Multiphase flow and boiling heat transfer modelling of nanofluids in horizontal tubes embedded in a metal foam

© 2019 Elsevier Masson SAS The aim of this numerical study is to evaluate the boiling process of nanofluid in horizontal tubes in the presence of a metal foam as porous medium and represent the experimental work of Zhao et al. in a numerical aspect with a different range of dependent variables. High conductive metal foams are employed to increase the rate of heat transfer and enhance the boiling performance in the domain. Two-phase mixture model is used to simulate the characteristics of nanofluid and solve the governing equations in a two-phase flow and boiling heat transfer problem. R134a and ZnO are considered as the base-fluid and nanoparticles, respectively. The characteristics of metal foam including the porosity and pore density as well as operating conditions including the fluid flow including the velocity, induced heat flux and concentration of nanoparticles on the pressure drop, vapour volume fraction and heat transfer coefficient are examined. The results show the positive effect of the metal foam on vapour production and overall heat transfer coefficient of the nanofluid in the pipe outlet; however, due to the flow resistance as a result of porous medium addition, a higher pressure drop is achieved. For the heat flux of 19 kW/m2 and inlet velocity of 0.05 m/s, by using a metal foam with the porosity of 70% and pore density of 20PPI, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 7.1%, 9.4% and 82.7%, respectively, compared with the case of without metal foam. However, by using the porosity of 90%, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 1.6%, 3.5%, and 7.0%, respectively. Consequently, according to the developed results in this paper, a system with a moderate to low porosity with a high to moderate pore density is recommended which is finally determined based on the required vapour production and allowed pressure drop

Investigation into the operation of a cement works precalciner vessel

This Ph.D thesis describes an investigation into the operation of the Blue Circle Cauldon Works precalciner vessel. The vessel is part of the cement making plant and it serves the purpose of providing a furnace that is maintained at approximately 900C to calcine the limestone in the raw meal prior to cementation in the kiln. At Cauldon, tyre chips are used as a support fuel. It was the aim of this work to predict the likely behaviour of the gases and particles in the precalciner by using Computational Fluid Dynamics modelling. The commercial code Fluent was used. Investigation of the likely trajectories and combustion behaviour of tyre chips was particularly important. In this way it was hoped that a method of assessing the suitability of alternative waste fuels for incineration in the precalciner might be established. CFD models were constructed that simulated the precalciner with all main reactions and energy exchanges occurring. The first model predicted the behaviour of the precalciner burning coal and the full load of raw meal. Subsequent models assessed the sensitivity of the first model to changes in the boundary conditions. Further models were developed together with experimental work to assess the combustion and aerodynamic behaviour of the tyre chips. Alternative injection points for the tyre chips were investigated. This type of precalciner had not previously been modelled elsewhere. Similar models were found for the operation of the precalciner without tyre chips but the geometrical accuracy of the models was improved in this work due to improvements in commercial CFD code. No similar study of tyre chip combustion has been made elsewhere. CFD was used successfully to model a precalciner vessel with tyre chip and coal combustion and the full raw meal loading simulated. Adjustments can be made quickly to the model to assess minor geometrical alterations. Alternative fuel injection points can be quickly assessed using the model

The stability of immiscible viscous fingering in Hele-Shaw cells with spatially varying permeability

In this paper, we investigate the stability of immiscible viscous fingering in Hele-Shaw cells with spatially varying permeability, across a range of capillary numbers. We utilise a coupled boundary element - radial basis function (BE –RBF) numerical method that adapts and moves with the growing interface, providing an efficient, high accuracy scheme to track the interfacial displacement of immiscible fluids. By comparing the interfacial evolution and growth rate in varying permeability cells to that in uniform cells, we can assess the relative stability of the perturbations as a consequence of the variable permeability. Numerical experiments in Hele-Shaw cells with gradually varying permeability highlight 3 aperture effects that control the interfacial stability: (1) Gradients in the capillary pressure (2) Local changes in fluid mobility (3) Variation in the viscous pressure gradient. In low capillary number regimes, we find that aperture effect 1 and 2 dominate, which (relatively) stabilise interfacial perturbations in converging geometries and destabilise perturbations in diverging geometries. In high capillary number regimes, aperture effect 3 dominates meaning the relative stability transitions; the interface is destabilised in converging cells and stabilised in diverging cells. We find an upper bound critical capillary number Cagt at which the relative stability transitions in our gradually varying cell as 1000<Cagt<1250, which is independent of both α and ϵ0. This result is much lower than the value of Cagt=9139 predicted by linear stability theory, due to significant non-linear perturbation growth. This transition links the results found in previous works performed at low and high capillary numbers, providing new insight into the viscous fingering instability in variable permeability cells. To conclude, we present simulations in Hele-Shaw cells with large geometric heterogeneities and anisotropy, in order to demonstrate the significant fluid re-distribution that can occur due to localised variations in cell permeability. Using periodic permeability distributions, we show the significant re-distribution of fluid that can occur due to large capillary pressure gradients in the capillary limit, and the channelling of flow that can occur in the viscous limit along anisotropic features

