Article

Vortices evolution in round pockets of modern machine tools

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Abstract

Vortices evolutions in oil pockets of machine tools were investigated by using a particle image velocimetry (PIV) system and numerical simulations. The influences of Reynolds number (Re), aspect ratio of pocket depth‐to‐nozzle radius (e), and clearance height (h) on the vortices characteristics were studied. Up to three interacting vortices appear, namely, the primary vortex, the secondary vortex, and the tertiary vortex. The vortices evolve in a complex manner. The primary vortex appears at 15 < Re < 1610 and dominates the whole pocket flow at relatively high Re. The secondary vortex appears at Re > 210. The vortices size increases with e increasing. Moreover, the vortices detachment and reattachment positions were also been investigated. The vortices structures have significant influences on the distributions of pressures and wall shear stresses on the upper disc. The results further the understanding of vortices evolutions in oil pockets and will guide engineering applications related to the round pocket flow.

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... The presence of recirculation zones (RZs) can cause entrapment of material and may compromise hygienic manufacture. For example, regions of recirculating fluid have been observed in industrially relevant flows, including oil pockets used in the hydrostatic supporting system of ultrahigh precision computer numerical control (CNC) machines (Shen et al., 2019), and are known to exist within valve housings (Jensen and Friis, 2005) and T-pieces . It is therefore essential to understand the origins and the influence of RZs on the observed cleaning behaviour (Nadge and Govardhan, 2014). ...
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Studies on the effect of wall shear stress on soil and biofilm attachment and removal from a surface are one of the many applications of radial axisymmetrical flow. The particular nature of this flow allows taking advantage of a wide range of wall shear stress applied at the analyzed surface in a single experiment. This type of experiments provides a critical radius up to which soil removal occurs. Good models are, however, still needed to convert the experimental data into critical wall shear stress. Analytical models are already available for creeping flow but Computational Fluid Dynamics must be applied for experiments performed at higher Reynolds numbers. The present study is a numerical analysis of the radial axisymmetrical flow for aspect ratios of 0.125, 0.25, 0.5 and 1 with inlet pipe Reynolds numbers varying from 0 to 2000, aiming at computing the wall shear stress distribution at any distance from the center. The simulations provided a thorough description of the complex flow pattern encountered close to the inlet section, which were validated for the laminar regime by dye injection. A total of up to four recirculation zones were identified in both numerical and experimental investigations. The experimental positions of these recirculation zones corresponded well to the numerical predictions. Based on this work, a map of the flow for the different aspect ratios was developed, which can be particularly interesting for the design of experimental devices involving axisymmetrical flow.
Article
Convective heat transfer from a moving isothermal hot plate due to confined slot-jet impingement is investigated numerically. Two-dimensional turbulent flow is considered. The rectangular flow geometry consists of a confining adiabatic wall placed parallel to the moving impingement surface with the slot-jet located in the middle of the confining wall. The k − ε turbulence model with enhanced wall treatment is used for the turbulence computations. The problem parameters are the jet exit Reynolds number, ranging from 5000 to 20,000, the normalized plate velocity, ranging from 0 to 2, and the normalized distance of separation between the impingement plate and the jet exit, ranging from 6 to 8. The computed flow patterns and isotherms for various combinations of these parameters are analysed to qualitatively understand the effect of the plate motion on the heat transfer phenomena. The distribution of the local and average Nusselt numbers and the skin friction coefficients at the hot moving surface for above combinations of the flow parameters are presented. Results are compared against corresponding cases for heat transfer from a stationary plate. The analysis reveals that the average Nusselt number increases considerably with the jet exit Reynolds number as well as with the plate velocity. The average skin friction coefficient, on the other hand, is relatively insensitive to the Reynolds number but increases significantly with the plate velocity.
Article
A computational study of the impingement of a thermally inhomogeneous turbulent jet on a solid plate, using large-eddy simulation, is reported. We investigate the case of a plane jet of water issuing from a plane channel into an enclosed pool and impinging normally on a perspex plate 1.8 jet-widths downstream. It is shown that the dynamics of the turbulence in this particular geometry results in the temperature variations at the plate surface having very high lateral correlation, so that lateral conduction of heat within the plate fails to have any significant effect on the transmission of thermal fluctuations from the fluid into the plate. By this means a simple one-dimensional model of the thermal, interaction between the media may be justified.
Article
The influence of natural convection on the local and average heat transfer at increasing temperature differences between the jet and the target plate from confined impinging jets has been experimentally and numerically investigated. Local Nusselt numbers were obtained numerically for jet Reynolds numbers in the range of 250–1000, and jet to target spacings of 2, 4, 8, 12 jet diameters at various modified Grashof numbers, to determine the effect of buoyancy induced natural convection. To determine the overall effect of natural convection on the average heat transfer, experiments have been conducted for Re numbers in the range 250–5000 and dimensionless jet to target spacing 2, 4, 6, 8, and 12 at increasing modified Grashof numbers. It has been determined that buoyancy induced natural convection might have opposing or assisting influence on local heat transfer at different locations of the target plate. It has also been shown that especially at low jet inlet velocities the average heat transfer coefficient at the highest modified Grashof number, where the natural convection is effective, is higher than the value corresponding to the lowest Grashof number at which buoyancy effects are negligible, by as much as 37%.
