Circular Jet Impingement Research Articles

Heat Transfer of Oscillating-Move Pin-Fin Heat Sinks with Circular Impinging Jets

This work experimentally investigated the effect of the oscillation move on the heat transfer enhancement of the pin-fin heat sinks with circular impinging jets. The forced convective cooling system usually applies the steady flow to pass through a stationary object. This work mounted the pin-fin heat sinks onto an oscillating platform with the impinging air jets. The experimental results indicate that strokes of the platform moving up and down as well as left and right are too long to enhance the overall heat transfer of the pin-fin heat sinks.

Fluid flow characteristics of single inclined circular jet impingement for ultra-fast cooling

The fluid flow characteristics of the single bunch inclined jet impingement were investigated with different jet flow velocities, nozzle diameters, jet angles and jet-to-target distances for ultra-fast cooling technology. The results show that the peak pressure varying significantly from nearly 0.5 to above 13.4 kPa locates at the stagnation point with different jet diameters, and the radius of impact pressure affected zone is small promoted from 46 to 81 mm in transverse direction, and 50 to 91 mm in longitude direction when the jet flow velocity changes from 5 to 20 m/s. However, the fluid flow velocity is relatively smaller near the stagnation point, and increases gradually along the radius outwards, then declines. There is an obvious anisotropic characteristic that the flow velocity component along the jet direction is about twice of the contrary one where the jet anlge is 60°, jet diameter is 5 mm, jet length is 8 mm and jet height is 50 mm.

Experimental study on critical heat flux of high-velocity circular jet impingement boiling on the nano-characteristic stagnation zone

This experimental study investigates the critical heat flux (CHF) of high-velocity circular jet impingement boiling on the nano-characteristic surface of the stagnation zone with different surface wettability. The wettability of the copper surface is varied to hydrophilic or hydrophobic nano-characteristic surfaces by modifying surface topography and chemistry. The effect of impact velocity, subcooling and solid–liquid contact angle (CA) on the CHF are studied thoroughly. The experimental relationship between CA and CHF of the heat transfer surface is summarized by further discussion. The semi-theoretical correlations proposed by the author previously for predicting the CHF of saturated and subcooled circular jet impingement boiling on the stagnation zone are improved and expanded to higher impact velocity and wider CA range. The comparison results indicate the predicted value of the improved correlations agrees well with the experimental data.

Numerical Simulation of the Jet Impingement Cooling of a Circular Cylinder

A numerical investigation was carried out on circular jet impingement heat transfer from a constant temperature circular cylinder to understand the major parameters which influence the fluid flow and heat transfer characteristics. In this study, air was considered as the working fluid. The flow was considered to be three-dimensional, incompressible, and turbulent. To select a suitable turbulence model for the parametric study, numerical simulations were carried out with standard k-ϵ, standard k-ω, RNG k-ϵ, Realizable k-ϵ, and SST k-ω turbulence models for modeling Reynolds stress terms. Simulations were also carried out using four low Reynolds number models. The results obtained using these models were compared with the available experimental results of jet impingement heat transfer from circular cylinder. It was identified that the RNG k-ϵ model predicts heat transfer characteristics better compared to all other turbulence models considered in this study. Using this turbulence model, a parametric study was carried out for the Reynolds number (Re d ), defined based on the diameter of the nozzle ranging from 10,000 to 50,000. The ratio of distance between the nozzle exit and the cylinder surface to the diameter of the jet (h/d) was varied from 4 to 16 and the ratio of nozzle diameter to cylinder diameter (d/D) varied from 0.11 to 0.25. For a fixed Re d and d/D, the stagnation point Nusselt number increases as h/d decreases. The stagnation point Nusselt number decreases as d/D increases for a fixed value of Re d and h/d. The effects of change in h/d and d/D are significant only near the stagnation region.

열간압연 이후 주행하는 고온 강재의 냉각해석에서 소재의 물성변화 효과 연구

열연 판재 제조과정에서는 제품의 강도와 인성을 제어하기 위해 압연 직후 800℃ 이상의 고온 소재를 수냉각 방식으로 급속 냉각시킨다. 이 과정에는 소재 표면과 냉각수 사이의 비등 열전달 현상과 소재 표면에 쌓이는 체류수의 자유표면 유동, 소재의 고속 주행 등 매우 복잡한 물리현상들을 포함하고 있다. 본 연구에서는 이 모든 물리 현상들을 수치적으로 모델링한 해석 모델을 적용하여 기본 열전달 해석을 수행한다. 실제로 소재는 냉각에 의해 내부에서 오스테나이트로부터 페라이트로 상변태가 일어나고 이로 인해 소재의 국부적인 열물성치의 변화가 발생하지만 본 연구에서는 상변태를 직접 푸는 방법 대신 이미 알려진 소재의 온도에 따른 물성변화 곡선을 이용하여 냉각해석을 수행하고 이를 기존의 일정 물성치 조건에서 해석한 결과와 비교하여 소재의 물성변화가 소재 냉각에 미치는 영향과 상변태 해석의 필요성에 대해 검토하였다.

