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In the present paper, a fundamental analysis on the effect of the channel aspect ratio on the bubble dynamics and heat transfer characteristics for the early transient stages of the bubble growth within confined microchannels of rectangular cross-section, under saturated flow boiling conditions, is conducted, utilising high resolution, 3D, transient, conjugate heat transfer simulations. The open-source toolbox OpenFOAM is applied for the simulations, utilising a custom, user-enhanced, diabatic Volume OF Fluid (VOF) solver. Two different series of numerical simulations are performed, focused on a single nucleation event from a single nucleation site and a single nucleation event from multiple, arbitrarily located, nucleation sites, respectively. In each series, three different values of channel aspect ratio are considered, corresponding to a narrow, a square, and a wide microchannel. For the first series, the simulations are performed for a low, a medium, and a high value of applied heat flux and mass flux. For the second series, only the lower values of applied heat flux and mass flux are used for each channel aspect ratio, since this constitutes the worst-case scenario from the overall heat transfer performance point of view, amongst the cases examined in the first series of simulations. The micro-passage aspect ratio has a significant effect in the generated bubble dynamics during the onset of the nucleate boiling regime, as the bubbles grow within the confined liquid crossflow. This alteration of the generated interfacial dynamics, in effect, regulates the size and position of the contact areas of the generated bubbles with the microchannel walls, with a direct effect in the individual contribution and therefore, the balance between the contact line and the liquid film evaporation mechanisms. Moreover, the work presents the quantification of the effect of the solid domain thermal inertia on the whole process and in particular on the local Nusselt numbers. It is evident that considering conjugate heat transfer in numerical simulations of flow boiling is compulsory in order to predict the physical processes in a correct form.

Prediction of meltpool features in Laser-Based Powder Bed Fusion (LB-PBF) is a complex non-linear multiple phase dynamic problem. In this investigation, numerical simulations and analytical models are offered to predict meltpool temperature and to provide a methodology to estimate melt track quality. By determining the meltpool temperature, different rheological phenomena including recoil pressure can be controlled. Recoil pressure is known to drive the keyhole and conduction modes in LB-PBF which is an important factor to qualify the melt track. A numerical simulation was carried out using Discrete Element Method (DEM) with a range of process parameters and absorptivity ratios; allowing observation of the variation of meltpool temperature and free surface morphology, as calculated by the volume-of-fluid (VOF) method. A spatially thermophysical-based analytical model is developed to estimate meltpool temperature, based on LB-PBF process parameters and thermophysical properties of the material. These results are compared with experimentally observed meltpool depth for IN718 specimens and found to have a good accuracy. The numerical and analytic results show good agreement in the conduction mode to estimate the meltpool temperature and related phenomena such as recoil pressure to control the melt track and layering quality. The analytical model does not accurately predict the keyhole mode which may be explained by evaporation of chemical elements in the examined material.

In this paper, the improved incompressible smoothed particle hydrodynamics (ISPH) method is used to conduct a numerical simulation for the buoyancy driven flow inside an enclosure including a cross shape that is filled with moving/fixed solid particles. In this study, a Cu-water nanofluid is assumed as a working fluid. The sidewalls of the cavity are cooled, the horizontal cavity walls are adiabatic and the cross shape with the inner solid particles were heated. Different cases are taken into account based on the temperature conditions on the inner solid particles, namely, fixed and cold solid particles, moving and cold solid particles, fixed and hot solid particles and moving and hot solid particles. The controlling parameters in this study are dimensions of the cross shape L cross , depth of the solid particles L solid , the Rayleigh number Ra and the nanoparticles volume fraction ϕ . The main results revealed that the decrease in the cross shape lengths by 0.6 increases values of the stream function by 27.8% and the isotherms lines are enhanced. In addition, the case of the cold and moving solid particles gives the higher rate of the heat transfer while the lowest rate of the heat transfer is observed in case of fixed and cold solid particles.

This paper presents a mathematical deduction of a new improved model for heat transfer during condensation inside tubes. This new model has been developed with the aid of the Gaussian Equation for an infinite straight line, considering this relation with the differential equations that govern the heat transfer process. The proposed model was verified by comparison with available experimental data of 22 different fluids, including various refrigerants, water, and organic substances, which condense inside vertical, inclined and horizontal tubes. The proposed model is valid for an reduced pressure values ranging from 0.0008 to 0.91, values of Reynolds number for single-phase between 68 and 84,827 and for Reynolds number for two-phase between 900 and 59,4373, a range of internal diameters ranging from 2 to 50 mm, vapor quality from 0.01 to 0.99, P r values for single-phase from 1 to 18 and mass flux rates in the ranges of 3 to 850 kg/(m 2 s). The mean deviation found for the analyzed data for horizontal tubes was 11.8%, while for the vertical and inclined tubes data the mean deviation was 13.0%. In all cases, the agreement of the proposed model is good enough to be considered satisfactory for practical design.

A new type of three-dimensional pin–fin plate with elliptical cross-section was proposed. Condensation heat transfer of FC-72 in a narrow rectangular channel with the proposed pin fins was experimentally studied in normal gravity and microgravity. The effects of pin geometry, thermal conductivity, mass flow rate and gravity on condensation heat transfer were investigated. The visualization experiment under microgravity was carried out and the condensate behaviour was observed by high CCD camera. The results showed that all the elliptical pin-fin plates exhibited substantially better performance than the flat plate. The influence of pin geometry and thermal conductivity on condensation heat transfer coefficient was great. The smaller pin size and higher thermal conductivity were favourable for condensation enhancement. The average condensation heat transfer coefficient increased with the increase of mass flow rate. In microgravity, obvious fluctuations and climbs occurred in gas-liquid interface. However, the condensate behaviour on the condensing surface was unconspicuous. For unsteady state, the vapor-side temperature difference increased evidently in microgravity. Microgravity had a certain effect on temperature evolution of the condensing base. Short-term microgravity degraded the condensation heat transfer coefficient in quasi-steady state or unsteady state, but had little effect on pulsating state.

