Heat transfer enhancement for laminar natural convection along a vertical plate due to sub-millimeter-bubble injection

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.     

Experimental investigation of water laminar mixed-convection flow with sub-millimeter bubbles in a vertical channel

This paper describes flow and heat transfer characteristics of laminar mixed-convection flows of water with sub-millimeter bubbles in a vertical channel. We use thermocouples and a particle tracking velocimetry technique for the temperature and velocity measurements. The working fluid used is tap water, and hydrogen bubbles generated by electrolysis of the water are used as the sub-millimeter bubbles. The Reynolds number of the main flow ranges from 100 to 200. The ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection (the heat transfer coefficient ratio) ranges from 1.24 to 1.38. The heat transfer coefficient ratio decreases with the increase in the Reynolds number. We conclude from velocity measurements that this decrease is mainly caused by a decrease in the advection effect due to sub-millimeter bubbles.     

Experimental study on turbulent natural convection heat transfer in water with sub-millimeter-bubble injection

Using thermocouples and a particle tracking velocimetry technique, temperature and velocity measurements are conducted to investigate flow and heat transfer characteristics of turbulent natural convection from a vertical heated plate in water with sub-millimeter-bubble injection. Hydrogen-bubbles generated by the electrolysis of water are used as the sub-millimeter-bubbles. In the turbulent region, the heat transfer deterioration occurs for a bubble flow rate Q = 33 mm³/s, while the heat transfer enhancement occurs for Q = 56 mm³/s. Temperature and velocity measurements suggest that the former is caused by a delay of the transition due to the bubble-induced upward flow. On the other hand, the latter is mainly due to two factors: one is the enhancement of the rotation of eddies in the outer layer, and the other is the increase in the gradient of the streamwise liquid velocity at the heated wall. These are caused by bubbles, which are located in the inner layer, rising at high speed.     

A method of concurrent thermographic–photographic visualization of flow boiling in a minichannel

A method is developed to capture the distribution of surface temperature while simultaneously imaging the bubble motions in diabatic flow boiling in a horizontal minichannel. Liquid crystal thermography is used to obtain highly resolved surface temperature measurements on the uniformly heated upper surface of the channel. High-speed images of the flow field are acquired simultaneously and are overlaid with the thermal images. The local surface temperature and heat transfer coefficient can be analyzed with the knowledge of the nucleation site density and location, and bubble motion and size evolution. The horizontal channel is 1.2 mm high × 23 mm wide × 357 mm long, and the working fluids are Novec 649 and R-11. Optical access is through a machined glass plate which forms the bottom of the channel. The top surface is an electrically heated 76 μm-thick Hastelloy foil held in place by a water-cooled aluminum and glass frame. The heat loss resulting from this construction is computed using a conduction model in Fluent. The model is driven by temperature measurements on the foil, glass plate and aluminum frame. This model produces a corrected value for the local surface heat flux and enables the computation of the bulk fluid temperature and heat transfer coefficient along the channel. The streamwise evolution of the heat transfer coefficient for single-phase laminar flow is compared to theoretical values for a uniform-flux boundary condition. Examples of the use of the facility for visualizing subcooled two-phase flows are presented. These examples include measurements of the surface temperature distribution around active nucleation sites and the construction of boiling curves for locations along the test surface. Points on the curve can be associated with specific image sequences so that the role of mechanisms such as nucleation and the sliding of confined bubbles may be discerned.     

Artificial boiling heat transfer in the free convection to carbonic acid solution

Free convection phenomenon has been experimentally investigated around a horizontal rod heater in carbonic acid solution. Because of the tendency of the solution to desorb carbon dioxide gas when temperature is increased, bubbles appear when cylinder surface is heated. The bubbles consists mainly carbon dioxide and also a negligible amount of water vapor. The results present that dissolved carbon dioxide in water significantly enhances the heat transfer coefficient in compare to pure free convection regime. This is mainly due to the microscale mixing on the heat transfer surface, which is induced by bubble formation. In this investigation, experiments are performed at different bulk temperatures between 288 and 333 K and heat fluxes up to 400 kW m⁻² at atmospheric pressure. Bubble departure diameter, nucleation site density and heat transfer coefficient have been experimentally measured. A model has been proposed to predict the heat transfer coefficient.     

