Reverse Flow Boundary Research Articles

Experimental study on the influence of inlet pressure on the flow parameters at the reverse flow boundary of vortex tube and its relation with energy separation performance

The performance of the vortex tube (also known as the Ranque-Hilsch tube) is significantly influenced by the inlet pressure and the internal flow processes, which can be categorized into two distinct flows: the center and the outer flow. The reverse flow boundary, acting as the interface between the two flows, plays a crucial role in energy separation. However, the research on the location of the reverse flow boundary and the associated flow parameters and energy transfer at this boundary are still insufficient. Therefore, this research employs the measurement method of interpolation probe to analyze the distribution of the reverse flow boundary and the specific flow parameters at this boundary. These parameters include static pressure gradient, angular velocity, and static temperature gradient, which substantially impact energy separation. A qualitative analysis of the energy transfer process in the vortex tube by examining mass flow in the reverse area and tangential and axial velocities are further delved. This analysis covers momentum transfer, heat transfer, and turbulent heat transfer processes resulting from the compression and expansion processes. The findings provide a research direction for exploring the energy separation performance of vortex tubes.

Analysis of the flow mixing and energy separation in a Ranque–Hilsch vortex tube based on an area ratio study

Ranque-Hilsch vortex tube is a pressure-driven thermal device that is an essential component of waste pressure utilization, especially in gas industry. However, the lack of an effective optimization design method and relative low efficiency restrict its development. In this study, the flow field and energy separation of five area ratios (AR) from 0.02–0.32 with the same inlet areas were analyzed based on the AR study of a vortex tube through experiments and numerical simulation. To determine the correlation of area ratio and energy separation, the basic flow field distributions are obtained and the flow structure is analyzed. The results show that for the fifth tube, denoted by AR5, the inlet mass flowrate remains largely constant (min = 9.27–9.24 g/s) with extremely small changes in the back pressure at hot exit (Ph = 11.2–18 kPa); in contrast, for AR1, the back pressure at hot exit (Ph = 10–148 kPa) and inlet mass flowrate (min = 6.8–4.26 g/s) vary significantly with increasing cold mass fraction. Moreover, despite using the same moderate cold orifice ratio of 0.48 for all the five tubes, back flow can occur at the cold exit even at an extremely large cold mass fraction of 0.82 for AR5. In addition, various flow structures and mixing and their corresponding causes inside the vortex tube are summarized. The comparison of the flow and thermal behaviors under the five ARs reveals opposite working patterns of reverse flow boundary and off-optimization under the non-flow mixing condition. These conflicting working patterns of reverse flow boundary can be attributed to the multi-parameter interaction of vortex tube. Thus, the criteria for tube optimization may unilaterally fail unless the precession frequency is studied. Finally, a performance evaluation index that quantifies the energy separation performance is proposed. The index serves to improve the tube optimization criteria and further develop the optimization of vortex tubes.

Numerical study on the cooling performance and inlet mass flow rate per unit area of Ranque–Hilsch vortex tubes with different area ratios

The development of the Ranque-Hilsch vortex tube has stagnated since the 1950s, when Hilsch improved the vortex tube proposed by Ranque. Consequently, the performance of vortex tubes has remained constant for the past 70 years. In addition to the unclear flow structure and energy separation mechanism of vortex tubes, the lack of a mature design method for vortex tubes has also limited its widespread adoption for industrial applications. The inlet nozzle and main tube parameters have been studied extensively, and matching the inlet with the main tube is considered an important step in the design of a vortex tube; however, this process has not been studied extensively. In addition, although the temperature drop at the cold exit is a commonly adopted performance benchmark, it is not possible to establish a close relationship between this parameter and the geometric parameters of a vortex tube. This study investigates the relationship between the cooling performance and the area ratio of a vortex tube. The results indicate that the inlet mass flowrate per unit area has a positive correlation with the cooling performance, and its influence on the performance of a vortex tube is higher than that of the distribution of the reverse flow boundary. Therefore, the inlet mass flowrate per unit area can potentially be a geometric performance benchmark. However, further studies are required to develop better guidelines for optimizing the design and to obtain a deeper understanding of the energy separation mechanism of vortex tubes.

