Laser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit condit...Laser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 × 104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = Vj/Uo of 22.5, and an inter-jet spacing of S = 6D. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that is most relevant for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.展开更多
Local heat transfer is predicted in turbulent axisymmetric jets, impinging onto a flat plate. A non-linear k-e model is used, in which both the constitutive law for the turbulent stresses and the transport equation fo...Local heat transfer is predicted in turbulent axisymmetric jets, impinging onto a flat plate. A non-linear k-e model is used, in which both the constitutive law for the turbulent stresses and the transport equation for the turbulent dissipation rate e have an important contribution in the improved heat transfer predictions. The shape of the Nusselt number profiles, expressing dimensionless heat transfer, as well as the stagnation point value, are well predicted for different distances between the nozzle exit and the plate. Accurate flow field predictions are the basis for good heat transfer predictions. For a fixed Reynolds number, the influence of the nozzle-plate distance is well captured. For a fixed distance, the influence of the Reynolds number is correctly reproduced. Comparisons are made to a low-Reynolds standard k-e model and the v2-f model. A thorough discussion is found in [4]. Only a summary of those results is discussed here, while some new results are also presented.展开更多
An active cooling solution based on close-looped micro impinging jet is proposed for high power light emitting diodes (LEDs). In this system, a micro pump is utilized to enable the fluid circulation, impinging jet is ...An active cooling solution based on close-looped micro impinging jet is proposed for high power light emitting diodes (LEDs). In this system, a micro pump is utilized to enable the fluid circulation, impinging jet is used for heat exchange between LED chips and the present system. To check the feasibility of the present cooling system, the preliminary experiments are conducted without the intention of parameter optimization on micro jet device and other system components. The experiment results demonstrate that the present cooling system can achieve good cooling effect. For a 16.4 W input power, the surface temperature of 2 by 2 LED array is just 44.2°C after 10 min operation, much lower than 112.2°C, which is measured without any active cooling techniques at the same input power. Experimental results also show that increase in the flow rate of micro pump will greatly enhance the heat transfer efficiency, however, it will increase power consumption. Therefore, it should have a trade-off between the flow rate and the power consumption. To find a suitable numerical model for next step parameter optimization, numerical simulation on the above experiment system is also conducted in this paper. The comparison between numerical and experiment results is presented. For two by two chip array, when the input power is 4 W, the surface average temperature achieved by a steady numerical simulation is 34°C, which is close to the value of 32.8°C obtained by surface experiment test. The simulation results also demonstrate that the micro jet device in the present cooling system needs parameter optimization.展开更多
为了研究液体火箭发动机中撞击式喷嘴的雾化问题,采用CLSVOF(Coupled Level-Set and Volume-of-Fluid Method)方法对撞击式喷嘴的雾化过程进行了数值仿真模拟,重点考察了撞击角对撞击式喷嘴雾化特性的影响。结果表明,通过CLSVOF方法能...为了研究液体火箭发动机中撞击式喷嘴的雾化问题,采用CLSVOF(Coupled Level-Set and Volume-of-Fluid Method)方法对撞击式喷嘴的雾化过程进行了数值仿真模拟,重点考察了撞击角对撞击式喷嘴雾化特性的影响。结果表明,通过CLSVOF方法能对射流撞击雾化形态进行较好的捕捉;分析整个雾化区域的液相分布,将射流撞击雾化过程中的液相分布大致分为三个区域:撞击前两股独立射流区域、撞击后形成的液膜区域、液膜破碎后液丝和液滴的生成区域;射流撞击雾化除了气液界面上速度差引起的不稳定之外,还存在由射流湍流或撞击波引起的其它不稳定因素;液膜破碎长度随撞击角的增大而减小,液膜表面上的表面波幅值及液膜破碎程度则随撞击角的增大而增大;射流撞击在撞击点位置处所形成的速度差对液膜的破碎和液膜上表面波幅值的大小起到了关键作用。展开更多
文摘Laser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 × 104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = Vj/Uo of 22.5, and an inter-jet spacing of S = 6D. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that is most relevant for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.
文摘Local heat transfer is predicted in turbulent axisymmetric jets, impinging onto a flat plate. A non-linear k-e model is used, in which both the constitutive law for the turbulent stresses and the transport equation for the turbulent dissipation rate e have an important contribution in the improved heat transfer predictions. The shape of the Nusselt number profiles, expressing dimensionless heat transfer, as well as the stagnation point value, are well predicted for different distances between the nozzle exit and the plate. Accurate flow field predictions are the basis for good heat transfer predictions. For a fixed Reynolds number, the influence of the nozzle-plate distance is well captured. For a fixed distance, the influence of the Reynolds number is correctly reproduced. Comparisons are made to a low-Reynolds standard k-e model and the v2-f model. A thorough discussion is found in [4]. Only a summary of those results is discussed here, while some new results are also presented.
基金Supported by the Key Technology R&D Program of Hubei Province, China (Grant No. 2006AA103A04)
文摘An active cooling solution based on close-looped micro impinging jet is proposed for high power light emitting diodes (LEDs). In this system, a micro pump is utilized to enable the fluid circulation, impinging jet is used for heat exchange between LED chips and the present system. To check the feasibility of the present cooling system, the preliminary experiments are conducted without the intention of parameter optimization on micro jet device and other system components. The experiment results demonstrate that the present cooling system can achieve good cooling effect. For a 16.4 W input power, the surface temperature of 2 by 2 LED array is just 44.2°C after 10 min operation, much lower than 112.2°C, which is measured without any active cooling techniques at the same input power. Experimental results also show that increase in the flow rate of micro pump will greatly enhance the heat transfer efficiency, however, it will increase power consumption. Therefore, it should have a trade-off between the flow rate and the power consumption. To find a suitable numerical model for next step parameter optimization, numerical simulation on the above experiment system is also conducted in this paper. The comparison between numerical and experiment results is presented. For two by two chip array, when the input power is 4 W, the surface average temperature achieved by a steady numerical simulation is 34°C, which is close to the value of 32.8°C obtained by surface experiment test. The simulation results also demonstrate that the micro jet device in the present cooling system needs parameter optimization.
文摘为了研究液体火箭发动机中撞击式喷嘴的雾化问题,采用CLSVOF(Coupled Level-Set and Volume-of-Fluid Method)方法对撞击式喷嘴的雾化过程进行了数值仿真模拟,重点考察了撞击角对撞击式喷嘴雾化特性的影响。结果表明,通过CLSVOF方法能对射流撞击雾化形态进行较好的捕捉;分析整个雾化区域的液相分布,将射流撞击雾化过程中的液相分布大致分为三个区域:撞击前两股独立射流区域、撞击后形成的液膜区域、液膜破碎后液丝和液滴的生成区域;射流撞击雾化除了气液界面上速度差引起的不稳定之外,还存在由射流湍流或撞击波引起的其它不稳定因素;液膜破碎长度随撞击角的增大而减小,液膜表面上的表面波幅值及液膜破碎程度则随撞击角的增大而增大;射流撞击在撞击点位置处所形成的速度差对液膜的破碎和液膜上表面波幅值的大小起到了关键作用。
