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research-article
J. Heat Transfer   doi: 10.1115/1.4041088
Heterogeneous nucleate boiling over a flat surface has been studied through complete numerical simulations. During the growth and departure of the vapour bubble, the interface is tracked following a coupled level-set and volume of fluid (CLSVOF) approach. A microlayer evaporation model due to Sato and Niceno ["A depletable micro-layer model for nucleate pool boiling," Journal of Computational physics 300, 20-52 (2015)] has been deployed in this investigation. A detailed study of the changes in microlayer structure as a result of different boiling scenario has been performed. The departure diameter is found to increase with increase in substrate superheat. The predicted departure diameter has been compared with the available experimental and analytical results. A power-law curve has been obtained for the growth rate of bubble depending on the degree of superheat at the wall. The space-time averaged wall-heat flux at different values of superheat temperature of substrate is obtained. Bubble growth during subcooled boiling at low and intermediate subcooled degrees has been observed through simulations. The variations in bubble dynamics after departure in saturated and subcooled liquid states have been compared.
TOPICS: Bubbles, Nucleate boiling, Subcooling, Boiling, Engineering simulation, Evaporation, Nucleate pool boiling, Dynamics (Mechanics), Heat, Temperature, Fluids, Computer simulation, Spacetime, Simulation, Computational physics
research-article
J. Heat Transfer   doi: 10.1115/1.4041048
It is a common practice to use ideal thermal boundary conditions to investigate natural convection. They correspond to a very good conducting wall and to a very bad conducting wall. In particular, this has been the case in natural convection of viscoelastic fluids. In this paper these conditions are generalized by taking into account the finite thermal conductivity and thickness of the wall in natural convection of a viscoelastic Jeffreys fluid heated from below. The goal is to present more realistic results related with experimental conditions. The critical Rayleigh number $R_c$, frequency of oscillation $\omega_c$ and wavenumber $k_c$, have been plotted varying the properties of the wall from the case of walls of very good thermal conductivity to very poor conducting walls. In order to understand the convective phenomena, two parameters are fixed and the other one will be varied among the non dimensional relaxation time $F$, the relative retardation time $E$ and the Prandtl number $Pr$ of the viscoelastic fluid. The role of the relative retardation time $E$ on the thermal instability is discussed in detail.
TOPICS: Thermal conductivity, Natural convection, Fluids, Viscoelastic fluids, Oscillations, Relaxation (Physics), Rayleigh number, Boundary-value problems, Prandtl number
research-article
J. Heat Transfer   doi: 10.1115/1.4041047
This paper presents a numerical investigation of the film-cooling performance of a kind of diffusion hole with a fusiform cross-section. Relative to the rectangular diffusion hole, the up- and/or downstream wall of the fusiform diffusion hole is outer convex. Under the same metering section area, six fusiform diffusion holes were divided into two groups with cross-sectional widths of W=1.7D and W=2.0D, respectively. Three fusiform cross-section shapes in each group included only downstream wall outer convex, only upstream wall outer convex, or a combination of both. Simulations were performed in a flat plate model using a 3D steady CFD method under an engine-representative condition. The simulation results showed that the fusiform diffusion hole with only an outer convex upstream wall migrates the coolant laterally toward the hole centerline, and then forms or enhances a tri-peak effectiveness pattern. Conversely, the fusiform diffusion hole with an outer convex downstream wall intensely expands the coolant to the hole two sides, and results in a bi-peak effectiveness pattern, regardless of the upstream wall shape. Compared with the rectangular diffusion holes, the fusiform diffusion holes with only an upstream wall outer convex significantly increase the overall effectiveness at high blowing ratios. The increased magnitude is approximately 20% for the hole of W=1.7D at M=2.5. Besides, the fusiform diffusion holes with an outer convex upstream wall increase the discharge coefficient about 5%, within the moderate to high blowing ratio range.
