Numerical study of the radiation effect on the jet array impinging heat transfer in a feeding pipe

The present study deals with the turbine casing radiation effect on the impinging cooling encountered in the blade tip active clearance control (ACC) system of aero-engine turbine. To this end, numerical simulations are carried out for a simplified model, that is, a pipe with a single row of impinging jets. The effects of the nozzle-to-plate distance to the diameter of the impinging hole (H/d?=?2–8), the number of the holes (n?=?17–68), the impinging wall temperature (T_p?=?400–800?K), and the inlet Reynolds number (Re?=?5,000–20,000) on the flow and heat transfer are investigated. Analysis is performed on the radiation heat transfer effects on the corresponding flow fields and surface heat flux distributions. The results indicate that, with the radiation included in the simulations, the mass flow rate of the cooling jet near the end of the pipe decreases significantly under the conditions of high wall temperature and small nozzle-to-plate distance. Radiation heat transfer should be accounted for in the numerical study for the casing cooling as it affects the flow and heat transfer remarkably. When the nozzle-to-plate distance is relatively large, such as H/d is larger than 8, the radiative heating leads to uniform heat flux and the radiative heating can suppress the uneven distributions of the heat flux.     

Influence of a pulsation on heat transfer and flow structure in submerged impinging jets

An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances.The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency.At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%.But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient.     

Heat transfer to impinging round jets with triangular tabs

Experiments were performed to characterize the heat transfer enhancement produced by adding arrays of triangular tabs to the exit of turbulent round impinging jets issuing from a long pipe. For small nozzle-to-plate distances the local heat transfer was increased more than 25% in a series of distinct regions surrounding the impingement region. The largest increase in the average Nusselt number occurred for a nozzle-to-plate distance of approximately 4 diameter. In this case, the average Nusselt number was increased by 20% for the impingement region but only approximately 10% for the region with a radius of 3 jet diameters. Measurements of the velocity field were performed in free jets with tab arrays to investigate how the tabs modify the development of the flow.     

Heat transfer of a row of three butane/air flame jets impinging on a flat plate

Experiments were performed to investigate the heat transfer characteristics of a row of three premixed, laminar, butane/air flame jets impinging on a water-cooled flat plate. The between-jet interference was found to reduce the heat transfer rate in the jet-to-jet interacting zone due to the depressed combustion. The interference became stronger when the jet-to-jet spacing and/or the nozzle-to-plate distance were/was small. The positive pressure existed in the between-jet interacting zone caused the asymmetric flame and heat transfer distribution of the side jet. The meeting point of the spreading wall jets of the central and the side jets did not occur at the midpoint of the neighboring jets, but at a location shifted slightly outwards. The maximum local heat flux and the maximum area-averaged heat flux occurred at a moderate nozzle-to-plate distance of 5d with a moderate jet-to-jet spacing of 5d. The lowest area-averaged heat flux was produced when both the jet-to-jet spacing and the nozzle-to-plate distance were small. Comparing with a single jet under the same experimental conditions, the heat transfer rates in both the stagnation point and the maximum heat transfer point were shown to be enhanced in a row of three-jet-impingement system. The present study provided detailed information on the heat transfer characteristics of a row of three in-line impinging flame jets, which had rarely been reported in previous study.     

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.     

Measurements on steady state heat transfer and flow structure and new correlations for heat and mass transfer in submerged impinging jets

An experimental investigation on flow structure and heat transfer from a single round jet impinging perpendicularly on a flat plate has been performed. Heat transfer has been studied by means of thermography. The influence of nozzle-to-plate distance and Reynolds number on local heat transfer coefficient has been investigated. Based on the experimental results of this investigation as well as on experimental data from the literature, correlations for heat transfer coefficients have been developed. Similar correlations are presented for heat transfer between a two-dimensional impinging jet and a flat plate, based on literature data. Flow structure in a free jet has also been examined.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL LAMINAR,SQUARE TWIN-JET IMPINGEMENT ON A FLAT PLATE,FLOW STRUCTURE,AND HEAT TRANSFER

The flow and heat transfer characteristics of impinging laminar square twin jets have been investigated numerically through the solution of three-dimensional Navier-Stokes and energy equations in a steady state. The simulations have been carried out for jet-to-jet spacings of 4, 6, and 8 and for nozzle-exit-to-plate distances between 0.25D and 5D. The calculated results show that the flow structure of square twin jets impinging on a heated plate is strongly affected by the jet-to-plate distance. In addition, for very small jet-to-plate distances (L z , 0.25D), no upwash fountain flow can form at the collision point where the jets are merely diverted in the transverse direction. For such nozzle-to-plate distances the wall jet fills the whole gap between the plates with no vortex motion around the twin jets.     

