Important Heat Transfer Mechanism Research Articles

Optimal parameters of Monte Carlo ray tracing solver with line-by-line spectral database for radiation modeling in fire

Thermal radiation is an important heat transfer mechanism in fire, but its modeling has often been over-simplified due to computational expense. In this study, we assess whether the Monte Carlo ray tracing (MCRT) method using the line-by-line (LBL) spectral database can serve as a fast and accurate candidate for solving radiation in fire. Using a snapshot extracted from a previous well-resolved simulation of a small turbulent pool fire, the radiation field is solved using a recently developed MCRT-LBL solver with three varying parameters: the number of rays per computational cell, the energy emission scheme, and the resolution of the LBL database. The energy equation is subsequently solved for one time step to assess the propagation of statistical noise into the temperature field. The radiative absorption is well predicted even with only one ray emitted per cell. Adaptive emission, where more rays are emitted at locations with stronger volumetric radiative emission, converges faster than the standard approach, where the same number of rays are emitted from every cell, although the advantage of the former is only pronounced when a sufficiently large number of rays are emitted. The statistical noise is smoothed in the calculation of the radiative heat source because the radiative emission is always predicted accurately regardless of the solver’s parameters. Further damping is provided by the energy equation, yielding a temperature field with negligible noise. Coarsening the LBL database only marginally affects the accuracy of the results, but it reduces the memory required to store the data to less than 10% of that of the original database. Lastly, when soot dominates radiative emission, the statistical noise of the radiation source is further reduced. In conclusion, the study indicates reasonable feasibility for applying MCRT-LBL in fire modeling, and we will further demonstrate this possibility in future studies using larger fires.

Computational fluid dynamics approaches: An overview

This special article is a collection of 28 review articles from different fluid flow approaches. The present article summarizes the recent research developments regarding the theoretical and experimental investigations about thermal conductivity of different nanofluids. The current study analyzes several factors those strongly affecting thermal conductivity of nanofluids include solid volume fraction, temperature, particle size, particle type, particle shape, different base fluids, magnetic field, pH, surfactant and ultrasonic time. In addition, different reasonably attractive models contributing augmentation of thermal conductivity of nanofluids are invoked. Finally, important heat transfer mechanisms namely Brownian motion, nanoclustering, thermophoresis, osmophoresis and interfacial nano-layer responsible for significant role in ameliorating the thermal conductivity and therefore the heat transfer characteristics of nanofluids are discussed.

Quantitative evaluation of radiative heat transfer from reactor surface to multiphase reaction medium in a supercritical water gasification reactor for coal

Supercritical water gasification (SCWG) of coal is a clean coal utilization technology, which converts organic matter in coal into hydrogen-rich gases through a series of thermo-chemical reactions. In this process, thermal radiation is an important heat transfer mechanism and significantly affects thermal efficiency and gasification performance. This paper aims to develop a comprehensive numerical model integrating with solving radiative transfer to precisely simulate the process of SCWG of coal, and quantitatively evaluate non-grey radiative heat exchange between reactor surface and multiphase reaction medium. The simulation results are validated against experimental data. It is shown that under practical operating conditions, thermal radiation accounts for 30 % – 42 % of total heat exchange in the reactor. More than 94 % of radiant energy lies in the spectral range of 1.99 – 50.0 μm, which just covers the three most important radiation bands of supercritical water. Because both coal particles and supercritical water participate in radiative heat transfer, enhancing radiative heat transfer can effectively improve reaction medium temperature and gasification rate. On average, for every 20 % increase in the proportion of radiative heat transfer in the reactor, fluid medium temperature increases by around 7.1 K, and the gasification reaction rate rises by 9.5 %. These findings would provide valuable guidance for reactor design and optimization in practice.

Transient Numerical Simulation the Friction Stir Welding Process of Aa 2017 Alloys, Contact Parameter Effects on Thermal History and Material Flow

The present work is a 3D transient numerical simulation of material flow and heat transfer during the FSW process. The equations of energy, conservation of mass and momentum were solved in the Eulerian coordinate, in which the spatially variable thermo-physical properties and the non-Newtonian viscosity for the flow of material was calculated taking into account the deformation velocity, the temperature and the properties of the material depending on the temperature. In the purpose of calibrating the proposed model, the effects of contact conditions at the tool / work-piece interface such as the friction coefficient and the slip coefficient on thermal cycles, distribution and the evolution of the maximum value of the temperature have been studied. However, the results obtained show a significant asymmetry of the temperature and mass distribution. Convective heat transport was an important heat transfer mechanism near the tool surface. Furthermore, the numerically simulated temperature value, cooling rates and the length of the thermo-mechanically and thermally affected zone increase with increasing slip coefficient.

