Effects of single and multi-walled carbon nano tubes on water and engine oil based rotating fluids with internal heating

Three dimensional (3D) flow of a rotating Water and Engine Oil Based fluid is analyzed. Effects of nanometer (nm) sized Single (SWCNT) and multiwalled carbon nanotubes (MWCNT) are examined on flow and heat transfer characteristics. Thermofluid performance within Water and Engine Oil base fluid are investigated using thermal conductivity and viscosity of both single and multiple wall carbon nanotubes (CNTs) of similar volume. The governing partial differential equations are converted into nonlinear, coupled, ordinary differential equations which are solved numerically using a combination of Quasi-linearization and Chebyshev pseudo-spectral methods. Effects of relevant parameters on physical quantities are examined. The obtained results revealed an enhancement in temperature as well as corresponding thermal boundary layer thickness with volume fraction of both types of carbon nanotubes. Due to higher density and thermal conductivity, SWCNTs offer higher skin friction and Nusselt number. Engine oil base fluid depicted higher heat transfer rate and local skin friction than water based fluid for both type of carbon nanotubes. Residual error are plotted to check the accuracy of obtained results. Comparison with previously published literature is made and an excellent agreement is observed which validate our applied numerical scheme.     

Natural convection of micropolar fluid in an enclosure with boundary element method

The contribution deals with numerical simulation of natural convection in micropolar fluids, describing flow of suspensions with rigid and underformable particles with own rotation. The micropolar fluid flow theory is incorporated into the framework of a velocity–vorticity formulation of Navier–Stokes equations. The governing equations are derived in differential and integral form, resulting from the application of a boundary element method (BEM). In integral transformations, the diffusion-convection fundamental solution for flow kinetics, including vorticity transport, heat transport and microrotation transport, is implemented. The natural convection test case is the benchmark case of natural convection in a square cavity, and computations are performed for Rayleigh number values up to 10⁷. The results show, which microrotation of particles in suspension in general decreases overall heat transfer from the heated wall and should not therefore be neglected when computing heat and fluid flow of micropolar fluids.     

Influence of Fe nanoparticles diameters on the structure and electron emission studies of carbon nanotubes and multilayer graphene

In this paper we report the effect of Fe film thickness on the growth, structure and electron emission characteristics of carbon nanotubes (CNTs) and multilayer graphene deposited on Si substrate. It is observed that the number of graphitic shells in carbon nanostructures (CNs) varies with the thickness of the catalyst depending on the average size of nanoparticles. Further, the Fe nanoparticles do not catalyze beyond a particular size of nanoclusters leading to the formation of multilayer graphene structure, instead of carbon nanotubes (CNTs). It is observed that the crystallinity of CNs enhances upon increasing the catalyst thickness. Multilayer graphene structures show improved crystallinity in comparison to CNTs as graphitic to defect mode intensity ratio (I_D/I_G) decreases from 1.2 to 0.8. However, I_2D/I_G value for multilayer graphene is found to be 1.1 confirming the presence of at least 10 layers of graphene in these samples. CNTs with smaller diameter show better electron emission properties with enhancement factor (γ_C = 2.8 × 10³) in comparison to multilayer graphene structure (γ_C = 1.5 × 10³). The better emission characteristics in CNTs are explained due to combination of electrons from edges as well as centers in comparison to the multilayer graphene.     

MHD boundary-layer flow of a micropolar fluid past a wedge with variable wall temperature

Summary The steady laminar MHD boundary-layer flow past a wedge immersed in an incompressible micropolar fluid in the presence of a variable magnetic field is investigated. The governing partial differential equations are transformed to the ordinary differential equations using similarity variables, and then solved numerically using a finite-difference scheme known as the Keller-box method. Numerical results show that the micropolar fluids display drag reduction and consequently reduce the heat transfer rate at the surface, compared to the Newtonian fluids. The opposite trends are observed for the effects of the magnetic field on the fluid flow and heat transfer characteristics.     