Experimental and numerical investigation of jet impingement cooling of a circular cylinder

The results obtained from both the experimental and numerical investigations of turbulent circular air jet impingement cooling of a circular heated cylinder are presented. At the surface of the heated cylinder, a constant heat flux condition was maintained. The Reynolds number, Red, defined based on the nozzle diameter, was varied from 10,000 to 25,000. The geometric parameters such as the non-dimensional distance between the nozzle exit and the circular cylinder, h/d, and the ratio of nozzle diameter to the diameter of the heated target cylinder, d/D, were investigated for the range of 4–16 and 0.11–0.25, respectively. The local Nusselt number variation along the circumferential and axial directions obtained from the experimental studies are reported. To understand the flow features and to obtain the temperature and local Nusselt number distributions over the surface of the heated cylinder, a numerical study was also performed. It was observed that the stagnation Nusselt number increases monotonically as the h/d decreases and the effects of h/d and d/D are significant only in the jet impinging region. Based on the experimental results, a correlation for the stagnation Nusselt number has also been developed.

Numerical Study of the Effect of Nozzle Arrangement on Cooling Process in Running Hot Steel Strip after Hot Rolling

In the steel strip manufacturing process, a high temperature steel strip at a temperature of more than 800°C is rapidly cooled by circular impinging water jets. The cooling rate is an important parameter that characterizes the desired microstructure and mechanical properties of the steel strip. In the cooling process, the nozzle arrangement of the water jet system can be easily changed as necessary and heat transfer characteristics can be controlled by adjusting the nozzle arrangement. In this study, inline and staggered nozzle arrangements are adopted. By using the numerical method developed in a previous study, cooling behaviors such as the temperature distribution of the plate surface, the cooling history of the plate, and the average heat flux are determined and compared quantitatively for each nozzle arrangement.

Thermal boundary layer analysis corresponding to droplet train impingement

Simulations of droplet train impingement on a pre-wetted solid surface heated from below are used to study the thermal boundary layer behavior over a parameter space which includes variations in Reynolds, Peclet, and Weber numbers, as well as variations in inter-droplet spacing and initial liquid film thickness. Computationally, a modified version of the Volume-of-Fluid method is developed and employed in this study. The solver is validated against closed form solutions and additional experimental data from the literature. In combination with the simulations, an analytical representation is also developed and compared to the computations yielding favorable agreement. Results show that the boundary layer thickness is mostly affected by changes in inter-droplet spacing, Reynolds, and Peclet number, and influenced minimally by variations in Weber number and initial film thickness. In fact, it is explicitly demonstrated in the analysis that the impact velocity has the greatest effect in local heat transfer. An analytical expression for the Nusselt number radial profile is also developed. It shows that the Nusselt number scales as ∼ Re1/2, and its radial dependence is \documentclass[12pt]{minimal}\begin{document}$\sim \sqrt{r}$\end{document}∼r, which is the same as the circular jet impingement case. The notable difference in the present Nusselt number relationship is the role of inter-droplet spacing, which plays a significant role in the current configuration.

Scour formation due to simultaneous circular impinging jet and wall jet

Experiments involving both a circular wall jet and an impinging jet were conducted, as occurring at irrigation control structures or at dams with combined crest and low-level spillways. The effects of the non-dimensional parameters on the scour hole dimensions have been investigated. It was found that the effect of the expansion ratio of the channel width to the jet diameter is negligible for values larger than 22.5 and 35, depending on the experimental conditions. The scour depth decreases with increasing tailwater depth and the horizontal distance between the impinging jet and the origin of the wall jet. The scour length and the distance between the end of the ridge and the wall jet origin increase with decreasing densimetric Froude number of the impinging jet. The opposite trend occurs for decreasing densimetric Froude number of the wall jet. An empirical relationship for estimating the scour depth due to the simultaneous jet action is given.

Experimental investigation for enhancement of heat transfer from cooling of electronic components by circular air jet impingement

An experimental investigation is carried out to study the enhancement of heat transfer from surface of the electronic components by impingement of a circular air jet. Local and stagnation Nusselt number on the impinged surface of the electronic components are presented for different nozzle configurations. Reynolds number, based on nozzle diameter (d) is varied between 5,500 and 28,500 and nozzle-to-electronic component spacing from 2 to 10 nozzle diameters. The measured data were correlated into a simple equation and compared to the predictions of several other correlations proposed by other researchers. The heat transfer mechanisms involved in the enhanced performance are discussed. The study provides a lot of useful information for the application of impinging jet heat transfer in electronic industry.

