Showing papers in "Heat and Mass Transfer in 2006"
TL;DR: In this paper, the authors summarized and reviewed a great deal of information from the literature on dispersion in packed beds, and provided empirical correlations for the prediction of the dispersion coefficients (D T and D L) over the entire range of practical values of Sc and Pem.
Abstract: The phenomenon of dispersion (transverse and longitudinal) in packed beds is summarized and reviewed for a great deal of information from the literature. Dispersion plays an important part, for example, in contaminant transport in ground water flows, in miscible displacement of oil and gas and in reactant and product transport in packed bed reactors. There are several variables that must be considered, in the analysis of dispersion in packed beds, like the length of the packed column, viscosity and density of the fluid, ratio of column diameter to particle diameter, ratio of column length to particle diameter, particle size distribution, particle shape, effect of fluid velocity and effect of temperature (or Schmidt number). Empirical correlations are presented for the prediction of the dispersion coefficients (D
T and D
L) over the entire range of practical values of Sc and Pem, and works on transverse and longitudinal dispersion of non-Newtonian fluids in packed beds are also considered.
436 citations
TL;DR: In this paper, two methods are discussed in detail which can provide the information about the entropy production with different degrees of accuracy, which can be determined in the post processing phase of a CFD calculation.
Abstract: Computational fluid dynamics (CFD) solutions of turbulent convective heat transfer problems based on the mass, momentum and energy conservation principle provide all information to calculate the entropy production rate in such a transfer process. It can be determined in the post processing phase of a CFD calculation. Two methods are discussed in detail which can provide the information about the entropy production with different degrees of accuracy.
218 citations
Journal Article•
TL;DR: In this article, the application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm2 PEM fuel cell is described. And the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.
Abstract: Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm2 PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180° bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.
179 citations
TL;DR: In this paper, the momentum and heat transfer in a laminar liquid film on a horizontal stretching sheet is analyzed by the Homotopy analysis method (HAM), and analytic series solutions are given and compared with numerical results given by other authors.
Abstract: The momentum and heat transfer in a laminar liquid film on a horizontal stretching sheet is analyzed by the Homotopy analysis method (HAM). Analytic series solutions are given and compared with numerical results given by other authors. The good agreement between them shows the effectiveness of HAM to the problem of liquid film on an unsteady stretching surface.
170 citations
TL;DR: This study paves the road for the researchers in the area of MRI and drug delivery to develop comprehensive models based on porous media theory utilizing fewer assumptions as compared to other approaches.
Abstract: Pertinent works associated with magnetic resonance imaging (MRI) and drug delivery are reviewed in this work to demonstrate the role of transport theory in porous media in advancing the progress in biomedical applications. Diffusion process is considered significant in many therapies such as delivering drugs to the brain. Progress in development of the diffusion equation using local volume-averaging technique and evaluation of the applications associated with the diffusion equation are analyzed. Tortuosity and porosity have a significant effect on the diffusion transport. Different relevant models of tortuosity are presented and mathematical modeling of drug release from biodegradable delivery systems are analyzed in this investigation. New models for the kinetics of drug release from porous biodegradable polymeric microspheres under bulk erosion and surface erosion of the polymer matrix are presented in this study. Diffusion of the dissolved drug, dissolution of the drug from the solid phase, and erosion of the polymer matrix are found to play a central role in controlling the overall drug release process. This study paves the road for the researchers in the area of MRI and drug delivery to develop comprehensive models based on porous media theory utilizing fewer assumptions as compared to other approaches.
124 citations
TL;DR: In this paper, an asymmetrically confined circular cylinder in a plane channel is numerically investigated using FLUENT for the ranges of Reynolds numbers as 10≤Re≤500, of the blockage ratio as 0.125≤γ≤1 for a constant value of the Prandtl number of 0.744.
