Modeling and Experimental Investigation of the Hydraulic Jump in Liquid Film Formed by an Impinging Two-Phase Air-Water Jet

A theoretical analysis and experimental investigation of the phenomenon of impingement of a circular liquid aerosol jet on a plate have been carried out. In the case considered, the spreading of liquid on a plate is caused by inertia and air drag force; for supercritical film flow in some conditions, a hydraulic jump is formed. The circular hydraulic jump exhibits behavior quite different than commonly observed in planar jumps. A new theoretical model of the phenomena of hydraulic jump has been formulated. The experiments exhibit that the liquid layer formed by an aerosol experiences a hydraulic jump at a location more downstream than the film created by a single-phase liquid jet. Theoretical results were compared with our experimental data, and the results seem to be satisfied.     

高温差下卷吸作用对冲击射流换热影响数值研究

基于V2F湍流模型计算研究了卷吸作用对高温差下圆管冲击射流换热的影响,首先通过计算结果与实验值的对比验证模型方法的有效性,然后分析了基于绝热壁面温度计算的努赛尔数和射流有效度随射流和环境的温差以及雷诺数的变化,并研究了取不同定性温度对计算结果的影响。计算结果表明,高温差下定性温度取为射流温度时,基于绝热壁面温度计算的努赛尔数与射流和环境之间的温差近似无关,有效度也与雷诺数无关,但有效度随射流和环境的温差变化较大。因此,在温差较低时,依据射流和环境温度相同时的换热工况得到射流和环境温度不同时的换热工况是可行的,但温差越大,由该方法带来的误差也越大。     

Three‐dimensional turbulent forced convection around a hot cubic block exposed to a cross‐flow and an impinging jet

A numerical study of a three‐dimensional, turbulent, forced convection flow around a hot cubic block exposed to cross‐flow and an impinging jet is carried out. The standard k‐ε turbulence model is used to study the effects of Reynolds number ratio on the flow and heat transfer. For each value of the Reynolds number of the jet, the Reynolds number ratio is equal to 1, 1.5, and 2. The influence of the channel height and the jet axis location are also examined. The governing equations are solved by using Ansys Fluent software 14.5. Results show that the heat transfer increases with the increase in the Reynolds number ratio. At the top of the cube, better cooling occurs with an increase in the speed of the impinging jet. A reduction in the height of the channel and the displacement of the axis of the jet toward the channel inlet improve the heat transfer. Our simulations are compared with experimental data found in the literature, using different turbulence models.     

A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

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.     

Rewetting of Vertical Hot Surface during Round Water Jet Impingement Cooling

A stainless steel vertical surface of 0.25 mm thickness at 800 ± 10° C initial temperature was quenched by jet impingement technique. The rewetting phenomenon of the surface was investigated for the jet of 2.5 – 4.8 mm diameter and jet Reynolds number of 5000–24000. The observations are made from the stagnation point to the 24 mm downstream spatial locations, for both upside and downside directions. The quenching performance of the test surface was evaluated on the basis of different rewetting parameters i.e. rewetting temperature, wetting delay, and rewetting velocity. It has been observed that with the rise in jet Reynolds number and jet diameter, the surface rewetting performance increases. A correlation has also been proposed to determine the dimensionless rewetting velocity that predicts the experimental data within an error band of ±20 percent.     

Numerical study of flow and thermal oscillations in buoyant twin jets

Non-isothermal twin parallel jets in horizontal orientation, are studied numerically to ascertain the mean flow structure and the oscillation characteristics of temperature and velocity fields. The analysis is carried out for Reynolds number ranging between 9000 and 12,000 and Grashof number between 50 and 1000. The simulated results compare well with available experimental data on axial velocity distribution and jet merger distance. Subsequently, a parametric study is presented to bring out the effects of Reynolds number, nozzle spacing and jet inlet temperature.     

Numerical Analysis of Jet Impingement Heat Transfer at High Jet Reynolds Number and Large Temperature Difference

Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer was investigated at a jet Reynolds number of 1.66 × 10⁵ and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up to about 100% were observed. Furthermore, the variation in stagnation point heat transfer was examined for jet Reynolds numbers in the range from 1.10 × 10⁵ to 6.64 × 10⁵. Based on the investigations, a correlation is suggested between the stagnation point Nusselt number, the jet Reynolds number, and the turbulence intensity at the jet inlet for impinging jet flows at high jet Reynolds numbers.     

