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In this study, an experiment on condensation heat transfer was conducted to develop a condensation model considering the structure of separated flow patterns. Multidimensional local condensation heat transfer parameters were measured in pure saturated steam at pressures of 1–5 bar and mass fluxes of 10–50 kg/m2 s in an inclinable tube with an inner diameter of 40 mm and a length of 3 m. A heat partition angle, which separated the upper and lower heat transfer areas for film condensation and convective heat transfer in stratified flow, was obtained based on the inflection point of the circumferential distribution function of local heat flux. A new condensation heat transfer model consisting of the heat partition angle and heat transfer coefficient correlations was developed based on the local heat transfer data. The experimental data for model development were obtained using circular tubes with inner diameters of 30–45 mm and inclination angles of 0–10° under pure saturated steam conditions at pressures of 1–67 bar and mass fluxes of 10–329 kg/m2 s. The model predicted the average heat transfer coefficient with an average deviation of 6.2% against the present experimental data.

This study introduces Kagome lattice structures to replace pin fins or ribs in a wedge-shaped channel, which represents a turbine-blade trailing edge, for heat transfer enhancement. Present simulation methods are verified against available experimental data. The local and overall thermo-fluidic characteristics of the four designed channels, at five Reynolds numbers and a given porosity, are investigated numerically and compared. The results reveal that the proposed structures exhibit 6–71% higher overall Nusselt numbers relative to the original structure but with similar pressure loss. The heat transfer enhancement is attributed to the fact that the first array of Kagome cores separates more high momentum fluid to the tip region, which results in the reduction of recirculation and increases convective heat transfer.

Results of an extensive experimental investigation performed to study the effect of different values of wall heat flux in a rectangular PCM (phase change material) test cell on the melting process are presented. A new experimental system consisting of a rectangular cross-section test cell formed from polycarbonate sheet, copper plates and mica heaters was constructed. During experiments uniform wall heat flux (q″wall = 675, 960 and 1295 W/m2) were applied to both the left and right sides of the test cell. Thermocouples were used to measure the temperature at different locations inside the PCM and on the surface of the copper plates and an infrared camera was used to measure the polycarbonate sheet external surface temperature distribution. The results show the expected strong correlation between the magnitude of wall heat flux and the melt fraction in the PCM as it drives the convective heat transfer. The transparent polycarbonate wall makes it possible to observe the location of the solid/liquid interface and determine melt fractions. The experiments have produced a significant experimental data set for the validation of numerical models simulating the solid/liquid phase change process and PCM melting in geometrical configurations relevant to, for example, latent heat thermal energy storage systems.

A good knowledge on thermal boundary layer thickness and mass flux are important features in the development of membrane distillation pilot modules. The present work poses as a comprehensive approach for modelling both during the process’ steady state, whilst cherishing simplicity as it does so through a simple, intuitive software – namely Calc from Libreoffice. The mass transfer model allows prediction of average mass flux, being obtained by tailoring a mass transfer equation to be used together with an overall-macroscopic energy balance for the system. Within such approach, five different convective heat transfer correlations were investigated and a mass transfer coefficient of 6.02 × 10 - 7 k g · m - 2 · s - 1 · P a - 1 was found for the PTFE supported membrane used in the experiments. Thermal boundary layer thickness was estimated using the integral method, primarily under the assumption of constant local heat flux along the membrane extension. Thermal boundary layer thickness was coherently modelled in the middle portion of the channels, region where model results agreed with the assumptions made with 80% accuracy or higher, pointing for correspondingly low variation of heat flux with space within such region. The applicability of the temperature profile proposed in the present work in portraying the reality of permeate and feed in membrane distillation processes was discussed.

The migration of a neutrally buoyant particle in a shear flow with thermal convection is studied with the lattice Boltzmann method. Due to the effect of thermal convection, the equilibrium position of the particle is different qualitatively from the isothermal case. The dependence of the equilibrium position of the particle on the Grashof number is not monotonic, and four driving forces, which affect the migration of the particle, are investigated. Since the migration of the particle is affected by both forced convection and thermal convection, with the Reynolds number increasing, forced convection becomes stronger, which weakens the effect of thermal convection, as a result, the equilibrium position of the particle is closer to the isothermal case.

Thermal management of high heat flux devices in many areas has promoted the development of advanced microchannel heat sinks. This study proposed a novel type of periodic expanded-constrained microchannels (PECM) heat sink with flow separation and convergence passages. Its convective heat transfer and pressure drop performance was explored by both experimental tests and numerical simulations, and its cooling efficiency was compared with rectangular microchannels. Forced convection tests were conducted at Reynolds number of 150–820 and two levels of heat fluxes using deionized water. Results indicated that the PECM samples showed significant heat transfer enhancement, i.e., 50% to 117%, compared to the rectangular counterpart in the test ranges. Such enhancement can be attributed to that the periodic expanded-constrained configurations of PECM induced periodic interruption and redevelopment of thermal and hydraulic boundary layers, as well as good flow separation, fluid acceleration and mixing. Moreover, the heat transfer enhancement of PECM was not accompanied with the expense of pressure drop penalty, and all the PECM samples reduced the pressure drop for 10–74% compared to the rectangular counterpart. Besides, the effect of microchannel gap width in the main flow passages on the thermal and hydraulic performance was also assessed by preparing three samples with microchannel gap widths of 0.5, 0.8 and 1.0 mm. It was found that the pressure drop and thermal resistance of PECM increased monotonically with decreasing the microchannel gap widths. The sample with the largest microchannel gap width of 1.0 mm presented the best overall thermal and hydraulic performance.

