Ozden Turan

Impinging round jet studies in a cylindrical enclosure with and without a porous layer: Part I—Flow visualisations and simulations

Abstract The interaction of turbulently flowing fluids and porous media occurs in many problems of practical interest. However, the engineering science literature appears to be devoid of either experimental or theoretical studies of such systems. In this paper, extensive flow visualisation experiments and comparisons with computational fluid dynamics (CFD) simulations are reported for these systems. In such systems, the turbulence in the adjacent fluid region can persist in the porous medium depending on its permeability and porosity. In the present study, turbulence is generated by using a round water jet that impinges on a porous foam. Due to the opaque nature of the porous medium, visualisations are carried out only in the fluid layer. However, the flow field in the fluid layer is affected by the flow in the porous medium, especially when the porous foam has a high permeability. Visualisations have been carried out to qualitatively evaluate the effect of the permeability of the porous medium, the height of the fluid layer and the thickness of the porous medium. A mathematical model of the system is formulated which incorporates two different turbulence models and a laminar model for the porous medium. A low-Reynolds number k – e turbulence model is used for the fluid layer in all cases. The resulting CFD predictions reflect well the effects of the changes in the permeability of the porous medium as well as the height of the fluid layer. However, the predictions are not as reliable in showing the changes due to the thickness of the porous medium. Predictions with one of the turbulence models with only Darcy damping in the turbulence transport equations for the porous medium is shown to give better qualitative comparisons for the gross flow patterns. Part II of this paper presents laser Doppler velocimetry measurements for the same system and comparisons of these measurements with the CFD simulations for a quantitative evaluation of the mathematical model.

Impinging round jet studies in a cylindrical enclosure with and without a porous layer: Part II—LDV measurements and simulations

Abstract This paper presents laser Doppler velocimetry (LDV) measurements and computational fluid dynamics (CFD) simulations for the same system as in Part I, a turbulent fluid layer overlying a saturated porous medium. The measurements are made to consolidate on the flow visualisation experiments which was used to qualitatively evaluate the numerical model in Part I. Comparisons between the simulations presented in Part I and the LDV measurements are made in Part II, for a quantitative evaluation of the numerical model. The simulations were carried out with two different turbulence models and one laminar flow model for the porous medium. For all simulations, a low-Reynolds number k – e turbulence model is used for the fluid layer. Predictions with one of the turbulence models with only Darcy damping for the porous medium is found to be quantitatively superior. Predictions with the laminar flow model and another turbulence model with Darcy and Forchheimer modification terms in the turbulence transport equations for the porous medium was similar. Simulations with these two models gave better results for the highest fluid height investigated, especially close to the interface between the fluid layer and the porous medium. The turbulence kinetic energy profiles in the fluid layer showed relatively good agreement with predictions by all three models for the porous medium.

Energy Dissipation with Sloshing for Absorber Design

Sloshing is the low frequency oscillation of the free surface of a liquid in a partially full container. Due to its detrimental effects, efforts are usually made in the direction of suppressing sloshing. In addition, intentionally induced sloshing may be employed as an effective energy sink to provide protection for resonant structures exposed to excessive vibration levels. It is generally reported that sloshing absorbers with shallow levels of liquid are more effective energy dissipators than those with deep levels. However, there has not yet been a study to reveal the mechanism of energy dissipation for practical applications, although there has been ample empirical proof for effectiveness. One of the limitations from a numerical perspective lies with the difficulty in predicting extreme free surface behaviour by traditional grid based computational methods. The objective of this paper is to report initial observations in this direction using Smoothed Particle Hydrodynamics (SPH). SPH is a Lagrangian method of solving the equations for fluid flow, that is suitable for modeling free surface phenomena such as sloshing due to its grid-free nature. Results are reported in this paper in the form of numerical case studies.Copyright

CFD modelling of natural convection heat and mass transfer in hygroscopic porous media

Abstract This paper presents the derivation of a model to predict heat and mass transfer in a system consisting of a turbulently flowing fluid overlying a saturated hygroscopic porous medium. Comparisons with experimental and numerical simulations have been carried out to check the accuracy of components of the model. Finally, a case study using silica gel as a representative hygroscopic porous medium is presented as an application of the model. It is shown that moisture is convected from the warm interior of a bulk of porous medium to the relatively cool periphery. This result has profound practical implications when the hygroscopic medium is stored agricultural produce as the region of high moisture content may become moldy.

Liquid Film Falling on Horizontal Plain Cylinders: Numerical Study of Heat Transfer in Unsaturated Porous Media

In this article, numerical investigation of liquid film falling over three horizontal plain cylinders is presented. The flow Reynolds number of 50 to 3000 and cylinder diameters of 0.022 m, 0.123 m, and 0.1 m are employed. Numerical predictions are obtained using FLUENT, a proprietary computational fluid dynamics (CFD) code for 2D configurations of the three cylinders. The mathematical approach is based on the volume of fluid (VOF) method to account for two fluid phases, namely air and water. The research reported in this paper is motivated by the possibility of using detailed numerical simulations of the phenomena that occur in beds of irrigated porous media to cool horticultural produce. The effect of liquid flow rate (Reynolds number) and the cylinder diameter on heat transfer coefficient have been studied. It is found that increasing liquid flow rate or Reynolds number results in an increase in the heat transfer coefficients, and decreasing the cylinder diameter results in an increase in the heat transfer coefficient. The numerical results provide an insight into the cooling mechanisms within beds of unsaturated porous media. This research is part of an overall study of horizontal cylinders falling film flow and heat transfer.

