Heat transfer enhancement of square-array jet impingement in a confined crossflow channel by using extended jet pipe and integrated acoustic actuator

A test investigation is performed to the convective heat transfer in a confined crossflow channel with a specific dimensionless height of 3, produced from a 2×2 square-array impinging jets with dimensionless pitches of 4. Particular focus is played on the heat transfer enhancement by using the passively extended jet pipes and the actively center-positioned synthetic jet in the continuous-jet square array, as well as their combination schemes. Square-array jet Reynolds number (Re) ranges from 3 000 to 10 000, and the synthetic jet acoustic actuator is driven at a fixed frequency of 250 Hz. Correspondingly, the synthetic jet velocity ratios (defined as the ratio of synthetic jet characteristic velocity to square-array jet ejecting velocity) are varied from 2.0 to 0.6. From the jet pipe extension, the dimensionless normal distance between jet outlet and targeting wall is adjusted in a range of 1–3. Within the scope of this study, the heat transfer enhancement roles are clearly illustrated. Under Re=3 000, the synthetic jet integration demonstrates a significantly stronger heat transfer augment role than the jet pipe extension, taking on dominant heat transfer enhancement mechanism in the combination scheme. With respect to the baseline situation (no synthetic jet integration and no jet pipe extension), the area-averaged Nusselt number on a specified zone could be increased up to 200% when the crossflow velocity ratio (defined as the ratio of crossflow inlet velocity to square-array jet ejecting velocity) beyond 0.67. Whereas under Re=10 000, the jet pipe extension plays dominant heat transfer enhancement mechanism on the otherwise. the area-averaged Nusselt number could be increased up to 100% at crossflow velocity ratio of 0.5 in relation to the baseline situation. Meanwhile, in the square array with the extended jet pipes, the role of synthetic jet integration is very faint. The most possibilities wherein the combination of synthetic jet integration and jet pipe extension could exhibit obviously its significance on heat transfer enhancement appear when both schemes display equivalent heat transfer augment roles. For instance, under Re=5 000 and crossflow velocity ratio ranging from 0.6 to 0.8, the combination scheme shows an obviously further improvement on heat transfer enhancement, in related to the single scheme either in active or passive.