This study experimentally investigates the effect of different values of wall heat flux intensity on the melting of RT44HC Phase Change Material (PCM) in a rectangular test cell. A new novel experimental test rig to provide accurate data for the validation of numerical models of phase change was developed. The designed and constructed test rig consists of a horizontal rectangular cross-section test cell formed from polycarbonate sheet with copper plates and mica heaters to provide controlled uniform wall heat flux. Experiments were performed for three constant uniform wall heat flux values (q″wall = 675, 960 and 1295 W/m2) applied to both left and right sides of the test cell. An imaging technique was used to visualize and record the movement of the solid-liquid interface using a Canon EOS DSLR Camera. The results obtained show a strong correlation between the magnitude of wall heat flux which drives the convective heat transfer and melt fraction development in the PCM. The results also show that increasing the input power from 675 W/m2 to 960 W/m2 to 1295 W/m2 reduces the total time for the melting process by 26.3% and 42.10% respectively. The raw data set comprised of measured temperatures and observation of melt fraction development provide a useful data set for validation of numerical models aiming to simulate the melting process in a rectangular cross-section test cell.

We present numerical simulations of three-dimensional thermal convective flows in a cubic cell at high Rayleigh number using thermal lattice Boltzmann (LB) method. The thermal LB model is based on double distribution function approach, which consists of a D3Q19 model for the Navier-Stokes equations to simulate fluid flows and a D3Q7 model for the convection-diffusion equation to simulate heat transfer. Relaxation parameters are adjusted to achieve the isotropy of the fourth-order error term in the thermal LB model. Two types of thermal convective flows are considered: one is laminar thermal convection in side-heated convection cell, which is heated from one vertical side and cooled from the other vertical side; while the other is turbulent thermal convection in Rayleigh-B\'enard convection cell, which is heated from the bottom and cooled from the top. In side-heated convection cell, steady results of hydrodynamic quantities and Nusselt numbers are presented at Rayleigh numbers of $10^6$ and $10^7$, and Prandtl number of 0.71, where the mesh sizes are up to $257^3$; in Rayleigh-B\'enard convection cell, statistical averaged results of Reynolds and Nusselt numbers, as well as kinetic and thermal energy dissipation rates are presented at Rayleigh numbers of $10^6$, $3\times 10^6$, and $10^7$, and Prandtl numbers of 0.7 and 7, where the nodes within thermal boundary layer are around 8. Compared with existing benchmark data obtained by other methods, the present LB model can give consistent results., Comment: 33 pages, 8 figures

High Rayleigh number thermal convection in tilted rectangular enclosures of different aspect ratios ( 1 , 3 , 6 , and 10) and angles of inclination ( 0 ° , 30 ° , 60 ° , 90 ° , 120 ° , and 150 ° ) is experimentally studied for 1.85 × 10 6 ⩽ Ra ⩽ 1.04 × 10 11 employing compressed gases. For a fixed Rayleigh number, aspect ratio is found to have a discernible effect on Nusselt number for all the cases, except for the horizontal enclosure case ( 0 ° ), and this decreasing effect becomes stronger as the angle of inclination increases. Nusselt number is observed to monotonously decrease as the angle of inclination is increased, for any given Rayleigh number. A new set of correlations for Nusselt number, valid for the investigated range of studied variables, is proposed. An Ra cos θ scaling is also found to work reasonably well for angles of inclination less than 60 ° .

The internal convective heat transfer characteristics of n-decane at supercritical pressure in a vertical square tube with a hydraulic diameter of 1.8 mm were experimentally investigated. The external wall temperatures of the square tube were measured with welded thermocouples whereas the internal wall temperatures were calculated with space marching method. In this experiment, the operating pressure ranged from 3 to 5 MPa and the heat flux from 100 to 500 kW/m2. Besides, the flow direction in the vertical tube was also switched in order to examine the effects of buoyancy. Moreover, the circumferential uniformity of heat transfer was also discussed. The results showed that the effects of buoyancy on the heat transfer were negligible once the local buoyancy number was less than a threshold. After the threshold, the buoyancy promoted the heat transfer in the downward flow configuration, but weakened the heat transfer in the upward flow one. The reason could be inferred that the buoyancy would influence the intensity of turbulent kinetic energy and the heat transfer near the wall. Finally, new heat transfer correlations for vertical upward and downward flow in square tubes were proposed.

The study reports the experimental results of warming-up and evaporation of homogeneous and heterogeneous water droplets at high temperatures, 100–600 °C. The heterogeneous water droplet is formed by covering a solid opaque graphite particle by a thin water layer. Differences between the warming-up mechanisms of the homogeneous and heterogeneous water droplets with an initial liquid volume from 10 μl to 20 μl at the convective heating are shown. The contactless method Planar Laser-Induced Fluorescence allows the analysis of the temperature distributions inside the homogeneous droplets and inside a liquid phase of the heterogeneous droplets, laying emphasis on the novelty of the research. A maximum temperature in the central part of the liquid phase of the heterogeneous droplets is approx. 90 °C; a maximum temperature in the homogeneous droplets is approx. 50 °C. The heterogeneous droplet heats up by 20 ± 3% longer than the homogeneous one. Moreover, a warming-up rate of the liquid phase of the heterogeneous droplet is less than that of the homogeneous water droplet by 85%. However, the size decreasing rate of the heterogeneous droplets is sometimes higher than that of the homogeneous droplets by 80%. The mean and instant evaporation rate of homogeneous and heterogeneous water droplets are compared. The study discusses an influence of convection in the liquid phase of the heterogeneous droplets on evaporation characteristics. The time of the complete water evaporation decreases by 40% due to a solid inclusion presence. A dimensionless criterion of the convective heat transfer enhancement in a water droplet containing the solid opaque inclusion is used to generalize the experimental data. The findings obtained are critically important to improve the existing high-temperature technologies and methods of water purification and to develop innovative ones.

The boiling convective heat transfer of dielectric fluid of FC-72 in a microchannel with etched micro fins is experimentally studied. The microchannel test surface is a square of 10 mm size with a height of 100 μm. The surface of the microchannel is covered with parallel-nucleated fins of a cubic size of 100 μm. The fins are nucleated with the central pore size of 60 μm and an opening of 45 μm. The fins are arranged in the equal space of 400 μm. There is a planar electrical element below the microchannel, which produces an adjustable surface heat flux below the channel. The dielectric liquid enters the test microchannel and flows over the test surface and the fins. In the experiments, the mass velocity (94–275 kg/m2 s) and heat flux (0–6 W/cm2) are tested with a saturation temperature of either 35 or 50 °C. The heat transfer, the outlet vapor quality, and the pressure drop across the microchannel are measured and reported. The boiling behavior of FC-72 over the etched fins is also investigated using photos. The results show that the liquid first go through some superheat states and then the boiling phase change starts. When the boiling heat transfer occurs, a significant pressure-drop can be observed across the microchannel, but the surface temperature difference would also drop significantly. In the case of very low mass velocity of 94 kg/m2 s, the minimum superheat temperature-difference of 4.7 °C is observed.