Simple model for flow field division and flow structure calculation in a vortex tube

The flow pattern and energy transfer process in the vortex tube are extremely complex, resulting in the difficulty of the direct calculation and prediction of the flow structure and temperature separation performance of a vortex tube. In view of this, research idea of dividing the vortex tube flow field into six regions was proposed, corresponding simplifications were made in different regions according to the flow features, and fluid parameters were coupled at the boundaries. An equation for the axial velocity component in the hot tube region was developed based on the characteristic that the axial velocity profiles under different conditions present similar patterns, the calculation method of reverse flow boundary which separates the working fluid into hot and cold fluids was described, and a set of relatively simple calculation methods for the internal flow field of vortex tubes was established based on the partition model. Three-dimensional velocity distributions inside the vortex tube under different working conditions were computed and analyzed. Further, the model was evaluated and validated through experimental data. The results showed good agreements of calculated data and experimental velocity fields. This work proposed a convenient calculation method for the estimation of the flow behavior inside a vortex tube and provide reliable predictions.

LES study on the working mechanism of large-scale precessing vortices and energy separation process of Ranque-Hilsch vortex tube

In this paper, a large eddy simulation model (LES) was adopted to analyse the large-scale precessing vortices and energy separation in a Ranque–Hilsch vortex tube. By comparing the time-averaged axial velocity field between the Laser Doppler velocimetry data and the numerical results, the feasibility of the LES model was confirmed. The 2D instant flow field presents two vortex structures of secondary circulation flow at the cold end, and precessing vortices inside the tube. It also reveals two opposite flows in the axial and radial directions, and the precessing vortices formed by the reverse flow turn towards the main flow. Then, the 3D coherent structure inside the vortex tube was obtained by the vortex identification method for the first time. The basic vortex structure inside the vortex tube consists of the vortex body, detached vortex, and fragmentised detached vortex. Through the evolution of the 3D vortex structure, it was found that the detached vortices experienced a life cycle of growth, shedding, fragmenting, and vanishing. Benefitting from the vortex shedding processes, the precessing vortices overcame the adverse radial pressure gradient and drove part of the reverse flow towards the main flow from the low-pressure zone to the high-pressure zone. Through these processes, the vortex core precession that drives the reciprocating motion of the gas parcels near the reverse flow boundary was established, and energy could be transferred from the inner layer to the outer layer through a heat pump cycle.

Hydrodynamic Lubricant Film Separation During Codirectional and Counter-Directional Rotations of Disengaged Wet Clutch Packs

Abstract The parasitic drag losses incurred by wet clutches, used in transmission systems, can significantly affect vehicular powertrain efficiency. This paper presents a novel implicit solution for hydrodynamic parasitic drag losses of disengaged clutches. These are generated by conjunctional friction, taking into account lubricant film separation during codirectional and counter-directional disk pair rotations. Lubricant film rupture is considered through application of incipient reverse flow boundary condition, which is representative of lubricant film separation. The results point to the operating conditions at which significant power losses occur. In particular, the time efficient model is able to represent the small losses incurred during codirectional rotation of disk pairs.

Numerical study of bluntness effects on laminar leading edge separation in hypersonic flow

Bluntness effects on laminar hypersonic leading edge separation are investigated numerically at Mach number $M\approx 10$, unit Reynolds number $Re=1.3\times 10^{6}~\text{m}^{-1}$, specific enthalpy $h_{o}=3.1~\text{MJ}~\text{kg}^{-1}$ and wall-to-stagnation temperature ratio $T_{w}/T_{o}=0.1$. Such effects are important from an experimental point of view and because bluntness can affect a separated flow favourably or adversely. In this study, two blunt leading edge cases of small radius ($15~\unicode[STIX]{x03BC}\text{m}$) and large radius ($100~\unicode[STIX]{x03BC}\text{m}$) are investigated. A comparison with the idealised sharp leading edge case is also given. General flow features and surface parameters such as the shear stress, pressure and heat flux are presented and analysed. The results have also been interpreted in terms of Cheng’s displacement-bluntness similitude parameter affecting the size of separation. Previous experiments by Holden delineated small and large bluntness effects based on Cheng’s parameter and considering small to moderate separated regions. In this study, leading edge separation was found to suppress the favourable effect of bluntness in delaying separation. Bluntness, furthermore, seemed to promote the appearance of secondary vortices within a main separated region. Analysis of reverse flow boundary layer profiles such as velocity, pressure and temperature is also given. It is shown that bluntness accentuates large transverse gradients. This in turn adversely effects the reverse flow boundary layer leading to the appearance of secondary vortices.

