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Dynamics >>24
Staying
cool with STAR-CD
Sergei Shulepov, Philips, Netherlands
Introduction
The
Center for Industrial Technologies of Philips (Philips/CFT) assists
Philips product divisions in development of new, and optimization
of existing industrial processes and products. A number of competences
at Philips/CFT heavily rely upon high-end, multi-physics computational
software. In problems related to thermo-fluid behavior, (micro)
fluidics, phase transitions, and multi-physics in general, STAR-CD
is used.
In this article, as an example of application of STAR-CD within Philips, modeling of ultra-high pressure lamps (UHP) is considered. Results represented here are based upon a joined development of Philips/CFT and Philips UHP/Turnhout.
Model
description
In Fig.1 for illustration
purposes, a photograph of UHP lamp is given. An UHP lamp consists
of the burner, reflector and closing glass. The closed UHP lamp
has two electrical contacts (front and back contact), and burner
is fixed within the reflector, using cement of a special composition.
Typically, lamps of this type have the power input ranging from
120 through 150 W, and are manufactured with either parabolic or
elliptic reflectors. However, for special applications, lamps of
deviating reflector shape and power can be produced.
This type of lamp is used, for example, in projection televisions and rear projectors. Currently, there is clear trend towards miniaturization and, at the same time, safety of applications, and, as a consequence, of the lamps. In order to improve the lifetime and safety of smaller products, thermal housekeeping of special lamps has to be well understood, and thermal behavior of the lamp in an application must be well controlled.
Fig. 1. Cross section of an UHP lamp.
The
UHP burner physics is described in [1], and, in our modeling, results
of developments within Philips Research/Aachen have been used. We
have not modeled the electrical discharge in the burner, but rather
used, as an input, the power/frequency distribution of radiation
resulting from such of modeling. The UHP burner is made of quartz,
which is semi-transparent to the plasma radiation. Quartz starts
to absorb typically above 4 mm wave length. This means that quartz
is also semi-transparent to the infrared radiation. As a consequence,
the phonon thermal conductivity will be enhanced at higher temperatures
(typically, higher than about 250°C) by the “photon” conductivity.
For optically “thick” materials, a well-known Roseland approach
can be used to describe this phenomenon. In optically “thin” materials,
this effect may be neglected. Quartz burners are, however, neither
optically “thick” nor optically “thin”. Therefore, we have,
used “ in-house” developed (in collaboration with Philips/Central
Development of Lamps (CDL)), semi-phenomenological model to describe
this phenomenon.
Fig. 2. Air re-circulation inside
the enclosed UHP lamp. Direction of the re-circulation is schematically
given in inset b).
At the inner surface of reflector, an optically reflective coating is applied. This coating reflects the visible portion of irradiation coming out of the burner. However, the reflector is semi-transparent to the rest of irradiation spectrum. Semi-transparency of the reflector and burner can be expressed in terms of the Lambert-Beer law, i.e. a fraction of the total power absorbed in materials is proportional to exp(-ax), where a is an effective attenuation constant, and x is the optical length of radiation beam passing through the material. This absorption has been implemented in STAR-CD, based on the geometry given.
Furthermore, it is not possible at the moment to model specular reflections within STAR-CD. Therefore, an optical analysis software ASAP (Breault research [2]) has been used for ray-tracing to identify possible hot spots due to specular reflections. The results have been translated into volumetric sources, and introduced into STAR-CD model.
Thermal properties of materials are all non-linear functions of temperature. Air has been modeled as an ideal gas, with all properties depending on the temperature.
Numerical implementation
Temperature in closed UHP lamps can be rather high. The outer burner surface can reach about 1000°C, whereas typical temperature at the outer reflector surface can be about 300-350°C. The coldest part of reflector can have temperatures about 180-200°C. This means that inside the closed UHP lamp an intensive air re-circulation takes place (see Fig. 2a/b). On the other hand, the air plume(s) around the outer reflector surface may be unstable, especially around the reflector neck, where the temperature gradient is the largest. All together, this means that the modeling of the lamp operation at “steady” conditions can be quite difficult.
Because
of the lamp dimensions, a low-Reynolds number turbulence model has
been utilized in our computations. We found that the k-e, low-Reynolds
number model yielded rather good results. Finally, the standard
version of this model has been used, which delivered best accuracy/speed
performance.
Fig. 3. Thermal plumes around an UHP lamp
In Fig. 3. temperature distribution around a closed UHP lamp is given. One can see that there are typically two plumes: one rising along the front glass of the closed lamp, and another – around the lamp neck. In these figure, a snapshot of an iso-thermal surface (at 80 oC) is given to indicate the temperature distribution in the plume.
Results
In
Figs. 4, comparison of the temperature distribution obtained from
the simulations and measured experimentally is given for the outer
surface of a reflector. Experiments have been carried out using
AGEMA900 infrared camera equipped with an infrared filter cutting
off frequencies lower than 4.7 mm. This setup assures that the temperature
of the surface of a semi-transparent material is measured, and it
is not affected by the thermal radiation from bulk. Measurements
have been verified using conventional thermocouples. At Philips
UHP/Turnhout, a special testing program has been developed to control
most critical (from the thermal point of view) features of closed
UHP lamps. This testing program allows for an evaluation of the
product lifetime.
Fig. 4. Comparison of the temperature distribution
over the reflector. a) STAR-CD simulations, b) experimental measurements
Comparison of the model results with data from these tests showed that the model developed is able of predicting the temperature distribution in a closed UHP lamp with accuracy of about 5-7%. Using this model approach, behavior of UHP lamps in different applications has been analyzed. In this way, an optimal cooling concept can be designed within a very short time for a given application.
Conclusions
CFD models, combined with experimental validation and testing under factory conditions, reduce dramatically time required to develop an optimized product for new applications.
STAR-CD gives freedom of geometrical modeling, required in industrial environment, combined with the state-of-art physical models in the area of CFD simulations. This combination allows simulating very tiny details, which is necessary for an accurate prediction of the behavior of critical features of products.
Using STAR-CD, we were able to predict thermal behavior of such a complex product as closed UHP lamps in different applications with accuracy better than about 7%.
This accuracy allows for virtual prototyping of new generation of products. Optimal cooling of lamps in different applications can readily be designed in this way.
References
[1]
H. Moench, Optical Modeling of UHP lamps, Modeling and Characterization
of Light Sources, Proceedings of SPIE Vol. #4775,
2002.
[2]
www.breault.com
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