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.     

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.     

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.     

Numerical Analysis of Heat Transfer from a Moving Surface Due to Impingement of Slot Jets

Heat transfer from a moving surface with uniform wall temperature due to impingement of series of slot jets has been investigated numerically. In the present paper, transition–shear stress transport model has been used for numerical simulations, which can predict the heat transfer in laminar as well as turbulent flows. This model is adopted here to study the transport phenomenon and predict the transition from laminar to turbulent flow seamlessly under different surface velocities. The present model with stationary surface is validated with the correlation given by Martin for series of slot jets. It has also shown good agreement with existing data for both laminar and turbulent slot jets, and is further studied to understand the heat transfer under wide range of flow conditions and the effect of surface velocity on flow regime. The range of Reynolds number is from 100 to 5,000, whereas surface velocity varied up to six times the jet velocity at the nozzle exit. It has been observed that at high surface velocities the heat transfer from the moving wall is more than stationary case. The transition from laminar to turbulent regime is found to be starting at a Reynolds number of 400 and turns completely turbulent at a Reynolds number of 3,000. Q-criterion is used to confirm the transition zone by observing the breaking of vortices at higher Reynolds number.     

Local heat transfer distribution on a smooth flat plate impinged by a slot jet

Experimental investigation of local heat transfer distribution on a smooth flat plate impinged by a normal slot jet is conducted. Present study concentrates on the influence of jet-to-plate spacing (z/b) and Reynolds number on the fluid flow and heat transfer distribution. A single slot jet with an aspect ratio (l/b) of about 50 is chosen to get the fully developed flow at the nozzle exit. Reynolds number based on slot width is varied from 4200 to 12,000 and jet-to-plate spacing (z/b) is varied from 0.5 to 12. The local heat transfer coefficients are estimated from the thermal images obtained from infrared thermal imaging camera. Measurement for the static wall pressure is carried out for various jet-to-plate spacings at a Reynolds number of 12,000. Normalized value of turbulence and velocity are measured using hot wire anemometer along the streamwise direction (x/b) for jet-to-plate spacings (z/b) of 1, 2, 4, 6, 8, 10 and 12. The entire flow field is divided into three regimes namely stagnation region (laminar boundary layer associated with favorable pressure gradient), transition region (associated with increase in turbulence intensities and heat transfer) and turbulent wall jet region. Semi-empirical correlation for the Nusselt number in the stagnation region is proposed. Heat transfer characteristics in the transition region are explained based on the fluid dynamic behavior from the hot wire measurements. Semi-empirical correlation for the Nusselt number in the wall jet region is presented using the velocity profile obtained from the hot wire measurements.     

Thermal transport phenomena over a slot-perforated flat surface in pulsating free stream

A theoretical study is performed to investigate unsteady, two-dimensional, incompressible thermal-fluid flow over both sides of a slot-perforated flat surface, which is placed in a pulsating free stream. The effects of the pulsating Strouhal number, the Reynolds number Re, and the ratio of the slot width, d, to the plate thickness, δ, on the heat transfer performance and the velocity and thermal fields are disclosed. It is found from the study that: (i) when the free stream is pulsated, the alternating change in the fluid flow disturbs the thermal boundary layer formed along the plate and induces mixing of the upper and lower streams of the plate downstream from the slot, resulting in an amplification of heat-transfer performance; (ii) heat-transfer performance at the rear plate is induced with an increase in d/δ and Re; and (iii) heat transfer performance is intensified with an increase in fSr.     

Impingement heat transfer at a circular cylinder due to an offset or non-offset slot jet

Heat transfer coefficients were measured on a circular cylinder subjected to the crossflow impingement of a slot jet. In one set of experiments, the symmetry plane of the jet was aligned with the axis of the cylinder, while in other experiments the jet was offset from the cylinder. In addition to the offset, parametric variations were also made for the width of the jet-inducing slot, the distance between the slot and the cylinder, and the Reynolds number. Supplementary flow visualization experiments showed that even in the presence of offset, the jet impinged on the cylinder, although not at the cylinder apex as in the aligned case. It was found that the heat transfer coefficient increased with slot width and Reynolds number but decreased with slot-to-cylinder separation distance and offset. The effect of offset is accentuated for narrow slots and at small slot-to-cylinder separation distances. The largest measured offset-related reduction in the heat transfer coefficient was slightly in excess of 50%.     

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.     

