Stability analysis of thermo-bioconvection in suspensions of gravitactic microorganisms in a fluid layer

In this paper, we investigate the effect of heating or cooling from below on the stability of a suspension of motile gravitactic microorganisms in a shallow fluid layer. The linear perturbation theory is used to obtain the stability diagram and the critical conditions for the onset of convection. It is found that the thermo-effects may either stabilize or destabilize the suspension, and decrease or increase the wavelength of the bioconvective pattern.     

Numerical investigation of bioconvection of gravitactic microorganisms in an isotropic porous medium

A new continuum model is formulated for bioconvection in a dilute suspension of swimming, gravitactic microorganisms in a porous medium. “Bioconvection” is the name given to pattern-forming convective motions set up in suspensions of swimming microorganisms. “Gravitaxis” means that microorganisms tend to swim against the gravity. The aim of this paper is to analyze collective behavior and pattern formation in populations of swimming microorganisms. The existence and stability of a two-dimensional plume in a tall, narrow chamber with stress-free sidewalls is investigated. Governing equations include Darcy law as well as microorganism conservation equation. A conservative finite-difference scheme is used to solve these equations numerically.     

Bioconvection of gravitactic microorganisms in a fluid layer

A study is made of the spontaneous pattern formation of gravitactic microorganisms in a horizontal fluid layer. The linear stability theory is used to determine the onset of convection in terms of the Rayleigh number and the swimming velocity. It is found that the onset of convection in a gravitactic suspension may be very different from that of Bénard cells under the well-known fixed-flux heating condition.     

Bioconvection of gravitactic microorganisms in a vertical cylinder

Patterns formation of gravitactic microorganism in a vertical cylinder is described by the Navier–Stokes equation coupled with the microorganism conservation equation. The control volume method is used to solve numerically these equations. It is found that when the Peclet number is decreased, the critical Rayleigh number also decreases to approach the value corresponding to Bénard convection under fixed-flux heating condition. However, at high Peclet numbers, the development convection is very different from that of Bénard convection. The most fundamental difference is that, while Bénard convection is a supercritical instability, the gravitactic bioconvection is shown to be a subcritical bifurcation from the diffusion state.     

Thermo-bioconvection in a suspension of gravitactic micro-organisms in vertical cylinders

We investigate the effect of heating or cooling from below at constant temperature and constant heat flux on the development of gravitactic bioconvection in vertical cylinders with stress free sidewalls. The governing equations are the continuity equation, the Navier–Stokes equations with the Boussinesq approximation, the diffusion equation for the motile micro-organisms and the energy equation. The control volume method is used to solve numerically the complete set of governing equations. The governing parameters are the thermal and bioconvection Rayleigh numbers, the bioconvection Peclet number, the Lewis number, the Schmidt number and the aspect ratio. We found that subcritical bifurcations of bioconvection became supercritical bifurcations when the thermal Rayleigh number Ra_T is different than zero. For Ra_T < 0, i.e. for cooling from below, we have opposing buoyancy forces, the convection is decreased and the critical thermo-bioconvection Rayleigh number is increased with respect to that of bioconvection. For Ra_T > 0, i.e. for heating from below, we have cooperating buoyancy forces, the convection is increased and the critical thermo-bioconvection Rayleigh number is decreased with respect to that of bioconvection. Heating and cooling from below at constant temperature and heat flux modify considerably the pattern formation of the gravitactic bioconvection.     

Bioconvection of gravitactic micro-organisms in rectangular enclosures

This paper investigates the gravitactic bioconvection in rectangular enclosures. The governing equations are the continuity, the Navier–Stokes equations with the Boussinesq approximation and the diffusion equation for the motile micro-organisms. The control volume method is used to solve numerically the complete set of governing equations. The effects of bioconvection Peclet number from 0.1 to 10 and the aspect ratio from 1 to 5 are investigated on the onset of bioconvection. It was found that the bifurcation was subcritical in all cases. The critical Rayleigh number is decreased with increasing bioconvection Peclet number and with increasing aspect ratio.     

