Characteristics of the recirculation zone in a cylindrical confined space and its suppression by an annular orifice plate

The accumulation of pollutants in the recirculation zone can worsen ventilation.It is critical to reduce recirculation zones to improve the ventilation efficiency of buildings.However,the variation rule of the recirculation zone in a cylindrical confined space(CCS)is unclear,and there are few solutions to suppress or eliminate the recirculation zone at present.In this paper,an annular deflector orifice plate for suppressing the recirculation zone was developed based on the structural characteristics of the CCS.This device is simple in structure and can be used flexibly.Through experiments and numerical simulations,the variation rule of the recirculation zone length and the influence of structural parameters of the device on the vortex suppression were explored.Firstly,empirical formulas for calculating the length of the recirculation zone in the CCS were obtained.In addition,it was proved that placing the annular orifice plate inside the CCS effectively reduced the recirculation zone and improved the ventilation efficiency.Compared to the system without the annular orifice plate,the dimensionless length of the recirculation zone was decreased by 76.3%,and the time to completely discharge the pollutants from the CCS was decreased by 16.7%.Finally,parameters of the annular orifice plate that form the best vortex suppression effect were proposed:the porosity range was 40%–50%,uniform in shape with equal ring spacing,and placed more than one inlet diameter away from the inlet.The results help guide the ventilation design of CCS.

Understanding of tip clearance flow structure in high speed mixed flow compressor

This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet total pressure ratio of 3.8 with 39,836 rpm rotor speed.The four different tip configurations namely the constant(l Z 0.016 and 0.019)and variable(l Z 0.011(inlet)-0.019(exit)and 0.019(inlet)-0.022(exit))tip clearances were numerically analysed using available experimental data-set.The numerical investigation reveals that in contrast to the classic jet-wake pattern,two anomalous velocity profiles formed at the impeller exit which results in pressure losses in the vaneless diffuser.Near the impeller inlet,the tip leakage flow rolls up to discrete tip leakage vortex structure for each tip clearance configuration.This results in the formation of a region of momentum deficit,recirculation zone,which gets weakened as it moves downstream.The tip clearance configuration is observed to profoundly influence the extent and vorticity of the tip leakage vortex.In the splitter blade passage,the tip leakage flow and Coriolis flow interact with passage flow,resulting in the formation of two secondary passage vortices that move downstream along the pressure and suction surface of the splitter blade.The tip clearance configuration directly influences the impeller exit jetwake pattern by modulating the secondary passage vortices trajectory and vorticity.Moreover,off-design analysis for tip clearances l Z 0.016 and l Z 0.019,depict distinctive tip leakage vortex characteristics.When operating near the stall conditions(80%of design mass flow rate),l Z 0.019 exhibits bubble shape tip leakage vortex breakdown occurring near the impeller inlet.This result in a substantial change in the tip leakage vortex nature;expansion of the recirculation zone and early weakening of the vorticity in the tip leakage vortex.It is observed that vortex breakdown plays a vital role in characteristics of the passage flow field structure and compressor performance near the stall conditions.

Experimental Investigations of the Flow Field Structure and Interactions between Sectors of a Double-Swirl Low-Emission Combustor:Effects of Main Stage Swirl Intensity and Venturi Angle

Effect of venturi angle and main stage swirl intensity on flow field and interactions were investigated using PIV.The results showed the difference between the side sector and the middle sector was caused by interactions.The interactions were stronger with the rise of the main stage swirl intensity.When the swirl intensity was 0.7 and 0.9,there was little difference of the width of the recirculation zone.But the flow field had a great difference when the swirl intensity was 0.5 and 0.7,which means that when the swirl intensity was small,the swirl intensity had a great influence on the flow field.Venturi angle had a great influence on the flow field structure and interactions when the venturi angle was big,such as 90°.The venturi angle just had a small influence on the width of the recirculation zone when the venturi angle was relatively small,such as 28°and 52°.The velocity of the center plane between two sectors(plane 3)was small.There was a recirculation zone at upstream of the center plane between two sectors(plane 3)when the swirl intensity was 0.7 and 0.9,whereas not one when the swirl intensity was 0.5.The above was induced by interactions.In addition,the velocity of plane 1,plane 3,and plane 5 when the venturi angle was 52°was smaller than that of 28°and 90°.

