Study On Convective Heat Transfer Characteristics Of Lobed-Nozzle Jet Impingement

In-flight icing is a serious hazard to the flight safety under natural icing conditions.Ice accretion will not only increase the weight of the flight,but also the aerodynamic profile and thus degrade the flying performance.At present,the most widely used method of anti-icing is the hot-air anti-icing system,which is closely related to the convective heat transfer ability of hot air.As the hot air used for anti-icing purpose is bled from the engine compressor,therefore,using less hot air to achieve efficient thermal anti-icing efficiency is a remarkable issue for the anti-icing system development.For this propose,the nozzle shape of jet impingement in the anti-icing system is focused in the current study.By using numerical methid,the jet impingement heat transfer characteristics of the lobed and round nozzle are studied and the influences of lobed nozzle structure on heat transfer characteristics are also analyzed.The results show that the role of lobed nozzle on jet impingement is tightly dependent on the impinging distance.The lobed nozzle produces array streamwise vortices downstream the nozzle outlet to enhance the mixing of jet with the surrounding air,resulting in distinct features of jet development and impingement heat transfer with the round jet.At a small impact spacing(such as H/d =2 and 4),the internal lobed jet has the highest convection heat transfer,the lobed jet is lower,and the circular jet is the weakest.When the impact spacing ratio reaches 6,the heat transfer coefficient of the circular jet flow is the highest,and the internal lobed jet is weaker than the lobed jet.When the impact spacing ratio reaches 8,the heat transfer of the internal lobed jet in the stagnation area is significantly weaker than that of the lobed jet,while the heat transfer of the lobed jet and the circular jet are basically the same.Numerical simulation is used to study the jet impingement heat transfer on a specific concave surface by lobed nozzle under a typical jet Reynolds number of 10000.The differences of flow and heat transfer between the concave and flat targeting surfaces are revealed.Meanwhile,the influence of lobed nozzle arrangement and target width-to-height ratio are also analyzed.The results show that concave surface curvature has an obvious effect on jet impingement heat transfer.On the concave surface,the wall jet generally forms isolated bubbles in the vicinity of the leading edge of the concave surface due to the curvature effect.The velocity distribution of the impinging jet approaching to the target surface is affected,thus causing the convective heat transfer to be weaker than that on the flat target surface.On the basis of numerical simulation,an experimental test is made under some typical situations.The effects of jet Reynolds number,the impinging diatance and target shape on the jet impingement heat transfer are taken into considerations.The experimental results show that the effects of the above factors on heat transfer charateristics have the same regularity as the the corresponding numerical results.The jet Reynolds number has a significant influence on the respectively optimal impinging distance and convective heat transfer in the vicinity of stagnation point for the lobed and round nozzles.