Flow and heat transfer on cryogenic flow boiling during tube quenching under upward and downward flow

The gravity effects on quenching of tube by cryogenic fluids for the development of cryogenic fluid management on orbit are studied. In this paper, the effects of the tube diameter, the flow directions, and the mass velocity on the tube quenching using liquid nitrogen are investigated systematically in the terrestrial conditions. The experiments are performed by the mass velocity between 100–600 kg/m²s in downward and upward flow directions by using three difference inner diameters of the transparent heated tube (7, 10, 13.6 mm) for measuring fluid behavior observations and heat transfer measurements simultaneously. The results indicate that the difference between the minimum heat fluxes under downward and upward flow conditions increased as the mass velocity increased. These characteristics of heat transfer were caused by filamentary flow pattern that was found in only downward flow and high mass velocity conditions.     

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

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

Investigation on Heat Transfer Properties of Supercritical Water in a Rock Fracture for Enhanced Geothermal Systems

Supercritical water (SCW) has shown promise as a working fluid to extract heat from hot dry rock (HDR); however, fundamental research on its heat transfer characteristics in HDR fractures is still required. A 2D heat transfer model that considers the variable thermophysical properties was updated to numerically investigate the effects of mass flow rate, thermal reservoir temperature, and fracture aperture size on the heat transfer characteristics of SCW flow through a single HDR fracture. The heat transfer performance of SCW and supercritical CO₂ (scCO₂) was compared under the same conditions. The results indicate that the heat transmission performance of SCW is superior to scCO₂ at high temperature and high pressure. It is essential to synthesize the thermal reservoir temperature and pressure, site conditions, and heat transmission fluids during HDR development.     

Hall effect on MHD flow and heat transfer over a stretching sheet with variable thickness

We investigate the MHD flow and heat transfer of an electrically conducting fluid over a stretching sheet with variable thickness. The wall temperature and the wall velocity are assumed to vary. The effects of external magnetic field along the sheet and the Hall currents are considered. The governing equations are solved numerically using an implicit finite difference scheme. The obtained numerical results are compared with the available results in the literature for some special cases and the results are found to be in very good agreement. The effects of the physical parameters on the velocity and temperature fields are presented graphically and analyzed. The effect of the Hall current gives rise to a cross flow. Moreover, the Hall current and the magnetic field have strong effect on the flow and heat transfer characteristics, i.e., shear stress and the Nusselt number.     

An experimental investigation and modelling of flow boiling heat transfer of isobutane-compressor oil solution in a horizontal smooth tube

Experimental results of local heat transfer coefficients for the boiling of working fluids (solutions of R600a with mineral naphthenic oil ISO VG 15) in a smooth tube with a small diameter (5.4 mm) are presented. The experiments have been performed in the following ranges: for the inlet pressure from 65.7 kPa to 82.2 kPa, for the heat flux from 2500 to 3300 W m⁻², and for the mass velocity of the working fluid from 11.90 to 15.99 kg m⁻² s⁻¹). The quantitative estimation in reduction of the heat transfer coefficient of the wetted surface in the evaporator at a high oil concentration in the mixture is examined. The influence of heat flux and mass velocities on the values of the local heat transfer coefficients is analyzed. The equation for the modelling of the local heat transfer coefficient for boiling of an isobutane/compressor oil solution flow in the tube is suggested.     

Analysis of Silver Nanoparticles in Engine Oil: Atangana–Baleanu Fractional Model

The present article aims to examine the heat and mass distribution in a free convection flow of electrically conducted, generalized Jeffrey nanofluid in a heated rotatory system. The flow analysis is considered in the presence of thermal radiation and the transverse magnetic field of strength B₀. The medium is porous accepting generalized Darcy’s law. The motion of the fluid is due to the cosine oscillations of the plate. Nanofluid has been formed by the uniform dispersing of the Silver nanoparticles in regular engine oil. The problem has been modeled in the form of classical partial differential equations and then generalized by replacing time derivative with Atangana–Baleanu (AB) time-fractional derivative. Upon taking the Laplace transform technique (LTT) and using physical boundary conditions, exact expressions have been obtained for momentum, energy, and concentration distributions. The impact of a number of parameters on fluid flow is shown graphically. The numerical tables have been computed for variation in the rate of heat and mass transfer with respect to rooted parameters. Finally, the classical solution is recovered by taking the fractional parameter approaching unity. It is worth noting that by adding silver nanoparticles in regular engine oil, its heat transfer rate increased by 14.59%, which will improve the life and workability of the engine.     

