Wake effects from a rising air bubble impacting a horizontal heated surface (original) (raw)
Related papers
Bubble Enhanced Heat Transfer from a Vertical Heated Surface
Journal of Enhanced Heat Transfer, 2008
A rising bubble in a liquid can greatly enhance heat transfer from heated surfaces by acting like a bluff body, displacing fluid as it moves and via the wake generated by the bubble, increasing the mixing of the liquid. The current research quantifies the effect a single free rising ellipsoidal air bubble has on heat transfer from a vertical heated block immersed in water. By measuring the time varying heat transfer and tracking the bubble dynamics, further understanding of the heat transfer mechanism is achieved. Both the plane in which the bubble oscillates and the proximity of the bubble path are shown to influence the surface heat transfer.
Bouncing bubble dynamics and associated enhancement of heat transfer
Journal of Physics: Conference Series, 2012
Heat transfer enhancement resulting from the effects of two phase flow can play a significant role in convective cooling. To date, the interaction between a rising gas bubble and a horizontal surface has received limited attention. Available research has been focused on bubble dynamics, although the associated heat transfer has not been reported. To address this, this study investigates the effect of a single bubble bouncing against a heated horizontal surface. Local heat transfer measurements have been performed for four orifice to surface distances, with a bubble injection orifice of 1 mm in diameter. High-speed photography and infrared thermography have been utilized to investigate the path of the bubble and the associated heat transfer.
Forced convection in the wakes of impacting and sliding bubbles
Heat and Mass Transfer, 2017
Both vapour and gas bubbles are known to significantly increase heat transfer rates between a heated surface and the surrounding fluid, even with no phase change. The cooling structures observed are highly temporal, intricate and complex, with a full description of the surface cooling phenomena not yet available. The current study uses high speed infrared thermography to measure the surface temperature and determine the convective heat flux enhancement associated with the interaction of a single air bubble with a heated, inclined surface. This process can be discretised into the initial impact, in which enhancement levels in excess of 20 times natural convection are observed, and the subsequent sliding behaviour, with more moderate maximum enhancement levels of 8 times natural convection. In both cases, localised regions of suppressed heat transfer are also observed due to the recirculation of warm fluid displaced from the thermal boundary layer with the surface. The cooling patterns observed herein are consistent with the interaction between an undulating wake containing multiple hairpin vortex loops and the thermal boundary layer that exists under the surface, with the initial nature of this enhancement and suppression dependent on the particular point on its rising path at which the bubble impacts the surface.
Bubble-wake interactions of a sliding bubble pair and the mechanisms of heat transfer
International Journal of Heat and Mass Transfer, 2017
An experimental investigation is reported for the bubble-wake interactions that occur between an in-line air bubble pair sliding under an inclined surface in quiescent water. Three experimental techniques are utilised to study this flow: time-resolved particle image velocimetry (PIV), a new edge-based bubble tracking algorithm incorporating high speed video and high speed infrared thermography. These techniques allow for a novel characterisation of sliding bubble-wake interactions in terms of their associated fluid motion, the fluid-induced changes in the trailing bubble interface and the resulting surface convective heat transfer. As these interactions are ubiquitous to multiphase flows, such knowledge is pertinent to many industrial applications, including the optimisation of two-phase cooling systems. This work has revealed that for an intermediate bubble size, in-line bubble pairs adopt a configuration in which their paths are 180°out of phase. Upon entering the fluid shed from the near wake of the leading bubble at each local extremum, the trailing bubble is accelerated both in the direction of buoyancy and in the spanwise direction corresponding to that of the shed fluid structure. This causes significant, high-frequency changes in the interface of the trailing bubble, which recoils and rebounds during this interaction. Surface heating adds further complexity to the bubble-wake interaction process due to the disruption of the thermal boundary layer at the surface. It is found that the trailing bubble can momentarily decrease local convective heat transfer levels by displacing the cool fluid introduced to the surface by the leading bubble. However, the amplified fluid mixing and local heat transfer enhancement of 7-8 times natural convection levels observed at the trailing bubbles mean that the net effect of the trailing bubble is to enhance convective heat transfer.
