Temperature-dependent Fluid Properties Research Articles

Improved Nu number correlations for gas flow in monolith reactors using temperature-dependent fluid properties

This paper reports an investigation of heat and mass transfer coupled with fluid flow inside monolith honeycomb substrates. Many important aspects of multiscale models of a monolith are investigated. Three common geometrical representations of monolith channels are tested, and their advantages and disadvantages are analysed. A detailed computational model and a large set of computational experiments, covering a broad range of substrate void fractions, channel Reynolds numbers and heating rates is used. Results assuming constant and temperature-dependent fluid properties are reported and compared. At a high heating rate, assuming constant fluid properties leads to a significant error in the estimation of Nusselt and Sherwood numbers in the entry region. At a constant wall temperature and a high heating rate, there is a minimum in the Nusselt curve that classical models are not able to describe. Hence, a novel correlation that shows excellent agreement with channel data is presented together with a methodology to fit it. Additionally, empirical models to estimate the placement and value of the minimum Nusselt are proposed. These results contribute significantly to the improvement of lumped parameter multiscale models of monolith catalytic reactors.

Topology optimization design of nanofluid-cooled microchannel heat sink with temperature-dependent fluid properties

The geometric structure of a two-dimensional (2-D) nanofluid-cooled microchannel heat sink (NMHS) is optimized using a topology optimization method. We describe the flow and heat transfer in the NMHS as a single-phase nanofluid-based convective heat transfer model. Since the thermophysical properties of the nanofluid strongly depend on temperature, the temperature-dependent fluid properties are considered in the model. A heat transfer maximization problem is studied under a constant pressure difference using a density based topology optimization method to optimize configurations of NMHS. In the optimization, the material density is adopted as the design variable to control the variation of the fluid domain. An adjoint-based sensitivity analysis method is applied to obtain the gradient information used for updating the design variable. In numerical examples, the effects of temperature-dependent fluid properties on the optimal configurations of the NMHS are first investigated. Additionally, the effects of the characteristics of the nanofluid, such as the base fluid, nanoparticle volume fraction and diameter, on optimized designs are assessed. The numerical results reveal that the temperature-dependent fluid properties can significantly affect the optimal results; more branched flow channels are produced in the optimal configurations as the pressure difference or heat generation coefficient increases; and more complex optimal configuration can be obtained by reducing the nanoparticle volume fraction. From an engineering standpoint, this study provides an applicable optimization method for the design of a well-performing NMHS.

Influence of temperature-dependent properties on a gravity-driven thin film along inclined plate

AbstractA numerical investigation into the effect of temperature-dependent fluid properties on the thin film flow along an inclined heated plate is presented. The equations governing the coupled flow and heat transfer are formulated based on couple stress non-Newtonian model. Solutions of the coupled nonlinear differential equations are tackled numerically by using the combination of shooting method and the Fehlberg-Runge-Kutta method. Findings are presented graphically and discussed precisely.

Characterization of temperature-dependent fluid properties in compressible viscous fluid flow induced by oscillation of disk

Abstract Time-dependent compressible flow of viscous fluid with temperature dependent density, thermal conductivity and viscosity is considered. The motion in fluid is generated due to periodic oscillations of rotating disk. The normalized system of partial differential equations is achieved by adopting similarity transformations. The obtained system is then solved numerically using Successive over Relaxation (SOR) method and finite difference schemes. The local similar solutions are presented. One-dimensional graphs, two-dimensional contours and three-dimensional flow phenomenon are drawn graphically. Tabular description of various physical parameters is also presented.

Heat transfer degradation of buoyancy involved convective RP-3 hydrocarbon fuel in vertical tubes with various diameters under supercritical pressure

The flow and heat transfer characteristics of hydrocarbon fuel flowing upward in vertical circular tubes with various diameters under supercritical pressure were numerically investigated. The buoyance effects in the vertical tubes were analyzed and discussed. A simulation tool for predicting the supercritical turbulent internal flow and heat transfer was developed and validated with experimental data. A four-species surrogate model of the fuel RP-3 was used to calculate the temperature-dependent fluid properties. The turbulence was modelled with the Launder-Sharma (LS) low-Reynolds number turbulence model. The tube diameter ranged from 2 mm to 15 mm in order to analyze the size effect. The simulation results showed that the tube diameter significantly affected the heat transfer deterioration, especially for low mass flux cases. Based on the predicted velocity profiles, the mechanism of the size effect on the heat transfer deterioration was discussed and analyzed. In the tube with a small diameter, the accelerated thin layer took up a significant share of the cross section of the tube. However, the buoyancy effect dominated in the tube with a large diameter due to the large gradient from the thin layer to the core of the main flow. It was found that the threshold for the size effect was around 10 mm, below which the size effect was significant.