Elastic and Thermoelastic Effects on Thermal Water Convection in Fracture Zones

AbstractNatural groundwater convection in fractures is an important mechanism of mass and heat transfer in the subsurface, locally altering temperature by several tens of degrees. The thermoelastic stresses resulting from these thermal anomalies induce thermal strains, which in turn alter the transmissivity (permeability times thickness) of the fracture and, therefore, the convective flow within. We investigate the effect of thermal strains on fracture convection patterns using a three‐dimensional thermo‐hydraulic‐mechanical numerical model, implementing the Barton‐Bandis relationship between fracture transmissivity and effective normal stress, which results in a downward‐narrowing of the fractures. When thermoelasticity is not taken into account, convection forms narrow upflow zones and wide downflow zones. Decreasing fracture stiffness results in similar upflow/downflow patterns, but restricted to the shallow portions of the fracture, creating relatively minor thermal changes. When thermo‐elasticity is included in the model, thermal strains induced by cool downflow zones create narrow high‐transmissivity channels within the fracture, allowing convective flow to reach greater depths and significantly reducing the geothermal gradient near the fracture. Fracture stiffness is a key parameter in determining convection depth and thermal perturbation strength for a given set of host rock mechanical properties. When fracture stiffness is below some threshold, subsequent contractive thermoelastic strains were found to induce tensile failure of the host rock below the fracture, propagating and deepening the fracture into the host rock. This observation provides support to the previously proposed concept of convective downward migration.

Evaluation of heat transfer during the process of heating water in containers using a solar cooker box-type

This work presents the experimental results and the evaluation of the heat transfer mechanisms by conduction, convection, and radiation that occur during the water heating process inside four containers with volumes of 250, 500, 750, and 1000 ml. using as for this process a box-type solar cooker. For the experimental measurements, a type k thermocouple arrangement was used, located in the water, the exterior surfaces of the containers, and the glass that forms part of the solar cooker cover. For the acquisition of the data perceived by the thermocouples, a Compact Field Point from National Instruments was implemented and for the measurement of global solar radiation, an Eppley 8-48 radiometer was used. The results obtained from the evaluation show that the most important heat transfer mechanism is by conduction located in the exterior glass of the solar cooker cover and the convection heat transfer in this same glass is the one with the lowest values. The evaluation of the results achieved contributes to the knowledge of the operation of solar cookers, which can contribute to the design and improvements of these solar energy devices.

Thermal Conductivity of Nanofluids-A Comprehensive Review

The present study deals with a comprehensive review on the enhancement of effective thermal conductivity of nanofluids. The present article summarizes the recent research developments regarding the theoretical and experimental investigations about thermal conductivity of different nanofluids. The current study analyzes several factors those strongly affecting thermal conductivity of nanofluids include solid volume fraction, temperature, particle size, particle type, particle shape, different base fluids, magnetic field, pH, surfactant and ultrasonic time. In addition, different reasonably attractive models contributing augmentation of thermal conductivity of nanofluids are invoked. Finally, important heat transfer mechanisms namely Brownian motion, nanoclustering, thermophoresis, osmophoresis and interfacial nano-layer responsible for significant role in ameliorating the thermal conductivity and therefore the heat transfer characteristics of nanofluids are discussed.

Effects of Included Angle on Local Pool Boiling of V-Shape Tubes

The effect of the angle included between the V-shaped tubes on the local pool boiling heat transfer of the tube surface submerged in saturated water at atmospheric pressure is experimentally studied. The tube is made of stainless steel, the surface is smoothed through machining, and the diameter and length of the heating section are 19mm and 400mm, respectively. The included angle is adjusted between 2 and 18 degrees. The three V-shaped tubes consist of (1) top horizontal and bottom slope, (2) vertical symmetry and (3) top slope and bottom horizontal. As the shape changes, the effect of the flow moving from the bottom tube to the top changes, and a distinct difference in heat transfer according to the shape is observed. When the distance between the upper tube and the lower tube is the smallest, the bundle effect is found to be greatest. The most important heat transfer mechanism is judged by liquid agitation due to convective flow and bubble movement, and the concentration and dispersion of flow and turbulence development are also considered as important factors.

