Variable Thermophysical Properties Research Articles

Novel heat pump assisted distillation configurations for energy and economic savings and their application on propylene/propane and I-butane/N-butane separation systems

Various configurations of vapor recompression heat pumps were previously proposed for enhancing its thermodynamic and/or economic performance. The outcomes of prior publications were documented using fundamentally distinct separation systems, making it challenging for process engineers to decisively determine the optimal processing options for a particular separation task. This difficulty arises from the variations in thermophysical properties exhibited by the materials involved. Despite limited research on bottom flashing heat pumps, they offer advantages over vapor recompression such as lower operating pressure and temperature in process sections. This can result in cost savings and safer operation. In this paper, novel configurations of bottom flashing are proposed, based on the ideas and schemes, previously proposed for vapor recompression. The novel processing systems were proposed and applied on propylene/propane and i-butane/n-butane separation processes as samples of close-boiling distillation systems. This study showed that the bottom flashing systems showed greater economic performances compared to vapor recompression for i-butane/n-butane separation. total annual costs savings of 19.0% was obtained, using the novel configuration of heat recuperated bottom flashing configuration on isobutane/n-butane separation. Also, annualized costs of the systems based on bottom flashing were 10.9%, 4.2% and 7.3% lower than their equivalent vapor recompression processes.

Phase change in a one-dimensional functionally graded material

Functionally graded materials (FGMs) are a class of materials with engineered microstructure to achieve spatial variation in properties so as to serve a desired function. While FGMs have been well-researched for their structural properties, there is a lack of investigation of FGMs in solid-liquid phase change heat transfer processes. A well-designed functionally graded phase change material (FG-PCM) may overcome some of the well-known limitations of the phase change process. In order to investigate this possibility, this work presents a theoretical model to predict phase change propagation in a one-dimensional FG-PCM containing an arbitrary spatial variation in thermal conductivity, thermal diffusivity and latent heat. The problem is solved using the method of eigenfunction expansion to determine the temperature distribution, wherein, spatial variation in thermophysical properties listed above is accounted for by deriving a first-order matrix ordinary differential equation in the coefficients of the series solution. Results are found to correctly reduce to the well-known Stefan solution under special conditions. The impact of variation in thermophysical properties is investigated in detail. It is found that while spatial variation in thermal conductivity and latent heat has a significant effect on the rate of phase change, the impact of the thermal diffusivity function is negligible for realistic values of the Stefan number. The effect of the Stefan number governing this problem on the rate of melting is investigated. The nature of phase change under linear (1+aξ), polynomial (1+aξn) and exponential (eaξ) functional forms of thermophysical properties is investigated. It is shown that an appropriate choice of these spatial functions may help overcome the well-known self-limiting nature of phase change and produce a nearly-linear phase change propagation curve. For example, results show 33% increase in phase change propagation for a linear (1+5ξ) variation in thermal conductivity compared to baseline. The theoretical tools developed here may help design and optimize functionally graded PCMs towards improved phase change heat transfer in applications of practical interest.

Thermo-hydraulic and entropy generation analysis of nanofluid flow with variable properties in various duct cross sections: a 3-D two-phase approach

A three-dimensional computational fluid dynamics (CFD) study is carried out to explore the effect of duct cross section on the thermo-hydraulic performance of various ducts. A finite volume-based scheme with an SST k-omega model and mixture model (two-phase model) was used to obtain more realistic results. A two-phase mixture model was used to consider the movement between base fluid and nanoparticles. Al2O3 nanoparticle having a volume fraction of 0.01% and 42 nm as particle size, the heat transfer and friction factor characteristic are studied for turbulent flow regime (3000 < Re < 9000) with variable thermo-physical properties. A maximum enhancement of 86% in heat transfer rate is obtained for the serpentine duct compared to the conventional circular duct at Re = 4500. Owing to a significantly lower increase in pressure drop, the elliptical duct has the highest thermo-hydraulic performance parameter of 1.54 relative to the circular duct. Further, to analyze the heat transfer quality, the entropy generation rate is studied, and it is observed that the square duct reported the highest with an increase of 60% and the elliptical duct the lowest with a reduction of 54% relative to the circular duct. This study can aid in choosing the duct geometry to enhance the heat transfer rate with nanofluid for applications such as solar-thermal, heat exchangers, etc.