The onset of nanofluid bioconvection in a suspension containing both nanoparticles and gyrotactic microorganisms

The purpose of this paper is to study the onset of bioconvection in a horizontal layer filled with a nanofluid that also contains gyrotactic microorganisms. The idea is to use microorganisms to induce or enhance convection in a nanofluid. A linear instability analysis is used to solve this problem. A Galerkin method is utilized to obtain an analytical solution for the critical Rayleigh number for the non-oscillatory situation. As in the case of a regular nanofluid (without the microorganisms), the presence of nanoparticles can either reduce or increase the value of the critical Rayleigh number, depending on whether the basic nanoparticle distribution is top-heavy or bottom-heavy. In contrast, the effect of gyrotactic microorganisms is always destabilizing.     

The onset of bioconvection in a suspension of negatively geotactic microorganisms with high-frequency vertical vibration

The effect of vertical vibration on the stability of a dilute suspension of negatively geotactic microorganisms in a fluid layer of finite depth is investigated. For the case of high-frequency vibration, solutions of governing equations are decomposed into two components: one which varies slowly with time and a second which varies rapidly with time. An averaging method is utilized to derive the equations describing the mean flow. Linear stability analysis is used to investigate stability of the obtained averaged equations.     

Numerical investigation on the performance of microencapsulated phase change material suspension applied to liquid cold plates

A three-dimensional thermo-hydrodynamic coupling model is established to study the performance of microencapsulated phase change material suspension (MEPCMS) applied to liquid cold plates. The cooling effect of three liquid cold plates with different flow channels is compared. And the liquid cold plate with the 3-elbow flow channel is chose to analyze the performance when using MEPCMS as coolant. The results show that compared with pure water, the MEPCMS not only can lower the maximum temperature of the liquid cold plate, improve the temperature uniformity, but also lessoning the load of the cooling system.     

Numerical investigation of an evaporating meniscus in a channel

A detailed numerical model is developed that describes heat and mass transfer from a meniscus to open air. The model accounts for the effects of evaporation at the interface, vapor transport through air, thermocapillary convection, and natural convection in air. Evaporation at the interface is modeled using kinetic theory, while vapor transport in air is computed by solving the complete species transport equation. Since the vapor pressure at the liquid–gas interface depends on both evaporation and the vapor transport in air, the equations are solved in an iterative manner. Evaporation is strongest at the triple line due to the highest local vapor diffusion gradient in this region. This differential evaporation, coupled with the low thermal resistance near the triple line, results in a temperature gradient along the interface that creates thermocapillary convection. The numerical results obtained show satisfactory agreement with experimental data for the evaporation rate and the temperature profile. Additionally, results from a simplified model neglecting thermocapillary convection are compared with the full solution, thus delineating the importance of thermocapillary convection-induced mixing in the energy transfer process. The present generalized model may easily be extended to other geometries and hence may be used in the design of two-phase cooling devices.     

The onset of bioconvection in a suspension of gyrotactic microorganisms in a fluid layer of finite depth heated from below

The aim of this paper is to investigate the effect of heating from below on the stability of a suspension of motile gyrotactic microorganisms in a fluid layer of finite depth. This problem is relevant to a number of geophysical applications, such as investigation of the dynamics of some species of thermophiles (heat-loving microorganisms) living in hot springs. It is established that heating from below makes the system more unstable and helps the development of bioconvection. By performing a linear stability analysis, a correlation for the critical bioconvection Rayleigh number is obtained.     