We study the flow characteristics of natural convection induced by exothermic reaction in an annulus bounded by two wavy wall cylinders. With a view to achieving the target the dimensionless equations are transformed for the present problem using suitable functions. We solve the new system of equations with the use of finite difference method. Numerical results demonstrate that an increase of the number of crests or troughs of the inner or outer walls of the enclosed domain considerably reduces the strength of streamlines and the maximum temperature. Due to the augmentation of the Frank-Kamenetskii number the intensity of the streamlines and the maximum temperature are found to increase but the minimum value of the concentration decreases. When the Rayleigh number for thermal diffusion is increased, the magnitude of streamlines noticeably increases, whereas there is little effect on the isotherms and isolines of concentration. For larger value of the amplitude of the inner or outer cylinder walls the intensity of streamlines considerably decreases, but the isotherms and the isolines of concentration change a little bit. The remarkable findings are that when Le = 0.5 and 1.0, the inner and outer major vortices contain two and three eyes respectively. But the streamlines for Le = 2.0 show distinct characteristics from those for Le = 0.5 and 1.0. In this case, there occur two elongated and two elliptic vortices adjacent to the inner and outer cylinder walls. The inner elongated vortices hold a single eddy whereas the elongated vortices neighboring the outer cylinder consist of two eddies. Contrary to this, in presence of thermal convection or combined convection for thermal and mass diffusion the inner and outer major vortices have two and three eddies respectively. However, the convection due to mass diffusion yields an astonishing result. Two vortices are recognized near the outer wall while a single vortex is observed adjacent to the inner wall. Both of the inner and the outer elongated vortices consist of two eyes. It is worthy of mentioning that the concept of constructing such a geometry by means of simple transformations might encourage the researchers to think for generating a more complex geometry. Moreover, these results could be useful for the advancement in science, engineering and technology.

The experiment investigation of laminar forced convection of nanofluids with Al2O3, TiO2, ZrO2 and diamond nanoparticles was carried out. Nanoparticle concentration varied in the range from 0.25 to 6 vol% in the experiments. The nanoparticle size ranged from 5 to 150 nm. The dependence of heat transfer coefficient and pressure drop from the concentration, size, material of the nanoparticles was studied. It was shown that adding nanoparticles to the coolant significantly influences the heat transfer coefficient in the laminar flow regime. It is shown that with increasing nanoparticles concentration, the local and average heat transfer coefficients at a fixed Reynolds number increase. The effect of nanoparticles material on the heat transfer coefficient has been also demonstrated. It was shown the effect of properties of the base fluid on the heat transfer coefficient nanofluids was considered. The effect of rheology nanofluids on the heat transfer coefficient was analyzed.

The subchannel code ATHAS-LMR has been improved for analyzing thermal-hydraulics characteristics of flow blockage events in the fuel assembly of a sodium-cooled fast reactor (SFR). Based on the in-depth study of the phenomenon of flow blockage and combined with the characteristics of fast reactor assembly, enhanced models for the wire-wrap spacer and convective heat transfer have been added to account for changes in transverse flow in the presence of blockages. And appropriate transverse mixing and local resistance model are also implemented in ATHAS-LMR-FB. The improved code has been assessed against experimental data obtained at adiabatic and diabatic conditions in SFR assemblies with simulated flow blockages. Good agreement between predictions and experimental data on velocity and temperature distributions downstream of the blockage has been observed. Comparison of predictions of various analytical tools have also been performed. Predictions of the improved ATHAS-LMR code are shown to be compatible with those of other analytical tools. An in-depth analysis of effects of location and size of blockage has been carried out for the fuel assembly of the China Experimental Fast Reactor. Boiling has been predicted to occur for blockage ratios greater than 55.61% at the central location of the fuel assembly. Peak cladding and coolant temperatures are predicted at the axial location 70% of the rod length (just downstream of the maximum local power position of the non-uniform axial power profile).

A two-dimensional (2D) numerical simulation is presented to study the effect of magnetic field on Ag-MgO nanofluid forced convection and heat transfer in a channel with active heaters and coolers. A Fortran code according to Lattice Boltzmann method (LBM) is developed for this purpose. The effects of thermal arrangement (Case1, 2 and 3), block side length (0.3 ≤ h ≤ 0.5), Reynolds number (50 ≤ Re ≤ 100), Hartmann number (0 ≤ Ha ≤ 60) and volume fraction of nanoparticles (0 ≤ ϕ ≤ 0.02) on flow pattern and heat transfer characteristics are analyzed systematically. The obtained results showed that the highest value of local Nusselt number occurs at the junction of the heater and the cooler due to the high temperature gradient, followed by the sharp corner of heaters and coolers. Moreover, the heat transfer at the heater sharp corner is higher than that of the cooler sharp corner. The average Nusselt numbers indicated that the rate of heat transfer increases with increasing ϕ or decreasing Ha. Finally, the heat transfer rate in Case 1 is more than Case 3 and Case 2.

Continuous hydrothermal pretreatment in tubular reactors is a key process for the production of biofuel from microalgae biomass. Complex heat transfer characteristics of microalgae slurries, which consist of micron-sized cells suspended in water, are a challenge for the industrialization of hydrothermal pretreatment systems. In this study, the convective heat transfer characteristics of microalgae slurries flowing through a uniformly heated circular tube were experimentally investigated for the first time. The results revealed that the microalgae cell concentration and cell motion in the solid–liquid two-phase flow were the two main factors affecting the convective heat transfer characteristics of microalgae slurries in laminar tube flow. Considering the effects of the cell concentration, cell motion, viscosity variation with temperature and shear rate, and Peclet number, a novel correlation was proposed to predict the Nusselt number of microalgae slurries in laminar tube flow. In addition, a heat transfer deterioration phenomenon was observed under forced convection subcooled boiling conditions. This study provides guidance for the design and optimization of tubular heat exchangers in continuous hydrothermal pretreatment systems.