Effects of a heated strip arrangement on heat transfer rate in cubical enclosures

This work addresses a numerical approach based on the finite volume method and a full multi-grid technique to study three-dimensional flow structures and heat transfer rates in cubical cavity partially heated from one wall. The working fluid is air so that the Prandtl number equates to 0.71. Numerical solutions are generated for representative values of the controlling Rayleigh number inside the range 10³ ≤ Ra ≤ 10⁷. The heating occurs with a heated strip placed inside the enclosure. Three scenarios are investigated: (i) the heated source is mounted on the bottom horizontal wall, (ii) the heated source is mounted horizontally on the lateral wall, and (iii) the heated source is mounted vertically on the lateral wall. Effects of heated strip position in the enclosure on heat transfer are studied. It is shown that suitable configuration which generates highest heat transfer rate through the heated strip is depending on Rayleigh number. Results are presented in the form of projection of flow lines and isotherms plots as well as the variation of the Nusselt number and the average temperature at the heat source surface for all configurations computed in this study.     

Influence of oil on nucleate pool boiling heat transfer of refrigerant on metal foam covers

Nucleate pool boiling heat transfer characteristics of refrigerant/oil mixture on metal foam covers were experimentally investigated. The refrigerant is R113, and the oil is VG68. The copper foams, having ppi (pores per inch) of 10 and 20, porosity from 90% to 98%, and thickness of 5 mm, are selected in this study. Experimental conditions include a saturation pressure of 101 kPa, oil concentrations from 0 to 5%, and heat fluxes from 0 to 80 kW m⁻². The experimental results indicate that the nucleate pool boiling heat transfer coefficient on copper foam covers is larger than that on flat heated surface by a maximum of 160% under the present experimental conditions; the presence of oil deteriorates the nucleate pool boiling heat transfer on copper foam covers by a maximum of 15% under the present experimental conditions, and the deterioration of oil on nucleate pool boiling heat transfer on copper foam covers is lower than that on a flat heated surface. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant/oil mixture on copper foam cover is developed, and it agrees with 95% of the experimental data within a deviation of ±20%.     

Flow of a thermomicropolar fluid with stretch

The linear theory of micropolar fluids with stretch including the heat conduction and heat dissipation effects and restrictions imposed by the second law of thermodynamics are stated. The final equations of motion are discretized and solved for the case of a flow between two parallel and heated horizontal plates, stationary with respect to the flow. The effect of microstructure on heat transfer and fluid flow is studied thoroughly, including the contribution of terms involving inertial spin.

The microrotation boundary conditions are discussed along with their influence on the velocity, gyration, and microstretch fields as well as on heat transfer. An interesting and experimentally observed phenomenon involving the appearance of two maximums in the velocity profile is noted.     

A pumped supercritical helium flow loop for heat transfer studies

The construction and operation of a flow loop is described in which a 0.5 ls^? centrifugal pump circulates supercritical helium through a 1 m long, 18 mm id heated test section instrumented with 18 carbon resistance thermometers. Based on the heat transfer measurements obtained (published in detail elsewhere) some observations are made on deviations from the standard Dittus Boelter heat transfer correlation caused by helium's variable properties, and on possible buoyancy induced reductions in heat transfer particularly for radially inward flow in rotating machines.     

Thermal boundary layers over a shrinking sheet: an analytical solution

In this paper, the heat transfer over a shrinking sheet with mass transfer is studied. The flow is induced by a sheet shrinking with a linear velocity distribution from the slot. The fluid flow solution given by previous researchers is an exact solution of the whole Navier–Stokes equations. By ignoring the viscous dissipation terms, exact analytical solutions of the boundary layer energy equation were obtained for two cases including a prescribed power-law wall temperature case and a prescribed power-law wall heat flux case. The solutions were expressed by Kummer’s function. Closed-form solutions were found and presented for some special parameters. The effects of the Prandtl number, the wall mass transfer parameter, the power index on the wall heat flux, the wall temperature, and the temperature distribution in the fluids were investigated. The heat transfer problem for the algebraically decaying flow over a shrinking sheet was also studied and compared with the exponentially decaying flow profiles. It was found that the heat transfer over a shrinking sheet was significantly different from that of a stretching surface. Interesting and complicated heat transfer characteristics were observed for a positive power index value for both power-law wall temperature and power-law wall heat flux cases. Some solutions involving negative temperature values were observed and these solutions may not physically exist in a real word.     