On the continuous nature of phase change in near-critical carbon dioxide

Unique boiling behavior at near-critical conditions can leverage the large heat transfer coefficients associated with flow boiling while mitigating some of the disadvantages encountered in two-phase flows, presenting opportunities for enhancing the stability and effectiveness of cooling systems. Contemporary understanding treats subcritical boiling and supercritical heat transfer as two distinct regimes, separated by the critical point. Recent studies investigating microchannel flow boiling near the critical point challenge this notion, suggesting a significant alteration in subcritical phase change heat transfer mechanisms, predominantly driven by variations in thermophysical properties near the critical point. However, a generalized understanding of heat transfer behavior and the phase change physics near the critical point is lacking. The current work presents a new perspective to elucidate underlying fluid phenomena associated with near-critical (0.85<PR<1.02) microscale (Dh=500μm) phase change over parameterized experimental conditions (G=100−1000kgm−2s−1; q″=2−100Wcm−2). Through high-speed (8000fps) side-view microscopic imaging and simultaneous heat transfer measurements, unique flow behavior was discovered at PR>0.95, including supercritical-like phenomena and evidence of homogeneous nucleation which form the physical basis for altered heat transfer behavior. The visualized flow physics and heat transfer behavior suggest a continuous transition from subcritical phase change to supercritical mechanisms near the critical point. As homogeneous nucleation becomes the dominant phase change mechanism, the near-wall heat transfer condition transitions from nucleate boiling to single-phase vapor convection. However, due to significant thermophysical property variations near the critical point, this single-phase convection condition exhibits fluid physics and heat transfer behavior virtually indistinguishable from supercritical conditions. These insights significantly disrupt the conventional understanding of a sharp transition in fluid behavior at the critical point. The gradual change in flow boiling mechanisms near the critical point presents opportunities for application-specific tuning of heat transfer performance through pressure variation alone.

Subsurface Thermophysical Properties of Europa’s Leading and Trailing Hemispheres as Revealed by ALMA

We present best-fit values of porosity—and the corresponding effective thermal inertiae—determined from three different depths in Europa’s near-subsurface (∼1–20 cm). The porosity of the upper ∼20 cm of Europa’s subsurface varies between 75% and 50% (Γeff ≈ 50–140 J m−2 K−1 s−1/2) on the leading hemisphere and 50%–40% (Γeff ≈ 140–180 J m−2 K−1 s−1/2) on the trailing hemisphere. Residual maps produced by comparison with these models reveal thermally anomalous features that cannot be reproduced by globally homogeneous porosity models. These regions are compared to Europa’s surface terrain and known compositional variations. We find that some instances of warm thermal anomalies are co-located with known geographical or compositional features on both the leading and trailing hemisphere; cool temperature anomalies are well correlated with surfaces previously observed to contain pure, crystalline water ice and the expansive rays of Pwyll crater. Anomalous regions correspond to locations with subsurface properties different from those of our best-fit models, such as potentially elevated thermal inertia, decreased emissivity, or more porous regolith. We also find that ALMA observations at ∼3 mm sound below the thermal skin depth of Europa (∼10–15 cm) for a range of porosity values, and thus do not exhibit features indicative of diurnal variability or residuals similar to other frequency bands. Future observations of Europa at higher angular resolution may reveal additional locations of variable subsurface thermophysical properties, while those at other wavelengths will inform our understanding of the regolith compaction length and the effects of external processes on the shallow subsurface.

