A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Annular synthetic jet used for impinging flow mass-transfer

An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble.     

Wall shear rates and mass transfer in impinging jets: Comparison of circular convergent and cross-shaped orifice nozzles

This article presents a study on the wall shear rate and mass transfer of impinging jets on a flat plate. The performance of a cross-shaped orifice nozzle was compared with a reference convergent circular nozzle having similar equivalent diameter. An array of electrodiffusion micro probes inserted into the plate was used for wall shear rates measurements. Mass transfer in the impinging region was calculated from the measured wall shear rates for a Reynolds number around 5500 and over a range of streamwise distances between the nozzle and the impinging plane within 1 to 5 nozzle equivalent diameters. The obtained Sherwood number of the reference convergent nozzle is close to the one given by Chin and Tsang (1978) [5]. The most important observation in the present investigation is that the wall shear rates and the mass transfer in the impingement region of the cross-shaped orifice nozzle are up to 175% and 40%, respectively, higher than that of the convergent nozzle. The performance of the cross-shaped orifice jet is probably related to its particular vortex dynamics characteristic of the near exit region. All the results confirm that the jet passive control enhance the mass transfer.     

Axisymmetric impinging jet excited by a synthetic jet system

A controlled impinging jet is a promising tool for various heat/mass transfer applications, such as drying technologies or cooling of highly loaded electronic devices or gas turbine blades. An axisymmetric air jet was excited using a system of four synthetic jets distributed around the circumference of the primary nozzle. First, the control synthetic jets were measured alone. After an adjustment, the primary axisymmetric jet was excited to the helical or bifurcating modes, and its behavior was studied experimentally including an impingement effect to the wall. For comparison purposes, a reference steady (unforced) jet from the same nozzle was also measured. The flow visualization, hot-wire anemometry, PIV, and naphthalene sublimation techniques were used. The main purpose was to investigate the influence of the actuation on the impingement heat transfer at the Reynolds numbers 1600 and 5000.The effects of the Strouhal number and nozzle-to-wall spacing on a distribution of the local heat transfer were evaluated. The most significant effects were found at the Strouhal numbers 0.14–0.32 at the ratio of the control to primary jet momentum rates only 0.24–2.4%. Under small nozzle-to-wall spacing H/D = 2, the excitation led to heat transfer increase in the stagnation area – the most prominent enhancement 40% was found at the stagnation point. Under moderate nozzle-to-wall spacing H/D = 6, the excitation made more uniform the Nusselt number distribution by means of a substantial reduction of the stagnation heat transfer rate.     

Jet impingement onto a conical cavity: Effects of annular nozzle outer angle and jet velocity on heat transfer and skin friction

Jet impingement onto a conical cavity results in complicated flow structure in the region of the cavity. Depending on the nozzle geometric configurations and jet velocities, enhancement in the heat transfer rates from the cavity surface is possible. In the present study, annular nozzle and jet impingement onto a conical cavity are considered and heat transfer rates from the cavity surfaces are examined for various jet velocities, two outer angles of the annular nozzle, and two cavity depths. A numerical scheme adopting the control volume approach is used to simulate the flow situation and predict the heat transfer rates. It is found that increasing jet velocity at the nozzle exit modifies the flow structure in the cavity while altering the heat transfer rates and skin friction; in which case, increasing nozzle outer angle and jet velocity enhances the heat transfer rates and skin friction.     

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.     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.     

Numerical simulation of flow and heat transfer from slot jets impinging on a cylindrical convex surface

Flow and heat transfer characteristics of slot jets impingement to a cylindrical convex surface are numerically investigated.Suitable turbulence models have been determined through comparison with the experimental data.Flow structures are described and impingement heat transfer characteristics are discussed.The effects of Re,H/B and D/B on single-slot jets impingement heat transfer are analyzed and heat transfer characteristics of multiple-slot jets are investigated.The results show that:Gas flows along the convex surface and boundary layer separation occurs in both single and multiple-slot jets impingement.A maximum stagnation Nu appears at H/B=8 and the local Nu decreases with increasing H/B in the region far away from the stagnation.The Nu in the stagnation region decreases with increasing D/B but the Nu is nearly the same in the region far away from the stagnation.Pressure gradient is an important factor on heat transfer enhancement.Correlations of the Num for single-slot,double-slot and quadric-slot jets impinging on a convex surface are obtained.It indicates the effects of Re and D/B on Num could become more important in less slot jets impingement.     

