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Pulsating impinging jets can have significant influence on transfer processes. A number of studies have been done on heat transfer in a pulsating impinging jet but very divergent and sometimes contradictory results have been reported. In the present study, the flow and temperature field under a single confined pulsating turbulent impinging jet are determined numerically by the finite volume method. Effects of Pulsation function parameters (frequency and amplitude) and various geometries on the flow characteristics and heat transfer rate from hot surface are discussed. Results of simulation show that flow pulsation has different effects in various flow zones. Pulsation of jet improve cooling performance in the wall jet zone and simultaneously reduce heat transfer in the stagnation zone. As expected the effect of flow pulsation decays with distance from the jet nozzle. Under certain conditions flow oscillation adversely affects the heat transfer in comparison with the steady jet at the same mean Reynolds number. In pulsating jets we can introduce a critical frequency correspond to St ¼ 0.26. The amount of heat transfer at the frequencies corresponding St ¼ 0.26 is higher than other cases. In general, it is concluded that cooling performance of oscillating impinging jet is enhanced by increase in the frequency and amplitude of oscillation as well as decrease in nozzle to plate distance.

Pulsed jets in various configurations have shown potential for improving transport phenomena. In this study, a system of confined laminar two-dimensional pulsed impinging streams of air is simulated numerically by solving the governing conservation equations using the control volume method. The key parameters examined in this study are as follows: frequency and amplitude of pulsation, mean jet Reynolds number and phase difference between the nozzle exit velocity profiles. The effects of these parameters are computed and discussed comprehensively. Temperature is used as a passive tracer to quantify the degree of mixing in the system. Results show that flow pulsation has significant effects on the flow field, vortex formation and secondary structures, which are generated. These vortex structures influence the thermal shear layer and improve the mixing behavior. A better mixing index is observed as a result of the formation of larger vortices due to increased amplitude of the pulsation velocity amplitude. In the range of parameters tested, the frequency of the pulsation is found to have negligible effect on the mixing behavior of the system. Also, it is observed that by introducing a phase difference between the two jet velocity profiles, the stagnation point oscillates between the two jets and, in general, the system with phase differences shows better mixing behavior.

The motion and drying characteristics of a single particle in a novel two-dimensional pulsed opposing jet contactor (POJC) are modeled and discussed. Hot air is used as the drying medium. To simulate particle drying, the gas phase and dispersed phase conservation equations are considered in the Eulerian reference frame and the Lagrangian reference frame, respectively. The RNG turbulence model is used to determine the turbulent characteristics of the gas phase. The particle motion is described by the BBO (Basset-Boussinesq-Oseen) equation. The effects of the key parameters, such as the jet Reynolds number, amplitude of pulsation, frequency of pulsation, particle diameter, location of release of particle from one jet as well as velocity profile on residence time (RT) and particle penetration depth (PN) into the opposite jet, are examined. Results show that POJC has strong potential for particulate heat transfer as well as drying; it can improve evaporation rate relative to the corresponding steady OJC by up to 30% as a result of increased residence time in the impingement zone within the parameter ranges simulated.