Constant Wall Heat Flux Conditions Research Articles

Flow and Heat Transfer Characteristics in the Entry and Stabilized Flow Regions of a Circular Channel Rotating About a Parallel Axis

Abstract Cooling of heavy-duty electrical machines such as generators and motors is crucial for the smooth operations without thermal runaway. A commonly employed technique for the cooling of rotors in these machines is to place channels at different radial locations for the continuous passage of coolant. These channels are therefore rotating about a parallel axis, and the rotation-induced forces alter the flow and thermal behavior of the coolant compared to stationary channels. The present study reports a detailed numerical investigation on a long circular channel rotating about a parallel axis. The objective is to analyze the flow, heat transfer, and rotation-induced forces (Coriolis and centrifugal forces) in the entry region as well as in the region where flow is stable (the term ‘stable’ is used rather than ‘developed’ due to the presence of secondary flows in this region). The rotating channel was subjected to constant wall heat flux and constant wall temperature conditions at different Rotation numbers of 0, 0.15, 0.4, and 0.6. The Coriolis force is observed to be strong enough in the entry region to influence the flow. In the “stable flow” region, the centrifugal force becomes more dominant and forms counter-rotating secondary vortex pair, which causes circumferential variation in the Nusselt number. The flow and heat transfer characteristics for constant wall heat flux and wall temperature boundaries are the same for rotation conditions with similar values of rotational Grashof number. A correlation is presented for the circumferential variation of the Nusselt number in the stable flow region.

Investigation of forced convection of a novel hybrid nanofluid containing NEPCM in a square microchannel: Application of single-phase and Eulerian-Eulerian two-phase models

In the present study, laminar forced convection flow and heat transfer of novel hybrid nanofluid of Al 2 O 3 -n-octadecane/water through a horizontal microchannel under a constant wall heat flux condition are investigated. N-octadecane is regarded as a nano-encapsulated phase change material (NEPCM). Simulations are performed utilizing single-phase and Eulerian-Eulerian two-phase models. Governing equations are discretized by finite volume method and then solved using SIMPLE algorithm. Effects of nanoparticles volume fraction ( φ ), Reynolds number ( Re ), and melting range ( T mr ) on thermal and hydrodynamic parameters of hybrid nanofluid are evaluated. It is shown that predictions by Eulerian-Eulerian approach are more accurate than those by single-phase approach when compared to available experimental data. Addition of Al 2 O 3 and n-octadecane nanoparticles to water-based fluid leads to a reduction in fluid and wall temperatures, which is intensified at higher φ and Re . Also, these nanoparticles do not have a significant effect on hydrodynamic development of flow, while they delay its thermal development. NEPCM with a higher T mr only slightly improves thermal performance of hybrid nanofluid, which is more noticeable in single-phase procedure. Total efficiency of hybrid nanofluid ( η hnf ) is amplified by raising φ for both approaches. This augmentation is more evident in Eulerian-Eulerian model due to better thermal and hydrodynamic performances. Maximum increases in η hnf concerning single-phase and Eulerian-Eulerian models are 27% and 60%, respectively, occurring at φ Al 2 O 3 = 1 % and φ n ‐ octadecane = 10 % . Therefore, this type of hybrid nanofluids, because of its enhanced thermo-physical properties, can be considered as a suitable candidate for thermal energy storage, heat transfer, and renewable energy applications.