Experimental study of the impacts of cold mass fraction on internal parameters of a vortex tube

In order to get the internal parameters of a vortex tube, a large-scale vortex tube was designed and an experimental device was built. A five-hole probe and thermocouples were used to obtain the three-dimensional velocities, the static pressure, static temperature and total temperature distributions inside the vortex tube. Four different cold mass fraction conditions (0.2, 0.4, 0.6 and 0.8) were chosen and the impacts on the internal parameters of the vortex tube were discussed. Different from the traditional view, the tangential velocity was considered to be the steady Burgers vortex form. A reverse flow boundary was found, and the location of which was changed at different operation conditions and axial positions. Further, it was found that the lowest static temperature existed near the nozzle outlet, and a new static temperature difference distribution law was firstly proposed experimentally.

Experimental investigation of flow structure and energy separation of Ranque–Hilsch vortex tube with LDV measurement

In this study, transparent vortex tubes of diameter 30 mm were designed for the main tube visualisation and a 2D laser Doppler velocimetry (LDV) measurement was adopted to investigate the mean flow field on the meridian plane. The distributions of the axial and radial velocities, and particularly the reverse flow boundary, were obtained for understanding the characteristics of large-scale vortex structures. Various operation conditions with different cold mass fractions (0.1–0.9), tube lengths (360 mm, 600 mm, and 900 mm), and inlet pressures (0.1 bar and 0.2 bar) were studied to reveal how the reverse flow boundary affects the energy separation performance. The LDV results show that the mean axial velocity distribution present good axial symmetry, and increasing the cold mass fraction results in an expansion of the reverse flow boundary as reported for previous numerical calculations. A deterioration in the energy separation performance due to an excessively short or long tube was revealed, and the proposed tube optimization criteria were proven.

Analysis of the unsteady heat and mass transfer processes in a Ranque–Hilsch vortex tube: Tube optimization criteria

Although numerous experiments and numerical calculations have been carried out in order to study, with different parameters, the relationship between the flow structure and energy separation performance in a vortex tube, there has been less progress on its design optimization, which is due to the lack of a systematic analysis of the flow structure inside the vortex tube. According to the evidence, the large-scale vortex structures resulting from the vortex breakdown and vortex core precession determine the heat and mass transfer processes and flow structure in the vortex tube, and they are sensitive to the operation conditions. To study the vortex structure, attention can be paid to the shape of the reverse flow boundary and the location of the stagnation point, because they are the bond between the hot and cold streams. Based on the understanding of the energy separation mechanism and different flow structures inside a vortex tube, this study proposed a flow structure inside the main tube that bears the best energy separation performance as the optimization criteria for the main tube design. Based on a basic description of the counter flow structure in a confined slender tube, a preliminary quantitative tube design procedure can be realized to achieve the key parameters of the main tube, such as length, diameter, and diameter of the cold exit.

A Improved Particle Swarm Optimization Algorithm for the Application with Boundary-Layer Problems of Power-Law Fluids

In fluid mechanics, how to solve power-law fluids in ordinary differential equations is always a concerned and difficult problem. we use generally a shooting method to tackle the boundary-layer problems under a suction/injection as well as a reverse flow boundary conditions. A improved particle swarm optimization algorithm (ISPO) is proposed for solving the parameter estimation problems of the multiple solutions in fluid mechanics. This algorithm has improved greatly in precision and the success rate. In this paper, multiple solutions can be found through changing accuracy and search coverage and multi-iterations of computer. Parameter estimation problems of the multiple solutions of ordinary differential equations are calculated, and the result has great accuracy and this method is practical.