Deep borehole heat exchangers (DBHE) provide an effective solution for ground coupled heat pump (GCHP) systems in cold climate region where heating is dominant. Concerning the analytical heat transfer models, the simplification that borehole wall temperature is constant along the depth of ground heat exchangers (GHE) which the existing quasi-three-dimensional models have assumed in the application of shallow borehole GHE, can no longer be accepted in the application of DBHE due to the geothermal gradient in deep ground. Making this simplification cannot give the real temperature distribution of circulating fluid along the depth of DBHE. Therefore, this paper developed a new analytical model for DBHE with coaxial pipes by successfully addressing the increasing borehole wall temperature using the convolution theorem, so that the widely employed quasi-three-dimensional models for shallow borehole GHE is extended for DBHE with coaxial pipes for the first time. The new analytical model is validated by comparing with an existing numerical model. Using the newly developed analytical model, the trends revealing the relationships between thermal performance of DBHE and various parameters are firstly plotted. Because of the high accuracy and quick calculation, this new analytical model can be used as a benchmark for numerical models. More importantly, the proposed analytical model can be an effective tool for the design and optimization of DBHE, since current numerical models are always calculation-demanding, time-consuming and difficult for engineers and designers to use. Also, the method this paper proposed to address the varying borehole wall temperature can certainly be employed to improve the existing quasi-three-dimensional models for shallow borehole GHE so that they can be applicable in some other cases, for example, GHE installed in layered soils, or affected by underground seepage flow in partial depth.

Experimental and numerical studies on the effect of heat transfer to an open cavity placed in supersonic crossflow are presented. Cavity floor is subjected to the desired amount of heat flux and the consequent effects in the flow dynamics inside the cavity and its neighborhood are systematically assessed. Cavity flowfield has been visualized using high-speed Schlieren imaging and pressure on the cavity floor has been measured using both steady and unsteady pressure transducers. Two-dimensional unsteady compressible turbulent flow field has been numerically simulated using a Harten-Lax-van Leer-Contact (HLLC) scheme based unstructured finite volume method solver. The numerical method has been validated using both experimental data available in the literature as well as the wall pressure measured in the present study. Significant changes in flow dynamics such as the growth of the recirculation region, shearlayer oscillation, shearlayer impingement on the aft wall of the cavity, and frequency of cavity induced oscillations have been observed. Experimental observations are well complemented by the numerical studies and could reveal the physics of alteration in cavity flow dynamics with the heat addition.

This paper addresses the significance of and need for understanding the thermal impulse response function for (1) validating in-situ thermocouple models; and, (2) forming a “parameter free” inverse heat conduction methodology. The former application is the focus of the present study but it will be evident how to implement the findings of this presentation into the latter situation. The experimental component of this study utilizes a new small-sample, high-accuracy, electrical heating test facility for producing a quantifiable and accurate heat flux source. The impulse response kernel at the probe site is extracted and used for verifying the proposed in-depth thermocouple model. A linear, first-order thermocouple model is proposed based on the orientation of the probe and limited test temperature range. The location of the thermocouple and time constant are assumed known from some independent means or experiments. The impulse response function is determined and compared with the kernel of the resulting solution of the heat equation using the first-order model. Both kernels should nearly replicate if the model is not physically deficient. This preliminary investigation demonstrates a new means for (1) reconstructing the impulse response function and (2) validating a thermocouple model. Further, this experimentally generated impulse function can be used for resolving inverse heat conduction problems.

Pressure has a huge impact on the performance of pool boiling heat transfer due to the change in thermofluid properties of water. The specific volume of water and the latent heat of vaporization decrease with increasing system pressure. High pressure pool boiling experiments were conducted for water on a horizontal smoothed copper surface at pressures ranging from 0 psig (0 kPa) to 60 psig (413.7 kPa). The hydrodynamic behavior of bubbles was studied at high pressure using the flow visualization technique. The primary bubble size, detachment frequencies, and nucleation site densities were studied at each pressure. The performance of the pool boiling curve was critically analyzed with different predictive models. To compare the pressure data and validate with the existing model, Stephan and Abdelsalam’s model was used. At low heat flux, no changes in the heat transfer coefficient were noticed. At high flux, a 50%, 75%, 125%, and 175% enhancement in the heat transfer coefficient was found against the atmospheric pressure (0 kPa) for 15 psig (103.4 kPa), 30 psig (206.8 kPa), 45 psig (310.2 kPa), and 60 psig (413.7 kPa) respectively. The critical heat flux value was also measured and compared to the predicted theories. A new mechanistic lift model, similar to the Zuber’s model, was used to explain the dry out mechanisms at high pressure in which pressure delays the surface dry out behavior. The results show that change in thermodynamic properties of water due to pressure not only change the hydrodynamic behavior, but also impacts the formation of the thermal boundary layer. The present study shows that there is a need for a theoretical model to predict the critical heat flux and the heat transfer coefficient at high pressure. At high pressure the contact angle shows a dynamic behavior due to change in the surface tension of liquid. The current research work aims to be a fundamental ground to develop for manufactures/designers of high pressurized boilers.

Understanding the influence of fuel composition and air turbulence on evaporation of bi-component fuel droplet is the aim of the current investigation. Experiments were carried out with a single fuel droplet (a mixture of n-heptane and ethanol) suspended using the cross-wire technique within a box of turbulence facility that is capable of generating near zero-mean homogeneous and isotropic turbulence at its center. The fuel composition (i.e. the volume fraction of ethanol) as well as turbulent intensity within the chamber were varied over a wide range. The images of the evaporating droplet were recorded using back-light illumination. The initial droplet size was about 30–40 times smaller than the integral length scale of air turbulence, while it was about 10 times larger than the Kolmogorov length scale. For each fuel composition, five different turbulent conditions were considered. While the effect of turbulence was found to enhance droplet evaporation rate for all cases, the extent of its influence depends on fuel vapor mass diffusivity in air, and, in addition, on the initial fuel composition of the binary mixtures. The evaporation constant for the first stage of evaporation is sensitive to both air turbulence level and composition, while that for the second stage of evaporation shows a specific trend with fuel composition for all cases of turbulent intensity. Finally, an experimental correlation for normalized evaporation constant is presented.

The influences of blowing ratio, rotational speed, Reynolds number, and injection angle on film cooling were experimentally studied on turbine blade suction side near leading edge. One row of round holes is located at streamwise location of 13.5% on suction side. Three injection angles, 30°, 45°, and 60°, were tested. Investigations were conducted with three rotational speeds, 300 rpm, 400 rpm, and 500 rpm with blowing ratio varying from 0.5 to 3.0. Reynolds numbers corresponding to rotational speeds are 20,300, 27,200, and 33,800, respectively. Steady-state Thermochromic Liquid Crystal thermography was adopted to measure the adiabatic film cooling effectiveness. The pressure distributions and the second flow of the whole fluid domain were numerical simulated. Results showed that the blade mid-span has the best film coverage. Film cooling effectiveness descends with the increasing blowing ratio. Rotation can deflect the film trajectories toward the tip of the blade. The corresponding Reynolds number increases with the increase of rotational speed, which improves the film cooling performance. The coolant ejects with a smaller injection angle and a smaller blowing ratio can produce weaker inward film defections and so a better film coverage.

