Numerical simulation of nanofluid flow over diamond-shaped elements in tandem in laminar and turbulent flow

In this paper, the Al2O3-water nanofluid flow in laminar and turbulent flows inside tubes fitted with diamond-shaped turbulators is numerically modeled. The nanofluid flow is modeled by employing a two-phase mixture method and applying the constant heat flux boundary condition at tube walls. In the results, the effects of different parameters such as the geometry of turbulators, volume fraction and diameter of nanoparticles, etc. on the flow field in the tubes have been investigated. The obtained results indicate that, with the reduction of tail length ratio (TR) and increase of vertex angle of turbulators (θ), the heat transfer coefficient as well as the wall shear stress increase. Similarly, with the reduction of TR and increase of θ, the amount of secondary flows, vortices and the turbulent kinetic energy increase. Moreover, the increase in the volume fraction of nanoparticles and the reduction of nanoparticles diameter lead to the increase of the heat transfer coefficient and wall shear stress

Numerical simulation and parametric study of laminar mixed convection nanofluid flow in flat tubes using two phase mixture model

In this article, the laminar mixed convection of Al2O3-Water nanofluid flow in a horizontal flat tube has been numerically simulated. The two-phase mixture model has been employed to solve the nanofluid flow, and constant heat flux has been considered as the wall boundary condition. The effects of different and important parameters such as the Reynolds number (Re), Grashof number (Gr), nanoparticles volume fraction (Φ) and nanoparticle diameter (dp) on the thermal and hydrodynamic performances of nanofluid flow have been analyzed. The results of numerical simulation were compared with similar existing data and good agreement is observed between them. It will be demonstrated that the Nusselt number (Nu) and the friction factor (Cf) are different for each of the upper, lower, left and right walls of the flat tube. The increase of Re, Gr and f and the reduction of dp lead to the increase of Nu. Similarly, the increase of Re and f results in the increase of Cf. Therefore, the best way to increase the amount of heat transfer in flat tubes using nanofluids is to increase the Gr and reduce the dp

Optimal arrangement of rotating hot cylinders in a compact heat exchanger based on energy and exergy analysis

The chief aim of this research is to explore the various rotation patterns of hot cylinders in four non-linear structures in laminar flow regime with different rotational speed(ω). For this purpose, using the finite volume method, the thermo-hydraulic parameters such as Nusselt number (Nu), friction factor, and thermo-hydraulic performance parameter (THP) of the cooling fluid, water, were investigated. This is a three-dimensional numerical study on a compact heat exchanger in which all possible rotation patterns of hot cylinders in non-linear structures were analyzed based on energy and exergy approaches. The results showed that in each non-linear structure of the cylinders, with increasing the rotational speed, at least one of the rotation patterns had a better thermo-hydraulic performance. In this regard, 22 possible patterns are investigated in a way that one of which provides a better performance based on first and second law of thermodynamic. In the seventh rotation pattern, which is known as C27 and belongs to the second non-linear structure, the thermo- hydraulic parameter (THP) is improved by 38% compared to the linear structure of rotating hot cylinder

Effect of Inserting Coiled Wires in Tubes on the Fluid Flow and Heat Transfer Performance of Nanofluids

In the present study, numerical study of Al2O3-water nanofluid flow in different coiled wire inserted tubes are performed to investigate the effects of inserting coiled wires in tubes on the fluid dynamic and heat transfer performance ofv nanofluids. The numerical simulations of nanofluids are performed using two phase mixture model. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. The effects of inserting coiled wires in tubes on different parameters such as heat transfer coefficient, pressure drop, temperature distribution, velocity distribution and secondary flows are presented and discussed. The results show that using coiled wire in tubes leading to increase in  about 13.44% but increase the Δp about 14.66% with respect to the flow without nanofluid and coiled wire. Similarly, using nanofluid leading to increase in  about 5.52% but increase the  Δp about 8.92%. Finally, using both of the mentioned heat transfer enhancement mechanisms leading to increase in  about 17.51% but increase the value of Δp about 22.86%

Study of the critical velocity of the tunnels using an analytical approach

Fire safety is one of the major design issues of tunnel engineers. Longitudinal ventilation is a common method for exhausting smoke and hot gases in tunnel fires. The minimum ventilation air velocity along the tunnel to prevent smoke back layering is called the critical velocity. This critical velocity is the key element in designing the longitudinal exhaust system. Several researchers have studied the impact of different parameters on critical velocity, including tunnel geometry, tunnel height, and fire magnitude, numerical and experimentally. In this study, an analytical solution was used to solve a third-order non-linear differential equation to determine the critical velocity of the tilted tunnel. Dimensional analysis can significantly reduce the costs associated with the experimental study in the full-scale tunnels. The Froude number representing the power of the buoyancy force against the inertial force is widely used in the critical velocity studies. This study validated our analytical results with critical values obtained from an experimental and numerical simulation in a scaled model with a ratio of 1:8. The results showed that the values calculated for the critical velocity employing an analytical solution were lower than the numerical and experimental studies’ values. Data from the latest international standard was used to enhance the precision of the critical velocity calculation. We showed that using a modified Froude number can significantly increase the accuracy of the analytical solution. The critical velocity values obtained using the modified Froude number were then compared with experimental results from full-scale tests. This study emphasized that the analytical solution for the critical velocity saves a significant amount of time compared to the iterative solutions while keeping the accuracy in a reasonable range