HARK formulation for entropy optimized convective flow beyond constant thermophysical properties

Background and objectiveOhmic heating has a pivotal role in food processing industry. For example, Ohmic heating in chocolate industry for cocoa beans shell is significant substrate for bioactive compounds recovery sustainability. In addition, the Soret and Dufour effects have importance in chemical and petroleum processes involving separation treatments, convective movements in lakes and solar ponds, solidification, humidity migration in building, drying processes, crystal growth and many others. In view of such applications here we discuss the hydromagnetic mixed convection flow with variable thermophysical properties. Formulation for Reiner-Rivlin material is made. Entropy generation in presence of dissipation, magnetohydrodynamics and radiation are discussed. Thermal relation consists of heat generation and dissipation. Soret and Dufour outcomes are under consideration. Ohmic heating is attended. Binary chemical reactive flow has been accounted. Convective conditions are implemented. MethodologyBy utilization of appropriate variables, we get the ordinary differential equations. ND-solve method is implemented to provide computations. ResultsEvaluation of different variables for flow, concentration, entropy rate and temperature are studied. Analysis of surface drag force and solutal and thermal transport rates via influential variables are examined. An augmentation in velocity is noted for variable viscosity and material parameters. Reverse effect holds for thermal field and flow with variation in magnetic variable. An intensification in thermal distribution and entropy is found through radiation and thermal Biot number. A reverse impact is observed for concentration through Schmidt and Soret numbers. Higher variable mass diffusivity parameter correspond to improves concentration and entropy rate. Brinkman number has increasing trend for entropy rate.

INFLUENCE OF BUBBLE GROWTH AND LIQUID FILM INSTABILITIES ON DROPLET IMPACT PHENOMENA UNDER SATURATED BOILING REGIMES

Evaporation and boiling are processes that occur in many industrial applications involving multiphase flows. For liquid films, however, studies are scarce regarding heat and mass transfer mechanisms and require further research. The main objective of this work is to evaluate bubble formation and detachment, followed by the impact phenomena. Therefore, an experimental setup was built and adapted for this purpose. A borosilicate glass impact surface is placed over a heat source, which consists of an aluminum block with four embedded cartridge heaters that heat the liquid film by conduction. Water and n-heptane are the fluids adopted for the experimental study, as the differences in thermophysical properties allow for a wider range of experiments. Study cases include dimensionless temperatures of &amp;theta; &amp;#62; 0.6 for similar impact conditions. In terms of bubble formation, n-heptane displays smaller bubble diameters and higher release rates, whereas water exhibits larger bubbles and lower rates. Qualitatively, liquid film temperatures close to the saturation temperature do not reveal a direct influence on the crown development and posterior secondary atomization. For later stages of the impact, the central jet height and breakup are influenced by the film temperature, which is associated with the variation of thermophysical properties.

Multi-agent simulation of doughnut deep fat frying considering two domain heating media and sample flipping

Two domain heating media and sample flipping during processing were considered when developing an agent-based model to explain coupled heat and mass transfer phenomena during deep fat frying of doughnuts. The model was validated by comparing the moisture content, oil content and temperature profiles obtained from the experimental results with those obtained from the model. Results of this study showed that the water content of crumb raised to 60% (based on dry weight) whereas, it decreased to less than 10% in the case of doughnut crust during deep fat frying. Simulated profile of oil penetration illustrated that the oil content of different parts of crust were not equal and were affected by frying temperature and crust structure. In general, as the surface of doughnut (a porous material) was heated from the surface, evaporation zones were formed in the thinner parts of the crust and gradually formed oil penetrating areas. Moreover, experimental and simulated data indicated that flipping of samples in the middle of processing time had an important effect on heat and mass transfer during frying. Variation of thermophysical properties in each part of doughnut had a unique behavior. The changes in the density, specific heat capacity and thermal conductivity of crumb followed a sigmoid pattern; whereas, a dominant falling rate period with some variations was observed in crust. Moreover, any changes in moisture content and temperature of crust occurred faster than the crumb. The output of simulation was in a good agreement with the experimental data. With the power of simulation now available for design, the results of this study greatly improve the design of fried foods and frying processes.