Numerical investigation of airflow through a Savonius rotor

The aim of this report is to present a model of a rigid‐rotor system based on computational fluid dynamics (CFD), which is applied on a vertical axis wind turbine (VAWT) research. Its originality results from the use of the average value of the variable rotational speed method taken in a periodic steady‐state (PSS) of the VAWT rotor instead of the classical fixed rotational speed method. This approach was chosen in order to determine the mechanical and aerodynamic parameters of the wind turbine. The modeling method uses an implicit Euler iterative solution strategy, which resolves the coupling between fixed and moving rotor domains. The main methods that were adopted are based on the three‐dimensional modeling of the interaction of the fluid flow with a rigid‐rotor. The strategy consists of using the Reynolds averaged Navier Stokes (RANS) equations with the standard k‐ ? and SST k‐ ω models to solve the fluid flow problem. To perform the rigid‐rotor motion in a fluid, the one degree of freedom (1‐DOF) method was applied. In the present study, the steady‐state and dynamic CFD simulations of the Savonius rotor are adopted to contribute to the validation elements of the VAWT models that are used. The dynamic study allows the investigation of the rotor behavior and the relation between velocity, pressure, and vorticity fields in and around the rotor blades. The flow fields generated by the rotation of the Savonius rotor were investigated in the half revolution period of the rotor angle θ from 0° to 180°. In this range of θ, the focus is on generating and dissipating vortices. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical investigation of pulsating flow around a discrete heater in a channel

In this study, the forced convection heat transfer around a discrete heater located in a channel subjected to laminar pulsating air flow is numerically investigated. Simulations are conducted for six different frequencies and three different amplitudes, while the Reynolds number (Re = 125) and Prandtl number (Pr = 0.71) remain constant for all cases. The impact of the important governing parameters such as the Womersley number (Wo) and the amplitude of flow pulsation (A_o) on heat transfer rate from discrete heaters is examined in detail. The instant velocity and temperature profiles are obtained to determine of the role of dimensionless parameters for pulsating flow. The numerical results show that thermal transport from the heater is greatly affected by the frequency and amplitude of the flow pulsation. The results given are dimensionless parameters.     

Numerical and experimental investigation of a mild combustion burner

An industrial burner operating in the MILD combustion regime through internal recirculation of exhaust gases has been characterized numerically. To develop a self-sufficient numerical model of the burner, two subroutines are coupled to the CFD solver to model the air preheater section and heat losses from the burner through radiation. The resulting model is validated against experimental data on species concentration and temperature. A 3-dimensional CFD model of the burner is compared to an axisymmetric model, which allows considerable computational saving, but neglects some important burner features such as the presence of recirculation windows. Errors associated with the axisymmetric model are evaluated and discussed, as well as possible simplified procedures for engineering purposes. Modifications of the burner geometry are investigated numerically and suggested in order to enhance its performances. Such modifications are aimed at improving exhaust gases recirculation which is driven by the inlet air jet momentum. The burner is found to produce only 30 ppm_v of NO when operating in MILD combustion mode. For the same air preheating the NO emissions would be of approximately 1000 ppm_v in flame combustion mode. It is also shown that the burner ensures more homogeneous temperature distribution in the outer surfaces with respect to flame operation, and this is attractive for burners used in furnaces devoted to materials' thermal treatment processes. The effect of air excess on the combustion regime is also discussed.     

Numerical investigation of a non-premixed hollow rotating detonation engine

Rotating detonation engines (RDEs) are widely studied because of their compact configurations and high thermal cycle efficiency. In this paper, a series of numerical investigations of a non-premixed hollow RDE are performed. The transient explicit density-based solver in ANSYS Fluent is used to perform the simulations. For a hollow RDE without Laval nozzle, there is only one rotating detonation wave in the combustion chamber. Compared to the traditional annular RDE, the mixing quality is deteriorated, and the thrust of the engine decreases and becomes more unstable. When the hollow RDE is attached with a Laval nozzle, there are two rotating detonation waves in the combustion chamber. The pressure within the combustion chamber increases while the axial velocity decreases. The mixing quality is improved. The height of detonation waves decreases with larger contraction ratio of the nozzle. A Laval nozzle is beneficial for a hollow RDE to achieve steadier operation and higher thrust output. When the contraction ratio is 4, the propulsive performance of the engine is the highest. The maximum thrust achieved is 840 N.     

