Turbulent mixed flow applying CFD in electronic cooling

The commercial application CFX® v. 10.0 was used to assess the thermal parameters of electronic devices in which several speed values were explored in a model with a turbulent mixed forced cooling flow. The finite-volume method was used to solve the problem equations throughout the physical domain with the standard k-є turbulence model. It was observed that the maximum temperature is not affected by influence of heat flux in the laminar and turbulent fluxes until heat flux equal 1,000 and maximum dimensionless temperature increase abruptly with heat flux to laminar and turbulent flows for injection dimensionless velocity equal 1, 2, 4, 6, 8, and 10. The results obtained allowed identifying the highest temperature when the system is submitted to combine forced and free convection, making possible to apply control actions, avoiding thermal damages to the devices that work with this cooling process.


Introduction
The heat transfer in enclosures has been studied for several engineering applications.
Results have been presented in research surveys such as in Bruchberg et al. (1976), Kakaç et al. (1987), and it has become a main topic in convective heat transfer textbooks (BEJAN, 1984).
Usually the enclosures are closed and natural convection is the single heat transfer mechanism.However, there are several applications in passive solar heating, energy conservation in building and cooling of electronic equipment, in which open cavities are applied (PENOT, 1982;HESS;HENZE, 1984;CHAN;TIEN, 1985).Ramesh and Merzkirch (2001)  the three-dimensional free convection in a vertical channel with spatially periodic, flush-mounted heat sources; Cheng and Lin (2005) present an optimization method of thermoelectric coolers using genetic algorithms and Vasiliev (2006) presents a short review on the micro and miniature heat pipes used as electronic component coolers.Devices applied for the cooling of electronic equipments are frequently based on forced convection (SPARROW et al., 1985).Altemani and Chaves (1988)  This study can be applied in many industrial applications such as solar heating, energy conservation in buildings, refrigeration of electronic equipment and other systems in which heat transfer occurs by forced convection.So, for cooling purposes, the results obtained allowed identifying the highest temperature when the system is submitted to combined forced and free convection, making possible to apply control actions, avoiding thermal damages to the devices that work with this cooling process.

Analysis
The conservation equations of mass, momentum and energy, as well as their boundary conditions, will be expressed for the system indicated in  where Rep is the porous wall Reynolds number.
The equations of the turbulent kinetic energy and its dissipation, Equations ( 6) and ( 7), were solved only in the fluid region.The turbulent kinetic energy is set to zero at solid walls, and its normal gradients are prescribed as zero at the other boundaries.The equation of dissipation is not solved at nodes that are adjacent to the wall.

Material and methods
The methodology consists in solving the physical problem using the finite volume CFD code CFX ® v. 10.0 from Ansys to discretize the three-dimensional partial differential equations of the mathematical model (PATANKAR, 1980).Scalar quantities (P, q, k and e) are stored in the centre of these volumes, whereas the components of the velocity For the simulation of cooling process it was used an electronic cooling with dimensions listed in Table 1.jection velocity Up, it can be seen that the influence of heat flow is stronger than forced convection after heat flow equal 1,000.So, for cooling purposes, the results obtained allowed identifying the highest temperature when the system is submitted to combine forced and free convection, making possible to apply control actions, avoiding thermal damages to the devices that work with this cooling process.
present a study of steady, combined laminar natural convection and surface radiation from side-vented open cavities with top opening; Gunes and Liakopoulos (2003) study present a spectral element method: present a numerical study of heat transfer inside a semi porous two-dimensional rectangular open cavity for both local and average Nusselt numbers at the heated wall and for the isotherms and streamlines of the fluid flowing inside the open cavity.Bessaih and Kadja (2000) described a numerical simulation of conjugate, turbulent natural convection air cooling of three heated components mounted on a vertical channel and showed the study of reduced temperatures.Silva et al. (2007) presented a work about a numerical analysis of the heat transfer inside a semi porous two-dimensional rectangular open cavity, where forced and natural convection were considered.This paper presents a continued work (SILVA et al., 2007) where about a turbulent numerical analysis of the heat transfer inside a semi porous three-dimensional rectangular open cavity.This is made by two vertical parallel plates opened at the top and closed at the bottom by a uniform heat flow, as indicated in Figure 1.One of the vertical plates is porous and there is a normal forced fluid flow passing through.The opposite vertical plate supplies the same uniform heat flow to the cavity.In addition to the forced convection, the analysis considered the influence of natural convection effects.The maximum temperatures were obtained for the uniformly heated plate and to the bottom for the parameters like injection velocity and heat flow.

Figure 1 .
Figure 1.One of the vertical plates is porous and there is a normal forced fluid flowing through it.The opposite vertical plate supplies the same uniform heat flow to the cavity: a uniform tempera-

Figure 1 :
Figure 1: Coordinate system and thermal boundary conditions of the open cavity (U, V, and W) are stored on the faces.Power law profiles are used for the dependent variables to ensure realistic results.Convergence of the numerical solution at consecutive time steps was reached when the mass, momentum and energy residuals were chosen after performing grid independence tests for a mesh with 12,062 nodes and 63,984 tetrahedral elements.It is considered a goal error equal to 10 -3 for mass flow, velocity, turbulence and turbulent dissipation energy in CFX ® v. 10.0.

Figure 2
Figure 2 express the results of maximum temperature inside cavity in function of injection dimensionless velocity Up and heat flow parameters in the surface to laminar and turbulent flows.The effect of heat flow in the behavior of maximum dimensionless temperature is shown on Figure 2 for injection velocity Up equal 1, 2, 4, 6, 8 and 10.It is evident that the maximum dimensionless temperature is affected by heat flow by influence of forced convection to laminar and turbulent flows.Until heat flow equal 1,000 the maximum dimensionless temperature is not affected by heat flow by influence of forced convection to laminar and turbulent flows.After 1,000 the maximum dimensionless temperature is affected by influence of heat flow for injection dimensionless velocity Up equal 1, 2, 4, 6, 8 and 10.After heat flow equal 1,000, the maximum dimensionless temperature increases abruptly with heat flow to laminar and turbulent flows for injection dimensionless velocity Up equal 1, 2, 4, 6, 8, and 10.