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.     

Heat transfer between an impinging air jet and an impinged water surface

Impingement of producer gas obtained from rice-husk gasification onto a water surface has been found to be very effective in removing tar and ash from the gas. Correlations to estimate tar- and dust-removal efficiencies have been developed. An advantage of impingement is that the hot gas jet is cooled. Here, we describe experiments to study heat transfer when heated ambient air impinges on the water surface. An equation to estimate the overall heat-transfer coefficient has been derived. This coefficient is expressed in terms of Nu and is correlated with the following dimensionless groups: v_a/v_w, H_n/D_n, D_c/D_n, and Re. The correlation obtained by minimizing the absolute error is Nu=0.0148 (v_a/v_w)^−0.16 × (H_n/D_n)^−1.3 (D_c/D_n)^−1/2 Re^1.2.     

Heat transfer characteristics of a slot jet impinging on a semi-circular convex surface

Surface heat transfer characteristics of a heated slot jet impinging on a semi-circular convex surface have been investigated by using the transient heating liquid crystal technique. Free jet velocity, turbulence and temperature characteristics have been determined by using a combination of an X-wire and a cold wire anemometry. The parametric effects of jet Reynolds number (Re_W) ranging from 5600 to 13,200 and the dimensionless slot nozzle-to-impingement surface distance (Y/W) ranges from 2 to 10 on the local circumferential heat transfer have been studied. Local circumferential Nusselt number (Nu_S) decreases with increasing the dimensionless circumferential distance (S/W) from its maximum value at the stagnation point up to S/W=3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3?S/W?4.2 which coincided with a secondary peak in heat transfer. Correlations of local and average Nusselt numbers with Re_W, Y/W and S/W have been established for the stagnation point and the circumferential distribution. The rate of decay of average circumferential Nusselt numbers around the semi-circular convex surface is much faster than that which occurs laterally along the flat surface. As Y/W increases, the effect of surface curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced.     

Heat transfer of impinging air and liquid nitrogen mist jet onto superheated flat surface

This experimental study performs the detailed heat transfer measurements of an impinging air-liquid nitrogen mist jet onto a superheated flat surface at atmospheric pressure with reference to the design of an instant freezing facility. A selection of experimental results illustrates the interacting effects of jet Reynolds number, mass flow ratio of air to liquid nitrogen flows and separation distance on the spatial distributions of heat transfer over the impinging surface. Mechanism associated with phase change of impacting droplets generates an enhanced and uniformly distributed heat transfer region centered on the stagnation point. A narrow oval-ring region encapsulating the enhanced core transits heat transfer from the wetting regime of complete evaporation to the non-wetting rebound regime. Stagnation heat transfer augmentation factor in the range of 1.2-2.8 times of the air-jet level is achieved. An empirical correlation based on the experimental data, which is physically consistent, has been developed to permit the evaluation of stagnation heat transfer.     

Heat transfer from a pulsed laminar impinging jet

A numerical investigation has been performed to study the effect of flow pulsations on time-averaged Nusselt number under a laminar impinging jet. The parameters considered are as follows: time-averaged jet Reynolds number (100 ≤ Re ≤ 1000), frequency of pulsation (1 ≤ f ≤ 20 Hz), and nozzle-to-target spacing (4 ≤ H/d ≤ 9). The combination of Re = 300, f = 5 Hz and H/d = 9 was found to give the best heat transfer performance. Interestingly, it was found that the onset of separation at the wall jet region of pulsating impinging jet is associated with the point of constant Nusselt number during the oscillation cycle. Downstream of the separation point in the wall jet region, the Nusselt number waveform fluctuates out of phase with the inlet velocity. Within one oscillation, large vortices existing during the minimum velocity state are broken into two smaller vortices when the flow is accelerated to reach the maximum velocity, after which the two vortices merge again when the flow decelerates back to the minimum velocity.     

Heat transfer from a plate impinging swirl jet

Heat transfer and flow visualization experiments were conducted to investigate the performance of swirling and multi‐channel impinging jets and compare the results with those of a multi‐channel impinging jet (MCIJ) and conventional impinging jet (CIJ) for the present work at the same conditions. Swirling impinging jets (SIJs) employed the fixed blade lengths of 12.3 mm with four blades at the exit of the housing tube to divert the air flow through four narrow channels with a desired swirl angle (θ of 22.5, 41 and 50°). The MCIJ jet had the same dimensions as the SIJs, except that the narrow channels in the solid insert were vertical (θ=0°). The local and surface average Nusselt numbers of MCIJ were generally higher than those of the CIJ and SIJs. The SIJs, however, demonstrated significant improvement in radial uniformity of heat transfer compared to the MCIJ and CIJ. In the region of 2.7?X/D?0 for H/D=8 and Re=20 000, the average Nusselt number for the MCIJ was 11, 33, 72 and 98 per cent higher than that of the CIJ, θ=22.5, θ=41 and θ=50°, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat transfer characteristics of three interacting methane/air flame jets impinging on a flat surface