Large eddy simulation of turbulent heat transfer in a non-isothermal channel: Effects of temperature-dependent viscosity and thermal conductivity

In this work, we perform large eddy simulations (LES) to study the influence of variable viscosity and thermal conductivity on forced convection in a non-isothermal channel flow. To prevent thermal dilatational effect, the gas density is assumed to be constant. The temperature ratio T2/T1 is varied from 1.01 to 2.2, where T2 and T1 are the temperatures of hot and cold walls, respectively. The mean turbulent Reynolds number is kept the same at 395. The results indicated that the mean flow fields are significantly affected by the temperature-dependent fluid properties. Despite the modified velocity and temperature profiles, it is interesting to note that the molecular momentum and heat transport across the channel remain unchanged. Meanwhile, pronounced differences are exhibited for various turbulence statistics such as root-mean-square velocity and temperature fluctuations, Reynolds shear stress, and correlation between streamwise velocity and temperature. Compared with the isothermal flows, it is also found that the presence of the temperature gradient tends to diminish heat transfer. With increasing the temperature ratio, the Nusselt numbers for both sides are reduced. Moreover, the hot side has a higher Nusselt number than the one at the cold side.

Horizontal landfill gas wells: Geometry, physics of flow and connection with the atmosphere

Landfills form an important link in the chain of waste management. Sound engineering design and effective gas collection can only be attained via a thorough understanding of flow in different parts of the landfill. This study addresses gas flow induced by perforated horizontal wells commonly used for landfill gas collection. Planar flow through a porous medium of varying properties is solved numerically, yielding a twofold insight: quantitative deviations establish the validity and limitations of theoretical models, whilst qualitative comparison illustrates the impact of various flow parameters. It is found that an accurate modelling of geometry and medium resistance is essential in determination of a correct pressure distribution that can then be used to optimise gas collection. Gravity, an often overlooked effect, is shown to be non-negligible. The error source and bounds are given and explained for different landfill configurations. By contrast, temperature is of a lesser importance by an order of magnitude, implying that comprehensive spatial modelling of the exothermal reactions generating the gas as well as temperature dependent fluid properties such as viscosity are of a secondary interest. The well radius of influence, i.e. the maximal collection distance, is a fundamental design parameter impacting the interchange of fluid with the atmosphere. The numerical solution permits to obtain optimal values for effective collection and minimise flux between the landfill and atmosphere. Based upon a comprehensive numerical simulation an analytical approximation is suggested as an accessible design tool. The formula enables the computation of pressure profiles within the landfill and involves a minimal number of required input parameters. The surface flux and radius of influence can also be determined thereby.

Probing rate estimation methods for multiphase flow through surface chokes

Choke is an essential device that controls flow rates at either subsurface or surface. Many models and correlations have appeared over decades handling multiphase flow through surface chokes. But, direct comparison of their relative performance has been a few, if any. This study evaluates several models and correlations to explore their relative performances. As expected, models anchored in thermodynamic principles outperformed others. Improvements in modeling followed the initial findings.We studied several models to explore their relative merits and ease of use in field settings. Seven different data sets gathered from laboratory and field settings, involving about 1000 independent data points, constituted the essence of this study. The study found the importance of PVT data in any flow through choke calculations. Specifically, we found that changes in density and heat capacity of fluids with pressure and temperature should be part of any rigorous effort for computation of flow rates.To ensure reliability and consistency in solutions, a model based on thermodynamic considerations deserve preference for generalized applications because it can handle pressure and temperature-dependent fluid properties. In this context, the recognition of choke-discharge coefficient's dependence on flow rate and Reynolds number constitutes an important element in ensuring the reliable outcome of results.

Effect of Bifurcation on Thermal Characteristics of Convergent-Divergent Shaped Microchannel

A microchannel heat sink with convergent-divergent (CD) shape and bifurcation is presented, and flow and heat transfer characteristics are analyzed for Re ranging from 120 to 900. The three-dimensional governing equations for the conjugate heat transfer with temperature-dependent solid and fluid properties are solved using the finite volume method. Comparisons are carried out for four cases, namely, rectangular shape with and without bifurcation and CD shape with and without bifurcation. The pressure drop, flow structure, and average Nusselt number are analyzed in detail, and the thermal resistance and overall performance are compared. It is shown that the CD shape with bifurcation has more uniform and lower temperature at the bottom wall and better heat transfer performance compared to other geometries. The heat transfer augmentation in the CD shaped microchannel with bifurcation can be attributed not only to the accelerated and redirected flow toward the constant cross section segment but also to periodically interrupted and redeveloped thermal boundary-layers due to bifurcation. It is also shown that increasing Re leads to thinning of thermal boundary-layers resulting in an enhanced heat transfer in terms of an increased average Nusselt number from 38% to 74%. However, there is an increased pressure drop due to channel shape and obstacle in fluid flow. Further, due to a high pressure drop penalty at high Re, CD shaped microchannel with bifurcation loses its heat transfer effectiveness.