Farklı Isı ve Basınç Koşulları Altında Düz Bir Boru İçin Tek Yönlü Akışkan-Yapı Etkileşimleri Analizi

Numerical studies on stress, deformation, and damages due to fluid flow have been highly carried out using Fluid-Structure Interaction (FSI) in recent years. FSI is highly efficient in investigating a solid domain deformed by the fluid flow. In this study, a one-way fluid-structure interaction study is performed by a straight pipe under different pressure and thermal conditions. Here, the thermophysical properties of the fluid and mechanical properties of the solid domain can be subjected to change during fluid flow. An aluminum straight pipe with a 1 mm wall thickness is operated under 1 Bar, 5 Bar, and 10 Bar with three different surface temperatures -10ºC, 20ºC, and 50ºC. This study aims to investigate the structural variation of aluminum by the temperature and pressure change of operating fluid in the pipe. Variation of thermophysical properties of fluid by heated pipe surface is integrated into the numerical analysis by generated functions. Numerical analysis showed that the variation of temperature in operating fluid highly affects the fluid characteristic and the structural response of the solid domain by different temperatures. An increase in the operating pressure caused maximum deformation to approximately %100 from 1 Bar to 5 Bar, and approximately %120 from 1 Bar to 10 Bar for the adiabatic process as expected but in the heating conditions stress is nearly three times higher than cooling conditions. As a result, one-way FSI solutions are highly effective in investigating the deformed solid domain as a result of flow, thermal, and operating conditions.

A compressible flow solver for turbomachinery of the real gases with strongly variable properties

Accurate prediction of real gas flows with strongly variable properties is an essential prerequisite for turbomachinery performance analysis. This paper presents a density-based solver for real gas flows in turbomachinery. The solver employs a third-order discretization scheme to improve computational accuracy. Real gas equations of state and look-up table are utilized in the solver to account for the strong nonlinear variations of thermophysical properties. The preconditioning method is adopted for simulations, including low-Mach flow regions, to address convergence difficulties. The implicit Time Consistent Preconditioned Gauss-Seidel Scheme (TCPGS) is employed for the time-stepping. The solver is validated by transonic compressor cascade wind tunnel experiments. Numerical test cases are adopted to evaluate the solver performance, including NASA Rotor 67, NASA Rotor 37, real gas flows in the Laval nozzle, and compressor seal. It has been proved that the numerical test results are consistent with published technical data. The advantages of this solver include numerical stability, computational efficiency, and physical accuracy, which indicate its applicability in the turbomachinery design process for real gases.

Effects of CO2 variable thermophysical properties and phase behavior on CO2 geological storage: A numerical case study

CO2 injection into the reservoir can effectively reduce greenhouse gas emissions. The thermophysical parameters and phase behavior of CO2 are highly sensitive to pressure and temperature, resulting in drastic change in wellbore and reservoir, and thus affecting the CO2 storage effect. To study the impact of CO2 thermophysical properties and phase behavior on CO2 geological storage, we established a fully coupled 1D+3D wellbore coupled 3D reservoir model. Based on the analysis of non–isothermal flow characteristics, CO2 thermophysical properties and phase behavior, the influence of engineering parameters on the key factors of CO2 geological storage is discussed. The findings indicate that the density and thermal capacity of CO2 are the most important thermophysical parameters that affect wellhead pressure and bottomhole temperature respectively. The efficiency of CO2 storage is greatly influenced by the density and viscosity of CO2, more so than any other thermophysical parameters. CO2 phase behavior and permeability change increase CO2 seepage capacity by 3.50 % and 8.79 % respectively. Engineering parameters will affect the thermophysical properties and permeability of bottom–hole CO2, thereby affecting the injectability and storage efficiency of CO2. This research can offer suggestions and theoretical groundwork for the improved design of CO2 geological storage.

Development and validation of a local thermal non-equilibrium model for high-temperature thermal energy storage in packed beds