The Lie-group shooting method for boundary-layer problems with suction/injection/reverse flow conditions for power-law fluids

For power-law fluids we propose a Lie-group shooting method to tackle the boundary-layer problems under a suction/injection as well as a reverse flow boundary conditions. The Crocco transformation is employed to reduce the third-order equation to a second-order ordinary differential equation, and then through a symmetric extension to a boundary value problem with constant boundary conditions, which can be solved numerically by the Lie-group shooting method. However, the resulting equation is singular, which might be difficult to solve, and we propose a technique to overcome the initial impulse caused by the singularity using a very small time-step integration at the first few time steps. Because we can express the missing initial condition through a closed-form formula in terms of the weighting factor r∈(0,1), the Lie-group shooting method is very effective for searching the multiple-solutions of a reverse flow boundary condition.

Investigation of the separation bubble formed behind the sharp leading edge of a flat plate at incidence

Detailed measurements of the separation bubble formed behind the sharp leading edge of a flat plate at low speeds and incidence are reported. The Reynolds number based on chord length ranged from 0.1 × 105 to 5.5 × 105. Extensive use of laser Doppler anemometry allowed detailed velocity measurements throughout the bubble. The particular advantages of laser Doppler anemometry in this application were its ability to define flow direction without ambiguity and its non-intrusiveness. It allowed the mean reattachment point to be accurately determined. The static pressure distribution along the plate was also measured. The length of the separation bubble was primarily determined by the plate incidence, although small variations occurred with Reynolds number because of its influence on the rate of entrainment and growth of the shear layer. Above about 105, the Reynolds number effect was no longer evident. The reverse flow boundary layer in the bubble exhibited signs of periodic stabilization before separating close to the leading edge, forming a small secondary bubble rotating in the opposite sense to the main bubble.

An experimental study of the effect of the blow-in rate on the size of the flow separation region

Experimental results are presented concerning the effect of the air blow-in rate on the size of the flow separation region downstream of a rearward-facing step at different step heights. The stream function ϕ is found from the experimental velocity profiles, the streamline ϕ = 0 being taken as the boundary line. It is shown that the separation region increases as the blow-in rate is increased. Generalization of the experimental results for different blow-in rates and step heights has made it possible to obtain an analytical expression describing the location of some characteristic lines in the separation region (boundary streamline, reverse-flow boundary, line of reverse-flow maximum velocity, line of variable-velocity layer thickness, and the displacement thickness line). Velocity profiles are obtained by means of a hot-wire anemometer. Analytical expressions are written as polynomials with unknown coefficients. The results obtained may be helpful in developing techniques for the treatment of heat transfer under flow separation conditions.

Study of Drift in an Asymmetric Linear Hydraulic Motor

It is well known that in a hydraulic servomechanism the ram exhibits in the open-loop a linear motion in addition to its usual oscillatory motion for a sinusoidal input. This phenomenon, called drift, gives rise to an apparent instability of the system and the study of the dynamic behaviour becomes difficult. A theoretical investigation into the causes and remedy of this phenomenon is made on a loaded system with an asymmetric linear hydraulic motor controlled by an open-centre three-way spool valve. The original non-linear system equation in dimensionless form is derived taking into account the effects of leakage, compressibility (both fixed and varying) and area ratio, and solved numerically by the fourth-order Runge–Kutta method. It is shown that the determination of the reverse flow boundary is essential prior to the study of drift, since the rate of drift will be maximum near the boundary frequency. It is also shown that while the effect of compressibility due to the change in initial volume is mainly responsible for drift for an area ratio of 1 : 2, the effect of area ratio itself is predominant for an area ratio not equal to 1 : 2. It is observed that in most of the cases the drift can be removed by giving a proper input shift to the valve. A method of calculating the input shift required is suggested and the effects of different system parameters on its magnitude are studied.