Ice slurry is attracting increasing attention owing to its high cooling rate, heat transfer capability and energy storage density. This paper provides a state-of-the-art review of ice slurry flow and heat transfer characterization in typical components of cooling systems including pipes, fittings, valves, pumps and heat exchangers by experimental measurements, analytical quantifications and numerical simulations. Generally, the experimental measurements concerned the flow pattern, pressure drop, rheology and heat transfer of ice slurry, and the measurement data were usually used to develop and validate analytical and numerical models. The analytical quantification was commonly performed by the principles of non-Newtonian fluid mechanics, which simplified the multiphase flow into a single-phase flow and can predict the friction factors and heat transfer coefficients. The numerical simulation employed multiphase flow dynamics to establish the governing equations for ice particles and carrier fluids in ice slurry flow and heat transfer with phase change, thus providing predictions on different physical fields of ice slurry flow. It indicated that the experimental measurement, analytical quantification and numerical simulation presented different advantages and disadvantages in describing ice slurry flow and heat transfer. The combination of numerical simulation with the experiment measurement and analytical quantification is promising to improve the performance of each single method in terms of accuracy, efficiency and generality. Moreover, the development of optimization strategies to maximize the performance of cooling system with ice slurry is essential.

Heat transfer performances of the impinging jet arrays issued from the nozzle plates without and with the two types of the interconnected grooves were experimentally studied at jet Reynolds numbers ( Re j ) and nozzle-to-plate distances (S) in the respective ranges of 1500–20,000 and 0.1–8 times of the nozzle diameters ( d j ). Owing to the significant spent flow confinement formulated between the smooth nozzle plate and the impinging surface at S/ d j d j effectively resumed the HTE characteristic in association with the impinging jet array. At large S/ d j , the additive ambient fluid entrainment from the grooves enriched the jet momentum to boost the heat transfer level from that produced by the smooth nozzle plate. Both the Nusselt number level and the heat transfer uniformity for the smooth and grooved nozzle plates were comparatively studied with the Re j and S/ d j effects examined. In conformity with the experimental observations for the isolated and coupled Re j and S/ d j effects, the regression-type analysis was performed to generate a set of empirical correlations that permitted the evaluation of the average Nusselt number ( Nu ) over the central jet region for each of the present impinging jet arrays.

The molten fuel is first discharged into a drain tank as the molten salt reactor is shut down. Some heat transfer tubes need to be placed in the drain tank accounting for the generation of decay heat by the fission products. The tube of special structure has no need to pass through the entire drain tank. It’s a two-layer assembly where a center tube is inserted in a thimble. A single prototype tube without scaling is studied in this paper. The results show that the absorption ratio by the center tube is approximately unchanged. There is a transition zone in the nucleate boiling region before the linear change of superheat and heat flux. The ONB data was used for the validation of the models and an improved correlation was developed.

An analytical investigation for the onset of convection in a vertical saturated porous layer distant from the axis of rotation is presented when vibration is present. A linear stability analysis is used to determine the convection threshold in terms of the critical Rayleigh number for variation within the porous layer and for the offset distance from the axis of rotation. The results indicate that the presence of vibration stabilizes the convection across the range of offset distances. The effect of decreasing the offset distance (from very large to almost zero) serves to destabilize the critical Rayleigh number due to offset distance and to stabilize the Rayleigh number for variations within the porous layer.

Described in this paper is a numerical investigation on the concept for improving thermal performance of internal cooling by placing the convergent and divergent slit ribs. Five different geometrical models are investigated, including the ribs of rectangular slits and trapezoidal slits with different convergent and divergent angles. The effects of slit shape and its convergent or divergent angles on thermal performance of internal cooling are evaluated with the Reynolds numbers ranging from 10,000 to 25,000. Results obtained show that the turbulence intensity in cases with the smallest angle trapezoidal slits are in the highest level, which produces the highest level of heat transfer enhancement and highest level of pressure loss. The thermal performance index, which comprehensively evaluates the thermal performance of internal cooling, shows that the thermal performance in cases with the smallest angle trapezoidal slits are in the highest level due to the increased heat transfer enhancement and limited increase of pressure loss.

An analytical investigation in a vertical fluid saturated porous layer distant from the axis of rotation is presented when vibration occurs in the presence of a stable temperature gradient. A linear stability analysis is used to determine the convection threshold in terms of the critical Rayleigh number for variation within the porous layer and for the offset distance from the axis of rotation. The results indicate that the presence of vibration in a stable system serves to reduce the Rayleigh numbers (both within the porous layer and due to offset from the axis of rotation), thus destabilizing the system when a stabilizing temperature gradient exists.

This paper reviews recent advances in pool and flow boiling of dilute emulsions, including initial theories on the boiling mechanisms. The current consensus is that pool boiling is initiated on nucleation sites on the surface and that increased heat transfer coefficients are a result of mixing produced by vapor bubbles in the thermal boundary layer. Visualization of the boiling process substantiates this conclusion. Current understanding of the influence of various quantities on the convective heat transfer coefficient is enumerated with respect to the underlying thermodynamics and energy transport. Prior research on flow boiling is limited in comparison, and much more needs to be done with respect to the boiling and enhancement mechanism. Various models for boiling of emulsions are discussed, including a RANS model that incorporates droplet contact with the heated surface, collisions between droplets and bubbles, and chain boiling due to collision during the boiling process. Key modeling elements are how interphase transport and droplet interactions affect nucleation rates and droplet behavior within the viscous and thermal boundary layers at and near a heated surface. It is clear now that boiling of a suspended phase with a lower saturation temperature than the continuous phase can produce marked increases in overall heat transfer coefficients. A re-conceptualization of two-phase heat exchange equipment is a future possibility, and various applications are identified. Boiling of dilute emulsions may be an attractive cooling strategy when restrictions on size, weight and pumping power must be addressed, such as in thermal management of avionics systems.

Marangoni instability in well-defined non-spherical sessile droplet of 0.65cSt silicone oil evaporating at constant contact line mode is investigated through a series of experiments. Droplets of typical regular geometries (such as triangular, square, pentagonal and elliptical) and exotic shapes (such as kidney and positive-positive curvature) are tested. Cell patterns of Benard-Marangoni instability are distributed along the azimuthal direction non-uniformly in the droplets. These cells always occur first at the region where the curvature of the three-phase contact line (kcl) is the largest and they do not fill the whole droplet at once. Different from that in spherical droplet, there is no such a steady state in the non-spherical droplets and these BM cells always drift from the lower kcl regions towards the higher ones. The influences of kcl, the substrate temperature and the contact angle on the patterns of the Benard-Marangoni instability are analyzed. The critical contact angles for the onset of Marangoni instability are determined.