High-temperature thermal energy storage (TES) in packed beds is gaining interest for industrial energy recovery. The wide range of temperature distributions causes significant variations in thermophysical properties of the fluid and solid phases, leading to inaccuracies of classical TES models and heat transfer correlations. The objective of this work is to develop and validate a detailed but pragmatic model accounting for high-temperature effects. Based on a literature survey spanning over several communities interested in high-temperature porous media, we propose a generic local thermal non-equilibrium model for granulate porous media accounting for conservation of mass, momentum and energy (two-equation temperature model). The effective parameters needed to inform the model are the effective thermal conductivities of the different phases and the heat transfer coefficient. An experimental-numerical inverse analysis method is employed to determine these parameters. A dedicated experimental facility has been designed and built to study a model granulate made of glass bead of 16 mm diameter. Experiments are conducted using the Transient Single-Blow Technique (TSBT) by passing hot air (ranging from 293 K to 630 K) through cold particles at various mass flow rates, covering a Reynolds number range of 58 to 252. The new model was implemented in the Porous material Analysis Toolbox based on OpenFoam (PATO) used to compute the transient temperature fields. Two optimization algorithms were employed to determine the parameters by minimizing the error between experimental and simulated temperatures: a Latin Hypercube Sampling (LHS) method, and a local optimization method Adaptive nonlinear least-squares algorithm (NL2SOL). The results indicate that the value of heat transfer coefficient hv in the two-equation model falls in the range of 1.0 × 104∼1.93× 104 W/(m3 K) under the given conditions. The axial dispersion gas thermal conductivity was found to be around 5.9 and 67.1 times higher than the gas thermal conductivity at Peclet numbers of around 55 and 165, respectively. Furthermore, two improved correlations of Nusselt number (Nu=2+1.54Re(T)0.6Pr(T)1/3) and of axial dispersion gas thermal conductivity (kdis,∥=0.00053Re(T)2.21Pr(T)⋅kg) are proposed and validated for a range of Reynolds number from 58 to 252. The overall approach is therefore validated for the model granulate of the study, opening new perspectives towards more precise design and monitoring of high-temperature TES systems.

GenEOS: An accurate equation of state for the fast calculation of two-phase geofluids properties based on gene expression programming

Numerical simulation of two-phase multicomponent flows requires solving continuity, momentum, energy, and transport equations. Typically, these conservation equations are solved for computing the main variables of pressure, enthalpy, velocity, and composition. Variation of thermophysical properties (e.g., density, viscosity, etc.) as functions of the main variables necessitates introducing equations of state (EOS) to the modeling scheme, equating the number of unknowns and equations. The problem arises here as almost all the available EOSs in the literature receive temperature as an input, which is not a main variable. Guessing temperature, as an unknown input, imposes more iterations on the already iterative algorithm of the EOS and increases the computational cost. The primary focus of this study is to provide highly-precise, but fast EOS scheme for calculating two-phase fluid properties using artificial intelligence algorithms. In the first step, a Fugacity-Activity model is implemented to supply a supervised learning algorithm with a large dataset. The provided data are fed into a machine-learning (ML) model called gene expression programming (GEP). The outputs of this GEP model are high-preciseness explicit formulas for non-iterative computing of temperature and equilibrium constants. Testing the proposed GEP equations for 1,000,000 arbitrary sets of inputs revealed high accuracy in predicting desired outputs (e.g., < 0.6% error in calculating temperature). Implementing GEP equations in modeling platforms can result in ∼90% reduction in EOS-related computational cost. This ML-based EOS is a transparent box for computing thermophysical properties of two-phase mixtures containing H2O, CO2, CH4, N2, H2S, NaCl, KCl, CaCl2, and MgCl2.

Mesoscoping simulation of viscoelastic hybrid nanofluid having variable thermophysical properties in an inverted T-shaped enclosure with a localized heater

This research looked at a viscoelastic Bingham hybrid Al2O3-Ag/water nanofluid’s natural convection in an inverted-T-shaped enclosure. It has been done using the mesoscopic simulation via the multiple-relaxation-time (MRT)-lattice Boltzmann method (LBM). And it employed highly effective graphics processing unit (GPU) computing to proceed with the investigation. The hybrid nanofluid’s viscosity is determined by the size of the nanoparticles and the velocity gradient, and the thermal conductivity is the function of fluid temperature. A uniform temperature boundary condition is used to heat the middle of the enclosure’s bottom wall. Other portions of the bottom and top walls are adiabatic, where the right and left walls are kept cold. Different parameters, namely the Bingham number (0≤Bn≤1), Rayleigh number (Ra=104, 105, 106), and nanoparticles volume fraction (ϕ=0,0.02,0.04), and heated sources’ size (ϵ=1/5,2/5,3/5,4/5) are applied to investigate and observe the natural convection of the fluid. Numerical outcomes are attained from the simulation and used to generate contours, figures, graphs of velocity temperature, average Nusselt number, local Nusselt number, isotherms, and streamlines. Based on the outcomes, it is observed that enhancements in Rayleigh number, heated sources’ size (ϵ), and volume fraction (ϕ) speed up the flow of the fluid. On the other hand, the Bingham number’s increment slows down the flow and decreases the value of Nu¯. The average Nusselt number increases 170.25% while increasing the length of bottom heater from ϵ=1/5 to ϵ=4/5 at ϕ=0.02, Bn=1, and Ra=106. This configuration can be applicable to electrical system cooling, ventilation, heat-exchanging system and in many other practical implements.

