Change In Transfer Mechanism Research Articles

Experimental study on sub-cooled flow boiling heat transfer in inclined non-uniform heating tube bundle channel

Since the specific structure of the U-tube, non-uniform heating (NUH) occurs on the shell side of the U-tube steam generator or exchanger. When the above equipment are applied in ocean environments, they may be in an inclined condition. The coupled conditions of NUH and incline can enhance the complexity of the heat transfer on the shell side, especially in terms of the sub-cooled flow boiling, during which the heat transfer mechanism changes. In this paper, sub-cooled flow boiling in a tube bundle channel under the coupled conditions of NUH and incline are performed experimentally. The result shows that the Gungor-Winterson correlation is in good agreement with the experimental data under uniform heating (UH) conditions and the mean relative error is only about 10.05%, while the predicted ability is poor under the coupled conditions of NUH and incline. The heat transfer characteristics are affected by radial heat and mass transfer, convective flow, bubbles behavior, and bubbles migration, which are introduced by the coupled action of NUH and incline. At last, an improved Gungor-Winterson correlation is proposed for UH, NUH, and inclined NUH conditions. Its average relative error is only 5.56% in total, and less than 0.91% of data has a larger deviation than 15%.

Deceleration mechanism of downward flame spreading over inclined solid surface based on FireFOAM

FireFOAM solver is used to simulate downward flame spread on the surface of a single-wall corrugated board with different inclination angles (0° ∼ −30°), and to study countercurrent fire spread behavior and heat transfer control mechanism on solid surfaces under different inclination angles. The simulation results show that when the inclination angle is less than −20° the fire spread rate and mass loss rate will first decrease and then stabilize. Combining the flow field and temperature field, it can be found that the cold air entrained by the flame will have a cooling effect occurs on the distal end of the downstream surface during the fire spread process. By analyzing the two heat transfer modes of convection and radiation in the preheating zone, it is found that the heat flux received by the solid surface is dominated by convection, while the radiation heat flux is the main reason for the deceleration stage and the speed difference of quasi-steady-state spreading under different inclination angles. In the process of inclined downward fire spreading, the behavior of deceleration followed by uniformity appears because the heat transfer mechanism changes from ignition energy control to flame spread control.

Stress–strain behavior of marine calcareous soil-tire mixtures

The increase in the volume of scrap tires has led to environmental concerns in different locations of the world, particularly in the coastlines. Managing this solid waste, including reuse in civil engineering applications (e.g. geotechnical projects) can be an effective solution to solve the problem. A series of drained triaxial tests was performed on Qeshm calcareous soil obtained from Qeshm Island. Specimens were prepared at loose and dense relative densities and mixed with different percentages of tire crumbs (TCs), including 0%, 10%, 20%, 30%, and 50%. The variation ratio parameter was introduced to evaluate the effect of TCs on the engineering properties of the soil-tire mixture. Based on the results, the stress–strain behavior of the calcareous soil was strongly influenced by TCs contents. As the tire content (TC) increases, the stress transfer mechanism changes from particle-particle to tire-tire, which results in a shear strength reduction. For an example, the addition of 50% of TCs to the calcareous soil resulted in a 64% reduction in the maximum deviatoric stress under a confining pressure of 600 kPa. In addition, the secant modulus at maximum deviatoric strength of the soil-tire mixture with 50% of TCs decreased about 80%.

Experimental study on sub-cooled flow boiling heat transfer characteristics in non-uniform heating tube bundle channel

Since the primary coolant temperature decreases along the U-tube, so the secondary side fluid in the steam generator tubes will receive non-uniform heating (NUH) from the primary side. Compared with uniform heating (UH), NUH has a great influence on the heat transfer characteristics of the secondary side, especially in terms of the sub-cooled flow boiling, during which the heat transfer mechanism changes. In this paper, sub-cooled flow boiling in a tube bundle channel under NUH and UH conditions are performed experimentally. The results show that NUH makes the onset of nucleate boiling (ONB) occur ahead of time, and it also leads to a greater radial inhomogeneity of fluid temperature field than UH. Four Chen-type correlations were evaluated, and it is found that the Gungor-Winterton correlation is most suitable for experiments under UH condition. However, the Gungor-Winterton correlation has a relatively large deviation under NUH condition, and it is also difficult to capture the predicted trend, which means that it is not applicable to NUH conditions. Therefore, an improved Gungor-Winterton correlation is proposed for NUH conditions. Its average relative error is 8.02%, and less than 2.18% of data has larger deviation than 20%.