The Geyser Boiling Phenomenon (GBP) is commonly present in two-phase closed thermosyphon operation. It is characterized by the violent formation of a vapor bubble at the beginning of each intermittent boiling cycle. Given the nature of this type of boiling, the thermal performance of the thermosyphon is reduced and unwanted vibrations are perceived. In this work, the GBP was experimentally studied. A test bench comprised of a glass thermosyphon with a punctual heater was designed to favor the nucleation of a single bubble in a predetermined location. A high-speed camera allowed the qualitative analysis of bubble formation and sensors installed inside the thermosyphon allowed for the quantitative characterization of the phenomena. The analysis presented is concentrated on the bubble nucleation, growth and burst. The mechanisms that act during its formation are theorized. The reasons for the intensity of the bubble burst are presented, as well as a criterion to differentiate GBP from other kinds of boiling.

The distribution and mixing characteristics of hydrogen gas are important for assessing the hydrogen risk in nuclear power plants (NPPs) during severe accidents. To estimate such characteristics, analyzing the erosion mixing process of a stratified hydrogen layer subject to a turbulent impinging jet is necessary. In this paper, we propose a novel scaling law for the interface displacement of a stratified layer by erosion mixing as the result of a turbulent jet impingement in a long-term transient state. We show that interface displacement has nonlinear and time-dependent relationships with flow conditions such as the competitive force between the turbulent jet and stratified layer, as well as geometric parameters such as the vessel diameter. We extend the mass conservation law for the entrainment of a stratified layer over a long elapsed time by incrementally using a scaling law for the entrainment rate that is valid at a quasi-steady state. As a result, we show that the novel scaling law for the long-term interface displacement depends on the elapsed time, vessel diameter, and competitive force. The results obtained in previously reported containment flows experiments, as well as those obtained in the present study, are in good agreement with the prediction by the proposed scaling law, demonstrating that this law would be useful for predicting the behavior of the interface displacement of a stratified layer by erosion mixing under various conditions.

In this paper, two-phase drag models for a packed bed of uniform-size particles were suggested, and they were applied to the calculation of pressure drop and dryout heat flux. We provided physical basis for the two-phase flow regime model through the analysis of the interfacial friction ( F i ). The suggested model provides flow patterns representing bubbly, slug, and channel flow and considering three criteria including d 2 F i /d α 2 = 0, F i = maximum, and F i = 0. The results obtained from the three criteria were drawn with several observation-based experimental ones to generate the flow regime map (void fraction vs. particle diameter). Through the current flow regime map, we clearly saw the existence of channel flow in a packed bed with particles smaller than around 3.5 mm. Then, mechanistic interfacial friction models were developed on basis of the current two-phase flow map of bubbly flow, slug flow, channel flow and annular flow. The suggested interfacial friction models were validated with top- and bottom-flooding air-water experiments and boiling experiments. We found out that the capability of pressure drop estimation by the current model were significantly improved for a bed with small particles. Finally, a zero-dimensional dryout heat flux (DHF) model was derived using the suggested interfacial friction models, and validated against DHF experimental data for beds with 1-D configuration. The root-mean-square error (RMSE) of the suggested DHF model was 35%, which was the smallest among the RMSEs of the previous DHF models.

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.

The various influence factors of macrosegregation are complex and have been researched widely due to their undesirable effect on continuous casting slab. Based on an Eulerian approach, a multiphase solidification model combining turbulent fluid flow, heat transfer, microstructure evolution, solute transport with back diffusion and shell deformation were developed in this work to investigate the fluid flow and macrosegregation in continuous casting slabs under the effects of shell bulging and mechanical reduction. In this model, five phases of the slab were considered: the liquid phase, inter-dendritic melt phase of equiaxed grains, solid phase of equiaxed grains, inter-dendritic melt phase of columnar dendrites, and solid phase of columnar dendrites. The predicted temperature, shell thickness and solute element distribution were verified by the results of thermal infrared imaging, nail-shooting experiments, macrostructure analysis, and carbon-sulphur analysis. In this model, the asymmetrical bulging between two adjacent supporting rollers was considered, and its effect on the fluid flow and macrosegregation of the slab was investigated. The calculation results show that the positive centreline segregation considering the asymmetrical bulging profiles was more serious than that considering the regular sinusoidal shell profiles. Using this model, the slab macrosegregation was investigated with different reduction mechanisms in the mushy zone; a large reduction applied just before the solidification end could significantly reverse the flow of solute-enriched melt and reduce the macrosegregation. These results were also verified by an industrial application.

In scaling computer performance, closely integrating heterogeneous components in a single system-in-package assembly provides faster signal communication and a tighter footprint – but also generates more heat: state-of-the-art microelectronic devices can produce more than 1 kW cm−2. The next generation high-powered electronics will need two-phase liquid cooling, such as droplet evaporation, which utilizes the latent heat of vaporization to remove excessive heat. Compared to traditional single-phase cooling techniques, two-phase cooling offers both high efficiency and an exceptionally high heat dissipation rate. Although droplet evaporation has been explored for over a hundred years, many fundamental transport mechanisms are still not well understood. For example, most droplet evaporation studies focused only on spherical droplets, which possess a uniform curvature, κ (for a low Bond Number), along the meniscus interface. Evaporation from a non-spherical droplet, due to the change in perimeter-to-area ratio and the meniscus curvature, exhibits very different interfacial mass transport features from a spherical droplet. In particular, a higher perimeter-to-solid-liquid-area ratio will yield a larger fraction of thin film region and therefore a smaller thermal resistance in an evaporating droplet, while a high local curvature will facilitate a stronger local vapor diffusion rate. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on heated porous micropillar structures. We explore the equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes, using the Volume of Fluid (VOF) method, and we use a simplified Schrage model [1] to study the evaporative mass transport at the liquid-vapor interface. The results show that microdroplets evaporating on a triangular substrate possess 12.8% smaller effective film thickness compared to that on a circular substrate, due to a longer length of the contact line. During the evaporative heat transfer process, the triangular-based droplets also exhibit the smallest temperature difference between the droplets’ solid-liquid interface and ambient temperature, which leads to a higher heat transfer coefficient (21% larger than a spherical droplet at a supplied heat flux of 500 W/cm2). When the supplied heat increases to a higher value (e.g., 1000 W/cm2), the shape effect becomes less significant where the diffusion resistance is dominated by the liquid-vapor interfacial temperature instead of the meniscus curvature.