Numerical Investigation of Jet Impingement Cooling of a Flat Plate with Carbon Dioxide at Supercritical Pressures

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6 to 1.6 MW/m². The shear-stress transport turbulence model was used and the numerical model was validated by comparison with experimental results for jet impingement heating with hot water at supercritical pressures. Radial conduction in the jet impingement plate was also considered. The sharp variations of the thermal-physical properties of the fluid near the pseudocritical point significantly influence heat transfer on the target wall. For a given heat flux, the high specific heat near the wall for the proper inlet temperature and pressure maximizes the average heat transfer coefficient. For a given inlet temperature, the heat transfer coefficient remains almost unchanged with increasing surface heat flux at first and then decreases rapidly as the heat flux becomes higher due to the combined effects of the thinner high specific heat layer and the smaller thermal conductivity at higher temperature.     

Experimental investigation of flow boiling heat transfer of jet impingement on smooth and micro structured surfaces

Flow boiling heat transfer experiments using R134a were carried out for jet impingement on smooth and enhanced surfaces. The enhanced surfaces were circular micro pin fins, hydrofoil micro pin fins, and square micro pin fins. The effects of saturation pressure, heat flux, Reynolds number, pin fin geometry, pin fin array configuration, and surface aging on flow boiling heat transfer characteristics were investigated. Flow boiling experiments were carried out for two different saturation pressures, 820 kPa and 1090 kPa. Four jet exit velocities ranging from 1.1–4.05 m/s were investigated. Flow boiling jet impingement on smooth surfaces was characterized by large temperature overshoots, exhibiting boiling hysteresis. Flow boiling jet impingement on micro pin fins displayed large heat transfer coefficients. Heat transfer coefficients as high as 150,000 W/m² K were observed at a relatively low velocity of 2.2 m/s with the large (D = 125 μm) circular micro pin fins. Jet velocity, surface aging, and saturation pressure were found to have significant effects on the two-phase heat transfer characteristics. Subcooled nucleate boiling was found to be the dominant heat transfer mechanism.     

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.     

Flow field and heat transfer of a two‐dimensional impinging jet disturbed by an elastically suspended circular cylinder

The flow field around a circular cylinder elastically suspended with a cantilever‐type plate spring in the jet impingement region was visualized to investigate the mechanism of the impingement heat transfer. The impingement distance H was kept constant at 3 or 5 times as large as the jet slot width, h = 15 mm.The Reynolds number was fixed at 10,000, or 5000 in the case of flow visualization. The self‐induced periodic swing motion of the cylinder across the jet axis was caused by the interaction between the jet and the elastically suspended cylinder. It was found that this swing motion has direct effects on the flow and heat transfer characteristics of the stagnation region. The ensemble‐averaged values of the flow velocity and its fluctuations depended on the cylinder diameter and the impingement distance. The local Nusselt number in the case of H/h = 3 with the oscillating cylinder of the smallest diameter D = 4 mm was increased to 1.15 times as large as that without the cylinder. The interesting patterns of the intermittency function defined with a certain threshold level of turbulence intensity were obtained under the above experimental conditions. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 313–330, 2001     

Alterations to coherent flow structures and heat transfer due to pulsations in an impinging air-jet

An investigation was carried out to study the effect of flow pulsation on the characteristics of a planar air jet impinging normally on a heated surface. Such information was further utilized to determine the influence of flow characteristics in the plane of impingement on Nusselt number distribution. Time-resolved system properties were investigated with modern instrumentation that allowed instantaneous heat transfer and flow velocity measurements to be performed simultaneously. Based on good coherence function estimates between the signals, heat transfer measurements were used in return to infer flow dynamics near the impingement surface. Experiments were performed for steady and pulsating jets at jet Reynolds numbers of 1 000, 5 500, and 11 000, pulse frequencies up to 82 Hz (corresponding to Strouhal numbers below 0.13), and pulse amplitude at the nozzle exit up to 50 % of the mean flow velocity. Special techniques commonly used for periodically disturbed flow fields elucidated the dynamics of the pulse and associated coherent flow structures. Results indicated the parametric conditions for which alterations are expected in time-averaged heat transfer from the surface. Engineering applications include cooling of electronic packages and heat transfer to gas turbine blades.     