Experimental Research on Flow and Heat Transfer in Microchannel with Refrigerant HFO1234yf

An experiment was carried out to study the effect of refrigerant mass flow rate, heat flux density, and saturation temperature on boiling heat transfer coefficient and frictional pressure drop in microchannel. Flow patterns are captured by a high-speed camera. HFO1234yf was used in rectangular microchannels with a hydraulic diameter of 0.5 mm. The results showed that there were eight flow patterns in the experiment, and the change of flow pattern is closely related to the change of heat flux and mass flow rate. The higher the heat flux and mass flow rate, the more the flow patterns appear. Different flow patterns have different heat transfer mechanisms. The heat transfer mechanism is mostly nuclear boiling at the region of bubble flow, slug flow, elongated slug flow, and agitated flow; the heat transfer mechanism is film convection boiling at the region of annular flow, and it is vapor convection at the region of dryout flow. When the dryness is about 0.4, the heat transfer mechanism changes from nuclear boiling to film boiling. The friction resistance decreases with the increase of dryness and saturation temperature.

Hydrodynamic and adsorption properties of anionic acrylamide copolymers in water-salt media

Rheological properties and concentration cross-overs of anionic acrylamide copolymers in saline solutions (potassium chloride) were investigated by using capillary viscometer method. Area of non-overlapping coils between the crossover concentration and the concentration of fluctuation mesh formation was determined; it was shown that with increase of salt concentration this area practically disappears, i.e. mass transfer mechanism changes near the crossover concentration. It was shown that at low concentrations of potassium chloride increasing the content of ionic groups of macromolecules leads to reduction in the crossover concentration and increase in the effective volume. It is found that the kaolin adsorption capacity decreases when polymers are adsorbed from saline solution, and the adsorption constant is significantly higher in the presence of salt than in water.

Interfacial stress transfer mechanism of Cu-Zr amorphous films on polyimide substrates: Effect of deformation-induced devitrification

The fragmentation and interfacial stress transfer of Cu75Zr25 amorphous films with varying film thickness (h) on polyimide substrates were systematically investigated by using uniaxial tensile testing, in combination with in situ optical microscope and atomic force microscope characterizations. It is revealed that the interfacial stress transfer length increases monotonically as h increases. The interfacial stress transfer mechanism changes from elastic-plastic to approximately linear elastic at a critical point of h = 250 nm. This is related to h-dependent deformation-induced devitrification (DID) near the interfaces that the thinner Cu-Zr amorphous films display more DID. This study elucidates for the first time the effect of DID on the interfacial stress transfer, which will be helpful to design the interface with desired property for nanotechnological applications.

The role of surface tension and viscosity of the coolant on spray cooling performance of red-hot inclined steel plate

The dissolved surfactant in water droplet alters the surface tension, contact angle and viscosity and thereby heat transfer mechanism changes accordingly. The sliding characteristic of the water and vapor film resulting from the agglomeration of partially evaporated droplets on the hot plate are controlled by the aforesaid properties of the fluid. In addition to the above, the sliding characteristic further depends on the orientation of the plate. For the identification of appropriate conditions to get high heat transfer rates, the effect of viscosity, surface tension and orientation of the plate in the favorable direction of heat transfer needs to be investigated. The literature does not reveal the combined effect of the aforesaid process parameters on heat transfer rate. Therefore, in the current work an attempt has been made to reveal the effect of viscosity, surface tension and the orientation of the plate on heat transfer. To determine the effect of viscosity and surface tension in the presence of the foaming characteristics, three types of surfactants such as Sodiumdodecyl sulfate (SDS), Cetyltrimethylammonium bromide (CTAB) and Polyoxyethylene (20) sorbitan monolaurate (Tween 20) below critical micelle concentration have been used. The result reveals that the reduction of surface tension and viscosity up to 48% and 45% enhances the heat transfer rate (CHF from 1.39 MW/m2 to 1.58 MW/m2) at 30° plate inclination due to reduction in the residence time from 40 ms to 20 ms and enhancement in wettability characteristic (contact angle from 72.34° to 30.96°). The aforesaid result has been achieved at an optimum concentration of SDS (600 ppm), CTAB (180 ppm) and Tween 20 (42 ppm). In addition to the above, to identify the effect of viscosity and surface tension in the absence of foaming, ethanol has been selected as an additive. In comparison with Tween 20, the ethanol added water spray at an inclination of θ = 30° shows insignificant enhancement due to very small reduction and increment in surface tension and viscosity, respectively. The accuracy and enhancement are identified by comparing the obtained results with the information reported in the literature.