Abstract: Unsteady momentum and heat transfer from an asymmetrically confined circular cylinder in a plane channel is numerically investigated using FLUENT for the ranges of Reynolds numbers as 10≤Re≤500, of the blockage ratio as 0.1≤β≤0.4, and of the gap ratio as 0.125≤γ≤1 for a constant value of the Prandtl number of 0.744. The transition of the flow from steady to unsteady (characterized by critical Re) is determined as a function of γ and β. The effect of γ on the mean drag
$$(\bar{C}_{\rm D})$$
and lift
$$(\bar{C}_{\rm L})$$
coefficients, Strouhal number (St), and Nusselt number (Nu
w
) is studied. Critical Re was found to increase with decreasing γ for all values of β.
$$\bar{C}_{\rm D}$$
and St were found to increase with decreasing values of γ for fixed β and Re. The effect of decrease in γ on
$$\overline{Nu}_{w}$$
was found to be negligible for all blockage ratios investigated.
84 citations
TL;DR: A review of the phonon Boltzmann transport equation (BTE) in the relaxation time approximation has been employed to make thermal predictions in dielectrics and semiconductors at microscales and nano-scales as discussed by the authors.
Abstract: Fourier diffusion has been found to be inadequate for the prediction of heat conduction in modern microelectronics, where extreme miniaturization has led to feature sizes in the sub-micron range Over the past decade, the phonon Boltzmann transport equation (BTE) in the relaxation time approximation has been employed to make thermal predictions in dielectrics and semiconductors at micro-scales and nano-scales This paper presents a review of the BTE-based solution methods widely employed in the literature and recently developed by the authors First, the solution approaches based on the gray formulation of the BTE are presented The semi-gray approach, moments of the Boltzmann equation, the lattice Boltzmann approach, and the ballistic-diffusive approximation are also discussed Models which incorporate greater details of phonon dispersion are also presented Hotspot self-heating in sub-micron SOI transistors and transient electrostatic discharge in NMOS transistors are also examined Results, which illustrate the differences between some of these models reveal the importance of developing models that incorporate substantial details of phonon physics The impact of boundary conditions on thermal predictions is also investigated
79 citations
TL;DR: In this article, the steady nonlinear hydromagnetic flow of an incompressible, viscous and electrically conducting fluid with heat transfer over a surface of variable temperature stretching with a power-law velocity in the presence of variable transverse magnetic field is analyzed.
Abstract: The steady nonlinear hydromagnetic flow of an incompressible, viscous and electrically conducting fluid with heat transfer over a surface of variable temperature stretching with a power-law velocity in the presence of variable transverse magnetic field is analysed. Utilizing similarity transformation, governing nonlinear partial differential equations are transformed to nonlinear ordinary differential equations and they are numerically solved using fourth-order Runge–Kutta shooting method. Numerical solutions are illustrated graphically by means of graphs. The effects of magnetic field, stretching parameter and Prandtl number on velocity, skin friction, temperature distribution and rate of heat transfer are discussed.
74 citations
TL;DR: In this article, the authors presented a numerical prediction of a new 3D film cooling hole geometry, the converging slot-hole or console, which is designed in order to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems without loosing the mechanical strength of a row of discrete holes.
Abstract: This paper presents a numerical prediction of a new 3D film cooling hole geometry, the converging slot-hole or console. The console geometry is designed in order to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems without loosing the mechanical strength of a row of discrete holes. The cross section of the console changes from a circular shape at the inlet to a slot at the exit. Previous successful application of a new anisotropic DNS based two-layer turbulence model to cylindrical and shaped hole injections is extended to predict film cooling for the new console geometry. The suitability of the proposed turbulence model for film cooling flow is validated by comparing the computed and the measured wall-temperature distributions of cylindrical hole injections. The result shows that the anisotropic eddy-viscosity/diffusivity model can correctly predict the spanwise spreading of the temperature field and reduce the strength of the secondary vortices. Comparative computations of the adiabatic film cooling effectiveness associated with the three geometries tested in the present study (cylindrical, shaped, and console) show that the new console film-cooling hole geometry is definitely superior to the other geometries as shown by the uniform lateral spreading of the effectiveness with a slight enhancement downstream of the intersection of the two consoles.