Design analysis of an experimental jet impingement study by using Taguchi method

In current study design analysis of a specified impingement jet application is performed by use of Taguchi method. Some results from a previously done experimental study that includes the impingement of co-axial and single circular jets to the both roughened and smooth heated surfaces are handled. By use of Taguchi method, some results are obtained to show the effect of Reynolds numbers (Re = 20,000 and 40,000), non-dimensional surface to jet exit spacing (H/D = 2, 6, 8 and 10), surface roughness (rough or smooth surface) and jet type (co-axial or circular). Finally, the effects of Reynolds number, surface roughness, jet-to-plate spacing and jet type on impingement heat transfer, by means of average Nusselt number, are found to be respectively, 83, 2, 4.5 and 10.7%.

Experimental investigation of circular free-surface jet impingement quenching: Transient hydrodynamics and heat transfer

An experimental study of heat transfer during quenching of a cylindrical stainless steel test specimen has been performed. A subcooled water jet is directed onto the upward facing flat face of the cylinder. The test specimen is heated to an initial temperature slightly above 900 °C and then quenched. The resulting boiling curves and heat transfer distributions are presented for impingement velocities of 2.85 and 6.4 m/s (Re = 7900 and 18,900). High-speed imaging shows that three distinct regions on the quenched surface can be identified: an expanding circular wetted region surrounding the impinging point, annular transition zone just outside the wetting front, and a unwetted region outside this zone. The free-surface of the liquid in the wetted region is smooth in the nucleate and transition boiling regimes. The annular transition zone or the wetting front region outside the wetted region is characterized by a highly disturbed liquid–gas interface, which can be attributed to intense vapor generation. At the outer edge of the transition zone, the liquid is deflected away from the surface. The velocity of the wetting front significantly increases with the jet impact velocity, which indicates that the wetting front position is governed by the ability of the flowing liquid to transport the bubbles radially outwards from the wetted region.

Effects of the surface roughness on heat transfer of perpendicularly impinging co-axial jet

Current work presents the comparison of the cooling characteristics of roughened and smooth heated surfaces subjected to co-axial impinging jet. The work fluid is air and the data runs are performed for jet Reynolds numbers for 10,000, 20,000 and 40,000, and non-dimensional surface to jet exit spacing, H/D, from 1 to 10. The co-axial jet configuration is based on a fully developed pipe flow encountering a double-pipe arrangement and splitting between the two pipes. The inner to outer diameter ratio is 0.5. A straight pipe without inner section is used as the circular jet. The impingement of circular jets to the roughened and smooth surfaces is also performed for comparison. Average Nusslet numbers were obtained to show the heat transfer enhancement from the surface. A good agreement between the literature and present paper was obtained. As a result, average Nusselt number with co-axial jet impingement to the roughened surface increased by up to 27% comparing to the circular jet impingement. In addition, the average Nusselt number increased with roughened surface by up to 6% over the whole surface area, comparing to the smooth surface.

Numerical simulation of sediment erosion by submerged jets using an Eulerian model

The erosion of loose beds by submerged circular impinging vertical turbulent jets is simulated using an Eulerian two-phase model which implements Euler-Euler coupled governing equations for fluid and solid phases, and a modified k-ɛ turbulence closure for the fluid phase. Both flow-particle and particle-particle interactions are considered in this model. The predictions of eroded bed profiles agree well with previous laboratory measurements and self-designed experiments. Analysis of the simulated results reveals that the velocity field of the jet water varies with various scouring intensities, that the scour depth and shape are mainly influenced by the driving force of the water when the density, diameter and porosity of the sand are the same, and that the porosity is an important contributor to sediment erosion. In this study, the scour depth, the height of dune and the velocity of the pore water increase with increasing porosity.

Effects of Jet Obliquity on Hydraulic Jumps Formed by Impinging Circular Liquid Jets on a Moving Horizontal Plate

The present work attempts to investigate the effects of jet obliquity on the spatial patterns formed as a consequence of hydraulic jumps due to the impingement of circular liquid jets on continuously moving but nonaccelerating horizontal flat plates. Both the normal and the oblique impinging jets are considered, in order to characterize the contrasting features of the associated hydraulic jump mechanisms. Theoretical calculations are executed to obtain the locations of the jump, for different jet and plate velocities and jet inclination angles, using a depth-averaged momentum integral equation for shallow-free surface flows. Comparisons are subsequently made between the theoretical predictions and experimental observations reported in the literature, and a good agreement between these two can be observed. Special cases of a circular hydraulic jump when the target plate is stationary and the impinging jet is vertical, and elliptic hydraulic jumps when the target plate is stationary and the impinging jet is obliquely inclined, are also discussed. It is conjectured that flow due to impinging jets on a horizontal moving plate can be modeled as an equivalent flow due to an inclined impinging jet on stationary horizontal flat plates, with appropriate alterations in the jet velocity and the jet inclination angles.

