NUMERICAL INVESTIGATIONS OF HEAT TRANSFER OVER A MOVING SURFACE DUE TO IMPINGING KNIFE-JETS

Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.     

Confined swirling jet impingement onto an adiabatic wall

Impinging swirling jets generate interesting flow fields and depending on the magnitude of the swirl velocity, circulation cells develop in the region close to the solid wall. Moreover, axial momentum of the jet is influenced by the magnitude of the swirl velocity. This, in turn, results in considerable entropy generation in the flow field. In the present study, confined swirling jet impingement onto an adiabatic wall is investigated. The flow and temperature fields are computed numerically for various flow configurations. Different jet exit velocity profiles are considered and their effects on the flow field are examined. The entropy production due to different flow configurations is computed and the irreversibility ratios due to fluid friction and heat transfer are determined. It is found that the jet axis tilts towards the radial direction as swirl velocity increases and reducing the velocity profile number enhances the entropy generation due to heat transfer. The irreversibility ratio variation with the velocity profile number behaves opposite for the fluid friction and heat transfer.     

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.     

Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)

The use of a jet from an orifice nozzle with a saddle‐backed‐shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×10⁴. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/d_o?0.5 is observed for α=15^°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20243     

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.     

Characteristics of heat transfer of impingement system with swap swirl generator

This article presents the computational fluid dynamic simulation of the heat transfer characteristics induced by a swap swirl air‐jet generator on the impingement surface. The study was carried out for conventional and swap twist tape inserts of twisted ratio y = 2.93 with various swap angles (α = 30°, 60°, 90°) at a constant distance of nozzle diameter impingement plate (L = 2D). The results show that the Nusselt number of the swirl impingement air‐jet depends on the twisted tape swap angles and airflow rate. The results also showed that the swap angle of 90° gave notable uniform local heat transfer distribution compared with the typical twist tape and other swap twist tapes (α = 30°, 60°). In addition, the predicted results of the local heat transfer coefficient help explain the local turbulence intensity and generation to assist the industrial applications of swirl impingement air‐cooling jet.     

Effect of hybrid coaxial air and hydrogen jets on fuel mixing at supersonic crossflow

In this research study, a computational method is applied to examine the impacts of coaxial hybrid air and fuel jets on fuel mixing at the supersonic cross-flow of Mach = 4. This study examined the coaxial air and fuel jet effects on main parameters i. e. circulation, mixing efficiency, and fuel penetration. Computational Fluid Dynamic is employed for the modelling of the coaxial jet at cross supersonic flow. Reynolds Average Navier-Stocks equations with SST turbulence model for achieving hydrodynamic feature of the main model. Impacts of air-jet pressure and nozzle configurations on fuel distribution are also presented and the main effective factors for efficient fuel mixing condition are explained. Our results disclosed that injection of coaxial air and fuel jets at supersonic cross airflow significantly improves the fuel penetration and mixing inside the combustion chamber. Flow study analysis shows that the coaxial injector augments the spiral feature of the fuel jet, which surges fuel mixing downstream. Our circulation analysis confirms that circulation strength increases in far away from an injector by the injection of a coaxial air jet.     

横向射流情况下热套管对于三通管传热影响的研究

对射流垂直向下入射至主流中，射流管与主管直径比为0．16时，在宽广的射流与主流流速比下(R=0．01～1.5)，测量了3个不同区域内加装热套管前后换热系数随流速比变化的情况。通过实验发现：热套管对于不同区域的影响情况不相同。在射流管下游区域，热套管对于换热系数随流速比变化情况影响很小，在射流管与主管相接区域以及射流管内，热套管有比较明显的改变传热系数的作用，而且对于入射流体侧面的影响要大于对于入射流体迎风侧及背风侧的影响。在流速比较高时传热系数大幅度地下降。图6参5     

Boiling Heat Transfer of Multiple Impinging Jets on a Hot Moving Plate

In this research, boiling heat transfer on a hot moving plate caused by multiple impinging water jets in multiple jet rows is studied. An inverse heat conduction code is developed to analyze the readings of thermocouples that are implemented inside the plate in order to find the surface values of temperature and heat flux. Effects of nozzle stagger, plate velocity, and jet line spacing are studied. Nozzle stagger is found to affect the uniformity of heat transfer across the width of the plate. Jet line spacing can affect the heat transfer between two adjacent jet rows. Plate speed is important only in the higher entry temperatures and in the impingement zone.     

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.     

The Impingement Heat Transfer Data of Inclined Jet in Cooling Applications: A Review

On the impingement heat transfer data,the experimental studies of air and liquid jets impingement to the flat surfaces were collected and critically reviewed.The oblique impingements of both single circular and planar slot jets were considered in particular.The review focused on the surface where the jet impingement cooling technique was utilized.The nozzle exit Reynolds numbers based on the hydraulic diameter varied in the range of 1,500–52,000.The oblique angles relative to the plane surface and the dimensionless jet-to-plate spacing vary in the range of 15°–90°and 2–12 respectively.The review suggested that the magnitude of maximum heat transfer shifted more for air jets compared with the liquid jets.The drop in the inclination angle and the jet-to-plate separation led to the increase in the asymmetry of heat transfer distribution.The displacement of maximum Nusselt number(heat transfer)locations was found to be sensitive to the inclination angle and the smaller jet-to-plate distance.Also,the Nusselt number correlations proposed by various researchers were discussed and compared with the results of the cited references.     

