D. Bouris

2D LES OF VORTEX SHEDDING FROM A SQUARE CYLINDER

Abstract In the high Reynolds number flow past a square cylinder, turbulent fluctuations are superimposed on the periodic vortex shedding motion making numerical calculation of the flow a difficult task. In the present work a two-dimensional large eddy simulation is performed with no-slip boundary conditions at the solid walls. A filtering procedure is introduced in frequency space to separate the periodic from the turbulent fluctuations and the kinetic energy of both is calculated along the centerline behind the rod. The drag coefficient, vortex shedding frequency and spacing of the vortices in the wake are also calculated and the results are validated against experimental measurements. It is found that the two-dimensional large eddy simulation using a fine grid resolution, especially in the near wall region, gives a good representation of the quasi-two-dimensional mechanisms of the flow since they are directly simulated instead of being modeled as with statistical turbulence models.

Numerical evaluation of alternate tube configurations for particle deposition rate reduction in heat exchanger tube bundles

Abstract The potential of two passive techniques, namely elliptic-shaped tubes and asymmetric tube bundle arrangement, for deposition rate reduction in lignite-fired utility boiler heat exchangers is numerically studied through comparison with an in-line tube arrangement with circular tubes. The simulation is based on a two-dimensional calculation of the turbulent two-phase flow in heat exchanger tube bundles using a subgrid-scale model for the small-scale turbulent structures and a particle adhesion model for the particle–surface interaction. Large-scale motion is found to be an important mechanism for tube surface fouling. Of the three tube bundle arrangements that were studied, the in-line tube bundle with elliptic-shaped tubes shows the lowest fouling rates and pressure drop. Taking advantage of these, heat transfer rates might be increased by placing more rows in the same area.

Two dimensional time dependent simulation of the subcritical flow in a staggered tube bundle using a subgrid scale model

Abstract Numerical calculation of subcritical flow in tube bundles is a difficult task since transitional effects as well as vortex shedding are important characteristics of the flow field. In the present paper a time dependent simulation using a subgrid scale model is performed in two dimensions for the subcritical flow through a staggered tube bundle. Previous time dependent simulations of the same flow using the k – e turbulence model unfortunately lead to a steady state solution, which underpredicted turbulence quantities and recirculation lengths. Thus a novel approach is introduced, using a subgrid scale model for the calculation of the eddy viscosity. The methodology that is used strongly resembles a large eddy simulation even though it is performed in two dimensional space. A filtering procedure is used to remove the vortex shedding frequency from the velocity data thus allowing a calculation of the turbulence fluctuations. Calculated vortex shedding frequencies, periodic and turbulence velocity fluctuations and integral time scales are in good agreement with experimental measurements.

Effects of fouling on the efficiency of heat exchangers in lignite utility boilers

Abstract The work presented in this article is directed towards the understanding of the mechanism of ash deposition on the surfaces of tubes of heat exchangers in lignite utility boilers and the evaluation of the influence of fouling on heat exchanger efficiency. For this purpose a numerical model was developed to predict the deposition of particles onto the heat transfer surfaces. The deposition model is combined with a numerical procedure that solves the two phase flow and temperature field around the tubes of the heat exchanger. Predictions of the flow field in both in-line and staggered tube bundles are validated through comparisons with experimental measurements that are conducted in laboratory model geometries. Experimental techniques include laser-sheet flow visualisation and laser Doppler Anemometry (LDA). Observations in full scale geometries of lignite utility boiler heat exchangers that are in current operation are provided by the industrial partner. The results of the work performed indicate that the maximum deposit height on the surface of the first tube is reached in about two weeks for a staggered tube bundle arrangement. It is the actual spacing of the tube arrangement that plays an important role in reducing the fouling rate while closely spaced tubes in in-line arrangements show signs of bridge formation between subsequent tube rows.

Numerical calculation of the effect of deposit formation on heat-exchanger efficiency

Abstract A numerically calculated deposit was used to identify the shape of a fouled tube and an orthogonal curvilinear grid was constructed in and around the deposit. The flow and temperature field was calculated and an implicit boundary condition, accounting for the effects of turbulence, was introduced at the solid-fluid interface for the simultaneous calculation of the temperature field in the fluid and the solid. Validation is presented against experimental and numerical calculations. Finally, a reduction of 8–17% in the heat transfer rate was found to occur for a deposit corresponding to 8 h of operation in a staggered tube bundle heat-exchanger of a lignite utility boiler. Thermal resistance of the deposit was also calculated.

3D Numerical Simulation of the Transient Thermal Behavior of a Simplified Building Envelope Under External Flow

Understanding building envelope performance and thermal mass effects is becoming increasingly important under the scope of low energy building construction and energy conservation. In the present paper, a three-dimensional computational fluid dynamics methodology is presented for the numerical simulation of the flow and heat transfer that determine the thermal behavior of simplified building envelopes. This is dominated by a conjugate heat transfer approach, which involves conduction, convection, solar heat gains, ambient temperature variation, and the effects of thermal radiation losses to the sky. Validation results include comparison both with measurements from fundamental laboratory studies of heat transfer from surface mounted cubes and with numerical results from well established commercial building energy simulation software. Numerical issues, such as temporal and spatial discretization, are addressed, and parametric studies are performed with regard to the effect of external flow Reynolds number and temperature variation in the building envelope, depending on the individual orientation of the external walls with respect to the flow and on the thermal properties of the building materials. Results from the parametric studies performed indicate that the transient three-dimensional calculations provide important information regarding the effect of external flow properties, such as the approaching flow temperature, velocity, and direction on the thermal behavior of the building envelope. In addition, it has been clearly demonstrated that the methodology is also capable of taking into account the complex effects of parameters such as the building material properties.

Numerical Comparative Study of Compressor Rotor and Stator Blade Deposition Rates

Compressor fouling is generally accepted to be an important factor when monitoring the efficiency of an engines operation. However, there are not many studies related to the local fouling behavior of the individual components of the compressor. In the present paper, the CFD-ACE software package is used for the flow field calculation and the results are utilized to calculate the deposition rates on the blade surfaces of conventional compressor stator and rotor. The deposition model takes into account the particle and surface material properties and the energy balance at the point of impact. A discussion is presented regarding the various mechanisms that produce the final deposition rate distribution and how the flow field and blade geometry affect it.

Controlling Unsteady Separation from a Cylinder by Non-Harmonic Perturbations

This study considers the effect of non-harmonic perturbations superimposed on the inflow velocity on the wake dynamics of a circular cylinder by means of two-dimensional numerical simulations. The cases considered encompass the ‘lock-on’ region in the frequency–amplitude parameter space which results in a strong interaction with the separated flow in the cylinder wake. Results for two different non-harmonic perturbation waveforms are compared with those corresponding to harmonically perturbed flows in terms of the dynamical states, drag and inertia coefficients and streamline patterns. It shown that the locked-on vortex formation is associated with large excursions of the separation point along the surface of the cylinder. The non-harmonic perturbations result in subtle changes in the location of the separation points, which however induce an appreciable shift in the phasing of the forces with respect to the imposed velocity perturbation.

Bluff Body Aerodynamics and Wake Control

In aerodynamics, a bluff body is one which has a length in the flow direction close or equal to that perpendicular to the flow direction. This spawns the characteristic that the contribution of skin friction is much lower than that of pressure to the integrated force acting on the body. Even a streamlined body such as an airfoil behaves much like a bluff body at large angles of incidence. A circular cylinder is a paradigm often employed for studying bluff body flows.