Large-scale laboratory experiment on erosion of sand beds by moving circular vertical jets

This study investigates the topographic deformation due to the erosion of a sand bed impinged by a moving submerged turbulent round jet in a large-scale laboratory. The test conditions represent the case of discharges beneath a vessel while operating in water with a limited clearance such as a shallow navigation channel. The jet moves horizontally and discharges water vertically downward towards the bed. The distance between the jet nozzle and the bed equals six times the jet diameter so the jet flow is in the potential core region. The speed of the jet horizontal motion was varied to examine its effect on the scour profile. The characteristic lengths of the scour profile in the asymptotic state were determined by modifying the empirical formulas in Aderibigbe and Rajaratnam [1996. Erosion of loose beds by submerged circular impinging vertical turbulent jets. Journal of Hydraulic Research 34(1), 19–33]. The maximum scour depth, the scour hole radius, and the ridge height were found to be a function of the ratio of the jet exit to jet translation velocities and were modeled using a hyperbolic function. Empirical equations describing the scour profile were developed and the scour profile was found to be self-similar when normalized by appropriate length scales.

A simple dissipation model of circular hydraulic jump

Considered has been the phenomenon of hydraulic jump formed as a result of circular jet impingement on the horizontal surface. The horizontal spreading of liquid is induced mainly by action of inertia forces. In the case of supercritical conditions of film flow the phenomenon of hydraulic jump may appear. Postulated model includes in the analysis dissipation effects present in the flow which lead to formation of the hydraulic jump. The model is derived from the analysis of Bernoulli equation. Some preliminary analysis on the formation of the type I and type II hydraulic jump, i.e., featuring formation of one or two eddies, has also been given. Presented model was compared with available database of experimental results. Satisfactory consistency has been achieved.

Application of pressure and temperature sensitive paints for study of heat transfer to a circular impinging air jet

Experimental and computational studies are performed to study pressure and temperature distributions and flow patterns on impingement target surface subject to a single impinging air jet from a plenum. The experiments cover a range of jet-to-target plate distance, Z / D , from 1.5 to 12 for Reynolds number range from 10 000 to 60 000. The main objective is to investigate the optimal jet-to-target distance ( Z / D ) for stagnation point heat transfer and location of second peak in local heat transfer at small Z / D value under the current jet impingement configuration. Pressure and temperature sensitive paints measurements techniques are implemented to obtain the distribution of pressure and temperature on target surface. The optimal separation distance ( Z / D ) for stagnation region Nu is found to be about 5. The location of the second peak in local Nu for Z / D of 1.5 is at radial location ( r / D ) of about 1.8. The area averaged Nusselt numbers are compared to those calculated using correlation from open literature. The agreement varies with Z / D and jet Reynolds number. Experimental results are also compared to computational (CFD) prediction using realizable k– ε turbulence model.

Hydraulic jumps due to oblique impingement of circular liquid jets on a flat horizontal surface

An obliquely inclined circular water jet, impinging on a flat horizontal surface, confers a series of hydraulic jump profiles, pertaining to different jet inclinations and jet velocities. These jump profiles are non-circular, and can be broadly grouped into two categories, based on the angle of jet inclination, φ, made with horizontal. Jumps corrosponding to the range (25° < φ≤ 90°) are observed to be bounded by smooth curves, whereas those corresponding to φ≤ 25° are characterized by distinct corners. The present work attempts to find a geometric and hydrodynamic characterization of the spatial patterns formed as a consequence of such non-circular hydraulic jump profiles. Flow-visualization experiments are conducted to depict the shape of demarcating boundaries between supercritical and subcritical flows, and the corresponding radial jump locations are obtained. Theoretical calculations are also executed to obtain the radial locations of the jumps with geometrically smooth profiles. Comparisons are subsequently made between the theoretical predictions and the experimental observations, and a good agreement between these two can be observed. Jumps with corners, however, turn out to be comprised of strikingly contrasting profiles, which can be attributed to the ‘jump–jet’ interaction and the ‘jump-jump’ interaction mechanisms. A phenomenological explanation is also provided, by drawing an analogy from the theory of shock-wave interactions.

0410 閉鎖空間における単孔衝突噴流の伝熱特性に関する研究(S47 次世代ガスタービン,S47 次世代ガスタービン)

0410 閉鎖空間における単孔衝突噴流の伝熱特性に関する研究(S47 次世代ガスタービン,S47 次世代ガスタービン)