Wu-Shung Fu

Thermal enhancement in laminar channel flow with a porous block

Abstract A study of a porous block mounted on a heated wall in a laminar flow channel to enhance convection heat transfer rate was investigated numerically. A numerical method of SIMPLEC is adopted to solve governing equations, as for the energy equation, one-equation thermal model with Van Driests wall function is considered. The parameters that include porosity e, particle diameter D p , Reynolds number Re , and blocked ratio HP are studied, and for simulating more realistically, the porosity is taken into consideration as variable. All the non-Darcian effects including the channeling effects, solid boundary effects, and inertial effects are also considered. The effects of the above parameters on the thermal performance of the heated wall are examined in detail. The results indicate that for HP = 0.5 case the thermal performances are enhanced by using a porous block with higher porosity and particle diameter. However, the results are the opposite for HP = 1.0.

Numerical investigation of heat transfer from a heated oscillating cylinder in a cross flow

Abstract A numerical simulation is performed to study the flow structures and heat transfer characteristics of a heated transversely oscillating cylinder in a cross flow. The variations of flow and thermal fields are classified into a class of moving boundary problems. The moving interfaces between the fluid and cylinder have been considered. An arbitrary Lagrangian–Eulerian kinematic description method is adopted to describe the flow and thermal fields. A penalty consistent finite element formulation is applied to solve the governing equations. The subsequent developments of the vortex shedding and heat transfer characteristics around the heated cylinder are presented in detail. The effects of Reynolds number, oscillating amplitude, oscillating speed on the flow structures and heat transfer characteristics are examined. The results show that the interaction between the oscillating cylinder and vortex shedding from the cylinder dominates the state of the wake. The flow and thermal fields may approach a periodic state in lock-in regime. The heat transfer of the cylinder in the lock-in regime is enhanced remarkably.

Thermal performances of different shape porous blocks under an impinging jet

Abstract A study of the enhancement of the convection heat transfer of a laminar slot jet impinging on a porous block mounted on a heated region was investigated numerically. A numerical method (SIMPLEC) was adopted to solve the governing equations, and a one-equation thermal model with Van Driests wall function was considered for solving the energy equation. Three different shape porous blocks (rectangle, convex and concave) were studied. The results indicated that the heat transfer is mainly affected by a fluid flowing near the heated region. For a lower porous block, the heat transfer is enhanced by the three types of porous block. However, for a higher porous block, heat transfer is only enhanced by the concave porous block.

Numerical investigation of heat transfer characteristics of the heated blocks in the channel with a transversely oscillating cylinder

Abstract A numerical simulation is performed to study the influence of an oscillating cylinder on the heat transfer from heated blocks in a channel flow. An arbitrary Lagrangian–Eulerian kinematics description method is adopted to describe the flow and thermal fields. A penalty consistent finite element formulation is applied to solve the governing equations. The effects of Reynolds number, oscillating amplitude and oscillating frequency on the heat transfer characteristics of the heated wall are examined. The results show that the heat transfer from heated blocks is enhanced remarkably as the oscillating frequency of the cylinder is in lock-in region.

TRANSIENT LAMINAR NATURAL CONVECTION IN AN ENCLOSURE PARTITIONED BY AN ADIABATIC BAFFLE

Transient laminar natural convection in a two-dimensional enclosure partitioned by an adiabatic baffle is investigated numerically, A penalty finite-element method with a Newton-Raphson iteration algorithm and a backward difference scheme dealing with the time term are adopted to solve governing equations. The effects of the baffle and Rayleigh number on the heat transfer mechanism are found to be substantial during the transient process for Rayleigh numbers of 104 and 106. However, the variations of the heat transfer mechanism occur mainly in the first one-third of the time period of the transient, in spite of the presence, absence, or location of a baffle.

ENHANCEMENT OF NATURAL-CONVECTION HEAT-TRANSFER OF AN ENCLOSURE BY A ROTATING CIRCULAR-CYLINDER

Abstract Enhancement of natural convection heat transfer in an enclosure by a rotating cylinder is investigated numerically. A penalty finite-element method with a Newton-Raphson iteration algorithm is adopted to solve the governing equations with the boundary conditions, and the accuracy of solutions is sensitively affected by the grid system combining curve-sided and rectangular quadrilateral elements. Since the flow fields of fluids are induced by mutual interaction between the natural convection and the rotating cylinder, the contribution of the rotating cylinder to natural convection heat transfer depends on the direction of rotation of the cylinder. For the counter-clockwise rotating cylinder situation, the contribution is found to be substantial when the value of Gr Re 2 is larger than 100: however, for the clockwise rotation cylinder situation, the contribution is hardly found even when the value of Gr Re 2 is equal to 1.