Hydromagnetic Rarefied Fluid Flow over a Wedge in the Presence of Surface Slip and Thermal Radiation

An analysis is presented to investigate the effects of thermal radiation on a convective slip flow of an electrically conducting slightly rarefied fluid, having temperature dependent fluid properties, over a wedge with a thermal jump at the surface of the boundary in the presence of a transverse magnetic field. The reduced equations are solved numerically using the finite difference code that implements the 3-stage Lobatto IIIa formula for the partitioned Runge-Kutta method. Numerical results for the dimensionless velocity and temperature as well as for the skin friction coefficient and the Nusselt number are presented through graphs and tables for pertinent parameters to show interesting aspects of the solution.

Water Absorption in Cementitious Materials at Different Temperatures

Abstract This paper presents results from an experimental investigation that assessed water absorption in cementitious materials at different temperatures. This work aims to determine whether the absorption behavior may be explained by considering only temperature related variations in the fluid properties (surface tension, viscosity, and density). The calculated water absorption curves at 5°C and 40°C (based on adjusting the absorption that was measured at 23°C using the temperature dependent fluid properties) exhibit agreement with the experimental data for the secondary sorption. This indicates that the change of fluid properties as a function of temperature is the primary reason for the difference in behavior. As a result, it appears possible to predict the rate of secondary absorption of cementitious materials at various temperatures with a simple correction. The initial sorptivity also showed a temperature dependency; however, the dependency of the mortars with water-to-cement ratios of 0.30 and 0.50 was 50 % and 64 % greater than that obtained using only the temperature dependent fluid properties. This indicates that, in addition to variations in the properties of the fluid, the interaction between the fluid and cementitious materials may also have a temperature dependence that influences the initial sorptivity. The transition between initial and secondary sorption (i.e., nick point) occurs at an earlier time with increasing temperature due to the increased rate of fluid absorption.

Development and validation of Nusselt number and friction factor correlations for laminar flow in semi-circular zigzag channel of printed circuit heat exchanger

Friction factor and heat transfer correlations have been developed for printed circuit heat exchangers (PCHEs) as a function of geometric parameters. These correlations summarize the thermal hydraulic performance of PCHEs and allow for accurate determination of their cost and effectiveness. Although there have been previous studies of the thermal-hydraulic performance of zigzag channel PCHEs, explicit correlations for friction factor or Nusselt number as a function of geometric parameters have not yet been reported. In this study, computational fluid dynamic (CFD) predictions were used to aid in the development of friction factor and Nusselt number correlations for laminar flow in semicircular zigzag-channel PCHEs. Two CFD models were developed to investigate the thermal-hydraulic characteristics of fluid flow in zigzag channels. A single-channel isothermal CFD model was used to investigate friction factors in PCHE zigzag channels. A two-channel CFD model was used to investigate the Nusselt number in zigzag channels and the effect of temperature-dependent fluid properties on the pressure loss. The effects of geometric parameters such as relative length ratio, zigzag angle and radius of curvature of bend were investigated. From the extensive CFD analysis database, friction factor and Nusselt number correlations were developed by implementing a least squares method with a non-linear Generalized Reduced Gradient algorithm. The friction factor correlation is valid for 50≤Re≤2000, 5°≤α≤45° and 4.09≤lR/Dh≤32.73. The Nusselt number correlations are valid for 200≤Re≤2000, Pr≤1.0, 5°≤α≤45° and 4.09≤lR/Dh≤12.27. The correlations were validated against experimental data from The Ohio State University (OSU) and Korea Advanced Institute of Science and Technology (KAIST). The friction factor for zigzag-channel PCHEs is mainly influenced by the zigzag channel geometry while the Nusselt number is influenced by the overall heat exchanger design including the plenum sections.

