Abstract

Multiple impinging jets are used in many industrial applications for cooling, heating or drying. For the single impinging jet a variety of experimental data and numerical results are available in the literature. In industrial applications a variety of geometric arrangements (nozzle shapes and arrangements) as well as operating conditions (e.g. moving surfaces) are used and the data available therefor is much scarcer and often empirical. The flow and heat transfer for multiple jets impinging on moving sheets, as used in paper drying, is analysed in order to optimize the performance of the jets. Numerical analysis is performed to investigate the relevant flow features and assess the influence of the main parameters and optimize the design. For simple repetitive geometries (e.g. round or planar nozzle) LES is a feasible approach. For arrays of multiple different jets this is still too expensive and a RANS approach is used. The simulations are performed with the k-omega SST model and validated with respect to experimental data and LES simulations. The local heat transfer is influenced by the turbulence in the adjacent bulk flow. Thus the results are influenced by the quality of the turbulence modeling, i.e. the turbulence model used and its implementation. This is shown for the secondary peak in the Nusseltnumber distribution for the impinging round jet. The flow for slot jets at moderate Reynoldsnumbers is analysed with LES. For higher Reynoldsnumbers and for arrays of round jets the k-omega SST model is used. The numerical results are used to gain some insight into the flow phenomena influencing the heat transfer. The heat transfer for particular designs can be predicted. Using numerical results correlation equation of dimensionless numbers are derived. The numerical model can also be used as an input into an optimization algorithm for the design of a nozzle field. Thus a good solution with regard to minimum energy requirement for the drying process and minimum strain for the sheets can be found. In a first step it is possible to improve the operating conditions, flow rate, air temperature, sheet velocity. In a second step the arrangement of the nozzles, i.e. spacing and distance to surface can be optimized. Finally using this numerical approach also the nozzle orifices can be designed accordingly.