Modelling additive transport in metal halide lamps

In 1912 Charles Steinmetz was granted a patent for a new light source. By adding small amounts of sodium, lithium, rubidium and potassium to a mercury lamp he was able to modify the light output from "an extremely disagreeable colour" to "a soft, brilliant, white light". Much later, at the New York world trade fair in 1964 General Electric was the first to introduced a commercial lamp based on the same principle. The light emitting metallic elements are introduced as components of halide salts. Hence, they are called metal halide lamps. The physics behind discharge lamps of this type, however, is still a matter of active investigation. One well-known phenomenon is that, when operated vertically, the metal halides in the lamp tend to demix; the concentration of metal halides in the gas phase is much greater at the bottom of the lamp. Demixing, or segregation as it is also called, has a negative impact on the lamp’s efficacy. It is currently avoided by using lamp designs with very small or very large aspect ratios. Gaining more insight into the process of demixing would allow a broader range of lamp designs with still better luminous efficacies. The demixing is caused by a competition between convection and diffusion. The centre of the lamp must be hot to produce as much light as possible. The walls must stay relatively cool to avoid them weakening and releasing the mercury vapour. Thus, large temperature gradients are present in the lamp, driving convective flows. In the hot centre the molecules are dissociated into atoms. The atoms are smaller and more mobile than the molecules. The atoms are dragged up by the convective currents while diffusing outward. Because of their larger mobility, however, the atoms do not reach the top of the lamp. The result is a larger concentration of metal additives at the walls and at the bottom of the lamp than at the centre and the top of the lamp. This thesis describes the process of demixing in a self consistent and quantitative manner using state-of-the-art computational methods. The competition between convection and diffusion is studied using a variety of models built with the plasma modelling toolkit Plasimo. Using Plasimo allows for the construction of models in a modular fashion. Partial models are used to study the conveci vtive flow as a result of the temperature gradients, the chemical composition as a function of temperature and pressure, and the radiation transport on the lamp. A grand model is formed by combining modules for ray tracing, elemental diffusion, convective flow and the temperature equation. The model result is validated against experiments done by colleagues: Experiments which have been carried out in Eindhoven, at the Argonne National Laboratories in the USA, and in the International Space Station. Cross validation with theoretical work has also been performed. Axial demixing is shown to be the result of the competition between axial convection and radial diffusion. This competition is best expressed by the dimensionless Peclet number. When the Peclet number is approximately equal to unity, axial segregation is strongest. The degree of axial segregation is best expressed by the dimensionless segregation depth t . The largest value of t depends on the element under study and on the position in the discharge where the molecules dissociate to form ions

Departures from local thermodynamic equilibrium in HID lamps

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Modelling additive transport in metal halide lamps

In 1912 Charles Steinmetz was granted a patent for a new light source. By adding small amounts of sodium, lithium, rubidium and potassium to a mercury lamp he was able to modify the light output from "an extremely disagreeable colour" to "a soft, brilliant, white light". Much later, at the New York world trade fair in 1964 General Electric was the first to introduced a commercial lamp based on the same principle. The light emitting metallic elements are introduced as components of halide salts. Hence, they are called metal halide lamps. The physics behind discharge lamps of this type, however, is still a matter of active investigation. One well-known phenomenon is that, when operated vertically, the metal halides in the lamp tend to demix; the concentration of metal halides in the gas phase is much greater at the bottom of the lamp. Demixing, or segregation as it is also called, has a negative impact on the lamp’s efficacy. It is currently avoided by using lamp designs with very small or very large aspect ratios. Gaining more insight into the process of demixing would allow a broader range of lamp designs with still better luminous efficacies. The demixing is caused by a competition between convection and diffusion. The centre of the lamp must be hot to produce as much light as possible. The walls must stay relatively cool to avoid them weakening and releasing the mercury vapour. Thus, large temperature gradients are present in the lamp, driving convective flows. In the hot centre the molecules are dissociated into atoms. The atoms are smaller and more mobile than the molecules. The atoms are dragged up by the convective currents while diffusing outward. Because of their larger mobility, however, the atoms do not reach the top of the lamp. The result is a larger concentration of metal additives at the walls and at the bottom of the lamp than at the centre and the top of the lamp. This thesis describes the process of demixing in a self consistent and quantitative manner using state-of-the-art computational methods. The competition between convection and diffusion is studied using a variety of models built with the plasma modelling toolkit Plasimo. Using Plasimo allows for the construction of models in a modular fashion. Partial models are used to study the conveci vtive flow as a result of the temperature gradients, the chemical composition as a function of temperature and pressure, and the radiation transport on the lamp. A grand model is formed by combining modules for ray tracing, elemental diffusion, convective flow and the temperature equation. The model result is validated against experiments done by colleagues: Experiments which have been carried out in Eindhoven, at the Argonne National Laboratories in the USA, and in the International Space Station. Cross validation with theoretical work has also been performed. Axial demixing is shown to be the result of the competition between axial convection and radial diffusion. This competition is best expressed by the dimensionless Peclet number. When the Peclet number is approximately equal to unity, axial segregation is strongest. The degree of axial segregation is best expressed by the dimensionless segregation depth t . The largest value of t depends on the element under study and on the position in the discharge where the molecules dissociate to form ions

