Derek Dunn-Rankin

Measurement and prediction of indoor air flow in a model room

Abstract In the interest of designing an efficient and acceptable indoor air environment in modern buildings, it is important to resolve the relationship between geometric room parameters and the air flow patterns produced by mechanical ventilation systems. Toward this end, we compare results from relatively simple three-dimensional numerical simulations (CFD) with laser Doppler anemometry (LDA) and particle image velocimetry (PIV) experimental measurements of indoor air flows in a one-tenth sub-scale model room. Laminar, k – e turbulence, and RNG k – e turbulence numerical models are used and evaluated with respect to their performance in simulating the flow in the model room, and results of the numerical simulations and velocimetry measurements show how obstructions can greatly influence the air flow and contaminant transport in a room. It is important, therefore, that obstructions be considered in ventilation design. Simulations predict the measured trends in a model room very well, with relative errors not much larger than 20%. In this study, the RNG model most accurately predicts the flow in a partitioned room, capturing the gross effects of a large flow obstruction.

Controlled Continuous Patterning of Polymeric Nanofibers on Three-Dimensional Substrates Using Low-Voltage Near-Field Electrospinning

We report on a continuous method for controlled electrospinning of polymeric nanofibers on two-dimensional (2D) and three dimensional (3D) substrates using low voltage near-field electrospinning (LV NFES). The method overcomes some of the drawbacks in more conventional near-field electrospinning by using a superelastic polymer ink formulation. The viscoelastic nature of our polymer ink enables continuous electrospinning at a very low voltage of 200 V, almost an order of magnitude lower than conventional NFES, thereby reducing bending instabilities and increasing control of the resulting polymer jet. In one application, polymeric nanofibers are freely suspended between microstructures of 3D carbon on Si substrates to illustrate wiring together 3D components in any desired pattern.

Miniature-scale liquid-fuel-film combustor

A new concept is explained for the miniaturization of cylindrical direct-injection liquid-fueled combustors wherein the fuel is flowed in a wall film that reduces heat losses, optimizes vaporization rate, and inhibits quenching. A simple analysis indicates that if a combustor of this type were part of a miniature engine, power levels from 10 W to 10 kW would be achievable with combustor volumes varying from a few hundred cubic millimeters to a few cubic centimeters and fuel flow rates varying from about 1 mg/s to 1 g/s. The combustor takes advantage of the fact that a surface area advantage of film combustion over spray combustion occurs for small volumes as the surface-to-volume ratio of the combustor increases. In addition, the wall film of fuel prevents heat losses while cooling the combustor surfaces. Photographs of a laboratory demonstration of the miniature film combustor concept with methanol and hepatane fuel and a mechanically swirled airflow are presented. The methanol flame required some methane gaseous fuel for stability, but the heptane fuel burned successfully alone. The experiments demonstrated externally and internally anchored flame modes, and a transition between them. The stability of internal burning was affected by internal fluid, dynamics and film dryout.

Effects of capillary spacing on EHD spraying from an array of cone jets

Abstract Electrohydrodynamic (EHD) sprays are fundamentally characterized by low liquid flows that are atomized into relatively small, monodisperse droplets. Since previous studies have shown that droplet size increases with increasing flow rate, the current work employs an array of capillaries intending to increase fluid throughput without increasing the size of the droplets produced. To do this, an array of four-capillary nozzles is examined by measuring the required potential needed for stable EHD spraying in the cone-jet mode as a function of capillary separation. In addition, a simple electrostatic model is used to support the experimental results, and to predict the behavior of a larger, 5×5 square array. Results show that the potential required for cone-jet spraying in a two-dimensional array of capillaries generally increases as the capillary spacing decreases (due to electrical shielding), but at very close spacing the potential can decrease if the neighboring capillaries are dry. This result suggests that EHD arrays can benefit from fine wire electrodes interspersed among the capillaries.

Ignition by excimer laser photolysis of ozone

Abstract We have ignited mixtures of hydrogen, oxygen, and ozone in closed cells with 248 nm radiation from a KrF excimer laser. Ozone, the only significant absorber in this system, absorbs a single photon and produces oxygen atoms which initiate combustion. A discretized, time-dependent Beers law model is used to demonstrate that the radical concentration immediately after photolysis is a function of laser power, ozone concentration, focal length, and separation between the lens and reaction cell. Spark schlieren photographs are used to visualize the ignition events and identify the ignition sites. The effects of equivalence ratio, pressure, and the initial gas temperature on the minimum ozone concentration needed to produce ignition are presented, and only the initial temperature has a significant effect. Modeling studies of the ignition process aid in the interpretation of the experimental results, and show that the ignition we observe is not due solely to thermal effects, but is strongly dependent on the number and type of radicals present initially after photolysis. Ignition using other hydrocarbons as fuels was also demonstrated.

