Homayun K. Navaz

The Application of Advanced Methods in Analyzing the Performance of the Air Curtain in a Refrigerated Display Case

Computational Fluid Dynamics (CFD) modeling is effectively coupled with the experimental technique of Digital Particle Image Velocimetry (DPIV), to study the flowfield characteristics and performance of the air curtain of a medium-temperature open vertical refrigerated display case used in supermarkets. A global comparison of the flowfield and quantification of the entrained air into the case indicate that there is a considerable amount of cold air spillage from a typical display case that is replaced by the ambient warm entrained air across the air curtain, lowering the energy efficiency of the case. The computational model that is developed from the marriage of CFD and DPIV techniques provides a reliable simulation tool that can be used for the design optimization of air curtains. A correct estimate of the infiltration rate by changing different parameters in a validated computational simulation model will provide a feasible tool for minimizing the spillage of the cold air, and thereby designing more energy efficient open display cases.

Sessile droplet spread into porous substrates—Determination of capillary pressure using a continuum approach

The problem of primary and secondary spread of sessile droplets into a porous substrate was formulated and solved numerically. A continuum approach for liquid- and gas-phases was utilized. The governing equations were discretized by finite difference method and solutions for both phases are obtained by marching in time using the fourth-order Runge-Kutta integration algorithm. This type of spread is a purely momentum-driven process that is caused by gradients both in capillary pressure and in saturation. A methodology was developed for finding the capillary pressure function for sessile droplets, which has not been described before. This approach was based on experimental data for a liquid/porous medium pair, and using universal, non-dimensional curves. Similar solutions were generated by the continuum approach and validated using experimental results. The model shows qualitative and quantitative agreement with experimental data. Although the focus of this work was to understand the interaction of chemical warfare agents with porous media, the approaches are universal and can be applied to determining the spread of any liquid into a porous material.

Experimental and Numerical Study of Spread and Sorption of VX Sessile Droplets into Medium Grain-Size Sand

The experimental measurement and modeling of liquid chemical agent spread and sorption on a porous substrate are described. Experimental results with the nerve agent O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX) demonstrate that the wetted imprint volume increases, even after the sessile drop volume is exhausted. This indicates the wetted imprint is only partially saturated, and a multiphase flow problem formulation is needed to predict the VX fate in porous substrates. Three characteristics and their changes in time: (i) sessile volume remaining, (ii) wetted imprint area on the sand surface where the droplet is deposited, and (iii) VX penetration depth into sand, are computed numerically and compared to experimentally measured values. A very good qualitative and quantitative agreement was found between the numerical and experimental results. These numerical and experimental methods can be used to determine the spread and sorption of hazardous materials into a variety of substrates.

On spread extent of sessile droplet into porous medium: Numerical solution and comparisons with experiments

The spread of a wetting liquid sessile droplet into porous medium is solved numerically using the capillary network model with a microforce balance boundary condition at the liquid/gas free interface in the porous medium. The spread starts as the porous medium imbibes the sessile liquid, followed by liquid additionally being spread inside the porous medium itself. After there is no remaining sessile liquid, the net flow across the porous medium boundaries is equal to zero. Either spread, with or without sessile liquid present at the porous medium surface, is rendered by local differences in capillary pressure. These local differences are accounted for by implementing the numerical solution over a heterogeneous capillary network, consisting of pores connected by throats. Both pores and throats follow predefined distribution functions. Once there is no sessile liquid present on the porous medium surface, it is found from a numerical solution that the liquid front can extend significantly in time, wetting ve...

Infiltration time and imprint shape of a sessile droplet imbibing porous medium

The infiltration of a sessile droplet into a homogeneous porous medium for a constant droplet base radius case is solved numerically, where the porous medium is represented as a capillary network consisting of pores and throats. For a homogeneous medium, the network is built of the spherical pores of constant radius, and the cylindrical throats of constant radius and height. Having such defined network, the droplet imbibes porous medium in a single-phase flow for which the free interface in porous medium is smooth, and the liquid phase permeability and the capillary pressure are constant. Using the numerical solution we carry out the parametric study in which: (i) liquid viscosity and surface tension, (ii) droplet volume and base radius, and (iii) porous medium porosity and permeability are varied. The droplet infiltration time, and the imprint shape that is given with two spheroid half-axes are calculated. The dimensionless analysis is utilized to correlate the droplet infiltration parameters from which master curves for the droplet infiltration time and the droplet imprint shape are obtained. Using the infiltration time correlation, both numerical and experimental results show a linear behavior.