Demixing in a metal halide lamp, results from modelling

Convection and diffusion in the discharge region of a metal halide lamp is studied using a computer model built with the plasma modeling package Plasimo. A model lamp contg. mercury and sodium iodide is studied. The effects of the total lamp pressure on the degree of segregation of the light emitting species are examd. and compared to a simpler model with a fixed temp. profile. Significant differences are obsd., justifying the use of the more complete approach. [on SciFinder (R)

A model for additive transport in metal halide lamps containing mercury and dysprosium tri-iodide

The distribution of additives in a metal halide lamp is examined through numerical modelling. A model for a lamp containing sodium iodide additives has been modified to study a discharge containing dysprosium tri-iodide salts. To study the complex chemistry the method of Gibbs minimization is used to decide which species have to be taken into account and to fill lookup tables with the chemical composition at different combinations of elemental abundance, lamp pressure and temperature. The results from the model with dysprosium additives were compared with earlier results from the lamp containing sodium additives and a simulation of a pure mercury lamp. It was found that radial segregation creates the conditions required for axial segregation. Radial segregation occurs due to the unequal diffusion of atoms and molecules. Under the right conditions convection currents in the lamp can cause axial demixing. These conditions depend on the ratio of axial convection and radial diffusion as expressed by the Peclet number. At a Peclet number of unity axial segregation is most pronounced. At low Peclet numbers radial segregation is at its worst, while axial segregation is not present. At large Peclet numbers the discharge becomes homogeneously mixed. The degree of axial segregation at a Peclet number of unity depends on the temperature at which the additive under consideration fully dissociates. If the molecules dissociate very close to the walls no molecules are transported by the convective currents in the lamp, and hence axial segregation is limited. If they dissociate further away from the walls in the area where the downward convective currents are strongest, more axial segregation is observed. © 2008 IOP Publishing Lt

Radial segregation in metal halide lamps; a critical analysis of the validity of the LTE approach

When modelling high pressure discharges assuming Local Thermal Equilibrium (LTE) greatly reduces the complexity of the problem. This requires of course, that the assumptoin of LTE holds. We examine an LTE model of a metal halide lamp containing dysprosium iodide and compare the results with experiments looking for possible invalidation of the model. Significant deviations are found from experimental results

Convection-diffursion confusion, treating self consistent diffusion with the controle volume method

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Radial segregation in metal halide lamps; validation of results from an LTE model using Plasimo with X-Ray Fluorescence

abstract A

Simulating metal halide lamps under varying accelerational conditions with plasimo

Segregation of the additives in metal halide lamps causes colour separation and reduces the efficacy of these lamps. Using the plasma modelling tool-kit. Plasimo, a model had been built to study radial and axial segregation in metal halide lamps under varyring gravity conditions