N. Fuhrmann

High-speed phosphor thermometry.

Phosphor thermometry is a semi-invasive surface temperature measurement technique utilising the luminescence properties of doped ceramic materials. Typically, these phosphor materials are coated onto the object of interest and are excited by a short UV laser pulse. Up to now, primarily Q-switched laser systems with repetition rates of 10 Hz were employed for excitation. Accordingly, this diagnostic tool was not applicable to resolve correlated temperature transients at time scales shorter than 100 ms. This contribution reports on the first realisation of a high-speed phosphor thermometry system employing a highly repetitive laser in the kHz regime and a fast decaying phosphor. A suitable material was characterised regarding its temperature lifetime characteristic and its measurement precision. Additionally, the influence of laser power on the phosphor coating was investigated in terms of heating effects. A demonstration of this high-speed technique has been conducted inside the thermally highly transient system of an optically accessible internal combustion engine. Temperatures have been measured with a repetition rate of 6 kHz corresponding to one sample per crank angle degree at 1000 rpm.

Two-dimensional surface temperature diagnostics in a full-metal engine using thermographic phosphors

An optical system consisting of a rigid borescope was developed to measure surface temperatures inside full-metal internal combustion engines. The measurement principle is predicated on lifetime-based phosphor thermometry of the material Gd3Ga5O12: Cr. The system is designed to resolve the luminescence decay of thermographic phosphors temporally and two-dimensionally by the use of a CMOS high-speed camera. The device allows the visualization of the temperature distribution in an area of 9 mm in diameter. An application of this optical system inside an internal combustion engine is demonstrated, yielding temperature maps under fired and motored conditions in a full-metal engine for the first time.

Gd3Ga5O12:Cr—a phosphor for two-dimensional thermometry in internal combustion engines

The lifetime of thermographic phosphors needs to be short enough to resolve the temperature time scales and long enough to be properly resolved by the respective detector. Up until now, there has not been a thermographic phosphor that exhibits an adequate temperature lifetime characteristic for two-dimensional thermometry inside internal combustion engines using high speed cameras. Hence, this study suggests the material Gd3Ga5O12:Cr for this purpose. The emission spectra and the temperature lifetime characteristics were determined and the lifetime is shown to be independent of the composition and the absolute pressure of the surrounding gas phase.

Phosphor thermometry at high repetition rates

Phosphor thermometry is a semi-invasive surface temperature measurement technique utilizing the luminescence properties of thermographic phosphors. Typically these ceramic materials are coated onto the object of interest and are excited by a short UV laser pulse. Photomultipliers and high-speed camera systems are used to transiently detect the subsequently emitted luminescence decay point wise or two-dimensionally resolved. Based on appropriate calibration measurements, the luminescence lifetime is converted to temperature. Up to now, primarily Q-switched laser systems with repetition rates of 10 Hz were employed for excitation. Accordingly, this diagnostic tool was not applicable to resolve correlated temperature transients at time scales shorter than 100 ms. For the first time, the authors realized a high-speed phosphor thermometry system combining a highly repetitive laser in the kHz regime and a fast decaying phosphor. A suitable material was characterized regarding its temperature lifetime characteri...

Spectral decomposition of phosphorescence decays

In phosphor thermometry, the fitting of decay curves is a key task in the robust and precise determination of temperatures. These decays are generally assumed to be mono-exponential in certain temporal boundaries, where fitting is performed. The present study suggests a multi-exponential method to determine the spectral distribution in terms of decay times in order to analyze phosphorescence decays and thereby complement the mono-exponential analysis. Therefore, two methods of choice are compared and verified using simulated data in the presence of noise. Addtionally, this spectral decomposition is applied to the thermographic phosphor Mg4FGeO6 : Mn and reveals changes in the exponential distributions of decay times upon a change of the excitation laser energy.

Planar droplet sizing for characterization of automotive sprays in port fuel injection applications using commercial fuel

Drop size measurements are crucial for characterizing practical sprays in automotive applications. Phase-Doppler (PD) techniques allow for highly accurate point-wise measurements of single droplets diameters and velocities. Statistical parameters such as mean diameters are recorded at single locations that are composed of thousands of individual droplet measurements. Therefore the assessment of an entire spray by the phase Doppler technique is time-consuming. For this reason there is some need for planar drop sizing methods in engineering applications that call for much faster spray characterizations. Simultaneous Mie scattering and planar laser-induced fluorescence (Mie/PLIF) is one choice that provides access to Sauter-mean-diameters (SMD). The advantage in reduced measuring times comes along with various challenging issues, such as accuracy limited by multiple-scattering, absorption effects, and calibration procedures. Its application especially to practically relevant sprays claims for real fuel composed of a mixture of various hydrocarbons with intense absorption bands in the ultraviolet spectral range. The purpose of this project is the adaptation of the Mie/PLIF technique to real-world requirements and is conducted in cooperation with BMW F1. For a multi-hole injector operated with commercial gasoline fuel and port-fuel injection conditions typical for motorsport engines it is shown that deviations between SMDs measured by phase Doppler and Mie/PLIF is below 20%. Savings of time are up to 80% justifying the limitation to measuring the SMD only.