TOPICS: Diffusion (Physics), Film cooling, Shapes, Coolants, Computational fluid dynamics, Engineering simulation, Discharge coefficient, Flat plates, Simulation results, Engines, Simulation
research-article
J. Heat Transfer   doi: 10.1115/1.4041049
Micro-rib is a very promising heat transfer enhancement method for the design of scramjet regenerative cooling channels. In this paper, a three dimensional numerical model has been built and validated to parametrically investigate the thermal behavior of transcritical n-Decane in mini cooling channels with micro-ribs under near critical pressure. The results have shown that the height and pitch of micro-rib perform a non-monotonic effect on the convective heat transfer coefficient of n-Decane inside the cooling channel and the optimal micro-rib parameters stay at relatively low values due to dramatic change of coolant thermophysical properties in the near critical region. Due to the severe thermal stratification and near critical conditions, there will be a significant recirculation zone in vertical direction near the micro-rib, and its interaction with the strong secondary flows in axial direction caused by limited channel width of mini-channel will largely enhance the local convective heat transfer and its downstream region. Besides, the dramatically changing thermophysical properties of n-Decane will lead to a locally remarkable heat transfer enhancement phenomenon similar to impingement cooling at the front edge of micro-ribs.
TOPICS: Pressure, Cooling, Convection, Heat transfer, Design, Impingement cooling, Thermal stratification, Scramjets, Computer simulation, Coolants, Flow (Dynamics)
research-article
J. Heat Transfer   doi: 10.1115/1.4040784
Heat transfer properties of two expanded polystyrene (EPS) samples of similar density, one without (white) and one with graphite opacifier particles (gray), are compared. Tomographic scans are used to obtain cell-sizes of the foams. Using established models for closed-cell polymer foams, the extinction coefficient and effective thermal conductivity are obtained. The effect of opacifiers is modeled using: (1) an effective refractive index for the polystyrene walls within a cell model for the EPS and (2) a superposition of extinction due to a particle cloud upon extinction predicted by the cell model, where particles are modeled as oblate spheroids, or equivalent volume, surface, or hydraulic diameter spheres. Modeled effective conductivities are compared with measurements done on a guarded hot-plate apparatus at sample mean temperatures in the range 0 °C to 40 °C. Typically, cells of the gray EPS are about 40% larger than those of the white EPS and the cell walls in the gray EPS are thicker. The refractive-index mixing model and the model with graphite opacifier particles as oblate-spheroids overpredict extinction, however, the mean error in the effective conductivity predicted by the oblate-spheroids model is only 2.7%. Equivalent volume/surface sphere models underpredict extinction, but still yield a low mean error in effective conductivity of around 4%. While the oblate spheroids model has a lower mean error, the computationally less expensive equivalent volume or equivalent surface models can also be recommended to model the inclusions.
TOPICS: Heat transfer, Radiation (Physics), Heat conduction, Modeling, Particulate matter, Errors, Thermal conductivity, Graphite, Refractive index, Foams (Chemistry), Density, Temperature, Polymer foams
research-article
J. Heat Transfer   doi: 10.1115/1.4040954
Natural convection heat transfer over horizontal solid cylinders has been studied numerically by varying the Rayleigh number in the range of (10^4<=Ra<=10^8) and (10^10<=Ra<=10^13) for both laminar and turbulent flows, respectively. The computations were carried out for three different geometries of 3, 6 and 10 cylinders in a stack arranged in a triangular manner having same characteristic length scale. The present numerical investigation on natural convention is able to capture a very interesting flow pattern and temperature field over the stack of horizontal cylinders which has never been reported in the literature so far. Visualization of plume structure over the horizontal cylinders has also been obtained pictorially in the present work. From the numerical results, it has been observed that the total heat transfer is marginally higher for 3 cylinders stack in the laminar range. In contrast, for turbulent flow, starting from Ra=10^10, heat transfer for 6 cylinders case is higher but when Ra exceeds 5×10^11 heat transfer for 10 cylinders stack is marginally higher. Whereas, average surface Nusselt number is higher for 3 cylinders stack compared to 6 and 10 cylinders cases for all range of Ra in both laminar and turbulent regime. A correlation for average Nusselt number has also been developed as a function of Rayleigh number which may be useful for researchers and industrial purposes.