Analytical modeling of hydrodynamic lubrication in a multiple-reduction drawing die

The hydrodynamic lubrication regime is reported to exist in numerous metal forming applications, such as wire drawing and hydrostatic extrusion, but it is difficult to achieve in the drawing of large diameter rods due to the relatively low drawing speeds common for larger parts. By creating a stable fluid film between the workpiece and the die during the drawing process friction and die wear could be significantly reduced, leading to energy savings, increased achievable reductions, and increased die life. An analytical model of the hydrodynamic drawing process is proposed which considers the geometry of the workpiece and die, as well as, the material properties (including work hardening effects), and (temperature dependent) fluid properties to determine the fluid film thickness over the reduction die. This model is then used to analyze several case studies, including a multiple reduction die with high pressure lubricant supplied to the space between the dies. It is shown that a stable fluid film can be established for low drawing speeds through the combination of a multiple reduction die and a supply of lubricant at high pressures to the inlet of the dies.

Models of heat transfer to fluids at supercritical pressure with influences of buoyancy and acceleration

Physically-based semi-empirical models of buoyancy-influenced and inertia-influenced heat transfer to fluids at supercritical pressure taking careful account of the temperature dependence of fluid properties are presented in this paper. The first model is concerned with buoyancy-influenced convective heat transfer in a vertical heated tube to fluid flowing either in the upwards direction (the buoyancy aided case) or the downwards direction (the buoyancy-opposed case). It provides a detailed picture of how the mode of heat transfer changes with increase of buoyancy influence from one of forced convection, when any such influence is negligibly small, to combined forced and free convection and then eventually to a mode of heat transfer which exhibits characteristics very similar to those of free convection. The second model is concerned with inertia-influenced heat transfer due to thermally-induced bulk flow acceleration. It shows how acceleration of a strongly heated flow can cause reduced turbulence production, deterioration of heat transfer effectiveness and even laminarisation of a turbulent flow. These two models yield criteria for screening experimental data in terms of the magnitude of buoyancy and inertia influences. A third model is presented which describes heat transfer under conditions where influences of buoyancy and inertia due to thermally-induced bulk flow acceleration are encountered together. It is shown how buoyancy-influenced flow and heat transfer can be modified as a result of thermally-induced bulk flow acceleration. The paper ends with a brief outline of how the models might be used for correlating data on buoyancy-influenced and inertia-influenced heat transfer taking account of the strong temperature dependence of fluid properties which can be encountered in the case of fluids at supercritical pressure.

Conductive sublayer of turbulent heat transfer for heating of water in a circular tube

The steady-state and transient turbulent heat transfer coefficients in circular platinum (Pt) test tubes (inner diameters: 3 and 6 mm; heated lengths: 66.5 and 100 mm and 69.6 mm, respectively) were systematically measured using an experimental water loop for a wide range of flow velocities, inlet liquid temperatures, Prandtl numbers, inlet pressures, and exponentially increasing heat inputs (Q 0 exp(t/τ), τ: exponential period). The Reynolds-averaged Navier–Stokes equations and the k–e turbulence model for unsteady turbulent heat transfer in circular test sections were numerically solved for heating of water with heated sections of diameter 3 and 6 mm and length 67 and 100 mm and 70 mm, respectively, by using computational fluid dynamics code under the same conditions as those in the experiment and with temperature-dependent thermophysical fluid properties. The thickness of the conductive sublayer, δ CSL,st and δ CSL [=(Δr) out /2], and the nondimensional thickness of the conductive sublayer, (y , + ) TEM [=(f F /2)0.5 ρ l u δ CSL,st /μ l ] and (y + ) TEM [=(f F /2)0.5 ρ l u δ CSL /μ l ], for steady-state and transient turbulent heat transfer at various heated length-to-inner diameter ratios, inlet liquid temperatures, and exponential periods were measured on the basis of the numerical solutions. The correlations of the thickness of the conductive sublayer, δ CSL,st , and nondimensional thickness of the conductive sublayer, (y , + ) TEM , for steady-state turbulent heat transfer and those of the thickness of the conductive sublayer, δ CSL , and nondimensional thickness of the conductive sublayer, (y + ) TEM , for transient turbulent heat transfer in a circular tube were derived.

The effect of surface thermal radiation on entropy generation in an open cavity with natural convection

In this work, a numerical study on entropy generation in a square open cavity with natural convection and surface thermal radiation is presented. The overall continuity, momentum, and energy equations for the gas phase in the open cavity were solved numerically by means of the finite-volume method. Temperature-dependent fluid properties were considered. During the calculations, the values of the Rayleigh number (Ra) were set in the range of 104–106. The temperature difference between the hot wall and the bulk fluid (ΔT) was varied between 50 and 400K, and was represented by a dimensionless temperature difference (φ) for the purpose of generalization of the present study. The results of this investigation indicate that surface thermal radiation increases the overall entropy generation rate between 33.52% and 560.87%, and thus cannot be neglected in the analysis of this type of system.

