Whole-Field Measuring Methods in Ventilated Rooms

The occupied zone of a ventilated room constitutes a large volume, and the temperature, velocity, and concentration cannot be mapped simultaneously over a whole field with standard point (local) measuring techniques. The situation is improving due to the introduction of whole-field measurement techniques. Whole-field techniques are relatively new and under development; therefore, this paper describes the principles behind the techniques and their pros and cons. All techniques are optical techniques in the sense that pictures are taken by an array of sensors responsive to different wavelengths of light depending on the application. The velocity components can be recorded with particle image velocimetry (PIV) or particle streak velocimetry (PSV). Both methods are based on adding tracer particles, which ideally follow the air motions; a digital camera records their displacement during a specified time interval. Recording the displacement corresponds to the Lagrangian formulation of fluid mechanics. With PIV, the displacement of groups of particles is recorded, in contrast to the PSV method, where the displacement of a single particle is recorded. PIV can provide high-resolution information over small regions, whereas the PSV method can cope with large areas. The two-dimensional concentration distributions of certain contaminants can be obtained by using tomography. The attenuation of light along a bundle of lines (rays) directed in a certain direction (view) and crossing the region with the contaminant is recorded. The measurements are repeated for several other angles. Based on the one-dimensional information in the form of attenuation of light along rays running in different directions, the two-dimensional concentration can be reconstructed by a reconstruction algorithm. The temperature distribution can be recorded and visualized with an infrared camera and a measuring screen that ideally attains the room air temperature. New development of the classical Schlieren technique has made it possible to visualize the air temperature distribution over large fields.

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