Concentrated solar thermal energy is a good alternative to mitigate the environmental impacts generated by the growing use of energy. There are several commercial solar power tower plants in the world operating with direct steam generation. These plants operate with cavity-type and external tubular receivers. The conventional tubular receivers consist of a set of vertical circular tubes through which the working fluid circulates. This study is focused to analyze in detail the direct steam generation in a vertical tube receiving concentrated solar radiation. The mathematical model is based on the equations of continuity, momentum and energy for each phase. The modified model of Rensselaer Polytechnic Institute was used for the conditions of critical heat flux coupled to a Eulerian two fluid model. In the modified model of Rensselaer Polytechnic Institute, the total heat flux is divided into four components from the wall to the liquid: (a) liquid-phase convective heat flux, (b) heat transfer due to quenching, (c) heat flux due to evaporation and (d) convective heat flux of the vapor phase. The mathematical model was solved with a computational fluid dynamics software. The results were validated with experimental data reported in the literature and a parametric study was carried out to determinate the effect of the mass flux and heat flux incident on the wall, on: the steam quality, the volumetric fraction, the enthalpies and temperatures of liquid and steam. The reduction of mass flux by 44% increased the output mass flow of steam 4.28 times, whereas the increase of concentrated solar flux by 40% increased the output mass flow of steam 4 times.

Three-dimensional flow and convective heat transfer was modelled and was discreted by FVM to investigate the effect of oscillation center position on heat transfer and flow internal a smooth round tube in the present study. The heat transfer performance was measured by Nusselt number and flow capacity was measured by Darcy friction factor. The numerical results showed that heat transfer was enhanced and the enhancement increased with increase in the distance between oscillation and inlet. More over the fluctuation of Nu increased as the distance between oscillation and inlet increased. However, flow resistance was lower than static tube when oscillation center at the first half of tube and higher than static tube when oscillation center at the second half of tube including center point. In addition, the secondary flow and radial velocity was created by oscillation motion, which was the reason why heat transfer was enhanced.

This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron-shaped offset wing structures. Heat transfer performance was measured for liquid water (510 ≤ Re ≤ 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm2. This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes.

Performance improvement of convective heat transfer processes is significant for energy conservation. Considering the trade-off between the heat transfer enhancement and the pumping power reduction, the optimization of convective heat transfer processes can be modeled as a multi-objective optimization problem in the entropy-based approach and a constrained optimization problem in the entransy-based approach. This article first reviews these two theories and then compares them from the perspectives of optimization criteria, optimization methods and optimization results. Studies have shown that simply analyzing the entropy generation rate or other entropy generation criteria cannot meet the diverse objectives of various practical applications. Besides, the minimum heat transfer entropy generation does not always lead to the maximum of convective heat transfer coefficient, as reason is also analyzed here. In contrast, the entransy dissipation extremum corresponds to the maximum convective heat transfer coefficient as has been shown mathematically. Moreover, the entransy dissipation extremum principle and the variational method can be combined to find the optimal flow and temperature fields with better heat transfer results than determined using the entropy generation minimization principle. The entransy optimization results can then be used to design better augmentation technologies for convective heat transfer processes. In summary, the entransy theory is more appropriate for optimizing convective heat transfer processes without heat-work conversion processes.

The Discrete Element Roughness Method (DERM) is evaluated as an engineering solution to the problem of convective heat transfer on rough surfaces. As part of the present work, DERM is incorporated into a general purpose compressible CFD code and explored as a way of modeling the sub-resolved roughness scales. In addition, DERM-model inputs are evaluated in detail and developed to represent sand-grain roughness (SGR). The results display good agreement in a number of validation cases. The overall results clearly indicate that DERM has potential to improve heat transfer predictions beyond the capability of SGR models, while only slightly increasing the computation time.

A comprehensive review and comparison on heatline concept and field synergy principle have been made based on more than two hundreds of related publications. The major conclusions are as follows. Both heatline concept and field synergy principle are important contributions to the developments of single-phase convective heat transfer theories. The role and function of heat line concept is to visualize the heat transfer path while that of field synergy principle is to reveal the fundamental mechanism of heat transfer enhancement and to guide the development of enhanced structures. None of them can be used to deduce the other, nor none of them can be derived from the other. Hence, there is no problem of mutual remake between them at all. If heatlines are constructed by solving a Poisson equation additional computational work should be done; However, either the synergy number or the synergy angle both can be obtained by using numerical results without additional computational work. Further research needs for both heatline concept and field synergy principle are also provided.

Experimental studies on convective heat transfer characteristics of supercritical R-134a flowing in a horizontal tube with an inner diameter 8 mm were conducted in the range of mass flux 500–1000 kg m−2 s−1, heat flux 20–100 kW m−2 s−1 and pressure 4.3–4.8 MPa. Local wall temperatures and convective heat transfer coefficients of the top and bottom surfaces of the test tube were obtained, and their variational trends with increasing heat flux and mass flux were discussed within the experimental range. To have a quantitative and more comprehensive analysis of convective heat transfer behaviors of supercritical fluids as compared with the conventional methods, two new field synergy analytical methods designated as TCEH and FCEH respectively were developed, motivated by the two basic concepts of the field synergy principle respectively, i.e., the analogy of convective heat transfer to conductive heat transfer and the three general criteria associated with convective heat transfer enhancements. Numerical simulations were performed within the identical parameter ranges, and the effects of buoyancy, heat flux and mass flux on convective heat transfer were analyzed using these two new methods. Finally some new insights into supercritical convective heat transfer were obtained.

