Thermophysical Properties Of Fluids Research Articles

A data set characterizing thermal resistance at evaporation from sintered porous media

Passive cooling devices like heat pipes play an increasingly important role in a vast field of cooling applications. For optimal design of heat pipe-based cooling systems, it is crucial to perform reliable simulations which adequately account for thermal resistance of evaporation from the embedded capillary structure. In this study, a data set which provides thermal resistance for water evaporating from sintered porous media was obtained using a micro model. The open source software Surface Evolver was used to calculate 1638 liquid meniscus shapes in a cubic arrangement of sintered spheres for various porosities, water levels, and contact angles. A heat conduction model accounting for temperature dependent thermophysical fluid properties was developed to compute corresponding thermal resistances. Phase change at the liquid surface was modeled using expressions based on the thermal-energy-dominant limit of the statistic rate theory. The results show a significant dependence of thermal resistance on vapor temperature, wick geometry, contact angle, and the shape of contact line. 8316 data points suggest high vapor temperatures, small grain sizes, low porosities, and small contact angles for minimal thermal resistance. Furthermore, a method for utilizing the data set in device level macro models is outlined.

Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO2 and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m2. The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.

Numerical Investigations of Different Tubular Microreactor Geometries for the Synthesis of Polymers under Unmixed Feed Condition

AbstractTwo tubular microreactor geometries—straight tube reactor (STR) and coil flow inverter reactor (CFIR)—are numerically studied with unmixed feed condition for the synthesis of poly(methyl methacrylate) and poly(styrene) by free radical polymerization. A new transformation rendering model dimensionless in terms of concentration is used during computational fluid dynamics modeling. For decoupled case of constant fluid thermo‐physical properties (FTPP) (density, viscosity, and thermal conductivity), significant differences between the two types of reactor are observed. CFIR is found to achieve higher number‐average chain lengths (DPn) while keeping polydispersity index (PDI) lower compared to STR. For the case of coupled FTPP variables with conversion (XM) and temperature, XM and DPn are found to have similar trends as in decoupled case but have systematically higher values while PDI values follows a different trend. CFIR is found to show better mixing compared to STR under similar conditions, thus allowing a better control over PDI under unmixed feed condition. An anomalous case of increased mixing in STR at low diffusion coefficient for coupled case is also observed. Overall, the importance of modeling the coupled case is clearly emphasized.

Optimization of the thermodynamic and thermophysical properties of the gas turbine cycle working fluid

A gas turbine cycle working fluids group based on their production technology and application is presented. The energy parameters analysis procedure of unclosed and closed gas turbine cycles is described. The comparison of the effect of the working fluids thermophysical and thermodynamic properties variations on the useful work l0 of both unclosed and closed gas turbine cycles is presented. The effect of the fuel gas composition variation on the useful cycle work l0 is revealed. The effect of the working fluid thermophysical properties variation on the gas turbine cycle optimal combination of thermodynamic properties (T3 and P3 ) is presented.

Experimental evaluation of thermophysical properties of oil‐based titania nanofluids for medium temperature solar collectors

AbstractThermal oils are widely used as working fluids in the medium temperature heat transfer applications including concentrating solar thermal collectors. However, the weak thermal characteristics of these oils are major drawbacks in their successful application in the medium‐high temperature solar collectors. Fortunately, the emergence of nanotechnology has provided the opportunity to alter thermo‐physical properties of base fluids by adding small amount of sub‐micron size solid particles possessing better properties. This paper presents an experimental investigation of thermophysical properties of an oil‐based nanofluid to be used in the medium temperature solar collector for enhanced thermal energy transport. The colloidal suspensions were prepared by dispersing different weight fractions (0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1.0 wt.%) of Titania nanoparticles in Therminol‐55 oil using two‐step method. Shear mixing and high energy ultrasonication was employed to achieve uniform mixing and de‐agglomeration of the nanoparticles in order to enhance the stability of the colloidal suspensions. Thermophysical properties of the nanofluids were determined as a function of nanoparticles concentrations in the temperature range of 25 °C–130 °C. The experimental results demonstrated substantial improvement in thermal conductivity of the nano‐oils with an increase in nanoparticles loading which further enhanced at higher temperatures. Dynamic viscosity and effective density displayed a decreasing trend against rising temperature which indicate the effectiveness of these nanofluids for medium temperature heat supply. Nano‐oils with superior thermal properties can improve the performance of medium temperature solar thermal collectors.

