Computational Heat Transfer Model Research Articles

Sensitivity study of forced convection bubbling in a transparent viscous fluid as a proxy for molten borosilicate glass

Computational fluid dynamic and heat transfer models are being developed for the waste glass melters that will be used to vitrify legacy tank waste at the Hanford site. The objective of this study is to validate the ability of these models to predict the forced convection bubbling in the molten glass at prototypic conditions and scale. Bubbling is used to homogenize the melt pool and increase the melt rate of the batch. Simulations were compared with experimental data obtained with a surrogate liquid under isothermal conditions in an acrylic tank. The simulation results quantify the projected bubble area at the free surface as a function of flow rate and nozzle depth and show agreement with experimental data. Additionally, the local sensitivities of the surface tension and viscosity are evaluated for their effects, since in the melter this parameter directly affects the heat transfer from the molten glass to the reactive batch (or cold cap) layer. From the sensitivity study performed, the viscosity exhibits twice as much influence on bubble projected surface area as the surface tension. In comparison to the grid resolution used in this study, these glass properties produce higher levels of uncertainty.

Computer‐aided design and experimental testing of continuous flow cooling of viscous foods

AbstractComputer‐aided studies, comprising computational fluid dynamics and heat transfer models, were used to simulate continuous flow cooling of viscous foods. Initially, model validation studies were conducted, giving a good correlation between simulation results and experimental data on cooling of sweet potato puree. Additional computer‐aided studies were performed to compare cooling efficiency of two different flow configurations, with the product flowing within the inner tube and the annulus of a tube‐in‐tube heat exchanger, respectively. Cooling of sweet potato puree was significantly improved for the annular product flow compared with the other case, resulting in a bulk temperature at the exit of the heat exchanger lower by 12 ± 4 °C. Therefore, the simulation results indicate that annular product flow could potentially improve cooling efficiency by 25% for sweet potato puree, due to the increased heat transfer to the environment.Practical applicationsDuring cooling of viscous foods, such as fruit and vegetable purees, laminar flow, and low thermal conductivity lead to a nonuniform, slow cooling process resulting in degradation of food quality. Improvement and optimization of the cooling process in a tube‐in‐tube heat exchanger, with an implementation of annular flow for the product, could potentially result in (1) enhancement of cooling efficiency, (2) reduction of cooling‐related product quality losses, and (3) improvement of sanitary conditions, by eliminating condensation formation.

Modelling the potential for multi-location in-sewer heat recovery at a city scale under different seasonal scenarios

A computational network heat transfer model was utilised to model the potential of heat energy recovery at multiple locations from a city scale combined sewer network. The uniqueness of this network model lies in its whole system validation and implementation for seasonal scenarios in a large sewer network. The network model was developed, on the basis of a previous single pipe heat transfer model, to make it suitable for application in large sewer networks and its performance was validated in this study by predicting the wastewater temperature variation across the network. Since heat energy recovery in sewers may impact negatively on wastewater treatment processes, the viability of large scale heat recovery was assessed by examining the distribution of the wastewater temperatures throughout a 3000 pipe network, serving a population equivalent of 79500, and at the wastewater treatment plant inlet. Three scenarios; winter, spring and summer were modelled to reflect seasonal variations. The model was run on an hourly basis during dry weather. The modelling results indicated that potential heat energy recovery of around 116, 160 & 207 MWh/day may be obtained in January, March and May respectively, without causing wastewater temperature either in the network or at the inlet of the wastewater treatment plant to reach a level that was unacceptable to the water utility.

Computational modeling of heat transfer and sintering behavior during direct metal laser sintering of AlSi10Mg alloy powder