Immersion effects in zinc ITS-90 fixed-point cells

The freezing point of zinc (419.527 °C) is an important defining fixed point of the International Temperature Scale of 1990 (ITS-90). To realize it, a crucible of high purity graphite containing an ingot of high purity zinc is placed in a glass container containing an inert gas, with a central re-entrant tube, also glass, to allow insertion of a standard platinum resistance thermometer (SPRT). A key problem with these cells is difficulty in obtaining good immersion of SPRTs; it is commonly thought that the glass construction causes ‘light piping’ whereby the glass could facilitate heat transfer away from the region close to the SPRT sensing element by radiation, both directly and via reflections. In this paper it is shown that at the zinc freezing temperature light piping via reflections between the surfaces of the glass components is not an important heat transfer mechanism. It is also shown that light piping via transmission within the glass components is insignificant because of attenuation at wavelengths where the magnitude of the Planck function is important. In the case of the zinc fixed point, direct line-of-sight radiative heat transfer and thermal effects within the ingot appear to be the most important contributors to spurious immersion effects. It is shown that where the immersion profile is poor, a short platinum coil wrapped around the SPRT in the vicinity of the sensing element to improve radial conduction and to block vertical radiative heat transfer substantially improves the performance.

Heat transfer and entropy generation analysis of nanofluids thermocapillary convection around a bubble in a cavity

In view of the fact that thermocapillary convection is an important heat transfer mechanism in nucleate boiling, and nanofluids is used to be heat transfer medium extensively. The present paper reports the numerical investigation of nanofluids thermocapillary convection around a gas bubble in a cavity, and the effects of nanoparticles volume fraction and size on flow characteristics, heat transfer performance and entropy generation are studied. The results show that, with more nanoparticles add to base fluid can enhance the heat transfer performance of gas bubble thermocapillary convection, the convective intensity is reduced, and the local entropy generation decreases and its distribution becomes more uniform. However, with larger nanoparticles diameter the results are just the opposite, namely, the heat transfer performance is weakened, the convective intensity is enhanced and the total entropy generation increases and its distribution becomes more nonuniform. These findings are of significance for the heat transfer enhancement in nanofluid boiling heat transfer.

Numerical Study of Heat Transfer and Entropy Generation of Nanofluids Buoyant-Thermocapillary Convection around a Gas Bubble

Buoyant-thermocapillary convection is an important heat transfer mechanism in nucleate boiling and nanofluids has been used as heat transfer medium extensively, so it is necessary to study the heat enhancement mechanism of nanofluids buoyant-thermocapillary convection around gas bubble. The present paper first reports the flow, heat transfer and entropy generation characteristics of nanofluids buoyant-thermocapillary convection around a gas bubble in a cavity, and the effects of nanoparticles volume fraction and size on buoyant-thermocapillary convection are discussed. The results show that, as nanoparticles volume fraction increases from 0 to 0.05, the convective intensity and entropy generation of buoyant-thermocapillary convection decreases gradually, and the average Nusselt number of bottom wall first increases and then decreases. As nanoparticles volume fraction is 0.02 the average Nusselt number is the largest and increased by 3.1% relative to that of pure fluid. As nanoparticles diameter increases from 20 nm to 80 nm, the temperature and flow fields indicate as the same structure, but the entropy generation increases gradually. The average Nusselt number of bottom wall increases slightly with nanoparticles diameter increasing, particularly, the average Nusselt number for nanoparticles diameter (dp) 80 nm is increased by 2.28% relative to that of dp = 20 nm.

Carbon-Based Materials for Thermoelectrics

This article reviews the recent progress towards achieving carbon-based thermoelectric materials. A wide range of experimental and computational studies on carbon allotropes and composites is covered in this review paper. Specifically, we discuss the strategies for engineering graphene, graphene nanoribbon, graphene nanomesh, graphene nanowiggle, carbon nanotube (CNT), fullerene, graphyne, and carbon quantum dot for better thermoelectric performance. Moreover, we discuss the most recent advances in CNT/graphene-polymer composites and the related challenges and solutions. We also highlight the important charge and heat transfer mechanisms in carbon-based materials and state-of-the-art strategies for enhancing thermoelectric performance. Finally, we provide an outlook towards the future of carbon-based thermoelectrics.

