Thomas D. Dreeben

Modelling of fluid-mechanical arc instability in pure-mercury HID lamps

A fully unsteady compressible 2D flow model is used to reproduce observed fluid-mechanical arc instability in a horizontally-running pure-mercury HID lamp. The model represents a 2D lamp cross-section normal to the arc, and assumes an infinitely long lamp. Departure from the steady condition is driven by oscillating lamp current. Sound-wave propagation and induced flows result from full coupling between the current, energy balance, ideal-gas law and conservation of mass and momentum, on two separate relevant time scales. Observed acoustically generated arc instability is reproduced with the model.

Simulation of vertical slot convection using 'one-dimensional turbulence'

Abstract One-dimensional turbulence (ODT) is used to model and simulate the buoyant turbulent flow in a vertical slot. ODT reproduces available Direct Numerical Simulation results on the Rayleigh number dependence of wall heat transfer and of other flow properties of interest. Extended ranges of Rayleigh and Prandtl numbers are investigated with ODT to explore the broader behavior of the flow, focusing on its connection to classical scaling arguments.

Two-dimensional streaming flows in high-intensity discharge lamps

High-intensity discharge (HID) lamps embody a practical application in which acoustically generated streaming flows are used to significantly improve energy efficiency. Streaming in these lamps is examined using finite-element simulations in conjunction with available experimental results on the basis of the assumption that the streaming motion is excited by two-dimensional acoustic standing waves. Neither the magnitude nor the direction of the time-averaged flows is adequately explained by existing theory. Consequently, a modified streaming analysis is proposed in which the fluctuating flow is driven by an oscillating pressure field rather by a moving boundary and convective terms in both the instantaneous and streaming flows are included. Density variations are also shown to be important to the generation of the observed and simulated streaming. This analysis highlights the differences between streaming flows in HID lamps and those described in canonical problems appearing elsewhere in the literature.

Experimental and Simulated Straightening of Metal Halide Arcs Using Power Modulation

Years of research in the fundamentals of acoustically generated flows in high-intensity-discharge arc tubes for enhancement have recently produced record-breaking performance in converting wall-plug power to useful white light. In this paper, simulated results show how power modulation produces straightened arcs in horizontally running lamps. A fully unsteady compressible 2-D flow model is used to reproduce observed instantaneous pressure and temperature oscillations, and the results are time averaged and rendered to visualize the bulk gas flows. Sound-wave propagation and induced flows result from full coupling between the oscillating current, the energy balance, the ideal-gas law, and the conservation of mass and momentum on two separate timescales. Experimental verifications of the instantaneous and time-averaged effects are included.

High-Resolution Modeling of Multiscale Transient Phenomena in Turbulent Boundary Layers

High fidelity numerical simulation of wall-bounded turbulence requires physically sound representation of the small scale unsteady processes governing near-wall momentum, heat, and mass transfer. Conventional wall treatments do not capture the diverse multiphysics flow regimes relevant to engineering applications. To obtain a robust yet computationally affordable near-wall submodel for turbulent flow computations, the fine-grained spatial structure and time evolution of the near-wall flow is simulated using a model formulated on a 1D domain corresponding to the wall-normal direction. This approach captures the strong variation of flow properties in the wall-normal direction and the transient interactions between this highly inhomogeneous region and the more nearly homogeneous (at fine scales) flow farther from the wall. The 1D simulation utilizes the One Dimensional Turbulence (ODT) methodology, whose formulation for the present application is described in detail. Demonstrations of ODT performance with regard to aspects of flow physics relevant to near-wall flow modeling are presented. The coupling of ODT to a large eddy simulation (LES) of confined turbulent flow is described, and the performance of the coupled formulation is demonstrated. It is concluded that this formulation has the potential to provide the fidelity needed for engineering applications at an affordable computational cost.

Scaling of low-aspect ratio HID lamps

This is an effort to characterize the lowest possible order approximation to how HID lamps of different wattages should scale. We are looking for broad, simple relationships that connect a lamps power to its dimensions for ceramic HID lamps of constant low-aspect ratio. We consider a class of lamps in which the arc-tube size and power vary together over an arbitrary range, but the operating pressure, temperature profiles and gas chemistries are all as identical as possible throughout the range. For this class of lamps, we characterize the scaling relationships between the rated power P and the arc-tube inner radius R. Corresponding wall-loading expressions are also derived.