Direct Metal Laser Sintering (DMLS) is one of the leading additive manufacturing processes, which produces complex metallic parts directly from the powder. One of the major problems of this rapid manufacturing process is an inhomogeneous temperature distribution, which leads to residual stress in the build part. Thus, temperature analyses must be performed, to better understand the temperature distribution and sintering behavior of the powder bed with a different laser recipe. In this study, a comprehensive three-dimensional numerical model was developed to understand the temperature distribution during direct metal laser sintering of AlSi10Mg alloy powder. The computer simulation was carried out in ANSYS 17.0 platform. Further, the effect of process parameters such as laser power and scan speed on the temperature distribution and sintering behavior were studied. From the simulation results, it was found that, when the laser power increased from 70 W to 190 W, the maximum temperature of the molten pool increased from 731 °C to 2672 °C, and the molten pool length changed from 0.286 mm to 2.167 mm. A reverse phenomenon was observed with an increase in scan speed. The sintering depth of the powder layer increases significantly from 0.061 mm to 0.872 mm with increasing the applied laser power, but decreased from 0.973 mm to 0.209 mm as a higher scan speed was applied. The developed model helps to optimize the powder layer thickness and minimize the wastage of excess powders during the sintering process.

(Invited) Microfluidics for Biomedical Devices By Two Examples

The need and the tremendous value of “very small” machines and devices were realized in late 1980’s. In the following decade, the indispensable utility of micro- or even nano-level transport phenomena in these machines was recognized, leading to the start of research and development in microflows. Researchers started investigating from a point where they deemed relevant body of knowledge on macroflows stood. Over the ensuing years, there has been significant amount of resource investment to, for example, the understanding of how the underlying physics differed as compared to macroflows and, just as importantly, what existing transport phenomena tools would be applicable. It was quickly observed that what significantly changed were the relative effects of, for example, very low Reynolds numbers, the domination of surface and interface tension, and even the continuum assumption. Much happened since then, hence, today we enjoy the ever ubiquities micro- and nano-fluidic devices this body of research and development have produced. The author took interest in this field in the early 2000’s. This presentation will briefly sample a few of our earlier studies which varied from “design optimization of a micro synthetic jet actuator to control microflow separation,” to “electrodiffusiophoretic motion of charged spherical particle in a nanopore.” The emphasis, however, will be on our current research on two biomedical devices, using in silico approach and collaborating with adjoining experimentations: (i) a convective polymerase chain reactor (cPCR); (ii) microfluidics-aided collagen fiber (MCF) fabrication. Our goal in the first project is to develop a methodology for a portable and affordable c-PCR design intended for point-of-care use. Computational fluid and heat transfer models are developed to simulate the prerequisite convective transport fields. The search space for optimality is determined based on a specified range of the selected design variables. The convection-diffusion equations are then recast to determine and minimize the time-to-double the reagent (in our case, the double-stranded DNA) by the candidate design. For the MCF device, we are developing a new microfluidic methodology to fabricate advanced telocollagen fibers. The ultimate goal is to obtain an advanced microfluidic-based telocollagen manufacturing technology suitable to produce ligament and tendon repair and replacement grafts. Our specific goal is to provide the in silico guidance to the fabrication of collagen fibrils, which are much finer and better axially aligned, since prior research has shown that the mechanical strength of such fibers would be more favorable in achieving their purpose.

Upward-and-downward spread of smoldering peat fire

Smoldering is the dominant combustion process in peat fire, releasing a large amount of carbon and smoke into the atmosphere. The spread of smoldering in peatland is a multi-dimensional process, which is slow, low-temperature, persistent, and difficult to detect. In this work, we investigate the upward spread of peat fire from the underground to the surface after forced ignition which is a relevant configuration but rarely studied. In the experiment, ignition is not possible if the igniter is deeper than 15 cm below the free surface, regardless of moisture content or density. Once ignited, the 1st-stage upward fire spread is initiated towards the free surface (opposed smoldering) with a peak temperature of 300 °C, leaving behind a char structure that does not collapse. Then, a 2nd-stage downward spread (forward smoldering) is activated with a peak temperature of 600 °C and regression of free surface. The upward spread is faster than the downward spread. The rates of both upward and downward spread decrease as the peat density or depth is increased. These experimental observations are successfully captured by a 1D computational model of heat and mass transfer with 5-step kinetics. Modelling results further suggest that (1) the oxygen diffusion controls the entire upward-to-downward spread of peat fire, (2) the oxidation of peat sustains the 1st-stage upward spread, and (3) the oxidation of char sustains the 2nd-stage downward spread. This is the first study investigating the upward spread of peat fire, which helps understand the persistence of peat fire and guide the fire prevention and suppression strategies.