受激液体射流的非轴对称模式

利用液体射流破碎的线性不稳定性理论，研究了受激液体射流特有的“局部频段非轴对称模式占优”现象，由一系列理论计算结果，得到了Re-We平面上的相应临界曲线，并根据这些曲线得出了受激液体射流出现非轴对称模式结构特征的物理条件。计算结果还表明，受激液体射流和自由液体射流的临界曲线有明显不同。初步的实验观察结果证实了理论的预测。     

Turbulent time scales in a nonpremixed turbulent jet flame by using high-repetition rate thermometry

High-repetition-rate (10-kHz), single-point laser Rayleigh scattering thermometry was used to measure temperature and thermal dissipation rate fluctuations in a turbulent nonpremixed jet flame at a Reynolds number of 15,200. The measurements were made along the jet-flame centerline for downstream locations in the vicinity of the flame tip and were aimed at obtaining one-dimensional (1D) energy and dissipation spectra and estimates of the integral and Batchelor scales. The signal-to-noise ratios of the measured correlations, energy spectra, and dissipation spectra were significantly improved by using two detectors to measure redundant scattering signals from the same point, followed by computing the cross-correlation of these signals. Measurements of the integral scales show good agreement with measurements in isothermal jets except near the stoichiometric flame length, where the flame scales are smaller. These smaller scales are attributed to the state relationship between temperature and mixture fraction. The cutoff frequency of the dissipation range (i.e., the Batchelor frequency) is determined from the 1D thermal dissipation spectra. The experimentally determined cutoff frequencies agree with those estimated using nonreacting jet scaling laws, provided an appropriate local Reynolds number is used that is significantly smaller than the jet exit Reynolds number. The resulting 1D energy and dissipation spectra at all downstream stations collapse when properly normalized and fit well to model spectra at a Taylor Reynolds number near 55. This Reynolds number is considerably smaller than the corresponding nonreacting jet value of 150. The measured energy and dissipation spectra exhibit significant overlap at intermediate frequencies, which indicates that the largest scales of the dissipation range may be significantly influenced by the large, energy containing scales. The time-series data are also used to determine the influence of the probe resolution on the temperature variance and mean 1D thermal dissipation rate. It is shown that the resolution requirements are similar to those in nonreacting jet flows, provided the difference in the local Reynolds number is considered.     

An experimental investigation of a block moving back and forth on a heat plate under a slot jet

This experimental study aims to investigate heat transfer phenomena caused by a moving block under a jet flow. The experimental apparatus includes three main systems of a jet flow, periodical movement and heating control. The work fluid is air and the data runs are performed for jet Reynolds numbers and speeds of moving block. The comparison between experimental and numerical results show good consistence, and the destruction of boundary layers discovered by previous authors’ numerical results is validated. The enhancement of heat transfer rate is generally accompanied with the increment of jet Reynolds number and speed of moving block.     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

液体射流结构特征的理论分析

本文利用线性不稳定性理论，在较大的韦伯数（Ｗｅ）和雷诺数（Ｒｅ）范围内对液体射流的结构特征进行了理论分析，研究结果表明：在一定韦伯数和雷诺数条件下，射流的高阶模式将成为最不稳定模式，射流的结构也将相应表现出该模式的特征，雷诺数减小对各阶模式有不同程度的抑制作用；圣零阶模式抑制较大，对高阶模式抑制较小，这是导致高阶模式成为最不稳定模式的原因；在小韦伯数和大雷诺数情况下，各阶模式几乎同样不稳定，这时射     

Effects of Mach number and Reynolds number on jet array impingement heat transfer

Experimental results from the present study show substantial, independent Mach number effects (as the Reynolds number is held constant) for an array of impinging jets. The present discharge coefficients, local and spatially averaged Nusselt numbers, and local and spatially averaged recovery factors are unique because (i) these data are obtained at constant Reynolds number as the Mach number is varied, and at constant Mach number as the Reynolds number is varied, and (ii) data are given for jet impingement Mach numbers up to 0.74, and for Reynolds numbers up to 60,000. As such, results are given for experimental conditions not previously examined, which are outside the range of applicability of existing correlations.     