Toward the detailed simulation of the heat transfer processes in unsaturated porous media

Trickle bed chemical reactors and equipment used to cool horticultural produce usually involve three phase porous media. The fluid dynamics and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships between dimensionless groups. The research reported in this paper is motivated by the possibility of using detailed numerical simulations of the phenomena that occur in beds of irrigated porous media to obviate the need for empirical correlations. Numerical predictions are obtained using a CFD code (FLUENT) for 2-D configurations of three cylinders. Local and mean heat transfer coefficients around these non-contacting horizontal cylinders are calculated numerically. The present results compare well with those available in the literature. The numerical results provide an insight into the cooling mechanisms within beds of unsaturated porous media.© 2009 ASME

Liquid jet impingement without and with heat transfer

Equipment used to cool horticultural produce often involves three-phase porous media. The flow field and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships amongst dimensionless groups. This work represents a first step towards the goal of harnessing the power of computational fluid dynamics (CFD) to better understand the heat transfer process that occur in beds of irrigated horticultural produce. The primary objective of the present study is to use numerical predictions towards reducing energy and cooling water requirement in cooling horticultural produce. In this paper, flow and heat transfer predictions are presented of a single slot liquid jet on flat and curved surfaces using a CFD code (FLUENT) for 2-D configurations. The effects of Reynolds number, nozzle to plate spacing, nozzle width and target surface configuration have been studied. Reynolds numbers of 250, 500, 700, 1800 and 1900 are studied where the liquid medium is water. Here, the Reynolds number is defined in terms of the hydraulic nozzle diameter, inlet jet velocity and fluid kinematic viscosity. The results show that Reynolds numbers, nozzle to plate spacing and nozzle width have a significant effect on the flow filed and heat transfer characteristics; whereas the target surface configuration at stagnation area has no substantial impact. The use of a numerical tool has enabled detailed investigation of these characteristics, which have not been available in the literature previously.Copyright

Flow Visualization and Heat Transfer Characteristics of Liquid Jet Impingement

Equipment used to cool horticultural produce often involves three-phase porous media. The flow field and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships amongst dimensionless groups. This work represents a first step towards the goal of harnessing the power of computational fluid dynamics (CFD) to better understand the heat transfer processes that occur in beds of irrigated horticultural produce. The primary objective of the present study is to use numerical predictions towards reducing the energy and cooling water requirement in cooling horticultural produce. In this paper, flow and heat transfer predictions are presented of a single slot liquid jet impinging on flat and curved surfaces using a CFD code (FLUENT) for 2D configurations. The effects of Reynolds number, nozzle to plate spacing, nozzle width, and target surface configuration have been studied. Reynolds numbers of 250, 375, 500, 700, 1000, 1500, 1800, and 1900 are studied where the liquid medium is water. Here, the Reynolds number is defined in terms of the hydraulic nozzle diameter, inlet jet velocity, and fluid kinematic viscosity. The results show that Reynolds numbers, nozzle to plate spacing, and nozzle width have a significant effect on the flow field and heat transfer characteristics, whereas the target surface configuration at the stagnation area has no substantial impact. The use of a numerical tool has resulted in a detailed investigation of these characteristics, which has not been available in the literature previously.

A sloshing absorber with a flexible container

Liquid sloshing may be employed for vibration control of resonant structures, similar to that of a classical tuned vibration absorber. For such a case, the sloshing frequency is tuned at a critical frequency of the structure in order to gain the benefits of the pressure forces as control forces. Such an absorber is practically free of maintenance. The work presented in this paper utilizes a flexible container partially filled with water, as the sloshing absorber. Numerical predictions are presented where a “tuned” flexible container can be advantageous over a rigid container for effective control.

A Particle Damper for Transient Oscillations

Tuned vibration absorbers, in various disguises, still form the basis of vibration suppression for light and flexible structures. These simple auxiliary components may be tuned at critical frequencies of the structure to be controlled. Tuning frequencies may be constant (in case of passive absorbers) or varied (in case of semi-active and active absorbers).Tuning ensures a strong interaction, and facilitates the transfer of the harmful energy from the problem structure to the absorber. The task then becomes to dissipate the transferred energy rapidly in the absorber, before it has a chance to return the energy back to the structure. Returning the energy back to the structure, when the rate of dissipation in the absorber is not fast enough, manifests itself as a beat, significantly deteriorating the control performance. The obvious solution to avoid the undesirable beat is to include dissipative components in the design of the tuned absorber. However, such an inclusion has two consequences. First, the effectiveness at the tuning frequency is sacrificed, at a level proportional to the amount of damping in the absorber. Second, and more critically, dissipative components are high maintenance components by nature, rendering the damped tuned absorber to be less practical. A particle damper is presented in this paper which can dissipate energy rapidly while maintaining the tuning of the absorber effectively. Simple experiments are detailed to demonstrate the tuning, and the level of dissipation.