查看更多>>摘要：Thermal management is restricting the further development of electronic devices. The rapid development of communication and chip manufacturing technologies results in an unprecedented increase on heat dissipation of electronics, which also makes the thermal management system consume a substantial amount of energy. Therefore, the manifold microchannel (MMC) is particularly important as an efficient thermal management scheme to enhance heat transfer and reduce the energy consumption. For this objective, the flow and heat transfer characteristics of MMCs with porous fins are investigated by numerical method in this study. The results show that the introduction of porous fins can reduce the pressure drop by up to 19%. In addition, the porous fins arranged on the outlet manifold side can significantly optimize the non-uniform flow distribution among the microchannels and strengthen the heat transfer of MMC. Furthermore, for MMC with the porous fins on the outlet manifold side with a proportion of 75%, the thermal resistance can be reduced by 19.8% and the PEC will increase by 46.2%, which performs the best among all the investigated cases.
Franco S.S.Henriquez J.R.Ochoa A.A.V.da Costa J.A.P....
15页
查看更多>>摘要：One of the reasons for replacing conventional expansion devices with electronic expansion valves in vapor compression refrigeration systems is associated with the fact that electronic valves promote savings in energy consumption. However, for the electronic expansion valve to be more efficient its PID controller must be properly tuned. Therefore, a refrigeration prototype of simple cycle by vapor compression, who uses R404A and an ethylene glycol solution as a secondary fluid, with nominal capacity of approximately 1 kW has been developed and analyzed. An optimization strategy of a black box model using Matlab? software to tune the PI control was used, while the Ziegler-Nichols method was also used to tune the PID control of the expansion device of the refrigeration prototype system. Developing a strategy to optimize the parameters of PI and PID controls to improve the energy efficiency ratio (EER) of refrigeration systems using superheat and valve opening as input variables is the novel contribution of this paper. The PI and PID control models improved the EER of the refrigeration system by 21% to 32% and the factory-set PI control by 28%, respectively. Applying the Ziegler-Nichols method for the PI and PID controls improved the EER by 10–17% and by 24 to 28%, respectively, compared to the factory-configured PI controller. The control parameters found with the discrete linear model provided better energy efficiency than the factory setting of the electronic expansion device and with the first Ziegler-Nichols, the PID control had a better EER than the one with the discrete linear model.”.
查看更多>>摘要：In order to improve the thermal conductivity of organic phase change materials and overcome the shortcomings of large supercooling degree and phase separation of inorganic phase change materials, the composites of sodium acetate, stearic acid and octadecyl alcohol as phase change materials were prepared by melt blending method, where disodium hydrogen phosphate dodecahydrate was used as nucleating agent and carboxymethyl cellulose as thickener in this work. XRD, DSC and T-history techniques were used to analyze the composite phase change materials. Sodium acetate, stearic acid and octadecyl alcohol are physically blended in the ternary composite phase change materials. Sodium acetate improves the thermal conductivity and latent heat of the eutectic mixtures of stearic acid and octadecyl alcohol, where the thermal conductivity and latent heat of the eutectic mixtures of stearic acid and octadecyl alcohol is increased by 384 % and 28%, respectively. Heat conduction and natural convection act together, but natural convection plays an important role in the melting process. While heat conduction plays an important part in the discharging process. Due to natural convection, the charging time of the composite phase change materials is less than the discharging time.
查看更多>>摘要：Experiments are carried out with different GITs under 10% and 25% load conditions to reveal the influence of gas injection timing (GIT) on combustion instability for a spark ignition natural gas engine under low load conditions. The phase space reconstruction and coefficient of cyclic variation (CCV) of the entire cycle in-cylinder pressure time series, the return maps of indicated mean effective pressure (IMEP) and CA50, the distributions of CA5 and CA90 are applied to analyze the combustion instability of the natural gas engine qualitatively and quantitatively. Results show that the combustion process of the natural gas engine presents chaotic characteristics. With increasing GIT from 0°CA to 90°CA ATDC, the CCV and the separation degree of phase space attractor initially increase and then decrease. The distributions of return map points for IMEP and CA50 initially disperse and then concentrate, and the combustion instability and chaotic characteristics are initially enhanced and then weaken. When the GIT is near 60°CA ATDC, the chaotic characteristics of combustion instability for the natural gas engine is the strongest. The reasons for the influence of GIT on combustion instability of the natural gas engine are identified based on 3D computational fluid dynamic (CFD) simulation, showing that the pressure difference between in-cylinder and the intake manifold under different GITs leads to the difference in the mixing degree of the natural gas and air as well as the CH4 concentration distribution of in-cylinder. This finding provides a theoretical basis for improving the lean-burn stability of natural gas engines.
NSTL
Elsevier