Article
An experiment combining flow visualization and temperature measurement is carried out here to investigate the possible presence of new inertia-driven vortex rolls and some unique characteristics of the time-dependent mixed convective vortex flow in a high-speed round air jet impinging onto a heated horizontal circular disk confined in a vertical cylindrical chamber. How the jet Reynolds and Rayleigh numbers and jet-to-disk separation distance affect the unique vortex flow characteristics is examined in detail. Specifically, the experiment is conducted for the jet Reynolds number varying from 0 to 1623 and Rayleigh number from 0 to 63,420 for the jet-to-disk separation distance fixed at 10.0, 20.0 and 30.0 mm. The results indicate that at sufficiently high Rej the inertia-driven tertiary and quaternary rolls can be induced aside from the primary and secondary rolls. At an even higher Rej the vortex flow becomes unstable due to the inertia-driven flow instability. Only for H = 20.0 mm the flow is also subjected to the buoyancy-driven instability for the ranges of the parameters covered here. Because of the simultaneous presence of the inertia- and buoyancy-driven flow instabilities, a reverse flow transition can take place in the chamber with H = 20.0 mm. At the large H of 30.0 mm the flow unsteadiness results from the mutual pushing and squeezing of the inertia- and buoyancy-driven rolls since they are relatively large and contact with each other. It is also noted that the critical Rej for the onset of unsteady flow increases with ΔT for H = 10.0 and 20.0 mm. But for H = 30.0 mm the opposite is true and raising ΔT can destabilize the vortex flow. Based on the present data, flow regime maps delineating the temporal state of the flow are provided and correlating equations for the boundaries separating various flow regimes are proposed.
Article
Experimental data for the rate of heat transfer from impinging turbulent jets with nozzle exit Reynolds numbers in the range of 5,000–124,000 have been collated and critically reviewed from the considerable body of literature available on the subject. The geometry considered is that of a single circular jet impinging orthogonally onto a plane surface for nozzle-to-plate distances from 1.2–16 nozzle diameters and over a flow region up to six nozzle diameters from the stagnation point. Existing correlations for local heat transfer coefficient express Nusselt number as a function of nozzle exit Reynolds number raised to a constant exponent. However, the available empirical data suggest that this exponent should be a function of nozzle-to-plate spacing and of the radial displacement from the stagnation point. A correlation for Nusselt number of the form suggested by this evidence has been derived using a selection of the data. The review also suggests that the Nusselt number is independent of nozzle-to-plate spacing up to a value of 12 nozzle diameters at radii greater than six nozzle diameters from the stagnation point. The results from a simple extrapolation for obtaining heat transfer coefficients in the wall jet region compare favourably with published data.
Article
A forced radially outward flow with secondary, buoyancy induced convection has been studied numerically in an axisymmetric geometry, consisting of two differentially heated, horizontal, coaxial, circular plates with a diameter of 25 times their mutual spacing. A forced laminar flow is supplied through the centre of the upper plate. The onset of thermal instability, leading to axisymmetric and three-dimensional rolls, has been determined as a function of the Reynolds, Prandtl and Rayleigh numbers.
Article
A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.
Article
In the present paper, flow and heat transfer characteristics of confined impinging slot jets have been numerically investigated using a SIMPLE-based segregated streamline upwind Petrov–Galerkin finite element method. For laminar jets, it is shown that the skin friction coefficient approaches the grid-independent Galerkin solution and that the present simulation induces negligible false diffusion in the flow field. For turbulent jets, the k–ω turbulence model is adopted. The streamwise mean velocity and the heat transfer coefficient respectively agree very well with existing experimental data within limited ranges of parameters.
Article
The flow field of an axisymmetric, confined and submerged turbulent jet impinging normally on a flat plate was studied experimentally using laser-Doppler velocimetry. Single jets of a perfluorinated dielectric liquid (FC-77) issuing from square edged, geometrically similar nozzles were used in the experiments with the radial outflow confined between parallel plates. The nozzle (length to diameter) aspect ratio was unity, giving rise to a still-developing velocity profile at the nozzle exit. Experiments were conducted with nozzle diameters of 3.18 and 6.35 mm, nozzle-to-target plate spacings of up to four jet diameters, and Reynolds numbers in the range of 8500 to 23000. The toroidal recirculation pattern in the outflow region characteristic of confined jets is mapped. Velocities and turbulence levels are presented over a fine measurement grid in the pre-impingement and wall-jet regions.
Article
Premixed methane-air mixture is introduced at the center of two parallel circular quartz plates separated by a millimeter scale distance (<or= 5mm). Two plates are heated with an external heater to create a positive temperature gradient condition along the flow direction. The combustion characteristics of lean methane-air mixtures are investigated at phi=0.67 and phi=0.85 in the laminar flow regime (Re approximately 500-1800). Contrary to the general perception of a stable premixed flame front at a radial location, a large variety of unstable and dynamic flame front patterns are observed at phi=0.67. At phi=0.85, stable flame propagation mode dominated the regime diagram and unstable flame propagation modes are observed under limited conditions. In these unstable flame propagation modes, single and multiple flame fronts rotate around the center at a rate of 15-50 Hz.
Mixed convection in radial flow between horizontal plates‐I. Numerical simulations
  • HV Santen
  • CR Kleijn
  • HEA Akker