On the Mechanism of Bubble Induced Forced Convective Heat Transfer Enhancement (original) (raw)
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Effect of Air Bubbles on Heat Transfer Coefficient in Turbulent Convection Flow
2017
Experimental and numerical studies have been conducted for the effect of injected air bubbles on the heat transfer coefficient through the water flow in a vertical pipe under the influence of uniform heat flux. The investigated parameters were water flow rate of (10, 14 and 18) lit/min, air flow rate of (1.5, 3 and 4) lit/min for subjected heat fluxes of (27264, 36316 and 45398) W/m2. The energy, momentum and continuity equations were solved numerically to describe the motion of flow. Turbulence models k-e was implemented. The mathematical model is using a CFD code Fluent (Ansys15). The water was used as continuous phase while the air was represented as dispersed. phase. The experimental work includes design, build and instrument a test rig for that purpose. Acircular vertical copper pipe test section of (length=0.7m, diameter= 0.05m, thickness= 1.5mm) is . designed and constructed, heated by an electrical heater fixed on its outer surface. Water . temperature at inlet is kept const...
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.
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.
This paper presents analysis of the heat transfer attendant upon the departure of a single steam bubble during pool boiling of water at atmospheric pressure. The flow of heat from a solid surface to liquid water during and immediately after bubble lift-off has been extracted from micro-scale measurements of the spatial and temporal variation of the temperature at the solid surface beneath the bubble. The numerical procedure used to extract the heat flux from the temperature variations at the solid surface has been assessed and verified, and applied to investigate the heat transfer during the bubble departure phase, and after the eventual bubble lift-off. Results confirm that fluid motion activated by a departing bubble is the cause of heat transfer enhancement. The phenomenon can be characterised as the process of rewetting, by an advancing liquid front, of a dry portion of wall area at the base of the bubble. The portion of wall area that is affected by the observed heat transfer augmentation mechanism has been found to be that of a circle of diameter roughly equal to half the bubble departure diameter. The current measurements enabled validation of interface-capturing numerical simulation of the hydrodynamics and heat transfer of single bubble formation and departure from a surface, including conjugate heat transfer in the solid substrate. From simulation results, the spatial and temporal variation of the heat flux at the solid surface beneath the bubble has been computed and monitored during bubble departure and after the eventual bubble lift-off. Heat transfer rates at bubble departure extracted from simulation have been found in good agreement with measurements. The simulation correctly captured experimental trends and was found to give an accurate estimate of the magnitude of the flows of heat to the liquid due to the bringing of cold fluid in the vicinity of the wall caused by the bubble departure process.
Experiments in Fluids, 2008
Sub-millimeter-bubble injection is one of the most promising techniques for enhancing heat transfer for the laminar natural convection of liquids. However, flow and heat transfer characteristics for laminar natural convection of water with sub-millimeter bubbles have not yet been fully understood. The purpose of this study is to experimentally clarify the effects of sub-millimeter-bubble injection on the laminar natural convection of water along a heated vertical plate. The use of thermocouples and a particle tracking velocimetry (PTV) technique are applied to temperature and velocity measurements, respectively. The temperature measurement shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection increases with an increase in the bubble flow rate or a decrease in the wall heat flux and that the ratio ranges from 1.35 to 1.85. Moreover, it is concluded from simultaneous measurement of temperature and velocity that the heat transfer enhancement is directly affected by flow modification due to bubbles rising near the heated vertical plate.
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.
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.
Augmentation of heat transfer in a bubble agitated vertical rectangular channel
Heat and Mass Transfer, 2012
This paper presents the results of an experimental study of convective heat transfer between three parallel vertical plates symmetrically spaced with and without bubble agitation to ascertain the degree of augmentation of the heat transfer coefficients due to agitation. The centre plate was electrically heated, while the other side plates were water-cooled forming two successive parallel vertical rectangular channels of dimensions 20 cm 9 3.5 cm 9 35 cm (length W, gap L, height H) each. At the bottom of the hot and cold plates air spargers were fitted. Water/ethylene glycol (100%) was used to fill the channels. The superficial gas velocity ranged from 0.0016 to 0.01 m/s. Top, bottom and sides of the channels were open to the water/ethylene glycol in the chamber which is the novel aspect of this study. Experimental data have been correlated as under: Natural convective heat transfer:
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.
International Journal of Multiphase Flow, 2016
This second of two companion papers presents an analysis of sliding bubble and wall heat transfer parameters measured during subcooled boiling in a square, vertical, upward flow channel. Bubbles were generated only from a single nucleation site for better observation of both the sliding bubble characteristics and their impact on wall heat transfer through optical measurement techniques. Specific interests include: (i) bubbles departure and subsequent growth while sliding, (ii) bubbles release frequency, (iii) coalescence of sliding bubbles, (iv) sliding bubbles velocity, (v) bubbles size distribution and (vi) wall heat transfer influenced by sliding bubbles. The results showed that sliding bubbles involve two distinct growth behaviors: (i) at low mass fluxes, sliding bubbles grew fast near the nucleation site, subsequently shrank, and then grew again, (ii) as mass flux increased, however, sliding bubbles grew more steadily. The bubbles originating from the single nucleation site coalesced frequently while sliding, which showed close relation with bubbles release frequency. The sliding bubble velocity near the nucleation site consistently decreased by increasing mass flux, while the observation often became reversed as the bubbles slid downstream due to the effect of interfacial drag. The sliding bubbles moved faster than the local liquid (i.e., u r <0) at low mass flux conditions, but it became reversed as the mass flux increased. The size distribution of sliding bubbles followed Gaussian distribution well both near and far from the nucleation site. The standard deviation of bubble size varied insignificantly through sliding compared to the changes in mean bubble size.