Active/Recently Completed Research Projects

Prof. Bakhtier Farouk

Department of Mechanical Engineering

and Mechanics

Drexel University

Philadelphia, PA 19104

Thermoacoustic Refrigeration

Thermoacoustic refrigerators are regenerative type systems that operate on a modified reverse Stirling cycle (the gas refrigeration cycle).  These thermoacoustic systems (in particular, the Pulse Tube Refrigerator or PTR) have gained increased importance in cryogenic cooling technologies.  The objectives of the current research are to develop fundamental understanding of the thermoacoustic effects for better design of thermoacoustic cryogenic refrigeration systems.

A schematic for the OPTR geometry used in the numerical simulations (a) no wall thickness and (b) wall thickness and/or thermal mass of component’s flanges considered

Schematic of the co-axial type OPTR geometry simulated

Thermoacoustic Waves in Supercritical Fluids

Thermoacoustic Phenomena

The interactions between acoustic waves and heat and mass transfer from objects falls into a general class known as pressure-driven convection, i.e., convection caused by the propagation of pressure waves. The current investigation is concerned with pressure waves produced by the rapid heating of one of the boundaries of an enclosure (thermoacoustic waves). A sharp rise in boundary temperature can cause pressure waves to propagate through a fluid in much the same way as does pushing a piston through a gas-filled pipe or by the vibrating surface of an acoustic driver. At low levels of gravity the unsteady-state rate of heat transfer caused by acoustic effects is greatly enhanced relative to pure conduction.

Acoustic waves (created by a vibrating surface) also can create bulk motion of the fluid, otherwise known as acoustic streaming. An experimental setup is being fabricated that will allow investigation of the acoustic convection in an enclosure fitted with piezoelectric drivers. Application of a sophisticated numerical algorithm that explicitly considers the acoustic along with the fluid flow and heat transfer is being applied to quantify such effects.

Another area of interest is in development of refrigeration using acoustic power (thermoacoustic refrigeration).

Instantaneous pressure field in an enclosure due to impulsive heating of a sidewall.

Propagation of a thermoacoustic wave in an enclosure.

Student: Murat Aktas

Undergraduate students: Benajmin Ott, Brian Schmidt

Sponsor: NSF (pending) NASA(pending)

Plasma-Assisted Net-Shape Deposition for Microfabrication

In this research project, we are developing a radically different microfabrication method, which does not require photolithography or high vacuum plasma reactors. We refer to the method as "Plasma-assisted Net Shape Deposition (PAND)". In this method, the desired "net shape" of a film is deposited onto the substrate using plasma-assisted chemical (or physical) vapor deposition. The deposition is carried out under atmospheric pressure and this eliminates the need for costly high vacuum reactors. The heart of the system consists of an extremely small atmospheric pressure ‘cold’ plasma ball, which is formed between a metal-wire electrode and a metal surface (or between two metal-wire electrodes). Recent investigations in our laboratory have shown that a small intense atmospheric pressure plasma ball (having a diameter of only few hundred microns) can be generated under suitable conditions. Such a micro-plasma ball is self-sustaining and of the 'cold' type - similar to the high vacuum plasma sources used in microchip manufacturing.

Both experimental and computational studies are underway to assess the viability of the proposed process. In the preliminary work, we are characterizing dc microdischarges formed between a metal-wire electrode and a metal surface and between two metal-wire electrodes. Effects of the metal-wire electrode diameters, spacing between the electrodes and the surrounding gases on the discharge characteristics are being studied. Comprehensive numerical simulations are being carried out to characterize the atmospheric pressure ‘micro-plasma’ discharge.

Student: Fang Yan

Post-doctoral associate: Jeong W. Yi

Undergraduate student: Jessica Giordani

Sponsor: NSF

Numerical Simulation of Microchannel Flows

of Gases and Liquids

With the rapid development of Microelectromechanical systems (MEMS), microchannel flow has been a subject of increasingly active research. The small size of MEMS poses unique challenge in the calculation of fluid flow and heat transfer. Traditional CFD techniques are often inaccurate for analyzing high Kn gas flow because of significant non-continuum effect. Gas/liquid two-phase flow in microchannel is characterized by capillary and surface tension effects.

Gas Flow

Direct Simulation Monte Carlo (DSMC) method offers an alternative to traditional CFD which retains its validity at high Kn.

Microchannel mixing flow investigation is valuable for MEMS-based fuel injectors and propulsion devices. The effects of inlet-outlet pressure difference and pressure ratio on mixing length are being systematically explored.