Prediction accuracy of thermodynamic properties using PC-SAFT for high-temperature organic Rankine cycle with siloxanes

High-temperature organic Rankine cycle (ORC) with a heat source temperature over 250 °C can be used as the bottom cycle to form a combined cycle with a heat engine such as gas turbine or internal combustion engine. The accurate calculation of thermo-physical properties of the working fluid is of great significance for the performance prediction of an ORC system. PC-SAFT Equation of State (EoS) based on statistical theory can be used to estimate the thermo-physical properties of different working fluids and even to develop new working fluids. However, its accuracy needs to be investigated comprehensively. In this study, the prediction accuracy for the thermal properties based on PC-SAFT for linear and cyclic siloxanes is evaluated and the influence of the uncertainties on the performance of a high-temperature ORC is estimated. First, a program based on the PC-SAFT is developed to compute the properties of 8 siloxanes (MM, MDM, MD2M, MD3M, MD4M, D4, D5, D6). Then, the parameters of the EoS are fitted. Next, the accuracies of PC-SAFT along with the saturation lines and in the superheated regions are determined and compared with the results of Peng-Robinson (PR) EoS. Finally, the ORC performance is estimated using PC-SAFT. The results indicate that the accuracy of PC-SAFT is better than PR EoS. Only the deviation for the compressibility factor in the superheated region is slightly higher than PR. When PC-SAFT is used for the performance prediction of ORC system, the prediction accuracy increases apparently with the increase of molecular chain length. The deviations of the thermal efficiency by PC-SAFT relative to that of Refprop are basically less than 8% for the siloxanes except MM.

A successful local fluid-to-fluid similarity theory for heat transfer to supercritical pressure fluids: merits and limitations

A similarity theory, proposed with a limited success some years ago and subsequently refined in a more complex form in further efforts, has been applied in a recent published work to perform Direct Numerical Simulations (DNS) of heat transfer to turbulent flow, with four different fluids at supercritical pressure. The obtained results showed an exceptionally good behaviour of the theory in the addressed cases, suggesting that the initial proposal, though it had only limited success in the cases considered at that time, possibly caught some of the basic features to be preserved in scaling.The theory, based on dimensionless definitions that provided a reasonable degree of universality in the analysis of flow stability, found immediate difficulties to be applied with a comparable success to heat transfer problems. These difficulties mainly stemmed from the fact that, while it is relatively easy to scale fluid density, having a major role in stability analyses, it is definitely much harder to scale at a comparable level of accuracy the fluid thermo-physical properties, relevant in heat transfer. The very good results obtained in the recent work by DNS stimulated new reflections that shed light on the merits and limitations on the old theory.The present paper, starting from these recent results and discussing them in front of RANS calculations, is aimed to highlight the promising features of this theory, envisaging the missing steps that should be completed to make it more general, in order to give to its consequences a higher level of universality.

Experimental study on heat transfer performance of pulsating heat pipes with hybrid working fluids

Pulsating heat pipe (PHP), a passive two-phase heat transfer device, has excellent application prospects in electronic cooling, renewable energy systems, heat recovery devices, etc. An experimental study was conducted to investigate the heat transfer performance of PHP with hybrid working fluids, i.e., ethanol-water mixtures (20, 30, 40, 50 and 75 vol%), graphene nanofluids (0.1, 0.3, 0.5, 0.75 and 1.0 mg/ml) and surfactant aqueous solutions (25, 50, 100, 200 and 500 ppm). For this purpose, liquid-phase exfoliated graphene (LPEG) was dispersed in 40 vol% ethanol-water mixture to prepare graphene nanofluids with excellent stability. The thermophysical properties of hybrid working fluids were then measured. Experimental results indicate that non-ionic surfactant, Pluronic® F-127, can cost-effectively exfoliate graphene from graphite using high-shear mixer. It provides a favorable condition for the large-scale application of graphene in heat transfer devices. Among ethanol-water mixtures, 30 vol% ethanol-water mixture exhibits the best thermal performance, i.e., Ep=16.01%. Graphene nanofluids as working fluids can decrease thermal resistance up to 25.16%, and the higher thermal resistance for 1.0 mg/ml graphene nanofluids is attributed to the higher dynamic viscosity and nano sheets accumulations. The heat transfer performance of PHP with surfactant aqueous solution is related to both type and concentration of surfactant. Compared with other surfactants (SDS, CTAC, Pluronic® F-127, PVP), Triton X-100 surfactant aqueous solution appears to be more effective in the improvement of thermal performance with the average enhancement ratio of 13.56%. These experimental results may help to choose the more appropriate working fluids for PHP.

