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.     

The effect of confinement on the flow and turbulent heat transfer in a mist impinging jet

Numerical study of the effect of confinement on a flow structure and heat transfer in an impinging mist jets with low mass fraction of droplets (M_L1 ? 1%) were presented. The turbulent mist jet is issued from a pipe and strikes into the center of the flat heated plate. Mathematical model is based on the steady-state RANS equations for the two-phase flow in Euler/Euler approach. Predictions were performed for the distances between the nozzle and the target plate x/(2R) = 0.5–10 and the initial droplets size (d₁ = 5–100 μm) at the varied Reynolds number based on the nozzle diameter, Re = (1.3–8) × 10⁴. Addition of droplets causes significant increase of heat transfer intensity in the vicinity of the jet stagnation point compared with the one-phase air impinging jet. The presence of the confinement upper surface decreases the wall friction and heat transfer rate, but the change of friction and heat transfer coefficients in the stagnation point is insignificant. The effect of confinement on the heat transfer is observed only in very small nozzle-to-plate distances (H/(2R) < 0.5) both in single-phase and mist impinging jets.     

Effect of confinement on heat transfer characteristics of a microscale impinging jet

This study experimentally investigates the local heat transfer characteristics of a microscale confined impinging air jet on a heated plate. The experimental parameters included the Reynolds number (Re_D = 1600–5600), the nozzle-to-plate spacing (H/D = 1–10), and the degree of confinement of the nozzle (D_C/D = 3, 6, 9, 12, 24, 48). The degree of confinement of the nozzle is a novel parameter. A reduction in the heat transfer rate was found for nozzles whose D_C/D values were 6, 9, 12, 24, and 48 as a result of the confinement effect at small nozzle-to-plate spacings. The confinement effect disappeared beyond H/D values of 2, 3, 4, 8, and 17 for D_C/D values of 6, 9, 12, 24, and 48, respectively. Flow characteristics were investigated by measuring pressure distributions along the wall. Subatmospheric pressure, which is evidence of the confinement effect, was observed for the confined nozzles. Correlations of the stagnation and average Nusselt numbers are proposed on the basis of the experimental results. Finally, a contour map that depicts the ratio of the Nusselt numbers of the unconfined and confined jets is presented. The contour map confirms that the confined jets have a smaller Nusselt number than the unconfined jets whenever the degree of confinement of the nozzle is large and the nozzle-to-plate spacing is small.     

Numerical study of transient conjugate heat transfer of a turbulent impinging jet

This study presents the numerical study of transient conjugate heat transfer in a high turbulence air jet impinging over a flat circular disk. The numerical simulation of transient, two-dimensional cylindrical coordinate, turbulent flow and heat transfer is adopted to test the accuracy of the theoretical model. The turbulent governing equations are resolved by the control-volume based finite-difference method with a power-low scheme, and the well-known low-Re κ–ω turbulence model to describe the turbulent structure. The SIMPLE algorithm is adopted to solve the pressure–velocity coupling. The parameters studied include turbulent flow Reynolds number (Re = 16,100–29,600), heated temperature of a circular disk (T_h = 373 K) or heat flux (q″ = 63–189 kW/m²), and orifice to heat-source spacing (H/D = 4–10). The numerical results of the transient impinging process indicate that the jet Reynolds number has a significant effect on the hydrodynamics and heat transfer, particularly in the stagnation region of an impinging jet. High turbulence values lead to greater heat transfer coefficients in the stagnation region and cause a bypass of the laminar-to-turbulent transition region in the wall jet region. Induced turbulence from the environment around the jet also influences the variation of the stagnation heat transfer. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.     

