The processes inside the arc tube of high-intensity discharge lamps are investigated by finite el... more The processes inside the arc tube of high-intensity discharge lamps are investigated by finite element simulations. The behavior of the gas mixture inside the arc tube is governed by differential equations describing mass, energy and charge conservation as well as the Helmholtz equation for the acoustic pressure and the Navier-Stokes equation for the flow driven by the buoyancy and the acoustic streaming force. The model is highly nonlinear and requires a recursion procedure to account for the impact of acoustic streaming on the temperature and other fields. The investigations reveal the presence of a hysteresis and the corresponding jump phenomenon, quite similar to a Duffing oscillator. The similarities and, in particular, the differences of the nonlinear behavior of the high-intensity discharge lamp to that of a Duffing oscillator are discussed. For large amplitudes the high-intensity discharge lamp exhibits a stiffening effect in contrast to the Duffing oscillator.
Computer Aided Optimum Design in Engineering X, 2007
The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustic... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustical cell. It can be considerably improved by taking advantage of cell resonances, i.e., the acoustical eigenmodes of the measuring chamber. In order to optimize a photoacoustic system, it is key to understand precisely the influence of optical excitation, sound wave generation, cell shape and microphone detection on the signal strength in the sample cell. In this paper, first steps towards the optimization of photoacoustic cells are presented. The evaluation of the objective function, the signal strength, is based on a finite element analysis of the pressure field. As a start, independent optimization of some key parameters are investigated.
Computer Aided Optimum Design in Engineering X, 2007
The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustic... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustical cell. It can be considerably improved by taking advantage of cell resonances, i.e., the acoustical eigenmodes of the measuring chamber. In order to optimize a photoacoustic system, it is key to understand precisely the influence of optical excitation, sound wave generation, cell shape and microphone detection on the signal strength in the sample cell. In this paper, first steps towards the optimization of photoacoustic cells are presented. The evaluation of the objective function, the signal strength, is based on a finite element analysis of the pressure field. As a start, independent optimization of some key parameters are investigated.
Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity d... more Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.
Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity d... more Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.
The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based... more The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based photoacoustic spectroscopy. The signal is modelled using the amplitude mode expansion method, which is based on eigenmode expansion and introduction of losses in form of loss factors. The measurement reproduced almost all the calculated resonances from the numerical models with fairly good agreement. The cause of the differences between the measured and the simulated resonances are explained. In addition, the amplitude mode expansion simulation model is established as a quicker and computationally less demanding photoacoustic simulation alternative to the viscothermal model. The resonance frequencies obtained from the two models deviate by less than 1.8%. It was noted that the relative height of the amplitudes of the two models depended on the location of the antinodes within the resonator.
The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based... more The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based photoacoustic spectroscopy. The signal is modelled using the amplitude mode expansion method, which is based on eigenmode expansion and introduction of losses in form of loss factors. The measurement reproduced almost all the calculated resonances from the numerical models with fairly good agreement. The cause of the differences between the measured and the simulated resonances are explained. In addition, the amplitude mode expansion simulation model is established as a quicker and computationally less demanding photoacoustic simulation alternative to the viscothermal model. The resonance frequencies obtained from the two models deviate by less than 1.8%. It was noted that the relative height of the amplitudes of the two models depended on the location of the antinodes within the resonator.
Computer Aided Optimum Design in Engineering XI, 2009
The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonato... more The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonator. Up to now, mainly photoacoustic sensors consisting of a number of cylindrical parts have been investigated (cylinder cells, H cells, T cells etc.). In this paper, a numerical shape optimization of the resonator cell of photoacoustic sensors is described. The approach considers all shapes that can be represented by a number of axisymmetrical truncated cones which are connected in a continuous way. In addition, the geometry of the cell is subjected to certain constraints, e.g. the laser beam should not be blocked during its passage through the cell. The purpose is to maximize the sensor's signal strength. The acoustic pressure at the microphone of the sensor represents the objective function and is calculated using an eigenmode expansion combined with a finite element calculation. The solution of the 9-dimensional nonlinear optimization problem is a resonator shape with a substantial quality improvement with reference to the well-known H cell.
