Thermal and Hydrodynamic Visualisation of a Water Jet Impinging on a Flat Surface using Microencapsulated Liquid Crystals

Thermochromic liquid crystals have been extensively applied to natural convection °ows for the visualisation of temperature and induced velocity ¯elds. In this study, results of the simultaneous visualisation of velocity and temperature in a forced convection °ow are presented. The single submerged impinging water jet was arti¯cially seeded with microencapsulated liquid crystals. A nozzle to plate spacing of three nozzle diameters and a turbulent Reynolds number of 35500 was considered. Visualisation of the °ow ¯eld revealed the thermal development of the impinging and resultant wall jet as well as the high temperature gradients that prevail at the impingement surface. In particular, entrainment of heated spent °uid at the nozzle exit and in the shear layer region was observed. Flow characteristics typical of turbulent impinging jets were visualised; vortices, initiated at the edge of the jet are transported to the stagnation region, a wall jet develops which rolls back and changes direction. The observed hysteresis in calibration curves for heating and cooling of the liquid crystals was quanti¯ed. The use of liquid crystals as particle tracers has been demonstrated as a useful tool for the simultaneous quantitative visualisation of °ow and temperature in a forced convection °ow. 1