Heat transfer correlations of perpendicularly impinging jets on a hemispherical-dimpled surface

Heat transfer results of an inline array of round jets impinging on a staggered array of hemispherical dimples are reported with the consideration of various parametric effects such as Reynolds number (Re_Dj), jet-to-plate spacing (H/D_j), dimple depth (d/D_d) and ratio of jet diameter to dimple projected diameter (D_j/D_d) for both impinging on dimples and impinging on flat portions. The results were normalized against those from a flat plate. The heat transfer was measured by using transient wideband liquid crystal method. Our previous work (Kanokjaruvijit and Martinez-Botas (2005) [1]) on the effect of crossflow scheme suggested that jet impingement coupled with channel-like flow formed by the crossflow helped enhance heat transfer on a dimpled surface; hence three sidewalls were installed to constrain the spent air to leave in one direction. Throughout the study, the pitch of the nozzle holes was kept constant at 4 jet diameters. The Reynolds number (Re_Dj) ranging from 5000 to 11,500, jet-to-plate spacing (H/D_j) varying from 1 to 12 jet diameters, dimple depths (d/D_d) of 0.15, 0.25 and 0.29, and dimple curvature (D_j/D_d) of 0.25, 0.50 and 1.15 were examined. The shallow dimples (d/D_d = 0.15) improved heat transfer significantly by 70% at H/D_j = 2 compared to that of the flat surface, while this value was 30% for the deep ones (d/D_d = 0.25). The improvement also occurred to the moderate and high D_j/D_d. Thereafter, the heat transfer results were correlated in dimensionless form by using logarithmic multiple regression. The correlations were reported with necessary statistics.     

Numerical study of turbulent slot jet impingement cooling on a semi-circular concave surface

An investigation of the flow field and heat transfer characteristics of a slot turbulent jet impinging on a semi-circular concave surface with uniform heat flux has been carried out numerically in this study. The turbulent governing equations are solved by a control-volume-based finite-difference method with a power-law scheme and the well-known k–ε turbulence model and its associate wall function to describe the turbulent structure. In addition, a body-fitted curvilinear coordinate system is employed to transform the physical domain into a computational domain.Numerical computations have been conducted with variations of jet exit Reynolds number Re_2B (5920 ? Re_2B ? 23,700), dimensionless jet-to-surface distance H/B (0.5 ? H/B ? 12), dimensionless jet width B/D (0.033 ? B/D ? 0.05) and the heat flux q″ (1663 W/m² ? q″ ? 5663 W/m²). The theoretical model developed is validated by comparing the numerical predictions with available experimental data in the literature. The variations of local Nusselt numbers along the semi-circular concave surface decrease monotonically from its maximum value at the stagnation point. The numerical results show that the local Nusselt numbers are reasonably predicted with a maximum discrepancy within 15%. As the Reynolds number fixes, the effect of the impingement distance (H/B) on the average Nusselt (Nu_avg) is not significant except at low H/B = 0.5. This study provides fundamental insight into turbulent slot jet impingement cooling on the semi-circular concave surface.     

Forced convective heat transfer with impinging slot jets of meso-scale

Measurements were made to investigate the localized heat transfer behavior of submerged slot jets. The experiments were performed with kerosene jets impinging on a vertical constant-heat-flux surface from a meso-scale slot nozzle 125 μm in width with Re = 600–1200 and nozzle-to-plate spacing Z/B = 2–20. Heat transfer coefficients at the stagnation line were measured and correlated as a function of jet Reynolds numbers and Prandtl numbers. Lateral distributions of local heat transfer coefficients were also determined and correlated. Non-monotonic variations and unusual behavior of local heat transfers were observed and attributed to the possible transition from a laminar to a turbulent flow. This transition takes place within an extremely short distance of 400–500 μm.     

Heat transfer of impinging jet-array over convex-dimpled surface

A detailed heat transfer measurement over a convex-dimpled surface of impinging jet-array with three eccentricities (E/H) between jet-centre and dimple-centre is performed. These surface dimples considerably modify heat transfers from smooth-walled scenarios due to different impinging topologies for jet array with modified inter-jet reactions. Heat transfer variations caused by adjusting jet Reynolds number (Re) and separation distance (S/D_j) over the ranges of 5000 ⩽ Re ⩽ 15,000 and 0.5 ⩽ S/D_j ⩽ 11 with three eccentricities of E/H = 0, 1/4 and 1/2 are examined. A selection of experimental data illustrates the isolated and interactive influences of Re, S/D_j and E/H on local and spatially averaged heat transfers. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to generate a set of heat transfer correlations, which permit the evaluations of spatially averaged Nusselt numbers over central jet region of dimpled impinging surface.     

