Characteristics of vortex flow in a low speed air jet impinging onto a heated disk in a vertical cylindrical chamber

An experiment combining flow visualization and transient temperature measurement is carried out to investigate the characteristics of the mixed convective vortex flow resulting from a low speed air jet impinging onto a heated horizontal circular disk confined in a vertical adiabatic cylindrical chamber. Attention is focused on the conditions leading to the onset of the inertia and buoyancy driven vortex rolls and the effects of governing nondimensional groups on the steady and time dependent vortex flow. More specifically, experiments are conducted for the jet Reynolds number varied from 0 to 1082 and Rayleigh number from 0 to 18,790 for two different injection pipes. The results show that typically the steady vortex flow in the processing chamber consists of two inertia-driven and one buoyancy-driven circular vortex rolls. The secondary inertia-driven roll only appears at high jet Reynolds numbers. At low buoyancy-to-inertia ratio Gr/Re_j² the vortex rolls are steady and axisymmetric. But at certain high Gr/Re_j² the vortex flow becomes unstable and the vortex rolls are somewhat deformed. Besides, new vortex rolls can be induced by the additional thermal rising from the heated disk and the splitting of the primary inertia-driven roll. The temporal characteristics of the time periodic vortex flows are examined in detail. In the region dominated by the new rolls the flow oscillates significantly. Finally, empirical equations are proposed to correlate the oscillation frequency of the time periodic flow, and the size and location of the vortex rolls. Furthermore, the conditions for the onset of the buoyancy driven rolls are given. A flow regime map is provided to delineate the temporal state of the vortex flow.     

Unstable vortex flow and new inertia-driven vortex rolls resulting from an air jet impinging onto a confined heated horizontal disk

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 Re_j the inertia-driven tertiary and quaternary rolls can be induced aside from the primary and secondary rolls. At an even higher Re_j 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 Re_j 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.     

A further investigation of effects of jet-disk separation distance on steady mixed convective vortex flow resulting from a confined impinging air jet

We extend our previous study [J.C. Hsieh, T.F. Lin, Effects of jet-to-disk separation distance on the characteristics of mixed convective vortex flow in an impinging air jet confined in a cylindrical chamber, Int. J. Heat Mass Transfer 48 (2005) 511–525] here to further investigate how the jet-disk separation distance H affects the mixed convective vortex flow resulting from a round air jet impinging onto a heated horizontal circular disk confined in a vertical cylindrical chamber. The experiment is conducted for the jet-disk separation distance varying from 40.0 to 60.0 mm and the jet flow rate is varied from 0 to 12.0 slpm (standard liter per minute) for the jet Reynolds number Re_j ranging from 0 to 1623. The temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0 °C for the Rayleigh number Ra ranging from 0 to 507,348. The data from the present study for the ratio H/D_j = 4–6 are compared with our previous study for H/D_j = 1–3. The results indicate that the critical jet Reynolds numbers for the onsets of the secondary and tertiary inertia-driven rolls and for the onset of the buoyancy-driven roll vary nonmonotonically with the jet-disk separation distance due to the complicate changes of the vortex flow structure with H. In the steady vortex flow, both the primary inertia-driven roll and the buoyancy-driven roll get larger at increasing jet-disk separation distance before they contact with each other for H/D_j = 1 and 2. But for H/D_j ≧ 3 the primary roll and buoyancy roll do not always grow at increasing H. Finally, empirical correlations are proposed for the critical conditions leading to the onsets of the inertia- and buoyancy-driven vortex rolls.     

Stationary transverse rolls and U-rolls in limiting low Reynolds number mixed convective air flow near the convective threshold in a horizontal flat duct