Experimental investigation of nanoparticles concentration, boiler temperature and flow rate on flow boiling of zinc bromide and acetone solution in a rectangular duct

Despite the increase in heat transfer properties of nano-fluids, they are not currently used in vapour absorption refrigeration systems (VARS), and there is little literature on the flow boiling behaviour of concentrated salt solutions with nano-particle suspension. A potential novel working fluid solution for a vapour absorption refrigeration unit capable of utilising very low grade waste heat is acetone and zinc bromide, and this fluid is investigated here as the salt solution with graphene nanoparticles in suspension in flow boiling similar to that found in VARS. Nanoparticle concentration, boiler temperature, and flow rate are investigated. The Rohsenow constant in the flow boiling correlation for the nanofluid acetone/ZnBr2 with graphene on a stainless steel surface is found to be 0.217. By increasing the particle concentration from 0 to 05 vol%, heat flux and heat transfer coefficient on the heated surface increase from 8638 W/m2 and 106 W/m2 K to 13164 W/m2 and 167 W/m2 K, respectively. The steady pressure of the system increases with increasing loading of the nanoparticles and consequently the saturation temperature increases. This is because of the increased vapour generation as a consequence of improved heat transfer properties. Heat transfer coefficient is linearly proportional to temperature difference between the fluid and wall (e.g. increases from 78 W/m2 K to 145 W/m2 K when the temperature difference increase from 102 K to 135 K) in the range tested and the heat flux correspondingly reflects a quadratic relationship with temperature difference. Increasing nanofluid flow rate reduces both the production of acetone in the condenser and the salt concentration in the strong solution reservoir. Regarding properties of the fluid, the density and the specific heat follow the simple mixture combination rule; the thermal conductivity of the nanofluid increases by 4.5% with increasing the loading the particles to 0.5 vol%, following reasonably well the correlation of Suganthi et al. (2014); the viscosity increases linearly with concentration of nanoparticles (e.g. increases from 3.22 m Pa s to 4.5 m Pa s by increasing the concentration from 0 to 0.5 vol%); the stability of the nano-salt-fluid is affected by the density of the base fluid. The nanofluid showing good stability for 4 h and during the circulation of the fluid in the rig. Over the range of temperatures tested, the salt solution demonstrates characteristics of nucleate boiling behaviour and offers significant improvement over the properties of the base fluid in terms of boiling effectiveness, indicating that it will provide improved operation in a VARS situation

Nano-particle deposition in the presence of electric field

The dispersion and deposition of nano-particles in laminar flows in the presence of an electric field were studied. The Eulerian-Lagrangian particle tracking method was used to simulate nano-particle motions under the one-way coupling assumption. For nano-particles in the size range of 5–200 nm, in addition to the Brownian excitation, the electrostatic and gravitational forces were included in the analysis. Different charging mechanisms including field and diffusion charging as well as the Boltzmann charge distributions were investigated. The simulation methodology was first validated for Brownian and electrostatic forces. For the combined field and diffusion charging, the simulation results showed that in the presence of an electric field of 10 kV/m, the electrostatic force dominates the Brownian effects. However, when the electric field was 1 kV/m, the Brownian motion strongly affected the particle dispersion and deposition processes. For the electric field intensity of 1 kV/m, for 10 nm and 100 nm particles, the deposition efficiencies for the combined effects of electrostatic and Brownian motion were, respectively, about 27% and 161.2% higher than the case in the absence of electric field. Furthermore, particles with the Boltzmann charge distribution had the maximum deposition for 20 nm particles

CFD assessment of the effect of nanoparticles on the heat transfer properties of acetone/ZnBr2 solution