Numerical study of the effect of magnetic field on Fe3O4 –water ferrofluid convection with thermal radiation

PurposeThe purpose of this paper is to examine the effect of magnetic field on ferrofluid convective mode with radiation.Design/methodology/approachViscosity of Fe3O4 ferrofluid is considered as a function of magnetic field. Solutions of the governing equations are obtained by a powerful numerical method, namely, control volume finite element method (CVFEM). Roles of radiation parameter (Rd), number of undulations (N), Fe3O4–water volume fraction (ϕ), Hartmann (Ha) and Rayleigh numbers are illustrated graphically. A correlation for Nuave is extracted.FindingsThe inner wall temperature decreases with increasing buoyancy forces, but increases with increasing Rd and Ha. Also increasing Rd results in increasing nanofluid motion. This influence is more evident when convection flow is dominant. As nanofluid temperature increases, the nanofluid begins moving from the warm surface to the outer one and dropping along the circular cylinder. At low Rayleigh number, conduction is more significant than convection. |Ψmax| increases as buoyancy force increases and it decreases as the Lorentz force increases. As Hartmann number increases, the center of the vortices moves to x = 0. As Ra increases, convection becomes stronger. Thus, |Ψmax| and temperature gradient increase with increasing Ra. As N increases, the distortion of isotherms reduces and vortices become weaker. Increasing Hartmann number results in a reduction in the thermal plume and the heat transfer mechanism changes from convection to conduction. Nusselt number decreases with increasing N ⋅ Nu decreases with increasing Lorentz force. At N = 5 , increasing the Lorentz force causes the main vortices to convert into three smaller ones. As the Lorentz force increases, the two upper vortices merge together and the thermal plume vanishes. The number of extrema in the Nuloc profile matches the existence of the thermal plume and the number of undulations. Nuave increases with increasing Rd. As buoyancy forces increase, the temperature decreases and in turn Nuave increases with increasing Ra.Originality/valueNanofluids are an innovative way to enhance radiation heat. In this paper, MHD Fe3O4–water nanofluid natural convection with radiation source term is examined. Magnetic field-dependent (MFD) viscosity is considered. Using the CVFEM, numerical simulations are carried out for various values of the radiation parameter (Rd = 0 to 0.8), volume fraction of Fe3O4–water (ϕ = 0 to 0.04), Rayleigh number (Ra = 103, 104 and 105), number of undulations (N = 3,4 and 5) and Hartmann number (Ha = 0 to 40).

A Model for Lubricant Transfer from Media to Head During Heat-Assisted Magnetic Recording (HAMR) Writing

One of the challenges in heat-assisted magnetic recording (HAMR) is the creation of write-induced head contamination at the near-field transducer. A possible mechanism for the formation of this contamination is the transfer of lubricant from the disk to the slider (lubricant pickup) due to temperature-driven evaporation/condensation and/or mechanical interactions. Here we develop a continuum model that predicts the head-to-disk lubricant transfer during HAMR writing. The model simultaneously determines the thermocapillary shear stress-driven deformation and evaporation of the lubricant film on the disk, the convection and diffusion of the vapor phase lubricant in the air bearing and the evolution of the condensed lubricant film on the slider. The model also considers molecular interactions between disk–lubricant, slider–lubricant and lubricant–lubricant in terms of disjoining pressure. We investigate the effect of media temperature, head temperature and initial lubricant thickness on the lubricant transfer process. We find that the transfer mechanism is initially largely thermally driven. The rate of slider lubricant accumulation can be significantly reduced by decreasing the media temperature. However, as the amount of lubricant accumulation increases with time, a change in the transfer mechanism occurs from thermally driven to molecular interactions driven. A similar change in transfer mechanism is predicted as the head–disk spacing is reduced. There exists a critical value of head lubricant thickness and a critical head–disk spacing at which dewetting of the disk lubricant begins, leading to enhanced pickup.

