Surface heat transfer due to sliding bubble motion

The presence of a rising bubble in a fluid can greatly enhance heat transfer from adjacent heated surfaces such as in shell and tube heat exchangers and chemical reactors. One specific case of this is when a bubble impacts and slides along the surface. The result is heat transfer enhancement by two main mechanisms: first, the bubble itself acting as a bluff body, and second, the wake generated behind the bubble leads to increasing mixing. The current research is concerned with measuring the heat transfer from a submerged heated surface that is subject to a sliding bubble flow. An ohmically heated 25 μm thick stainless steel foil, submerged in a water tank, forms the test surface. An air bubble is injected onto the lower surface of the test plate, it slides along its length and the effects are monitored by two methods. Thermochromic liquid crystals (TLC’s) are used in conjunction with a high speed camera to obtain a time varying 2D temperature map of the test surface. A second synchronised camera mounted below the foil records the bubble motion. Tests are performed at angles of 10°, 20° and 30° to the horizontal. This paper reports on the enhancement of the heat transfer due to the bubble. It has been found that the angle made between the heated surface and the horizontal influences heat transfer by changing the bubble’s motion. In general, a steeper angle leads to a higher bubble velocity, which results in greater heat transfer enhancement.     

Numerical study of bubble growth and boiling heat transfer on a microfinned surface

The bubble growth and boiling heat transfer on a microfinned surface are studied numerically by solving the conservation equations of mass, momentum and energy. The bubble shape is tracked by a sharp-interface level-set method, which is modified to include the effect of phase change and to treat the contact angle and microlayer heat flux on an immersed solid surface. The present computation demonstrates that the microfinned surface enhances boiling heat transfer significantly compared to a plain surface. The effects of fin spacing and height on the bubble growth and heat transfer are investigated to find the optimal conditions for boiling enhancement.     

Turbulent heat transfer on the surface of a steam-chugging bubble

The length and velocity scales of the turbulent eddies at the surface of a type of steam-chugging bubble (the detached bubble) are derived from a physical model. The corresponding turbulent heat transfer result, which contains no free parameters, is $\text{Nu}\text{= 0.04}\text{Re}{}^{\mathrm{<mtext>7</mtext><mtext>8</mtext>}}\text{Pr}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. A comparison with the available experimental data shows excellent agreement.     

Numerical study of bubble growth and wall heat transfer during flow boiling in a microchannel

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier–Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. Experiments have been conducted to validate the numerical model. The bubble growth rate and shapes show good agreement between numerical and experimental results. The numerical results show that the wall heat transfer increases with wall superheat but stays almost unaffected by the liquid flow rate. The liquid vapor surface tension value has little influence on bubble growth and wall heat transfer. However, the bubble with the lowest contact angle resulted in the highest wall heat transfer.     

Direct simulation of spray cooling: Effect of vapor bubble growth and liquid droplet impact on heat transfer

Numerical modeling of multiphase flow using level set method is discussed. The 2-D model considers the effect of surface tension between liquid and vapor, gravity, phase change and viscosity. The level set method is used to capture the movement of the free surface. The detail of incorporating the mechanism of phase change in the incompressible Navier–Stokes equations using the level set method is described. The governing equations are solved using the finite difference method. The computer model is used to study the spray cooling phenomenon in the micro environment of about 40 μm thick liquid layer with vapor bubble growing due to nucleation. The importance of studying the heat transfer mechanism in thin liquid film for spray cooling is identified. The flow and heat transfer details are presented for two cases: (1) when the vapor bubble grows due to nucleation and (2) merges with the vapor layer above the liquid layer and when a liquid droplet impacts the thin liquid layer with vapor bubble growing.     

Transient heat transfer and bubble dynamics in a pressurized fluidized bed

A flow visualization technique for studying the gas bubble dynamics in a pressurized fluidized bed was developed and used to quantify these dynamics at the surface of a vertical tube submerged in the bed. Transient heat flux measurements were made and correlated with bubble motion. As a result, it is concluded that the heat transfer process is strongly affected by bubble dynamics and is much more complex than any of the generally accepted models can predict. It is also shown that the overall bed operating conditions are the primary driver for local bubble/particle motion around the tube which significantly affects the time-dependent fluctuation in local heat transfer.     

