This paper assesses the suitability of the inflow Reynolds number defined by Re o ≡ U o D/ν (here U o and D are respectively the initial jet velocity and diameter while ν is kinematic viscosity) for a round air/air jet.Specifically an experimental investigation is performed for the influences of U o,D and Re o on the mean-velocity decay and spread coef-ficients (K u,K r) in the far field of a circular air jet into air from a smoothly contracting nozzle.Present measurements agree well with those previously obtained under similar inflow conditions.The relations K u ∝ U o and K r ∝ 1/U o for U o < 5 m/s appear to work,while each coefficient approaches asymptotically to a constant for U o > 6 m/s,regardless of the magnitudes of Re o and D.It is revealed that Re o may not be an appropriate dimensionless parameter to char-acterize the entire flow of a free air/air jet.This paper is the first paper that has challenged the suitability of Re o for turbulent free jets.
In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters(De) and their aspect ratios(AR) range from 1 to 6.5. Planar particle image velocimetry(PIV) is used to measure the velocity field at the same Reynolds number of Re = 5 × 104,where Re = Ue De/ν with Ue being the exit bulk velocity and ν the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core,expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet(AR = 6.5) produces the greatest decay rate of the near-field mean velocity, postpones the position at which the 鈥榓xis-switching鈥檖henomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.
Moderate or intense low oxygen dilution(MILD)combustion plays a significant role in the mitigation of combustion-generated pollutants and greenhouse gases whilst meeting thermal efficiency needs.However,due to the lack of the fundamental knowledge on this combustion,there is a misconception that MILD combustion should be established by high preheating of the air,which has limited its application.Our research and development on this combustion has been performed for several years. We have found that the requirements for establishing the MILD combustion are more relaxed than previously.It is also revealed that this combustion of different type,i.e.,non-premixed,partially premixed and fully premixed,can be achieved by firing various fuels(i.e.,gaseous,liquid and solid fuels).It is suggested that the application of the MILD combustion can be expanded significantly.The present review summarizes the progress and recent trend made in the R&D of this combustion and recommends further fundamental studies for improving our knowledge and widening its applications.
LI PengFeiMI JianChunDALLY B BWANG FeiFeiWANG LinLIU ZhaoHuiCHEN ShengZHENG ChuGuang
The present study experimentally investigated the near-field flow mixing characteristics of two turbulent jets issuing from equilateral triangular and circular orifice plates into effectively unbounded surroundings,respectively.Planar particle image velocimetry(PIV) was applied to measure the velocity field at the same Reynolds number of Re=50,000,where Re = UeDe /with Ue being the exit bulk velocity and the kinematic viscosity of fluid,D e the equivalent diameters.The instantaneous velocity,mean velocity,Reynolds stresses were obtained.From the mean velocity field,the centreline velocity decay rate and half-velocity width were derived.Comparing the mixing characteristics of the two jets,it is found that the triangular jet has a faster mixing rate than the circular counterpart.The triangular jet entrainments with the ambient fluid at a higher rate in the near field.This is evidenced by a shorter unmixed core,faster Reynolds stress and centreline turbulence intensity growth.The primary coherent structures in the near field are found to break down more rapidly in the triangular jet as compared to the circular jet.Over the entire measurement region,the triangular jet maintained a higher rate of decay and spread.Moreover,all components of Reynolds stress of the triangular jet appear to reach their peaks earlier,and then decay more rapidly than those of the circular jet.In addition,the axis-switching phenomenon is observed in the triangular jet.
XU MinYiZHANG JianPengMI JianChunNATHAN G JKALT P A M