Effect of surfactants on the convective heat transfer and pressure drop characteristics of ZnO/DIW nanofluids: An experimental study

The advancement of nanotechnology has demonstrated the ability of metal-oxide-based nanofluids (NFs) to produce high heat flux in microscale thermal applications. Convective heat transfer (HTC) and flow characteristics (pressure drop (ΔP) and friction factor (f)) of aqueous ZnO NFs' within a circular mini tube (Di = 1.0 mm, L = 330 mm) were analyzed. Experiments were carried out under steady-state and varying flow rates using 0.012–0.048 wt % of NFs and sodium hexametaphosphate (SHMP) and acetylacetone (ACAC) as surfactants (SFs). Laminar flow and constant wall heat flux conditions were used to assess NFs heat transfer properties, ΔP and f. The viscosity (VC) and thermal conductivity (TC) of NFs exhibited a strong dependence on the operating temperature and NFs concentration. VC and TC increased by increasing the NFs concentration and decreased by increasing the operating temperature. Maximum VC and TC enhancement of 16.75% and 23.70% were achieved for SHMP-stabilised NFs, respectively. The average HTC increased by increasing NFs loading and flow rate, with HTCmax of 17.0% noticed for ACAC-stabilised NFs. The ΔPmax and fmax were 16.0% and 12.0%, respectively. Experimental and theoretical results showed a maximum deviation of ±7.0% and ±4.0%, respectively.

EXPERIMENTAL AND NUMERICAL CONVECTIVE HEAT TRANSFER INVESTIGATION IN LAMINAR RECTANGULAR-CHANNEL FLOW ACROSS V-SHAPED GROOVES

The influence of staggered V-grooves on the hydrothermal performance of a rectangular-channel flow is systematically investigated through a combination of numerical and experimental approaches. The 3-D numerical simulation is developed adopting computational fluid dynamics (CFD) (ANSYS FLUENT) for a range of Reynolds numbers (Re) from 100 to 1000. The experiments are conducted on straight and V-grooved channels (with pitch-to-height and height-to-hydraulic diameter ratios of 2 and 0.75) for distilled water under constant wall heat flux conditions to validate the computational model. Additionally, the impact of V-shaped groove arrangements, forward V-grooved channel (FVGCH) and backward V-grooved channel (B-VGCH), on the flow and heat fields, as well as the effect of groove depths (<i>d</i> = 1.5, 2.5, and 3.5 mm), are also studied. In both experimental and numerical results, the performance evaluation criterion (PEC) grows with rising Reynolds numbers. The highest PEC values of the numerical and experimental findings for the F-VGCH are 2.18 and 2.29, respectively, at Re = 1000. Whereas the highest PEC values of the numerical and experimental results for the B-VGCH are 1.81 and 1.96, respectively, at the same Re (Re = 1000). In addition, the values of PEC for F-VGCH are greater than the PEC values of B-VGCH for all examined groove depths over the entire range of Reynolds numbers. Thus, the F-VGCH offers the best performance evaluation criterion in comparison with B-VGCH.

Silver-Water Nanofluid for Heat Transfer Applications

Introduction: Nanoscale materials have attracted a great deal of attention of many researchers and academician in mechanical engineering because of their size and shape dependent thermal properties. The attractiveness of its superior thermal properties compared to its base materials is the main reason for the effective utilization of nanomaterials for the energy systems. The rapid development and researches in nanotechnology led to the development of new class of heat transfer fluid called nanofluid for heat transfer enhancement. The nanoparticles dispersed in the fluid increase the surface area, heat capacity and thermal conductivity of the fluid making it very suitable for heat transfer enhancement (1). Methods: Silver being a safe material with higher thermal conductivity of 429 W/mK can be extensively used for the heat transfer applications. Silver-water nanofluid was prepared by dispersing silver nanoparticles of sizes 20, 30-50 and 50-80 nm in the volume concentrations of 0.01, 0.05 and 0.1%. Polyvinyl pyrrolidone (PVP) has been used as surfactant to stabilize the nanofluid. Thermal conductivity of nanofluid was measured by kD2 Pro thermal analyzer. The stability of nanofluid was analyzed by sedimentation technique, pH measurement, zeta potential and particle size distribution. The effect of surfactant on the thermal conductivity enhancement and stability of nanofluid was studied (1,2). Then the investigations on the heat transfer and pressure drop of silver-water nanofluid in a circular tube under constant wall heat flux condition were carried out in laminar and turbulent flow regimes. Results & Discussions: Among the three sizes of silver nanoparticles used in the investigation, nanofluid with 20 nm exhibited better thermal conductivity enhancement and stability due to lower size. The addition of surfactant improved the stability of nanofluid but caused reduction in thermal conductivity enhancement slightly. Increase of concentration of nanoparticles from 0.01 to 0.1% improved the thermal conductivity of nanofluid up to 50% (1). Experimental study on use of silver-water nanofluid for heat transfer applications showed enhancement in heat transfer coefficient of nanofluid up to 36% and 74% respectively for laminar and turbulent flow conditions. However, the addition of nanoparticles increases the pressure drop slightly. But the higher enhancement of heat transfer overcomes the increase of pressure drop of nanofluid. The proposed correlations for the prediction of Nusselt number and friction factor showed good agreement with the experimental results (3.4). Conclusions: Lower size and lower concentrations of silver water showed better hydrothermal performance of heat transfer systems. The effect of pressure drop and erosion problem due to the addition of nanoparticles will be minimized with low concentrations and lower size of silver nanoparticles. It can be a suitable nanofluid for the thermal energy systems requiring improvement in their performance. Keywords: Silver Nanoparticles, Nanofluid, Thermal conductivity, Heat transfer