Jet entrainment minimization in an air curtain of open refrigerated display case

Purpose – To address the effects of velocity profile at the discharge air grille (DAG) on the amount of entrained air into an open refrigerated display case (ORDC).Design/methodology/approach – The performance of an ORDC was studied by CFD, DPIV and LDV. The actual measured velocity profile at the DAG and total flow rate of the display case at its nominal operating conditions are used as guidelines throughout the CFD modeling.Findings – It was found that a skewed parabolic profile with the peak shifted towards the inner section of the case generates the minimum entrainment and demonstrates that with simple changes to the geometry of the DAG, a significant reduction in the entrainment rate could be achieved.Research limitations/implications – This study finds the optimum infiltration rate of a manufactured ORDC. A fundamental study is currently being done to address all the effective parameters that can affect the infiltration rate of any ORDCs.Originality/value – This paper presents this fact that the vel...

A New Approach to Teaching Undergraduate Thermal/Fluid Sciences—Courses in Applied Computational Fluid Dynamics and Compressible Flow

A new approach is taken in teaching elective courses in thermal/fluid sciences (TFS) specialities primarily to undergraduate students in mechanical, aerospace, and chemical engineering disciplines by incorporating new and advanced technology into the curriculum. Two courses are designed at mezzanine level for undergraduate and graduate students that utilize computational fluid dynamics (CFD) and experimental methods. Techniques to enable undergraduates to learn advanced course materials are discussed. A course in Applied Computational Fluid Dynamics (CFD) is designed that combines the basic concepts in algorithmic and numerical aspects of CFD with experimental verification and validation. The course provides a mathematical and physical understanding of the behaviour of the conservation equations for different flow regimes through lectures, projects and hands-on experience. Another course in modern compressible flow will enable students to utilize the CFD technology and measurement methods to find accurate solutions to a wide range and complex problems in gas dynamics. Students develop the capability to utilize this new and growing technology in their industrial career after graduation.

Numerical solution of wetting fluid spread into porous media

Purpose – The purpose of this paper is to develop a general numerical solution for the wetting fluid spread into porous media that can be used in solving of droplet spread into soils, printing applications, fuel cells, composite processing.Design/methodology/approach – A discrete capillary network model based on micro‐force balance is numerically implemented and the flow for an arbitrary capillary number can be solved. At the fluid interface, the boundary condition that accounts for the capillary pressure jump is used.Findings – The wetting fluid spread into porous medium starts as a single‐phase flow, and after some particular number of the porous medium characteristic length scales, the multi‐phase flow pattern occurs. Hence, in the principal flow direction, the phase content (saturation) decreases, and in the lower limit for the capillary number sufficiently small, the saturation should become constant. This qualitative saturation behavior is observed irrespective of the flow dimensionality, whereas th...

Dynamics of freely moving plates connected by a shallow liquid bridge

We study the dynamics of freely moving plates connected by a shallow liquid bridge via analytic and experimental methods. The gap between the plates is used as a small parameter within a lubrication approximation, reducing the problem to an Abel equation of the second kind. Analysis of the governing differential equation yields two novel physical phenomena: (1) An impulse-like peak in the force applied by the liquid bridge on the plates, obtained from a uniform asymptotic solution for small capillary numbers. (2) Both linear and non-linear oscillations of the system for the case of surfaces with low wettability, obtained from small perturbations of the system around the equilibrium point. An experimental setup examining the motion of freely moving plates was constructed, yielding experimental data which compared favorably with the analytic results and specifically displayed the predicted oscillations and impulse-like peak of the applied force. The application of the current analysis to the manipulation of solid bodies and possible future research directions are discussed.

The influence of capillary flow on the fate of evaporating wetted imprint of the sessile droplet in porous medium

The fate of a wetting liquid sessile droplet imbibed by a porous medium is formulated as a multiphase flow problem and a numerical solution is developed using the capillary network model with a microforce balance at the liquid∣gas interface. The liquid phase capillary flow and evaporation are solved simultaneously. An exclusive evidence for a multiphase flow is already found in the capillary flow, as a liquid wets a much larger volume of porous medium compared to the wetted volume, calculated by assuming that the medium imbibes the liquid in the single-phase flow. The physics of the multiphase capillary flow includes the formation of local gas clusters and liquid ganglia. The clusters and ganglia distribution is further altered by evaporation. The evaporation tends to shrink the ganglia sizes and open the gas clusters, both due to the liquid mass loss from the porous medium. Still, the capillarity tends to disperse the liquid back into the regions from where the liquid previously evaporated. These changes...