In this work, a multiple-relaxation-time (MRT) lattice Boltzmann (LB) equation model with Beam-Warming (B-W) scheme is proposed to simulate a multi-phase fluid system with Peng-Robinson (P-R) equation of state (EOS). The mathematical model of the multi-phase fluid flow is derived based on the NVT-based framework, where the Helmholtz free energy of P-R EOS is introduced. The nonideal force in the multi-phase flow is directly computed from the free energy in order to obtain a more compact formulation of hydrodynamic equations, which is termed as potential form. The MRT-LB model is developed based on the potential form of hydrodynamic equations, which can eliminate spurious currents effectively. In addition, to capture the tiny nonconvex perturbation from the linear trend of P-R model precisely, the B-W scheme is utilized in the present MRT-LB model, which gives rise to an adjustable Courant-Friedrichs-Lewy (CFL) number. Also, the second order accuracy can be naturally achieved by this scheme without any other requirements and numerical boundary conditions. In the numerical experiments, a realistic hydrocarbon component (isobutane) in three dimensional space is simulated by the proposed MRT-LB model. Numerical results show that the magnitude of spurious currents can be significantly reduced by the present MRT-LB model. In addition, our numerical predictions of surface tension agree well with the experimental data, which verify the effectiveness of the proposed MRT-LB model.

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.

Natural circulation two-phase flow characteristics are studied under stable and rolling motion conditions in a rod bundle channel. The mass quality, flow rate and frictional pressure drop variation results with increasing heat flux are given and the effects of system pressure and inlet subcooling are analyzed. The flow rate decline is observed under high mass quality conditions since the frictional pressure drop is large enough. The flow instability characteristics for different stages with increasing heat flux are given. The main reason for the flow instability characteristics variation is that the dominant factor changes from subcooled boiling to the pressure drop oscillation. The experimental results indicate that increasing the system pressure and the inlet resistance of the test section, including moving the compressible volume to a downstream position, can delay the onset of the flow instability. The time-average flow parameters under rolling motion conditions are mainly influenced by the stable mass quality and the rolling parameters. For high stable mass quality conditions, the time-average two-phase flow parameters decrease a little compared to stable results. With the stable mass quality decreasing, the time-average mass quality gradually increases with the rolling frequency. For further lower stable mass quality conditions, the time-average values are even larger than stable values. For the frictional resistance calculation, a few correlations are evaluated and recommended. Additionally, new correlations are fitted for stable and time-average two-phase frictional pressure drop prediction in high mass quality conditions.

Vortex generation has emerged as a promising passive method for enhancing air-side heat transfer. In the present work, two-dimensional numerical analysis is carried out to examine the thermo-fluid analysis of fin-and-tube compact heat exchangers with sinusoidal wavy and elliptical curved type rectangular winglet vortex generator (RWVG) having varying Reynolds number ranges from 400 to 1000. The wavy and curved VGs are also placed as up and down configurations with respect to flow direction. Thus the effect of the different common flow down (CFD) configurations of wavy-up, wavy-down, curved-up, curved-down and flat rectangular winglet is compared with baseline configuration (non-winglet case) for flow structure, temperature distribution and pressure distribution for fin-and-tube compact heat exchanger with seven inline circular tube arrangement. They are also compared with thermo-hydraulic performance criterions which include Nusselt number (Nu), Pressure drop (ΔP), friction factor (f), London area goodness factor (j/f). Appreciable improvement in heat transfer characteristics is observed with wavy and curved rectangular winglet with a moderate loss in pressure. Compared with the non-winglet baseline and flat RWVG case, the heat transfer performance of the fin-and-tube heat exchanger is significantly improved with wavy-up and down and curved-up and down configurations. Further, it is also noted that up-wavy RWVG showed the best heat transfer improvement compared to any other RWVG configuration considered. However, Wavy-up configuration has the lowest values of j/f than others and curved-down has higher values of j/f than others. Thus, curved-down RWVG found suitable than other cases of RWVG as far as London goodness factor strategy.

The distribution of the energy in the keyhole plasma arc welding process has significant effect on the final weld quality. In this study, a novel fully coupled plasma arc-keyhole-weld pool model is built to study the energy propagation in the keyhole plasma arc welding process. The numerical results show that: the conduction energy flux, electric energy flux and total energy flux from the arc to the top weld pool are larger than these from the arc to the bottom weld pool. Under the influence of the arc pressure and plasma shear stress, two dominant convective eddies exist inside the weld pool, and the molten metal flows backward at the top and bottom surfaces, so the energy is not accumulated on the top surface, but transported to the rear weld pool together with the molten metal. The conduction energy from the arc to the anode is much higher than the thermionic heating. The penetrated keyhole formation can’t increase the energy absorption, and 11.5% of the arc energy are lost by the efflux arc. The calculated energy efficiency for heating the anode in this study is only 60.7%.

The aim of this work is to improve numerical predictions of natural convection heat transfer inside of gas turbines. It is desirable for engineers to use numerical simulations be able to accurately predict the heat transfer inside of a gas turbine casing during engine shutdown. This will help them to understand and alleviate problems such as engine casing distortion. A natural convection test rig was constructed to provide simulation validation data. The rig represented a section of a large gas turbine casing and operated at the same Rayleigh number as a real engine. The casing rig was able to provide a unique dataset that could not be obtained through real engine measurements. Analysis of the casing rig was performed using a large eddy simulation and RANS simulations. It was found that the baseline RANS simulation did not predict the heat transfer accurately enough for engineering use. The simple gradient diffusion hypothesis (SGDH) turbulent heat flux model used in the baseline simulations was replaced by more advanced generalised gradient diffusion hypothesis (GGDH). A modification to the GGDH model, called GGDH+, was developed to account for buoyancy effects on the turbulent heat flux. The GGDH+ model was then able to give heat transfer predictions comparable to the LES results.