Effects of jet pattern on single-phase cooling performance of hybrid micro-channel/micro-circular-jet-impingement thermal management scheme

This study explores the single-phase cooling performance of a hybrid cooling module in which a series of micro-jets deposit coolant into each channel of a micro-channel heat sink. This creates symmetrical flow in each micro-channel, and the coolant is expelled through both ends of the micro-channel. Three micro-jet patterns are examined, decreasing-jet-size (relative to center of channel), equal-jet-size and increasing-jet-size. The performance of each pattern is examined experimentally and numerically using HFE 7100 as working fluid. Indirect refrigeration cooling is used to reduce the coolant’s temperature in order to produce low wall temperatures during high-flux heat dissipation. A single heat transfer coefficient correlation is found equally effective at correlating experimental data for all three jet patterns. Three-dimensional numerical simulation using the standard k–ε model shows excellent accuracy in predicting wall temperatures. Numerical results show the hybrid cooling module involves complex interactions of impinging jets and micro-channel flow. Increasing the coolant’s flow rate strengthens the contribution of jet impingement to the overall cooling performance, and decreases wall temperature. However, this advantage is realized at the expense of greater wall temperature gradients. The decreasing-jet-size pattern yields the highest convective heat transfer coefficients and lowest wall temperatures, while the equal-jet-size pattern provides the greatest uniformity in wall temperature. The increasing-jet-size pattern produces complex flow patterns and greater wall temperature gradients, which are caused by blockage of spent fluid flow due to the impingement from larger jets near the channel outlets.     

Streamwise distribution of the recovery factor and the local heat transfer coefficient to an impinging circular air jet

An investigation of the radial distribution of the recovery factor and the local heat transfer for an axisymmetric impinging air jet formed by a smooth nozzle is described. The recovery factor is dependent on the jet nozzle to impingement plate spacing, but is independent of jet Reynolds number. The maximum stagnation region heat transfer occurs at a nozzle to impingement plate spacing of about eight jet diameters. A correlation is obtained for the average heat transfer from the surface.     

Impinging premixed butane/air circular laminar flame jet--influence of impingement plate on heat transfer characteristics

Experimental studies were performed to study the heat transfer characteristics of an impingement flame jet system consisting of a premixed butane/air circular flame jet impinging vertically upward upon a horizontal rectangular plate at laminar flow condition. There were two impingement plates manufactured with brass and stainless steel respectively used in the present study. The integrated effects of Reynolds number and equivalence ratio of the air/fuel jet, and distance between the nozzle and the plate (i.e. nozzle-to-plate distance) on heat transfer characteristics of the flame jet system had been investigated. The influence in using impingement plate with different thermal conductivities, surface emissivities and roughnesses on heat flux received by the plate was examined via comparison, which had not been reported in previous literatures. A higher resistance to heat transfer had been encountered when the stainless steel impingement plate of lower thermal conductivity was used, which led to a significantly lower heat flux at the stagnation region. However, the heat flux distribution in the wall-jet region of the plate was only slightly affected by using different impingement plates. Because of the significantly lower heat transfer, more fuel was not required to consume and existed at the stagnation region of the stainless steel impingement plate, which would be burned latter in the wall-jet region to release its chemical energy and enhance the local heat flux there.     

A Numerical Investigation of Flow and Heat Transfer of Laminar Multiple Slot Jets Impinging on Multiple Protruding Heat Sources

Abstract

The flow and heat transfer behavior of laminar incompressible slot jets impingement cooling of an array of heated surfaces in a channel have been investigated numerically. The computations are done for a variety of values of slot jets Reynolds number, channel height and distance between two heated blocks. The influences of these geometrical and physical parameters are predicted. The results, streamline contour, velocity profile, isothermal contour, local Nusselt number, and average Nusselt number are compared and documented. The first and second recirculation cells size are gradually increased, and the highest heat transfer rate is attained when Reynolds number increased. However, the heat transfer rates are decreased when channel height increased. The peak local Nusselt number value is noticed at stagnation point of the first block by first jet, and the second peak local Nusselt number value is observed at fourth block by second jet. The distances between two blocks play a significant role in the downstream velocity which leads to create the strong recirculation cells in between the two heated blocks when the distance between the two blocks increased.     

Numerical simulation on the effect of jet nozzle position on impingement cooling of gas turbine blade leading edge

In this paper, Computational Fluid Dynamics (CFD) simulations are performed to investigate the impingement cooling on internal leading edge region which is stretched by the middle cross section of the first stage rotor blade of GE-E³ engine high pressure gas turbine. The simulations are carried out for a blade with a single row of circle jets at five different positions and seven different inlet flow Mach numbers. The results indicate that the global area weighted average Nusselt number at the blade leading edge increases with the increase of jet Mach number, and increases with the decrease of the distance between the jet nozzle and the pressure side. The correlation for the area weighted average Nusselt number as a function of the parameters is derived for the range of the parameters considered. The streamwise length weighted average Nusselt number and the spanwise length weighted average Nusselt number also increase with the decrease of the spacing between the jet nozzle and the pressure side, and increase with the increase of jet Mach number. The side entry jet is desirable to improve the performance of impingement cooling on turbine leading edge, but the arrangement of the jet nozzle and the shape of the internal cooling passage should be further optimized to improve the distribution of the heat transfer coefficient.     