Computer Aided Optimum Design in Engineering XI, 2009
The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonato... more The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonator. Up to now, mainly photoacoustic sensors consisting of a number of cylindrical parts have been investigated (cylinder cells, H cells, T cells etc.). In this paper, a numerical shape optimization of the resonator cell of photoacoustic sensors is described. The approach considers all shapes that can be represented by a number of axisymmetrical truncated cones which are connected in a continuous way. In addition, the geometry of the cell is subjected to certain constraints, e.g. the laser beam should not be blocked during its passage through the cell. The purpose is to maximize the sensor's signal strength. The acoustic pressure at the microphone of the sensor represents the objective function and is calculated using an eigenmode expansion combined with a finite element calculation. The solution of the 9-dimensional nonlinear optimization problem is a resonator shape with a substantial quality improvement with reference to the well-known H cell.
The processes inside the arc tube of high-intensity discharge lamps are investigated by finite el... more The processes inside the arc tube of high-intensity discharge lamps are investigated by finite element simulations. The behavior of the gas mixture inside the arc tube is governed by differential equations describing mass, energy and charge conservation as well as the Helmholtz equation for the acoustic pressure and the Navier-Stokes equation for the flow driven by the buoyancy and the acoustic streaming force. The model is highly nonlinear and requires a recursion procedure to account for the impact of acoustic streaming on the temperature and other fields. The investigations reveal the presence of a hysteresis and the corresponding jump phenomenon, quite similar to a Duffing oscillator. The similarities and, in particular, the differences of the nonlinear behavior of the high-intensity discharge lamp to that of a Duffing oscillator are discussed. For large amplitudes the high-intensity discharge lamp exhibits a stiffening effect in contrast to the Duffing oscillator.
The light flicker problem of high intensity discharge lamps is studied numerically and experiment... more The light flicker problem of high intensity discharge lamps is studied numerically and experimentally. It is shown that in some respects the systems behaves very similar to the forced Duffing oscillator with a softening spring. In particular, the jump phenomenon and hysteresis are observed in the simulations and in the experiments.
Operating high-intensity discharge lamps in the high frequency range (20−300 kHz) provides energy... more Operating high-intensity discharge lamps in the high frequency range (20−300 kHz) provides energy-saving and cost reduction potentials. However, commercially available lamp drivers do not make use of this operating strategy because light intensity fluctuations and even lamp destruction are possible. The reason for the fluctuating discharge arc are acoustic resonances in this frequency range that are excited in the arc tube. The acoustic resonances in turn generate a fluid flow that is caused by the acoustic streaming effect. Here, we present a 3D multiphysics model to determine the influence of acoustic streaming on the temperature field in the vicinity of an acoustic eigenfrequency. In that case a transition from stable to instable behavior occurs. The model is able to predict when light flicker can be expected. The results are in very good accordance with accompanying experiments.
The detection sensitivity of a photoacoustic gas sensor strongly depends on the design of the sam... more The detection sensitivity of a photoacoustic gas sensor strongly depends on the design of the sample cell. It can be considerably improved by taking advantage of acoustical cell resonances, i.e., the radiation is modulated at a frequency equivalent to an acoustical mode of the measuring chamber. Most contemporary PA systems enhance their signal likewise, thereby increasing sensitivity. Experimental investigations of different cell geometries are very time consuming and expensive. The Finite Element Method (FEM), however, enables a very efficient investigation of resonators. We developed a method to calculate the photoacoustic signal for arbitrary resonator shapes. This offers the possibility to compare different cells by numerical means. In our presentation we review the applied methods and compare the results with experimental data. Also, we would like to present some first results of a shape optimization.