TOPICS: Heat transfer, Natural convection, Cylinders, Turbulence, Rayleigh number, Visualization, Computation, Plumes (Fluid dynamics), Flow (Dynamics), Temperature
research-article
J. Heat Transfer   doi: 10.1115/1.4040955
A fuzzy inference method for estimating the transient heat flux distribution at the metal-mold interface in slab continuous casting is established in this paper. For the temporal and spatial distribution characteristic of the internal surface heat flux of continuous casting mold, a decentralized fuzzy inference (DFI) inverse algorithm with a temporal-spatial decoupling characteristic is proposed. For each temperature measurement point a set of decentralized fuzzy inference units are established. The fuzzy inference process is performed from the corresponding temperature measurement sequence. The decentralized inference results in the time domain are weighed and synthesized by dynamic response coefficients to obtain the time adjustment vector of the internal surface heat flux of mold. In the space domain, the time adjustment vectors are weighed and synthesized by the normal distribution function to obtain the space adjustment vector of the internal surface heat flux of mold. Numerical experiments are performed to study the effects of the number of measurement points and measurement errors on the inversion results, and they show the validity of the inverse method established in this paper.
TOPICS: Slabs, Casting, Heat flux, Temperature measurement, Metals, Transients (Dynamics), Algorithms, Dynamic response, Errors, Gaussian distribution
research-article
J. Heat Transfer   doi: 10.1115/1.4040956
An experimental study is performed to investigate water flow and heat transfer characteristics in silicon micro-pin-fin heat sinks with various fin configurations and a conventional microchannel. The heat sinks have different fin arrangements, fin shapes and fin pitches. The results show that the heat sinks have the better overall thermal-hydraulic performance including the heat transfer enhancement and the pressure drop penalty compared to the conventional microchannel. The effects of various fin configurations on the flow and heat transfer characteristics are studied. The linear relationship between fRe and Re is found for water flow through the heat sinks for the first time. A new friction factor correlation is further developed based on the linear relationship. Considering the effects of the various fin configurations on the Nusselt number, a new Nusselt number correlation is developed. The new correlations of friction factor and Nusselt number predict the experimental data well. An infrared thermo-imaging system was used to measure the temperature field of water in the micro-pin-fin heat sinks and the conventional microchannel. The infrared thermo-images show the more uniform temperature profile in the transverse direction for the micro-pin-fin heat sinks than that for the conventional microchannel, which indicates the better heat transfer performance of the heat sinks. The dominant mechanism of heat transfer enhancement caused by the micro-pin-fins is the hydrodynamic effects, including fluid disturbance as well as the breakage and re-initialization of the thermal boundary layer in flow direction.
TOPICS: Flow (Dynamics), Heat transfer, Heat sinks, Silicon, Water, Microchannels, Friction, Temperature, Fluids, Fins, Temperature profiles, Pressure drop, Shapes, Thermal boundary layers, Imaging
research-article
J. Heat Transfer   doi: 10.1115/1.4040957
The turbulent flow heat transfer and friction penalty in triangular cross-sectional duct is studied in the present article. The sharp corners of the duct are modified by converting it into circular shape. Five different models are designed and fabrication. The heat transfer through all the models is investigated and compared with the Model-1 (i.e. conventional triangular duct) under similar conditions. The curvature radius of rounded corners for different models is kept constant i.e. 0.33 times the duct height. The numerical simulations also performed and the obtained result is validated with the experimental findings and closed match seen between them. The velocity and temperature distribution is analyzed at particular location in the different models. Because of rounded corners, the velocity with higher magnitude is observed inside the duct (except corners) compared to conventional duct. Considerable increase in Nusselt number is seen in Model-5, Model-4, Model-3, and Model-2 by 191%, 41%, 19%, and 8% in comparison to Model-1, respectively, at higher Reynolds number (i.e. 17500). But, frictional penalty through the Model-5, Model-4, Model-3, and Model-2 is increased by 287%, 54%, 18%, and 12% , respectively, in comparison to Model-1 at lower Reynolds number (i.e. 3600).