Accuracy of measurements of curvature and apparent contact angle in a constrained vapor bubble heat exchanger

Improved experimental and analytical techniques were developed to measure the liquid pressure field in a meniscus formed in the right angled corner of a Constrained Vapor Bubble (CVB) heat exchanger. Based on the definition of the curvature, an analytical expression for the curvature as a function of the film thickness profile and the apparent contact angle was obtained. The error associated with the resolution of the image processing system was defined and several imaging factors affecting the error are discussed. The error associated with the system resolution, E_r, increases as the curvature increases; the apparent contact angle increases; the wavelength decreases; and the number of dark fringes used for the data fitting decreases. The accuracy of the data fitting can be used to also determine the region where the disjoining pressure and viscous stresses affect the results. Examples using pentane data are presented. The relatively large experimental cell size with regions of low capillary pressure was dictated by future use in microgravity.     

Heat transfer characteristics of high heat flux vapor chamber

To meet the challenge of heat spreading in electronic products, highly efficient high heat flux heat transfer vapor chambers have been manufactured and their heat transfer characteristics have been studied by a fast test system. A solid copper block with the same shape as the vapor chamber is used to compare the performance of the vapor chamber. The result shows that, it will take about 5 min to achieve a steady state in the fast test system. The heat transfer characteristics of the vapor chamber are more superior to those of the copper block. In this paper, total thermal resistance of the test system is used to evaluate the heat transfer characteristics of the vapor chamber, because it has already been used to consider both the spreading thermal resistance and the flatness of the vapor chamber.     

Heat transfer characteristics of high heat flux vapor chamber

To meet the challenge of heat spreading in electronic products, highly efficient high heat flux heat transfer vapor chambers have been manufactured and their heat transfer characteristics have been studied by a fast test system. A solid copper block with the same shape as the vapor chamber is used to compare the performance of the vapor chamber. The result shows that, it will take about 5min to achieve a steady state in the fast test system. The heat transfer characteristics of the vapor chamber are more superior to those of the copper block. In this paper, total thermal resistance of the test system is used to evaluate the heat transfer characteristics of the vapor chamber, because it has already been used to consider both the spreading thermal resistance and the flatness of the vapor chamber.     

Dynamics of a vapor bubble on a heated substrate

The dynamics of a vapor bubble between its liquid phase and a heated plate is studied in relation to the breakdown and recovery of the film boiling. By examining the expansion and the contraction of the vapor bubble the film boiling and transition boiling states are predicted. Conservation laws in the vapor, solid, and liquid phases are invoked along with fully nonlinear, coupled, free boundary conditions. These coupled system of equations are reduced to a single evolution equation for the local thickness of the vapor bubble by using a long-wave asymptotics, which is then solved numerically to yield the transient motion of the vapor bubble. Of the numerous parameters involved in this complex phenomenon we focus on the effects of the degree of superheat from the solid plate, that of the supercooling through the liquid, and the wetting/dewetting characteristics of the liquid on the solid plate. A material property of the substrate thus is incorporated into the criteria for the film boiling based on hydrodynamic models.     

Experimental study of the flow boiling heat transfer enhancement and pressure drop due to the bubble behavior restricted by a screen sheet

A unique method previously proposed by the authors was applied to the heat transfer augmentation in the flow boiling field. In this method a screen sheet was placed on the horizontal heated surface where bubble nucleation occurred. Generated vapor bubbles were trapped between the screen and the wall, became flat, and moved along the surface. This restricted bubble behavior caused the heat transfer enhancement. Three types of screen sheet were tested in the present experiment and the effect of the screen on the heat transfer and two‐phase flow characteristics was investigated. In two of these cases, the screen was displaced upward by the bubble nucleation. Compared with the ordinary flow boiling case, heat transfer was enhanced by a factor of 1.2 to 6 within the present experimental range. Using a simple flow model, it was made clear that the effect of the height of the displaced screen was important in evaluating the increase in pressure drop. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 319–329, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10094     

Flow and heat transfer of liquid plug and neighboring vapor slugs in a pulsating heat pipe

A model of fluid flow and heat transfer on liquid slug and neighboring vapor plugs in a pulsating heat pipe (PHP) is proposed. A new energy equation for the liquid slug is built by aid of Lagrange method. The shear stress term related with the fluid flow state is included in the motion equation of the liquid slug. A sensitive heat term is replaced by a phase change term in the energy equation of the vapor plug. Based on our analysis on the displacement variation of the liquid slug with time, it is known that the harmonic force acting on the liquid slug in PHPs is the pressure difference between the vapor plugs. The flow oscillation can be considered as a forced damping vibration of one degree of freedom system. The phase difference of the oscillating flow between with and without the gravity effect can reach 45°. The amplitude and angular frequency of flow oscillation is irrespective with the initial displacement of liquid slug. If the flow pattern remains strictly slug flow in the entire system, the contribution of the sensible heat exchange to the total heat transfer of the PHP is about 80%.     