A general unit conversion system based on the analogy with the Planck unit system is put forward in this article for most physical quantities in lattice Boltzmann simulations. Comparing with the traditional unit conversion methods, the approach has more systematic, programmatic and standardized operating process and can be applied in programming easily. In addition, a two-dimension exemplary simulation of convective heat transfer in tube banks is implemented to illustrate the feasibility and advantage of the approach, with verification of coupled flow and heat transfer by the empirical formula in literature. Furthermore, a uniform scheme for three kinds of boundary conditions for heat and mass transfer is developed and applied herein.

The present study considers the influence of unsteady flow structures in natural convection on the heat transfer from a vertical flat plate to air. The mean and momentary intensity of convective heat transfer were determined experimentally for specific variations of the boundary conditions. For cyclic and step-like changing operation modes of a direct-electrically heated, 2 m-high copper plate, laminar, transitional and turbulent flow regimes were considered. Compared to a quasi-stationary approach, mean deviations of up to 22.0% for cyclic variations and up to 30.2% for step-like changes occur during an unsteady phase. The extent of the deviations shows a dependence on the mean Grashof number, as well as further influencing factors. Phenomenological observations of the unsteady convection flow in the form of visualizations show a temporarily increased mixing of the boundary layer, which also deviates distinctly from a quasi-stationary development. In particular, a temporary occurrence of Kelvin-Helmholtz vortices can be observed.

The instantaneous convective heat transfer rate resulting from the passage of a single elongated bubble propagating in an inclined pipe with a constant translational velocity is examined experimentally for various pipe inclinations and liquid flow rates. Local instantaneous heat transfer rates in the upper and lower parts of the pipe were determined using wall temperature measurements by an IR camera. The ensemble-averaged values of the heat transfer rate are presented as a function of the distance from either the bubble tip or bottom. Optical sensors synchronized with the IR camera were used for this purpose. Possible mechanisms that govern the enhancement of the heat transfer rate during the passage of an elongated bubble are discussed.

The subject of convective heat transfer in micro-/nano-scale channels is of utmost importance for the effective cooling of small-scale electronic devices with high power density. The overall heat convection in these channels largely depends on interfacial (wall-fluid) thermal transport. In this work, we demonstrate that a carefully designed surface topology can enhance heat dissipation in nanochannels. Specifically, we found that both uniformly and randomly distributed surface roughness can substantially increase the overall heat transfer between the wall and fluid in the nanochannels. Moreover, we reveal that depositing a coating layer, of which the vibrational density of states bridge those of the walls and the flowing fluid, can significantly enhance wall-fluid interfacial heat transfer. This work reveals the critical effect of surface topology and material choice of the channel wall on heat dissipation in nanochannels, which is important for the cooling of small-scale devices.

Moderate Rayleigh number thermal convection for horizontal rectangular enclosure is experimentally studied for 1.85 × 10 6 ⩽ Ra ⩽ 1.04 × 10 11 . The best fit for the investigated range of Rayleigh numbers is found to be Nu = 0.067 Ra 0.330 when a sidewall material of very low thermal conductivity is employed. The effect of sidewall conductance and aspect ratio on the Nu - Ra relationship is also scrutinized.

The buoyancy has great influence on the convective heat transfer of supercritical pressure fluids whose thermal properties change drastically near the pseudocritical point. Thus, numerical investigations on the coupled heat transfer characteristics of supercritical pressure CO2 (S-CO2) in horizontal semicircular channels are conducted in the present study. The effect of the secondary flow induced by buoyancy on the local thermal performance is analyzed in detail, and the dimensionless number Se, which represents the absolute vorticity flux in the main flow, is employed to describe the secondary flow and estimate the buoyancy effect. The numerical results show that the buoyancy effect gets smaller with the increase of the mass flow rate, and the heat transfer performance in the asymmetric flow is superior to that in symmetric flows at low mass flow rate. The buoyancy could significantly improve the thermal performance on the top wall but deteriorate that on the bottom wall in the hot side, which is exactly opposite in the cold side. The local wall heat transfer enhancement could be attributed to thinner thermal boundary layer due to the secondary flow, and the entransy dissipation theory could well explain the local heat transfer behavior. The criterion Se/Re gives better prediction for the buoyancy effect on both overall and local heat transfer than the existing ones.

Packed beds with large tube-to-particle diameter ratios (D/d) suffer from poor lateral heat transport. Consequently, for applications with internal heat generation, fluid with high temperature concentrates in the center of packed beds, which is unfavorable to the design of high efficiency reactors. To address this problem, a novel method is developed: a free channel—flow guiding conduit (FGC)—is constructed in the center of a packed bed using highly fluid-penetrable material. Therefore, in the presented work, CFD simulations are carried out to study the influences of FGC to a conventional packed bed on packing structure, fluid flow distribution as well as heat transfer. It shows that cold fluid with high velocities is guided directly into the original hot zones where fluid temperature is generally higher in a conventional packing. Fluid flow in the lateral direction can be promoted with the Venturi effect. Moreover, with the FGC in proper sizes, pressure drop can be reduced about 16–26%; heat transfer coefficients merely drop 4–6%. However, although favorable effects could be achieved using the FGC, it is also observed that if an oversized flow guiding conduit was imposed into a packed bed, loss of effective cooling for particles would be also resulted since excess fluid could bypass through the conduit. Consequently, unexpected fluid temperature distribution could get some recovery. These findings are helpful for understanding of transport phenomena in packed beds as well as the design of high efficiency reactors.

Stable Fe3O4-water nanofluids are prepared and sedimentation observation is used to investigate its stability. A convection heat transfer experiment is set up to investigate the thermo-hydraulic performances of Fe3O4-H2O nanofluids in a circular tube considering the effects of different nanoparticle mass fractions (ω = 1.0%, 3.0% and 5.0%), Reynolds Numbers (Re = 600–11000) and paralleled magnetic induction intensities (B = 0G, 100G, 200G and 300G). The result is that Nusselt number is proportional to nanoparticle mass fraction but opposite trend is found with the increasing paralleled magnetic induction intensity. Nanofluids with ω = 5.0% show the best performance of heat transfer. The resistance coefficient increases with mass fraction and can be enhanced by magnetic field further. Nanofluids with ω = 5.0% under magnetic induction intensity B = 300G show the largest resistance coefficient. A comprehensive evaluation index and an exergy efficiency evaluation plot are developed to discuss the thermo-hydraulic performances of nanofluids from quantity and quality. The thermo-hydraulic performance increases with nanoparticle mass fraction but decreases with paralleled magnetic induction intensity.

