Staying cool with STAR-CD

Fig. 1. Cross section of an UHP lamp.

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.

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