Numerical investigation of extinction in a counterflow nonpremixed flame perturbed by a vortex

The features of extinction in a CH₄/N₂-air strained counterflow nonpremixed flame perturbed by a vortex were investigated numerically. First, the extinction behaviors using two augmented reduced mechanisms (ARM) and their original full reaction mechanisms (the Miller and Bowman mechanism and GRI-Mech 3.0) were investigated with the numerical results for a steady counterflow flame and unsteady flamelet equations. The modified ARM, based on Miller and Bowman's mechanism (MB-ARM), and adjusted to predict an extinction limit reasonably, was the most suitable mechanism for the unsteady simulation, taking into consideration computational cost, stiffness during the ignition process, and prediction performance for an unsteady flame with sinusoidal transient disturbances. The unsteady 2D computations with the modified MB-ARM showed that fuel- and air-side vortices caused the unsteady effect, and a flame interacting with a vortex was extinguished at a much higher scalar dissipation rate than a steady flame. Moreover, an air-side vortex extinguished the flame more rapidly than a fuel-side vortex, since the air-side vortex was much stronger than the fuel-side vortex, given the same vortex jet velocity conditions. In addition, the degree of the unsteady effect experienced by a flame could be clearly understood by introducing characteristic time scales for the flame, vortex, and convective-diffusive layer.     

Numerical investigation of a convective hybrid nanofluids around a rotating sheet

Hybrid nanofluids are formulated with various kinds of base fluids. They are designed to provide good heat transfer performance. They can achieve this by dispersing various kinds of nanoparticles in the base materials. This new technology of formulating hybrid nanofluids has a wide range of applications in various industries such as solar energy, medical equipment, and aerospace. Keeping these applications in view, this study provides an insight into the effects of convective heat transport on a Hybrid nanofluid, across a rotating sheet with a variable heat source. In this investigation, the governing boundary layer partial differential equations were modified into the ordinary differential equations, by using the proper similarity transformations. Later, they were solved numerically, with the support of the Lobatto IIIA technique in MATLAB. The influence of the Richardson number on flow parameters was studied, and it was discovered that increasing Ri increases the velocity while decreasing temperature and concentration profiles. The impact of various other flow parameters on the flow fields and also on the behavior of Nusselt number, coefficient skin friction, and Sherwood number were studied and represented graphically. The outcomes were found to be in excellent accord when compared with quoted studies.     

Numerical investigation of the intercooler of a two-stage refrigerant compressor

This study performs a numerical study to examine the flow mixing characteristics subject to slot-injection and hole-injection of an intercooler applicable to a two-stage refrigerant compressor. The effect of injected angle and velocity is also investigated. The result indicates that the temperature distribution of a hole-injection is more uniform while the velocity distribution less uniform at a vertically injected arrangement. Larger injection angles generate bigger flow separation zones which results in velocity non-uniformity, whereas smaller injection angles give rise to a less velocity non-uniformity. As the injection velocity rises, both the temperature and velocity non-uniformity of the slot-injection type increase significantly whereas the injection velocity has negligible influence on the relevant uniformity for a hole-injection type. It is also found that the existence of the secondary vortices allows the hole-injection type to have a more uniform temperature and velocity distribution than that of the slot-injection type.     

Numerical investigation of a PCM-based heat sink with internal fins

The present study explores numerically the process of melting of a phase-change material (PCM) in a heat storage unit with internal fins open to air at its top. Heat is transferred to the unit through its horizontal base, to which vertical fins made of aluminum are attached. The phase-change material is stored between the fins. Its properties used in the simulations, including the melting temperature of 23-25 °C, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax.A detailed parametric investigation is performed for melting in a relatively small system, 5-10 mm high, where the fin thickness varies from 0.15 mm to 1.2 mm, and the thickness of the PCM layers between the fins varies from 0.5 mm to 4 mm. The ratio of the PCM layer to fin thickness is held constant. The temperature of the base varies from 6 °C to 24 °C above the mean melting temperature of the PCM.Transient three- and two-dimensional simulations are performed using the Fluent 6.0 software, yielding temperature evolution in the fins and the PCM. The computational results show how the transient phase-change process, expressed in terms of the volume melt fraction of the PCM, depends on the thermal and geometrical parameters of the system, which relate to the temperature difference between the base and the mean melting temperature, and to the thickness and height of the fins.In search for generalization, dimensional analysis of the results is performed and presented as the Nusselt numbers and melt fractions vs. the Fourier and Stefan numbers and fin parameters. In some cases, the effect of Rayleigh number is significant and demonstrated.