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.     

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.     

Boiling Heat Transfer of Multiple Impinging Jets on a Hot Moving Plate

In this research, boiling heat transfer on a hot moving plate caused by multiple impinging water jets in multiple jet rows is studied. An inverse heat conduction code is developed to analyze the readings of thermocouples that are implemented inside the plate in order to find the surface values of temperature and heat flux. Effects of nozzle stagger, plate velocity, and jet line spacing are studied. Nozzle stagger is found to affect the uniformity of heat transfer across the width of the plate. Jet line spacing can affect the heat transfer between two adjacent jet rows. Plate speed is important only in the higher entry temperatures and in the impingement zone.     

A numerically-based parametric study of heat transfer off an inclined surface subject to impinging airflow

Impinging jets may be used to achieve enhanced local heat transfer for convective heating, cooling, or drying. The issuing jet may contact the surface normally or obliquely. Factors such as jet attachment, surface angle, jet angle and size, separation distance between jet orifice and surface of impingement, and trajectory influence heat transfer dramatically. This study addresses the thermal problem of jet impingement on an inclined surface and is motivated by the practical application of air jets issuing out of a defroster’s nozzles and impinging on the inclined windshield surface of a vehicle. The effects of incoming fluid velocity, openings’ geometry (circular vs. rectangular), number of openings, angle that the inclined surface makes with the horizontal plane and angle of impinging jet on heat transfer are examined. Fluid mechanics and heat transfer characteristics are exhibited in details for a configuration with three rectangular openings. A comparative study for other configurations is also featured. The results are correlated in terms of governing dimensionless parameters through numerically-based correlations that are useful for predicting heat transfer on an inclined surface subject to impinging airflow.     

Annular Impinging Jet Controlled by Radial Synthetic Jets

This experimental study focuses on generation and control of annular impinging jets. The annular nozzle used in the investigations was designed with an active flow control system using 12 synthetic jets issuing radially from the central nozzle body. Measurements of the control effects were made on the impingement wall. The data acquisition involved wall pressure and wall mass transfer (by the naphthalene sublimation technique) using air as the working fluid. Also measured was time-mean flow velocity (by a Pitot probe) in the jet flow field. Moreover, flow visualization was carried out. Two main flow-field patterns (A and B) were identified. The patterns differ in the size of the separated-flow recirculation regions that develop attached to the nozzle central body: While pattern A is characterized by a quite small recirculation region (bubble) extending not far from the nozzle exit, pattern B exhibits a large recirculation region, reaching up to the impingement wall, on which it forms a stagnation circle. The control action modifies the flow field, resulting in changes of the corresponding heat/mass transfer distributions. The convective transfer rate on the stagnation circle can be demonstrably enhanced by 20% at a moderate nozzle-to-wall distance, equal to 0.6 of the nozzle outer diameter.     

Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets

Experimental studies were carried out to investigate the flame shape and the heat transfer and wall pressure characteristics of a pair of laminar premixed butane/air flame jets impinging vertically upon a horizontal water-cooled flat plate at jet Reynolds numbers of 800, 1000 and 1200, respectively. Equivalence ratio of the butane/air mixture was maintained constantly at unity. The flame shape, the pressure distribution on the impingement plate and the heat transfer from the flame to the plate were greatly influenced by the interference occurred between the two flame jets. This interference caused a sharp pressure peak at the between-jet midpoint and the positive pressures at the between-jet area, which led to the separation of the wall jet from the impingement plate after collision. Such interference became more significant when the non-dimensional jet-to-jet spacing (S/d) and the nozzle-to-plate distance (H/d) were reduced. Heat transfer in the interaction zone between the jets was at the lowest rate due to this interference at the smallest S/d ratio of 2.6, resulting from the separation of the high-temperature inner reaction zone of the flame from the impingement plate. On the other hand, the interference enhanced the heat transfer in the interaction zone between the jets when the S/d ratio was greater than 5, by enhancing the heat transfer coefficient. The average heat flux of the impingement plate was found to increase significantly with the increasing H/d ratio until H/d=6. The present study provided detailed information on flame shape and the heat transfer and wall pressure characteristics of a twin laminar pre-mixed impinging circular flame jets, which has rarely been reported in previous studies.     

Heat/mass transfer in rotating impingement/effusion cooling with rib turbulators

A detailed measurement of the heat/mass transfer coefficients on the ribbed surfaces for the rotating impingement/effusion cooling system has been conducted. Three different jet orientations (front, leading, and trailing) were investigated at the same rotating speed and impinging jet Reynolds number of 3000. A naphthalene sublimation method was used to obtain local heat/mass transfer coefficients. Regardless of rib turbulators, the leading and trailing orientations lead to totally changed heat/mass transfer distributions due to the jet deflection, while the Sh distributions of the front orientation were similar to those of the stationary case. For leading and trailing orientations, the influence of crossflow, which deflected wall jets, decreased due to the blockage effect of the rib turbulators. Therefore, the wall jets spread more widely and the interaction between adjacent wall jets along spanwise direction became stronger, enhancing the heat/mass transfer compared to that on smooth surface.     

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.     