Study on heat transfer performance of spray cooling: model and analysis

In consideration of droplet–film impaction, film formation, film motion, bubble boiling (both wall nucleation bubbles and secondary nucleation bubbles), droplet–bubble interaction, bulk air convection and radiation, a model to predict the heat and mass transfer in spray cooling was presented in this paper. The droplet–film impaction was modeled based on an empirical correlation related with droplet Weber number. The film formation, film motion, bubble growth, and bubble motion were modeled based on dynamics fundamentals. The model was validated by the experimental results provided in this paper, and a favorable comparison was demonstrated with a deviation below 10%. The film thickness, film velocity, and non-uniform surface temperature distribution were obtained numerically, and then analyzed. A parameters sensitivity analysis was made to obtain the influence of spray angle, surface heat flux density, and spray flow rate on the surface temperature distribution, respectively. It can be concluded that the heat transfer induced by droplet–film impaction and film-surface convection is dominant in spray cooling under conditions that the heated surface is not superheated. However, the effect of boiling bubbles increases rapidly while the heated surface becomes superheated.     

Experimental study on the forced convective flow in a channel with heated blocks in tandem

This study experimentally examines the forced convective flow over two sequentially heated blocks mounted on one principal wall of a channel. The experiments, involving mass transfer, were carried out via the naphthalene sublimation technique (NST). By virtue of the analogy between heat and mass transfer, the results can then be converted to determine the heat transfer. In the experiments, the block spacings were set at 2, 4, 6, 8, 12, 16, and 22 and the Reynolds numbers were set at 1300 and 10⁴ which correspond to the laminar and the turbulent convective flow cases, respectively. Results show that the Sherwood number increases or decreases monotonically along the block surfaces in the laminar convection cases; while the hump and sharp increase in the Sherwood number can be found in the turbulent convection cases. This is attributed to the reattachment of the separating bubble and the flow impingement, respectively. Comparison between the experimental and numerical results is made and the effect of the block spacing on heat transfer is discussed.     

Temperature gradient driven flow experiments of two interacting bubbles on a hot wall

This publication deals with the thermocapillary convection of two bubbles in a close proximity under a heated wall. The resultant toroidal vortex rings of the bubbles interfere and cause a distinctive threedimensional flow pattern. Additionally we observed the penetration depth of this flow configuration in dependence on the bubble spacing. Liquid crystal tracer particles serve for simultaneous flow- and temperature monitoring. This novel method is described in some detail. It is a very helpful tool for analyzing heat and mass transfer in liquids. Received on 25 April 1997     

Interaction of mixed convection in two-sided lid driven differentially heated square enclosure with radiation in presence of participating medium

The current study addresses the mathematical modeling aspects of transport phenomena in steady, two-dimensional, laminar flow accompanied by heat transfer in a lid-driven differentially heated cavity in presence of radiatively absorbing, emitting and scattering gray medium. The walls of the enclosure are considered to be opaque, diffusive and gray. Mixed convection is the outcome of the interaction of forced convection induced by the moving vertical hot and cold wall with the natural convection induced due to the differentially heated enclosure. Two different orientations of the wall movement have been considered to simulate opposing and aiding mixed convection phenomenon and to study its interaction with radiation. Vorticity-stream function formulation of N–S equation has been employed. The discrete ordinate method has been used in modeling the radiative transport equation followed with finite volume method as discretisation technique. The effect of influencing parameters on fluid flow and heat transfer has been studied.     

Series solutions of boundary-layer flows in porous media with lateral mass flux

Approximate analytical solutions for free convection boundary layers on a heated vertical plate with lateral mass flux embedded in a saturated porous medium are presented using the modified Adomian decomposition method and Padé technique. Several values of the wall temperature exponent for illustrating the effects of suction/injection parameter on the flow and heat transfer are considered. This study also includes the influence of the exponent on an impermeable surface. The results obtained are comparable to the exact analytical solutions and elucidate reliability and efficiency of the technique.     