Numerical study on the effect of jet spacing on the Swirl flow and heat transfer in the turbine airfoil leading edge region

ABSTRACT

In this paper, flow and heat transfer of a swirl chamber that models an internal cooling passage for a gas turbine airfoil leading edge is studied with numerical simulation. The geometry consists of a circular pipe, and rectangular section inlets that lead inlet flow to impinge tangentially on the circular pipe. The effects of the ratio of jet spacing to swirl chamber radius and Reynolds numbers on swirl cooling performance are investigated. The results indicate how the pressure loss and globally averaged Nusselt number on the swirl chamber wall increase with increases of Reynolds number and the ratio of jet spacing to swirl chamber radius. A Nusselt number correlation on these parameters is suggested. Also shown is how Nusselt numbers on the swirl chamber surface increase with the ratio of jet spacing to swirl chamber radius.     

Forced convective heat transfer by swirling impinging jets issuing from nozzles equipped with twisted tapes

Forced convective heat transfer on the impinged plate associated with swirling impinging jets (SIJ) issuing from nozzles inserted by twisted tapes has been investigated. Swirling impingement jets with several swirl rates were generated by twisted tapes at different twist ratios (y/W = 3, 4, 5, and 6). The experiments were performed by locating nozzle at 4 different jet-to-plate spacings of L/D = 2, 4, 6 and 8. A jet Reynolds number varied between 4000 and 16000. Attributing to a high momentum transfer rate, an efficient heat transfer was obtained by using the jet with a small jet-to-plate spacing and the twist tape with a large twist ratio at high Reynolds number. At small jet-to-plate spacings (L/D = 2 and 4), swirling impinging jets gave higher heat transfer rate than conventional impinging jets while at large jet-to-plate spacings (L/D = 6 and 8), the opposed result was obtained. Over the range examined, only SIJ induced by the twisted tape at a twist ratio of 6 consistently provided higher average Nusselt numbers than CIJ.     

Jet impingement heat transfer – Part I: Mean and root-mean-square heat transfer and velocity distributions

Impinging jets provide a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The current work forms the first stage of a two part investigation of heat transfer distributions from a heated flat surface subject to an impinging air jet for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit spacing, H/D, from 0.5 to 8. In the present paper, the relative magnitudes of the local heat transfer coefficients are compared to the fluctuating components and to the mean and root-mean-square local velocity components. It has been shown that at low nozzle to surface spacings (<2 diameters) secondary peaks in the radial heat transfer distributions are due to an abrupt increase in turbulence in the wall jet. In particular the velocity fluctuations normal to the impingement surface have a controlling influence on the enhancement in the wall jet.     

NUMERICAL INVESTIGATION OF HEAT TRANSFER UNDER CONFINED IMPINGING TURBULENT SLOT JETS

This work numerically in vestigates confined impinging turbulent slot jets. Eight turbulence models, including one standard and seven low-Reynolds-number k-epsilon models, are employed and tested to predict the heat transfer performance of multiple impinging jets. Validation results indicate that the prediction by each turbulence model depends on grid distribution and numerical scheme used in spatial discretization. In addition, spent fluid exits are set between impinging jets to reduce the cross-flow effect in degradation of the heat transfer of downstream impinging jets. The overall heat transfer performance can be enhanced by proper spent fluid removal.     

CFD and correlations of the heat transfer from a wall at constant temperature to an impinging swirling jet

Numerical simulations of the impingement of a swirling jet against a heated solid wall at a prescribed temperature are presented in order to propose correlations of the heat transfer coefficients along the heated wall as a function of the jet Reynolds number Re, jet swirl intensity S_i, jet average turbulent intensity I_avg, and jet to wall spacing H. The swirling jet used as boundary condition of the numerical simulations is the one described by Ortega-Casanova et al. [1]. It is created by a experimental nozzle (whose exit diameter is D) and with the swirl given to the jet by moving swirl blades: different blade orientations give jets with different swirl intensities. In Ortega-Casanova et al. [1], the jet velocity components (measured by means of a LDA system) just at the nozzle exit and their mathematical models are also presented for seven Reynolds numbers and each nozzle configuration. The LDA measurements show the jet is axisymmetric and highly turbulent. For those reasons, axisymmetric flow and turbulent models are used in the simulations. The same seven Reynolds numbers and three nozzle-to-wall distances are simulated numerically in this work. Taking into account the blade orientations, the Reynolds numbers and the nozzle-to-wall distances, a total of 63 different simulations have been carried out. From them, correlations of the area-weighted average Nusselt number Nu_avg and the stagnation point Nusselt number Nu₀ as a function of the dimensionless parameter Re (ranging from around 7000 to 20 000), S_i (ranging from around 0.015 to 0.45), I_avg (ranging from around 10 to 40%), and H/D (=5, 10 and 30), are proposed. The results presented in Ortega-Casanova [2], where the heat transfer when other blade orientation is studied, have been also taken into account to obtain some of the proposed correlations.     