Effects of jet pattern on single-phase cooling performance of hybrid micro-channel/micro-circular-jet-impingement thermal management scheme

This study explores the single-phase cooling performance of a hybrid cooling module in which a series of micro-jets deposit coolant into each channel of a micro-channel heat sink. This creates symmetrical flow in each micro-channel, and the coolant is expelled through both ends of the micro-channel. Three micro-jet patterns are examined, decreasing-jet-size (relative to center of channel), equal-jet-size and increasing-jet-size. The performance of each pattern is examined experimentally and numerically using HFE 7100 as working fluid. Indirect refrigeration cooling is used to reduce the coolant’s temperature in order to produce low wall temperatures during high-flux heat dissipation. A single heat transfer coefficient correlation is found equally effective at correlating experimental data for all three jet patterns. Three-dimensional numerical simulation using the standard k–ε model shows excellent accuracy in predicting wall temperatures. Numerical results show the hybrid cooling module involves complex interactions of impinging jets and micro-channel flow. Increasing the coolant’s flow rate strengthens the contribution of jet impingement to the overall cooling performance, and decreases wall temperature. However, this advantage is realized at the expense of greater wall temperature gradients. The decreasing-jet-size pattern yields the highest convective heat transfer coefficients and lowest wall temperatures, while the equal-jet-size pattern provides the greatest uniformity in wall temperature. The increasing-jet-size pattern produces complex flow patterns and greater wall temperature gradients, which are caused by blockage of spent fluid flow due to the impingement from larger jets near the channel outlets.     

Local heat/mass transfer and flow characteristics of array impinging jets with effusion holes ejecting spent air

The present study investigates the effects of spent air flows with and without effusion holes on heat/mass transfer on a target plate for array impinging jets. For a conventional type of array impinging jets without effusion holes, the spent air of the injected jets forms a cross-flow within the confined space and affects significantly the downstream jet flow. The injection plate of array impinging jets is modified having effusion holes to prevent the cross-flow of the spent air where the spent air is discharged through the effusion holes after impingement on the target plate. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate using a heat and mass transfer analogy. The flow patterns of the array impinging jets are calculated numerically and compared for the cases without and with the effusion holes. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to the strong effects of cross-flow and re-entrainments of spent air. However, uniform distributions and enhancements of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the effect of cross-flow is weak and the distributions and levels of heat/mass transfer coefficients are similar for both cases.     

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.     

Study on flow and heat transfer of multiple impingement jets

Multiple jets are often used to obtain high heat transfer in a wide area. Heat and flow behavior of multiple jets show more complicated characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer coefficients over the impingement plate were measured using an infrared radiometer when four jets impinged to the impingement plate for various jets arrangements and separating distances from jet exit to the impingement plate. The flow fields were also visualized by means of a water table. The heat transfer characteristics are made clear from the results of the instantaneous heat transfer contour or temperature fluctuation values. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 419–431, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20073     

The role of jet inlet geometry in impinging jet heat transfer,modeling and experiments

Effects of jet inlet geometry and aspect ratio on local and average heat transfer characteristics of totally nine confined impinging jets have been investigated experimentally using thermochromic liquid crystals and numerically by using a 3-D low Reynolds number k–? model. Experimental study by using liquid crystals for temperature measurement was conducted for three different jet exit geometries (circular, elliptic, rectangular). In addition, simulations were performed at the same mass flow rate for totally nine jet exit geometries including circular, elliptic and rectangular jets with different aspect ratios for dimensionless jet to plate distances 2, 6, and 12.As the aspect ratio of equal cross-sectional area elliptic and rectangular jets increases, heat transfer enhancement in the stagnation region was obtained. As a result higher aspect ratio jets can be used as a passive enhancement technique for localized heating or cooling especially at small jet to plate distances. Wall jet region comprises very large portion of the impinging plate under study and generally lower heat transfer rates were attained for higher aspect ratio jets in this region especially at small jet to plate distances. Therefore as the aspect ratio increases, lower average heat transfer rates were acquired. The effect of aspect ratio on local and average heat transfer decreases with increasing jet to plate distance. Even though the mass flow rate is the same, heat transfer rate of rectangular jets were reduced with increasing the cross-sectional area. With increasing jet to plate distance very similar heat transfer characteristics were observed along the major and minor axis directions.     

阵列射流冲击冷却流场与温度场的数值模拟

采用数值模拟方法对冲击冷却的流动和传热过程进行了三维数值研究。特别研究了在冲击孔叉排方式下,相邻孔间距、冲击距离以及射流入口雷诺数对冲击表面冷却流动传热特性的影响规律。     

Flow structure,wall pressure and heat transfer characteristics of impinging annular jet with/without steady swirling

The detailed impinging annular jet characteristics with/without induced swirling motion are experimentally studied. Obtained flow and wall pressure data confirm the classical observation for conventional annular impinging jet, with the flow exhibiting simple round impinging jet-like features at sufficiently large separation distances. In contrast, no such transition occurs in the presence of swirling. At short and intermediate separation distances, the swirling annular jet causes more non-uniform distributions of wall pressure and heat transfer on the impingement plate compared to the conventional annular jet. However, the reverse holds at large separation distances.     

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

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

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.     

Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)

The use of a jet from an orifice nozzle with a saddle‐backed‐shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×10⁴. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/d_o?0.5 is observed for α=15^°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20243     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.