For the reasons of energy efficiency and mate-rial cost reduction one would prefer to drive high-... more For the reasons of energy efficiency and mate-rial cost reduction one would prefer to drive high-intensity discharge lamps at frequencies of about 300 kHz. Operat-ing lamps at these high frequencies bears the risk of stim-ulating acoustic resonances inside the arc tube, which can result in low frequency light flicker and even lamp de-struction. The acoustic streaming effect has been iden-tified as the link between the high frequency resonances and the low frequency flicker. A highly coupled 3D mul-tiphysics model has been set up to calculate the acous-tic streaming velocity field inside the arc tube of high-intensity discharge lamps. This velocity field is an impor-tant quantity for the understanding of the lamp behavior and a prerequisite for a forthcoming linear stability anal-ysis, which will be used to identify light flicker.
Introduction: High-intensity discharge (HID) lamps will in the foreseeable future be important li... more Introduction: High-intensity discharge (HID) lamps will in the foreseeable future be important light sources despite a growing market share of LEDs. Cost and energy efficient high frequency (300 kHz) operation is hampered by the excitation of acoustic resonances inside the arc tube which results in low frequency (10 Hz) light flicker. Our aim is to calculate the acoustic streaming (AS) velocity field, which is related to the sound waves, and link it to arc flicker. In contrast to the approach in [1] the model is 3 dimensional. FE Model: The model comprises three parts. The first part consists of a set of equations, which serve to calculate the temperature distribution inside the arc tube (Figure 2). In the second part the acoustic response is calculated via expanding the acoustic pressure in eigenmodes. The sound velocity is space dependent since it depends on the temperature field. Damping effects are included via loss factors [2]. In a last step the AS velocity field is calculated...
A procedure for the numerical calculation of photoacoustic signals is introduced. It is based on ... more A procedure for the numerical calculation of photoacoustic signals is introduced. It is based on the fi nite element method and uses an expansion of the signal into acoustical eigenmodes of the measuring cell. Loss is included by the incorporation of quality factors. Surface and volume loss effects due to viscosity and thermal conductivity are considered. The method is verifi ed for cylindrical cells with excellent accordance. Application to photoacoustic cells of unconventional shape yields good agreement with experimental data.
The detection sensitivity of photoacoustic sensors strongly depends on the design of the sample c... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the sample cell. It can be considerably improved by taking advantage of acoustical resonances of the measuring chamber. In order to optimize a photoacoustic system, it is key to precisely understand the distribution of pressure in the sample cell. Therefore, we have investigated photoacoustic cells of different geometries and compared experimental results to finite element method (FEM) analyses. In order to verify reliability of the FEM results, we performed analytical and numerical eigenfrequency calculations of simple cylindrical cavities and found them to be in excellent agreement (precision better than 1‰). Consecutively, experimental resonance data of a more sophisticated new cell geometry was compared to FEM results. For these results, as well, an excellent conformity was achieved. Even complicated eigenmodes were successfully identified.
A stationary compressible three-dimensional (3D) model of photothermal processes inside an arc tu... more A stationary compressible three-dimensional (3D) model of photothermal processes inside an arc tube of a high-intensity discharge lamp is developed on the basis of the finite element method. It takes plasma, electrodes, and the tube wall into account and enables simulation of acoustic phenomena. The temperature profile of the discharge arc is used as a marker for the emission of visible light. Complementary, experimental investigations are conducted at different modulation frequencies. A photodetector array is used to record 2D information about the light intensity distribution. The shape and length of the discharge arc are determined and compared to numerical results.
