Study Of Laminar Mixed Convection Research Articles

Numerical simulation of mixed convection of a nanofluid in a circular pipe with different numerical models

In this paper, we present a numerical study of laminar mixed convection of a nanofluid in a pipe and compare the results to experimental measurements. Mechanisms that control the behavior of nanoparticles in the base fluid and the fluid motion are not very well known. Thus, it is important to know, which mathematical model describes the nanofluid best. For this study, we used two numerical models, that are based on the Euler–Euler modelling of multiphase flow. The comparison of results has shown that none of the numerical models could accurately model the flow of a nanofluid. Hence, to better model the nanofluid flow, we varied the nanoparticle concentration distribution at the inlet. The results show that the temperature field in the fluid and in the pipe depends strongly on the nanoarticle concentration distribution.

Experimental and numerical study of laminar mixed convection from a horizontal isothermal elliptic cylinder

The problem of laminar mixed convective heat transfer from a horizontal isothermal elliptic cylinder has been experimentally and numerically investigated. The test elliptic cylinder has major and minor diameters of 70.3 mm and 34.9 mm respectively with an axis ratio of 0.496. The buoyancy due to the temperature difference between the cylinder surface and incoming air produces a Grashof number of 2.1 × 106. Via the incoming cold air velocity, the generated Reynolds number varies from 145 to 1322.8. Hence, the corresponding Richardson number is in the range of 1.2–100. The combined effect of both upward natural convection and the forced convection has been executed for angle of attack from 0° (assisting flow) to 180° (opposing flow) with an interval of 30°. The numerical solution via the finite volume method (FVM) has been accomplished to validate the obtained experimental results. The numerical results are validated by previous experimental results which show good agreement. Results are presented in the form of average and local Nusselt number around the circumference of the elliptic cylinder. In addition, new empirical correlation is obtained for the average Nusselt number as a function of Richardson number and the attack angle.

Experimental study of laminar mixed convection in a rod bundle with mixing vane spacer grids

Heat transfer by mixed convection in a rod bundle occurs when convection is affected by both the buoyancy and inertial forces. Mixed convection can be assumed when the Richardson number (Ri=Gr/Re2) is on the order of unity, indicating that both forced and natural convection are important contributors to heat transfer. In the present study, data obtained from the Rod Bundle Heat Transfer (RBHT) facility was used to determine the heat transfer coefficient in the mixed convection regime, which was found to be significantly larger than those expected assuming purely forced convection based on the inlet flow rate. The inlet Reynolds (Re) number for the tests ranged from 500 to 1300, while the Grashof (Gr) number varied from 1.5×105 to 3.8×106 yielding 0.25<Ri<4.3. Using results from RBHT test along with the correlation from the FLECHT-SEASET test program for laminar forced convection, a new correlation ​is proposed for mixed convection in a rod bundle. The new correlation accounts for the enhancement of heat transfer relative to laminar forced convection.

Numerical study of laminar mixed convection in a square open cavity

The Lattice Boltzmann Method (LBM) is used to study steady and unsteady laminar flow in a channel with an open square cavity and a heated bottom wall in two dimensions, under mixed convection flow conditions. LBM is compared to results obtained by ANSYS-FLUENT for validation. Temperature, velocity and Nusselt number agree very well in the range of Reynolds and Richardson numbers studied, i.e. 50⩽Re⩽1000 and 0.01⩽Ri⩽10. Our observations indicate that the effect of the buoyancy force is negligible for Ri⩽0.1, for all values of the Reynolds number considered. For Ri=1, 10 buoyancy effects are important, which combined with a high enough Re (≳200 in our study), causes the development of the upstream secondary vortex and the stratification of the flow into two main recirculating cells. As previously observed in earlier studies, for high enough Ri the recirculation is no longer encapsulated, the flow becomes unsteady, and an oscillatory instability develops. This is observed in our simulations starting from Re=500, Ri=10. The analysis of the unsteady regime reveals a very rich phenomenology where the geometry of the problem couples with the oscillatory thermal instability. This regime is characterized by the periodic emission of pairs of vortices generated from the upper downstream vertex of the square cavity, and pseudoperiodic variations of the Nusselt number which persist at least up to Re=1500, while the two main vortices remain in the cavity. Our observations extend previous studies and shed a new light on the characteristics of the oscillatory instability and the role of the Reynolds and Richardson numbers.

A numerical study of entry region laminar mixed convection over shrouded vertical fin arrays

A computational study of laminar mixed convection over a shrouded vertical rectangular fin array attached to a vertical base has been performed. Maintaining the base-fin system above the surrounding temperature, a fan velocity is imposed to enhance the heat transfer through the mixed convection process. Present study finds the effects of clearance spacing, fin spacing, fin length, Reynolds number and Grashof number on the thermal performance of the base-fin system. Mixed convection inlet velocity is decoupled to forced and natural convection velocity components and the resulting pressure drop across the duct length arises purely due to the forced convection velocity component. Thus, Reynolds number is estimated based on forced convection velocity component as the inlet velocity does not vanish even in pure natural convection. Computed local Nusselt number shows sharp drop near the entrance and reaches a fully developed value after a certain streamwise distance from the entrance. Further, fully developed local Nusselt number shows a clear maximum at the clearance spacing of 0.075 and 0.15 for the inter-fin spacing of 0.3 and 0.5 respectively. Pressure drop across the duct, induced natural convection velocity and overall Nusselt number are well correlated with the governing parameters of the problem.

