Environmental Wind Tunnels17is effective and widely applied, however this technique essentially requires two nested vacuum systems and is therefore generally expensive and complex to construct. Efficient vacuum compatible thermal (cryogenic) insulation is available. One example is multi-layer thin film super-insulation developed at CERN-CryoLab and commercially available (JEHIER). Despite its high price relative to ambient pressure insulation, it is simple to apply multilayer super insulation within an environmental chamber design, it is also reasonably efficient and affordable compared to a multi-walled vacuum insulation solution. In a closed circuit systems the frictional loss of power within wind tunnel can cause significant heating at elevated wind speeds (of the order 100W/m3). This can become problematic for thermal control systems in such situations. Specifically heating of the gas will ensue and heat deposition on surfaces in contact with the gas. This must be considered when designing thermal control systems if stable temperatures are to be achieved. As mentioned previously with respect to sensors, in addition to a thermal sensor system and a cooling/heating system an automated (intelligent) control network needs to support these sub-systems in order to achieve effective thermal control. In many research and industrial applications it is necessary to perform complex thermal cycling. Such cycling may involve specific thermal ramp rates and extend over long periods of time (days) necessitating computer control. Even in a thermally controlled system where effective thermal insulation has been employed, effective thermal conduction to the sample has been used and sufficient heat is exchanged, this does not ensure thermal stability since typically the cooling system will not be continuously in operation and will have a certain time delay between activation and the onset of cooling. In practice therefore thermal control will consist of a feedback system of thermal sensors and thermal control which will introduce oscillation of the test section temperature. Additionally ensuring thermal stability may not ensure thermal uniformity as the application of cooling may not be physically at the same location as the source of heating or thermal loss which therefore leads to spatial temperature gradients. An obvious method to both stabilize the test section (or sample) temperature and achieve improved thermal uniformity is the use of a massive conductive (metal) test section element or sample mounting section. Here the thermal inertia of the mass achieves thermal stability and the high thermal conductivity of the mass ensures uniformity. It is however often difficult to find space to house such a massive element and the cooling/heating time of this element will necessarily be long. In the case of the AWTSI and AWTSII facilities a compromise has been reached between the desired stability/uniformity and the available space/required response time. In the case of the AWTS-II facility at a temperature of around -120C the uniformity of the test region is around 15C/m and the stability is around 2C despite the use of a 0.1m thick sample (aluminum) mounting plate.8. Ice formation and sensingThe transport of water vapor is often an important physical parameter for environmental simulators, both with regard to industrial and research applications. For example on Mars the desiccation of surface materials and subsequent frost formation (re-hydration) may lead to geophysical changes in the surface materials, specifically salt crust formation (which are widely observed) or even erosion. Similarly man made materials can be susceptible to weathering by the transport of water vapor from the surface. For the control of humidity at low temperatures it is necessary to both cool the sample and another element within the