Heat Transfer and Friction Factor Characteristics of Turbulent Flow through a Conical Heat Exchanger

In this present energy demand scenario, the use of a waste heat recovery device like regenerator, recuperator, reheaters in the industrial processes is gaining much more importance due to improvement in thermal efficiency, and consequently reduction in thermal pollution. In the power plant, the waste heat carried by hot flue gases through the chimney remains untapped which can be recovered through the use of a conical heat exchanger at the inner portion of the chimney. Also, few customized designs of the conical coil heat exchanger can have the potential to be used to recover the waste heat in other industrial processes like chemical reactors, electrical contractors, HVAC applications, condenser, boiler, and evaporator, etc. In the present study, different arrangements of the conical coil heat exchanger are analyzed using a numerical technique. A conical coil of different geometrical configurations viz. at three different cone angles 45°, 90°and 135°, and at three different pitches 20 mm, 25 mm, and 30 mm are considered. The results for heat transfer and friction factor are presented for the flow of fluid of different Reynolds numbers. The impact of geometrical parameters as well as flow parameters on pressure drop and heat transfer under constant wall heat flux conditions are presented. The average heat transfer and pressure drop values are observed to be 175 to 225% more than the smooth tube values with increased pressure drop. Also, the results indicate that there is a nonlinear increase in both Nusselt number and friction factor with the increase in cone angle and pitch of conical coil.

Performance evaluation of convective heat transfer and laminar flow of non-Newtonian MWCNTs in a circular tube

Nanofluids are often designed to improve their heat transfer characteristics. However, there are some challenges regarding the increase in their viscosity and the change of the rheology. Therefore, this study investigates the laminar flow of non-Newtonian MWCNTs nanofluids in a circular tube under a constant wall heat flux condition. The rheological behavior and thermal conductivity of the nanofluids are modelled using suitable correlations derived from an experimental database. A numerical approach by finite volume method (FVM) is used for modelling the heat transfer and the laminar flow conditions in this study. The thermal behavior and the non-Newtonian characteristics of the MWCNTs nanofluids are evaluated concerning heat transfer, pressure drop and the overall energy efficiency of the system. The obtained results show that the heat transfer performance is enhanced by adding nanoparticles to the base fluid, reporting an average heat transfer enhancement of about 8.7% and 12.4% for 0.25% and 0.5% concentrations of MWCNTs, respectively. Also, it was found an average increase in pressure drop of about 9.1% and 15.3% for 0.25% and 0.5% concentrations of MWCNTs, respectively. Moreover, the overall energy efficiency of the system is enhanced by using MWCNTs nanofluids for high Reynolds number (1000 ≤ Re ≤ 2000), reaching a maximum enhancement at Re = 1000. The methodology here developed and validated, establishes an important breakthrough concerning modelling and prediction of MWCNTs nanofluids towards its tailoring for subsequent heat transfer studies and applications.