Measurements of crosswind influence on a natural draft dry cooling tower for a solar thermal power plant

Crosswind is a significant concern for natural draft dry cooling towers. The concern is more serious for shorter towers. Therefore, the crosswind influence is a significant threat to the use of natural draft dry cooling towers in concentrating solar thermal power plants, which are generally built at sizes smaller than conventional fossil-fired plants and employ relatively shorter towers. While some numerical studies and small lab-scale test reports exist, very few full scale experimental studies have been reported for conventional cooling towers and none for relatively short cooling towers suitable for renewable thermal power plants. To address this gap, a 20-m tall fully instrumented natural draft dry cooling tower was built by the University of Queensland. The tower was designed to serve a future 1-MWe concentrating solar thermal plant on the same site. Its performance was tested under different ambient temperatures and crosswind speeds. The detailed experimental data of the crosswind condition, air temperature distribution inside and outside of the cooling tower and the cooling performance are presented. The experimental data demonstrate the substantial yet complex impact of the crosswind on cooling tower performance. Significant non-uniformities in air and hot water temperature distributions and strong air vortices inside the tower were observed in high crosswind speeds. Unlike tall cooling towers used in large conventional plants, the cooling tower performance does not monotonously decrease with the increase of the crosswind speed. In fact, after the tower performance drops to its lowest level at a wind speed around 5 m/s, the trend is reversed and further increases in the crosswind speed help the tower performance. Analysis shows that this reversal occurs because the tower heat transfer mechanism changes. As crosswind rises above the critical speed, the airflow inside the cooling tower becomes increasingly controlled by the crosswind instead of the natural draft.

Rapid Cellular Crystal Growth of TiAl-Based Intermetallic without Peritectic Reaction by Melt-Quenching in Ga–In Liquid

A rapid cellular microstructure of Ti-48Al-2Cr-2Nb (in atom %) intermetallic was grown without peritectic reaction by the method of melt-quenching in Ga–In liquid. After characterization of the microstructures and phase constituents, it is observed that the cellular crystals mainly consist of the α2 phase and directionally grew in the outer layer of the melt droplet, with the growth length and cellular spacing about 358–460 μm and 0.68–3.6 μm, respectively. Upon detailed analysis of the cellular growth process, it is found that the formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga–In liquid, mainly determined by the heat transfer process; the dominant heat transfer mechanism changes from heat convection to heat conduction at a growth distance of about 70 μm. By using the semireverse method, the dependence of the cooling rate on the cellular growth distance can be estimated accurately and conveniently, which ranges from 2.61 × 106 to 1.26 × 105 K/s. Th...

Concentration-Dependent Proton Transfer Mechanisms in Aqueous NaOH Solutions: From Acceptor-Driven to Donor-Driven and Back.

Proton transfer processes play an important role in many fields of chemistry. In dilute basic aqueous solutions, proton transfer from water molecules to hydroxide ions is aided by "presolvation", i.e., thermal fluctuations that modify the hydrogen-bonding environment around the proton-receiving OH(-) ion to become more similar to that of a neutral H2O molecule. In particular at high concentrations, however, the underlying mechanisms and especially the role of the counterions are little understood. As a prototypical case, we investigate aqueous NaOH solutions using molecular dynamics simulations employing a reactive high-dimensional neural-network potential constructed from density functional theory reference data. We find that with increasing concentration the predominant proton transfer mechanism changes from being "acceptor-driven", i.e., governed by the presolvation of OH(-), to "donor-driven", i.e., governed by the presolvation of H2O, and back to acceptor-driven near the room-temperature solubility limit of 19 mol/L, which corresponds to an extremely solvent-deficient system containing only about one H2O molecule per ion. Specifically, we identify concentration ranges where the proton transfer rate is mostly affected by OH(-) losing an accepted hydrogen bond, OH(-) forming a donated hydrogen bond, H2O forming an accepted hydrogen bond, or H2O losing a coordinated Na(+). Presolvation also manifests itself in the shortening of the Na(+)-OH2 distances, in that the Na(+) "pushes" one of the H2O protons away.

A CFD Simulation of Hydrogen Production in Microreactors

In this study, the modeling of hydrogen production process in microreactors by methanol-steam reforming reaction is investigated. The catalytic reaction of methanol-steam reforming producing hydrogen is simulated considering a 3D geometry for the microreactor. To calculate diffusion among species, mixture average correlations are compared to Stephan-Maxwell equations. The reactions occurring inside the microreactor include reforming of methanol with steam, methanol decomposition, and a reaction between carbon dioxide and hydrogen. The main objectives of this study are the prediction of temperature profile along the microreactor using either mixture average method or Stephan-Maxwell one and the comparison between the present predictions and some existing experimental data. The simulation results indicate that Stephan-Maxwell method conforms more suitably to the experimental results. The difference is more at lower feed flow rates since, when the flow rate increases, mass transfer mechanism changes from diffusion to convection, which in turn reduces the difference.

