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In a model tunnel the influence of the thermal boundaries on the so-called back layering is investigated. The back layering is driven by density differences and consists of hot gases propagating against the ventilation of a tunnel. Increased heat loss to the ceiling should therefore reduce the extent of the hot gas propagating upstream. The thermal properties of the ceiling are altered by removing insulating panels. Three measurement techniques are used to detect the presence of hot gas regions below the ceiling. Thin film heat flux sensors measure the heat flux from the gas to the ceiling at 6 locations. They also provide ceiling surface temperature measurements. Optically interrogated thermochromic liquid crystal foils allow to measure the back layering length continuously. Some of the foils are slightly detached from the ceiling in order to indicate air rather than ceiling temperature. Comparing the results obtained from these foils to fast heat flux sensor readings shows good agreement. The so-called heated grid technique has been developed further. Wires are immersed in a flow field and interrogated using an infrared camera. Adaptive background compensation has been introduced. This method simplifies the calibration procedure needed. At the same time it expands the possible application of the technique by allowing for changing backgrounds and distances between the wires and the camera. A total of 8 wires are used to indicate air temperature 1 cm below the ceiling. A ceiling panel has been fitted with sensing elements to support all three measurement techniques. This panel has then been placed in the model tunnel at IFD. The test section has been placed upstream of a heated jet, allowing to observe hot gases propagating against a cross flow. Measurements have been performed during the initial 300 s after switching on the heating of the jet. The results indicate a reduction of the back layering length with increased heat loss. In the present configuration the effect remains, however, weak.

Influence of Thermal Boundary Conditions on Transient Back Layering in a Scale Tunnel