Dependence of Conjugate Heat Transfer in Ribbed Channel on Thermal Conductivity of Channel Wall: An LES Study

A series of large eddy simulations was conducted to analyze conjugate heat transfer characteristics in a ribbed channel. The cross section of the rib is square and the blockage ratio is 0.1. The pitch between the ribs is 10 times the rib height. The Reynolds number of the channel is 30,000. In the simulations, the effect of the thermal resistance of the solid wall of the channel on convective heat transfer was observed in the turbulent flow regime. The numerical method used was based on the immersed boundary method and the concept of effective conductivity is introduced. When the conductivity ratio between the solid wall and the fluid (K*) exceeded 100, the heat transfer characteristics resembled those for an isothermal wall, and the cold core fluid impinging and flow recirculation mainly influenced the convective heat transfer. For K* ≤ 10, the effect of the cold core fluid impinging became weak and the vortices at the rib corners strongly influenced the convective heat transfer; the heat transfer characteristics were therefore considerably different from those for an isothermal wall. At K* = 100, temperature fluctuations at the upstream edge of the rib reached 2%, and at K* = 1, temperature fluctuations in the solid region were similar to those in the fluid region. The rib promoted heat transfer up to K* = 100, but not for K* ≤ 10. The Biot number based on the channel wall thickness appears to adequately explain the variation of the heat transfer characteristics with K*.

Maxwell Nanofluid Flow over an Infinite Vertical Plate with Ramped and Isothermal Wall Temperature and Concentration

The aim of this study is to investigate how heat and mass transfer impacts the unsteady incompressible flow of Maxwell fluid. An infinite vertical plate with ramped and isothermal wall temperature and concentration boundary conditions is considered with the Maxwell fluid. Furthermore, in this study, engine oil has been taken as a base fluid due to its enormous applications in modern science and technologies. To see the importance of nanofluids, we have suspended molybdenum disulfide in engine oil base fluid to enhance its heat transfer rate. To investigate the flow regime, the system of equations was derived in the form of partial differential equations. The exact solutions to the complex system are obtained using the Laplace transform technique. Graphically, the impact of different embedded parameters on velocity, temperature, and concentration distributions has been shown. Through using the graphical analysis, we were interested in comparing the velocity, temperature, and concentration profiles for ramped and isothermal wall temperature and concentration. The magnitude of velocity, temperature, and concentration distributions is greater for an isothermal wall and less for a ramped wall, according to our observations. We observed that adding molybdenum disulfide nanoparticles to the engine oil increased the heat transfer up to 12.899%. Finally, the corresponding skin friction, Nusselt number, and Sherwood number have been calculated and presented in a tabular form.

Assessing IDDES-Based Wall-Modeled Large-Eddy Simulation (WMLES) for Separated Flows with Heat Transfer

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction when it acts as WMLES, as claimed by the original authors. However, the model tested only on flow configurations with no heat transfer. This study takes a systematic approach to assess the performance of the IDDES model for separated flows with heat transfer. Separated flows on an isothermal wall and walls with mild and intense heat fluxes are considered. For the case of the wall with heat flux, the skin friction and Stanton number are underpredicted by the IDDES model however, the underprediction is less significant for the isothermal wall case. The simulations of the cases with intense wall heat transfer reveal an interesting dependence on the heat flux level supplied; as the heat flux increases, the IDDES model declines to predict the accurate skin friction.

An exact analysis of unsteady MHD free convection flow of some nanofluids with ramped wall velocity and ramped wall temperature accounting heat radiation and injection/consumption

This article investigates the influence of ramped wall velocity and ramped wall temperature on time dependent, magnetohydrodynamic (MHD) natural convection flow of some nanofluids close to an infinitely long vertical plate nested in porous medium. Combination of water as base fluid and three types of nanoparticles named as copper, titanium dioxide and aluminum oxide is taken into account. Impacts of non linear thermal radiation flux and heat injection/consumption are also evaluated. The solutions of principal equations of mass and heat transfer are computed in close form by applying Laplace transform. The physical features of connected parameters are discussed and elucidated with the assistance of graphs. The expressions for Nusselt number and skin friction are also calculated and control of pertinent parameters on both phenomenons is presented in tables. A comparative study is performed for ramped wall and isothermal wall to evaluate the application extent of both boundary conditions.