Orientation effects on conjugate natural convection heat transfer from an LED bulb: A numerical study
Introduction
Natural (free) convection heat transfer is a combination of diffusion and advection, in which the fluid motion is triggered by the density gradient in the fluid [1]. Natural convection does not involve any external source, such as a fan or a pump, to generate the fluid motion. Natural convection heat transfer plays an important role in the cooling of electronic equipment since it is a reliable, quiet, and cost-effective solution [2,3]. Nowadays, natural convection is a widely used cooling technique in LED bulbs [3,4]. LED bulbs have replaced the conventional light source because of their low power consumption, high luminous efficacy, and long life. About 75–85% of the input power to an LED package is being wasted as heat [5,6]. Along with the LED packages, a notable amount of heat is generated by the LED driver [7]. Thus, if this generated heat is not removed in a proper way, it can lead to decrease in the life-span of the LED. Heat dissipation in low power LEDs is not an issue, but in today's high-power LEDs, heat dissipation must be considered [[8], [9], [10]]. Fig. 1(a) shows a typical LED bulb with a B22 base cap and the internal components of the LED bulb are shown in Fig. 1(b).
Many experimental and numerical studies have been conducted on heat transfer from LED bulbs in the recent past. Nguyen et al. [7] have presented an effective and straightforward technique to decrease the driver temperature of a 16 W LED bulb by filling the interior space between the driver and the walls of the bulb's heat sink with potting materials. A finite element method was used to study the heat transfer process from the LED bulb. Both simulation and experiments were conducted to study the effect of potting material on the LED chip's temperature and it was shown that the thermal conductivity of the potting material has a strong influence on the driver's temperature. For simulation, a constant heat transfer coefficient of 3.5 W/m2-K was used on the external surface. Calderón et al. [11] have developed a set of technical specifications and test procedures for LED lighting. Jakovenko et al. [12] have presented the thermal analysis of an 8 W retrofit LED Lamp. Simulations were carried out in ANSYS and CoventorWare software and validated the thermal distribution with the thermal measurement of commercial 8 W LED lamp. A constant heat transfer coefficient of 5 W/m2-K was used for all external surfaces of the bulb. Perpina et al. [13] studied the thermal influence of LED driver on a retrofit 8 W LED lamp and has compared the simulation results with the experiments. The model was also investigated under two alternative thermal designs. The thermal and flow fields inside an 8 W LED lamp has been numerically investigated by Huang et al. [14] using ANSYS Fluent. The chip temperature of 75.1 °C and a maximum air velocity of 97.3 mm/s inside the bulb with two sets of air circulation have been observed in their study.
Luo et al. [15] have conducted experiments to obtain the surface temperatures of a 4 W LED bulb. The experimental data were used to develop a thermal analysis method for the 4 W LED bulb. This method was used for the thermal design of a 16 W LED bulb and simulations were also done to verify the design. Kulha et al. [16] have performed thermo-mechanical simulations on a low power LED lamp using the finite element method (FEM) and validated it with experimental data of commercially available LED lamp. For the simulations, the convective heat transfer coefficient of 5 W/m2-K was used on the external surface of the lamp that was exposed to air. The effect of orientation of a heated object strongly affects the buoyancy effect and the consequent natural convection heat transfer [17,18]. Park and Lee [19] have numerically investigated the orientation effect on heat transfer of a radial heat sink with a chimney and validated with the available experimental data. The orientation angle of the heat sink was varied from 0° to 180° and the effect of orientation on performance factor was studied. A heat transfer correlation was also developed in their study using regression analysis. Jang et al. [20] reported the effect of orientation on heat loss from a cylindrical heat sink used in LED bulbs. The angle of the heat sink was varied from 0° to 90° and its effect on Nusselt number and drag coefficient was investigated. A correlation was developed to predict the Nusselt number.
Based on the literature survey, it has been observed that several studies have been numerically done to examine the temperature distribution of specific LED bulbs and how it was affected by adding potting material. However, most of the studies were based on the assumption of a constant heat transfer coefficient on the external surface of the bulb. Few studies have been done to show the internal heat transfer characteristics and how it was affected by the integration of a driver. Furthermore, some studies were conducted to investigate both internal and external heat transfer for a specific LED bulb without considering the effect of the orientation of the bulb [[14], [15], [16]]. Some literatures have reported the effect of orientation of the heat sink without considering the entire LED bulb [19,20]. However, none of them have reported any studies that effectively capture the natural convection heat transfer from the LED bulb. Thus, the primary objective of the present study is to numerically investigate the effect of orientation on laminar natural convection heat transfer from an LED bulb.
Section snippets
Problem description
An LED bulb of A60 shape with a B22 base cap is taken for the present study since A60 is the commonly used shape of the bulb. Here, A refers to A-series of bulb shapes, and 60 refers to the nominal major diameter of the bulb in millimeters [21]. B22 refers to the Bayonet cap, which is one of the widely used mounts for bulbs. Here, 22 refers to the diameter of the base cap in millimeters [22]. The schematic of the front view and dimensions of the bulb are shown in Fig. 2. The LED bulb is divided
Governing equations
In this work, the governing equations for a steady, laminar, three-dimensional, and incompressible flow of a Newtonian fluid are considered for the conservation of mass (continuity), linear momentum (Navier-Stokes), and thermal energy. These are written in the Cartesian coordinate system as follows:
Numerical procedure
The entire flow domain was divided into a finite number of cells and the governing differential Eqs. (1)–(6) were applied to each cell. Differential equations were then integrated over each cell and a finite volume method was used for discretization, which gives a set of algebraic equations. These algebraic equations, with the help of specified boundary conditions, were solved in ANSYS Fluent 15.0 [23], which is an algebraic multigrid solver. Pressure and velocity were coupled using the
Effect of Rayleigh number and orientation on thermal field
The thermal field around the LED bulb is visualized in terms of temperature contours obtained from the simulations. The temperature contours on the symmetry plane of the computational domain are shown in Fig. 8 and Fig. 9. Here, two cases are analyzed. In the first case, the effect of Rayleigh number on the thermal field for three different orientations of the bulb is studied. Fig. 8 shows the temperature contours as a function of Ra=103, 105, and 107 for three different orientations ϕ=0°, 90°,
Conclusions
Natural convection heat transfer from an LED bulb in the laminar flow regime has been numerically studied to elucidate the effect of orientation of the bulb on both momentum and heat transfer characteristics. Simulations are performed for seven orientations of the bulb, ϕ=0°, 30°, 60°, 90°, 120°, 150°, and 180°. Rayleigh numbers of 103,105, and 107 are used for each orientation. Important observations from the present work can be enumerated as follows:
- a.
Thermal plumes are seen to be thin at high
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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