TRANSIENT THERMAL-CONVECTION IN AN ENCLOSURE INDUCED SIMULTANEOUSLY BY GRAVITY AND VIBRATION

Abstract Transient thermal convection in a two-dimensional square enclosure induced simultaneously by gravity and vibration is investigated numerically. The enclosure, which is filled with air under a terrestrial environment, is insulated at both horizontal walls and kept at constant temperature at the vertical walls. For time t=0 , the fluid is stationary with the same temperature as the vertical walls T c ; as t > 0 , the left wall temperature is raised to T h and the enclosure is vibrated with a constant frequency Ω and amplitude b simultaneously. In order to study the effect of vibration frequency on the transient thermal convection, four vibration frequencies (100, 900, 1100, 5000) are considered with fixed Rayleigh number ( Ra=10 4 ) and vibrational Grashof number ( G = 10 6 ). The results show that the transient process, from the stationary state to the steady flow state, is shortened by increasing the vibration frequency, and both the flow field and heat transfer mechanism are mainly determined by the vortex shedding rate, which has the same frequency as the vibration frequency near the upper and lower corners of the hot wall. For ω=100 , a single main cell is formed and alternates the rotating direction with the variation of the buoyancy force direction. For ω=5000 , the buoyancy force induced by the vibration is definitely dominant and the development of temperature distribution from left to right sides is initially symmetric at the center line of the vertical wall; afterwards, an instability of the thermal boundary layer causes an overshoot of the total Nusselt number and an increase of flow intensity before the periodic solution is approached. For ω=900 and 1100, the vortices shed continuously and alternately from the upper and lower corners near the hot wall, which causes the variation of the total Nusselt number to be irregular and inconsistent on the hot and cold walls for the ω=900 case; for the ω=1100 case the total Nusselt number varies irregularly in the transient processes, but a periodic solution is obtained at steady state.

Numerical investigation of heat transfer of a heated channel with an oscillating cylinder

A numerical simulation is performed to study the influence on the heat transfer rate of the heated wall in the channel with an oscillating cylinder. An arbitrary Lagrangian-Eulerian kinematic description method is adopted to describe the flow and thermal fields. A penalty consistent finite-element formulation is applied to solve the governing equations. The effects of Reynolds number, oscillating amplitude, oscillating frequency, eccentric ratio, and blockage on the heat transfer characteristics of the heated wall are examined. The results show that not only the region relating to heat transfer is enlarged substantially, but also that the heat transfer rate of this region is enhanced remarkably.

Effects of a random porosity model on heat transfer performance of porous media

Abstract Due to the non-uniform distribution and fractural structure of the beads inside a porous medium, the porosity distributed in the porous medium is random for most realistic situations. Therefore, the effects of the porosity distributed casually inside a porous block mounted on a heated region with a laminar slot impinging jet on flow and thermal fields are investigated numerically. A numerical method of SIMPLEC is adopted to solve governing equations, as for the energy equation, a one-equation thermal model with Van Driests wall function is adopted. All the non-Darcian effects including the solid boundary and inertial effects are considered and three different porosity models of constant, variable and random are examined. The results indicate that the relationship between the local Nusselt number Nu x and the near wall local porosity e x is a negative correlation. Consequently, in order to enhance the thermal performance of the porous medium, the porosity near the solid plate should be smaller to make the conductive heat transfer to be dominant.

A new model for heat transfer of fins swinging back and forth in a flow

Abstract In this paper, a new concept of an electronic device cooling method is proposed. In this method, extremely thin fins are used for swinging back and forth in a flowing fluid. The boundary layers attaching on the fins are then contracted and disturbed, and the heat transfer rate of the fins can be enhanced remarkably. The dynamic behavior between the fins and fluid is classified into a class of the moving boundary problems. A Galerkin finite element formulation with an arbitrary Lagrangian–Eulerian kinematic description method is adopted to solve this problem. The parameters of velocities of the fluid and the swinging speed of the fins are employed to investigate the variations of the flow and thermal fields. The results show that the velocity and thermal boundary layers may be contracted and disturbed, which results in a significant heat transfer enhancement, being attained.