Mixed convective heat transfer enhancement in a ventilated cavity by flow modulation via rotating plate

The present study numerically explores the mixed convection phenomena in a differentially heated ventilated square cavity with active flow modulation via a rotating plate. Forced convection flow in the cavity is attained by maintaining external fluid flow through an opening at the bottom of the left cavity wall while leaving it through another opening at the right cavity wall. A counter-clockwise rotating plate at the center of the cavity acts as an active flow modulator. Moving mesh approach is used for the rotation of the plate and the numerical solution is achieved using arbitrary Lagrangian-Eulerian finite element formulation with a quadrilateral discretization scheme. Transient parametric simulations have been performed for various frequency of the rotating plate for a fixed Reynolds number (Re) of 100 based on maximum inlet flow velocity while the Richardson number (Ri) is maintained at unity. Heat transfer performance has been evaluated in terms of spatially averaged Nusselt number and time-averaged Nusselt number along the heated wall. Power spectrum analysis in the frequency domain obtained from the fast Fourier transform analysis indicates that thermal frequency and plate frequency start to deviate from each other at higher values of velocity ratio (>4).     

Research on the effect of Reynolds correlation in natural convection film condensation

Film condensation is a vital phenomenon in the nuclear engineering applications,such as the gas-steam pressurizer design,and heat removing on containment in the case of postulated accident.Reynolds number in film condensation can be calculated from either the mass relation or the energy relation,but few researches have distinguished the difference between them at present.This paper studies the effect of Reynolds correlation in the natural convection film condensation on the outer tube.The general forms of the heat transfer coefficient correlation of film condensation are developed in different flow regimes.By simultaneously solving a set of the heat transfer coefficient correlations with Remass and Reenergy,the general expressions for Remass and Reenergy and the relation between the corresponding heat transfer coefficients are obtained.In the laminar and wavefree flow regime,Remass and Reenergy are equivalent,while in the laminar and wavy flow regime,Remass is much smaller than Reenergy,and the deviation of the corresponding average heat transfer coefficients is about 30％ at the maximum.In the turbulent flow regime,the relation of Remass and Reenergy is greatly influenced by Prandtl number.The relative deviation of their average heat transfer coefficients is the nonlinear function of Reynolds number and Prandtl number.Compared with experimental results,the heat transfer coefficient calculated from Reenergy is more accurate.     

A stencil of the finite-difference method for the 2D convection diffusion equation and its new iterative scheme

The paper gives the numerical stencil for the two-dimensional convection diffusion equation and the technique of elimination, and builds up the new iterative scheme to solve the implicit difference equation. The scheme's convergence and its higher rate of convergence than the Jacobi iteration are proved. And the numerical example indicates that the new scheme has the same parallelism and a higher rate of convergence than the Jacobi iteration.     

Mesoscopic approach for steady‐state free convection in a diamond array

The purpose of the present paper is testing an in‐house efficiency algorithm based on lattice Boltzmann method (LBM) and using it to resolve the obtained coupled nondimensional governing equations to analyze two‐dimensional free convection inside a cold outer cavity subjected to a heated cylindrical diamond array. Steady state or oscillatory results are obtained using the Bhatnagar‐Gross‐Krook collision model associated to the thermal LBM. Both the velocity and temperature fields are solved using the D2Q9 models. With different Rayleigh numbers (Ra), the tested free convection can either achieve to steady state or oscillatory. We extended our in house Fortran 90 code using curved boundary conditions and implemented them into a cavity with a diamond array. The numerical simulations were done using distinct Ra (10⁶ and 10 ⁷) and distances between the four neighboring circular cylinders aligned in a diamond array. The effects of several physical parameters, including Ra and position of the hot body array on flow and heat transfer characteristics are investigated. The obtained results are highlighted in the form of streamlines, isotherms, and velocities plots. We show in this paper the stability and the efficiency of the LBM to deal with a complex geometry and its ability to reach suitable convergence criteria for high Ra (10 ⁶ and 10 ⁷). The numerical results indicate that LBM can simulate numerical problems with a high Ra reaching a steady state where we can depict the change of the flow pattern and enhancement of the heat transfer in the presence of heated diamond array.     

Alternating group explicit method with exponential-type for the diffusion–convection equation

Based on the semi-explicit asymmetric exponential schemes constructed by the author, a new alternating group explicit (AGE) method with exponential-type for the numerical solution of the convection–diffusion equation is derived in the paper. The method has the obvious property of parallelism and is unconditionally stable. The results of numerical examples are given to show the effectiveness of the present methods that are in preference to the Evans and Abdullah' method in [Evans, D.J. and Abdullah, A.R., 1985, A new explicit method for the diffusion–convection equation. Computers & Mathematics with Application, 11, 145–154].     