Natural convection heat transfer from a bundle of heated circular cylinders with staggered arrangement immersed in molten solar salt

A two-dimensional steady-state natural convection heat transfer from a bundle of closely spaced isothermally heated cylinders in a staggered arrangement immersed in molten solar salt was investigated in this study for a laminar flow regime using numerical analysis. The flow and temperature fields and heat transfer from the cylinder bundle were predicted as functions of tube spacing and flow conditions. It was determined that flow patterns and heat transfer are strongly affected by the transverse and longitudinal cylinder-to-cylinder spacing ST and SL, Rayleigh number and thermo-physical properties of the heat storage fluid. Performance maps in the form of contour diagrams for the average Nusselt number Nu and dimensionless heat transfer volumetric density are presented for a staggered bundle of circular horizontal cylinders immersed in a molten solar salt.The variation of the heat transfer enhancement for the staggered bundle of horizontal cylinders shows that there is an optimal transverse cylinder spacing resulting in the highest heat transfer enhancement. The heat exchanger geometry was optimized by determining the optimal set of the tube (cylinder) spacings SL/D and ST/D in the bundle resulting in the maximum volumetric heat transfer rate q ̃, and smallest size, and lowest cost of a heat exchanger. A set of correlations were developed for the maximum and optimum dimensionless heat transfer volumetric densities as functions of the Rayleigh number RaD. The analyzed geometries are important in the solar heating and storage technology, nuclear reactor safety, and waste heat recovery applications.

Operation characteristic of supercritical carbon dioxide-cooled reactor system under coordination control scheme

Nuclear power plant is completely independent of air and has high power density, which improves unmanned marine vehicle maneuvering performance, maximum speed, and concealment. The supercritical carbon dioxide-cooled reactor system that has the advantages of compact system and high cycle efficiency is a promising energy conversion system candidate for generation IV reactors. However, due to the dramatic variations of supercritical fluid thermophysical properties, the operation of supercritical carbon dioxide-cooled reactor system is complex and it is difficult to control it. In this article, a mathematical model of supercritical carbon dioxide-cooled reactor system was established using the modified RELAP5 code, and two traditional coordination control schemes were used to study the system operation characteristics under load drop transient condition. Each key system parameter response characteristics was investigated. Finally, based on the research results, an optimized operation scheme that was proved to have better system control effect was proposed.

Thermal conduction of the magnetic fluids mixing micrometer size particles

In this study, thermophysical properties of magnetic fluids mixing micrometer size spherical copper particles were investigated experimentally. To examine the behavior of dispersed micrometer size copper particles in the magnetized magnetic fluid, visualization experiments were performed by using a dark field microscope. From the visualization experiments, cluster formations of the copper particles in the magnetic field direction in the magnetic fluid under applied uniform magnetic field were observed. The thermal conductivity of the test fluid was measured by using the transient hot wire method. Thermal conductivity of the magnetic fluid with randomly dispersing copper particles is larger than the thermal conductivity of the base magnetic fluid. In case of the magnetic fluid mixed 12 vol% of spherical copper particles, it was estimated that the thermal conductivity in the field direction increases by 1.7 times or more of the thermal conductivity of the fluid in the absence of magnetic field. The Landau-Lifshitz equation for estimating the relationship between the thermal conductivity and the internal structure was expanded to consider spherical shape of the dispersed particles in the fluid and the particle distribution state was estimated by using the expanded Landau-Lifshitz equation.