Influence of rotating twisted tape turbulator and nanofluids on heat transfer characteristics in a tube

ABSTRACT In this study, the heat transfer characteristics of water and three different water based nano fluids comprising of copper oxide (CuO), zinc oxide (ZnO) and titanium dioxide (TiO2) with constant 0.2% volume concentration (φ) of respective nanoparticles in a bare tube and bare tube fitted with “rotating twisted tape turbulator (RTTT)” of twist ratio (y = 2.88) inside, was examined. The flow rate of the working fluids (1–6 LPM) and the rotational speed of the twisted tape turbulator (TTT) (0-300 RPM) were considered as variable constraints and their influence on heat transfer characteristics were evaluated. It was detected that for all the working fluids, the convective heat transfer coefficient intensifies with upsurge in the flow rate and rotational speed of the TTT. A constant wall heat flux condition was used in the contemporary investigation. The literature review reveals that most of the research work focuses on the study of heat transfer characteristics in a bare tube with TTT. Minimal research work has been carried out on the usage of ‘rotating twisted tape turbulators’ using nano fluids as working fluid to augment the heat transfer characteristics. Further, the results indicate that the heat transfer characteristics may be enhanced by rotating the twisted tape turbulator at 300 RPM in a plain tube and by maintaining the flow rate of the working fluids at 6 LPM.

Mesoscale simulations of saturated flow boiling heat transfer in a horizontal microchannel

The bubble dynamics and their effects on flow boiling heat transfer in a horizontal channel are investigated under constant wall temperature and constant wall heat flux conditions. It is found that both the bubble departure time and departure diameter decrease with increasing the inertial force for all the considered heating modes. Moreover, the bubble departs from the heater with a residual for the constant wall temperature condition so the heat transfer efficiency for the constant wall temperature condition depends mainly on the bubble growth rate and the vapor thickness. While for the constant heat flux condition, the bubble slides away from the heater accompany with waiting periods. So the heat transfer performance for this condition is mainly affected by the growth and waiting times. Besides, the results also show there were two contradictory trends of heat transfer efficiency under different wettabilities for the considered heat modes.

Numerical study of heat transfer augmentation in an axially rotating pipe equipped with Kenics mixer

This paper reports fluid flow and heat transfer in an axially rotating pipe equipped with Kenics mixer under constant wall heat flux condition. Numerical simulations were processed with the commercial finite volume CFD code, ANSYS Fluent 19.2. The realizable k-ε turbulent model was used to close the Reynolds Average Navier-Stokes (RANS) equations. Buoyancy effects (βΔT) and gravity (g = 9.81 m/s2) were considered in the simulations. In the range of axial Reynolds numbers between 10,000 and 30,000 (10,000 ≤ Rea ≤ 30,000), the Kenics mixer enhanced the Nusselt number of the rotating pipe by 41–50%, 38–47% and 33–42% at rotation rates of 1, 2 and 3 respectively. Enhancement in Nusselt number was linked to additional swirl created by the Kenics mixer insert which improved fluid mixing and boundary layer disturbance. CFD results also showed that increasing the axial Reynold number augmented the Nusselt number but decreased the friction factor.