The processes inside the arc tube of high-intensity discharge lamps are investigated by finite el... more The processes inside the arc tube of high-intensity discharge lamps are investigated by finite element simulations. The behavior of the gas mixture inside the arc tube is governed by differential equations describing mass, energy and charge conservation as well as the Helmholtz equation for the acoustic pressure and the Navier-Stokes equation for the flow driven by the buoyancy and the acoustic streaming force. The model is highly nonlinear and requires a recursion procedure to account for the impact of acoustic streaming on the temperature and other fields. The investigations reveal the presence of a hysteresis and the corresponding jump phenomenon, quite similar to a Duffing oscillator. The similarities and, in particular, the differences of the nonlinear behavior of the high-intensity discharge lamp to that of a Duffing oscillator are discussed. For large amplitudes the high-intensity discharge lamp exhibits a stiffening effect in contrast to the Duffing oscillator.
Computer Aided Optimum Design in Engineering X, 2007
The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustic... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustical cell. It can be considerably improved by taking advantage of cell resonances, i.e., the acoustical eigenmodes of the measuring chamber. In order to optimize a photoacoustic system, it is key to understand precisely the influence of optical excitation, sound wave generation, cell shape and microphone detection on the signal strength in the sample cell. In this paper, first steps towards the optimization of photoacoustic cells are presented. The evaluation of the objective function, the signal strength, is based on a finite element analysis of the pressure field. As a start, independent optimization of some key parameters are investigated.
Computer Aided Optimum Design in Engineering X, 2007
The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustic... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the acoustical cell. It can be considerably improved by taking advantage of cell resonances, i.e., the acoustical eigenmodes of the measuring chamber. In order to optimize a photoacoustic system, it is key to understand precisely the influence of optical excitation, sound wave generation, cell shape and microphone detection on the signal strength in the sample cell. In this paper, first steps towards the optimization of photoacoustic cells are presented. The evaluation of the objective function, the signal strength, is based on a finite element analysis of the pressure field. As a start, independent optimization of some key parameters are investigated.
Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity d... more Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.
Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity d... more Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.
The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based... more The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based photoacoustic spectroscopy. The signal is modelled using the amplitude mode expansion method, which is based on eigenmode expansion and introduction of losses in form of loss factors. The measurement reproduced almost all the calculated resonances from the numerical models with fairly good agreement. The cause of the differences between the measured and the simulated resonances are explained. In addition, the amplitude mode expansion simulation model is established as a quicker and computationally less demanding photoacoustic simulation alternative to the viscothermal model. The resonance frequencies obtained from the two models deviate by less than 1.8%. It was noted that the relative height of the amplitudes of the two models depended on the location of the antinodes within the resonator.
The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based... more The photoacoustic signal in a closed T-cell resonator is generated and measured using laser based photoacoustic spectroscopy. The signal is modelled using the amplitude mode expansion method, which is based on eigenmode expansion and introduction of losses in form of loss factors. The measurement reproduced almost all the calculated resonances from the numerical models with fairly good agreement. The cause of the differences between the measured and the simulated resonances are explained. In addition, the amplitude mode expansion simulation model is established as a quicker and computationally less demanding photoacoustic simulation alternative to the viscothermal model. The resonance frequencies obtained from the two models deviate by less than 1.8%. It was noted that the relative height of the amplitudes of the two models depended on the location of the antinodes within the resonator.
Computer Aided Optimum Design in Engineering XI, 2009
The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonato... more The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonator. Up to now, mainly photoacoustic sensors consisting of a number of cylindrical parts have been investigated (cylinder cells, H cells, T cells etc.). In this paper, a numerical shape optimization of the resonator cell of photoacoustic sensors is described. The approach considers all shapes that can be represented by a number of axisymmetrical truncated cones which are connected in a continuous way. In addition, the geometry of the cell is subjected to certain constraints, e.g. the laser beam should not be blocked during its passage through the cell. The purpose is to maximize the sensor's signal strength. The acoustic pressure at the microphone of the sensor represents the objective function and is calculated using an eigenmode expansion combined with a finite element calculation. The solution of the 9-dimensional nonlinear optimization problem is a resonator shape with a substantial quality improvement with reference to the well-known H cell.