Comments on “Effects of temperature dependent fluid properties and variable Prandtl number on the transient convective flow due to a porous rotating disk by M. S. Alam, S. M. Chapal Hossain, M. M. Rahman [Meccanica (2014) 49:2439–2451

In this letter, we submit our comments on recently published paper titled “Effects of temperature dependent fluid properties and variable Prandtl number on the transient convective flow due to a porous rotating disk by Alam et al. (Meccanica 49: 2439–2451, 2014)”. Authors of this paper have attempted to present similarity solutions in the paper. We comment in this letter is that the similarity transformations considered in Alam et al. (Meccanica 49: 2439–2451, 2014) are not correct and thus results are leading to invalid conclusions.

Further Investigation on Laminar Forced Convection of Nanofluid Flows in a Uniformly Heated Pipe Using Direct Numerical Simulations

In the present paper, laminar forced convection nanofluid flows in a uniformly heated horizontal tube were revisited by direct numerical simulations. Single and two-phase models were employed with constant and temperature-dependent properties. Comparisons with experimental data showed that the mixture model performs better than the single-phase model in the all cases studied. Temperature-dependent fluid properties also resulted in a better prediction of the thermal field. Particular attention was paid to the grid arrangement. The two-phase model was used then confidently to investigate the influence of the nanoparticle size on the heat and fluid flow with a particular emphasis on the sedimentation process. Four nanoparticle diameters were considered: 10, 42, 100 and 200 nm for both copper-water and alumina/water nanofluids. For the largest diameter d n p = 200 nm, the Cu nanoparticles were more sedimented by around 80%, while the A l 2 O 3 nanoparticles sedimented only by 2 . 5 %. Besides, it was found that increasing the Reynolds number improved the heat transfer rate, while it decreased the friction factor allowing the nanoparticles to stay more dispersed in the base fluid. The effect of nanoparticle type on the heat transfer coefficient was also investigated for six different water-based nanofluids. Results showed that the Cu-water nanofluid achieved the highest heat transfer coefficient, followed by C, A l 2 O 3 , C u O , T i O 2 , and S i O 2 , respectively. All results were presented and discussed for four different values of the concentration in nanoparticles, namely φ = 0 , 0 . 6 % , 1 % and 1 . 6 % . Empirical correlations for the friction coefficient and the average Nusselt number were also provided summarizing all the presented results.

Effects of Variable Fluid Properties on MHD Flow and Heat Transfer over a Stretching Sheet with Variable Thickness

AbstractThe influence of temperature-dependent fluid properties on flow and heat transfer of an electrically conducting fluid over a stretching sheet with variable thickness in the presence of a transverse magnetic field is analyzed. Using similarity transformations, the governing coupled non-linear partial differential equations (momentum and energy equations) are transformed into a system of coupled non-linear ordinary differential equations and are solved numerically by Keller-box method. For increasing values of the wall thickness parameter, the analysis reveals quite interesting flow and heat transfer patterns. The effects of the temperature dependent viscosity, the wall velocity power index, the thermal conductivity, the wall temperature parameter and the Prandtl number on the flow and temperature fields are presented. The obtained numerical results are compared with the available results in the literature for some special cases and are found to be in excellent agreement. The skin friction and the wall temperature gradient are presented for different values of the physical parameters and the salient features are analyzed.

Effects of temperature-dependent properties on wire-coating from a bath of FENE-P fluid

An analysis is carried out to study theoretically the coating of wire by drawing it from a pool of viscoelastic fluid in a pressure type-die under non-isothermal conditions. The constitutive equation of FENE-P model is used to characterize the rheology of the polymer melt. The equations describing the flow and heat transfer inside the die are solved (a) analytically for constant physical properties (b) numerically for temperature-dependent fluid properties. In the former case, the analytical expressions of velocity, volumetric flow rate, final radius of the coated wire and force required to pull the wire are reported. In the latter case, the fluid viscosity and relaxation time are considered as a function of temperature. The well-known Nahme–Griffith law is used to capture such dependence. Shooting method with Runge–Kutta–Fehlberg algorithm is employed to solve the coupled energy and momentum equations. The effects of different parameters on the velocity and the temperature distributions, volumetric flow rate, final thickness of the coated wire and force necessary to pull the wire are illustrated graphically for both the cases of constant and variable physical properties.