Computational fluid dynamics modeling and experimental validation of heat transfer and fluid flow in the recovery boiler superheater region

Development of predictive computational fluid dynamics (CFD) methods for recovery boilers would be highly beneficial for the development of such very large-scale energy production applications. Herein, unique experimental data is compared with a developed CFD framework demonstrating the predictive character of the present simulations. The novelty of the work consists of the following: (1) We report two sets of previously unpublished full-scale temperature and flow field measurements from the recovery boiler superheater region. The data from these challenging measurements is very valuable, since reported experimental data on recovery boilers is scarce in literature. (2) We introduce a detailed, three-dimensional CFD model for the recovery boiler superheater region. The results of the model are verified computationally and validated with the experimental data. (3) We demonstrate the added-value of the developed CFD model with a detailed analysis of the three-dimensional flow field and heat transfer results. In addition, we consider the implications of the three-dimensional solution for the estimation of fouling.

An optimal design approach for the annular involute-profile cross wavy primary surface recuperator in microturbine and an application case study

In this work, a computational model of heat transfer and pressure drop is established for the design optimization of the annular involute-profile cross wavy primary surface (CWPS) recuperator in microturbine. The genetic algorithm method is employed to solve the optimization problem of annular CWPS recuperator with multiple design variables. Thus, an optimal design approach for this kind of recuperator is formulated. The validity of the computational model is checked by comparing the calculation results with the experimental data of C30 and C65 prototype recuperators (Capstone Turbine Corporation). With the total relative pressure loss as the optimization target, the developed method is applied to the optimal design of the recuperator in a 300 kW power-level microturbine. In comparison with the original design, the optimal design results in a significant reduction of the pressure loss in the case that all constraints are satisfied, which indicates the formulated optimal design approach is effective for the annular CWPS recuperator. By replacing the optimization target, the optimal design approach presented in this work can be also applied to the single-objective optimization for other targets (compactness, heat transfer area, weight, volume, effectiveness, etc.) and even the multi-objective optimization according to the particular requirements of different microturbines.

Charge and discharge behavior of elemental sulfur in isochoric high temperature thermal energy storage systems

Thermal energy storage with elemental sulfur is a low-cost alternative to molten salts for many medium to high-temperature energy applications (200–600 °C). In this effort, by examining elemental sulfur stored isochorically inside isolated pipes, we find that sulfur provides attractive charge/discharge performance since it operates in the liquid-vapor regime at the temperatures relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The isolated pipe configuration is relevant to shell-and-tube thermal battery applications where the heat transfer fluid flows over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena. The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. The model results evaluate the influence of pipe diameter on charge and discharge times, heat transfer rate, and Nusselt number due to buoyancy driven convection currents. Depending on the Rayleigh number (pipe diameter), the average Nusselt number obtained for discharge is 3–14 times higher than proposed solid-liquid phase change technologies based on molten salt, which are limited in their performance due to conduction based solidification and low thermal conductivity. The results show competing trade-offs between increase in heat transfer coefficient, thermal energy stored in sulfur, and increase in charge and discharge time with increase in pipe diameter. A preferred pipe diameter can be determined for target applications based on their requirements and these competing trade-offs. A validated fundamental correlation for Nusselt number as a function of Rayleigh number for charge and discharge is developed that can be used to design the sulfur-based thermal storage system for transient operation.

Experimental and numerical investigation on the impact of vacuum level on effective thermal conductivity and microstructure of carbon fibre reinforced epoxy composites manufactured by vacuum bag method

The physical properties of epoxy based carbon reinforced composites are highly anisotropic due to their directional structure and dependent on the manufacturing process parameters. Thermal conductivity was found to be dependent on the void volume fractions, which appear as a result of the insufficient vacuum level. In the proposed paper the multi-scale computational model of heat transfer across the carbon fiber-epoxy resin composite is proposed. The meso-scale effective thermal conductivities are determined with analytical formulae for isotropic and anisotropic media, the latter takes into account thermal resistance at the interface of fibres and epoxy resin. Proposed model is utilized to determine the effective thermal conductivity in the direction perpendicular to plies of composite. The influence of void fractions and the thickness of the composite on the effective thermal conductivity is investigated. The numerical outcomes underestimate the real variation in conductivity, which can be caused by change in carbon volume fraction of samples manufactured at different vacuum levels which was not considered in numerical computation.

