Imaging flow during impingement of differentially heated jets over a flat surface

Abstract

The present study investigates thermal fluctuations occurring in the mixing region of two adjacent differentially heated water jets impinging on a horizontal stainless steel surface. The nozzle diameters of the individual jets are equal. The jets are immersed in a pool of water at the ambient temperature. The Reynolds number range considered in the experiments is 2.09×103–2.51×104 for each jet, while the Richardson number is in the range of 9.58×10−3–1.38×10−2. Light intensity fluctuations in the interfacial region of the jets are imaged using the shadowgraph technique. Particle image velocimetry (PIV) has been carried out in a plane passing across the two jets. The effect of various flow rates near the nozzles and the impingement plate are visualized. Time-averaged and RMS temperature fluctuations on the exposed face of the impingement plate are measured using thermocouples. Spectra of light intensity fluctuations determined from the shadowgraph data are correlated with those in wall temperature.Light intensity spectra grow in amplitude in the flow direction, as additional jet instabilities set in. The spectra are broadband with distinct peaks, typical of high Reynolds number jets. Time-averaged and RMS wall temperature distribution are quite distinct from those of light intensity within the impinging jets. Consistent with a wall-jet behavior, the largest RMS wall temperature occurs near the stagnation point and decays away from it. The gradient in the time-averaged wall temperature is the highest for equal velocity jets and occurs at the mid-plane location. This is also the location of the peak in RMS wall temperature. Among all velocity ratios, the equal velocity configuration shows the highest RMS wall temperature. Here, PIV imaging shows a recirculation pattern in the time-averaged velocity near the stagnation point. When the flow rates are unequal, the higher velocity jet provides a blanket on the lower velocity side. The wall temperature fluctuations depend on the interaction between the cold and the hot jets and the location of the impingement plate.JEL Classification: K. Thermal hydraulics