Combined experimental flow visualization and temperature measurement are carried out in the present study to explore the buoyancy driven vortex flow patterns in a limiting low Reynolds number mixed convective air flow through a bottom heated horizontal flat duct. In Particular, attention is paid to the flow approaching the natural convection limit (Re=0) for Re=1.0 and 2.0 with the Rayleigh number near the critical level for the onset of convection for 1200?Ra?4000. Results from the flow visualization have revealed two unfamiliar vortex flow patterns which were not seen in our earlier study [Int. J. Heat Mass Transfer 44 (4) (2001) 705]. One is characterized by the stable stationary transverse rolls in the duct entry and stable longitudinal rolls in the downstream. Another is in the form of U-rolls. The relations of these two patterns with those reported in the literature from analytical, numerical and experimental studies are discussed. Moreover, stable longitudinal rolls along with nonperiodic traversing waves, and mixed longitudinal and transverse rolls as well as irregular cells which appear in the higher Reynolds number for 3.0?Re?5.0 are also noted here. The temporal and spatial characteristics of the unfamiliar vortex flows are inspected in detail. In addition, the flow formation processes leading to the two unfamiliar vortex flow structures are also examined carefully. During the flow formation we noted merging of longitudinal and transverse rolls to form U-rolls, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the generation of the longitudinal and transverse rolls. Finally, a flow regime map is provided to delineate various vortex flow structures observed in this study and in the previous study (cf. the above-mentioned reference) driven by the slightly supercritical and subcritical buoyancies for 1.0?Re?5.0.     

Visualization of flow and heat transfer characteristics for swirling impinging jet

Flow and heat transfer characteristics of swirling impinging jet (SIJ) were studied experimentally at constant nozzle-to-plate distance of L = 4D. The swirling jet is generated by inserting twisted tapes within a pipe nozzle. Effects of swirl on the impinged surface are investigated at twist ratios (y/W) of ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52. The flow patterns of the free swirling jet and the swirling impinging jet were visualized by mixing dye with the jet flow. Distributions of temperature and convective heat transfer coefficient on the impinged surface were measured with thermochromic liquid crystal (TLC) sheet and image processing technique. Additionally, an oil film technique was performed as a complementary technique for flow visualization on the impinged surface. The experimental results reveal that there appear to be two peaks of heat transfer in the jet impingement region. The heat transfer enhancements in jet impingement region can be achieved at a low twist ratio of 3.64 which corresponds to the swirl number of 0.4.     

Simulation of turbulent impinging jet into a cylindrical chamber with and without a porous layer at the bottom

Turbulent impinging jets on heated surfaces are widely used in industry to modify local heat transfer coefficients. The addition of a porous substrate covering the surface contributes to a better flow distribution, which favors many engineering applications. Motivated by this, this work shows numerical results for a turbulent impinging jet into a cylindrical enclosure with and without a porous layer at the bottom. The macroscopic time-averaged equations for mass and momentum are obtained based on a concept called double decomposition, which considers spatial deviations and temporal fluctuations of flow properties. Turbulence is handled with a macroscopic k–ε model, which uses the same set of equations for both the fluid layer and the porous matrix. The numerical technique employed is the control volume method in conjunction with a boundary-fitted coordinate system. One unique computational grid is used to compute the entire heterogeneous medium. The SIMPLE algorithm is applied to relax the system of algebraic equations. Results indicate that the permeability of the porous layer and the height of the fluid layer significantly affect the flow pattern. The effect of the porous layer thickness was less pronounced in affecting the flow behavior in the fluid layer.     

Structure of mixed convective longitudinal vortex air flow driven by a heated circular plate embedded in the bottom of a horizontal flat duct

In this study experimental flow visualization combined with transient temperature measurement are conducted to investigate the structure of the buoyancy driven longitudinal vortex rolls in low Reynolds number mixed convective air flow through a horizontal flat duct with an isothermally heated circular disk embedded in the bottom plate of the duct for the Reynolds number ranging from 15.1 to 99.2 and Rayleigh number from 3506 to 29,493. How the circular geometry of the heated surface affects the longitudinal vortex flow characteristics is investigated in detail. The results indicate that the longitudinal vortex rolls (L-rolls) in the duct core are induced at more upstream locations than those near the duct sides, which is completely opposite to those induced in a duct with a uniformly heated bottom. Besides, the thermals driven by the circular heated surface are not evenly spaced in the spanwise direction and are slightly asymmetric. It is of interest to note that at a given Rayleigh number Ra the thermals are unstable at high Reynolds numbers, suggesting the existence of the inertia driven instability. Thus the L-rolls evolved from these thermals are also unstable with the presence of nonperiodic generation and disappearance of new L-rolls. But at slightly lower Re the thermals and L-rolls are steady and regular. The vortex flow becomes unstable and irregular for a further reduction in the Reynolds number, which obviously results from the buoyancy driven instability. The simultaneous presence of these two instability mechanisms explains the appearance of the reverse steady-unsteady transition in the vortex flow.Based on the present data, a flow regime map is given to delineate various L-roll patterns driven by the circular heated plate. In addition, the boundaries separating these patterns are empirically correlated. Empirical correlations for the onset points of the L-rolls are also provided.     