Experimental investigation on the heat transfer of an impinging inverse diffusion flame

This paper presents the results of an experimental study on the heat transfer characteristics of an inverse diffusion flame (IDF) impinging vertically upwards on a horizontal copper plate. The IDF burner used in the experiment has a central air jet surrounded circumferentially by 12 outer fuel jets. The heat flux at the stagnation point and the radial distribution of heat flux were measured with a heat flux sensor. The effects of Reynolds number, overall equivalence ratio, and nozzle-to-plate distance on the heat flux were investigated. The area-averaged heat flux and the heat transfer efficiency were calculated from the radial heat flux within a radial distance of 50 mm from the stagnation point of the flame, for air jet Reynolds number (Re_air) of 2000, 2500 and 3000, for overall equivalence ratios (Φ) of 0.8–1.8, at normalized nozzle-to-plate distances (H/d_IDF) between 4 and 10. Similar experiments were carried out on a circular premixed impinging flame for comparison.It was found that, for the impinging IDF, for Φ of 1.2 or higher, the area-averaged heat flux increased as the Re_air or Φ was increased while the heat transfer efficiency decreased when these two parameters increased. Thus for the IDF, the maximum heat transfer efficiency occurred at Re_air = 2000 and Φ = 1.2. At lower Φ, the heat transfer efficiency could increase when Φ was decreased. For the range of H/d_IDF investigated, there was certain variation in the heat transfer efficiency with H/d_IDF. The heat transfer efficiency of the premixed flame has a peak value at Φ = 1.0 at H/d_P = 2 and decreases at higher Φ and higher H/d_P. The IDF could have comparable or even higher heat transfer efficiency than a premixed flame.     

Thermal and fluid dynamic behaviors of confined laminar impinging slot jets with nanofluids

Heat transfer enhancement technologies play an important role in research and industrial fields; thus, they have been widely applied to many applications as in refrigeration, automotive, aerospace, and process industry. For example, heat transfer can be passively enhanced by increasing the thermal conductivity of the working fluids, adopting nanofluids, or actively by employing impinging jets.In this paper a numerical analysis on confined impinging slot jets working with pure water or water/Al₂O₃ based nanofluids is presented. The flow is laminar and a constant uniform temperature is applied on the target surface. The single-phase model approach has been adopted in order to describe the nanofluid behavior and different particle volume concentrations have been considered. Moreover, simulations have been performed for different geometric ratios in order to take into account the confining effects and Reynolds numbers. The behavior of the system has been analyzed in terms of average and local convective heat transfer coefficient, Nusselt number, and required pumping power profiles. Correlations for stagnation point and average Nusselt number for 100 ≤ Re ≤ 400, 0% ≤ ϕ ≤ 5% and 4 ≤ H/W ≤ 10 are provided.     

Heat transfer correlation for hexagonal and in-line arrays of impinging jets

The paper reports on the results of heat transfer measurements in hexagonal and in-line arrays of impinging jets for Reynolds numbers (based on the nozzle diameter D_m) ranging from 5 × 10³ to 2 × 10⁴. Liquid crystal thermography (LCT) was used to determine the temperature distribution on the flat impingement plate. The distance between the impingement plate and the nozzle exit plane varied between 3D_m and 10D_m, while the spacing between the nozzles varied between 2D_m and 6D_m. The experiments indicate that the multiple-jet heat transfer is strongly influenced by jet interactions, which, in turn, depend on the parameters mentioned above. The data set was used to construct a new correlation for the (area-averaged) Nusselt number that takes the interactions into account.     