THERMOPHYSICAL PROPERTIES OF DEEP EUTECTIC SOLVENT-CARBON NANOTUBES (DES-CNT) BASED NANOLUBRICANT

Lubrication is an important aspect when it comes to machinery and equipment in the industry where nanolubricants are extensively being explored as the future of lubricants. Nanolubricants are new-engineered fluids, which are dispersed with nanoparticles that aids in the interface movement by forming a protective layer hence reducing the wear and friction and dissipate the heat generated due to friction. In this research, a new synthetic lubricant is formulated by mixing two or more components of hydrogen bond donors and halide salt that achieves lower melting points compared to individual components. In this study, Glycerol and Polyethylene Glycol 600 (PEG) acts as the HBDs with common salt of Choline Chloride (ChCl). The nanolubricants were synthesised via two-step method by dispersing three different concentration of Functionalised Multi Walled Carbon Nanotubes (F-MWCNTs) of 0.01 wt%, 0.05 wt% and 0.10 wt% with the aid of ultrasonication. The stability of the suspensions were studied via optical microscopy and visual observation. The addition of F-MWCNT increased the resultant dynamic viscosity of the nanolubricant whereas the density was not much affected. Besides that, thermal conductivity showed positive enhancement for glycerol DES nanolubricant and negative enhancement for PEG DES nanolubricant. Thus, this study concludes that the thermophysical properties of DES base fluids and MWCNT have potential to be used as lubricants due to its enhanced physical properties.

Fundamental limits of jumping droplet heat transfer

Liquid-vapor phase-change cooling has a significant potential to facilitate the development of highly dense electronics by leveraging latent heat during the phase transition to remove heat from hotspots. A promising form of liquid–vapor phase-change cooling is coalescence-induced jumping droplet condensation, where droplet growth results in coalescence and gravity-independent jumping from the cold surface due to capillary-inertial energy conversion. Once the departed droplets reach the hotspot, heat is extracted via evaporation and through vapor return, subsequently spreading to the cold surface via condensation. Realizing the full potential of jumping droplet cooling requires a detailed understanding of the physics governing the process. Here, we examine the fundamental thermal and hydrodynamic limits of jumping droplet condensation. We demonstrate that jumping is mainly governed by the rate of droplet growth and fluid thermophysical properties. Timescale analysis demonstrates that the upper bound of water vapor jumping droplet condensation critical heat flux is ∼ 20 kW/cm2, significantly higher than that experimentally observed thus far due to surface structure limitations. Analysis of a wide range of available working fluids shows that liquid metals such as Li, Na, and Hg can obtain superior performance when compared to water.

Experimental comparison of the heat transfer of carbon dioxide under subcritical and supercritical pressures

The carbon dioxide (CO2) trans-critical cycle is widely used in refrigerating systems, air conditioning, and heat pumps. Heat transfer behaviors at subcritical and supercritical pressures play essential roles in trans-critical thermohydraulic systems. In past decades, boiling heat transfer at subcritical pressure and `single-phase' convective heat transfer at supercritical pressure have been broadly discussed; however, evaluation of the heat transfer difference of CO2 between supercritical and subcritical pressures remains lacking. In this study, experimental investigations are conducted to explore the heat transfer characteristics of the vertical flow of CO2 in a wide pressure range (6.2 MPa–8.0 MPa) within a circular tube (inner diameter = 5 mm). The range of operating conditions includes a mass flux (G) between 0 and 190 kg/m2s and a heat flux (q) of up to 105 kW/m2. Parametric effects (e.g., heat flux, inlet temperature, and mass flux) within these experimental conditions are also analyzed. Measurement results show that the inner-wall temperature at subcritical pressure is higher than that at supercritical pressure when a boiling crisis occurs. Closer to the critical pressure, a boiling crisis occurs more easily at lower heat fluxes, but the corresponding temperature peak declines. Higher temperatures are measured farther from critical pressure, indicating that overheating induced by boiling heat transfer at subcritical pressure is far more significant than that governed by the deterioration of convective heat transfer at supercritical pressure. Additionally, a qualitative analysis of physical mechanisms governing observed heat transfer differences is performed, confirming variations in fluid thermophysical properties and a bubble-like flow pattern.

Entropy Generation Minimization Analysis of Solar Salt, Sodium, and Lead–Bismuth Eutectic as High Temperature Heat Transfer Fluids

Abstract The amount of entropy generation in heat transfer devices impacts their operation economy and should therefore be minimized during the design phase. Entropy generation also depends on the individual thermophysical properties of the heat transfer fluid (HTF). An entropy generation minimization analysis of three different liquid coolants, namely, solar salt (SS), sodium, and lead-bismuth eutectic (LBE) is thus performed for fully turbulent flow in a circular tube under circumferentially uniform heat flux by considering a heat rate, inlet and outlet temperatures and heat flux densities typical of a concentrated solar tower plant. The Reynolds number is determined at which the proper combination of Nusselt number and friction factor minimizing the entropy generation, and consequently the exergy loss, is obtained as well as the best thermodynamically performing coolant fluid over the operating range of Reynolds numbers. Sodium can operate at 60% lower entropy generation than solar salt while providing a smaller wall-to-bulk temperature difference. Despite its high thermal conductivity, LBE performs similarly to solar salt. However, it can be advantageous compared to solar salt if operated at higher temperatures that cannot be achieved by the latter due its thermal stability limit.