68 citations
TL;DR: In this paper, the authors developed a mathematical model to analyse the heat exchanges in four different types of solar air collectors, and they showed that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collectors.
Abstract: Sensitivity analysis is a mathematical tool, first developed for optimization methods, which aim is to characterize a system response through the variations of its output parameters following modifications imposed on the input parameters of the system. Such an analysis may quickly become laborious when the thermal model under consideration is complex or the number of input parameters is high. In this paper, we develop a mathematical model to analyse the heat exchanges in four different types of solar air collectors. When building this thermal model we show that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of an important dimensionless parameter, referred to as the thermal performance factor of the collector, that compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. An important result which can be drawn from this study is that the heat transfer model developed is robust enough to be used for thermal design studies of most known flat plate solar air heaters, but also of flat plate solar water collectors and linear solar concentrators.
60 citations
TL;DR: In this article, the variation of thermal conductance of contact has been investigated as a function of apparent contact pressure experimentally, and the experimental results are compared with the theoretical predictions of an existing theory.
Abstract: The variation of thermal conductance of contact has been investigated as a function of apparent contact pressure experimentally. Experimental data has been obtained for steel, brass, copper and aluminum test pieces having different surface roughness over a wide range of contact pressures. Experimental results are compared with the theoretical predictions of an existing theory. Comparison revealed good agreement of trend with the experimental data, however, numerical values vary widely. Theory can predict the experimental results within an over-all error of less than 35%.
TL;DR: In this paper, an innovative diffusion driven desalination process was described where evaporation of seawater is driven by diffusion within a packed bed, and the authors described the evaporative heat and mass transfer analysis for the packed bed.
Abstract: An innovative Diffusion Driven Desalination process was recently described where evaporation of seawater is driven by diffusion within a packed bed. This work describes the evaporative heat and mass transfer analysis for the packed bed. Temperature and humidity data have been collected over a range of flow conditions at the inlet and outlet of the packed bed. The analysis agrees very well with the experimental data collected during this investigation and that which has been reported in the literature.
TL;DR: In this paper, a numerical model based on the finite difference method was used for the solution of mass, momentum and energy equations for a coupled laminar natural convection with radiation in air-filled square enclosure heated from below and cooled from above.
Abstract: Coupled laminar natural convection with radiation in air-filled square enclosure heated from below and cooled from above is studied numerically for a wide variety of radiative boundary conditions at the sidewalls. A numerical model based on the finite difference method was used for the solution of mass, momentum and energy equations. The surface-to-surface method was used to calculate the radiative heat transfer. Simulations were performed for two values of the emissivities of the active and insulated walls (ɛ1=0.05 or 0.85, ɛ2=0.05 or 0.85) and Rayleigh numbers ranging from 103 to 2.3×106 . The influence of those parameters on the flow and temperature patterns and heat transfer rates are analyzed and discussed for different steady-state solutions. The existing ranges of these solutions are reported for the four different cases considered. It is founded that, for a fixed Ra, the global heat transfer across the enclosure depends only on the magnitude of the emissivity of the active walls. The oscillatory behavior, characterizing the unsteady-state solutions during the transitions from bicellular flows to the unicellular flow are observed and discussed.
TL;DR: In this paper, two-fluid magnetoconvection in a vertical enclosure consisting of two regions, one electrically conducting and another electrically nonconducting, is investigated analytically.
Abstract: Two-fluid magnetoconvection in a vertical enclosure consisting of two regions, one electrically conducting and another electrically non-conducting is investigated analytically. The transport properties of both fluids are assumed constant. The resulting coupled non-linear equations are solved using regular perturbation method. It is found that the effect of negative electric load parameter E is to aid the flow while the effect of positive E is to oppose the flow as compared to the case when E=0. The effects of various physical parameters such as Grashof number, viscosity ratio and width ratio are presented for heat absorption and heat generation case.
