| Jet impingement heat transfer is considered as a promising heat transfer enhancement technique,which has been widely applied in a wide variety of practical applications that aim to achieve intense heating,cooling or drying rates.Pulsed jet is a typical unsateay jet.Its impingement heat transfer cheracteristics is affected by more complicated factores when compared to the steady jet.Aiming at the further requirement in the pulsed-jet impingement heat transfer enhancement,a plused jet impingment scheme with the use of additional jet chamber was proposed in current study.Experimental and numerical syudies were performed for the single-inlet plused-excitation jet impingment heat transfer on a confined flat target.Further,some shaped orifices(such as lobed orifice,center-satellite array orifice,and combined fork-cashew orifice)were investigated.The main research efforts and corresponding conclusions in current study are summarized as the following four aspects:Firstly,experimental and numerical investigations were performed to study the impingement heat transfer on a confined flat target surface produced by single-inlet plused-excitation jet with an additional jet chamber.The effects of plusation frequency(f),duty cycle(DC),Reynolds number(Re),dimensionless nozzle-to-surface distance(H/d)and jet-chamber geometric parameters on the plused-jet impingement heat transfer were determined.The current experimental test was conducted under the following conditions,such as 5Hz≤f≤40Hz,5000≤Re≤15000,0.2≤DC≤0.8 and 2≤H/d≤10.In the numerical simulations,the above parameter ranges were relatively extended,such as5Hz≤f≤200Hz,5000≤Re≤20000 and 0.2≤DC≤1.0.With respect to the baseline pulsed-jet impingement without an additional jet chamber,the pulsed-jet with an additional jet chamber is found to enhance the convective heat transfer with a 8%～20% increase of the stagnation Nusselt number,especially under small nozzle-to-surface distances.Due to the presence of an additional jet chamber,the exiting jet velocity profile at the orifice outlet is varied with respect to the baseline case,making the time-averaged ejecting velocity increase in the central zone but decrease in the edge zone.The additional jet chamber introduces two dominate mechanisms for enhancing plused-jet impingement heat transfer.At the jet-on process the vena contracta effect near the jet orifice behaves to increase the approaching velocity towards the target surface.At the jet-off process the inertia role inside the jet chamber behaves to maintain the jet ejection.Among the current research conditions,the relatively optimal pulsation parameters were demonstrated to be f=80Hz and DC=0.8.For the pulsed-jet impingement on a confined target,H/d=4 was a relatively optimal jet-to-surface distance.Secondly,by using lobed orifice which has the same exit area or equivalent diameter with the baseline round orifice,a series of experimental tests were performed for the impingement heat transfer on a confined flat target surface produced by single-inlet plused-excitation jet with an additional jet chamber.The effects of lobed-orifice geomrtric parameters(such as lobe number,lobe aspect ratio)on the plused-jet impingement heat transfer were determined under the yipical operational parameters(such as plusation frequency,duty cycle,Reynolds number,dimensionless nozzle-to-surface distance).Simultaneously,some numerical simulations were conducted to illustrate the roles of lobed-orifice on the flow dynamics of plused-jet impingement.The results show that the lobed orifice shows an obvious action on the plused-jet impingement heat transfer enhancement under a large Reynolds number and a small jet impingement distance when compared to the round orifice,owing to that it posses the active role on fludic excitation similar to the tab protrusion.For the 3-lobe lobed orifice,the local Nusselt number distribution takes on two ―triangle-like pattern‖ feature at a small dimensionless nozzle-to-surface distance.The apexes of outer triangle are staggered to the inner triangle.This feature is also vaguely discernible for the 4-lobe and 6-lobe orifices,but replaced by―tetragonal‖ patten and ―hexagonal‖ pattern respectively.However,for the 8-lobe lobed orifice,the ring-shaped local Nusselt number distribution is demonstrated,nearly the same as that of a round-orifice jet impingement.As the increase of nozzle-to-surface distance,the non-uniformity of local convective heat transfer in the circumfertial direction disappears gradually,characterized by a nearly axisymmetric local Nusselt number distribution.Thirdly,by using center-satellite array orifice which has the same exit area or equivalent diameter with the baseline round orifice,experimental tests were performed for the single-inlet plused-jet impingement heat transfer on a confined flat target surface.The effects of geomrtric parameters of center-satellite array orifice(such as center-satellite area ratio,satellite number and arrangement mode)on the plused-jet impingement heat transfer were determined under the typical operational parameters of plused jet.Simultaneously,some numerical simulations were conducted to illustrate the roles of center-satellite array orifice on the flow dynamics and local Nusselts distribution feature.For the center-satellite array orifice,it is found that the time-averaged electing velocity from the center hole is obviously higher than that form the satellite hole.Beside,the jet issuing from the satellite hole is subjected to the crossflow effect due to the wall jet after center-hole jet impingement,so that its flow is deflected toward radial direction.Under a small nozzle-to-surface distance,the center-hole jet produces the highest local convective heat transfer zone and the surrounding satellite-hole jets produced several islands with higher convective heat transfer coefficient.Under larger nozzle-to-surface distances,the local islands produced by the satellite jets disappear,replaced by a “star-shaped” Nusselts number distribution feature.Among the present satellite-hole arrangement modes,it is found that two modes show better convective heat transfer at small nozzle-to-surface distances,namely,1-6 mode and 1-4-4mode.With respect to the lobed orifice,the center-satellite array orifice does not show any advantages generally.At last,a combined fork-cashew orifice was put forward according to the above orifice flow field characteristics.Around the inner fork-hole,corresponding cashew-shaped holes were arranged where each branch of the fork hole was located at the interval of the adiacent cashew holes.Three structural schemes were designed in the viewing of fork-hole type,namely,three-branch,slot-shaped and four-branch-shaped.By using the combined fork-cashew orifice which has the same exit area or equivalent diameter with the baseline round orifice,experimental tests were performed for the pulsed-jet impingement heat transfer on a confined flat target surface.Simultaneously,some numerical simulations were conducted to illustrate the roles of combined fork-cashew orifice on the flow dynamics and local Nusselts distribution feature.The results show that four-branch-shaped fork-cashew orifice is the best one for achieveing the highest convection heat transfer among the present three structural schemes.As each branch of the fork hole is located at the interval of the adiacent cashew holes,the crossflow due to the wall jet after inner fork-hole jet impingement introduces little effect on the outer jet impingement issuing forn the cashew hole.Generally,the four-branch-shaped fork-cashew orifice has a certain advantage over the lobed orifice and center-satellite array orifice on the pulsed-jet impingement heat transfer enhancement under high Reynolds numbers. |
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