TOPICS: Heat transfer, Corners (Structural elements), Ducts, Fluid dynamics, Reynolds number, Friction, Shapes, Temperature distribution, Turbulence, Computer simulation, Manufacturing
research-article
Andrey Gusarov
J. Heat Transfer   doi: 10.1115/1.4040958
The statistical multiphase approach proposed in the first part of this work to evaluate radiative properties of composite materials is applied to porous structures of opaque material and biological tissues. Radiative thermal conductivity is calculated for the bundle of circular rods, packed pebble beds, and metal foams. The results generally agree with the reference calculations by other methods. The small difference can be explained by different approaches to scattering and assumptions about the temperature distribution. Attenuation of light in skin tissues is calculated by the diffusion approximation. The attenuation coefficient generally agrees with the reference Monte Carlo simulation. The difference observed at certain combination of parameters can be due to the assumption of regular arrangement of vessels at the Monte Carlo simulation.
TOPICS: Diffusion (Physics), Radiative heat transfer, Scattering (Physics), Composite materials, Simulation, Radiation scattering, Electromagnetic scattering, Thermal conductivity, Biological tissues, Approximation, Metal foams, Rods, Skin, Temperature distribution, Thin wall structures, Vessels
Technical Brief
J. Heat Transfer   doi: 10.1115/1.4040953
X-ray sources are used for both analytical and inspection applications. In X-ray photoelectron spectroscopy (XPS), aluminium Ka X-rays are generated through electron beam irradiation of a copper-based X-ray anode incorporating a surface film of aluminium. The maximum operating power of the X-ray anode is limited by the relatively low melting point of the aluminium. Hence, optimisation of the materials and design of the X-ray anode to transfer heat away from the aluminium thin film is key to maximising performance. Finite element analysis has been employed to model the heat transfer of a water-cooled copper-based X-ray anode with and without the use of a CVD (chemical vapour deposited) diamond heat spreader. The modelling approach was to construct a representative baseline model, then to vary different parameters systematically, solving for a steady state thermal condition, and observing the maximum temperature attained. The model indicates that a CVD diamond heat spreader (with isotropic thermal properties) brazed into the copper body reduces the maximum temperature in the 4 µm aluminium layer from 613 °C to 301 °C. Introducing realistic anisotropy in the TC (thermal conductivity) of the CVD diamond has no significant effect on heat transfer if the aluminium film is on the CVD diamond growth face (with the highest TC). However, if the aluminium layer is on the CVD diamond nucleation face (with the lowest TC), the maximum temperature is 575 °C, higher by 274 °C. Implications for anode design are discussed.
TOPICS: Heat transfer, X-rays, Aluminum, Anodes, Synthetic diamonds, Modeling, Flat heat pipes, Temperature, Copper, Thin films, Design, Finite element analysis, Optimization, Diamonds, Melting point, Steady state, Water, Heat, Inspection, Cathode ray oscilloscopes, Irradiation (Radiation exposure), Chemical vapor deposition, Electron beams, Photoelectron spectroscopy, Anisotropy, Nucleation (Physics), Thermal properties, Thermal conductivity
research-article
J. Heat Transfer   doi: 10.1115/1.4040783
This paper quantifies the pool boiling performance of R134a, R1234yf, R513A, and R450A on a flattened, horizontal reentrant cavity surface. The study showed that the boiling performance of R134a on the Turbo-ESP exceeded that of the replacement refrigerants for heat fluxes greater than 20 kWm-2. On average, the heat flux for R1234yf and R513A was 16 % and 19 % less than that for R134a, respectively, for R134a heat fluxes between 20 kWm-2 and 110 kWm-2. The heat flux for R450A was on average 57 % less than that of R134a for heat fluxes between 30 kWm-2 and 110 kWm-2. A model was developed to predict both single-component and multi-component pool boiling of the test refrigerants on the Turbo-ESP surface. The model accounts for viscosity effects on bubble population and uses the Fritz (1935) equation to account for increased vapor production with increasing superheat. Both loss of available superheat and mass transfer resistance effects were modeled for the refrigerant mixtures. For most heat fluxes, the model predicted the measured superheat to within ± 0.31 K.