Geometrical effect on laminar mixed convection in the entrance region of shrouded arrays of heated rectangular blocks

Abstract

The effect of geometric parameter on laminar mixed convection in the entrance region of shrouded arrays of heated rectangular blocks is approached numerically for large Prandtl number fluid. The problem considered is related to convective cooling of electronic components mounted on horizontal circuit boards. Typical development of streamline and isotherm, block wall temperature distribution, and local Nusselt number are presented. It is found that the secondary flow leads to a significant enhancement in heat transfer. The multiple eddies above the block, induced by the combined geometric and buoyancy effect, lead to a more uniform block wall temperature distribution and then reduce the magnitude of the highest wall temperature.     

Lattice Boltzmann simulation of MHD mixed convection in a two‐sided lid‐driven square cavity

In this paper the effects of a magnetic field on mixed convection flow in a two‐sided lid‐driven cavity have been analyzed by the lattice Boltzmann method (LBM). The Hartmann number varied from Ha = 0 to 100. The study has been conducted for different Richardson numbers (Ri) from 0.01 to 100 while the direction of the magnetic field was investigated in the x‐direction. Consequences demonstrate that the heat transfer augments with an increment of the Richardson number for different Hartmann numbers for two cases. The heat transfer declines with the growth of the magnetic field for various Richardson numbers for two cases. The difference between the values of heat transfer for the two cases at variant parameters is negligible but the trend of fluid flow for the two cases is multifarious. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20402     

Calculation of Heat Transfer Coefficients on a Flat Plate by Pseudo Direct Numerical Simulation of Turbulence

Pseudo direct numerical simulation applying the finite difference method is performed on a fully-developed turbulent flow through parallel flat walls heated by uniform flux. The study is aimed at examining the influence brought to the turbulent field calculation by differences in the level of spatial resolution obtainable in the streamwise and spanwise directions. All in all, satisfactory results are obtained for the mean profiles and second-order moments of velocity and temperature, as well as on the Nusselt number of various flows presenting-various Prandtl numbers. Differences in the level of spatial resolution prove to affect little the accuracy of calculated results.     

Experimental and numerical studies on periodic convection flow and heat transfer in a lid-driven arc-shape cavity

This paper presents experimental and numerical studies on periodic convection flow and heat transfer in a lid-driven arc-shape cavity with temperature differential. Three cases were considered: Gr = 2 × 10⁵, 5 × 10⁵ and 1.2 × 10⁶ at Re = 100 (Gr = Grashof number; Re = Reynolds number). The mathematical model was proposed in our previous study. The current study performs an experiment to validate this model, to corroborate the existence of the periodic flow, and to more deeply probe the internal flow and temperature characteristics. The experimental setup primarily comprised an arc-shape cavity, a moving lid, a thermo-system, a smoke generator and an image acquisition system. The periodic convection flow in the cavity was visualized using kerosene smoke. The numerical and experimental results consistently reveal that the periodic flow pattern was observed in the case with Gr = 5 × 10⁵, whereas the steady-state flow pattern took place in the other two cases (Gr = 2 × 10⁵ and Gr = 1.2 × 10⁶). The numerical simulation produced reasonable and satisfactory agreement with the experiment for the periodic flow pattern and period. The difference between the predicted and measured periods is less than 5%. The transport properties, such as average kinetic energy, overall Nusselt number, stream function, phase space trajectory, local kinetic energy, velocity history and temperature distribution, were further analyzed and discussed in this paper. The proposed numerical simulation not only confirms the experimental observation, but also enhances the understanding of periodic convection in an arc-shape cavity subjected to a moving lid and temperature differential.     

Numerical and experimental investigation of a thermosiphon solar water heater system thermal performance used in domestic applications

The thermosiphon is a passive heat exchange method, which circulates a fluid within a system without the need for any electrical or mechanical pumps. The thermosiphon is based on natural convection where the thermal expansion occurs when the temperature difference has a corresponding difference in density across the loop. Thermosiphons are used in different applications such as solar energy collection, automotive systems, and electronics. The current study aims to investigate thermosiphon thermal performance used in domestic applications. The thermal performance of a thermosiphon has been studied by many researchers; however, according to the knowledge of the authors, the influence of the amount of the working fluid on the thermal output has not yet been investigated. Therefore, the influence of the amount of working fluid within the riser pipe has been investigated on the thermal performance of the thermosiphon. In the current study, a computational fluid dynamics model is involved. This model has been validated by comparison with experimental findings. The maximum variation between numerical and experimental results is 14.2% and 11.2% for the working fluid at the inlet and outlet of the absorber pipe, respectively. Furthermore, the results show that the amount of working fluid inside the closed thermosiphon has a great influence on the thermal performance of the system. Additionally, it is found that Case-B, when the amount of working fluid is less than by 10% compared to the traditional model, is the best case among all cases under study. Furthermore, a correlation equation to predict water temperature at the exit of the absorber pipe has been established with an accuracy of 95.05%.