Investigation of heat transfer in partially filled horizontal drums

The use and the improvement of classical mathematical models for heat transfer in particulate systems can facilitate the industrial large-scale application of rotating drums as bioreactors for solid-state cultivation (SSC). In the current paper, a one-phase two-dimensional heat transfer model is proposed for that radial and angular temperature profiles can be forecasted along time in a horizontal drum, partially filled with glass beads, subjected to intermittent rotation. The most important heat transfer mechanisms were calculated and discussed. Experiments have been carried out in order to provide experimental data of temperature at several radial and angular positions to be compared to the ones predicted by simulation. Three parameters were adjusted to obtain the best fit between simulated and experimental temperature data: hs, hw and k (convective heat transfer coefficient bed surface-headspace, convective heat transfer coefficient wall-to-bed and effective thermal conductivity of the bed, respectively). A factorial central composite design (CCD) was used to optimize the search for the best set of parameters, having the determination coefficient as optimization criteria. For the values of the fitted parameters hs, hw and k, experimental and predicted temperatures were very close, addressing that the model here can be used as a basic feature for further studies of heat transfer in SSC drum bioreactors and other applications.

Quantification of the influence of parameters determining radiative heat transfer in an oxy-fuel operated boiler

Radiative heat transfer is a very important heat transfer mechanism in pulverized coal combustion. To identify the influence of parameters determining radiatve heat transfer and to give recommendations on the required accuracy of corresponding submodels, a 3D-periodic oxy-fuel pulverized coal combustion test case is investigated. Measurement values determined by the authors or elaborate submodels are applied for each parameter and compared to simplified models or empirical constants. To investigate the interaction between particle radiation and the strong spectral dependence of gas radiation in oxy-fuel scenarios, a comparison between spectrally averaged and spectrally resolved calculations performed. To the best knowledge of the authors, for the first time the contribution of the parameters determining radiative heat transfer are quantified and compared in one comprehensive study.The results indicate a strong influence of coal particle emissivity and scattering phase function as well as the projected particle surface on the radiative source term. For the wall heat flux, the largest influences were found for ash and coal particle emissivity, projected particle surface and the scattering phase function. Additionally, the difference between coal particle and gas temperature was found to have a significant influence on wall heat flux. A comparison of spectrally averaged to spectrally resolved results and the corresponding models for gas radiation (WSGGM and SNBM) yielded similar trends for the influence of each parameter. Thus, based on the models and parameters involved in this study, a spectrally averaged approach seems to be of sufficient accuracy to describe radiative heat transfer in oxy-fuel combustion systems.

Direct numerical simulation of low-Prandtl number turbulent convection above a wavy wall

Turbulent forced convection is investigated by Direct Numerical Simulation in a channel with one sinusoidal wavy wall and one flat wall. Fluid flow and heat transfer are periodically fully developed, the simulated Reynolds number of the bulk velocity and the hydraulic diameter is Re=18, 880 while three Prandtl numbers are considered, i.e. Pr=0.025, Pr=0.2, and Pr=0.71. The fluid flow is characterized by separation, reattachment and a shear layer downstream the wave peak, these are conditions relevant for turbulent heat transfer and passive scalar transport applications.In the range of Péclet numbers investigated, the most important heat transfer mechanism is by mean flow advection. Accordingly, the peak heat transfer region is in the upslope part of the domain. The separation bubble instead acts as a barrier to convection and the heat transfer rate is minimum close to separation. An a priori analysis is performed in order to assess the accuracy of turbulent heat transfer models based on the Generalized Gradient Diffusion Hypothesis.

Innovative radiation-based direct heat exchanger (DHX) for liquid metal cooled reactors

Considering the importance of safety features in the development of Generation IV nuclear reactors, an innovative and passive decay heat removal system (DHRS) has been proposed for liquid metal cooled reactors. The attention is here focused on the direct heat exchanger (DHX) of the system constituted by a bayonet tube that allows to remove the decay heat from the primary coolant; both primary and secondary fluids flow in natural circulation. Since each bayonet tube is equipped with a vacuum gap, the most important heat transfer mechanism characterizing the DHX is radiation. Furthermore, the presence of the vacuum gap guarantees a physical separation and a complete decoupling between primary and secondary fluids, enhancing the safety features of the whole system. Several CFD analyses have been carried out in order to obtain a characterization of the DHX both for sodium and lead cooled fast reactors, in order to optimize the DHX geometry on the basis of the specific application, and the results are discussed in the paper.