Experimental research on the performance of nanofluid flow through conically coiled tubes

In this paper, the thermal and hydrodynamic characteristics of SiO2 / water nanofluids flow through conically coiled tubes are examined experimentally. An experimental rig is constructed and set of experiments were carried out in order to investigate the heat transfer and pressure drop of SiO2-water nanofluids flow under constant wall heat flux condition through various conically coiled tubes. To better understand the fluid flow physics, the thermal conductivity and dynamic viscosity of nanofluids in different temperatures and volume fractions are measured experimentally. The results show that both heat transfer rate and pressure drop increase with increment in nanoparticle concentration. Moreover, the effects of cone angle and coil pitch were studied on heat transfer and pressure drop. Results indicate that, lower cone angle brings about more heat transfer and pressure drop. The maximum increase of 26% and 117% in heat transfer and pressure drop were recorded, respectively. Eventually, empirical correlations were presented for prediction of Nusslet number and friction factor in conically coiled tubes.

3D numerical analysis of thermal-hydraulic behaviors of turbulent flow inside twisted square ducts

Heat transfer, local distributions of Nusselt number, flow structure, and friction characteristics of twisted square ducts are presented. Numerical analysis was carried out to investigate the influence of the twist ratio (TR = p/D = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0) on the thermal-hydraulic performance of twisted square ducts under constant wall heat flux condition for Reynolds numbers based on the hydraulic diameter of the twisted square duct ranging from 3000 to 20 000. The straight square duct was also analyzed for comparison. The numerical results showed that the twisted square ducts were more efficient in heat transfer than the straight square ducts because the swirl flow helped to increase fluid mixing and reduce thermal layer boundary thickness. The decrease of the twist ratio led to the increase in the Nusselt number and friction factor due to the higher frequency of swirl flow. As compared to the straight square duct, the twisted square ducts with TR = 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0 improved heat transfer by 52, 49.82, 45.85, 42.22, 39.54, 35.41, and 31.77 %, respectively. Among the studied twisted ducts, the ones with twist ratio TR = 3.5 offered the maximum thermal enhancement factor of 1.42 at Re = 3000. In addition, the results also revealed that the twisted square ducts are thermo-hydraulically superior to the straight square ducts.

Heat Transfer Enhancement in an Axially Rotating Pipe with Twisted Tape Insert

This paper addresses heat transfer enhancement in an axially rotating pipe under constant wall heat flux condition. Numerical simulations were processed using the Reynolds Average Navier-Stoke (RANS) solver of ANSYS Fluent 19.2. The realizable k-e turbulence model was used to provide closure to the RANS equations. Buoyancy effects (β∆T) and gravity (g) were considered in the numerical analysis. Parametric runs were made for axial Reynolds numbers between 10000 and 30000 (10000 ≤ Rea ≤ 30000) at various rotation rates (0 ≤ N ≤ 3) using water as the working fluid. CFD results showed that increasing the rotation rate (N) augmented the Nusselt number. Nusselt number augmentation was associated to increase in the strength of secondary flow. The influence of twisted tape insert on the thermal performance of the rotating pipe was also investigated. Twisted tape produced additional swirl to further enhance the heat transfer rate. The intensity of swirl created by the inserted tape depended on the twist ratio. Three different twist ratios (P/W) of 3, 5 and 7 (P is the pitch and W is the width of the tape) were examined. Simulation results revealed that decreasing the twist ratio of the tape augmented the heat transfer rate.

Fully Developed Transport Constants of Annular Tubes, With Application to the Entrance Region

Abstract Description of the laminar thermal entry problem in annular tubes has historically been limited to a few geometric cases that require piecing together classical Graetz series and Lévêque series solutions to span all values of the Graetz number. The current work uses a recently developed generalized correlation to describe the full range of Graetz numbers for any annular tube geometry. However, the correlation requires fully developed Nusselt number values that have only been accurately reported in tabular and graphical forms. Exact analytic solutions for the constant wall heat flux condition are developed in this work, and simplified correlations are proposed for all wall conditions that reproduce exact Nusselt number solutions to within ± 0.4%. Using these results, a modified version of the generalized Graetz problem correlation is developed to reproduce the most published Nusselt numbers for the thermal entry problem in an annular tube to be within ± 5%.

