel wall pressure to elucidate any possible relation to extravasations. Five different types of contrast for Iodine fluxes ranging at 1.5–2.2 gI/s were modelled through 18 G and 20 G cannulae placed in an ideal vein at two different orientation angles. Results demonstrate that the least viscous contrast media generate the least maximum wall shear stress as well as the lowest total pressure for the same flow rate. This supports the empirical clinical observations and hypothesis that more viscous contrast media are responsible for a higher percentage of contrast extravasations. In addition, results support the clinical hypothesis that a catheter tip directed obliquely to the vein wall always produces the highest maximum wall shear stress and total pressure due to impingement of the contrast jet on the vessel wall

CFD modelling of biomass gasification with a volatile break-up approach

Gasification thermochemical processes of biomass in a 20 kW downdraft gasifier are investigated using a robust two-dimensional (2D) computational fluid dynamics (CFD) modelling method. The model includes all the four zones of the gasifier namely drying, pyrolysis, oxidation and reduction. A step-by-step approach is proposed to evaluate the composition of different gas species as a result of the volatile break-up during gasification. However, selecting suitable chemical reactions for the CFD modelling becomes challenging as the commonly used reactions in kinetic study showed discrepancy in predicting the synthesis gas compositions. A revised set of chemical mechanisms is therefore proposed in the study and the robustness of the approach is examined with results validated against data from literature. The study reports how the air equivalence ratio (ER) affects the gasifier temperature and also the composition of producer gases. The model is then applied to investigate the syngas production of various biomass feedstocks sourced from Scottish agricultural sites