Effects of the imbibitional flow of steam on oil recovery from carbonate reservoirs

Carbonate reservoirs contain over 90% of oil within an extremely tight matrix and are usually oil-wet, which makes them unfavorable for oil production. During water injection, water does not imbibe into the matrix, but flows preferentially through the fractures, resulting in very low oil recoveries. It is proposed that steam or hot water is injected, inducing a wettability change and rendering the matrix water-wet. Then, water can move into the matrix by capillary imbibition. From this perspective, it is crucial to analyze the contributions of conductive and convective heat transfer during steam injection.The mechanism of wettability changes is accounted for when steam is injected into carbonate reservoirs. The effect of strong capillary pressure in a tight matrix on the oil recovery is also examined. The purpose of this study is to evaluate the rate of conduction and convection and to determine optimum conditions that maximize heat transfer and wettability alteration in the tight matrix. An analytical solution and numerical results were compared for heat conduction within the matrix during steam injection. Investigations were also performed to determine the optimum injection time, steam injection pressure and steam quality required to make the carbonate matrix water-wet and maximize oil recovery. The results from this study show that convectional flow might be the most important heat-transfer mechanism to shorten the matrix wettability converting time. Based on analysis of the thermal process, an optimum steam injection strategy can be designed for a given matrix permeability and fracture spacing.

Heat transfer from small objects in microgravity: Experiments and analysis

Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes. One important mechanism of heat transfer in the above processes is natural convection which leads to heat transfer rates many times larger than that of pure conduction. Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers. Existing correlations for external geometries predict a progressively increasing contribution of natural convection to heat transfer with respect to gravity (starting from zero gravity). To test the validity of these correlations, experiments are performed for the estimation of heat transfer rates at low gravity. Heat pulses are given to a miniature thermistor with a nearly spheroidal shape immersed in a liquid and its thermal response is registered during heating in parabolic flights. The contribution of natural convection to heat transfer is undoubtedly estimated from runs in which acceleration varies from 0 to 1.8 g. Surprisingly enough, the experiments showed that the Rayleigh number must take a minimum value before non-negligible effect of natural convection on heat transfer appears (existence of a threshold Rayleigh number). In the absence of natural convection (below Ra thr) the experimental thermal response curves can be successfully described by approximating solutions of the transient heat conduction equation for the spheroidal geometry of the thermistor. Apparently, additional research is needed regarding the natural convection around sub-millimeter objects for small Rayleigh numbers.

Modeling Peat Thermal Regime of an Ombrotrophic Peatland with Hummock–Hollow Microtopography

The theory of conductive heat transfer cannot explain different attenuations of the daily amplitude of peat temperatures (TS) in hummocks (detectable below the 20‐cm depth) and hollows (disappearing above the 10‐cm depth). Large readily drained macropores in the upper fibric peat determine a large air permeability and hence may enhance heat transfer by air convection in porous media, driven by temperature gradients between hummock sides and interiors. In this study, the ecosys model was used to simulate a peat thermal regime at Mer Bleue peatland, Ontario, Canada. It was hypothesized that adding the air‐convective heat transfer to conductive plus water‐convective heat transfers would improve simulations of TS The results for TS, ground heat fluxes, G, and sensible heat fluxes, H, modeled with and without air‐convective heat transfer were tested with continuous hourly measurements from 2000 to 2004 using thermocouples, heat flux plates, and eddy covariance. Simulated air‐convective heat transfer caused an average increase in G and a corresponding decrease in H of ∼20 W m−2 from the simulated conductive plus water‐convective heat transfer. Hastened soil warming in hummocks resulted in better agreement between measured and simulated hummock TS values with (RMSD of 2.23°C) than without air‐convective heat (RMSD of 2.54°C). Enhanced hummock TS caused an indirect increase in hollow TS in the model with (RMSD of 1.68°C) compared to without air‐convective heat (RMSD of 1.82°C). Our results suggest that air convection is probably an important mechanism of heat transfer in peat hummocks and should be included in peatland biogeochemical models.

APPLICATION OF THE MONTE CARLO METHOD IN THE SOLUTION OF RADIATION HEAT TRANSFER IN PARTICIPATING MEDIA

The temperatures of the gases produced in combustion processes are very high so thermal radiation constitutes an important heat transfer mechanism in industrial furnaces. Most furnaces can be modeled as gray enclosures containing non-gray gases. The radiation heat transfer can be obtained with the aid of the weighted-sum-of-gray-gases model, determining the zonal exchange areas for each of the gray gases considered in the sum. For some enclosures with simple geometries, there are correlations to obtain the direct exchange-areas which can be used to determine the total exchange areas. However, for enclosures with complex geometries, determining the direct exchange areas can become a difficult task. In this case, the use of the Monte Carlo method is advantageous, for it allows one to approach geometric complexities without additional complications. Therefore, the method was applied to compute total exchange areas in enclosures containing participating media. Two cases were considered: cylindrical enclosures and enclosures that have generic geometries formed from cube combinations. The results presented a good agreement with solutions available in the literature.