Effect of wall temperature modulation on the heat transfer characteristics of droplet-train flow inside a rectangular microchannel

Abstract The numerical studies of water–oil two-phase slug flow inside a two-dimensional vertical microchannel subjected to modulated wall temperature boundary conditions have been discussed in the present paper. Many researchers have contributed their efforts in exploring the characteristics of Taylor flows inside microchannel under constant wall heat flux or isothermal wall conditions. However, there is no study available in the literature which discusses the impact of modulated thermal wall boundary conditions on the heat transfer behavior of slug flows inside microchannels. Hence, to bridge this gap, an effort has been made to understand the heat transfer characteristics of the flow under sinusoidal wall temperature conditions. Initially, a single phase flow and heat transfer study was performed in microchannels, and the results of the fully developed velocity profile and heat transfer rate were validated with benchmark analytical results. Then an optimal selection of the combination of sinusoidal thermal wall boundary conditions has been made for the two-phase slug flow study. Later, the effects of amplitude (0

Comprehensive simulation of nanofluid flow and heat transfer in straight ribbed microtube using single-phase and two-phase models for choosing the best conditions

In this paper, the hydrodynamics and heat transfer parameters of nanofluids are investigated using CFD analysis. Laminar convective heat transfer of alumina–water nanofluids with 0, 1%, and 2% volume fraction in a straight microtube heat sink under constant wall heat flux condition is studied. This work is performed in two parts. In the first part, the single-phase and two-phase approaches have been used for modeling heat transfer of pure water and alumina–water nanofluids in a straight microtube. The results of simulation are compared with the experimental data. The results showed that CFD predictions via a two-phase model show better agreement with the experimental measurements. For nanofluid with 1% concentration, the average relative error between the experimental data and CFD result based on a two-phase model is 5.85%, while for nanofluid with 2% concentration that is 2.54%. In the second part of this work, the effects of ribs through the microtube are investigated. The effect of the geometry on the Nusselt number and friction factor in the microtube is studied. We found that spiral pitch increment increases the thermal performance by an average of 19.8%. Finally, the best performance is obtained for the ribs height 1 mm and the pitch 1.5 mm.

A Historical Misperception on Calculating the Average Convection Coefficient in Tubes With Constant Wall Heat Flux

The historical approach to averaging the convection coefficient in tubes of constant wall heat flux leads to quantitative errors in short tubes as high as 12.5% for convection into fully developed flows and 33.3% for convection into hydrodynamically developing flows. This mistake can be found in teaching texts and monographs on heat transfer, as well as in major handbooks. Using the correctly defined relationship between local and average convection coefficients, eight new correlations are presented for fully developed and developing flows in round tubes and between parallel plates for the constant wall heat flux condition. These new correlations are within 2% of exact solutions for fully developed flows and within 6% of first principle calculations for hydrodynamically developing flows.

Convective heat transfer studies on helically corrugated tubes with spiraled rod inserts using TiO2/DI water nanofluids

Convective heat transfer and friction factor studies are experimentally carried out in a smooth and five helically corrugated tubes of different heights and pitches of corrugation with spiraled rod inserts. The experiment is conducted under turbulent flow (Re = 4800–8900) and constant wall heat flux conditions. Deionized (DI) water and titanium dioxide (TiO2)/DI water nanofluids are used as working fluids. The average size of TiO2 nanoparticles is 32 nm. Two volume concentrations of nanofluids (0.25 and 0.5%) are used in this study. The combined effects of nanofluids, inserts and corrugation in tubes on Nusselt number and friction factor are investigated. The results indicate that (i) the addition of TiO2 nanoparticles in DI water upsurges the heat transfer rate, which increases with nanofluids volume concentrations; (ii) use of inserts and corrugation in tubes enhances the heat transfer rate further; (iii) among the corrugated tubes, the tube having highest corrugation height (hc = 1 mm) and lowest pitch (pc = 8 mm) with spiraled rod insert of smaller pitch (pi = 30 mm) shows the maximum thermal performance factor of 1.56 for 0.5% volume concentration of nanofluids.

