No abstract

A numerical study of the heat transfer and flow resistance performance of Al 2 O 3 -water nanofluids in selfoscillating hot runners with different chamber lengths was performed. The control volume method based on the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm was used to numerically solve the governing equations of the two-dimensional computational domain. The inlet flow rate was determined according to the Reynolds number (10;000 < Re < 50;000), and the uniform temperature was applied to the wall of the hot runner. The effects of chamber length and volume fraction on the heat transfer performance of nanofluids vortex pulsation were described by temperature, thermal boundary-layer thickness, pulsation frequency, Nusselt number, pressure drop, and heat transfer performance evaluation index. The simulation outcomes indicated that the counterflow vortex can reduce the thermal boundary-layer thickness and increase the mixing degree between the central mainstream region and the wall. It was also observed that the heat transfer performance increases with the increase of the chamber length. When L∕d 1 5.6, the heat transfer performance is the highest, and the main frequency (f 398.01 HZ) at the center of the chamber corresponds to the pressure pulsation amplitude of 35,982.6 Pa.

The fluid movement motion has an important influence on the evolution of the pulsating flow in the hot runner. Using the Large Eddy Simulation numerical method, the instantaneous velocity, wall shear stress, boundary layer thickness and Nu number of hot runner section under different structural parameters at an inlet pressure of 5000 Pa were studied. The research results showed that the backflow vortex can be formed in the hot runner, and the fluid at the axis center of hot runner can form a pulsating flow under the squeezing action of the backflow vortex. The pulsating flow had a strong disturbance effect on the fluid around the axis center and accelerated the heat exchange between the fluid around the axis center and the wall. The disturbance effect of pulsating flow gradually strengthened with the flow of the main flow to the downstream. When d2/d1 was 1-1.8, the wall shear stress first increased and then decreased, and the wall heat transfer efficiency first increased and then decreased. The maximum wall shear stress was 36.4Pa. When L/D was 0.45-0.65, the boundary layer thickness first decreased and then increased, and the heat transfer efficiency first increased and then decreased. The minimum boundary layer thickness was 0.392mm and the maximum Nu number was 138. When d2/d1=1.4 and L/D=0.55, the maximum comprehensive evaluation factor reached 1.241, and the heat transfer efficiency was increased by 24.1%.