Heat Transfer Performance Comparisons of Supercritical Carbon Dioxide and NaCl–KCl–ZnCl2 Eutectic Salts for Solar s-CO2 Brayton Cycle

Supercritical carbon dioxide (s-CO2) and the new generation molten salts can both be used as the heat transfer fluid in the solar supercritical carbon dioxide Brayton cycle. One of the main concerns in the design of solar s-CO2 Brayton cycle is that the working fluids need to meet the desired heat transfer coefficient to reach the high operating temperature, responding to the high concentrated heat flux. In this paper, to satisfy the SunShot Initiative’s goal of a 10 MWe solar s-CO2 Brayton cycle power plant, the receiver is designed to heat the s-CO2 and the NaCl–KCl–ZnCl2 eutectic salts from 500 ºC to 750 ºC to gain heat of 20 MWth. A tubular receiver made of pressureproof Haynes 230 is assumed. A one-dimensional computational heat transfer model is developed. The influences of receiver operating and structure parameters on the performance of the HTFs are evaluated. Finally, the outlet temperature, the heat gain, the solar field efficiency and the solar exergy efficiency for the s-CO2 and the new molten salts are compared. It is shown that both the fluids can reach the desired temperature and heat gain under special operating and structure parameters, but their relevant effectiveness to convert solar radiation into heat are different. The results obtained in this study are expected to provide references for the further development of the solar s-CO2 Brayton cycle technology.

Ammonia escape mass transfer and heat transfer characteristics of CO2 absorption in packed absorbing column

Ammonia escape is one of the main barriers to CO2 capture by aqueous ammonia solution, so it is of benefit to the practical CO2 capture engineering to clarify the effects of running parameters on the mass transfer of ammonia escape and heat transfer performance in packed absorbing column. Based on the representative elementary volume approach and pseudo-single-liquid model, a computational model of heat and mass transfer for the CO2 capture by aqueous ammonia solution in an industrial packed column is developed, by which the ammonia and CO2 volume fractions and temperatures, ammonia escape mass transfer coefficient and heat transfer per unit volume are obtained. The results show that the ammonia escape volumetric mass transfer coefficient is mainly influenced by the liquid inlet temperature and flow rate, inlet CO2 volume fraction and flow rate. What’s more, the ammonia concentration plays a key role in the ammonia escape amount, and the heat transfer per unit volume is greatly dominated by the gas flow rate and liquid inlet temperature. This work can contribute to the ammonia escape inhibition and high CO2 removal efficiency for the industrial application of CO2 capture by ammonia solution in packed columns.

Computational modelling and analysis of heat and mass transfer in MHD flow past the upper part of a paraboloid of revolution

The flow and heat transfer nature of MHD dissipative flow past the upper part of a non-melting paraboloid of revolution with non-uniform heat source/sink, thermophoresis and Brownian moment effects are investigated numerically. The buoyancy induced on the flow is considered in such way that the surface is neither vertical/horizontal nor wedge/cone. The equations governing the flow are solved numerically by employing the RKF integration scheme after suitable transformations. Graphical results are obtained to discuss the behaviour of flow, temperature and concentration fields at various parameters of interest. The reduced wall friction, local Nusselt and Sherwood numbers are computed. It is found that the non-uniform heat source/sink parameters regulate the thermal boundary layer. Thermophoresis and Brownian moment parameters act as controlling parameters of heat and mass transfer rate.