An experimental study has been conducted for three interacting methane/air flame jets (arranged in a triangular configuration) impinging normally on a flat surface. Surface heat flux distributions have been determined for various dimensionless inter-jet spacings (S/d = 3, 4, 6 and 7.58) and separation distances between the exit plane of the burners and the target plate (H/d = 2, 2.6, 5 and 7). All experiments were conducted for stoichiometric mixture at a Reynolds number of 800. The surface heat flux distributions were intimately related to flame shapes. For small inter-jet spacings and small separation distances, flames were deflected outward from the centroid of the triangular arrangement due to strong interaction between the jets. The heating was quite non-uniform at very large inter-jet spacings. Zones of low heat flux were obtained when the tip of inner reaction zones were intercepted by the plate (H/d = 2). There were sharp peaks in the heat flux distribution when the tips of the inner reaction zones just touched the impingement surface (H/d = 2.6). Heat flux distribution was non-uniform at small separation distances (H/d = 2 and 2.6). For the system of flame jets under consideration, the optimum configuration, considering the magnitude of the average heat flux and the uniformity in the heat flux distribution, was corresponding to H/d = 5 and S/d = 3.     

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 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.     

Heat transfer and fluid flow characteristics in a swirling impinging jet

An experimental study on heat transfer and fluid flow has been carried out for a swirling round impinging jet. A thermosensitive liquid crystal sheet was used for the heat transfer measurements and the three velocity components were measured with LDV in the stagnation region for cases where the Swirl number Sw = 0.0, 0.22, and 0.45 at the Reynolds number Re = 8100. The formation of recirculation flow due to a swirl near the impinging wall was found to deteriorate the heat transfer coefficient in the stagnation region and results in a more uniform distribution of the Nusselt number with an increasing Swirl number. The heat transfer mechanism of the swirling impinging jet is discussed based on the flow characteristics of the mean velocities and turbulence quantities. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 324–335, 2005; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20068     

Heat transfer to a row of impinging jets in consideration of optimization

In the current study the heated-thin-foil technique is used jointly with infrared thermography to evaluate accurately the heat transfer characteristics of one row of jets impinging on a flat plate. The impingement is confined by the test section and spent air is constrained to exit in only one direction. The configuration and parameters variation range are similar to those encountered in turbine internal cooling systems. The results are analysed in terms of local and averaged Nusselt number keeping in mind the design engineer preoccupation of minimizing the amount of cooling air taken from the compressor. The influence of the impingement distance Z/d, injection Reynolds number Re and spanwise spacing p/d between two holes of the row is presented. An optimum impingement distance is pointed out and some qualitative design rules for the two other parameters are underlined.     

Heat transfer characteristics of an array of impinging pre-mixed slot flame jets

An experimental study was carried out to investigate the shape and the heat transfer characteristics of an array of three laminar pre-mixed butane/air slot flame jets impinging upwards normally on a horizontal water-cooled flat plate. The effects of jet-to-jet spacing and nozzle-to-plate distance were examined at the Reynolds number (Re) of 1000 and the equivalence ratio (?) of unity. Comparisons of the heat transfer characteristics between single and multiple slot flame jets, as well as multiple slot and round jets, were made. The between-jet interference decreased with increasing jet-to-jet spacing (s/d_e) and nozzle-to-plate distance (H/d_e). Strong interference was obtained at s/d_e = 1 and H/d_e = 2, at which the central jet was suppressed while the side jets were deflected towards their free sides. In addition, there was no minimum heat flux found in the inter-jet interacting zone, instead, a peak heat flux was obtained. Thermal performance was reduced when H/d_e became smaller than the length of the conical luminous reaction zone of the flame. A maximum average heat flux occurred at the moderate jet-to-jet spacing of s/d_e = 2.5 at Re = 1000, ? = 1 and H/d_e = 2. The resultant heat flux distribution of the central jet of a multiple slot jets system was higher than that of a single slot jet when the jet-to-jet spacing was small, but this advantage in thermal performance diminished when the jet-to-jet spacing was increased. Besides, the area-averaged heat flux of the multiple slot flame jets was higher than that of the multiple round flame jets arranged at the same geometric configuration.     

Heat transfer mechanism of a swirling impinging jet in a stagnation region

Heat transfer characteristics of a swirling impinging jet have been experimentally examined using a combined particle image velocimetry (PIV) and laser‐induced fluorescence (LIF) technique for simultaneous measurement of velocity and temperature fields. The present study shows that the radial width of the jet stretches with increasing swirl intensity, and that the stretching phenomenon contributes to the maximum local heat transfer coefficient. At the stagnation region, the flow near the heated surface is mixed intermittently by reverse flows toward upstream, and spatial distributions of temperature are correlated with instantaneous velocity vector maps. The dynamic behavior of recirculation zones, attributed to swirl number Sw and impinging distance, mainly determines the turbulent heat transfer at the stagnation region. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 663–673, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10120     

Heat transfer in dusty fluid with suspended hybrid nanoparticles over a melting surface