Numerical Study of Laminar Mixed Convection of a Nanofluid in a Horizontal Curved Tube with Constant Heat Flux and Mass Flow Rate

Fully developed laminar mixed convection of a nanofluid (water/Al2O3) in a horizontal curved tube is numerically investigated. Three-dimensional elliptic governing equations have been solved to show how nanoparticle concentration affects on thermal and hydrodynamic parameters while these parameters are impressed by centrifugal and buoyancy forces under constant mass flow rate and heat flux. Comparisons with previously published experimental works on horizontal curved tubes show good agreements between the results. Results which are obtained using the two – phase mixture model indicate that adding the nanoparticles causes changes in the properties of nanofluid and finally increases the temperature of the flow. Furthermore, increasing nanoparticles volume fraction at first augments the heat transfer coefficient of nanofluid and then, for higher concentration of particles, decreases this thermal parameter of nanofluid.

Adaptive network-based fuzzy inference system analysis of mixed convection in a two-sided lid-driven cavity filled with a nanofluid

A numerical study of laminar mixed convection in a two-sided lid-driven cavity filled with a water–Al 2O 3 nanofluid is presented. The top and bottom walls of the cavity are kept at different temperatures and can slide in the same or opposite direction. The vertical walls are thermally insulated. An Adaptive Network-based Fuzzy Inference System (ANFIS) approach is developed, trained and validated using the results of a Computational Fluid Dynamics (CFD) analysis. The results show that ANFIS can successfully be used to predict the fluid velocity and temperature as well as the heat transfer rate of the cavity, with reduced computation time and without compromising the accuracy.

Numerical study of laminar mixed convection of a nanofluid in a horizontal tube using two-phase mixture model

Laminar mixed convection of a nanofluid consists of water and Al 2O 3 in a horizontal tube has been studied numerically. Two-phase mixture model has been used to investigate hydrodynamic and thermal behaviors of the nanofluid over wide range of the Grashof and Reynolds numbers. Comparisons with previously published experimental and numerical works on mixed convection in horizontal tubes show good agreements between the results. For a given particles mean diameter the effects of nanoparticles volume fractions on the hydrodynamics and thermal parameters are presented and discussed. It is shown that at the fully developed region the nanoparticle concentration does not have significant effects on the hydrodynamics parameters. However, its effects on the thermal parameters are important. Concentration of the nanoparticles is higher at the bottom of the tube and also at the near wall region.

Numerical study of laminar mixed convection of a nanofluid in horizontal curved tubes

Fully developed laminar mixed convection of a nanofluid consisting of water and Al 2O 3 in a horizontal curved tube have been studied numerically. Three-dimensional elliptic governing equations have been used. Simultaneous effects of the buoyancy force, centrifugal force and nanoparticles concentration has been presented and discussed. The nanoparticles volume fraction does not have a direct effect on the secondary flow, axial velocity and the skin friction coefficient. However, its effect on the entire fluid temperature could affect the hydrodynamic parameters when the order of magnitude of the buoyancy force becomes significant compared to the centrifugal force. For a given Reynold number, buoyancy force has a negative effect on the Nusselt number while the nanoparticles concentration has a positive effect on the heat transfer enhancement and also on the skin friction reduction.

Experimental study of laminar mixed convection in the entrance region of a horizontal semicircular duct

Combined free and forced convection is experimentally investigated for laminar water flow in the entrance region of a horizontal semicircular duet. With uniform heat input axially, measurements are made of axial and circumferential wall temperature variations, together with pressure drops across the heated section, in order to study the effects of the buoyancy-induced secondary flow. For mean Reynolds numbers ranging from 400 to 1600, a factor of over two for friction factor increase and a factor of over four for heat transfer enhancement are found at high heating rates (mean Rayleigh numbers up to 4.6 × 10 8). Local and fully developed Nusselt numbers show substantial circumferential variations and increase with increasing heat flux level. Correlations are provided which disclose some key features of the laminar mixed convection regime.

A theoretical study of laminar mixed convection from a horizontal cylinder in a cross stream

The problem of laminar mixed convection from a horizontal isothermal cylinder is considered. The free stream direction is assumed to be horizontal and perpendicular to the cylinder axis. The study is based on the solution of the full Navier-Stokes and energy equations for 2-dim. flow of a Boussinesq fluid. The free stream is assumed to start impulsively from rest and the velocity and thermal boundary layers are developed in time until reaching steady conditions. The investigation covered the ranges of Reynolds number 1 < Re < 40 and Grashof numbers up to Gr = 5 Re 2 while keeping Prandtl number at a constant value of 0.7. Comparison of results with previous experimental correlations shows a good agreement. The streamline and isotherm patterns are plotted and different aspects of the phenomenon are discussed.