Heat transfer during pulsating liquid jet impingement onto a vertical wall

Liquid jet impingement is used for cooling and cleaning in various industrial branches. The advantages of jet impingement include high heat and mass transport rates in the vicinity of the impingement point. Pulsating liquid jets impinging on horizontal substrates with a pulsation frequency around 100 Hz have been shown to increase the cooling efficiency in comparison to jets with continuous mass flow rates. The influence of jet pulsation on cooling efficiency for impingement of horizontal jets onto vertical walls has not yet been investigated. In the case of a vertical heated wall, gravity contributes to the liquid flow pattern. In particular, if the time span between two pulses is sufficiently long, the liquid drainage from the region above the impingement point can contribute to heat transport without increasing the average flow rate of the cooling medium. In this work, the influence of pulsations on heat transfer during impingement of a horizontal liquid jet onto a vertical wall is investigated experimentally for the pulsation frequency range 1–5 Hz. The results regarding increase of heat transfer efficiency are related to flow patterns developing by impingement of successive pulses, as well as to the liquid splattering.

Stability of a moving radial liquid sheet: experiments

A recent theory (Tirumkudulu & Paramati, Phys. Fluids, vol. 25, 2013, 102107) for a radially expanding liquid sheet, that accounts for liquid inertia, interfacial tension and thinning of the liquid sheet while ignoring the inertia of the surrounding gas and viscous effects, shows that such a sheet is convectively unstable to small sinuous disturbances at all frequencies and Weber numbers $(We\equiv {\it\rho}_{l}U^{2}h/{\it\sigma})$. Here, ${\it\rho}_{l}$ and ${\it\sigma}$ are the density and surface tension of the liquid, respectively, $U$ is the speed of the liquid jet, and $h$ is the local sheet thickness. In this study we use a simple non-contact optical technique based on laser-induced fluorescence (LIF) to measure the instantaneous local sheet thickness and displacement of a circular sheet produced by head-on impingement of two laminar jets. When the impingement point is disturbed via acoustic forcing, sinuous waves produced close to the impingement point travel radially outwards. The phase speed of the sinuous wave decreases while the amplitude grows as they propagate radially outwards. Our experimental technique was unable to detect thickness modulations in the presence of forcing, suggesting that the modulations could be smaller than the resolution of our experimental technique. The measured phase speed of the sinuous wave envelope matches with theoretical predictions while there is a qualitative agreement in the case of spatial growth. We show that there is a range of frequencies over which the sheet is unstable due to both aerodynamic interaction and thinning effects, while outside this range, thinning effects dominate. These results imply that a full theory that describes the dynamics of a radially expanding liquid sheet should account for both effects.

Heat Transfer Performance of an Air-Assisted Impinging Water Jet Under a Fixed Pumping Power Constraint

Air injection into a liquid impinging jet has been shown to be a method of improving non-phase change heat transfer rates by up to twice the normal amount. Previous work has shown that there exists an optimal operating point in terms of the volumetric fraction of air injection when the pumping power is held constant because of an optimal two-phase flow pattern. However, previous work focused on heat transfer from the impingement point only, and neglected performance at other points. The present work studies the local heat transfer performance of an air-assisted water jet, at the impingement point and at positions moving radially outward, under constant pumping power conditions. The area-averaged heat transfer is also considered. Heat transfer at the stagnation point is shown to be optimized between β = 0.1∼0.2, where a bubbly flow pattern exists. Nuavg(r/D ≤ 1) is optimized when the flow pattern was plug-flow and off-center peaks in Nur exist. Nuavg(r/D > 1) is optimized when the water is accelerated by the injected air, but splattering is avoided. Flow patterns have no direct effect outside the impingement region.