DNS investigation of flow and heat transfer characteristics of supercritical carbon dioxide over a backward-facing step

This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO2 forced convection, and SCO2 mixed convection. All cases were conducted with an inlet Reynolds number of 4805, and the expansion rate of BFS is set at 1.5. Mean statistics like velocity, temperature, Reynolds stress, turbulent heat flux, and thermophysical properties are given and discussed. The results show that the turbulent and heat transfer characteristics of SCO2 over a backward-facing step are significantly different from those of conventional fluids due to the dramatic changes in its thermophysical properties near the pseudo-critical temperature. In the case of SCO2 forced convection, though the large thermophysical property variations do not have an obvious impact on the mean flow field, the near-wall mean density decreases largely and thus Reynolds stress and turbulent heat flux near the wall are reduced significantly, which in turn leads to a substantial reduction in the local skin friction coefficient and Nusselt number. In the SCO2 mixed convection, buoyancy substantially distorts the flow, resulting in a significant reduction in the size of the recirculation zone. Simultaneously, buoyancy acts as an additional driving force leading to the formation of a wall jet. These combined effects induce intensified turbulence near the heated wall, which enhances the turbulent heat flux and facilitates efficient heat transfer. The instantaneous vortical structures based on Q criterion also clearly show the intensified turbulence in the near-wall region.

Study on the convective heat transfer characteristics of supercritical CO2 in mini-channels under unilateral heating conditions for application in a compact solar receiver

The mini-channel in compact solar receiver deliver the high performance in both heat transfer (HT) and pressure resistance. However, the severe variation of thermophysical properties of supercritical CO2 (S-CO2) coupling with the complex nonuniform distribution of solar flux leads to the abnormal flow and heat transfer (FHT) performance of S-CO2 in mini-channels. The present work numerically investigated the detailed FHT of S-CO2 in the horizontal mini-channel with one-side heating. The impacts of buoyancy and flow acceleration on FHT of S-CO2 are demonstrated in detail. The results show that the buoyancy effects can lead to the enhancement on circumferentially average heat transfer performance of S-CO2 while the local deterioration at the top. In contrast, the thermal acceleration effects can worsen the S-CO2 HT. The buoyancy and thermal acceleration effects can be ignored when Ri ≤ 2.7 × 10−3 and Ac ≤ 7.8 × 10−8, respectively. The empirical formula for S-CO2 HT in the mini-channel under single-side heating conditions is developed, with a maximum error of 4.5%.

On the flow field and heat transfer characteristics of a supercritical CO2-cooled microchannel heat sink under high heat flux conditions

Supercritical CO2 (sCO2) is a suitable choice as a coolant due to its favorable thermodynamic characteristics (lower density and viscosity, and higher specific heat) compared to water. In the current investigation, sCO2 is considered a coolant in a micro heat sink under high heat fluxes 75, 100, 125, and 150 W/cm2 and the operating pressure 8 MPa. The effects of the operating conditions, including the change in coolant mass flux (1250–3125 kg/s-m2), the change in the inlet temperature (305-308 K) under the mentioned heat fluxes on the heat transfer coefficient, thermal resistance, pressure drop, pumping power, and performance evaluation criteria (PEC) are investigated numerically using the finite volume method. Results indicated that thermohydraulic performance of the sCO2-cooled heat sink depends on the inlet coolant temperature and the applied heat flux due to the dramatic variation of the local thermophysical properties. It was found that increase in the heat flux from 75 to 150 W/cm2 results in increase of the pumping power by 48%, diminishing the PEC from 2.35 to 0.5. Overall, sCO2-cooled heat sink performs better than water-cooled heat sink if the mass flow rate is high and the inlet temperature is below the pseudocritical temperature. Otherwise, using sCO2 is not beneficial, especially under higher heat flux conditions.