Investigation of thermo-physical fluid properties effect on binary fluid ejector performance

Supersonic Ejector (SE) is a thermally-driven fluidic compressor that replaces the electro-mechanical compressor in Reverse-Rankine refrigeration/heat pump cycles. These widely used thermal cycles account for billions of kWh of electric energy and produce hundreds of millions of metric tons of atmospheric carbon yearly in North America.As compared to mechanical compressors, ejectors are simple mechanical devices with no moving parts. It can be configured to provide residential and commercial space heating/cooling and water heating, industrial process heating/cooling, industrial distillation/desalination and drying. Rather than electricity, SE-based systems can make direct use of many forms of thermal energy including solar thermal, waste heat, biogas, or natural gas, depending on emission targets, price, or availability. It is known that the SE systems have a lower thermal efficiency as compared to mechanical compressor because of its lower performance at high compression ratios.Highly efficient ejector would thus play a critical role in unlocking the wide spread use of renewable energy such as waste heat, solar thermal, and geothermal. Even in the absence of renewable energy, such a device would enable fuel switching from electricity to natural gas, which would save 65 to 75% on energy costs, and relieve the power grid during peak times.In the present study, Computational Fluid Dynamics (CFD) is used to study the effect of fluids thermo-physical properties including molecular mass, viscosity and specific heat ratio on the performance of an ejector for distillation applications. It is found that molecular mass and specific heat ratio can significantly affect the entrainment ratio of the ejector and consequently the COP of the refrigeration system.

Numerical investigation on heat transfer of water spray cooling from single-phase to nucleate boiling region

In the present study, spray cooling heat transfer characteristics were numerically investigated under the heat flux ranging from 50 to 170 W/cm2. Effect of nozzle height and spray pressure was also studied in order to reveal the microscopic mechanisms of the heat transfer enhancement from single-phase to nucleate boiling regions. A two-phase flow heat transfer model was adopted based on the Euler-Lagrangian approach. The dependence of thermo-physical properties of fluid on the temperature was also taken into account. The comparison between simulated wall temperature and experimental data demonstrates a satisfactory agreement. The results show that the heat transfer coefficient increases monotonously with the heat flux. In the nucleate boiling region, the wall film is thinner with a smaller velocity compared with those in the single-phase region. Spray cooling heat transfer is enhanced evidently as the nozzle height decreases and the spray pressure increases. The most important feature which separates the present study from the literature is that the numerical simulation in this paper presents some detailed microscopic characteristics, e.g. the wall film thickness and velocity, which play an important role in spray cooling heat transfer.

Exploring Anomalous Fluid Behavior at the Nanoscale: Direct Visualization and Quantification via Nanofluidic Devices.