Natural convection due to solar radiation over a non-absorbing plate with and without heat losses

Natural convection over a non-reflecting, non-absorbing, ideally transparent semi-infinite vertical flat plate due to absorption of incident radiation (solar radiation) is considered. The absorbed radiation acts as a distributed source which initiates buoyancy-driven flow and convection in the absorbing layer. The plate when heated by the absorbing fluid loses heat to the surroundings from its external side. Solution of the governing equations of the flow under these circumstances is non-similar because of both the heat source term in the energy equation and the temperature boundary condition at the plate. A local non-similar technique is used to obtain solutions for a wide range of the dimensionless distance along the plate and of the dimensionless loss coefficient to the surroundings. The results show that the temperature distribution has a maximum temperature in the depth of the fluid rather than on the plate. A new definition for a local heat transfer coefficient between the plate and the absorbing fluid is introduced which is based on the local maximum temperature rise in the fluid. A formula to calculate this heat transfer coefficient is given for the anticipated range of the loss coefficient.     

A visual flow study of mixed convection in the entry region of a vertical internally heated annulus

The velocity distribution in laminar upward flow of water (Pr 7.25) in the entry of a vertical internally heated annulus (radius ratio 4:1) has been determined by visual observation. Photographic measurements have been made of the motion of hydrogen bubble clusters, which were generated by a carefully controlled process of electrolysis, to assess the effects of free convection effects on the forced flow.For heat fluxes up to 2500 W/m² and at a Reynolds number of approximately 450, local heat transfer coefficients have been obtained in a length of about 23 equivalent diameters. Heat transfer rate in the immediate entry was found to be insensitive to change in heat flux over the range of variables considered. As the distance downstream increased, the heat transfer rate was found to be dependent on the heat flux.     

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

A simultaneous PIV and heat transfer study of bubble interaction with free convection flow

Whole field velocity and point temperature and surface heat flux measurements were performed to characterise the interaction of a single rising ellipsoidal air bubble with the free convection flow from a heated flat surface immersed in water at different angles of inclination. Two thermocouples and a hot film sensor were used to characterise heat transfer from the surface, while a time-resolved digital particle image velocimetry technique was used to map the bubble induced flow in a plane parallel to the surface. Heat flux fluctuations, preceding and following the bubble passage, were shown to correlate with the variation in both local flow velocities and fluid temperatures. The largest increases in heat transfer were recorded when both flow and temperature effects combined to enhance the convective cooling simultaneously. Such conditions were shown to be most likely met when the block was inclined at 45°, thus forcing the bubble to slide closer to the heated surface and hence to the thermal boundary layer.     

Local heat transfer coefficients on the circumference of a tube in a gas fluidized bed

A heated horizontal heat transfer tube was installed 14.8 cm above the distributor plate in a square fluid bed measuring 30.5 × 30.5 cm. Four different Geldart B sized particle beds were used (sand of two different distributions, an abrasive and glass beads) and the bed was fluidized with cold air. The tube was instrumented with surface thermocouples around half of the tube circumference and with differential pressure ports that can be used to infer bubble presence. Numerical execution of the transient conduction equation for the tube allowed the local time-varying heat transfer coefficient to be extracted. Data confirm the presence of the stagnant zone on top of the tube associated with low superficial velocities. Auto-correlation of thermocouple data revealed bubble frequencies and the cross-correlation of thermal and pressure events confirmed the relationship between the bubbles and the heat transfer events. In keeping with the notion of a “Packet renewal” heat transfer model, the average heat transfer coefficient was found to vary in sympathy with the root-mean square amplitude of the transient heat transfer coefficient.     