Convective mass transfer from a horizontal rotating cylinder in a slot air jet flow

The effects of air jet impinging on the mass transfer characteristics from a rotating spinning cylinder surface were experimentally investigated. The effects of rotational Reynolds number, jet-exit Reynolds number , the nozzle width-to-cylinder diameter ratio , and the ratio of the distance between nozzle exit and the front of cylinder to nozzle width on the mean were determined. The phenomena of the first and second critical point was analyzed and validated. On the basis of experimental data, the correlation equation was obtained.     

Impingement heat transfer of staggered arrays of air jets confined in a channel

Impingement heat transfer of circular air jets confined in a channel was experimentally investigated. The impingement plate was exerted with a constant surface heat flux. Five jets, including one center jet and four neighboring jets, in staggered arrays were considered. The considered jet Reynolds number (Re) was in the range 5000–15,000; the jet height-to-jet diameter ratio (H/d) was in the range 1.0–4.0; the jet spacing-to-jet diameter ratio (S/d) was in the range 4.0–8.0; the jet plate width-to-jet diameter ratio (W/d) was in the range 6.25–18.75. Two jet plate length-to-jet diameter ratio (L/d), 31.7 and 83.3, were individually arranged. For the center jet with a specific Reynolds number, its stagnation Nusselt number was found to linearly increase with the jet Reynolds number of the four neighboring jets. For all the five jets with the same Reynolds number, the correlation result shows that the stagnation Nusselt number of the center jet is proportional to the 0.7 power of the Re and the −0.49 power of the W/d. A weak dependence of the stagnation Nusselt number on H/d, S/d and L/d was found.     

Experimental and Numerical Study on Heat Transfer with Impinging Circular Jet on a Convex Hemispherical Surface

In this paper, the experimental and numerical study has been carried out to investigate the water jet impingement on a convex hemispherical surface. The pressure and skin friction coefficient distributions on the impingement surface were analyzed numerically. A great deal of attention was paid to analyze the effects of the jet impingement exit velocity and the nozzle-to-surface distance on the heat transfer characteristics. A comparison of the heat transfer coefficient was performed between the jet impingement on the convex surface and the flat plate. The results show that the Nusselt number for the convex surface is higher than that for the flat surface. The local Nusselt number is decreased monotonically from its maximum value at the stagnation point for lower Reynolds numbers. A secondary maxima occurs for higher Reynolds numbers. The experiment and simulation were performed with the following parameters: the jet impingement Reynolds number of Re = 1947–19478, and the nozzle-to-surface distance of L/D = 2.5–25.     

Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

Study of heat transfer for a pair of rectangular jets impinging on an inclined surface

Critical design parameters in jet impingement heat transfer like nozzle hydraulic diameter, jet angle and velocity, physical properties of the fluid, and nozzle-to-target plane spacing are the subject. This paper identifies the dominant fluid-thermal characteristics of a pair of rectangular air jets impinging on an inclined surface. Heat transfer modes and flow characteristics are studied with eight different Reynolds numbers ranging from 500 to 20 000. Local and average Nusselt numbers are evaluated with two different boundary conditions on three specified lines located on the inclined surface. The correlation between stagnation Nusselt number and Reynolds number is presented. Turbulent intensity and wall y⁺ distributions are compared on three lines parallel to the incline. The effect of jet impingement angle on local and average Nusselt number is also documented. Finally, a correlation between the average Nusselt number, nozzle exit Reynolds number and the jet angle is documented.     

Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

Role of entrainment in the stabilisation of jet-in-hot-coflow flames

The aim of the research on the Delft jet-in-hot coflow (DJHC) burner is to gain understanding in the interplay of turbulence and chemistry in conditions as encountered in devices operating in flameless combustion mode, and to test the validity of numerical models when applied to these flameless combustion conditions. Datasets on velocities, temperatures and qualitative OH data of several Dutch natural gas flames in the DJHC burner have been obtained and are discussed in this paper. It was found that the mean velocity and turbulent stresses are not significantly affected by the chemical reactions, which is in line with the very moderate increase of mean temperatures in the flames. Even at heights where flame structures are present, peak temperatures do not always approach the adiabatic flame temperature. With both flame luminescence and OH-PLIF measurements, it is seen that chemical reactions begin to occur at a lower location when the jet velocity (and thereby the jet Reynolds number) is increased. By analysing the velocity and temperature data in the near-nozzle region, the entrainment of coflow fluid into the turbulent jet has been quantified. The increased entrainment of the higher Reynolds number jet, in combination with the positive temperature gradient in radial direction in the near field of the jet, is shown to be responsible for the decrease of the height where reactions start to occur.