One of the obstacles of ground-based astronomical observatories is a phenomenon known as “self-induced seeing”. It consists of the degradation of image quality, mostly resulting in an increased blurring of objects and a reduction of contrast in long exposure images. This occurs when thermal and wind disturbances create fluctuating layers of refractive indices within the optical beam path. With this in mind, a small hot object in close proximity to the secondary mirror could have potentially disastrous consequences for the accuracy of the telescope. A key requirement for the system is that the surface temperature of the Heat Stop must never be more that 10°C higher than the temperature of the ambient air so as to prevent buoyancy-induced flows from creating turbulent disturbances that would result in “self-induced seeing”.

As part of the design process, the team at thermal management specialist Thermacore was required to demonstrate the efficacy and robustness of their Heat Stop cooling system across the full range of potential operating conditions, as well as in some “failure mode” scenarios in which the failure of some other component had resulted in the telescope being aligned outside of its design range.

The surface temperatures (and generated flow around the Heat Stop) depend on a number of interacting physical phenomena. In simulating the Heat Stop assembly, the Thermacore engineers had to take into account multiphase flow within the porous metal heat exchangers, conjugate heat-transfer through the Heat Stop assembly and the interior wick structure, and both natural convection and radiation heat transfer around the Heat Stop.

All about Porous Metal Heat Exchangers

The past decade has witnessed a significant growth in the number of applications for a new category of heat transfer device known as “porous metal heat exchangers”. These devices, when used in conjunction with a pumped single-phase coolant or a pumped gas, take advantage of a porous layer of a thermally conductive medium located beneath the heat transfer surface to effect efficient heat transfer. In this device, convective heat transfer to the selected coolant combines with the “fin effect” produced by the large surface area of the conductive porous structure to produce efficient heat transfer. A porous media heat exchanger can be used to dissipate very large heat fluxes, such as those encountered in this solar telescope, or can be used to provide very efficient heat transfer at much lower heat fluxes.

The increased surface area, however, is obtained at the expense of increased flow resistance or pressure drop. To overcome the constricted flow paths, multiple closely-spaced inlets and outlets are used. The pore sizes are sufficiently small to prevent a flow “short-circuit” near the walls. The thickness of the porous structure depicted below is typically on the order of 0.020-0.050.

A significant effort has been made in recent years to improve the fundamental understanding of convective forced flow heat transfer in porous metal heat exchangers, which has resulted in improved understanding of governing principles, and has thus opened new applications for these devices such as cooling the DKIST solar telescope.