Flow characteristics and heat transfer performances of a semi-confined impinging array of jets: effect of nozzle geometry

The flow and heat transfer characteristics of confined jet array impingement with crossflow is investigated. Discrete impingement pressure measurements are used to obtain the jet orifice discharge flow coefficient. Digital particle image velocimetry (DPIV) and flow visualization are used to determine the flow characteristics. Two thermal boundary conditions at the impinging surface are presented: an isothermal surface, and a uniform heat flux, where thermocouple and thermochromic liquid crystal methods were used, respectively, to determine the local heat transfer coefficient. Two nozzle geometries are studied, circular and cusped ellipse. Based on the interaction with the jet impingement at the surface, the crossflow is shown to influence the heat transfer results. The two thermal boundary conditions differ in overall heat transfer correlation with the jet Reynolds number. Detailed velocity data show that the flow development from the cusped ellipse nozzle affects the wall region flow more than the circular nozzle, as influenced by the crossflow interactions. The overall heat transfer for the uniform heat flux boundary condition is found to increase for the cusped ellipse orifice.     

Heat transfer due to double laminar slot jets impingement onto an isothermal wall within one side closed long duct

In this study, heat transfer due to double impinging vertical slot jets onto an isothermal wall was investigated numerically for laminar flow regime. Navier–Stokes and energy equations were discretized with a finite volume procedure on a non-staggered grid arrangement using SIMPLEM (SIMPLE-Modified) algorithm. The effect of the jet Reynolds number, the jet-isothermal bottom wall spacing, and the distance between two jets on heat transfer and flow field was examined. Air was chosen as the working fluid (Pr = 0.71). It is found that multi-cellular flow is formed in the impingement region due to interaction between two jets and entrainment effects in the duct. The mean Nusselt number increases almost linearly with increasing of Reynolds number at isothermal surface. When Reynolds number of the first jet is higher than second one the heat transfer is enhanced significantly.     

Local heat/mass transfer and flow characteristics of array impinging jets with effusion holes ejecting spent air

The present study investigates the effects of spent air flows with and without effusion holes on heat/mass transfer on a target plate for array impinging jets. For a conventional type of array impinging jets without effusion holes, the spent air of the injected jets forms a cross-flow within the confined space and affects significantly the downstream jet flow. The injection plate of array impinging jets is modified having effusion holes to prevent the cross-flow of the spent air where the spent air is discharged through the effusion holes after impingement on the target plate. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate using a heat and mass transfer analogy. The flow patterns of the array impinging jets are calculated numerically and compared for the cases without and with the effusion holes. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to the strong effects of cross-flow and re-entrainments of spent air. However, uniform distributions and enhancements of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the effect of cross-flow is weak and the distributions and levels of heat/mass transfer coefficients are similar for both cases.     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

Aerodynamic and mass transfer characteristics of an annular bistable impinging jet with a fluidic flip-flop control

An annular nozzle has been designed on the basis of fluidic principles. The nozzle forms actively controlled air jet. Numerical and experimental investigations were performed in several subsequent steps, namely numerical simulation (using a commercial CFD code FLUENT), geometry adaptation, models manufacturing, flow visualization, hot-wire measurement, and mass transfer (naphthalene sublimation) experiment. A “jet switching” possibility has been discussed, an undesirable hysteresis effect has been suppressed. Present collaborative numerical and experimental investigations have resulted in a better understanding of mechanisms involved in controlled impinging jets, as well as in a further improvement of the particular nozzle geometry.     

Patterns of Fluid Flow in a Shell-and-Tube Heat Exchanger

The oil-lampblack technique was employed to reveal the patterns of fluid flow in a shell-and-tube heat exchanger. Flow patterns were visualized adjacent to the shell wall, to the baffle plates, and to each tube of the array. From the flow patterns adjacent to the shell, three recirculation zones were identified. The patterns adjacent to the baffles were similar to those for cross flow over a tube bank, with curvature-related deviations near the shell. The flow patterns adjacent to the tubes showed, typically, a stagnation line/circumferential flow system on the upstream face of each tube and a recirculation tone dominating the tee side. Turn-related deviations from these patterns were in evidence at the window region.     

Transient thermal structure, turbulence, and heat transfer in a reattaching slot jet flow

The role of turbulent fluctuations on mean heat transfer coefficient in a reattaching slot jet flow is studied experimentally. Convective heat transfer rate and near-wall fluid flow are examined in the recirculation, reattachment, and post-reattachment regions for two nozzle-to-surface spacings of 0.25 and 0.75 times the width of the nozzle bottom plate. In the reattachment region, results indicate a strong correspondence between variances of near-wall velocity fluctuation and peak heat transfer rate for both spacings. Thermal structures that vary in the spanwise direction are identified in the recirculation region from low-frequency transient infrared thermographs of the heated surface. While these thermal structures are confined to regions in the vicinity of nozzle bottom plate for the low nozzle spacing, they span the entire recirculation region at larger spacings. Thermal streaks are observed past reattachment for the larger nozzle spacing, suggesting a periodic breakup and re-formation of the jet curtain. The scaling of heat transfer distribution is affected by the flow structure in the geometrically non-similar area of the recirculating flow beneath the nozzle. A correlation for peak Nusselt number is presented.