Integral properties of temperature variance production in a turbulent channel flow with passive scalar transport

This paper investigates the integral properties of the mean temperature dissipation α(∂Θ/∂y)2 and the temperature variance production Pθ=Rvθ(∂Θ/∂y) in fully developed turbulent channel flow, and in particular the effects of Prandtl number Pr=ν/α and Reynolds number Reτ=δuτ/ν (and/or Peclet number Peτ=ReτPr). Here Θ is the mean temperature and Rvθ is the wall-normal turbulent heat flux. y is the wall-normal direction and δ is the channel half-height. ν and α are the fluid viscosity and thermal diffusivity, respectively. uτ is the friction velocity. The main findings of the present work include: (1) Identities are derived from the mean thermal energy equation for the global integral of the mean temperature dissipation and temperature variance production for four heating conditions: constant heat source (CHS), constant wall heat flux (CHF), constant temperature difference (CTD), and constant wall temperature (CTD). Identities for the CHS and CHF conditions are consistent with those derived by Pirozzoli et al. (2016) and Abe and Antonia (2017). The identities are verified using publicly available direct numerical simulation (DNS) data under three heating conditions CHS, CHF, and CTD. (2) The global integral of the mean temperature dissipation is found to depend strongly on the fluid Prandtl number, but it is largely independent of the Reynolds number of the flow. In inner-scaling, this global integral can be approximated as(1/Pr)∫0δ+(∂Θ+/∂y+)2dy+≈9.13Pr1-n where Θ+=Θ/θτ is the inner-scaled mean temperature and θτ is the friction temperature. y+=yuτ/ν is the inner-scaled wall-normal distance. n≈1/2 for Pr≲1 and n≈1/3 for Pr≳1. The strong dependence of (1/Pr)∫0δ+(∂Θ+/∂y+)2dy+ on the Prandtl number is directly rooted in the molecular thermal diffusion sub-layer. (3) The global integral of temperature variance production is found to strongly depend on both Prandtl number and Reynolds number. (3.1) At sufficiently high Reynolds number and Peclet number, the integral of temperature variance production is found to exhibit a logarithmic-like layer similar to that of the mean temperature profile. (3.2) The effect of Prandtl number is mainly caused by the molecular thermal diffusion sub-layer and buffer layer. (3.3) Between the peak wall-normal turbulent heat flux Rvθ location, ymt, and the channel centerline, δ, the integral of temperature variance production increases with Peclet number in a logarithmic-like fashion. (3.4) At low Prandtl number Pr≲1, the integral of temperature variance production is found to be equally partitioned around ymt.

Non-Newtonian Fluid Flow and Heat Transfer in a Semicircular Microtube Induced by Electroosmosis and Pressure Gradient

The hydrodynamic and thermal characteristics of electroosmotic and pressure-driven flows of power-law fluids are examined in a semicircular microchannel under the constant wall heat flux condition. For sufficiently large values of the electrokinetic radius, the Debye length is thin; the active flow within the electric double layer (EDL) drags the rest of the liquid due to frictional forces arising from the fluid viscosity, and consequently a plug-like velocity profile is attained. The velocity ratio can affect the pure electrokinetic flow as well as the flow rate depending on the applied pressure gradient direction. Since the effective viscosity of shear-thinning fluids near the wall is quite small compared to the shear-thickening fluids, the former exhibits higher dimensionless velocities than the later close to the wall; the reverse is true at the middle section. Poiseuille number increases with increasing the flow behavior index and/or the electrokinetic radius. Due to the comparatively stronger axial advection and radial diffusion in shear-thinning fluids, better temperature uniformity is achieved in the channel. Reduction of Nusselt number continues as far as the fully developed region where it remains unchanged; as the electrokinetic radius tends to infinity, Nusselt number approaches a particular value (not depending on the flow behavior index).