Forced convective flows of Casson hybrid nanofluid and entropy production over diverging channel with variable thermophysical properties

The heat transfer occurring on boundary layer flows during dilute suspension of Cu–Al2O3 nanoparticles-based non-Newtonian Casson hybrid nanofluid over diverging channel is to be characterized in this model. The governing equations comprise continuity, momentum, energy and concentration equations which incorporated variable viscosity and magnetic effects in the momentum equation, variable thermal conductivity, chemical reaction, thermal radiative heat flux, uniform magnetic field, Joule heating and viscous dissipative effects in the energy equation and Brownian motion and thermophoretic effects in the concentration equation. The modeled equations are nondimensionalized using nonsimilar variables and linearized using quasilinearization technique. The system of linearized partial differential equations is solved numerically using implicit finite difference and successively iterated with the help of Varga’s algorithm. The physical impacts of viscous dissipation parameter (Ec), Brownian motion ([Formula: see text]) on velocity, temperature, concentration and Schmidt number (Sc) influences on skin friction, Nusselt number, Sherwood number profiles, Bejan lines and total entropy production profiles are simulated graphically. The velocity profiles are enhanced and the temperature profile declines for augmented values of Eckert number. Moreover, for augmented values of Schmidt number the heat and mass transfer rates are enhanced and the Bejan lines dropped a decreasing trend whereas total entropy production is augmented near the wall region. The improved values of the Schmidt number physically increased Sc in the heat transfer rate. The ascending profile of the mass transfer rate with increased values of the viscous dissipation parameter Ec demonstrated that the graph is raised for larger values of the viscous dissipating parameter (Ec). The physical behavior of incremental mass transfer rate led to the conclusion that the relative diffusivity of nanoparticles is characterized by both nondimensional numbers Ec and Sc which are directly proportional to the viscous forces.

The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation for the CO2 around pseudo-critical region

Trans-critical CO2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When (Δτa/τw)<1&(Δτ/τw)tot>1 or 0.1<(Δτ/τw)tot<1, the turbulence is partially re-laminarized. When(Δτa/τw)>1&(Δτa/τw)>>(Δτbo/τw), the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary . The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects.

Numerical analysis of mixed convection phenomena in heat transfer to supercritical pressure carbon dioxide inside a horizontal miniature tube

To clarify and evaluate the influence of buoyancy force on the heat transfer to SCO2 flowing in a horizontal miniature tube, in this study, mixed convection phenomena with strong variations in thermophysical properties were numerically studied using an in-house code. The numerical model was validated by comparing the predicted wall temperature and available experimental data obtained for SCO2 at different heat fluxes, and the results showed good agreement. The effects of the heat-to-mass flux ratio on the heat transfer characteristics and heat transfer mechanism were investigated under operating conditions having mass flux, heat flux, pressure, and inlet temperature of 600–1200 kg‧m−2‧s−1, 25–300 kW‧m−2, 8 MPa and 303 K, respectively. A transition in the heat transfer regime from enhanced to normal was found as the heat-to-mass flux ratio increased to 83.33 J·kg−1. A pseudo-trans-critical process occurred in the tube at a heat-to-mass flux ratio above 125 J‧kg−1, and the heat transfer to SCO2 could be classified into three stages based on the reduced bulk fluid temperature. Particularly, the wall temperature was inversely proportional to the mass flux within the reduced bulk fluid temperature ranging of 0.992–1.0. Analysis of the heat transfer mechanism showed that the location of the large heat capacity in the boundary layer and the buoyancy force were responsible for the variation in the heat transfer characteristics. The applicability of the existing buoyancy criterion was examined, and BuP = 6 was found to be reasonable for predicting the onset of the influence of the buoyancy force on heat transfer.