Purpose of the work
Thermographic investigations often find application in energy consulting or for identifying the causes of problems that are related to building physics. Since this measuring technique is complex, measurement inaccuracies and errors are not uncommon. Thus in order to obtain a meaningful measurement, the object under investigation must be in a steady-state condition as far as this is possible. Further, environmental influences such as rain, snow, fog, solar irradiation and wind have a major impact on the measurement situation. These are decisive for measurement quality. On the one hand, air flows can change rapidly in direction and speed; on the other, turbulence can also develop, especially at higher wind speeds. Inside buildings, the convective influence on surfaces depends mainly on radiators, furnishings and floor plans but also on fan-assisted ventilation systems. In addition to wind, interior convection in particular has a major influence on the thermal transfer resistance of the building element and ultimately on the outcome of the measurement. In this paper, the convective influence on thermographic measurement of building surfaces shall be investigated in theory but also on the basis of practice.
Method of approach
The influence of convection was investigated in the lab and in a practical setting. In the lab, air flow trials using blowers from the field of aeronautics were conducted on various building elements with differing surface structures and emissivities. For this purpose, flow was directed onto the objects under investigation from various directions at a range of pressure levels.
Additionally, turbulence was induced at the surfaces. Then measurements in the field were conducted inside and outside of a residential building. Floor plans and furnishings as well as scenarios for building usage were considered in equal measure. By applying numerical flow simulation (CFD), mathematical approaches were developed and used to validate the practical trials.
Content of the contribution
Due to air flow, the thermal transfer resistance of building elements varies greatly so that absolute measurements – that is quantitative measurements with a minimal risk of measuring errors – are only possible to a limited extent. In addition there may also be changes in the thermal signature of building element surfaces. This means that any problems relating to building construction or building physics will not be detected. Comprehensive descriptions of the test rig and the results obtained are presented.
Results and evaluation of their significance
The findings made it possible to establish in concrete terms the influence of convection on the measurement result. A procedure was developed for use by measurement technicians and thermographers.
The result of thermographic measurements is significantly influenced by convection. Up to now, this has not been taken sufficiently into account by measurement technicians and specialist staff. With the help of the procedural guidelines, it is possible to show the influence of convection on the measurement result.
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