This study explores the influence of counter current airflow on evaporative heat transfer performance of falling films on a horizontal elliptical tube. The numerical analysis was based on the volume-of-fluid simulations to catch the dispersal evolution of descending films in conjunction with user defined function to model the evaporation effect at the water surface to the ambiance. The calculated results of heat transfer coefficients were in reasonably good agreement with the measured data from the literature for validation of the computational model. Calculations were performed to elaborate the interaction mechanism of counter current airflows with the formation process and heat transfer of evaporative liquid films on the tubes. In essence, upward air streams can induce the shear stresses at the water-air interface to form thicker wavy films. The counter current airflow has a more pronounced effect on heat transfer enhancement in the lower part of the tube than the upper part. Numerical experiments were also extended to assess the thermal performance of water film evaporators by systematically varying the inlet liquid mass flow rate of 0.093–0.186 kg/m-s and counter current airflow velocity of 0–3 m/s, respectively. The simulations results reveal steeper temperature gradients at the wall to enhance heat transfer outcomes, achieving an averaged heat transfer coefficient of 4.15 kW/m2 K for Γ = 0.149 kg/m-s and Vair = 3 m/s.

The special topology of the X-lattice induces a large-scale primary spiral flow and three kinds of secondary flow skewed towards and away from the endwall regions and behind each ligament. To make full use of secondary flows produced in different regions, dimples, protrusions and pin fins are separately introduced to the endwall of a sandwich panel to further improve the overall cooling performance. Forced air convection in the six sandwich panels is numerically investigated based on a validation of the numerical model against experimental data available in the literature. Results reveal that the case with pin fins installed in the region upstream of the crosses in the X-lattice (Region A) exhibits the best overall heat transfer performance, while the case with dimples installed downstream of the crosses in the X-lattice (Region B) shows the poorest heat transfer. In terms of the pressure drop, the introduction of protrusions or pin fins in Region A slightly decreases the pressure drop within a certain range of Reynolds number, but the other four cases show an increase in pressure drop. Taking the pressure drop and heat transfer into consideration, a proper combination of the X-lattice and the elements can enhance the overall thermal performance of the sandwich panel. The modifications of the fluid flow and local heat transfer characteristics by the added elements are well analyzed to understand the underlying mechanisms.

A conjugate heat transfer simulation of transient turbulent flow in a scaled turbine valve, which usually occurs in the fast start-up processes of coal-fired power plants, was performed against experimental validation. A high-temperature (615 °C) experimental system with a scaled (1:3) turbine valve was set up at Pennsylvania State University. Eighty thermocouples were flush-mounted in streamwise and circumferential directions inside the valve body, and spatio-temporally varying temperature and temperature gradients were acquired as the mainstream temperature and pressure rapidly varied. A simulation using the shear stress transport model showed considerably better agreement with the measured temperature than the standard k - e model and the Realizable k - e model; the numerical errors in valve top, valve chamber, valve seat and valve diffuser were below 1%, 2%, 5% and 8%, respectively. However, the largest errors located in the upper diffuser were confirmed to be associated with alternating oscillations of the annular attachment jet along the diffuser surfaces. Further investigations of transient thermal behaviors demonstrated that the instability of large-scale vortical structures inside the valve diffuser significantly enhanced heat transfer between the valve body and the air flow. In addition, upstream straighteners enabled the formation of separated secondary flow structures inside the diffuser, resulting in non-uniform heat transfer along the valve’s circumferential direction.

The three–phase gas–liquid–solid flow, caused by hydrate decomposition in cuttings is a main concern during drilling through gas–hydrate reservoir. In this study, a transient gas–liquid–solid flow model is developed considering the coupling interactions between hydrate dynamic decomposition, cuttings transport and heat transfer in multiphase flow. Using this model, the transient gas–liquid–solid flow behaviors are investigated. Numerical simulations show that the decomposition rate of hydrate in formation is only 1/140 of that in annular cuttings for a unit depth, therefore, the influences of hydrate decomposition in hydrate layers can be neglected. Hydrate particles undergo three processes from bottom hole to wellhead in annulus: non–decomposition, slow decomposition and rapid decomposition. In annulus where the depth is more than 400 m, hydrates decompose slowly and the decomposed gas hardly expands due to the high pressure. While, if the hydrates and decomposed gas return upwards to the position where the depth less than 400 m, the gas void fraction increases significantly, not only due to the faster decomposition rate of hydrates but also due to the more intense expansion of decomposed gas. After the hydrate particles return upwards to the wellhead, the behaviors of gas–liquid–solid flow tend to be a quasi–stable state. If there is no backpressure device at the wellhead, that is, the wellhead backpressure is 0 MPa, the gas void fraction at the wellhead can reach 0.68, which is enough to cause blowout accident. Increasing wellhead backpressure to 2 MPa through managed pressure devices and lowering the inlet temperature of drilling fluid to 17.5 °C except adjusting drilling fluid density can manage the gas void fraction within 10%.

The thermal management for a lithium ion battery cell plays a pivotal role in enhancing the cell performance and reliability for electric vehicles. In this work, a novel serpentine-channel liquid cooling plate with double inlets and outlets is developed for better managing an undesirable temperature distribution of a cell module. With a simplified model for the cell module, numerical analyses are implemented using the software of FloEFD to study effects of flow directions, flow rates and channel widths of the cooling plate on cell temperature distribution under different operating conditions; Likewise, a ratio of power consumption as a non-dimensional number is defined to analyze the hydraulic performance of the developed cooling plate. Results show that locations of the inlet and the outlet as well as flow directions have great impacts on the cell temperature distribution and the ratio of power consumption of the cooling plate. Increasing fluid flow rate substantially decreases the maximum temperature rise of the cell module while it has little effect on the temperature distribution. Moreover, the channel width of the cooling plate has a strong influence on its ratio of power consumption as well as the cell temperature distribution but it has a weak influence on the cell maximum temperature rise. Interestingly, the developed serpentine-channel cooling plate offers one a new method to design a lithium ion battery thermal management system for controlling temperature distribution of a cell module.

Superheated steam is the main medium in the piping system of steam power plants. In these pipelines, perforated plates are widely used to regulate the flow rate, pressure, and also reduce aerodynamics noise of steam. For the superheated steam flow, the flow resistance and aerodynamic noise are two key parameters of the perforated plates. In order to investigate the flow resistance and steam aerodynamic characteristics, this paper presents a systematic study of superheated steam and water through multi-stage perforated plates under a wide range of operating conditions with both experimental and numerical methods. The experiments are conducted out to validate the applicability of the adopted numerical models. Then numerical methods are used to investigate the water flow resistance and the Mach number distributions of the steam flow with different structural parameters. The results show that with the increase of the holes diameter, the pressure drop decreases and the Mach number increases. Mach number drops significantly when the position of side holes moves towards the edge of the perforated plate. A critical stage distance is found, at which the pressure drop has the maximum value. Besides, the occurrence probability of supersonic steam flow increases as the decrease of stage distance under a restricted space, and the disturbed length of supersonic steam flow is longer when the stage distance is smaller. This work can be referred by someone who deals with perforated plate design in steam piping systems or related research topics.