Numerical thermal analysis and optimization of a water jet impingement cooling with VOF two-phase approach

The fluid flow and heat transfer characteristics of a free-surface liquid jet impingement cooling have been investigated numerically. The slot jet with water impinging normally on a flat plate is employed. To describe the turbulent structure, the turbulent governing equations are solved by a control-volume-based finite-difference method with a power-law scheme and the well-known turbulence model, which are associated with wall function. Numerical computations have been conducted with variations of jet exit Reynolds number (11,000 ≤ Re_d ≤ 17,000), dimensionless jet-to-surface distance (3 ≤ H/d₀ ≤ 12), dimensionless jet width (1 ≤ B/d₀ ≤ 2), and the heat flux (140 kW/m² ≤ q″ ≤ 280 kW/m²). The theoretical model developed is validated by comparing the numerical predictions with available experimental data in the literature. Under the studied ranges, the variations of local Nusselt numbers by hydraulic diameter Nu_d of the dimensionless jet-to-surface distance 3 ≤ H/d₀ ≤ 12 along the flat plate decrease monotonically from its maximum value at the stagnation point. In addition, the shape of the inlet area and jet-to-surface distance are optimized by using the response surface methodology (RSM) and the genetic algorithm (GA) method after solutions are carefully validated with available experimental results in the literature. Based on the optimal results, the optimum condition is in H/d₀ = 7.86 and B/d₀ = 2 for this physical model.     

Enhancement of heat transfer coefficients by actuation against an impinging jet

Recent technological developments have lead to significant increase in the generated heat by electronic and optical components. The removal of high heat fluxes can be successfully treated by several methods, e.g. impinging jets. Further improvement is offered by incorporating arrays of jets or causing jets to pulsate. The research reported herein introduces a new method which is based on actuation of a slab against a two dimensional steady, impinging, laminar, liquid micro-jet. This leads to enhanced heat transfer in the wall region of the jet. An experimental setup which included a piezoelectric (PZT) actuator, a dedicated silicon chip and a steady, slot, impinging jet, was assembled. Using a high speed infrared (IR) radiometer, the cooling process of the chip was recorded and the heat transfer enhancement values were determined for normalized actuation amplitudes, Reynolds and Strouhal numbers in the ranges of 0.45 < δ < 0.75, 756 < Re < 1260 and 0 < St < 0.052, respectively. It was experimentally found that heat transfer coefficients were enhanced by up to 34%.     

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.     

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.     

Effects of jet plate size and plate spacing on the stagnation Nusselt number for a confined circular air jet impinging on a flat surface

This work deals with the effects of jet plate size and plate spacing (jet height) on the heat transfer characteristics for a confined circular air jet vertically impinging on a flat plate. The jet after impingement was restricted to flow in two opposite directions. A constant surface heat flux of 1000 W/m² was arranged. Totally 88 experiments were performed. Jet orifices individually with diameter of 1.5, 3, 6 and 9 mm were adopted. Jet Reynolds number (Re) was in the range 10,000–30,000 and plate spacing-to-jet diameter ratio (H/d) was in the range 1–6. Eleven jet plate width-to-jet diameter ratios (W/d = 4.17–41.7) and seven jet plate length-to-jet diameter ratios (L/d = 5.5–166.7) were individually considered. The measured data were correlated into a simple equation. It was found that the stagnation Nusselt number is proportional to the 0.638 power of the Re and inversely proportional to the 0.3 power of the H/d. The stagnation Nusselt number was also found to be a function of exp[−0.044(W/d) − 0.011(L/d)]. Through comparisons among the present obtained data and documented results, it may infer that, for a jet impingement, the impingement-plate heating condition and flow arrangement of the jet after impingement are two important factors affecting the dependence of the stagnation Nusselt number on H/d.     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.     