Experimental Investigations of the Flow Field Structure and Interactions between Sectors of a Double-Swirl Low-Emission Combustor

In this paper,the flow field characteristics of a double-swirl low-emission combustor were analyzed by using Particle Imaging Velocimetry(PIV)technology in an optical three-sector combustor test rig.The interactions between sectors and the flow field structure were explained.The results illustrated that there was a big difference between the flow field structures of the middle sector and the side sector under the same pressure drop,which was mainly induced by the interactions between sectors.The interactions made the swirl intensity of the middle sector weaker than that of the side sector,which made the recirculation zone of the middle sector be smaller than that of the side sector.With the increase of swirler pressure drop,the jet velocity at the exit of the swirler,the jet expansion angle,the width of the recirculation zone and the recirculating speed of the central axis became larger,enhancing the interactions between air streams from middle sector and side sector.The flow velocity in the central plane between sectors was small,especially the radial velocity,mainly because of the loss of the swirl intensity by the interactions between flow field of adjacent sectors.The expansion angle determined the position of the vortex in the primary recirculation zone;the axial and radial position of the vortex move downstream and radial outward with the increase of the jet expansion angle.The results of the mechanism of flow field organization in this study can be used to support the design of new low-emission combustor.

斜爆轰解耦引起的波复合体不稳定性

斜爆轰波(ODW)在冲压发动机中的应用已得到广泛的研究,但对于其在受限空间中的不稳定性研究较少.最近,观察到由ODW解耦引起的波复合体,本研究考察了具有回流区的波复合体的不稳定性.结果表明,具有回流区的波复合体是振荡的,但振荡是周期性的,而不是随机的,这是以前从未报道过的.本文详细分析了周期和频率与拐角位置和角度的关系,以及回流区核心的不规则运动轨迹.通过快速傅里叶变换进一步定量分析,揭示了两个不稳定性的来源:一个是与周围波复合体运动相关的回流区的基本振荡;另一个是由回流区内部的自激发导致,这使其运动轨迹比外部波结构更复杂。

The Impact of Key Factors on Flow Characteristics of 14 MW Reverse-Swirl Pulverized Coal Burner

Reverse-swirl(RS)burner which has been industrialized couples reverses jet and swirl flow for the stabilization of flame.Using Dantec multichannel constant-temperature anemometer,experiments on airflow characteristics were implemented on a 1:2 scaled burner model with different values in terms of reverse primary air(RPA)ratio and swirl inner secondary air(SISA)ratio.It was found that the shape of annular coupled recirculation zone(ACRZ)had stayed symmetrical all the time.The RPA ratio was the main factor that had an impact on the values of axial and RMS velocity as well as the radial velocity direction of ACRZ.Both RPA ratio and SISA ratio had a great impact on the area of ACRZ,relative reverse flow rate,mixing between SISA and outer secondary air(OSA)as well as swirling ability of the airflow.The area of ACRZ reached its peak when the RPA ratio was 11.92%or SISA ratio was 17.03%;however,when the RPA ratio and SISA ratio reached 14.86%and 28.41%respectively,the combination of RPA and SISA became relatively favorable;besides,ACRZ area,relative reverse flow and swirling ability became suitable and the mixing between SISA and OSA was relatively delayed.The research was of great practical and theoretical importance to the design and operation of RS burner.

Cold Gas-particle Flows in a New Swirl Pulverized-coal Burner by PDPA Measurement

A new type of swirl burner has been developed to stabilize pulverized\|coal combustion by burning different types of coal at different loads and to reduce NO x formation during combustion. The burner uses a device to concentrate the coal powder in the primary\|air tube that divides the primary coal\|air into two streams with different pulverized\|coal concentrations. This paper reports the measurement of gas\|particle flows at the exit of the different swirl burners using a 3\|D Phase Doppler Particle Anemometer (PDPA). The effect of different geometrical configurations on the two\|phase flow field is studied. The results that give the two\|phase flow fields and particle concentrations show the superiority of the new swirl burner.