This paper presents a computational modeling of ten different configurations of microchannel heat sink (MCHS) with single phase liquid flow in order to compare their performance with the conventional MCHS. New configurations with a longitudinal and transverse arrangement of microchannels relative to the base plate, different number of coolant passages, cocurrent and countercurrent coolant supply were studied to obtain maximum thermal performance and uniform temperature distribution. The formation of the developing flow zones in a number of new configurations allowed to achieve higher Nusselt numbers and low thermal resistances at the same pumping power in comparison with conventional MCHS. The proposed methods of coolant supply make it possible to reduce the temperature gradients and improve the temperature uniformity on the base due to better distribution of coolant.

Inspired by Namib desert beetle’s bumpy structures, condensing droplet shedding from various super-hydrophobic bumps with a hydrophilic patch present on their top was numerically studied using the lattice Boltzmann method (LBM). The droplet grows on the top patch due to condensation and then sheds from the bump due to gravity. A parametric study has been conducted. It was revealed that the bump shape, diameter, height, inclination and wettability all affect the shedding volume (volume at which the droplet sheds from the bump) of the droplet. The shedding volume decreases with the bump height till a threshold height, defined as the critical bump height, beyond which the shedding volume does not change any more. With the same patch area, the shedding volume is smaller on the bump with a hemispherical top compared to the bump with a circular cylindrical top. The shedding volume increases with bump diameter, but decreases with the patch contact angle and surface inclination angle. The same trends were also found for the critical bump height. Furthermore, compared to milli-scale bumps, droplet removal followed by shedding on micro-scale bumps was found to be inefficient. Based on the simulation results, a scaling law was obtained by data fitting to estimate the critical bump height for bumps with a cylindrical top, which can be used to guide the design of desert-beetle-inspired bumps for efficient water collection from fog.

The present study initially considered the double wavy (DW) shape as the geometric disturbance to control fluid flow and the heat transfer. The large eddy simulation (LES) was performed to solve the filtered momentum and energy equations at the subcritical Reynolds number of 3000 and the Prandtl number of 0.7. In order to evaluate the effect of DW geometry on the force coefficients and the heat transfer, the smooth (CY), and symmetric wavy (SW) and asymmetric wavy (ASW) cylinders are considered for the purpose of the comparison. The DW cylinder achieved the smallest mean drag and the largest suppression of the lift fluctuation among the different geometric disturbances. The Nusselt number of the DW cylinder has the smallest mean and fluctuation values. The spanwise local Nusselt number is strongly correlated to the geometric disturbance, which is consistent with the SW and ASW cylinders. The DW cylinder provided smaller spanwise local Nusselt number over the span than the SW and ASW cylinders. The DW cylinder attenuates the heat transfer in comparison with the circular cylinder. The wake modification by the DW cylinder associates to the attenuation of the heat transfer.

In this paper we analyse the jet of non-Newtonian power law fluids emerging from a tube with a diameter D and impinging on a flat plate, which is receiving from the other side a constant heat flux. To that end, different numerical simulations have been carried out with the one dimensional fully developed axisymmetric velocity profile used as boundary condition to model the jet. The aim of the work is to quantify the heat (through the Nusselt number) and momentum (through the friction coefficient on the plate) transfer processes between the jet and the plate as a function of the Reynolds number ( Re = 50 , 100 , 200 ), the tube-to-plate distance ( H / D = 1 , 2 , 4 ) and the power law index ( n = 0.8 , 0.9 , 1.0 , 1.1 , 1.2 ). The values given to the power law index allowed to explore both the shear thickening and the shear thinning behaviours. Results showed that a larger tube-to-plate separation decreases both the transference of heat and momentum; additionally, increasing the Reynolds number improves the heat transfer in detriment of momentum transfer; and, similarly to this latter input parameter, lowering the power law index, increases the heat transfer and worsens the momentum transfer. These results are summarised in three mathematical correlations consisting of potential functions of the input parameters, i.e. Re , H / D and n. However, as it is usually done, the Prandtl number Pr will be used instead of n.

A thorough understanding of the internal flow dynamics inside a poppet valve makes sense for an improved control performance. Hence, a numerical simulation for the cavitating jet is performed with main concern on the cavitation-vortex interaction. The periodic cavitation structure, along with corresponding variation in vorticity distribution and large scale eddy, is presented for the quasi-steady flow from numerical results. As expected, cavitation inception exhibits great dependence on coherent structure, which is organized as paired vortex rings. Besides, cavitation is persistently located at core of vortex, indicating strong correlation between cavitation and vortex dynamics. The cavitation-vortex correlation is further discussed with analysis of vorticity transport equation, involving vortex stretching, dilatation and baroclinic torque terms. At incepting stage, vortex stretching is the dominant factor, responsible for vortex growth and the elliptical shape of cavitation ring. In comparison, the dilatation term could produce either enhancement or suppression in local vorticity, depending on the volumetric variation induced by cavitation. During the collapse stage, the bubble collapse creates baroclinic vorticity, and contributes to 3-dimensional vorticity. The most significant finding of the present study is the suppression of vorticity during the growing process of cavitation, contributing to an enhanced understanding of the cavitation-vortex interaction.

This paper numerically investigates the effects of the film cooling hole diameter on the flow and heat transfer characteristics of the impingement/effusion cooling and double swirl/effusion cooling. Impingement jet arrays at three jet Reynolds numbers, 10,000, 15,000, 20,000, are employed. The target channel consists of a semicircular channel in impingement/effusion cooling and two partially overlapping cylinders in double swirl/effusion cooling. Three arrays of film cooling hole rows are established on the target surface under two arrangements. Four film cooling hole diameters, 0.4, 0.6, 0.8 and 1.0 times the jet hole diameter, are considered. The flow structure and flow development inside the target channel are compared and analysed. The heat transfer performance are discussed and compared. Results show that the effusion air distribution and Nusselt number distribution is more uniform in double swirl/effusion cooling. With the application of the double swirl channel, about 20–33% increase in overall averaged Nusselt number of the whole target channel and about 12–20% increase in spatially averaged Nusselt number at the effective cooling region are obtained. With the application of the film cooling holes, the maximum increase in spatially averaged Nusselt number at the effective cooling region is 10.3% in impingement/effusion cooling and 4.7% in double swirl/effusion cooling.

