Development and experimental validation of numerical heat transfer models for impingement jets

In con­ti­nuous strip lines and in cham­ber fur­naces for the heat tre­at­ment of steel, alu­mi­ni­um and cop­per strip, nozz­le sys­tems are used to heat and cool the strips. The nozz­les are direc­ted at the strip in such a way that the resul­ting impinge­ment flow ensu­res the hig­hest pos­si­ble and homo­ge­neous heat trans­fer. The heat trans­fer bet­ween the strip and the gas of the nozz­le flow takes place main­ly con­vec­tively. To assess the heat trans­fer of the impinge­ment flow, a heat trans­fer coef­fi­ci­ent (HTC) α is defi­ned and is repre­sen­ted dimen­si­on­less­ly by the Nußelt num­ber Nu. The heat trans­fer in impinge­ment flows depends on seve­ral fac­tors such as the nozz­le exit velo­ci­ty, the mate­ri­al values of the flu­id and the geo­me­try of the nozz­le systems.

Figu­re of a (a) round nozz­le and (b) slot nozz­le sys­tem with the rele­vant geo­me­tric para­me­ters h: strip distance, L: nozz­le length, W: nozz­le width, s: spacing

The plan­ned pro­ject should con­tri­bu­te to the deve­lo­p­ment of a favoura­ble and effi­ci­ent design opti­on for nozz­le sys­tems. For this pur­po­se, an inno­va­ti­ve, nume­ri­cal model­ling approach is to be used with which the heat trans­fer bet­ween impact jets and the impact sur­face can be cal­cu­la­ted with suf­fi­ci­ent accu­ra­cy. At pre­sent, this is only pos­si­ble with a non-eco­no­mic­al com­pu­ta­tio­nal effort. The vali­da­ti­on and eva­lua­ti­on of the models takes place with an expe­ri­men­tal para­me­ter stu­dy. The tur­bu­lence struc­tures of the impact flow can be recor­ded with the help of opti­cal flow mea­su­re­ment methods. The local heat trans­fer bet­ween the impinge­ment flow and the impinge­ment sur­face can also be inves­ti­ga­ted using sui­ta­ble mea­su­re­ment tech­no­lo­gy. Both mea­su­re­ment methods men­tio­ned can be car­ri­ed out simul­ta­neous­ly in an expe­ri­men­tal set­up at the IOB. This makes it pos­si­ble to direct­ly ana­ly­se the flow and the local Nußelt number.

Figu­re of the heat trans­fer test rig for the mea­su­re­ment of Nußelt numbers.
Left: Prin­ci­ple sketch of the test rig, bot­tom right: Pho­to of the test rig
Top right: Sam­ple Nu dis­tri­bu­ti­on for com­bi­ned nozz­le sys­tem of round and slot nozzles