A potential novel working fluid for vapour absorption refrigeration utilising very low grade waste heat, is based on acetone and zinc bromide as the salt solution. A Computational Fluid Dynamics (CFD) model is presented of the fluid with zinc oxide nano-particles in a flat tube flow. A two phase type of model represents the zinc oxide nano-particles as a distinct fluid phase. The cases of laminar and turbulent flow are explored numerically for a wide range of acetone and nanoparticles concentrations. The velocity is varied between 1.5 and 6 ms−1, representing typical heat exchanger conditions. Reynolds number depends significantly on the solution concentration. Heat transfer coefficient increases with Re, by turbulent mixing, and with the concentration of nanoparticles and of acetone by the enhanced thermal diffusivity. The shear wall stress is not affected by changing the concentration of nano-particles. The nano-fluid is demonstrated to work well for heat transfer enhancement over the base fluid; the further issue of suspension of the nano-particles in the solution is explored experimentally. The nano-fluid can be achieved by ultra-sonic excitation, with a settling time in the order of several hours. Subject to the particle suspension time being increased, this fluid combination is a good candidate for the application considered

CFD multiphase modelling of the acetone condensation and evaporation process in a horizontal circular tube

With increasing demands on energy efficiency, the use of low grade waste heat using vapour absorption refrigeration systems (VARS) are receiving renewed interest. One idea is to use the combination of acetone and zinc bromide as the salt solution, which allows use of temperatures in the order of 10s of C above ambient conditions. This work numerically models acetone phase change in the evaporator and condenser in order to indicate how improvements can be made in these components of the system. ANSYS® Fluent finite volume method CFD is used to produce volume of fluid (VOF) and mixture multiphase flow models to investigate the evaporation and the condensation of acetone in a horizontal circular tube. Different velocities and temperatures were taken in each process to explore the effect of these variables in the system. A user defined function (UDF) is used to calculate the volume fraction of the phases. For the evaporation case, the heat transfer coefficient increases with increasing velocity and the temperature difference between the inlet flow and the wall, as expected. The mass transfer rate decreases with increasing the flow rate or decreasing the wall temperature, from 0.045 kg/m 3 .s at 0.01 m/s to 0.016 kg/m 3 .s at 0.06 m/s and it drops from 0.044 to 0.023 kg/m 3 .s by changing the temperature just from 300 to 298 K. This demonstrates a reduction in specific heat transfer to the liquid despite the higher wall heat transfer coefficient. In the condenser, vapour quality decreases along the tube as liquid acetone is created with reduced flow rate. Vapour volume fraction at the outlet section drops from 0.74 to 0.168 by increasing the ingoing velocity from 0.01 to 0.06 m/s. Increasing the rate of condensation will increase the liquid in the evaporator, which increase the evaporation rate then increase the performance of the VARS. This demonstrates the importance of controlling the temperature and the flow rate in the VARS for generate more refrigerants

Applicability of mechanical tests for biomass pellet characterisation for bioenergy applications

In this paper, the applicability of mechanical tests for biomass pellet characterisation was investigated. Pellet durability, quasi-static (low strain rate), and dynamic (high strain rate) mechanical tests were applied to mixed wood, eucalyptus, sunflower, miscanthus, and steam exploded and microwaved pellets, and compared to their Hardgrove Grindability Index (HGI), and milling energies for knife and ring-roller mills. The dynamic mechanical response of biomass pellets was obtained using a novel application of the Split Hopkinson pressure bar. Similar mechanical properties were obtained for all pellets, apart from steam-exploded pellets, which were significantly higher. The quasi-static rigidity (Young’s modulus) was highest in the axial orientation and lowest in flexure. The dynamic mechanical strength and rigidity were highest in the diametral orientation. Pellet strength was found to be greater at high strain rates. The diametral Young’s Modulus was virtually identical at low and high strain rates for eucalyptus, mixed wood, sunflower, and microwave pellets, while the axial Young’s Modulus was lower at high strain rates. Correlations were derived between the milling energy in knife and ring roller mills for pellet durability, and quasi-static and dynamic pellet strength. Pellet durability and diametral quasi-static strain was correlated with HGI. In summary, pellet durability and mechanical tests at low and high strain rates can provide an indication of how a pellet will break down in a mill

A global picture of quantum de Sitter space

Perturbative gravity about a de Sitter background motivates a global picture of quantum dynamics in `eternal de Sitter space,' the theory of states which are asymptotically de Sitter to both future and past. Eternal de Sitter physics is described by a finite dimensional Hilbert space in which each state is precisely invariant under the full de Sitter group. This resolves a previously-noted tension between de Sitter symmetry and finite entropy. Observables, implications for Boltzmann brains, and Poincare recurrences are briefly discussed.Comment: 17 pages, 1 figure. v2: minor changes, references added. v3: minor changes to correspond to PRD versio