Heat transfer of a non-Newtonian fluid (Carbopol aqueous solution) in transitional pipe flow

An experimental study of the forced convection heat transfer for non-Newtonian fluid flow in a pipe is presented. We focus particularly on the transitional regime. A wall boundary heating condition of heat flux is imposed. The non-Newtonian fluid used is Carbopol (polyacrylic acid) aqueous solutions. Detailed rheology as well as the variation of the rheological parameters with temperature are reported. Newtonian and shear thinning fluids are also tested for comparative purposes. The characterization of the flow and the thermal convection is made via the pressure drop and the wall temperature measurements over a range of Reynolds number from laminar to turbulent regime. Our measurements show that the non-Newtonian character stabilizes the flow, i.e., the critical Reynolds number to transitional flow increases with shear thinning and yield stress. The heat transfer coefficients are given and compared with heat transfer laws for different regime flows. Details when the heat transfer coefficient loses rapidly its local dependence on the Reynolds number are analyzed.     

Radial heat transfer behavior of impinging submerged circular jets

Experiments were performed to investigate the radial heat transfer behaviors of impinging submerged circular jets. Local heat transfer rate at several fixed radial locations and different nozzle-to-plate spacings were correlated and compared. Results reveal that with the jet being far from the stagnation point, the coefficient in the correlation Nu ∼ Re decreases while the exponent characterizing the flow pattern of the working liquid increases.     

Heat transport in a sandwiched set-up of MHD Newtonian–Casson–Newtonian fluids through a porous medium

The time-dependent sandwiched flow of immiscible fluids plays a vital role in petroleum extraction, bio-fluid mechanics, blood rheology, and so forth. Owing to their applications, the present study focuses on exploring the unsteady flow of Casson fluid through porous media sandwiched between Newtonian fluids in the channel. The channel is placed in the horizontal position, wherein the Casson fluid layer is in the central region, and Newtonian fluids are in the upper and lower regions. The governing system of equations is numerically solved using the finite difference method to obtain flow and heat transfer profiles. The influence of vital parameters on flow and heat transfer is discussed. Furthermore, the parameters of engineering interest, that is, fluid flow rate, skin friction, and Nusselt number are also analyzed in this study.     

Transient rotating flow over a moving surface with a magnetic field

The transient flow and heat transfer on a moving surface in a rotating fluid in the presence of a magnetic field have been investigated. The unsteadiness in the flow field has been introduced by the sudden change in the surface velocity or the fluid angular velocity. The parabolic partial differential equations governing the unsteady flow and heat transfer have been solved by using an implicit finite-difference scheme in combination with the quasilinearization technique. The computations have been carried out from the initial steady state to the final steady state. The effects of the sudden change in the surface velocity on the flow and heat transfer are found to be more significant than those of the impulsive change in the angular velocity of the fluid. When the surface velocity is suddenly reduced, the surface shear stress is found to vanish in a small time interval after the start of the impulsive motion, but it does not imply flow separation. The surface shear stress for the primary flow increases with the magnetic field and the fluid angular velocity, but the surface heat transfer decreases. The surface shear stress for the secondary flow increases with the angular velocity of the fluid, but decreases with increasing magnetic field.     

Product Literature

Abstract

In this paper, the effects of viscous dissipation, Joule heating, and magnetic field on the Hiemenz flow of a micropolar incompressible, viscous, electrically conducting fluid impinging normal to a plane are investigated. Numerical solutions for the governing momentum, angular momentum, and energy equations are given. A discussion has been provided for the effects of Hartman number, Prandtl number, Eckert number, and the micropolar parameters on two-dimensional flow of a fluid near a stagnation point (Hiemenz flow). Results for the details of the velocity, angular velocity, and temperature distributions as well as the skin friction, wall couples stress, rate of heat transfer, and thermal boundary layer thickness are shown graphically.     

Experimental and numerical study on the occurrence of off-stagnation peak in heat flux for laminar methane/air flame impinging on a flat surface

A combined experimental and numerical study has been conducted to investigate the occurrence of off-stagnation peak for laminar methane/air flame impinging on a flat surface. Experiments were conducted for three tube burners of internal diameter 8 mm, 9.7 mm and 12 mm. Radial heat flux distributions were compared (experimentally) for different burner diameters under identical operating conditions (with firing rates of 0.25 kW, 0.40 kW and 0.50 kW, ? = 1 and H = 40 mm). An off-stagnation point peak in heat flux was observed for some of the configurations in the present study which is in accordance with the previous findings. This off-stagnation point peak is a function of stand-off distance between the exit plane of the burner and the plate and also the distance between flame-tip and the plate. A satisfactory explanation is presented to explain the existence of this off-stagnation peak with the help of results of numerical simulation carried out with commercial CFD code FLUENT. It is concluded that this off-stagnation peak in heat flux is primarily due to the peak in the axial velocity profile close to the impingement surface.     

