Numerical simulation of the cooling efficiency of circular serrated water-cooled LEDs using CFD in plant factory

LEDs are more commonly used than fluorescent lamps in plant factories with artificial light for energy savings. But the LEDs cannot convert the input power to light at 100 % efficiency. Part of energy can be converted into heat, and then be transferred to the ambient environment, in terms of heat conduction, radiation, and convection. However, heat dissipation of LEDs has become a great challenge, as the power increased while the volume of LEDs reduced. In this study, a circular serrated water-cooled LED was designed to transmit the heat generated by LEDs in time for a longer service life. A three-dimensional Computational Fluid Dynamic (CFD) model was developed to assess the design, where the LED bubbles were set as the internal heat source. The electrical efficiency was assumed to be 32% and 49% in the red and blue LEDs, respectively. The heat flux of 1.7×107 W/m3 was calculated, according to the number of lamp beads and the electrical to light conversion efficiency. The constructed grids were approximately 1 162 800 for each case, including 220 881 nodes with a minimum element size of 2 mm. Much finer meshes were automatically imposed near the bubbles with proximity and curvature size functions in meshing. The SIMPLE was selected for the pressure-velocity coupling. A least-square cell-based scheme was used for the gradient term in spatial discretization. The second-order scheme was applied for the pressure term. The second-order upwind discretization schemes were used for momentum and energy equations, whereas, the first-order upwind discretization schemes were used for turbulence equations, mainly for higher accuracy. The convergence criterion was set as 10-6 on energy and 10-3 on continuity, momentum and viscous terms. Inlet and outlet boundary conditions were set for the numerical solution using the velocity-inlet and pressure-outlet. The inlet water velocity and water temperature were set as 0.2 m/s and 24 ℃, respectively. The simulated value of the LED water-cooled lamp was close to the measured value, with the maximum error of 16.4%, indicating that the CFD model could accurately simulate the temperature distribution of each structure of the lamp. The validated model was used to simulate the influence of different water flow velocities on the temperature distribution and water flow pressure drop in a water-cooled LED lamp. The results showed that the temperature distribution of bubbles and water flow was relatively uniform, and the structure of the lamp was reasonable. The heat released by the bead chip was quickly transferred into the water flow; when the inlet velocity of the lamp increased from 0.10 to 0.25 m/s, and the difference of water temperature between the inlet and outlet dropped from 1.4 ℃ to 0.5 ℃. Therefore, a series of connected lamps were calculated, according to the temperature difference between the inlet water and the ambient air, when the water-cooled LED lamps were connected in series. The inlet flow velocity also improved the flow resistance, where the resistance coefficient of lamps to the water flow was 2.2.