TL;DR: In this article, the problem of steady, laminar, natural convective flow of a viscous fluid in an inclined enclosure with partitions is considered and a numerical solution based on the finite volume method is obtained.
Abstract: The problem of steady, laminar, natural convective flow of a viscous fluid in an inclined enclosure with partitions is considered. Transverse gradient of temperature is applied on the two opposing regular walls of the inclined enclosure while the other walls are maintained adiabatic. The problem is formulated in terms of the vorticity-stream function procedure. A numerical solution based on the finite volume method is obtained. Representative results illustrating the effects of the enclosure inclination angle and the degree of irregularity on the contour maps of the streamlines and temperature are reported and discussed. In addition, results for the average Nusselt number at the heated wall of the enclosure and the difference of extreme stream-function values are presented and discussed for various Rayleigh numbers, inclination angles and dimensionless partition heights.
TL;DR: In this article, the effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the flow structure are reported, and it is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and the peaks in heat transfer coefficients for low spacings in impinging jets.
Abstract: The flow characteristics of both confined and unconfined air jets, impinging normally onto a flat plate have been experimentally investigated. The mean and turbulence velocities, and surface pressures were measured for Reynolds numbers ranging from 30,000 to 50,000 and the nozzle-to-plate spacings in range of 0.2–6. Smoke-wire technique is used to visualize the flow behavior. The effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the flow structure are reported. In the case of confined jet, subatmospheric regions occur on both impingement and confinement surfaces at nozzle-to-plate spacings up to 2 for all Reynolds numbers in consideration and they lie up to nearly the same radial location at both surfaces. However, there is no evidence of the subatmospheric region in unconfined jet. It is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and the peaks in heat transfer coefficients for low spacings in impinging jets.
TL;DR: In this article, the potential of a high electric field was used to induce ice nucleus formation in aqueous solutions, which reduced the primary drying time during lyophilization.
Abstract: The potential of a high electric field was utilized to induce ice nucleus formation in aqueous solutions. Using this technique it was possible to reduce the primary drying time during lyophilization. Samples of 10% (w/v) hydroxyethylstarch (HES) solution were frozen at a constant rate of −1 K/min, while nucleation was initiated at temperatures of −1.5, −4.5 and −8.5°C. In contrast, spontaneous nucleation was observed in a range between −11.5 and −17.1°C. Electrically induced nucleus formation has proved to be a reliable method to start crystallization at a desired temperature. Continuous measurement of the weight allowed to determine the drying rate and to detect at which time primary drying was completed. The drying time and the drying rate were found to be strongly dependent on the nucleation temperature during freezing. A relation between the nucleation temperature, the structure of the frozen samples and the drying times could be established.
TL;DR: In this paper, the authors considered the viscous dissipation effect in a microchannel with constant wall temperature and showed that the Nusselt number may be either higher or lower than those of the continuum regime, depending on the competition between the effects of βvKn and β.
Abstract: Forced convection flow in a microchannel with constant wall temperature is studied, including viscous dissipation effect. The slip-flow regime is considered by incorporating both the velocity-slip and the temperature-jump conditions at the surface. The energy equation is solved for the developing temperature field using finite integral transform. To increase βvKn is to increase the slip velocity at the wall surface, and hence to decrease the friction factor. Effects of the parameters βvKn, β, and Br on the heat transfer results are illustrated and discussed in detail. For a fixed Br, the Nusselt number may be either higher or lower than those of the continuum regime, depending on the competition between the effects of βvKn and β. At a given βvKn the variation of local Nusselt number becomes more even when β becomes larger, accompanied by a shorter thermal entrance length. The fully developed Nusselt number decreases with increasing β irrelevant to βvKn. The increase in Nusselt number due to viscous heating is found to be more pronounced at small βvKn.