The effect of inclination on impinging jets at small nozzle-to-plate spacing

The effect of inclination on heat transfer characteristics of an impinging slot air jet is experimentally investigated. The effects of inclination angle (0° ? θ ? 40°) and dimensionless pumping power on the Nusselt number are considered. The focus is on cases where the nozzle-to-plate spacing is equal to or less than one nozzle diameter (H/d_h ? 1.0). The results show that the heat transfer characteristics of small nozzle-to-plate spacings are significantly different from those of large nozzle-to-plate spacings. In the cases of fixed flow rate conditions, the impingement point and average Nusselt numbers at small nozzle-to-plate spacing (H/d_h ? 1.0) increase as the inclination angle increases due to an increase in the pumping power, while the impingement point and average Nusselt numbers at large nozzle-to-plate spacing (H/d_h > 1.0) decrease as the inclination angle increases due to momentum loss of the wall jet. In the cases of fixed pumping power conditions, the impingement point and average Nusselt numbers at both of small and large nozzle-to-plate spacings are independent of the inclination angle. Based on the experimental results, correlations for the impingement point and average Nusselt numbers of the impinging jet are suggested as a function of the pumping power alone.     

Parametric Study and Optimization of Staggered Inclined Impinging Jets on a Concave Surface for Heat Transfer Augmentation

The characteristics of the fluid flow and heat transfer of staggered inclined impinging jets on a concave surface have been investigated numerically using three-dimensional Reynolds-averaged Navier-Stokes analysis using the shear stress transport turbulence model. Shape optimization of the impinging jet has been performed with a weighted-average surrogate model. A constant temperature condition has been applied to the concave surface. The inclination angle of the staggered jet nozzles and the distance between the jet nozzles are chosen as the design variables, and their effects on the heat transfer performance have been evaluated. It is found that the overall heat transfer increases with the pitch of vertical jet nozzles, and the staggered inclination of jet nozzles improves the heat transfer on the concave surface. For the optimization of the impinging jet, the area-averaged Nusselt number on the concave surface is set as the objective function. Latin hypercube sampling is used to determine the training points as a design of experiment, and the surrogate model is constructed using the objective function values at the training points. Sequential quadratic programming is used to search for the optimal point from the constructed surrogate model. Through the optimization, the heat transfer performance has been improved by nearly 60% compared to the reference design.     

Experimental investigation of free single jet impingement using Al2O3-water nanofluid

Four volume fractions Al₂O₃-water nanofluids (0.5%, 1%, 1.5% and 2%) are introduced into free single jet impingement experiment as working fluids. The Reynolds numbers, impact angles and nozzle-to-plate distances (H/D) are variable for investigating the heat transfer performance. As to get observation of flow characteristics in nanofluid, heat transferring performance would be studied in this case. Experimental results show that there is a relationship between convective heat transferring coefficient and nanoparticles suspendability within base fluid. Convective heat transfer coefficient is proportional to the extent of nanoparticles concentration, Reynolds number while it decreases with the increasing angle of impacting. In addition, considering the influence of the suspended nanoparticles and the condition of impinging jet, a heat transfer correlation has been proposed combining the influence of the suspended nanoparticles and the condition of impinging jet.     

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.     

Fully developed natural convection heat and mass transfer of a micropolar fluid in a vertical channel with asymmetric wall temperatures and concentrations

This work examines the effects of the vortex viscosity parameter and the buoyancy ratio on the fully developed natural convection heat and mass transfer of a micropolar fluid in a vertical channel with asymmetric wall temperatures and concentrations. The closed-form analytic solutions for the important characteristics of fluid flow, heat transfer, and mass transfer are derived. Increasing the vortex viscosity parameter tends to increase the magnitude of microrotation and thus decreases the fluid velocity in the vertical channel. Moreover, the volume flow rate, the total heat rate added to the fluid, and the total species rate added to the fluid for micropolar fluids are lower than those of Newtonian fluids.     

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.     

Turbulent impinging jet heat transfer enhancement due to intermittent pulsation

A numerical study was carried out of heat transfer under a pulsating turbulent slot impinging jet. The jet velocity was varied in an intermittent (on–off) fashion. The effects of the time-mean jet Reynolds number, temperature difference between the jet flow and the impinging surface, nozzle-to-target distance as well as the frequency on heat and mass transfer were examined. The numerical results indicate significant heat transfer enhancement due to intermittent pulsation of the jet flow over a wide range of conditions for both cooling and heating cases. Simulations of the flow and temperature fields show that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.