Numerical and experimental study of free convection above a sinusoidal horizontal plate

A numerical and experimental analysis is performed to study the laminar free convection above a horizontal plate facing upward subjected to an uniform heat flux. The surface of the plate, in contact with the fluid, is described by a sinusoidal profile. The natural convection equations are discretized, using an implicit finite difference technique, based on the finite volume approach. The SIMPLE algorithm assumes the linkage between velocities and pressure fields. The top and the lateral boundaries of the space, where free convection is developing, are determined by using an iterative procedure. The temperature fields of the fluid, over the plate, are visualized by an experimental device, which can realize a simultaneous measurement of the temperature and the position. Qualitative information about the natural convection flow above the plate is obtained by using a laser tomography technique. The numerical results show that the flow and the heat transfer are strongly affected by the amplitude, the period of the sinusoidal profile and the type of fluid. Comparisons between numerical and experimental results show a good qualitative agreement.     

Simulation of flow boiling in micro-channels: Effects of inlet flow rate and hot-spots

This paper presents the findings of a numerical study on the flow boiling in a micro-channel heat sink. The Navier-Stokes equations, energy equation, and the continuity equation are solved in a finite-volume framework using the front-tracking method. The numerical method is validated by comparison with the experimental results for a slug bubble growth, and vertical flow boiling. The numerical method is then used to study the effect of changing the inflow mass-velocity on the heat transfer coefficient, bubble size distribution, and the bubble nucleation frequency for a constant heat flux. The mean heat transfer coefficient of all the cases is found to be nearly twice that of the single-phase heat transfer coefficient. The bubble nucleation frequency is found to increase monotonically with the inflow mass-velocity. The bubble size distribution along the channel is found to become flatter as the mass-velocity is increased. We identify three distinct phases of the bubble evolution, namely the initial rapid growth phase, the boiling dominant phase, and finally the condensation dominant phase. Subsequently, the numerical method is used to study the effect of having a hot-spot near the bubble nucleation site on the heat transfer characteristics. It is found that the bubble nucleation frequency increases and the bubbles’ maximum volume decreases as the intensity of the hot-spot is increased for a fixed inlet flow rate. It is also observed that the average heat transfer coefficient does not change significantly with changing the intensity of the hot-spot, and that the bubble size distribution along the channel becomes flatter as the intensity of the hot-spot is increased.     

Simultaneous heat and mass transfer by natural convection from a plate embedded in a porous medium with thermal dispersion effects

The problem of steady, laminar, simultaneous heat and mass transfer by natural convection flow over a vertical permeable plate embedded in a uniform porous medium in the presence of inertia and thermal dispersion effects is investigated for the case of linear variations of both the wall temperature and concentration with the distance along the plate. Appropriate transformations are employed to transform the governing differential equations to a non-similar form. The transformed equations are solved numerically by an efficient implicit, iterative, finite-difference scheme. The obtained results are checked against previously published work on special cases of the problem and are found to be in good agreement. A parametric study illustrating the influence of the porous medium effects, heat generation or absorption, wall suction or injection, concentration to thermal buoyancy ratio, thermal dispersion parameter, and the Schmidt number on the fluid velocity, temperature and concentration as well as the skin-friction coefficient and the Nusselt and Sherwood numbers is conducted. The results of this parametric study are shown graphically and the physical aspects of the problem are highlighted and discussed.     

Adverse and favorable mixed convection heat transfer in a two-side heated square channel

Experimental heat transfer measurements and analysis for mixed convection in a vertical square channel are presented. Water flow directions are selected such that buoyancy assists or opposes the bulk flow pressure gradient. Unlike most previous experiments with symmetrically heated circular tubes, the present configuration uses an asymmetric heating condition (two sides heated and two sides insulated) and shows significant increase in the Nusselt number for both assisted and opposed flow conditions. Observed heat transfer coefficient distributions are different from the symmetrically heated channels; and this difference in heat transfer coefficient is attributed to the formation of buoyancy driven large-scale flow structures. In general, opposed flow shows higher heat transfer coefficients, and the Nusselt number ratio is observed to increase as Gr/Re or Gr/Re² ratios increase for both assisted and opposed flow conditions. A correlation based on the buoyancy parameter predicts the heat transfer pattern well in both assisted and opposed mixed convection. The range of Reynolds numbers discussed (Re=400–10,000) is of importance for direct numerical simulations and the details provided here can serve as the benchmark data required for complicated buoyancy affected turbulence simulations.