TL;DR: The behavior of one drop impinging on a hot surface by varying the surface temperature, the drop velocity and the position of the surface (horizontal and a inclined 45°) both at a temperature below and above the Leidenfrost temperature has been experimentally evaluated, estimating the temperature at which the drop rebounds as discussed by the authors.
Abstract: The behaviour of one drop impinging on a hot surface by varying the surface temperature, the drop velocity and the position of the surface (horizontal and a inclined 45°) both at a temperature below and above the Leidenfrost temperature has been experimentally evaluated, estimating the temperature at which the drop rebounds. A large influence on the drop velocity has been evidenced. The inclination of the surface decreases the critical value of the temperature above which the surface is not rewetted.
TL;DR: In this paper, the results of velocity measurements of natural convection in symmetrically heated vertical channel using the particle image velocimetry (PIV) system were presented.
Abstract: This paper presents the results of velocity measurements of natural convection in symmetrically heated vertical channel using the particle image velocimetry (PIV) system. Velocity measurements were conducted at three different sections on the horizontal plane to validate the flow two-dimensionality and at three different heights in the vertical plane to establish vertical mean velocity profiles. The results indicate a considerable influence of the Rayleigh number and aspect ratio on the mean velocity profile. The results also indicate significant diffusion rates of the vertical mean velocity component and normal Reynolds stresses towards the center of the channel.
TL;DR: The paper addresses some of the thermodynamic and heat transfer issues relevant in improving the performance of the thermoacoustic system, such as optimization for maximum COP, maximum cooling load and the role of the heat exchangers.
Abstract: During the past two decades the thermoacoustic refrigeration and prime mover cycles gained importance in a variety of refrigeration applications. Acoustic work, sound, can be used to generate temperature differences that allow the transport of heat from a low temperature reservoir to an ambient at higher temperature, thus forming a thermoacoustic refrigeration system. The thermoacoustic energy pumping cycle can also be reversed: temperature difference imposed along the stack plates can lead to sound generation. In this situation the thermoacoustic system operates as a prime mover. Sound generated by means of this thermoacoustic energy conversion process can be utilized to drive different types of refrigeration devices that require oscillatory flow for their operation, such as thermoacoustic refrigerators, pulse tubes and Stirling engines. In order for a thermoacoustic refrigeration or prime mover system as well as a thermoacoustic prime mover driving a non-thermoacoustic refrigeration system to be competitive on the current market, it has to be optimized in order to improve its overall performance. Optimization can involve improving the performance of the entire system as well as its components. The paper addresses some of the thermodynamic and heat transfer issues relevant in improving the performance of the thermoacoustic system, such as optimization for maximum COP, maximum cooling load and the role of the heat exchangers. Results obtained using the two optimization criteria are contrasted in the paper to illustrate the complexity of the optimization process.
TL;DR: In this article, an experiment was designed with an artificial nucleation site in a thin electrically heated wall featuring a two-dimensional, high resolution temperature measurement technique using unencapsulated thermochromic liquid crystals and a high speed colour camera.
Abstract: Heat transfer in nucleate boiling is strongly influenced by a very small circular area in the vicinity of the three phase contact line where a thin liquid film approaches the heated wall. This area is characterised by high evaporation rates which trigger a local temperature drop in the wall. The wall temperature drop can be computed using an existing nucleate boiling model. To verify the complex model and the underlying assumptions, an experiment was designed with an artificial nucleation site in a thin electrically heated wall featuring a two-dimensional, high resolution temperature measurement technique using unencapsulated thermochromic liquid crystals and a high speed colour camera. The shape of the bubble is observed simultaneously with a second high speed camera. Experiments were conducted in a low gravity environment of a parabolic flight, causing larger bubble departure diameters than in normal gravity environments. Thus, it was possible to measure the evolution of the predicted temperature drop in a transient boiling process.
TL;DR: In this article, the authors investigated the effect of channel geometry on the characteristic of the flow and thermal fields as well as the local and average Nusselt number variation in parallel-plate channels.