The dynamics of sliding air bubbles and the effects on surface heat transfer
International Journal of Heat and Mass Transfer, 2015
Freely rising and sliding bubbles have been found to increase local heat transfer coefficients from adjacent heated surfaces. The latter have been exploited in various industrial applications, such as shell and tube heat exchangers and chemical reactors. Although there is a relatively large body of work on bubbles, only a very small portion of this focuses on sliding bubbles. The current study intends to expand this by understanding both the motion and surface heat transfer characteristics of sliding bubbles. Herein, results are presented on bubbles of 4.75-9.14 mm equivalent spherical diameter sliding under both a heated and unheated surface, inclined at 30 relative to the horizontal. The sliding bubble path and shape oscillations are observed by a pair of high speed cameras. The frequency and amplitude of these oscillations are derived from analysis of the acquired images. Heat transfer is measured using a high speed infra-red camera synchronised to the video cameras, which is spatially and temporally aligned with the high speed images, allowing for the relationship between bubble motion and heat transfer to be observed. It has been found that bubbles in the range tested exhibit a sinusoidal motion. This motion is likely linked to the asymmetrical generation and shedding of vortices, with one vortex shed for each half period of path oscillation. It was observed that the bubble shape fluctuations were closely linked to the path oscillations and therefore vortex shedding. At higher bubble volumes, the bubble interface was found to recoil after a vortex is shed. There is little difference between bubble motion on a heated and unheated surface at lower bubble volumes, but at higher volumes a thinning of the bubble tips is observed along with a less-smooth interface, both of which are attributed to the thermal boundary layer at the surface. The bubble drag coefficient is also decreased when the surface is heated. Two-dimensional, time-varying surface heat transfer patterns reveal local heat transfer coefficient enhancements of up to 8 times that corresponding to natural convection levels, while the global surface-averaged heat transfer coefficient was up to twice that of natural convection. The mechanism of this heat transfer enhancement is a combination of forced convection by the sliding bubble and vortices shed at half the bubble path oscillation frequency that draw cool fluid from the bulk towards the surface. In the far wake, these vortices form isolated, elliptical regions of cooling that remain for many seconds after the bubble has passed. Interestingly, there are regions of the bubble wake where the heat transfer coefficient briefly drops below natural convection levels, highlighting the complexities of the fluid mechanics behind multiphase cooling.
Sliding bubble dynamics and the effects on surface heat transfer
Journal of physics, 2012
An investigation into the effects of a single sliding air bubble on heat transfer from a submerged, inclined surface has been undertaken. Existing literature has shown that both vapour and gas bubbles can increase heat transfer rates from adjacent heated surfaces. However, the mechanisms involved are complex and dynamic and in some cases poorly understood. The present study utilises high speed, high resolution, infrared thermography and video photography to measure two dimensional surface heat transfer and three dimensional bubble position and shape. This provides a unique insight into the complex interactions at the heated surface. Bubbles of volume 0.05, 0.1, 0.2 and 0.4 ml were released onto a surface inclined at 30 degrees to horizontal. Results confirmed that sliding bubbles can enhance heat transfer rates up to a factor of 9 and further insight was gained about the mechanisms behind this phenomenon. The enhancement effects were observed over large areas and persisted for a long duration with the bubble exhibiting complex shape and path oscillations. It is believed that the periodic wake structure present behind the sliding bubble affects the bubble motion and is responsible for the heat transfer effects observed. The nature of this wake is proposed to be that of a chain of horseshoe vortices. 2 The work presented in this paper was performed by the author during his time in Trinity College Dublin under the supervision of Prof. D B Murray as part of his Ph.D research.