TOPICS: Cavities, Pool boiling, Heat, Flux (Metallurgy), Refrigerants, Heat flux, Turbochargers, Bubbles, Boiling, Mass transfer, Vapors, Viscosity
research-article
J. Heat Transfer   doi: 10.1115/1.4040415
The present research aims to examine the micropolar nanofluid of Casson fluid between two parallel plates in a rotating system with effects of thermal radiation. The effects of Hall current on the micropolar nanofluids are have been taken into account. The flow of micropolar nanofluid has been assumed in steady state. The rudimentary governing equations have been changed to a set of differential nonlinear and coupled equations using suitable similarity variables. An optimal approach has been used to acquire the solution of the modelled problems. The convergence of the method has been shown numerically. The impact of the Skin friction on velocity profile, Nusslet number on temperature profile and Sherwood number on concentration profile have been studied. The influences of the Hall currents, rotation, Brownian motion and thermophoresis analysis of micropolar nanofluid have been mainly focused in this work. Moreover, for comprehension the physical presentation of the embedded parameters have been plotted and deliberated graphically.
TOPICS: Flow (Dynamics), Heat, Mass transfer, Fluids, Plates (structures), Nanofluids, Steady state, Temperature profiles, Rotation, Brownian motion, Skin friction (Fluid dynamics), Thermal radiation, Hall effect
research-article
J. Heat Transfer   doi: 10.1115/1.4040611
Nanostructured semiconducting materials are promising candidates for thermoelectrics due to their potential to suppress phonon transport while preserving electrical properties. Modeling phonon-boundary scattering in complex geometries is crucial for predicting materials with high conversion efficiency. However, the simultaneous presence of ballistic and diffusive phonons challenge the development of models that are both accurate and computationally tractable. Using the recently developed first-principles Boltzmann transport equation (BTE) approach, we investigate diffusive phonons in nanomaterials with wide mean-free-path (MFP) distributions. First, we derive the short MFP limit of the suppression function, showing that it does not necessarily recover the value predicted by standard diffusive transport, challenging previous assumptions. Second, we identify a Robin type boundary condition describing diffuse surfaces within Fourier's law, extending the validity of diffusive heat transport in terms of Knudsen numbers. Finally, we use this result to develop a hybrid Fourier/BTE approach to model realistic materials, obtaining good agreement with experiments. These results provide insight on thermal transport in materials that are within experimental reach and open opportunities for large-scale screening of nanostructured thermoelectric materials.
TOPICS: Phonons, Nanostructures , Boundary-value problems, Model development, Heat, Scattering (Physics), Semiconductors (Materials), Radiation scattering, Electromagnetic scattering, Electrical properties, Mean free path, Modeling, Nanomaterials
research-article
Junde Li
J. Heat Transfer   doi: 10.1115/1.4040647
A partial differential-integral equation has been derived to connect vapor condensations and the development of condensation film in both the tangential and axial directions in a horizontal circular condenser tube. A high order explicit numerical scheme is used to solve the strongly nonlinear equation, and a simple strategy is applied to avoid possible large errors from high order numerical differentiation when the condensate becomes stratified. A set of correlations covering both laminar and turbulent film condensations have been selected to connect the film thickness with the rate of condensation, and thus allow for the predictions of local heat transfer coefficients. The predicted heat-transfer coefficients of film condensation for refrigerant R134a and water vapor in horizontal circular mini- and macro-tubes, respectively, have both been compared with experimental results and results from the simulations of film condensations using computational fluid dynamics, and very good agreement has been found. Some of the predicted film condensations are well into the strong stratification regime, and the results show that, in general, the condensate is close to annular near the inlet of the condenser tube and becomes gradually stratified as the condensate travels further away from the inlet for all the simulated conditions. The results also show that condensate in the mini-tubes becomes stratified much earlier than that in the macro-tubes.
TOPICS: Condensation, Water vapor, Heat transfer, Vapors, Turbulence, Simulation, Condensed matter, Computational fluid dynamics, Engineering simulation, Modeling, Condensers (steam plant), Errors, Film condensation, Film thickness, Nonlinear equations, Refrigerants, Heat transfer coefficients
research-article
J. Heat Transfer   doi: 10.1115/1.4039915
This work presents a study of double-diffusive free convection in a porous square cavity under turbulent flow regime and with aiding drive. The thermal non-equilibrium model was employed to analyze the energy and mass transport across the enclosure. Governing equations were time- and volume averaged according to the double-decomposition concept. Analysis of a modified Lewis number, Lem, showed that for porous media this parameter presents opposite behavior when varying the thermal conductivity ratio or the Schmidt number, while maintaining the same value for Lem. Differently form free flow, the existence of the porous matrix contributes to the overall thermal diffusivity of the medium, whereas mass diffusivity is only effective within the fluid phase for an inert medium. Results indicated that increasing Lem through an increase in Sc reduces flow circulation inside porous cavities, reducing Nuw and increasing Shw. Results further indicate that increasing the buoyancy ratio N promotes circulation within the porous cavity, leading to an increase in turbulence levels within the boundary layers. Partial contributions of each phase of the porous cavity (solid and fluid) to the overall average Nusselt number becomes independent of N for higher values of the thermal conductivity ratio, ks/kf. Further, for high values of ks/kf, the average Nusselt number drops as N increases.