Computational modelling of flow and conjugate heat transfer of a drop impacting onto a cold wall

Abstract The flow and heat transfer in water drops impinging onto a dry cold surface has been computationally investigated. This situation pertains to aircraft icing or icing of power lines. The computational model solves the hydrodynamics using an algebraic implicit volume-of-fluid-based method for interface capturing with an extension for the simultaneous solution of the heat transfer within the drop and underlying substrate. The numerical solution is validated by computing a series of configurations of two-phase fluid flow and conjugate heat transfer for which experimental and analytical results are available in the literature. In a parametric study, the effect of the impact conditions and initial surface temperatures on the minimum liquid temperature reached and the heat transfer during drop impact are examined, representing the quantities relevant for nucleation and icing modelling. It is shown that these quantities do not depend on the impact conditions. However, the amount of heat transferred between the fluid and the wall is affected by the contact time and area available for heat transfer, which are determined by the impact conditions. Based on the numerical results, a semi-empirical model is developed, which accurately predicts the total heat transferred during a single impact event.

Spatially localized, see-through-wall temperature measurements in a flow reactor using radar REMPI.

See-through-wall coherent microwave scattering from resonance-enhanced multiphoton ionization (REMPI) for rotational temperature measurements of molecular oxygen has been developed and demonstrated in a flow reactor at atmospheric pressure. Through limited, single-ended optical access, a laser beam was focused to generate local ionization of molecular oxygen in a heated quartz flow reactor enclosed by ceramic heating elements. Coherent microwaves were transmitted, and the subsequent scattering off the laser-induced plasma was received, through the optically opaque ceramic heater walls and used to acquire rotational spectra of molecular oxygen and to determine temperature. Both axial and radial air-temperature profiles were obtained in the flow reactor with an accuracy of ±20 K⁢(±5%). The experimental results show good agreement with a steady-state computational heat transfer model. This technique shows great potential for non-invasive, high-fidelity measurement of spatially localized temperature and radical species concentration in combustion kinetic experiments and confined combustors constructed of advanced ceramic materials in which limited or non-existing optical access hinders usage of conventional optical diagnostic techniques to quantify thermal non-uniformity.

Numerical simulation of heat transfer during production of rutile titanium dioxide in a rotary kiln

This paper presents a computational heat transfer model of a rotary kiln used for the production of rutile titanium dioxide by the calcination of paste-like hydrous titanium dioxide. The work details the modelling of several chemical reactions occurring in the solid bed region along with turbulent convection of gas, radiation heat exchange among hot gas, refractory wall and the solid surface, and conduction in the refractory wall. Finite-difference techniques are used and the steady state thermal conditions are assumed. The kiln is divided into axial segments of equal length. The solution is of marching type and proceeds from the solid inlet to the solid outlet. The direction of gas flow is opposite to that of the solids. Mass balance of each species in the solid charge, and mass and energy balances of the solid and gas in an axial segment are used to obtain solids and gas temperatures, and species concentration at the exit of that segment. The kiln length predicted by the present model is 45.75m as compared to 45m of an actual kiln reported by Ginsberg and Modigell (2011). The steady-state axial gas and solid temperature profiles have been also satisfactorily validated with the numerical results of the aforementioned paper. The output data consist of refractory wall temperature distribution, the axial solids and gas temperature profiles, axial solids composition profile, the length required for drying of the solid charge and the total kiln length required to achieve 98% conversion of anatase TiO2 to rutile TiO2. A detailed parametric study with respect to the controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle and kiln rotational speed lent a good physical insight into the rutile-TiO2 production process in a rotary kiln.

Determining the optimal shaking rate of a reciprocal agitation sterilization system for liquid foods: A computational approach with experimental validation

A new canning process where a reciprocating agitation is carried out in horizontally oriented containers has been recently demonstrated to reduce processing times and enable energy savings with less degradation in the quality of processed food products. Reciprocal agitation by imposing additional forces enhances convective mixing with increased production efficiency. The reciprocal agitation uses the horizontal acceleration in addition to gravity and sum of these forces lead to a considerable increase in the heat transfer rates. In the literature, there have been experimental approaches to evaluate heat transfer enhancement. However, due to the balance among these forces, there might be an optimum reciprocal agitation rate for the increased heat transfer depending upon the physical properties of the liquid processed. Therefore, the objectives of this study were to determine the optimum agitation rates by developing a computational model for heat transfer. For this purpose, a multi-phase model simulation was performed using a finite volume method based on discretization of governing flow equations for liquid and gas phase in a non-inertial reference frame of moving mesh. Experimental studies for model validation were carried out in a reciprocally agitated retort using 98.2mm×115mm cans containing distilled water with 2% headspace as a model case. The model results were in agreement with the experimental data, and the optimum reciprocal agitation rate was determined. The results of this study are to be used to optimize the process with respect to improve the health-promoting compounds of processed foods.