Spin-up and Spin-Down Times of Rotational Air Flow Depended on Mean Air Velocity and Surface Roughness on Inner Surface in Cylindrical Vortex Chamber

The spin-up time TA in the spin-up process and spin-down time TD in the spin-down process of rotational air flow in the cylindrical vortex chamber with three kinds of the artificial surface roughness were investigated. One of the most different characteristics between the smooth and rough surface conditions was the spin-down time TD. Also the transient response of the tangential velocity on the spin-up process was analyzed numerically and this result was compared with the experimental results. The above stated results are reported here in detail.     

Heat transfer between an impinging air jet and an impinged water surface

Impingement of producer gas obtained from rice-husk gasification onto a water surface has been found to be very effective in removing tar and ash from the gas. Correlations to estimate tar- and dust-removal efficiencies have been developed. An advantage of impingement is that the hot gas jet is cooled. Here, we describe experiments to study heat transfer when heated ambient air impinges on the water surface. An equation to estimate the overall heat-transfer coefficient has been derived. This coefficient is expressed in terms of Nu and is correlated with the following dimensionless groups: v_a/v_w, H_n/D_n, D_c/D_n, and Re. The correlation obtained by minimizing the absolute error is Nu=0.0148 (v_a/v_w)^−0.16 × (H_n/D_n)^−1.3 (D_c/D_n)^−1/2 Re^1.2.     

Turbulent flow in the near field of a round impinging jet

This paper reports on detailed measurements of the turbulent flow in the stagnation region of a single impinging jet issuing from a round pipe with diameter D and a length of 76D. The distance between the pipe exit and the flat impingement plate is 2D, and the Reynolds number (based on the bulk velocity and pipe diameter) is 2.3 × 10⁴. Mean velocity components and Reynolds stresses were determined by using a two-component LDA. A modified one-component LDA was used to perform near-wall measurements with minimum wall distances of approximately 40 μm. PIV measurements were taken in a small field of view (approximately 4 × 5 mm²) to study instantaneous reversals in the near wall region.     

Heat transfer from an impinging premixed butane/air slot flame jet

Experiments were performed to study the heat transfer characteristics of a premixed butane/air slot flame jet impinging normally on a horizontal rectangular plate. The effects of Reynolds number and the nozzle-to-plate distance on heat transfer were examined. The Reynolds number varied from 800 to 1700, while the nozzle-to-plate distance ranged from 2d_e to 12d_e. Comparisons were made between the heat transfer characteristics of slot jets and circular jets under the same experimental conditions. It was found that the slot flame jet produces more uniform heat flux profile and larger averaged heat fluxes than the circular flame jet.     

过冷沸腾汽泡顶部射流可视化实验研究

通过商速摄像井配合激光PIV技术，对过冷沸腾中微小汽泡顶部射流进行细致现察，确认过冷沸腾中顶部射流的存在，测量显示其速度场，具体分析讨论此类射流与汽泡生长的联系。     

Natural convective oscillatory flow in cylindrical annuli

Detailed quantitative information is presented concerning the characteristics of a natural convective oscillatory flow in horizontal, cylindrical annuli. This unusual, three-dimensional oscillatory flow is described by giving amplitude, period, and wave length data as well as data showing when this type of flow has its inception from a previously stable flow at lower Grashof numbers. Visualization of the air flow was achieved by the use of tobacco smoke. Annulus pressure was varied from 13.8 to 275.8 kN/m² while the temperature difference between the cylinders was varied from 2.8 to 55.5 deg C yielding a Grashof number (based on inner-cylinder diameter) range from 290 to 2.7 × 10⁶. Correlation equations are given to facilitate data presentation and to allow estimation of the inception of this oscillatory flow and its subsequent amplitude, period, and wave length.     