Effect of Reynolds number on flow and heat transfer in incompressible forced convection over a 3D backward-facing step

A three-dimensional incompressible numerical model for the case of the 3D backward-facing step flow is established to investigate the characteristics of fluid flow and heat transfer in the low and middle Reynolds number ranges (200 ≤ Re≤1400). The governing equations, including continuous, unsteady Navier–Stokes and energy equations, are solved by the finite volume method in FLUENT. The simulation results show that the time averaged reattachment length reaches the peak value at Re = 1000, and subsequently decreases as the increase of Re. The formation of secondary peak Nu influenced by flow instability has a better contribution to the heat transfer at the center area. Taking away the hot fluid and carrying the cold fluid into the floor wall, which is caused by periodic instability, have positive effects on heat transfer enhancement.     

Optimization of field emission properties of carbon nanotubes by Taguchi method

It is the purpose of this study to evaluate the field emission property of carbon nanotubes (CNTs) prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. Nickel layer of 5 nm in thickness on 20-nm thickness titanium nitride film was transformed into discrete islands after hydrogen plasma pretreatment. CNTs were then grown up on Ni-coated areas by MPCVD. Through the practice of Taguchi method, superior CNT films with very low emission onset electric field, about 0.7 V/μm (at J = 10 μA/cm²), are attained without post-deposition treatment. It is found that microwave power has the most important influence on the field emission characteristics of CNT films. The increase of methane flow ratio will downgrade the degree of graphitization of CNT and thus its field emission characteristics. Scanning electron microscope and transmission electron microscopy (TEM) observation and energy dispersive X-ray spectrometer analysis reveal that CNT growth by MPCVD is based on tip-growth mechanism. TEM micrographs validate the hollow, bamboo-like structure of the multi-walled CNTs.     

Natural convection from a discrete heater in enclosures filled with a micropolar fluid

In this study, we numerically investigate the steady laminar natural convective flow and heat transfer of micropolar fluids in enclosures with a centrally located discrete heater in one of its sidewalls by applying a finite difference method. The other sidewall is kept at isothermal conditions, while horizontal walls are assumed to be insulated. Computations are carried out to investigate effects of the dimensionless heater length, the material parameter of the micropolar fluid, the Rayleigh number and the Prandtl number both for weak and strong concentration cases. Local results are presented in the form of streamline and isotherm plots as well as the variation of the local Nusselt number through the discrete heater. It was shown that micropolar fluids presented lower heat transfer values than those of the Newtonian fluids. An increase at the material parameter, K is shown to decrease the heat transfer. The results for K = 0, which corresponds to the Newtonian fluid case is compared with those available in the existing literature and, an excellent agreement is obtained.     

Onset of convection in the Rayleigh–Bénard flow under non-Boussinesq conditions: an asymptotic approach

The effect of temperature dependent fluid properties on the onset of convection in a horizontal fluid layer heated from below is investigated. Based on asymptotic expansions of all fluid properties with respect to temperature final results are deduced that are independent of specific property laws. They are general in nature and hold for all Newtonian fluids and all (small) heat transfer rates. A special point of interest is the widely used Boussinesq approximation. Our asymptotic results clearly show how this approximation affects the stability calculations of the flow under investigation.     

Forced flow boiling heat transfer of liquid hydrogen for superconductor cooling

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was first measured at the pressure of 0.7 MPa for wide ranges of flow rates and liquid temperatures. The heat transfer coefficients in non-boiling regime for each flow velocity were well in agreement with the Dittus–Boelter equation. The heat fluxes at the inception of boiling and the departure from nucleate boiling (DNB) heat fluxes are higher for higher flow velocity and subcooling. It was found that the trend of dependence of the DNB heat flux on flow velocity was expressed by the correlation derived by Hata et al. based on their data for subcooled flow boiling of water, although it has different propensity to subcooling.