Using numerical simulation to predict ventilation efficiency in a model room

A computational fluid dynamics (CFD) code is used to simulate the air currents and the contaminant decay inside a small scale model room with forced ventilation through a simple supply and return. The numerical results are validated with flow visualization experiments and local clearance rate measurements by laser extinction. The comparisons show excellent agreement on the inlet side of the room and fair agreement on the outlet side. Based on the concept of local exponential tracer decay and mixing factor (m), an arithmetic average of m is proposed as a measure of overall air ventilation efficiency ηm. The proposed definition has practical benefit in numerical simulation because it reduces computational time without sacrificing the concept of air ventilation effectiveness. Examples of application are demonstrated by eviluating ηm for different room configurations and ventilation arrangements. The calculated ηm correlate well with the time required for evacuating the contaminants in different room configurations. The results show that, for the room geometry studied, a ventilation system performs better when the inlet and the outlet are perpendicular to each other than when they are parallel to each other, and that partitions in the room can have a significant influence on ventilation performance.

Numerical simulation of two-dimensional blunt body sampling in viscous flow

Abstract This paper presents a numerical investigation of the aspiration efficiency of a cylindrical blunt body sampler with a forward facing suction opening. The paper: (1) describes and validates the numerical procedure for calculating laminar viscous flow around a two-dimensional blunt cylindrical sampler; (2) combines the flow field calculations with particle dynamics to determine the aerosol aspiration efficiency of the sampler as a function of the particle Stokes number, with a physical explanation for the shape of the aspiration efficiency curve; and (3) examines the effects of the suction to freestream velocity ratio, Reynolds number, and suction opening size on the viscous flow field and aspiration efficiency. The results of the study indicate the particle size ranges and flow conditions where the viscous boundary layer affects the aerosol sampling process. Comparing the results from the viscous calculations with results from inviscid calculations shows qualitative agreement but quantitative differences. In particular, the viscous calculation produces generally lower aspiration efficiencies, indicating that the viscous boundary layer can play an important role in particle inhalability.

Modelling electric field driven convection in small combustion plasmas and surrounding gases

Electric fields applied to flames can be used to impart force directly on the reaction zone and local surroundings due to the presence of flame ions. The resultant electric body force modifies the flow field surrounding the combustion environment. This paper describes a numerical simulation of the flow field of entrained gases and exhaust surrounding a gaseous fuel flame when it is exposed to electric fields by formulating the problem as a unipolar two fluid system (neutrals and positive ions). An analysis, performed at steady state, is verified by comparison to published experimental results (total current, current density profiles, and qualitative flow measurements). The results indicate a complex interaction between the combustion and current discharge environments where the flow field is dominated by the presence of a toroidal vortex structure and the discharge is dominated by space charge effects. In addition, a transient analysis of the flow field is performed, and four fundamental timescales in the system are identified. These timescales are (1) steady state discharge establishment, (2) plume establishment, (3) vortex establishment, and (4) heating of the feed tube. Timescales (1)–(3), being the fastest timescales of the system, are evaluated and compared. The results show good agreement between computational and analytical timescale values.

CARS thermometry in high temperature gradients

CARS is an effective non-intrusive technique for measuring gas temperature in combustion environments. In regions of high temperature gradient, however, the CARS signal is complicated by contributions from gas at different temperature. This paper examines theoretically the uncertainty associated with CARS thermometry in steep temperature gradients. In addition, the work compares the temperature predicted from CARS with the adiabatic mixed temperature of the gas resident in the measurement volume. This comparison helps indicate the maximum sample volume size allowed for accurate temperature measurements.

In Situ Light Scattering Measurements of Mainstream and Sidestream Cigarette Smoke

ABSTRACT This paper presents in situ and continuous size measurements for submicron cigarette smoke particles. The method, which can be applied to any ensemble of small particles narrowly distributed in size, uses the light scattered from the particles at angles of 60° and 120° and the ratio of polarization components of the scattered light at 55° to determine the mean particle size. Polystyrene latex spheres diluted in distilled water are used to calibrate the system. Good agreement exists between the experimental measurements and the theoretical calculations for the calibration particles. Based on the assumption that cigarette smoke particles cluster into locally uniform parcels during their formation as condensate, the light scattering system is employed to measure the mean size and size fluctuations of mainstream smoke and sidestream smoke from research cigarettes (1R3 and 1R3f). Similar light scattering techniques have been employed by other researchers, but this paper describes in situ measurements ...