To explore the inlet air cooling effect in a precooled turbine-based combined cycle engine, a numerical investigation for air/droplet fully coupled cooling is performed in the cooling section with a spray system. The accuracy of numerical simulation program for mass injection cooling methodology is qualified with the experimental data. Results indicate that an even spray arrangement is beneficial to the uniform distribution of flow field, and it is also the major factor resulting in the flow loss. Although the flow loss of dry air is irreversible in the flow process, the additional new energy of water vapor can contribute total energy of wet air mixture. In contrast, under the lowest requirement qualified of mass injection cooling at two high-altitude simulation environments, the total-pressure drop coefficients are 3.22–4.22% before mist injection and 2.87–3.86% after mist injection; the total-temperature drop coefficient is 10.09% and 16.29%, respectively, for dry air and wet air conditions. After optimization for spray apparatus, the flow loss can decrease by about 55.84% before mist injection and about 64.53% after mist injection. Additionally, a little influence is for flow field temperature. Therefore, water injection cooling can effectively improve the flow field characteristics due to a larger total-temperature drop and a smaller total-pressure loss in the cooling section.

This paper presents a novel numerical framework based on the accurate spectral element method (SEM) for electro-thermo-hydrodynamics (ETHD), in which the coupled electric, flow, and heat fields are solved simultaneously. The electric field potential is estimated by the classic Galerkin variation with the electric field vectors evaluated by the spectral approximation, and the streamline upwind Petrov-Galerkin variation and consistent approximate upwind technique are applied to stabilize the numerical simulation of charge density transport. As for the flow and heat transfer, the improved time-splitting method developed in our previous work is applied to deal with the coupled pressure and velocity in the momentum equations. Three carefully selected cases of analytical solution, pure thermal and pure electric convection, and electro-thermo-convection are performed to validate the algorithm. Then, the electro-thermo-convection in an enclosure with uniform temperature distribution is simulated and the average Nusselt numbers at different electric Rayleigh numbers are presented accurately. Additionally, the sinusoidal temperature boundary conditions are considered that the contours for stream function, temperature, and charge density are analyzed in detail to depict the heat transfer enhancement with ETHD effects.

The oscillatory instability of a 3D natural convection flow in a cold cubic enclosure containing a tandem of cold and hot vertically aligned cylinders is investigated in detail as a function of the distance between the cylinders. The study is performed by applying time integration of slightly supercritical flows. The flows undergo a transition to unsteadiness via either reflectional symmetry breaking or reflectional symmetry preserving Hopf bifurcation as a function of the distance between the cylinders. An extensive discussion of the observed instability scenarios is presented, and the different instability mechanisms are characterized in terms of the values of the main oscillating harmonics and the spatial distribution of the oscillating amplitudes of all the fields obtained for slightly supercritical flows.

Using the Eulerian-Eulerian two-fluid model (TFM) the hydrodynamics and heat transfer around different immersed bodies in a fluidized bed were analyzed. The kinetic theory of granular flow (KTGF) was used to simulate the solid phase. For hydrodynamic simulations, 3D, 2D Cartesian and 2D axisymmetric frameworks were examined and the differences in the results of these simulations were discussed. For the heat transfer analysis, due to the high computational demand of 3D simulations, the analyses were performed using 2D Cartesian and axisymmetric frameworks. For studying the effect of shape on the CFD results, two cases of spherical and cylindrical immersed bodies were simulated. In addition, two methods for calculating the surface-to-bed heat transfer coefficient (HTC) for 2D Cartesian and axisymmetric models were examined. The first (method I) is based on constant heat flux boundary condition, while the second one (method II) is based on the isothermal wall boundary condition. It was found that method I outperforms the second one for both 2D Cartesian and axisymmetric configurations in prediction of average HTC. It was shown that the simulation results were in closed agreement with the corresponding measured data. The findings revealed that the spherical immersed body, which has a better aerodynamic shape, produced higher HTC in bubbling fluidized bed with Geldart B particles. Finally, the impact of Archimedes number (Ar) on the surface-to-bed HTC was studied.

Effective thermal diffusivity and conductivity are two crucial parameters to determine inherent safety in porous pebble bed of High Temperature Gas-cooled Reactor. A full-radius-scale heat test facility has been developed to measure these thermal properties at high temperature. Two vacuum tests and two atmospheric-pressure helium tests up to 1200 °C have been conducted to determine the temperature-dependent parameters by the inverse method. Repeatable results are validated by both tests with different heating processes. Furthermore, the primary experimental errors from installation positions of thermocouples as well as random packing effect are reduced by retrieving positions in the inverse method. Sensitivity and uncertainty analyses are carried out in respective tests to give the reasonable descriptions of the results of effective thermal diffusivity and conductivity in these tests.

Buoyant flows are of importance in mass and energy transformation in mountain areas, and significantly influence on local climate. The mountain is modeled by a vertical cone and three dimensional numerical simulation is carried out for Rayleigh numbers of Ra = 7.14 × 106 –2.34 × 107 in which the vertical cone is heated to the specified temperature in order to simulate buoyant flows on the mountain heated by solar radiation. Based on the comparison between numerical and experimental results, buoyant flows on the vertical cone are characterized and the mechanisms are analyzed. Further, the thickness of the thermal boundary layer, the velocity of the plume front, the vorticity, the volume and the onset of the break of the plume cap, and heat and mass transfer are measured; a set of scaling laws are obtained and verified by numerical and experimental results.

Understanding multicomponent drop breakup during impact with walls is critical to the prediction and optimization of fuel-air mixture distribution in combustion systems. In combustors, drops impact walls over a range of wall temperatures and drop velocities, resulting in complex outcomes. In this paper, the regimes of drop impact are characterized for bicomponent and multicomponent fuel drops impacting hot walls. Mixtures of n-heptane and n-decane were used to represent low and high boiling point fuel components, respectively. The wall surface temperatures were varied from 27 to 400 °C with drop Weber numbers ranging up to ∼ 700 for a range of mixing ratios of n-heptane and n-decane. The drop impact events were recorded using high-speed imaging, allowing the identification of impact outcomes and classification into regimes of film deposition, nucleate boiling, and rebound at low Weber numbers, and splashing and breakup at high Weber numbers. In bicomponent mixtures of n-heptane and n-decane, increasing the fraction of the volatile component (n-heptane) results in secondary film breakup at low wall temperatures and a shift in the classified regimes of impact. The droplet size distribution following this secondary breakup was determined, and results show a decrease in the mean droplet size with increasing volatile concentration. Finally, commercial gasoline and diesel fuel drops were examined over similar wall temperature and Weber number ranges. The impact regimes are comparable to those identified for the well-defined bicomponent liquid mixture, in which more volatile components promote rebound and disintegration of the liquid.