Computer Aided Optimum Design in Engineering XI, 2009
The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonato... more The sensitivity of photoacoustic sensors strongly depends on the shape of the acoustical resonator. Up to now, mainly photoacoustic sensors consisting of a number of cylindrical parts have been investigated (cylinder cells, H cells, T cells etc.). In this paper, a numerical shape optimization of the resonator cell of photoacoustic sensors is described. The approach considers all shapes that can be represented by a number of axisymmetrical truncated cones which are connected in a continuous way. In addition, the geometry of the cell is subjected to certain constraints, e.g. the laser beam should not be blocked during its passage through the cell. The purpose is to maximize the sensor's signal strength. The acoustic pressure at the microphone of the sensor represents the objective function and is calculated using an eigenmode expansion combined with a finite element calculation. The solution of the 9-dimensional nonlinear optimization problem is a resonator shape with a substantial quality improvement with reference to the well-known H cell.
The processes inside the arc tube of high-intensity discharge lamps are investigated by finite el... more The processes inside the arc tube of high-intensity discharge lamps are investigated by finite element simulations. The behavior of the gas mixture inside the arc tube is governed by differential equations describing mass, energy and charge conservation as well as the Helmholtz equation for the acoustic pressure and the Navier-Stokes equation for the flow driven by the buoyancy and the acoustic streaming force. The model is highly nonlinear and requires a recursion procedure to account for the impact of acoustic streaming on the temperature and other fields. The investigations reveal the presence of a hysteresis and the corresponding jump phenomenon, quite similar to a Duffing oscillator. The similarities and, in particular, the differences of the nonlinear behavior of the high-intensity discharge lamp to that of a Duffing oscillator are discussed. For large amplitudes the high-intensity discharge lamp exhibits a stiffening effect in contrast to the Duffing oscillator.
The light flicker problem of high intensity discharge lamps is studied numerically and experiment... more The light flicker problem of high intensity discharge lamps is studied numerically and experimentally. It is shown that in some respects the systems behaves very similar to the forced Duffing oscillator with a softening spring. In particular, the jump phenomenon and hysteresis are observed in the simulations and in the experiments.
Operating high-intensity discharge lamps in the high frequency range (20−300 kHz) provides energy... more Operating high-intensity discharge lamps in the high frequency range (20−300 kHz) provides energy-saving and cost reduction potentials. However, commercially available lamp drivers do not make use of this operating strategy because light intensity fluctuations and even lamp destruction are possible. The reason for the fluctuating discharge arc are acoustic resonances in this frequency range that are excited in the arc tube. The acoustic resonances in turn generate a fluid flow that is caused by the acoustic streaming effect. Here, we present a 3D multiphysics model to determine the influence of acoustic streaming on the temperature field in the vicinity of an acoustic eigenfrequency. In that case a transition from stable to instable behavior occurs. The model is able to predict when light flicker can be expected. The results are in very good accordance with accompanying experiments.
The detection sensitivity of a photoacoustic gas sensor strongly depends on the design of the sam... more The detection sensitivity of a photoacoustic gas sensor strongly depends on the design of the sample cell. It can be considerably improved by taking advantage of acoustical cell resonances, i.e., the radiation is modulated at a frequency equivalent to an acoustical mode of the measuring chamber. Most contemporary PA systems enhance their signal likewise, thereby increasing sensitivity. Experimental investigations of different cell geometries are very time consuming and expensive. The Finite Element Method (FEM), however, enables a very efficient investigation of resonators. We developed a method to calculate the photoacoustic signal for arbitrary resonator shapes. This offers the possibility to compare different cells by numerical means. In our presentation we review the applied methods and compare the results with experimental data. Also, we would like to present some first results of a shape optimization.