Vibrational energy transfer between I2(BΠu+3,ν′=21) and He at very low temperatures: Impulsive versus complex formation mechanisms assisted by tunneling through the centrifugal barrier

The temperature dependence of the state-to-state vibrational relaxation rate constant (k(nu)(21-Delta nu)) for collisions between I(2)(B,nu(')=21) and He at very low kinetic energies was studied. The fluorescence from I(2)(B,nu(')=21-Delta nu(')) with Delta nu(')=1-5 indicates that in the temperature range of 0.6-8.2 K these states are populated by only one collision with He. The behavior of k(nu)(21-Delta nu) with temperature can be divided into two groups. The group with quantum changes Delta nu(')=1-3 shows scattering resonances in the low temperature region, with a general monotonical decrease of the rate constant with temperature, suggesting the importance of van der Waals interactions. This behavior is supported by the calculation of the probability of tunneling through the centrifugal barriers. For collisions in which 4-5 quanta are lost in a single event, there are no evidences of scattering resonances and the values of the relaxation rate constants could be determined only at the highest temperatures of this study. This suggests that relaxation occurs via impulsive collisions. The branching ratios for each channel are also temperature dependent and this behavior also suggests that the energy transfer mechanism changes with Delta nu(').

Thermally assisted atom transfer on surfaces

The low-temperature rates for site-to-site transfer of single atoms and molecules adsorbed on surfaces have been determined in recent scanning tunneling microscopy (STM) studies within the temperature regime where the dominant transfer mechanism changes from mostly activated transmission over to thermally assisted tunneling through the intersite (transition-state) barrier as the temperature is reduced. A model that has provided useful conceptual and quantitative insights into thermally assisted field emission of electrons is used here as the basis for a theory of site-to-site atom transfer in this temperature range where proper account of atomic tunneling and quantum reflection, for energies below and above the transition state barrier, is required. The predicted transfer rates, which are very sensitive to barrier shape as well as height, agree well with those observed in the STM studies of Co and Cu on surfaces in the interesting transition range which is relevant in the atom-by-atom fabrication of thermally stable surface nanostructures.

On filter design and critical thickness of synchrotron x-ray filters

A ‘‘critical thickness’’ for a synchrotron radiation x-ray filter exists. Because x-ray absorption in media is an exponential function of depth and because radiation and conduction both play a role in the cooling of the filter assembly, the heat transfer mechanism changes from radiation dominant to conduction dominant as the thickness increases. For a thin filter, radiative heat transfer is the main mechanism. The maximum temperature in the filter increases as the thickness increases due to the fact that the total heat load increases while the total area for radiative heat transfer remains the same. For a thick filter, conductive heat transfer is the main cooling mechanism. When the filter thickness increases, the heat absorption per unit thickness decreases and so does the maximum temperature. At a certain thickness, the temperature in the filter is the maximum. This is the critical thickness. For third-generation synchrotron radiation facilities, the maximum temperature and thermal stresses in a filter are the main factors considered in a filter assembly design. It is very important to avoid designing a filter inside the critical thickness range.

Hydrodynamics in Czochralski growth-computer analysis and experiments

Probable flows in a melt are reviewed. In a pure melt, four different flows, free convection, surface tension driven flow (Marangoni flow), forced convection caused by crystal rotation, and forced convection caused by crucible rotation, are to be considered. The mixed flow caused both by buoyant force and by crystal rotation is roughly divided into three classes with respect to the ratio Gr Re 2 , where Gr is the Grashof number due to the temperature difference between the crystal and crucible and Re is the Reynolds number of the rotating crystal. For a low Gr Re 2 , forced convection is dominant in the melt, while for a high Gr Re 2 , free convection is dominant. For an intermediate Gr Re 2 , free and forced convection coexist and they are separ ated by a stagnant surface. The temperature field is slightly dependent on the flow for low Prandtl number melts such as molten metals and semiconductors, while it strongly depends on the flow for high Prandtl number melts such as molten oxides. Two distinct types of temperature field are obtained with respect to the dominant convection and forced convection. For a fixed Grashof number, the heat transfer mechanism changes from free convection dominant mode to forced convection dominant mode as the Reynolds number increases. This explains the change of the interface shape with increasing crystal rotation rate and the abrupt inversion of the interface shape with increasing crystal diameter. A formula to predict the critical Reynolds number at which the interface shape changes from convex to concave is presented. The effect of heat dissipation from the melt free surface on the flow is also examined, and a reason for the difficulty encountered in the growth of some oxide crystals is offered. Finally, the effect of the crucible rotation on the flow is briefly described.