Background
The Optical Science Laboratory (OSL) was founded in 1985. OSL is a research and development centre within the Department of Physics and Astronomy at University College London (www.osl.ucl.ac.uk) , which undertakes work on the studies, design and construction of optics, optical instrumentation and associated activities.

The largest instrumentation project currently undertaken by the group is the design and building of the High Resolution Optical Spectrograph (HROS). Phase 1 of the project started on October 1 1995, with the Conceptual Design Review in November 1996, and commissioning in Chile is scheduled for 2002.

The object of the analysis was to determine the type of cooling required to keep the temperature difference across the optical fused silica components within a given temperature range, to show that their performance would be acceptable. The cooling would either be passive, relying on the natural and forced convection of the environment in which the HROS was working, or active, requiring the addition of some type of fan-assisted cooling device.

The graphic to the right illustrates the thermal contours in the prism element of HROS.

The analysis was performed in two stages. The first stage was the creation of a parameterised input file of a simplified representation of the HROS. This allowed OSL to modify a number of design parameters:

• Model dimensions and object locations
• Material properties
• Meshing and solution controls
• Load and boundary condition histories

Using this input file, the thermal characteristics of the HROS could be investigated by varying these elements of the design. This was made possible by the unique features of the ANSYS Parametric Design Language (APDL). Since the input file was a simple unformatted text file, composed of standard ANSYS commands, it was possible to incorporate the instructions for running the analysis within the file itself. Other guidance, such as the formulas used to calculate the initial time step for a transient analysis and to evaluate the heat transfer coefficients, was also included. No extra documentation was required. Once these studies had been performed, it was decided that a more detailed analysis of a passive cooling system should be performed. This was to be the second stage of the project. The graphics above and below illustrate the thermal contour plots for the whole assembly minus insulation and for the echelle respectively. The graphic above illustrates the temperatures at the hottest and coolest parts of the structure as well as the difference between them.

In the second stage, the original parameterised input file was developed to include the more detailed features of the HROS. Two transients were analysed to investigate the possible extremes of the thermal response of the structure: the first assumed no natural convection within the HROS, the second did. In both transients, forced convection was applied to the exterior surfaces of the structure and heat generation loads were specified for electronic equipment situated within the HROS itself.

The results confirmed that a passive cooling system would be sufficient to allow the optical components of the HROS to perform within their design limits. This meant that the additional expense that would have been incurred by the introduction of an active cooling system was averted.

Due to the enhancements in the latest release of the ANSYS program, it was possible to incorporate a simpler scheme for the definition of the boundary conditions and loading histories. The enhancements allow ANSYS users to specify boundary conditions and loads as a function of coordinate location, temperature, time or any user-defined variable.