This image shows the mixing process of two parallel gas streams (hydrogen and oxygen) in a microchannel. Hydrogen and oxygen streams with the same inlet pressures but different inlet streamwise velocities are led into the microchannel for mixing. The microchannel is 1 mm in width. Before entering the mixing chamber, the two streams are separated by a thin splitter plate of length 4.5 mm. Both streams enter the microchannel at 200 kPa. Hydrogen flows into the microchannel along the lower half with an average velocity 60.6 m/s and oxygen enters the microchannel at the upper inlet with an average velocity 19.2 m/s. The temperature of the microchannel walls is held at 300 K. The flow field is simulated by using the direct simulation Monte Carlo method (DSMC) technique. The flow field was simulated for a channel length of 16 mm. The outlet pressure for the microchannel is held at 50 kPa. Due to the large aspect ratio of the microchannel considered, only a portion of the computed flowfield (in the streamwise direction) is shown in the image. The image shows the velocity vectors and the shaded density contours near the inlet of the microchannel (from 3.7 mm to 5.2 mm). The location of the splitter, which extends up to 4.5 mm is also indicated in the figure. The mixing in the microchannel is diffusion dominated and no shear layer instability is observed. Significant back diffusion of hydrogen and oxygen (into the oxygen and hydrogen streams respectively) is observed within the region separated by the splitter plate. (also see Microscale Thermophysical Engineering, Vol. 6, No. 3, 2002, cover)

Liquid Flow

Numerical simulations are being performed to study the capillary electrophoresis flow in microchannels. Sample stream is focused during the loading step and driven into the separation channel during the dispensing step. Flow fields and species distribution are simulated for both the loading and the dispensing steps in a two dimensional cross channel device. The evolution of each sample species concentration at the end of the separation channel is predicted. The separation resolution is defined from the sample species concentration band retention time and band width. Different separation performances can be obtained by manipulating electric field strength. A series of simulations for different electric field distributions and field magnitudes in the channel are presented. The goal of these simulations is to identify the parameters providing optimal separation performance. The effect of both loading and dispensing schemes on species concentration and separation resolution is being studied

Concentration contours of a species at a given instant due to capillary electrophoresis

.

Student: Fang Yan

Sponsor: Unfunded

Electrospinning of nanofibers

A numerical model has been developed for an electrostatically driven liquid meniscus for a dielectric fluid. The model is able to calculate the shape of the liquid cone and the resulting jet, the velocity fields inside the liquid cone-jet, the electric fields in and outside the cone-jet, and the surface charge density at the liquid surface. The mathematical formulas with proper boundary conditions for the relevant physical processes are described in detail. The equations of continuity, momentum and electric potential are solved numerically with an iterative procedure developed for the model. The results of the present model fit well with experimental observations of the cone shape and jet formation.

We are now developing models for electrospinning of polymeric fluids that result in nanofibers.

Student: Position open

Sponsor: NSF IGERT Fellowship

A Comprehensive CFD Model for Hydrodynamics and Microbial Inactivation in Ozone Bubble Contactors

Public health concerns and recent and upcoming regulations are causing water utilities to consider alternatives to traditional chlorine-based drinking water disinfection schemes. Ideally, these alternative schemes will provide improved inactivation of problematic pathogenic organisms such as Cryptosporidium parvuum while reducing the output of potentially toxic byproducts formed through the interaction of the disinfectant with dissolved materials in the water. An alternative disinfectant in wide use in Europe and gaining increasing use in the Unitded States is ozone. In a typical pilot plant ozone contactor (shown in Figure 1) bubbles of ozone-enriched air are sparged into a down-ward flowing column of water. Ozone dissolves into the water from the bubbles and the dissolved ozone or its byproducts inactivate microorganisms in the water stream. Ozone still in the gas phase at the top of the bubble column is collected and destroyed.

The proposed research constitutes development, validation and application of a computational fluid dynamic model of the hydrodynamics, mass transfer, chemistry and microbial inactivation relevant to ozone bubble contactor operation. This work is motivated by the need to improve the disinfection process efficiency and ensure a sufficient reduction in viable pathogens while not incurring excessive cost (ozone generation is power-intensive) or unwieldy design (such as very tall contactors). Currently-used models are not sufficiently detailed to predict a high level of microbial inactivation or the complicated hydraulics of ozone bubble contactors.

