Abstract. Extremely severe and persistent haze occurred in January 2013 over eastern and northern China. The record-breaking high concentrations of fine particulate matter (PM 2.5 ) of more than 700 µg m −3 on hourly average and the persistence of the episodes have raised widespread, considerable public concerns. During that period, 7 of the top 10 polluted cities in China were within the Hebei Province. The three cities in southern Hebei (Shijiazhuang, Xingtai, and Handan) have been listed as the top three polluted cities according to the statistics for the first half of the year 2013. In this study, the Mesoscale Modeling System Generation 5 (MM5) and the Models-3/Community Multiscale Air Quality (CMAQ) modeling system are applied to simulate the 2013 severe winter regional hazes in East Asia and northern China at horizontal grid resolutions of 36 and 12 km, respectively, using the Multi-resolution Emission Inventory for China (MEIC). The source contributions of major source regions and sectors to PM 2.5 concentrations in the three most polluted cities in southern Hebei are quantified by aiming at the understanding of the sources of the severe haze pollution in this region, and the results are compared with December 2007, the haziest month in the period 2001-2010. Model evaluation against meteorological and air quality observations indicates an overall acceptable performance and the model tends to underpredict PM 2.5 and coarse particulate matter (PM 10 ) concentrations during the extremely polluted episodes. The MEIC inventory is proven to be a good estimation in terms of total emissions of cities but uncertainties exist in the spatial allocations of emissions into fine grid resolutions within cities. The source apportionment shows that emissions from northern Hebei and the Beijing-Tianjin city cluster are two major regional contributors to the pollution in January 2013 in Shijiazhuang, compared with those from Shanxi and northern Hebei for December 2007. For Xingtai and Handan, the emissions from northern Hebei and Henan are important. The industrial and domestic sources are the most significant local contributors, and the domestic and agricultural emissions from Shandong and Henan are non-negligible regional sources, especially for Xingtai and Handan. Even in the top two haziest months (i.e., January 2013 and December 2007), a large fraction of PM 2.5 in the three cities may originate from quite different regional sources. These results indicate the importance of establishing a regional joint framework of policymaking and action system to effectively mitigate air pollution in this area, not only over the Beijing-Tianjin-Hebei area, but also surrounding provinces such as Henan, Shandong, and Shanxi.
Recent studies have demonstrated that the aerothermal characteristics of turbine rotor blade tip under a transonic condition are qualitatively different from those under a low-speed subsonic condition. The cooling injection adds further complexity to the over-tip-leakage (OTL) transonic flow behavior and aerothermal performance, particularly for commonly studied shroudless tip configurations such as a squealer tip. However there has been no published experimental study of a cooled transonic squealer. The present study investigates the effect of cooling injection on a transonic squealer through a closely combined experimental and CFD effort. Part I of this two-part paper presents the first of the kind tip cooling experimental data obtained in a transonic linear cascade environment (exit Mach number 0.95). Transient thermal measurements are carried out for an uncooled squealer tip and six cooling configurations with different locations and numbers of discrete holes. High-resolution distributions of heat transfer coefficient and cooling effectiveness are obtained. ansysFluent is employed to perform numerical simulations for all the experimental cases. The mesh and turbulence modeling dependence is first evaluated before further computational studies are carried out. Both the experimental and computational results consistently illustrate strong interactions between the OTL flow and cooling injection. When the cooling injection (even with a relatively small amount) is introduced, distinctive series of stripes in surface heat transfer coefficient are observed with an opposite trend in the chordwise variations on the squealer cavity floor and on the suction surface rim. Both experimental and CFD results have also consistently shown interesting signatures of the strong OTL flow–cooling interactions in terms of the net heat flux reduction distribution in areas seemingly unreachable by the coolant. Further examinations and analyses of the related flow physics and underlining vortical flow structures will be presented in Part II.
A basic attribute for turbine blade film cooling is that coolant injected should be largely passively convected by the local base flow. However, the effective working of the conventional wisdom may be compromised when the cooling injection strongly interacts with the base flow. Rotor blade tip of a transonic high-pressure (HP) turbine is one of such challenging regions for which basic understanding of the relevant aerothermal behavior as a basis for effective heat transfer/cooling design is lacking. The need to increase our understanding and predictability for high-speed transonic blade tip has been underlined by some recent findings that tip heat transfer characteristics in a transonic flow are qualitatively different from those at a low speed. Although there have been extensive studies previously on squealer blade tip cooling, there have been no published experimental studies under a transonic flow condition. The present study investigates the effect of cooling injection on a transonic squealer tip through a closely combined experimental and computational fluid dynamics (CFD) effort. The experimental and computational results as presented in Part I have consistently revealed some distinctive aerothermal signatures of the strong coolant-base flow interactions. In this paper, as Part II, detailed analyses using the validated CFD solutions are conducted to identify, analyze, and understand the causal links between the aerothermal signatures and the driving flow structures and physical mechanisms. It is shown that the interactions between the coolant injection and the base over-tip leakage (OTL) flow in the squealer tip region are much stronger in the frontal subsonic region than the rear transonic region. The dominant vortical flow structure is a counter-rotating vortex pair (CRVP) associated with each discrete cooling injection. High HTC stripes on the cavity floor are directly linked to the impingement heat transfer augmentation associated with one leg of the CRVP, which is considerably enhanced by the near-floor fluid movement driven by the overall pressure gradient along the camber line (CAM). The strength of the coolant-base flow interaction as signified by the augmented values of the HTC stripes is seen to correlate to the interplay and balance between the OTL flow and the CRVP structure. As such, for the frontal subsonic part of the cavity, there is a prevailing spanwise inward flow initiated by the CRVP, which has profoundly changed the local base flow, leading to high HTC stripes on the cavity floor. On the other hand, for the rear high speed part, the high inertia of the OTL flow dominates; thus, the vortical flow disturbances associated with the CRVP are largely passively convected, leaving clear signatures on the top surface of the suction surface rim. A further interesting side effect of the strong interaction in the frontal subsonic region is that there is considerable net heat flux reduction (NHFR) in an area seemingly unreachable by the injected coolant. The present results have confirmed that this is due to the large reduction in the local HTC as a consequence of the upstream propagated impact of the strong coolant-base flow interactions.
In the present work, the effect of coolant injection on the over-tip-leakage (OTL) flow and squealer designs has been investigated in a transonic flow regime. After an experimental verification of the computational tool adopted for capturing transonic flow characteristics, a series of quasi-three-dimensional (3D) computational analyses were carried out to reveal and understand the cooling jet—OTL flow interaction at various hole locations and inclination angles. The results indicate that the performance rankings between flat tip and squealer tip designs might be altered by the addition of cooling injection. Full 3D conjugate heat transfer analyses demonstrate that partially replacing the squealer cavity with a simple flat shaped configuration in the rear transonic flow portion would offer a much improved coolability without paying extra aerodynamic penalty.
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