Numerical investigation on flow behavior and energy separation in a micro-scale vortex tube

Nader Rahbar, Mohsen Taherian, Mostafa Shateri, Sadegh Valipour
2015 Thermal Science  
There are a few experimental and numerical studies on the behavior of microscale vortex tubes. The intention of this work is to investigate the energy separation phenomenon in a micro-scale vortex tube by using the computational fluid dynamic. The flow is assumed as steady, turbulent, compressible ideal gas, and the shear-stress transport k-ω is used for modeling of turbulence phenomenon. The results show that 3-D computational fluid dynamic simulation is more accurate than 2-D axisymmetric
more » ... -D axisymmetric one. Moreover, optimum cold-mass ratios to maximize the refrigeration-power and isentropic-efficiency are evaluated. The results of static temperature, velocity magnitude, and pressure distributions show that the temperature-separation in the micro-scale vortex tube is a function of kinetic--energy variation and air-expansion in the radial direction. there are some applications for vortex tubes: cooling electronic devices, dehumidifying gas samples, cooling parts of machines, and setting solders [4] . Because of their special characteristics, vortex tubes have been an attractive subject for many scientists and researchers. So many experimental and numerical studies have been done to increase the performance or to better understanding of thermal separation in the vortex tubes. In an experimental study, Saidi and Valipour [5] concluded that the effective parameters of a vortex tube are divided into two different types: geometrical and thermophysical properties. They reported that the cold temperature and efficiency decreases when the number of inlet nozzles increases. In addition, the inlet pressure has a direct effect on the cold air temperature difference; meanwhile, there is an optimum-efficiency at a specific inlet pressure. Gao et al. [6] conducted some experiments to understand the cooling mechanism, also the pressure, temperature, and velocity distributions inside the vortex tubes. They found that rounding off the entrance can improve the performance of a vortex tubes. They also reported the existence of a secondary circulation inside the tube. In an experimental research, Aydin and Baki [7] reported that inlet pressure and cold fraction are the important parameters influencing the performance of the RHVT. These results were also reported in another study by Hamdan et al. [8]. Wu et al. [9] modified inlet nozzles and air passage of a RHVT and experimentally showed that these modifications could remarkably improve the performance of a vortex tube. In another study, Nimbalkar and Muller [10] used energy separation and energy flux separation efficiencies to investigate the characteristic of a vortex tube. They reported that the maximum value of energy separation was always happened at a 60% cold fraction irrespective of orifice diameter and inlet pressure. Valipour and Niazi [11] investigated the influence of main tube uniform curvature on the performance of a vortex tube. They reported that these effects depend on inlet pressure and cold mass ratio. Because of the complex nature of energy separation, it is extremely difficult and expensive to construct an experimental set-up for detailed investigation of the energy separation in vortex tubes. Many researchers tried to use computational fluid dynamics (CFD) to study the separation phenomenon in RHVT. Frohlingsdorf and Unger [12] used a 2-D axisymmetric model to simulate the compressible flow and energy separation with CFX flow solver. They reported that the application of k-ε model leads to substantial differences between measured and calculated tangential velocity profiles. They suggested that, for the calculation of turbulent viscosity, it is possible to replace k-ε model by the correlation from Keyes [13] . Behera et al. [14] used STAR-CD commercial code to study the flow behavior in a RHVT. They used RNG k-ε turbulence model to evaluate the velocity components, flow patterns and the optimum parameters of the vortex tube. Aljuwayhel et al. [15] used Fluent CFD solver to study the flow behavior within a counter-flow vortex tube. They showed that a work transfer due to the viscous-shear between hot and cold streams is the main reason for unique behavior of vortex tubes. They also reported that the choice of turbulence model has a large effect on estimating of the vortex tube's performance. Skye et al. [16] compared the results of RNG k-ε and standard k-ε turbulence models with the results of experimental measurements. They used a 2-D steady axisymmetric model and reported that the standard k-ε (with swirl) has a better prediction of the flow behavior. Eiamsa and Promvonge [4, 17] showed that the results of algebraic stress model (ASM) has a better agreement with experimental data than Standard k-ε model. Ameri and Behnia [18] used 2-D and 3-D RSM turbulence models to investigate the energy separation in a vortex tube. They find an optimum inlet-pressure for maximum efficiency. They also suggested the optimum dimensional values for their vortex tube. Farouk et al. [19, 20] used large eddy simulation (LES) technique to predict the gas flow, temperature
doi:10.2298/tsci120316206r fatcat:riscxnz5nncprdaq2wxdvo3lwm