Figure 1: Ozone Bubble Contactor Schematic

Student: Tim Bartrand

Co-Advisor: Professor C. N. Haas

Sponsor:

Recently Completed Research Projects

Prof. Bakhtier Farouk

Department of Mechanical Engineering

and Mechanics

Drexel University

Philadelphia, PA 19104

Plasma-Assisted Chemical Vapor Deposition

The principal objective of the proposed project is to develop a commercially viable thin film coating process of depositing stress-free diamond-like thin films. A PACVD method will be used along with an advanced in-situ monitoring instrumentation package to synthesize stress-free diamond-like thin films with high process reproducibility. Experimental investigations, mathematical modeling, and simulation will be carried out for the optimization of the plasma assisted chemical vapor deposition (PACVD) process that will lead to the development of effective synthesis of diamond-like carbon (DLC) films. Computational process modeling will complement the experimental efforts in the plasma assisted chemical vapor deposition of the DLC films. The plasma reactors are characterized by high vacuum conditions where the continuum concept may not hold. Comprehensive modeling of the process will include the consideration of the electron kinetics in the reactor, plasma chemistry of the deposition process and surface reactions. Both Monte Carlo simulation and hybrid (Monte Carlo/continuum) approaches will be used for developing the predictive models.

Predicted electron and ion densities in a radio-frequency capacitively-coupled PACVD Reactor

Predicted electron energy contours and electric field vectors in the same reactor

Sponsor: National Science Foundation

Graduate Student: Kallol Bera, Katsuya Nagayama

Modeling Combustion and Ash-deposition in a Coal-fired furnace.

Computed Temperature field in a down-fired coal furnace

Student: Kaustubh Chandratre

Transport Processes in a Jet Slurry Bubble Column

Slurry bubble columns are widely used in the hydroprocessing and fermentation industries. Typical examples include the coal liquefaction, hydrogenation of heavy oils, biological waste treatments etc. The design and scale-up of such reactors require a thorough understanding of hydrodynamics and mass transfer in the reactor. Multi-phase (gas/liquid/solid particles) flows in a slurry reactor are being studied numerically. Gas/liquid and gas/solid particles flows have been investigated in the past by various groups. However, three phase flows (as found in slurry bubble columns) have been analyzed via empirical models. The numerical models will give us predictions of phase separation and mixing efficiency of the reactors as functions of inlet parameters and geometry.

An experimental test rig is proposed for the measurement of mixing of the liquid phase and particle settling velocities. The measurements will be used to validate and refine the numerical models.

Student: Debabrata Mitra-Majumdar

Modeling of DC Arc Transfer Process for the Treatment and Recycling of Hazardous Solid Wastes

Thermodynamic equilibrium calculations are being performed to analyze the behavior of wastes to in plasma arc melters. A transport model is also under development to simulate the flow and temperature fields in the plasma arc and the molten bath in the melter.

Sponsor: Electro-Pyrolysis Incorporated and Svedala Industries

Student: Ashley Wenger

Measurement of Thermal Conductivity of Diamond Films

We are developing techniques for measuring thermal conductivity of thin diamond films. The method is based on the 'thermal comparator' technique and allows us to rapidly measure thermal conductivity of thin films produced by PACVD or other related techniques.

Sponsor: Diamonex Inc., Allentown, PA

Students: Claudio Mesyngier, Kumar Cheruparambil

Computational Fluid Dynamic Analysis of Flow/Organism Interactions

A collaborative research project has been initiated between Drexel University and the Natural Academy of Sciences to investigate flow-organism interactions for the black fly larvae. The application of computational fluid dynamics to organism behavior is unique. Specifically we are looking into the flow and turbulence behavior in shallow streams with an uneven floor.

Sponsor: National Science Foundation (current funding)

High Pressure Hydrogen Production

Hydrogen production and processing (as a clean fuel) is an area which is now receiving increased attention, both from the industry and the government. A novel process is being investigated where a liquid hydrocarbon jet is injected into a molten iron bath. The liquid jet undergoes phase change and chemical reaction in the bath to produce hydrogen which is collected on top of the bath surface.

Sponsor: Ashland Petroleum Company

Heat Transfer Studies in the Myocardium

Finite-volume time-domain computer simulations were performed to analyze the temperature distributions produced by radio frequency (RF) ablation in the heart. RF cardiac ablation is accomplished by thermal destruction of endocardial tissue due to resistive heating at the site of the tip electrode of the ablation catheter.

The present model simulates a cardiac ablation catheter electrode in contact with a block of tissue and a ground plate on the opposite side of the tissue. The tissue's thermal response to this heating modality is investigated numerically by solving the two-dimensional time-dependent bioheat equation.

Graduate Student: Barbara C. Zimmerman