Measurement of detailed heat transfer along rib-roughened surface under arrays of impinging elliptic jets

The objective of the present study is to examine the detailed heat transfer coefficient distributions over a ribbed-surface under impingement of elliptic jet arrays using a liquid crystal thermograph technique. Both continuous and broken V-shaped-rib configurations with different exit flow orientations were considered. To examine the angled rib effects, three angled ribs were discussed under jet-to-plate spacing Z = 3 for different Reynolds numbers. Measured results show that the local heat transfer rates over the ribbed-surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing crossflow effect. Compared to the results without ribs, the heat transfer over the ribbed-surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° V-shape ribs. In addition, the surface with continuous ribs provides a better impingement heat transfer than that with broken ribs.     

Experimental study of impinging heat transfer along rib-roughened walls by using transient liquid crystal technique

The objective of this work is to examine the detailed heat transfer coefficient distributions over a ribbed surface under impingement of in-line and staggered jet arrays by using a liquid crystal thermograph technique. In-line and staggered jet arrays with different exit flow orientations were considered. Three jet-to-target spacing Z of 3, 6 and 9 with in-line and staggered jet arrays were considered at jet Reynolds numbers of Re = 1500, 3000 and 4500 with three different exit flow orientations. In addition, the effects of rib configuration on the heat transfer distributions were discussed in detail. Results show that the local heat transfer rates over the ribbed surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing cross flow effect. Compared to the results without ribs, the heat transfer over the ribbed surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° angled ribs.     

Experimental study on the heat transfer under impinging elliptic jet array along a film hole surface using liquid crystal thermograph

In this research, the effects of jet geometry and the arrangement of film holes on the target plate on the impinging heat transfer are experimentally investigated in detail. A liquid crystal thermograph technology is employed in this study. The aspect ratios (AR) of elliptical jet with five different values, 4, 2, 1, 0.5, and 0.25, jet Reynolds number ranging from 2000 to 4000, and jet-to-target spacing ranging from 1.5 to 4.5 are considered to investigate impingement heat transfer performance. In addition, three arrangements of film hole on the target plates, named side-, middle- and staggered-types, are tested, respectively. The experimental results show that the Nu increases with the increase of jet Reynolds number. Better heat transfer is noted for the cases with smaller jet-to-plate spacing. For the effect of the arrangement of pores on the target surface, the heat transfer on middle-type plate is more significant than the other two for smaller jet-to-plate spacing. As for the effect of aspect ratio, results indicate that the optimal heat transfer performance is found with circular jet of AR = 1.     

Visualization of heat transfer from arrays of impinging jets

A visualization technique is used to measure the heat transfer coefficient distribution on a flat plate on which either a single jet or an array of jets impinges. Liquid crystals coated on a mylar sheet are used to locate isotherms on a heated surface. By adjusting the surface heat flux, contours of constant heat transfer coefficient are obtained.     

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.     

Flow and heat transfer of confined impingement jets cooling using a 3-D transient liquid crystal scheme

It has been shown that the heat transfer coefficients obtained from using the 1-D transient liquid crystal scheme are higher than those obtained from employing the 3-D scheme when surface heat transfer is highly nonuniform such as on a hot surface subject to jet impingement cooling. This is due to the fact that 1-D method does not include the lateral heat flows induced by local temperature gradients. The objective of this study is to provide a new database of heat transfer coefficient distribution on the jet impingement target surface in the confined cavity by employing a 3-D transient liquid crystal scheme. The study is performed with an 8 × 11 array of confined impinging jets with Reynolds numbers ranging from 1039 to 5175. The 1-D results are higher than the 3-D results with the local maximum and minimum heat transfer values being overvalued by about 15–20% and the overall heat transfer by approximately 12%. In addition, hot-film measurements of the flow structure are conducted to gain insight into the effects of cross-flow on heat transfer behavior. The surface mapping of heat transfer coefficient demonstrates a change from columnar pattern to a horizontal pattern and switching back to the columnar pattern as Reynolds number increased consecutively. This pattern switching is thought to be caused by the competition between jet penetration and the cross-flow buffering effect. A nonuniformity index is defined to provide a quantitative measure for cooling effectiveness for various cases. The results indicate that increased cross-flow degrades the heat transfer performance but increase uniformity.     

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.     

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.     