TOPICS: Porous materials, Turbulence, Equilibrium (Physics), Buoyancy, Convection, Cavities, Flow (Dynamics), Fluids, Thermal conductivity, Boundary layers, Thermal diffusivity, Natural convection
research-article
J. Heat Transfer   doi: 10.1115/1.4039642
Numerical simulations are carried out for fluid flow and natural convection heat transfer induced by a temperature difference between a hot inner cylinder with different geometries (i.e. circular; triangular; elliptic; rectangular; and rhombic) and a cold outer square enclosure filled with nanofluid superposed porous-nanofluid layers. The Darcy-Brinkman model is applied for the saturated porous layer with nanofluid. Moreover, the transport equations (mass, momentum, and energy) are solved numerically using the Galerkin weighted residual method by dividing the domain into two sets of equations for every layer with incorporating a non-uniform mesh size. The considered domains in this investigation are closely examined over a wide range of Rayleigh number (103 = Ra= 106), Darcy number (10-5 = Da = 10-1), the thickness of porous layer (0% = Xp = 100%), thermal conductivity ratio (1 = Rk = 20) and nanoparticle volume fraction (0 = ? = 0.1), respectively. The nanofluid is considered to be composed of Cu-nanoparticle and water as a base fluid. The results showed that the obtained total surfaces-averaged Nusselt numbers of the enclosure, in all cases, at the same operating conditions, the rate of heat transfer from the enclosure which the triangular cylinder is located inside is better. Also, as the thickness of the porous layer is increased from 20% to 80%, the free convection performance will decrease significantly (to about 50%) due to the hydrodynamic properties of the porous material.
TOPICS: Natural convection, Circular cylinders, Cylinders, Nanofluids, Heat transfer, Nanoparticles, Thermal conductivity, Momentum, Fluid dynamics, Temperature, Fluids, Porous materials, Computer simulation, Rayleigh number, Water
research-article
J. Heat Transfer   doi: 10.1115/1.4039585
High pressure/high temperature forced and mixed convection experiments have been performed with helium and nitrogen at temperatures and pressures up to 893K and 64 bar, respectively. The test section had a 16.8-mm ID flow channel in a 108-mm OD graphite column. Flow regimes included turbulent, transitional and laminar flows with the inlet Reynolds numbers ranging from 1,500 to 15,000. Due to strong heating, the local Reynolds number decreased by up to 50% over the 2.7-m test section. In addition, heat transfer degradation and flow laminarization caused by intense heating led to Nusselt numbers 20~50% lower than the values given by the modified Dittus-Boelter and modified Gnielinski correlations. Flow laminarization criteria were considered based on a dimensionless acceleration parameter (Kv) and buoyancy parameter (Bo*). Upward turbulent flows displayed higher wall temperatures than downward flows, due to the impact of flow laminarization which is not expected to affect buoyancy-opposed flows. Laminar Reynolds number flows presented an opposite behavior due to the enhancement of heat transfer for buoyancy-aided flows. At low Reynolds numbers, downward flows displayed higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases. In the entrance region of downward flows, convection heat transfer was reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancy-induced mixing caused higher convection heat transfer and lower wall temperatures.
TOPICS: Flow (Dynamics), High pressure (Physics), Graphite, High temperature, Buoyancy, Reynolds number, Wall temperature, Heating, Heat transfer, Turbulence, Convection, Mixed convection, Entrance region, Laminar flow, Temperature, Helium, Nitrogen