Numerical study of confined slot jet impinging on porous metallic foam heat sink

This study numerically investigates the impinging cooling of porous metallic foam heat sink. The analyzed parameters ranges comprise ε = 0.93/10 PPI Aluminum foam, L/W = 20, Pr = 0.7, H/W = 2–8, and Re = 100–40,000. The simulation results exhibit that when the Re is low (such as Re = 100), the Nu_max occurs at the stagnation point (i.e. X = 0). However, when the Reynolds number increases, the Nu_max would move downwards, i.e. the narrowest part between the recirculation zone and the heating surface. Besides, the extent to which the inlet thermal boundary condition influences the prediction accuracy of the Nusselt number increases with a decreasing H/W and forced convective effect. The application ranges of H/W and Re that the effect of the inlet thermal boundary condition can be neglected are proposed. Lastly, comparing our results with those in other studies reveals that the heat transfer performance of the Aluminum foam heat sink is 2–3 times as large as that without it. The thermal resistance is also 30% less than that of the plate fin heat sink for the same volumetric flow rate and the 5.3 mm jet nozzle width. Therefore, the porous Aluminum foam heat sink enhances the heat transfer performance of impinging cooling.     

Effect of jet direction on heat/mass transfer of rotating impingement jet

The objective of this study is to investigate the heat/mass transfer characteristics on various impinging jets under rotating condition. Two cooling schemes related to impingement jet are considered; array impingement jet cooling and impingement/effusion cooling. The test duct rotates at Ro = 0.075 with two different jet orientations and the jet Reynolds number is fixed at 5000. Two H/d configurations of 2.0 and 6.0 are conducted. The detailed heat/mass transfer coefficients on the target plate are measured by a naphthalene sublimation technique. The rotation changes the local heat/mass transfer characteristics due to the jet deflection and spreading phenomenon. For H/d = 6.0, the jet is strongly deflected at the leading orientation, resulting in the significant decrease in heat/mass transfer. At the axial orientation, the momentum of jet core decreases slightly due to jet spreading into the radial direction and consequently, the value of stagnation peak is a little lower than that of the stationary case. However, reduction of heat/mass transfer due to rotation disappears at a low H/d of 2.0. In the averaged Sh, the leading orientation with H/d = 6.0 shows 35% lower value than that of the stationary case whereas the other rotating cases lead to a similar value of the stationary case.     

Conjugated heat transfer on leading edge of a wedge-shaped concave wall internally impinged by hot jets from corrugated orifice plate

An experimental and numerical investigation is conducted to study the conjugated heat transfer performance on the leading edge of a wedge-shaped concave wall subjected to external cold flow and internal hot jets impingement. A corrugated impinging plate with an extended front-extended port inside the concave cavity is proposed for the purpose of heat transfer enhancement. The effects of corrugation length-to-diameter ratio (H_j/d) ranging from 5 to 11 and width-to-diameter ratio (W_j/d) ranging from 2.5 to 6 on the conjugated heat transfer performance are examined under some representative jet Reynolds numbers (Re_j) in the range of 7900–31,700. The results show that the corrugated impinging plate has a significant impact on improving the conjugated heat transfer performance in the vicinity of concave wall leading edge. The presence of corrugation plays two roles by reducing the jet impinging distance on one hand and aggravating the jet confinement on the other hand. Therefore, it produces more complicated jet impinging flow and convective heat transfer behaviors than the baseline case without corrugation. According to the tested results, the specified area-averaged heating effectiveness is increased approximately 6.3%–18.8% under Re_j = 7900 and 2.5%–9.4% Under Re_j = 31,700 respectively by increasing the corrugation length when W_j/d is fixed as 2.5. The specified area-averaged heating effectiveness is increased approximately 16.1%–22.1% under Re_j = 7900 and 7.7%–12.7% under Re_j = 31,700 respectively by increasing the corrugation width when H_j/d is fixed as 9. In general, the corrugation with larger length and width seems to perform the better heating effectiveness over the entire concave surface. The enhancement of heating effectiveness related to the baseline case behaves more significantly under a smaller jet Reynolds number.