Heat Transfer Enhancement from Bouncing Bubble Dynamics
2011
Heat transfer enhancement resulting from the effects of two phase flow can play an important role for convective cooling. Currently, the effect of bubble/surface interactions has received very little attention. To address this, this paper investigates the effect of a single bubble bouncing on a heated surface. Local heat transfer measurements have been performed for two bubble release heights of 15 and 25 mm, along with two levels of surface heat flux, 5800 W/m 2 and 9040 W/m 2 . High-speed photography and infrared thermography have been employed to investigate the path of the bubble and the resulting heat transfer effect when an interaction between the bubble and the heated surface takes place.
Mechanisms of heat transfer for axisymmetric bubble impingement and rebound
Heat and Mass Transfer, 2017
Heat transfer enhancement resulting from the impingement and rebound of bubbles in confined geometries can play an important role in heat transfer applications. Limited studies exist on the impact behaviour of large ellipsoidal bubbles against a horizontal surface, while the associated fluid flow field has received even less recognition. To address this, the current study investigates the dynamics of a single large ellipsoidal bubble impinging on a horizontal heated surface. The bouncing dynamics have been explored by utilising synchronised high-speed and IR photography. Due to the large bubble size in the present study only a bubble with a low release to surface distance was found to have a symmetric bouncing event. The results showed that separated wake structures initially cooled the surface before the wake structures become counter productive and convect warm fluid onto the previously cooled surface. Two cooling zones were observed; the inner region due to the bubble and the outer region due to the bubble's wake.
On the Mechanism of Bubble Induced Forced Convective Heat Transfer Enhancement
Frontiers in Heat and Mass Transfer
This article presents both an experimental and numerical study of both stationary and sliding bubbles in a horizontal duct with forced convection heat transfer. An experimental facility was fabricated using a fully transparent, electrically-heated test section in which the bubble dynamics and the thermal field on the heated wall can be acquired using high-speed cameras and Thermochromic Liquid Crystals (TLC). Experiments were conducted using the working fluid HFE 7000 for two different turbulent Reynolds numbers. The experimental temperature field in the span-wise direction is first compared to the numerically calculated temperature field of a bubble sliding near a wall and second to the temperature field calculated for a stationary bubble under the same flow and thermal conditions. In both cases the thermal field influence of the microlayer thickness, bubble shape, and the presence of multiple bubbles is investigated. An important outcome is that, unlike the sliding bubble case, the temperature field calculated in the stationary case is in agreement with the experimental results. The temperature field does not show any significant sensitivity to the micro-layer thickness or the bubble shape. It is concluded that the mechanism of heat transfer enhancement due to growing bubbles in forced convection is due to the flow perturbation induced by the bubble at the growth site or injection site rather than the thermal boundary layer disruption of the sliding bubbles. This is the reason flow boiling superposition correlations have success in predicting heat transfer without considering the bubble sliding process.
International Journal of Heat and Fluid Flow, 2009
Injection of sub-millimeter bubbles is considered a promising technique for enhancing natural convection heat transfer for liquids. So far, we have experimentally investigated heat transfer characteristics of laminar natural convection flows with sub-millimeter bubbles. However, the effects of the bubble size on the heat transfer have not yet been understood. The purpose of this study is to clarify the effects of the bubble size on the heat transfer enhancement for the laminar natural convection of water along a vertical heated plate with uniform heat flux. Temperature and velocity measurements, in which thermocouples and a particle tracking velocimetry technique are, respectively used, are conducted to investigate heat transfer and flow characteristics for different bubble sizes. Moreover, two-dimensional numerical simulations are performed to comprehensively understand the effects of bubble injection on the flow near the heated plate. The result shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection ranges from 1.3 to 2.2. The result also shows that for a constant bubble flow rate, the heat transfer coefficient ratio increases with a decrease in the mean bubble diameter. It is expected from our estimation based on both experimental data and simulation results that this increase results from an increase in the advection effect due to bubbles.