The melting effect with the magnetic field performs a significant role in various manufacturing and industrial applications, such as welding, casting, magma-solidification, nuclear engineering, and so forth. The present study focuses on the impact of the melting effect and magnetic field with inhomogeneous heat origination and sink. The formulation of the mathematical model is done by considering fluid with hybrid nanoparticles and dust particles in two different phases. We have considered Fe₂SO₄ and Cu as nanoparticles dispersed in the base fluid water along with suspended dust particles. The set of partial differential equations is reduced by using apt similarity variables and boundary conditions to obtain ordinary differential equations. The numerical solution is approximated using MATLAB-bvp4c adopting the shooting technique. The impact of numerous pertinent physical parameters on the velocity and thermal profiles is plotted and deliberated. Furthermore, the rate of heat flow and friction factor is also tabulated and visualized through the graphs. Streamlines are also drawn to know the behavior of the fluid flow. The rise in values of M_E quickly increases the velocity of the fluid motion but declines the thermal gradient and thickness of its related boundary layer. Also, inclining values of Pr enhance the thermal profile due to the impact of melting.     

Heat transfer of MHD flow over a wedge with a surface of mutable temperature

The objective of this study is to investigate the thermal distribution and heat transfer in the boundary layer of a wedge with a variable surface temperature in the presence of a magnetic field. To achieve this, we first used similarity solutions to transform the governing equations of magnetohydrodynamic flow for variable surface temperature conditions into ordinary differential equations. We then solved the resulting equations using the collocation method (CM) with different intensity magnetic fields and varying Hartmann (Ha) numbers and surface temperatures. The CM was further modified by incorporating boundary conditions. The results obtained from the solved equations were validated and compared with those obtained using the numerical Runge–Kutta fourth-order method and previous literature. Finally, we investigated the impact of various parameters on the friction coefficient (C_f) and Nusselt number (Nu), including the power of variable surface temperature (n), Prandtl (Pr) number, Eckert (Ec) number, the half angle of the wedge (φ), and Ha number. We considered values for these parameters within the ranges 0.5 ≤ n ≤ 1.5, 0.5 ≤ Pr ≤ 5, 0.001 ≤ Ec ≤ 0.002, 15° ≤ φ ≤ 60°, and 0 ≤ Ha ≤ 3. Our findings indicate that the slope of the boundary layer increases with increasing Ha or φ, resulting in an increase in C_f on the surface by up to 526%. The Nu number, calculated using the energy equation, increases up to 91.7%, 39.8%, and 1.43% with increasing Ha, n, and Ec, respectively, resulting in faster growth of the thermal boundary layer, which causes the thickness to decrease and the Nu number to rise. However, as φ increases, the Nu number drops on the surface, and the heat transfer behavior remains similar to that observed previously.     

Heat transfer in a turbulent slot jet flow impinging on concave surfaces

An experimental and numerical study is conducted to investigate turbulent slot jet impingement cooling characteristics on concave plates with varying surface curvature. Air is used as the impingement coolant. In the experimental work, a slot nozzle specially designed with a sixth degree polynomial in order to provide a uniform exit velocity profile was used. The experiments were carried out for the jet Reynolds numbers in the range of 3423 ≤ Re ≤ 9485, the dimensionless nozzle-to-surface distance range of 1 ≤ H/W ≤ 14 for dimensionless values of the curvature of impinging surfaces in the range of R/L = 0.5, 0.725, and 1.3 and a flat impingement surface. Constant heat flux was applied on the plates. Numerical computations were performed using the k-ε turbulence model with enhanced wall functions. For the ranges of the governing parameters studied, the stagnation, and local and average Nusselt numbers have been obtained both experimentally and numerically. The numerical results showed a reasonable agreement with the experimental data.     

Heat transfer enhancement from micro pin fins subjected to an impinging jet

An experimental investigation was carried out to study the single-phase stagnation point jet impingement heat transfer on smooth and micro pin fin structures using water and R134a. The experiments were carried out for a single jet (d_j = 2.0 mm) impinging on a 2 × 2 mm micro-heater over a wide range of Reynolds numbers. Both an unfinned and a micro structured impingement surfaces were investigated. The micro structures consisted of an array of 64 circular micro pin fins fabricated using MEMS microfabrication. The micro pin fins had diameters of 125 μm, heights of 230 μm, and pitches of 250 μm with an area enhancement of A_total/A_base = 2.44. The jet stand-off ratio and area ratio (A_j/A_base) were 0.86 and 0.785, respectively. Nusselt numbers were found to increase with increasing Reynolds numbers. Correlations from the literature for impingement zone Nusselt numbers were found to underpredict the experimental results. Significant enhancement of the heat transfer coefficients were observed as a result of the presence of the micro pin fins on the impingement surface. Enhancement factors as high as 3.03 or about 200% increase in the heat transfer coefficients were demonstrated. Enhancements are attributed to flow mixing, interruption of the boundary layers, and augmentation of turbulent transport.