Enhancement of Impinging Atomization by Microjet Injection

With increasing focus on environmental effects and the need for fuel diversity in gas turbines, good liquid atomization is increasingly important. It is known that impinging atomization is able to produce fine drops by impingement of fast liquid jets. However, the atomization characteristics deteriorate at lower injection velocities. In this study, for improving atomization characteristics under a wide range of injection velocity, an effective technique is verified utilizing a small amount of gas (microjet) injection. The microjet is supplied from a pressurized reservoir independent of the liquid supply system, and it is injected from the center of the liquid nozzles toward the impingement point. To clarify the flow field and the mechanism of the effectiveness, experimental visualizations and drop size measurements are carried out. It is found that atomization is remarkably promoted when the dynamic pressure of microjet overcomes that of the liquid at the impingement point. By the microjet injection with only 1% of liquid mass flow rate, Sauter mean diameter (SMD) becomes one-tenth of the original SMD. In addition, optimized atomization efficiency is successfully achieved when the dynamic pressure of the microjet is two times that of the liquid at the impingement point.

A Mechanism for the Rapid Development of Intracranial Aneurysms: A Case Study

BACKGROUND: Despite technical and diagnostic progress there are still open questions in the understanding of the pathophysiology of intracranial aneurysms. OBJECTIVE: Within 44 days we observed the de novo genesis and rupture of an aneurysm of the basilar artery in a patient. We performed computational fluid dynamics on 3-dimensional (3D) models of the inconspicuous vessel and the same vessel with aneurysm. Based on the simulations we propose a mechanism of genesis of fast-growing aneurysms. METHODS: Three-dimensional mesh models were built using computed tomography-angiography slices. Flow was modeled as a non-Newtonian blood model with shear-dependent dynamic viscosity. We investigated flow velocity, wall pressure, impingement point, wall shear stress (WSS), and asymmetric flows in 3D models of the vessel tree of the basilar artery. RESULTS: Impingement point and wall pressure had no clear relation to the origin of the aneurysm. The impingement point faded away during aneurysm growth. Instead we found an area of permanently low WSS in the original basilar artery. This location corresponded to the origin of the later developing aneurysm. Aneurysm growth was facilitated by an increasing overall expansion of the basilar tip and a constant decrease of WSS. CONCLUSION: Assuming a preexisting reduced resistibility of the vessel wall to pressure changes and an area of permanently low WSS, an increase in pressure induces geometrical changes. These cause changes of intravascular flow distribution, lowering the already low WSS in specific locations. This leads to endothelial damage in this area and to a decreasing stability of the vessel wall, causing aneurysm development, growth, and rupture.

Experimental Investigation of Wetted Zone Due an Impinging Circular Water Jet on a Moving Surface

Cooling liquid jet impingement has many industrial applications such as metal manufacturing where the shape and the size of wetting area around impingement point is important in heat transfer analysis. A series of experiments on a moving surface with an impinging industrial-scale circular water jet is conducted to explore experimentally the wetting region and the hydraulic jump as the front of wetted zone. The effects of surface motion on the wetting zone and the formation and location of hydraulic jump upstream the impingement point is examined by changing systematically the surface velocity and the jet flow rates. The surface motion impacted the radial evolution of wetted zone in all direction and decreased the radius of non-circular hydraulic jump. However, higher jet flow rate suppressed its effect. Both surface velocity and jet velocity or their velocity ratio has to be considered in adjusting jet space along a jet-line in industrial cooling application.

Transient Cooling of a Hot Steel Plate by an Inclined Bottom Jet

Controlled cooling on the runout table is a crucial component in the production of highly tailored steels since it has a strong influence on the final mechanical properties. High efficiency heat transfer in impinging jet cooling makes this an important method for heat transfer enhancement. The purpose of this study is to develop an experimental database for modelling of boiling heat transfer for bottom jet impingement that occurs during runout table cooling in a steel mill. Experiments have been carried out on a pilot scale runout table using stationary plates, with focus on the effect of water flow rate and nozzle inclination to the overall heat transfer rates. Volumetric flow rates and inclination angles are in the range of 35-55 l/min and 0-30º, respectively. Temperatures on the test plates are measured internally very close to the surface during cooling for the purpose of reducing thermal lag and receiving better data responsiveness. These measurements are taken at the impingement point and several streamwise distances from the impingement point. From the above measurements transient cooling data on the hot steel plate by bottom jet impingement has been analysed.