Computational modeling of heat and mass transfer in reactor during magnesium-thermic reduction of titanium

Computational modeling of heat and mass transfer in reactor during magnesium-thermic reduction of titanium

Computational modeling of heat transfer in an annular porous medium solar energy absorber with the P1-radiative differential approximation

We study the steady, laminar thermal convection flow in a participating, absorbing-emitting fluid-saturated porous medium occupying a cylindrical annulus with significant thermal radiation effects as a simulation of a solar energy absorber system. The dimensionless incompressible, viscous conservation equations for mass, axial momentum, radial momentum, heat conservation and radiative transfer equation are presented with appropriate boundary conditions in an axisymmetric (X, R) coordinate system. The Traugott P1-Differential radiative transfer model is used which reduces the general integro-differential equation for radiation heat transfer to a partial differential equation. The Darcy–Forcheimmer isotropic porous medium drag force model is employed to simulate resistance effects of the solar porous medium with constant permeability in both the radial (R) and axial (X) direction. A numerical finite difference (FTCS) scheme is used to compute the velocity (U,V), temperature (Θ) and dimensionless zero moment of intensity (I0) distributions for the effects of conduction–radiation parameter (N), Darcy parameter (Da), Forchheimer parameter (Fs), Rayleigh buoyancy number (Ra), aspect ratio (A) and Prandtl number (Pr). The computations have shown that increasing aspect ratio increases both axial and radial velocities and elevates the radiative moment of intensity. Increasing Darcy number accelerates both axial and radial flow whereas increasing Forchheimer number decelerates the axial and radial flow. Higher values of optical thickness induce a weak deceleration in the radial flow whereas they increase both axial flow velocity and temperature. Increasing optical thickness also reduces radial radiative moment of intensity at intermediate axial coordinate values but enhances them at low and high axial coordinate values. Extensive validation is conducted with the network thermo-electric simulation program RAD-SPICE. The model finds important applications in solar energy porous wafer absorber systems, crystal growth technologies and also chemical engineering thermal technologies.

Lumped Capacitance and Three-Dimensional Computational Fluid Dynamics Conjugate Heat Transfer Modeling of an Automotive Turbocharger

One-dimensional wave-action engine models have become an essential tool within engine development including stages of component selection, understanding system interactions, and control strategy development. Simple turbocharger models are seen as a weak link in the accuracy of these simulation tools, and advanced models have been proposed to account for phenomena including heat transfer. In order to run within a full engine code, these models are necessarily simple in structure yet are required to describe a highly complex 3D problem. This paper aims to assess the validity of one of the key assumptions in simple heat transfer models, namely, that the heat transfer between the compressor casing and intake air occurs only after the compression process. Initially, a sensitivity study was conducted on a simple lumped capacity thermal model of a turbocharger. A new partition parameter was introduced αA, which divides the internal wetted area of the compressor housing into pre- and postcompression. The sensitivity of heat fluxes to αA was quantified with respect to the sensitivity to turbine inlet temperature (TIT). At low speeds, the TIT was the dominant effect on compressor efficiency, whereas at high speed αA had a similar influence to TIT. However, modeling of the conduction within the compressor housing using an additional thermal resistance caused changes in heat flows of less than 10%. Three-dimensional computational fluid dynamics (CFD) analysis was undertaken using a number of cases approximating different values of αA. It was seen that when considering a case similar to αA = 0, meaning that heat transfer on the compressor side is considered to occur only after the compression process, significant temperature could build up in the impeller area of the compressor housing, indicating the importance of the precompression heat path. The 3D simulation was used to estimate a realistic value for αA which was suggested to be between 0.15 and 0.3. Using a value of this magnitude in the lumped capacitance model showed that at low speed there would be less than 1% point effect on apparent efficiency which would be negligible compared to the 8% point seen as a result of TIT. In contrast, at high speeds, the impact of αA was similar to that of TIT, both leading to approximately 1% point apparent efficiency error.