System Design And Experimental Investigation Of High Performance Minichannel Heatsink Cooling System

The microchannel heatsink has been widely used in the field of thermal control of spacecraft and cooling of electronics with high heat flux because of the advantages of compact structures, high surface-to-volume ratio in limited space, strong cooling abilities, easily implementation and safe working. The thesis introduced the operating principles and analyzed the working characteristics of the minichannel heatsink cooling systems. A series of experimental and numerical works are conduced to several different minichannel heatsink structures.According to different application requirements, the overall designs of the cooling system and the components structures are proposed in the thesis. Using the wire-cut method, the parallel rectangular-shaped minichannel array and the offset-fin minichannel structure heatsinks are fabricated. An experimental investigation is conducted to determine the heat transfer characteristics and cooling performance of the two kinds of minichannel structures machined into brass plates. The influence of minichannel in/outlet configuration, input heating power and property variations on the heat transfer behavior are analyzed experimentally. The experimental and numerical results show that the offset-fin structure obtains better cooling performance because the fluid flow disturbance restricts the development of the thermal boundary. The cooling fins are arranged into a staggered construction, when fluid flows in these alternating minichannels, the thermal boundary layer up-and downstream from the cooling fins. A new thermal boundary layer is restarted along the small pieces, resulting in a thinner thermal boundary layer. A high heat transfer coefficient is therefore obtained. The analytical results indicate that cooling performance increases to up 19.4% compared with the conventional straight ones. However, the vortex flow regions are found near the front and the rear edges of cooling fins, bringing the problem of larger pressure loss than the conventional straight ones. From the field synergy point, the heat transfer enhancement and flow resistance increment mechanisms of the offset-fin structures are also analyzed. The intersection angle between velocity and temperature gradient periodically changes along the up-and downstream from the cooling fins, and its value is far less than the conventional straight ones, improving heat transfer intensity. However, disturbances in the fluid simultaneously bring the issue of increased resistance, the intersection angle between velocity and velocity gradient is larger than the conventional straight ones. Both considering the heat transfer and flow conditions, the synergy relationship between temperature gradient and velocity gradient shows that staggered fin structure has a relatively optimum overall performance.Combined the purposes of extending the surface-to-volume ratio in heatsink and restricts the development of the thermal boundary for heat transfer enhancement. A novel multilayer staggered honeycomb minichannel heatsink is designed in the cost-effective way in the thesis. Multilayered metal plates each with rows of etched honeycomb cells in are stacked to form the well-designed staggered minichannels in the heatsink. Better cooling performance is obtains based on the fluid hydrodynamic mixing improvement for periodic breakup of the thermal boundary layer. At the same time, the stacking structure design is also an easily implemental way to obtain higher surface-to-volume ratio in limited space simultaneously reducing the fabrication difficulty for 3D structure. Experimental investigation is conducted to determine the heat transfer characteristics and cooling performance of the honeycomb minichannel cooling system. The system heat transfer performance is evaluated under various operation situations, which includes different flow rate, input heating power, working fluid medium, minichannel heatsink parameters and test system configurations. The influence factors for the system performance are analyzed and discussed experimentally. It finds the flow rate has a large influence for the system performance from the experiments. In order to obtain larger flow rate for better cooling performance under the limited pumping power condition, the system configurations and structures must be optimized to reduce flow resistance. Several improvements such the agreement of the connection pipe size between micropump and heat sink, increasing the pipe size of heatsink in/outlet entrance holes and the multi-inlets and outlets arrangement are taken to reduce the system resistance for the comparison experiments. It finds that the substrate temperature of the singleφ4mm inner diameter in/outlet heatsink design decreases 14.9℃compared to the 02mm inner diameter in/outlet ones under the same 140W input heating power. The effects of working fluid to the cooling system are also studied by comparing water and ethanol as working fluid. It shows that the fluid with high specific heat is more suited for the heat transfer application.By modeling the multilayer staggered honeycomb minichannel structures as porous media, the coupled heat transfer and flow characteristics of the heatsink are simulated and analyzed with the extended Darcy equation and the two-energy equation. The numerical results are in good accordance with the experimental ones. For the different in/outlet arrangements, the heatsink substrate temperature distribution and pressure drop are compared using the porous media model. The results show that the multi-inlets and outlets arrangement gets better cooling performance because the lower flow velocity under the constant flow rate reduces the pressure drop through the heatsink. However, although the cross multi-inlets and outlets design obtains the least pressure drop because of the reduction of flow length in the minichannels, there is found a local high temperature region in the center of the substrate also as the reduction of the flow rate through the minichannels. The experimental and numerical work is the fundament of developing the minichannel heatsink cooling system for high heat flux removal applications.