b'Engineer Innovation | Automotive & Transportationstop 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 Figure 3: The heat stop must dissipate 1,700 W at peak operating load, without ever allowing thethrough the heat stop assembly and the surface temperature to rise by more than 10C over ambient. interior wick structure, and both natural convection and radiation heat transfer around the heat stop.Although some of these configurations provided lower direct thermal loading, Thermacore engineers needed to demonstrate that asymmetries in thermal loading would not lead to local hot spots that might generate additional buoyancy-driven flow patterns (the potential source of a self-induced seeing problem). Using Simcenter STAR-CCM+, the Thermacore engineers were not only able to fulfill the design criteria demonstrating that the surface temperature of the reflector could be kept below 10C above ambient, but they Figure 4: Limiting air disturbance, whether thermally generated or as a result of the wind, playsalso demonstrated that the criteria was an important role in reducing self-induced seeing. sensible by visualizing the flow patterns generated within the enclosure as a result of natural convection.increased blurring of objects and a reduction of contrast in long exposureThe Thermacore team also examined the images. This occurs when thermal andinfluence of various atmospheric wind wind disturbances create fluctuatingloading scenarios, predicting flow layers of refractive indices within thepatterns and heat transfer across a range optical beam path. With this in mind, aof possible flow orientations, small hot object in close proximity to thedemonstrating that surface secondary mirror could have potentiallytemperatures and turbulence levels disastrous consequences for thewould not exceed design criteria.accuracy of the telescope. A key requirement for the system is that theAll about porous metal heat surface temperature of the heat stopexchangersmust never be more that 10C higherThe past decade has witnessed a than the temperature of the ambient airsignificant growth in the number of so as to prevent buoyancy-induced applications for a new category of heat flows from creating turbulenttransfer device known as porous metal disturbances that would result in self- heat exchangers." These devices, when induced seeing." used in conjunction with a pumped single-phase coolant or a pumped gas, As part of the design process,employ a porous layer of a thermally hermacore was required to demonstrateconductive medium beneath the heat the efficacy and robustness of their heattransfer surface to effect efficient heat 48'