Abstract: The problem of buoyancy driven turbulent flow in parallel-plate channels is investigated. The investigation is limited to vertical channels of uniform cross-section with different modes of heating. The details of the flow and thermal fields are obtained from the solution of the conservation equations of mass, momentum, and energy in addition to equations of the low Reynolds number turbulence model. The study covers Rayleigh number ranging from 105 to 107 and focuses on the effect of channel geometry on the characteristic of the flow and thermal fields as well as the local and average Nusselt number variation. A Nusselt number correlation has been developed in terms of a modified Rayleigh number and channel aspect ratio for the cases of symmetrically heated isothermal and isoflux conditions.
TL;DR: In this article, forced convection in a rectangular duct subjected to microwave heating is investigated, and three types of non-Newtonian liquids flowing through the duct are considered, specifically, apple sauce, skim milk, and tomato sauce.
Abstract: In this paper, forced convection in a rectangular duct subjected to microwave heating is investigated. Three types of non-Newtonian liquids flowing through the duct are considered, specifically, apple sauce, skim milk, and tomato sauce. A finite difference time domain method is used to solve Maxwell’s equations simulating the electromagnetic field. The three-dimensional temperature field is determined by solving the coupled momentum, energy, and Maxwell’s equations. Numerical results show that the heating pattern strongly depends on the dielectric properties of the fluid in the duct and the geometry of the microwave heating system.
TL;DR: In this article, the authors investigated the heat transfer characteristics of laminar and turbulent pulsating pipe flows under different conditions of Reynolds number, pulsation frequency, pulsator location and tube diameter.
Abstract: Heat transfer characteristics to both laminar and turbulent pulsating pipe flows under different conditions of Reynolds number, pulsation frequency, pulsator location and tube diameter were experimentally investigated. The tube wall of uniform heat flux condition was considered for both cases. Reynolds number varied from 750 to 12,320 while the frequency of pulsation ranged from 1 to 10 Hz. With locating the pulsator upstream of the inlet of the test section tube, results showed an increase in heat transfer rate due to pulsation by as much as 30% with flow Reynolds number of 1,643 and pulsation frequency of 1 Hz, depending on the upstream location of the pulsator valve. Closer the valve to the tested section inlet, the better improvement in the heat transfer coefficient is achieved. Upon comparing the heat transfer results of the upstream and the downstream pulsation, at Reynolds number of 1,366 and 1,643, low values of the relative mean Nusselt number were obtained with the upstream pulsation. Comparing the heat transfer results of the two studied test sections tubes for Reynolds number range from 8,000 to 12,000 and pulsation frequency range from 1.0 to 10 Hz showed that more improvement in heat transfer rate was observed with a larger tube diameter. For Reynolds number ranging from 8,000 to 12,000 and pulsation frequency of 10 Hz, an improvement in the relative mean Nusselt number of about 50% was obtained at Reynolds number of 8,000 for the large test section diameter of 50 mm. While, for the small test section diameter of 15 mm, at same conditions of Reynolds number and frequency, a reduction in the relative mean Nusselt number of up to 10% was obtained.
TL;DR: In this paper, the results of an experimental study on pressure drop during horizontal flow boiling of refrigerants R22, R507, R404A, R134a, R407C and R410A were reported.
Abstract: The paper reports the results of an experimental study on pressure drop during horizontal flow boiling of refrigerants R22, R507, R404A, R134a, R407C and R410A. The test section is a smooth, horizontal, stainless steel tube (6 mm I.D., 6 m length) uniformly heated by Joule effect. The experimental tests are carried out at an almost constant evaporating pressure of 7.0 bar varying the mass flow rate in the range 280–1,080 kg/m2 s. The experimental comparison clearly shown that the pressure drop of R22 is significantly higher as compared to all the other fluids. The results are compared against well-known pressure drop prediction methods. The available correlations can be used for both pure fluids and mixtures with no corrective factors, provided the mixture properties are evaluated at local compositions. The Chawla friction correlation is the best-fitting of our experimental data in combination with the heterogeneous momentum pressure drop model on the basis of the Rouhani-Axelsson void fraction correlation.