Effect of jet-jet spacing on convective heat transfer to confined, impinging arrays of axisymmetric air jets

The effects of jet-jet spacing (X_n/D), low nozzle-plate spacings (H/D = 0.25, 1.0 and 6.0) and spent air exits located between the jet orifices were studied on the magnitude and uniformity of the convective heat transfer coefficients for confined 3 × 3 square arrays of isothermal axisymmetric air jets impinging normally to a heated surface. Local and average Nusselt numbers are presented for Reynolds number range of 3500–20 400. The local Nusselt numbers illustrate the (non)uniformity of the heat transfer and aid in understanding the variations in the average Nusselt number. The jet-jet spacing affects the convective coefficient by varying the influence of the adjacent jet interference and fraction of the impingement surface covered by the wall jet. The addition of spent air exits increased the convective coefficient and influenced the location of the optimum separation distance. In addition, significant enhancement of the uniformity and the convective coefficients was observed at H/D = 0.25 and 1.0 when compared to H/D = 6.0.     

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 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.     

The effect of flow field and turbulence on heat transfer characteristics of confined circular and elliptic impinging jets

The flow field of confined circular and elliptic jets was studied experimentally with a Laser Doppler Anemometry (LDA) system. In addition, heat transfer characteristics were numerically investigated. Experiments were conducted with a circular jet and an elliptic jet of aspect ratio four, jet to target spacings of 2 and 6 jet diameters, and Reynolds number 10 000. The toroidal recirculation pattern was observed in the outflow region for both geometries at dimensionless jet to plate distance 2. Higher spreading rates in the minor axis direction of the elliptic jet have also been mapped. Along the target plate, different boundary layer profiles were obtained for circular and elliptic jets at H/d=2, but profiles became similar when dimensionless jet to plate distance was increased to 6. Positions of maximum radial and axial velocities and turbulence intensities have been determined for both geometries. For the confined circular and elliptic jet geometries, analysis of flow field measurements and numerical heat transfer results showed that inner peaks in local heat transfer closely relate to turbulence intensities in the jet and radial flow acceleration along the wall. Differences between the circular and elliptic jet, in terms of flow field and heat transfer characteristics, reduced with increase in the jet to plate distance.     

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.     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

Effect of Pressure Gradient on Heat Transfer of Impinging Submerged Circular Jets

An experimental and numerical study have been carried out to investigate the distribution of radial local heat transfer coefficients of impinging submerged circular jets. Good agreement is achieved between the experimental results and the predicted value. Results show that the outer peak usually occurs at the radial location of r/d= 1.8~2.0, in which transition from laminar to turbulence happens resulting from disappeared pressure gradient abruptly, and that the inner peak appears rigidly at r/d=0.5, where the boundary layer has a minimum thickness because of elevating pressure gradient.     

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.     

A numerical study of heat transfer performance of oscillatory impinging jets

Enhancement of heat transfer to a planar surface by oscillating jets is presented in this work. Two jets from adjacent slots are made to oscillate with the same frequency but with a phase shift of π/2. The two nozzles are idealisation of an array of oscillating jets. A two-dimensional model is developed using finite element methods to investigate the heat transfer performance with respect to the oscillation frequency, geometric parameters and the flows with Reynolds number in the range of 0 < Re ? 1200. The computational results show that the oscillatory flow jets achieve approximately 100% improvement of heat transfer efficiency over conventional steady flow jets. It is shown that the periodic disruption of the boundary layer leads to this improvement. Further, the analysis shows the existence of a range of frequencies that are effective depending on the separation between the nozzles. The study shows that frequencies as low as 1 Hz are effective depending on the nozzle separation.     

Local heat transfer characteristics of array impinging jets from elongated orifices

The aim of this research is to enhance the heat transfer on an impinged surface under an impinging jet array by minimizing a cross-flow effect. Conventional round orifices (aspect ratio, AR = 1) are substituted by the elongated orifices with aspect ratio AR = 4 and 8 with the same jet exit area. Two types of orifice arrangements; in-line and staggered arrays are compared. The experimental investigation was carried out at constant distance from orifice plate to impinged surface H = 2D_E (D_E is equivalent diameter of orifice). The heat transfer characteristic was visualized using thermochromic liquid crystal sheet (TLCs) and the Nusselt number distribution was evaluated by an image processing technique. The flow characteristic on the impinged surface was also visualized by oil film technique. The results show that the cross-flow in a case of the jets issued from the orifices with AR = 4 is considerably less significant than that in cases of the ones delivered from the orifices with AR = 1 and 8. At Reynolds number of 13,400, the Nusselt numbers for the jet arrays issued from the elongated orifices with AR = 4 with in-line and staggered arrangements are respectively 6.04% and 12.52% higher than those for the case of AR = 1.