Direct contact heat transfer with change of phase: Condensation of a bubble train

Solution of the coupled velocity and temperature fields associated with the condensation of a single or two-phase bubble train is used to obtain the bubbles' radii as a function of time (or height), frequency, temperature driving force and inerts concentration.The reliability of the solution procedure is demonstrated by its convergence at zero frequency to other solutions of single bubble condensation and by the good agreement of the calculated results with experimental data.     

The effects of bubble translation on vapor bubble growth in a superheated liquid

A theoretical analysis of vapor bubble growth in a uniformly superheated liquid has been carried out to determine the effects of translational motion of the bubble on the bubble growth rate. Assuming potential flow in the region surrounding the bubble the appropriate convective diffusion equation is solved by means of a new similarity transformation. The results of the theoretical analysis are compared with available experimental data and with analyses of the limiting cases of no bubble translation and quasi steady state bubble growth. The analysis is shown to reduce to the Plesset and Zwick or Scriven analysis for stationary growing bubbles. The effects of translation are found to be significant when the translational velocity is sufficiently high at moderate Jakob numbers, but for high Jakob numbers radial convection predominates and translation has little effect on the growth rates. The analysis predicts results in good agreement with experimental data available in the literature.     

Subcooled flow boiling heat transfer of R-134a and the associated bubble characteristics in a vertical plate heat exchanger

Subcooled flow boiling heat transfer characteristics of refrigerant R-134a in a vertical plate heat exchanger (PHE) are investigated experimentally in this study. Besides, the associated bubble characteristics are also inspected by visualizing the boiling flow in the vertical PHE. In the experiment two vertical counterflow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of subcooled refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the boiling heat flux, refrigerant mass flux, system pressure and inlet subcooling of R-134a on the subcooled boiling heat transfer are explored in detail. The results are presented in terms of the boiling curves and heat transfer coefficients. The measured data showed that the slopes of the boiling curves change significantly during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, increases in the inlet subcooling and saturation temperature only show slight improvement on the boiling heat transfer coefficient.The photos from the flow visualization reveal that at higher imposed heat flux the plate surface is covered with more bubbles and the bubble generation frequency is substantially higher, and the bubbles tend to coalesce to form big bubbles. But these big bubbles are prone to breaking up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also note that the bubbles nucleated from the plate are suppressed to a larger degree for higher inlet subcooling and mass flux. Finally, empirical correlations are proposed to correlate the present data for the heat transfer coefficient and the bubble departure diameter in terms of boiling, Froude, Reynolds and Jakob numbers.     

Microlayer thickness under a vapor bubble on a cylindrical probe

The thickness of a liquid microlayer underneath a vapor bubble on a heated, cylindrical probe was determined by simultaneously solving the fourth‐order differential equation for the microlayer thickness that incorporates the momentum and energy equations in the microlayer in conjunction with the pressure distribution in the microlayer and the evaporative heat flux at the interface. The analysis also considers the temperature gradient along the probe due to heat transfer in the probe. The results show that the microlayer on a cylindrical surface is very thin and short except for very low probe surface temperatures, superheated less than 1 K. The microlayer size and the evaporative heat flux both decrease rapidly as the surface temperature increases. The results show that most of the evaporation occurs along the curved portion of the interface. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 193–203, 2000     

Refrigerant flow boiling heat transfer in parallel microchannels as a function of local vapor quality

Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762 mm wide) of hydraulic diameter 1.09 mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0 kg m^?2 s^?1, which correspond to refrigerant mass flow rates of 0.5–2.0 g s^?1, and at three different pressures 400, 550 and 750 kPa corresponding to saturation temperatures of 8.9, 18.7, and 29 °C. The wall heat flux varies from 0 to 20 W/cm² in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%.