Nanofluidics is the study of fluids under nanoscale confinement, where small-scale effects dictate fluid physics and continuum assumptions are no longer fully valid. At this scale, because of large surface-area-to-volume ratios, the fluid interaction with boundaries becomes more pronounced, and both short-range steric/hydration forces and long-range van der Waals forces and electrostatic forces dictate fluid behavior. These forces lead to a spectrum of anomalous transport and thermodynamic phenomena such as ultrafast water flow, enhanced ion transport, extreme phase transition temperatures, and slow biomolecule diffusion, which have been the subject of extensive computational studies. Experimental quantification of these phenomena was also enabled by the advent of nanofluidic technology, which has transformed challenging nanoscale fluid measurements into facile optical and electrical recordings. Our groups' focus is to investigate nanoscale (2 to 103 nm) fluid behaviors in the context of fluid mechanics and thermodynamics through the development of novel nanofluidic tools, to examine the applicability of classical equations at the nanoscale, to identify the source of deviations, and to explore new physics emerging at this scale. In this Account, we summarize our recent findings regarding liquid transport, vaporization, and condensation of nanoscale-confined liquids. Our study of nanoscale water transport identified an additional resistance in hydrophilic nanochannels, attributed to the reduced cross-sectional area caused by the formation of an immobile hydration layer on the surfaces. In contrast, a reduction in flow resistance was discovered in graphene-coated hydrophobic nanochannels, due to water slippage on the graphene surface. In the context of vaporization, the kinetic-limited evaporation flux was measured and found to exceed the classical theoretical prediction by an order of magnitude in hydrophilic nanochannels/nanopores as a result of the thin film evaporation outside of the apertures. This factor was eliminated by modifying the hydrophobicity of the aperture's exterior surface, enabling the identification of the true kinetic limits inside nanoconfinements and a crucial confinement-dependent evaporation coefficient. The transport-limited evaporation dynamics was also quantified, where experimental results confirmed the parallel diffusion-convection resistance model in both single nanoconduits and nanoporous systems at high accuracy. Furthermore, we have extended our studies to different aspects of condensation in nanoscale-confined spaces. The initiation of condensation for a single-component hydrocarbon was observed to follow the Kelvin equation, whereas for hydrocarbon mixtures it deviated from classical theory because of surface-selective adsorption, which has been corroborated by simulations. Moreover, the condensation dynamics deviates from the bulk and is governed by either vapor transport or liquid transport depending on the confinement scale. Overall, by using novel nanofluidic devices and measurement strategies, our work explores and further verifies the applicability of classical fluid mechanics and thermodynamic equations such as the Navier-Stokes, Kelvin, and Hertz-Knudsen equations at the nanoscale. The results not only deepen our understanding of the fundamental physical phenomena of nanoscale fluids but also have important implications for various industrial applications such as water desalination, oil extraction/recovery, and thermal management. Looking forward, we see tremendous opportunities for nanofluidic devices in probing and quantifying nanoscale fluid thermophysical properties and more broadly enabling nanoscale chemistry and materials science.

Natural convection heat transfer from a bundle of in-line heated circular cylinders immersed in molten solar salt

A two-dimensional steady-state natural convection heat transfer from a bundle of closely spaced isothermally heated cylinders in the in-line arrangement was investigated in this study for a laminar flow regime using numerical analysis. It was determined that flow patterns and heat transfer are strongly affected by the transverse and longitudinal cylinder-to-cylinder spacing ST and SL, Rayleigh number, and thermo-physical properties of the heat storage fluid. Performance maps in the form of contour diagrams for the average Nusselt number Nu and dimensionless heat transfer volumetric density q ̃, and corresponding correlations are presented for an in-line bundle of 9 × 5 horizontal cylinders immersed in a molten solar salt.The variation of the heat transfer enhancement for the in-line bundle of horizontal cylinders shows that there is an optimal cylinder spacing resulting in the highest heat transfer enhancement. The heat exchanger geometry was optimized by determining the optimal set of the tube (cylinder) spacings SL/D and ST/D in the bundle resulting in the maximum volumetric heat transfer rate q ̃, and smallest size and lowest cost of a heat exchanger. A set of correlations were developed for the maximum and optimum dimensionless heat transfer volumetric densities as functions of the Rayleigh number RaD. The analyzed geometries are important in the solar heating and storage technology, nuclear reactor safety, and waste heat recovery applications.

The Effects of Some Thermo-physical Properties of Fluid on Heat and Mass Transfer Flow Past Semi-infinite Moving Vertical Plate with Viscous Dissipation

This paper examines the effect of some thermo-physical properties of fluid on heat and mass transfer flow past semi-infinite moving vertical plate. The fluid considered is optically thin such that the thermal radiative heat loss on the fluid is modeled using Rosseland approximation.The governing partial differential equations in dimensionless forms are solved numerically using the Method of Lines (MOL). The velocity, the temperature, and the concentration profiles of the flow are discussed numerically and presented. Numerical values of the skin-friction coefficient, Nusselt number, and Sherwood number at the plate are discussed numerically for various values of thermo-physical parameters and they are presented by the tables.The result shows that an increase in thermal radiation causes increase in velocity and temperature profiles of the flow, thus, the thermal radiation intensifies the convective flow. Also, an increase in Soret number causes increase in velocity and concentration profiles of the flow while the effect is negligible on temperature profile distribution. Similarly, an increase in Dufour number causes increase in velocity and temperature profiles of the flow.