Numerical study of injection strategy based on cavity combustor under oblique shock wave interference

In this study, the effect of jet injection strategy on the mixing performance of ethylene jets is investigated. Numerical simulations are carried out using the SST k-ω turbulence model and steady RANS. Mesh-independence verification is performed using experimental data from the open literature. The mixing performance, penetration depth, and total pressure loss of the ethylene jets with different injection angles and injection numbers are compared and analyzed. The analysis reveals that the interaction between the ethylene jets in the double-hole jet facilitated the mixing process and has a significant mixing effect in the vicinity of the nozzle. The mixing efficiency of the two-hole jet near the injection nozzle is about 43.4% higher than that of the 90° jet. The vertical incidence still dominates in the penetration depth. The ethylene diffusion distribution of the double-hole jet is more uniform, so the penetration depth of the double-hole jet does not increase, but decays more slowly downstream. In addition, the jet injection strategy has almost no effect on the total pressure loss.     

Heat transfer between an under-expanded jet and a cylindrical surface

This paper presents a selection of data from an investigation that was concerned with the heat transfer which occurs when an under-expanded jet impinges onto a heated cylindrical surface. The purpose of the study was to establish the thermal boundary conditions for calculating thermal stresses in heat transfer surfaces when subjected to high-speed cleaning jets. The heat transfer in the impingement zone of a high-speed jet is extremely high and when the presence of the surface interferes with the expansion of the jet, the radial and circumferential distributions of the heat transfer coefficient become complicated. If a highly under-expanded jet impinges upon the surface while the nozzle-to-surface spacing is small, z/D≈3, there is no longer a maximum stagnation heat transfer coefficient on the geometric axis of the jet, instead a stagnation ‘ring’ is formed with a radius of about one nozzle diameter. A selection of data is presented that shows how, particularly for z/D less than 10, the Nusselt number distribution has a very high peak value at, or near to, the geometric stagnation point and then falls away steeply in both the axial and circumferential directions. The high values of Nusselt number, and the large differences between the peak values on the front edge of the cylinder and the values at the rear of the cylinder, could lead to very substantial differential cooling rates and hence to significant thermal stresses being generated when high pressure air cleaning jets are used on high-temperature tubes. However, when the nozzle exit is placed more than 20 nozzle diameters away from the surface of the cylinder there is a significant reduction in the maximum Nusselt number and the overall distribution is much smoother; this will alleviate potential problems from thermal stresses.     

Effect of crossflow regulation by varying jet diameters in streamwise direction on jet impingement heat transfer under maximum crossflow condition

Jet impingement heat transfer has been studied numerically for a maximum crossflow condition using a 3?×?9 array of jets. Five-hole configurations have been studied for jet average Reynolds numbers ranging from 10,000 to 20,000. Crossflow has been mitigated by varying the jet diameters in the streamwise direction to reduce the impact of crossflow on downstream jet impingement. The design criteria for all five configurations were to keep the average of the jet diameters equal to the constant jet diameter configuration (baseline). It has been found that the configuration with increasing and then decreasing jet diameters provided higher levels of heat transfer with more uniform cooling when compared to the traditional constant diameter configuration and other configurations.     

Heat transfer characteristics of impinging jets: The influence of unsteadiness with different waveforms

A series of experiments are conducted in which specially designed periodic air jets are applied to a heated surface for the purpose of enhancing heat transfer relative to the corresponding steady air jet. The periodic jets are produced by a mass flow rate controller. The experiments are performed for steady jets and for specially designed periodic jets, including a combination of sinusoidal, triangular and rectangular jets at frequencies from 1.25 to 20 Hz, the triangular signal having a different symmetry (representing the ratio of time to increasing velocity in one cycle to total cycle time) and the rectangular jets having a variable duty cycle (representing the ratio of the pulse cycle on-time to off-time) at frequencies of 10 Hz and 20 Hz. Various unsteady jets show distinguishing frequency dependences and the step change, i.e. the sudden increase or decrease of signal, shows some advantageous influence on heat transfer improvement, especially negative step change. Therefore, the enhancement of combined signals lies between the performances of the individual signals. The enhancement for triangular (or sinusoidal) plus rectangular signals shows some improved performance compared with that of rectangular plus triangular (or sinusoidal) signals. The heat transfers are enhanced as Strouhal number increases from 0.004 to 0.068 on the whole for impinging jet with such combined signals. The signals of triangular jet with symmetry parameter of 0 and 1 have improved heat transfer, and the latter has a slightly better result than the former. The instantaneous changing rate of velocity also has an influence on heat transfer improvements. Thus the duty cycle of 1:2 has the best performance in terms of heat transfer enhancement in this study.