Dynamical Study of Pulsed Impinging Jet with Time Varying Heat Flux Boundary Condition

Pulsed jets in different configuration are potentially considered for enhancing transport phenomenon generally. Flow and temperature field in a pulsed impinging jet are simulated numerically by solving the governing equations using the control volume method. Ensemble Averaging Method as well as Phase Averaging has been employed for reporting the results in this study. In order to simulate a pulsating jet, inlet velocity profile was exerted as a time dependent sinusoidal and step signals. The results of this simulation showed an oscillatory jet could lead to an increase in jet development and its cross section with the wall and also a more uniform Nusselt profile would be obtained compared to the steady jet. For parametric investigations and extracting flow and thermal characteristics of a pulsed impinging jet, the effects of various parameters including flow frequency and amplitude and heat flux frequency were considered. It has been seen that Nusselt number varies by the changes in frequency, amplitude and the type of the excitation. It has been shown that the oscillating impinging jet has a better performance rather than the steady case when the excitation amplitude and frequency increase. Finally, it is also observed how a thermal field is going to respond with two pulsating inputs.     

Synchronal measurement of flow structure and heat transfer of impingement jet

This paper will present the characteristics of flow behavior and thermal field of both free and
impingement jet issued from a circular orifice nozzle at Re=8900.The flow behavior of a single round
jet and impingement jet was observed by smoke flow visualization recorded by a high speed camera
using 5000 frame per second.Heat transfer coefficient on the impingement surface was measured by
means of infrared camera （TVS-8500,Avio） with a two-dimensional array of Indeum-Antimony （In Sb）
sensors varying in the separation distance between the nozzle and the target plate.The heat transfer
coefficient changes in time and spatial.Therefore,the root mean square distribution of the heat
transfer was obtained from the data.As a result,it was confirmed that the longitudinal vortex was
observed outside of the ring vortex,and then the longitudinal vortex was penetrated in the jet
flow.Moreover,the high value of root mean square of the heat transfer coefficient has spread radially
in stripy manner,which is caused as the results of the longitudinal vortexes flowing in the radial
direction on the impingement plate.     

Heat Transfer and Thermal Characteristics Effects on Moving Plate Impinging from Cu-Water Nanofluid Jet

The present article is focused on modelling of flow and heat transfer behaviour of Cu-water nanofluid in a confined slot jet impingement on hot moving plate.Different parameters such as various moving plate velocities,nanoparticles at various concentrations,variation in turbulent Reynolds number and jet nozzle to plate distance have been considered to study the flow field and convective heat transfer performance of the system.Results of distribution of local and average Nusselt number and skin friction coefficients at the plate surface are shown to elucidate the heat transfer and fluid flow process.Qualitative analysis of both stream function and isotherm contours are carried out to perceive the flow pattern and heat transfer mechanism due to moving plate.The results revealed that average Nusselt number significantly rises with plate velocity in addition with jet inlet Reynolds number.Correlations of the average Nusselt numbers are presented.     

Large eddy simulation of turbulent heat transfer in a rotating two-pass smooth square channel with sharp 180° turns

Heat transfer in a rotating two-pass square channel with 180° sharp turns was numerically simulated by using the large eddy simulation with a Lagrangian dynamic subgrid-scale model. In the stationary condition, the heat transfer in and after the turn was increased due to the flow impingement, the flow separation and reattachment, and the reversed flow at the second outer corner. In the rotating condition, the high momentum fluid on the upstream pressure side formed one strong vortex in the turn that transported the high momentum and low temperature fluid to the suction surface side where the heat transfer became high.     

Heat transfer and friction coefficients in corrugated ducts confined by sinusoidal and arc curves

The thermally developing laminar forced-convection flow and heat transfer characteristics in corrugated ducts confined by sinusoidal and arc curves, which are often encountered in honeycomb desiccant wheels, are numerically investigated via the boundary-fitted coordinate system. The control volume-based finite difference technique is applied to obtain the solution utilizing the numerically generated boundary-fitted coordinates. According to this method, the complex domain in the physical plane is transformed into a regular square domain in the computational plane. Studied and graphically illustrated are the effects of aspect ratios and bending ratios of the ducts on the friction coefficients and heat transfer coefficients under uniform wall temperature (T) boundary conditions. For the special cases of corrugated ducts such as circular, square, and flat sinusoidal, the results are compared with some findings in the literature, and very good agreement is obtained. Furthermore, variations of the bulk temperature, Nusselt numbers, velocity, and temperature profiles in the entire thermal entry region are plotted.     

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.     

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.