1982, Al-Sanea, 1992, Barata, 1996). The numerical result of Al-Sanea (1992) on singlejet combined with crossflow showed that a crossflow degradesthe favourable characteristics of impinging jet. The impingement of confined single and twin turbulent jets through a crossflow was studied experimentally and numerically by Barata et al. (1996) to examine the characteristics of vortex and complex flows generated by the multiple jets in a crossflow. Nakabe et al. (1998) studied various aspects of the jet impingement heat transfer under the influence of the crossflow and observed that the Nusselt number distribution became a plateau-like pattern, which means that the enhanced region of heat transfer expanded more than the case of impinging jets without crossflow.
San and Lai (2001) investigated experimentally the configuration of five confined circular air jets in staggered arrays vertically impinging on a flat plate. An optimum ratio of jet-to-jet spacing was obtained. Heat transfer from a discrete heat source to multiple, normally impinging and confined air jet was experimentally investigated by Garimella and Schroeder (2001). They performed numerical simulations to gain insight into the fluid flow and heat transfer characteristics of inclined jet with crossflow impinging on a heated plate. The effect of the jet to crossflow velocity ratio on the flow and thermal fields were investigated.
Chen and Hwang (1991) experimentally studied a 2D heated plane jet in a crossflow, where the jet was confined in a channel. In this flow configuration, the jet injected from a narrow slot developed between the two sidewalls of the channel, without any clearance between the jet and walls. They reported about the two-dimensionality of the flow field, especially at the centre of the slot.
Sherif and Pletcher (1991) made an experimental investigation of a round heated jet in crossflow for different velocity ratios of 1, 2, 4 and 7. They analysed the jet wake thermal characteristics for those velocity ratios. They found the difference of the flow behaviour for small velocity ratios (R < 2) and large velocity ratios (R > 2). They observed the existence of a double vortex structure in the flow field for both the velocity ratios, but weaker in strength in small velocity ratio. Based on their results, they suggested that the velocity ratio R = 2 should be a borderline between the high and low velocity ratios. Nishiyama et al. (1993) reported the characteristics of temperature fluctuations in a slightly heated 2D jet issuing through a slot normally into a crossflow for different velocity ratios and they studied the effects of the velocity ratio on the mean and fluctuating temperature fields. They observed that the low velocity ratio jets behave like a wall jet and the high velocity ratio jets are lift-off jets with a recirculation region. They also derived a relation of the decay characteristics of the maximum temperature. The decay characteristics are represented in a following equation, where m means the slope of the decay.
max ( . / 2 D. / 2) m
j
T T
s D R s D R T T
α α
− −
= −
− (2.4) Where Tmaxis the maximum temperature, Tjis the jet temperature at the centre line and Tαis the crossflow temperature. The distance along streamline is s and sD is the distance of the jet along streamline. Said et al. (2003) performed numerical investigation of a round heated jet in crossflow using different turbulence models.
They reported the distribution of the velocity and temperature fields and mass fraction of different constituents of the jet. They observed that the velocity field fully controls the dilution of temperature and concentration of the jet. They also found a better performance of the Reynolds stress transport model compared to the two-equation models.
Like heated jets in crossflow some authors have also investigated the flow field of a cold jets in a warmer crossflowing ambient. The cold jets in hot crossflow may have
sinking trajectories when the buoyancy forces are dominant. Sherif and Pletcher (1998) performed the experimental investigation of cooled jets discharged into heated crossflowing ambient in a water channel. They presented the results in the region of jet and wake thermal regions in terms of the mean and root-mean-square temperature fluctuations profiles. They also compared the results with the heated jets discharged into cold crossflow. They observed similar temperature profiles in the region of jet discharge as in the case of heated jet in cold crossflow and farther downstream of jet discharge, the agreement between the two was poor.
The investigation of the scalar field of transverse jets has received somewhat less attention than the velocity and vorticity fields. Mean scalar fields were experimentally measured by Andreopoulos and Rodi (1984) and Niederhaus et al. (1997). Niederhaus et al. (1997) applied planar laser-induced fluorescence (PLIF) to obtain the scalar concentration fields in the cross-sections of the crossflowing jets in water, with velocity ratio R = 4.9 to 11.1. While applying PLIF in air-into-air crossflowing jets, Smith and Mungal (1998) mapped the concentration field in cross-sectional planes, the symmetry plane, and in planes parallel to the jet exit plane, for the velocity ratios ranging from 5 to 20. They also measured the vortex interaction region, mean trajectories and concentration decay, and overall structural features of mixing of a round jet in a crossflow. Su and Mungal (2004) provided a comprehensive view of the scalar and velocity fields in the developing region of the crossflowing turbulent jet in the gas phase, using planar imaging techniques. Simultaneous planar measurements of scalar mixing and 2D velocity fields permitted the detailed study of the developing region of turbulent crossflowing jets with velocity ratio R = 5.7. The results show that the intensity of the mixing, as quantified by the scalar variance and the magnitude of the turbulence scalar fluxes, is initially higher on the jet windward side, but eventually becomes higher on the wake side. Plesniak and Cusano (2005) performed an experimental investigation of a confined rectangular jet in crossflow to investigate the scalar mixing. They performed the systematic variation of three pertinent parameters, i.e., the momentum ratio, injection angle and development length. They observed the three regimes (wall jet, fully lifted jet and reattached jet) for the jet crossflow interaction and the resulting concentration field. The combined scalar concentration and velocity field data provided an understanding of the large-scale mixing and the role of coherent structures and their evolution. Recently Shan and Dimotakis (2006) investigated the Reynolds number dependence of the scalar mixing by examining the
probability distribution of the jet fluid in strong liquid-phase transverse jets at a fixed far-downstream location. In their study, the high-Schmidt-number mixing was compared between transverse jets and ordinary jets to investigate possible differences in the mixing for the fully developed (but finite Reynolds number) turbulent flows.