Heat transfer and fluid flow characteristics in a swirling impinging jet

An experimental study on heat transfer and fluid flow has been carried out for a swirling round impinging jet. A thermosensitive liquid crystal sheet was used for the heat transfer measurements and the three velocity components were measured with LDV in the stagnation region for cases where the Swirl number Sw = 0.0, 0.22, and 0.45 at the Reynolds number Re = 8100. The formation of recirculation flow due to a swirl near the impinging wall was found to deteriorate the heat transfer coefficient in the stagnation region and results in a more uniform distribution of the Nusselt number with an increasing Swirl number. The heat transfer mechanism of the swirling impinging jet is discussed based on the flow characteristics of the mean velocities and turbulence quantities. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 324–335, 2005; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20068     

Buoyancy driven vortex flow and its stability in mixed convection of air through a blocked horizontal flat duct heated from below

Experimental flow visualization combined with transient temperature measurement are carried out here to explore the possible stabilization of the buoyancy drive vortex flow in mixed convection of air in a bottom heated horizontal flat duct by placing a rectangular solid block on the duct bottom. Two acrylic blocks having dimensions 40 × 20 × 5 mm³ (block A) and 40 × 20 × 10 mm³ (block B) are tested. The blocks are placed on the longitudinal centerline of the duct bottom at selected locations. How the location and orientation of the rectangular block affect the stability of the regular vortex flow is investigated in detail. Experiments are conducted for the Reynolds number varying from 3 to 30 and Rayleigh number from 3000 to 6000, covering a wide range of the buoyancy-to-inertia ratio. For longitudinal vortex flow, the presence of either block near the duct entry causes the onset points of the longitudinal rolls to move significantly upstream especially for the roll pair directly behind the block. Besides, the longitudinal vortex flow in the exit portion of the duct is destabilized by the block. The transverse vortex flow is found to be only slightly affected by the block when it is placed in the exit half of the duct. Significant deformation of the transverse rolls is noted as they pass over the block. However, they restore to their regular shape in a short distance. Substantial decay in the transient flow oscillation results in the region right behind the block. Elsewhere the flow oscillates at nearly the same frequency and amplitude as that in the unblocked duct. When the block is placed near the duct entry, stabilization of the vortex flow behind the block is more pronounced. This flow stabilization is more prominent for block B with its height being twice of block A. Placing the block with its long sides normal to the forced flow direction can also enhance the flow stabilization. For the mixed longitudinal and transverse vortex flow, placing the block near the duct inlet causes the transverse rolls to change to regular or deformed longitudinal rolls in the duct depending on the buoyancy-to-inertia ratio and orientation of the block. The flow stabilization by the block is substantial. Again the stabilization of the mixed vortex flow can be enhanced by increasing the block height and length and by placing the block with its long sides normal to the forced flow direction.     

The LDA study of flow gas-dynamics in a vortex chamber

The experimental data on the turbulent structure of swirling flow in vortex chamber with diaphragm are presented. The data were made with the help of LDA method. The evolution of turbulence intensity in the vortex chamber is shown. The influence of initial conditions of the swirling flow forming on the turbulent properties are considered.     

A Study of Under-expanded Moist Air Jet Impinging on a Flat Plate

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is under-expanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with non-equilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics are not known satisfactorily. In the present study, the effect of the non-equilibrium condensation on the under-expanded air jet impinging on a vertical flat plate is investigated numerically in the case with non-equilibrium condensation, frequency of oscillation for the flow field becomes larger than that without the non-equilibrium condensation, and amplitudes of static pressure become small compared with those of dry air. Furthermore, the numerical results are compared with experimental ones.     

A study of the gas flow through air jet loom

Air jet loom,as one of the shuttleless looms,transports a yarn into warps using viscosity and kinetic energy of anair jet.Performance of this picking system depends on the ability of instantaneous inhalation/exhaust,configura-tion of nozzle,operation characteristics of a check valve,etc.In the recent past,many studies have been reportedon the air jet discharged from a nozzle exit,but studies for understanding the flow field characteristics associatedwith shear layer and shock wave/boundary layer interaction in the nozzle were not conducted enough.In this pa-per,a computational study was performed to explain the flow field in the air jet nozzle with an acceleration tubeand validated with previous experimental data available.The results obtained from the computational study showthat,in the supersonic flow regime,the flow field depends significantly on the length of acceleration tube.Asnozzle pressure ratio increases,drag force acting on the string also increases.For a longer acceleration tube,thetotal pressure loss is large,owing to the frictional loss.