TL;DR: In this paper, the effects of ultrasonic vibration on critical heat flux (CHF) have been experimentally investigated under natural convection condition, where flat bakelite plates coated with thin copper layer and distilled water are used as heated specimens and working fluid, respectively.
Abstract: The effects of ultrasonic vibration on critical heat flux (CHF) have been experimentally investigated under natural convection condition. Flat bakelite plates coated with thin copper layer and distilled water are used as heated specimens and working fluid, respectively. Measurements of CHF on flat heated surface were made with and without ultrasonic vibration applied to working fluid. An inclination angle of the heated surface and water subcooling are varied as well. Examined water subcoolings are 5°C, 20°C, 40°C and the angles are 0°, 10°, 20°, 45°, 90°, 180°. The measurements show that ultrasonic wave applied to water enhances CHF and its extent is dependent upon inclination angle as well as water subcooling. The rate of increase in CHF increases with an increase in water subcooling while it decreases with an increase in inclination angle. Visual observation shows that the cause of CHF augmentation is closely related with the dynamic behaviour of bubble generation and departure in acoustic field.
TL;DR: In this paper, the optimal fin spacing for three-row flat tube bank fin mounted with VGs was investigated and shown to be about 2 mm in industrial application for the configuration of fin studied.
Abstract: To reduce the size and the weight of heat exchangers, vortex generators (VGs) were punched on fin surface to improve the fin heat transfer performance. This paper is focused on the optimal fin spacing for three-row flat tube bank fin mounted with VGs. The results show, for commonly used fin materials and fin thickness, the optimal fin spacing is about 2 mm in industrial application for the configuration of tube bank fin studied.
TL;DR: In this paper, it has been suggested that small amounts of dissolved gas within the bulk liquid can accumulate within the vapor bubble, forming localized concentration gradients that results in a temperature gradient to form along the liquid-vapor interface that drives thermocapillary convection.
Abstract: The mechanisms by which thermocapillary convection arises during boiling of nominally pure fluids in low-g environments are currently not known. It has recently been suggested that small amounts of dissolved gas within the bulk liquid can accumulate within the vapor bubble, forming localized concentration gradients that results in a temperature gradient to form along the liquid–vapor interface that drives thermocapillary convection. This hypothesis was tested by boiling > 99.3% pure n-perfluorohexane with and without noncondensible gas in a low-g environment using a 7.0 × 7.0 mm2 microheater array to measure time and space resolved heat transfer at various wall superheats. The thermocapillary convection around the primary bubble that formed in the gassy fluid was found to be much weaker than in the degassed fluid, and the primary bubble diameter was much larger in the gassy fluid due to the accumulation of noncondensible gas within the bubble. The results suggest that the accumulation of noncondensible gas in the bubble can result in temperature variations along the interface but due to the increased vapor/gas bubble size, the driving thermocapillary temperature gradient along the interface is significantly reduced and result in much weaker thermocapillary flow. The highest CHF values in a reduced gravity environment (19 W/cm2) occurred when the fluid was highly subcooled and degassed.
TL;DR: In this article, the thickness of the thermal boundary layer on natural convection from a horizontal plate was experimentally measured and expressed as a function of Rayleigh number both in laminar and turbulent regimes.
Abstract: Thickness of the thermal boundary layer on natural convection from a horizontal plate was experimentally measured and expressed as a function of Rayleigh number both in laminar and turbulent regimes. Several parametric equations expressing the Nusselt number as a function of Rayleigh number were developed combining experimental data by other authors with the ones obtained in this work. The characteristic length was taken as the thickness of the thermal boundary layer in one equation and as the ratio of the area to the perimeter in another one. Both characteristic lengths correlated the data precisely in wide ranges of Rayleigh numbers.