Liquid metal is an important heat exchange media in high-power industrial installations. In the fusion reactor, large temperature difference generating in the liquid metal of facing high temperature plasma leads to external natural convection on the first wall. The heat load on the first wall is carried away by the liquid metal which covers on the first wall. The cooling process of unforced liquid metal on large-sized plate is regarded as the external natural convection. A numerical simulation is employed to investigate the external natural convection heat transfer of liquid metal on the vertical plate under the different horizontal magnetic field which is perpendicular to the temperature gradient. The heated plate is placed on one side of the cavity, and two horizontal walls adjacent to it are thermally insulating. The remaining walls are stationary temperature. The results derived from velocity boundary layer and temperature boundary layer analysis are presented as the important investigation subject at different Grashof numbers under the influence of magnetic field. It can be observed that magnetic field will promote or inhibit the liquid metal flow and heat transfer, and the thickness of boundary layer different from internal natural convection, it presents diverse results at different magnetic field intensities and vertical height positions of the heated plate. Under the influence of magnetic field, the thickness of velocity boundary layer increases first and then decreases, while the thickness of thermal boundary layer generally increases. Three modes are found when there is a magnetic field exerting effects on the liquid metal external natural convection. When there is a weak magnetic field and small Grashof number, the magnetic field will enhance natural convection heat transfer, when increasing the Grashof number, magnetic field will inhibit convection heat transfer but will not suppress the flow, the flow changes from three-dimensional to quasi-two-dimensional in a moderate magnetic field. A strong magnetic field restrains convection heat transfer and flow significantly.

In this study, a constitutive relation between fluid power flux and pressure gradient was discussed, and a conservative equation of fluid mechanical energy was introduced to describe convectional phenomena that is essential to convective heat and mass transfer. The multi-field synergy relations among velocity, pressure, temperature and component concentration were revealed according to the synergy equations based on the conservations of thermal energy, mechanical energy, component mass and fluid momentum to analyze physical characteristic of enhancing convective heat and mass transfer. A new experimental correlation of Eu number versus Re and Li numbers was obtained for the first time through theoretical derivation and experiment to test hydrodynamic performance of heat transfer tube. A circular tube inserted by longitudinal swirl flow generator with center-connected rectangle rods was investigated through numerical simulation and PIV experiment to validate the synergy relation in convective heat transfer enhancement.

The effect of radiative heat transfer on the thermocapillary flow of a high-Prandtl-number (high-Pr) liquid is studied. The flow geometry is a long liquid bridge (long LB) suspended between coaxial disks in microgravity (μg). Experimental data taken on the International Space Station are used, and the numerical simulations for the two-phase flow in the chamber containing the LB and the ambient gas (AG) are carried out. The local radiative heat transfer from the LB to its surroundings is incorporated into this two-phase flow simulation. The two-phase flow simulation allows for the evaluation of QHD, QCD and QLB, which are the heat transfer from the hot disk to the LB, from the LB to the cold disk and from the LB free surface to the surroundings, respectively. It is shown that the radiative heat transfer Qr contributes more to QLB than the convective heat transfer Qc. This relationship applies when the temperatures involved in the flow system are near room temperature. The heat balance changes depending on the directions and the magnitudes of QHD, QCD and QLB, and a dimensionless parameter called the “modified heat transfer ratio”, defined as χ = Q LB / max ( Q HD , Q CD ) to distinguish four conditions: a large heat-gain, a moderate heat-gain, a moderate heat-loss and a large heat-loss, which correspond to χ - 1 , - 1 ⩽ χ 0 , 0 ⩽ χ ⩽ 1 and 1 χ , respectively. It is found that a large heat-loss condition and a large heat-gain condition generate a secondary roll in the flow pattern near the cold disk and near the hot disk, respectively. The effect of radiative heat transfer on the flow pattern in the LB is examined in terms of χ r = Q r / max ( Q HD , Q CD ) to conclude that approximately 84% of the heat transfer from the LB to its surroundings is undertaken, under the present LB conditions, by radiative heat transfer in large heat gain and loss conditions where the secondary roll appears.

Convective heat transfer in turbulent flow in porous media is, in general, less understood compared to laminar and creeping flows. It is broadly integrated into industrial applications even though a lack of understanding in microscale. In the present study, we report a direct numerical simulation (DNS) based microscopic analysis of heat and fluid flow in synthetic porous media with an aim to link the macroscopic statistics and qualitative observations with pore-scale signatures of the same. DNS is conducted with two computational codes with a cautious validation on the microscale. The conventional finite-volume solver shows an excellent agreement with high-order spectral/hp element solvers. The synthetic porous media are comprised of square cylinders in a staggered array. Macroscopic observation indicates an increased pressure loss and increased Nusselt number (Nu) as a result of high Reynolds numbers. However, a further examination of the ratio of Stanton number to skin friction coefficient St / C f reveals that increasing Reynolds numbers lead to more pressure loss than improved heat transfer. Microscopic analysis shows that Nu is highest on the impinging surface, whereas lowest on the wake surface. In addition, it tends to be homogeneously distributed on surface area in the high porosity case. In the low porosity case, Nu on the sharp corners, is significantly higher than on other locations. Probability density function (PDF) of streamwise velocity fluctuation u ′ shows Gaussian like distribution within two standard deviation. The high porosity group owns a spike around zero abscissa on PDF of both u ′ and temperature fluctuation T ′ , which is an indication of less chaotic flow motions.