For the reasons of energy efficiency and mate-rial cost reduction one would prefer to drive high-... more For the reasons of energy efficiency and mate-rial cost reduction one would prefer to drive high-intensity discharge lamps at frequencies of about 300 kHz. Operat-ing lamps at these high frequencies bears the risk of stim-ulating acoustic resonances inside the arc tube, which can result in low frequency light flicker and even lamp de-struction. The acoustic streaming effect has been iden-tified as the link between the high frequency resonances and the low frequency flicker. A highly coupled 3D mul-tiphysics model has been set up to calculate the acous-tic streaming velocity field inside the arc tube of high-intensity discharge lamps. This velocity field is an impor-tant quantity for the understanding of the lamp behavior and a prerequisite for a forthcoming linear stability anal-ysis, which will be used to identify light flicker.
Introduction: High-intensity discharge (HID) lamps will in the foreseeable future be important li... more Introduction: High-intensity discharge (HID) lamps will in the foreseeable future be important light sources despite a growing market share of LEDs. Cost and energy efficient high frequency (300 kHz) operation is hampered by the excitation of acoustic resonances inside the arc tube which results in low frequency (10 Hz) light flicker. Our aim is to calculate the acoustic streaming (AS) velocity field, which is related to the sound waves, and link it to arc flicker. In contrast to the approach in [1] the model is 3 dimensional. FE Model: The model comprises three parts. The first part consists of a set of equations, which serve to calculate the temperature distribution inside the arc tube (Figure 2). In the second part the acoustic response is calculated via expanding the acoustic pressure in eigenmodes. The sound velocity is space dependent since it depends on the temperature field. Damping effects are included via loss factors [2]. In a last step the AS velocity field is calculated...
A procedure for the numerical calculation of photoacoustic signals is introduced. It is based on ... more A procedure for the numerical calculation of photoacoustic signals is introduced. It is based on the fi nite element method and uses an expansion of the signal into acoustical eigenmodes of the measuring cell. Loss is included by the incorporation of quality factors. Surface and volume loss effects due to viscosity and thermal conductivity are considered. The method is verifi ed for cylindrical cells with excellent accordance. Application to photoacoustic cells of unconventional shape yields good agreement with experimental data.
The detection sensitivity of photoacoustic sensors strongly depends on the design of the sample c... more The detection sensitivity of photoacoustic sensors strongly depends on the design of the sample cell. It can be considerably improved by taking advantage of acoustical resonances of the measuring chamber. In order to optimize a photoacoustic system, it is key to precisely understand the distribution of pressure in the sample cell. Therefore, we have investigated photoacoustic cells of different geometries and compared experimental results to finite element method (FEM) analyses. In order to verify reliability of the FEM results, we performed analytical and numerical eigenfrequency calculations of simple cylindrical cavities and found them to be in excellent agreement (precision better than 1‰). Consecutively, experimental resonance data of a more sophisticated new cell geometry was compared to FEM results. For these results, as well, an excellent conformity was achieved. Even complicated eigenmodes were successfully identified.
A stationary compressible three-dimensional (3D) model of photothermal processes inside an arc tu... more A stationary compressible three-dimensional (3D) model of photothermal processes inside an arc tube of a high-intensity discharge lamp is developed on the basis of the finite element method. It takes plasma, electrodes, and the tube wall into account and enables simulation of acoustic phenomena. The temperature profile of the discharge arc is used as a marker for the emission of visible light. Complementary, experimental investigations are conducted at different modulation frequencies. A photodetector array is used to record 2D information about the light intensity distribution. The shape and length of the discharge arc are determined and compared to numerical results.
The photoacoustic effect is based on resonant absorption of light by a sample and the transfer of... more The photoacoustic effect is based on resonant absorption of light by a sample and the transfer of the excitation energy into thermal energy via inelastic collisions of gas molecules. A modulated irradiation of the sample causes periodic pressure variations that can be detected by a microphone and measured using lock-in technique (Demtröder 2002). Photoacoustic spectroscopy finds many applications in the
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