An approximate mathematical model of the heat transfer processes in the laminar and turbulent boundary layers, which occur near the surface of an aircraft moving at the hypersonic speed in the Earth’s atmosphere, is derived. This mathematical model makes it possible to calculate the convective heat transfer on the surface of typical structural elements of modern perspective aircrafts. 2D versions of the calculations of convective heat fluxes for bodies of simple geometric shapes are performed.

Clean Energy Research Group, at the Department of Mechanical and Aeronautical Engineering of the University of Pretoria, and the DST in South Africa.

Multi-droplet impact on a thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. Results show that successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film.

Hypersonic Inflatable Aerodynamic Decelerator (HIAD) has shown its great potential for future planetary explorations. However, the HIAD surface deflections could both promote boundary-layer transition early and augment heating levels sharply, which poses challenges for survivability of a Thermal Protection System (TPS). The goal of this work is to assess different transition models for the prediction of hypersonic transitional flows over scalloping deformed HIAD surface and to seek the critical factor for their capabilities to provide a reference for the application and advancement. Three representative transition models are considered: γ-Reθ, k-ω-γ and kT-kL-ω. The results show that the undulating surface causes flow separations and reattachments in valleys and strong crossflow along the leeward. k-ω-γ model can correctly predict both the shapes and locations of the transition onsets. The transition zone predicted by γ-Reθ model is much small, while kT-kL-ω is incapable of transition prediction for such a complex and irregular configuration. Moreover, this study reveals that the crossflow instability plays a dominant role in the transition. The crossflow Reynolds number Recf, whose distributions are approximately consistent with the transition zone, could be a feasible crossflow strength indicator for the transition onsets on the undulating surface. Once k-ω-γ model excludes effects of crossflow instability, it predicts incorrect transition results for the deformed surface of HIAD. Besides, a detailed analysis shows that both the crossflow instability mode and the first disturbance mode in valleys are the major contributors to the transition on the leeward surface. Near the leeward ray, the transition is mainly determined by the first disturbance mode.

Computational Fluid Dynamics (CFD) analyses are carried out to investigate convective heat transfer of developing laminar flows of water and air in microchannels, for wide ranges of parameters (α = 1–10, Dh = 145–375 µm, L/Dh = 130–750, Q = 1–18 W, Prin = 0.7–11.2, and Rein = 130–900). The convergence of the results validated the numerical approach and the mesh grid refinement used and the accuracy is confirmed by the excellent comparison to reported experimental results. The results account for the azimuthal variations of the wall heat flux and temperature, and the changes of the fluid properties with temperature along the microchannel. The compiled database from the present analyses results are used to correlate the local Nuch(z), which is integrated along the microchannel length to obtain the average Nusselt number, Nu ‾ ch . A continuous correlation of Nu ‾ ch is developed in terms of Nusselt number for fully developed flow Nufd, Rein, α, and Prin, as: Nu ‾ ch = Nu fd [ 1.0 + 0.0363 / L ∗ 0.631 ] , Nu fd = 8.235 - 17.2 / 3.785 + α - 1 1.11 , and L ∗ = L / D h α 0.583 / Re in Pr in 1.2 The values of the thermal development length, ι th , for Nuch(z) are also correlated in terms of Dh, α, Rein, and Prin, as: ι th = 0.028 + 0.088 e - 0.45 α - 1 D h Re in Pr in The correlations of Nuch(z), Nu ‾ ch and ι th are in excellent agreement with the present numerical results and those reported by other investigators for Nuch(z) and Nu ‾ ch . The numerical values of Nu ‾ ch are 10–20% higher than those reported experimentally, assuming uniform wall and fluid temperatures along the microchannel, and neglecting the effects of the flow development and the azimuthal variations in the wall heat flux and temperature.

Based on the inverse heat transfer methodology, the quantitative identification of three-dimensional subsurface defect is studied by using the finite element method and fuzzy inference method. For the quantitative identification problem of subsurface defect, a fuzzy inference system is established. A set of fuzzy inference units are created, through which the decentralized fuzzy inference processes are executed from the deviations between measured and computed temperature to the corresponding inference components. These fuzzy inference components are weighted and synthesized to gain the compensations of the defect parameters. Numerical tests are performed to study the effects of defect size, the initial guess value, the measurement errors, the number of measurement points, the defect shape and the convective heat transfer on the identification results. Comparison with the Levenberg-Marquardt method (L-MM) is also conducted. The results show that the established method in this paper can significantly reduce the dependence of identification results on the number of measurement points and improve the anti-interference ability on the measurement errors, also has better anti-illposedness characteristic and higher computational efficiency.

Some experimental tests were conducted to reveal the enhancement of the ferrofluid heat transfer under a permanent magnetic field. This research aims to investigate the effect of various external magnetic fields on convective heat transfer characteristics of the ferrofluid (magnetic nanofluid). Comparison of theoretical predictions and experimental data were conducted to validate the rationality of the test results, and a good agreement with less than 10% deviations was found. The deviations from experimental data decrease with an increase of the Reynolds number (Re) from 391 to 805. Results from the case with 5 cannulas indicate that a continuous increase in the magnetic flux density (by increasing the quantity of the magnets) can improve the heat transfer enhancement significantly. The ferrofluids with a magnetic cannula shows heat transfer enhancements of 26.5% and 54.5% at Re = 391 and 805, respectively.

The present work investigates and compares the hydraulic and thermal performances of two tree-like networks with constant surface area constraint and constant volume constraint. The results find that the optimal channel diameter ratio to reach a minimum hydraulic resistance satisfies βm = N−2/5 for the tree-like network with surface area constraint, but βm = N−1/3 for the tree-like network with volume constraint. The convective heat transfer in both of the two tree-like networks with different size constraints is independent on the channel diameter and diameter ratio, but increases with the increasing channel length, channel length ratio, branching level and branching number. For the two tree-like networks with exactly the same channel length, length ratio, branching number and branching level but different channel diameter, the present work provides a segmenting model to choose the network with better hydraulic and thermal performances.

The ribbed surface is a widely used convection heat transfer enhancement technology. The geometric parameters of the ribbed surface have great influences on its heat transfer performance. It is crucial to determine the optimum geometric parameters. In the present work, the optimal arrangement of ribs mounted in a rectangular channel is provided by solving the inverse optimization problem with the entransy dissipation as the objective function. The parameter analysis based on the direct solution is firstly carried out to get an approximate range near the optimal pitch ratio which can provide the initial value for the inverse optimization problem; afterwards, the inverse optimization problem is solved by a modified simplified conjugate gradient method, and the accurate optimal pitch ratio is obtained; finally the optimization results are discussed in detail. The results indicate that, (1) the inverse optimization analysis presented in this paper can provide an accurate optimal value of the pitch ratio; (2) taking the entransy dissipation as the objective function could lead to a faster converge process and give a more stable result than taking the temperature data as the objective function; (3) the optimal value under the fixed heat flux boundary condition is slightly larger than that under the fixed temperature boundary condition.

Cooperative effect of the conductive and convective heat fluxes on the development of vortical structures is studied in a particle-dispersed natural convection. A numerical approach is proposed that visualizes the heat transfer paths through the fluid and finite-volume particles (i.e., non-point particles) to show the spatial extension of the heat fluxes and its influence on the flow field. The approach is applied to a weakly-convective dense particulate system under the Rayleigh number 10 5 containing more than 4000 particles (solid volume fraction 42.8%) of various conductivities. With highly-conductive particles (in comparison to the ambient fluid), local downward convection of a large-scale vortical flow strengthens the conductive heat flux near the bottom hot wall in the counter-convective direction by the increase of the vertical temperature gradient, and the observation of the spatial extension of the heat flux lines indicates that the increase of the solid volume fraction near the hot wall further strengthens the conductive heat flux. The paired upward convection causes strengthened conductive heat flux near the top cold wall by the same increase mechanism. The above interplay of the local heat fluxes in the dense solid-dispersed media are found to influence on the formation and transition of the large-scale vortical structures through the distribution of the moment of buoyancy in a horizontal plane. An analytical model for a low-dimensional vortex system shows that buoyancy gradient and vorticity diffusion could determine the time-development of the large-scale vortical structure.

A series of three-dimensional numerical simulations was conducted to investigate the convection inside an electromagnetically levitated silicon droplet under vertical, horizontal and rotational magnetic fields, respectively. The results show that the flow is totally axisymmetric and two counter-circulating vortexes appear in the droplet with an inner-outer distribution under vertical magnetic fields. The inner vortex brings fluids from the top area to bottom area directly, which is potentially responsible for the requirement of quite strong vertical magnetic fields in the experimental measurement of thermal conductivity of molten silicon. With horizontal magnetic fields, the vortexes inside the droplet turn to an up-down distribution, which is more beneficial in the thermal conductivity measurement since it prevents the direct convective heat transfer effectively. However, the horizontal magnetic field fails to suppress the convection effectively and the flow is intense and spatially imbalanced, leading to a less stable flow state. The rotational magnetic field combines the advantages of the formers, suppresses the convection along z-axis apparently and creates an up-down distribution of vortexes simultaneously. It suggests that the rotational magnetic field shows potential interest in the measurement of thermal conductivity of melts. Besides, the rotational magnetic field would evoke forced convection in azimuthal direction, which can be better controlled by varying magnetic intensity and rotating frequency. The azimuthal flow is expected to balance the rotation of melts induced by helicity of coils in experiments and shows potential advantages in melt stirring and solidification from undercooled melts.

Constructal theory has been widely used in performance optimizations of various problems. Among these problems, engineering problem is a hot topic for constructal theory. The developments of constructal theory about engineering problem in China over the past decade are reviewed in this paper. Multi-disciplinary, multi-objective and multi-scale constructal optimizations of various transfer processes in engineering, such as heat conduction and thermal insulation processes, fluid flow processes, convective heat transfer processes of fins, heat sources, cavities, cooling channels, vascular networks and heat exchangers, mass transfer processes of porous mediums, heat and mass transfer processes of solid-gas reactors and solid oxide fuel cells, iron and steel production processes and generalized transfer processes, have been conducted since 2006. Performance improvements are realized, new design requirements are satisfied, and more practical and generalized results are obtained after new research objects, model improvements, optimization objectives, boundary conditions, constraint conditions and transfer extensions are considered. It shows that the developments of constructal theory are still being made in China, which will provide more design guidelines not only for engineering problem, but also for natural and social problems.

In this study, local convection heat transfer technology was investigated using ionic wind. The characteristics of ionic wind using wire and plate electrodes are studied by experiment and numerical method. A particle image velocimetry (PIV) test was conducted in a wind tunnel to observe the flow behavior by the ionic wind on the heated surface. A new boundary layer was formed on the heated surface which was controlled by the ionic wind, regardless of the Reynolds number of bulk flow. The surface averaged heat transfer coefficient with the ionic wind increased by 11% at low Reynolds number (100–200) and decreased by 19% at high Reynolds number (2500–3500). It was also found that the zigzag type of wire arrangement enhanced the heat transfer coefficient by 40% compared with the single wire. It is thus concluded that ionic wind can be used for enhancing the convection heat transfer rate or insulating the local surface according to its operating condition.

The flow fields and convective heat transfer downstream of a backward facing step (BFS) with a Re H = 9630 were investigated using experimental methods. The BFS flow was perturbed using a synthetic jet actuator deployed at the separation point with a range of perturbation frequencies ( f A H / U o = 0.04 - 0.39 ) and amplitudes ( u A ′ / U o = 0.3 - 0.9 ). The momentum transport across the shear layer appeared to be enhanced by the perturbations, resulting in a shortened recirculation bubble, a larger curvature in the time-average streamline over the separation bubble and enhanced heat transfer rate in the attachment and recirculation regions. The distributions of the reattachment length, streamline curvature, static base pressure, turbulent kinetic energy and convective heat transfer rate all suggested there was a critical perturbation frequency f A H / U o ≈ 0.22 where the perturbation frequency matched the characteristic frequency of the attaching shear layer. Flow changed only slightly with the perturbation amplitude when f A H / U o ≳ 0.22 but changed significantly when f A H / U o ≲ 0.22 .