Degradation of gas diffusion layers through repetitive freezing

This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL without PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process.     

Experimental study on the performance of an inverter heat pump with a bypass orifice

An economical, fixed geometry, bypass orifice was invented to control more precisely the refrigerant flow in an inverter heat pump system. Flow characteristics of the bypass orifice were investigated as a function of orifice geometry and operating conditions. Experimental results of bypass orifices were compared with those of capillary tubes. The bypass orifice showed the best flow trend as a function of frequency: the flow slope of the bypass orifice with respect to frequency was the highest among the orifice, capillary, and bypass orifice. The performance of an inverter heat pump with a bypass orifice and then a capillary tube was measured with a variation of frequency in the psychrometric calorimeter. It was observed that the performance of the inverter heat pump was enhanced with the application of the bypass orifice in the system instead of a capillary tube. The improvement of COP in an inverter heat pump with a bypass orifice versus the capillary tube was prevalent at the low frequency region.     

Knowledge-aided Bayesian Rao detection approach in compound Gaussian noise

An asymptotically likelihood covariance matrix is usually used to detect a target in compound Gaussian noise. However, it is influenced greatly by the training sample support, and it ignores the prior distribution of the covariance matrix. For this problem, this paper proposes the knowledge-aided Bayesian Rao detection. The covariance matrix in compound Gaussian noise is modeled as a random matrix, the prior distribution of which satisfies the complex inverse Wishart distribution. With prior distribution, the maximum a-posterior estimation of the covariance matrix is derived. Then, Rao detection is obtained based on the maximum a-posterior estimation. Finally, the performance of the knowledge-aided Bayesian Rao detection approach is evaluated by Monte Carlo simulation. The simulation results show that the detection performance of the proposed approach outperforms the traditional detection approaches when the number of training samples is small in a complex Gaussian noise scenario.     

参数可调的联合子空间目标检测方法

基于Rao和Wald检验准则,本文推导了联合子空间（Union of Subspaces, UoS）检测器（UoS-Rao、UoS-Wald）,并通过引入一个可调参数提出一种可调检测器（UoS-Tunable）,从而实现联合子空间目标检测,通过调节参数来灵活调节检测器的检测性能、分类性能与选择性。当可调参数较小时,可以提高检测性能与分类性能,并提高其对信号失配的鲁棒性;当可调参数较大时,则会降低检测性能与分类性能,但会提高其对于信号失配的选择性。最后,仿真实验验证了所提方法的有效性。     

TWO-PHASE PRESSURE DROP OF CO2 IN MINI TUBES AND MICROCHANNELS

Two-phase pressure drops of CO₂ are investigated in mini tubes with inner diameters of 2.0 and 0.98 mm and in microchannels with hydraulic diameters from 1.08 to 1.54 mm. For the mini tubes, the tests were conducted with a variation of mass flux from 500 to 3570 kg/m²s, heat flux from 7 to 48 kW/m², while maintaining saturation temperatures at 0, 5, and 10°C. For the microchannels, mass flux was varied from 100 to 400 kg/m²s and heat flux was altered from 5 to 20 kW/m². A direct heating method was used to provide heat flux into the refrigerant. The pressure drop of CO₂ in the mini tubes shows very similar trends with those in large diameter tubes. Although the microchannel has a small hydraulic diameter, it shows a larger Chisholm parameter in two-phase multiplier. Based on the experimental data in this study, the Chisholm parameter in the Lockhart and Martinelli correlation is modified by considering diameter effects on the two-phase multiplier.     

Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm⁻² and mass fluxes of 200 to 530 kgm⁻² s⁻¹. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed.     

论现代煤炭企业图书资料知识转移的保障机制

实现现代煤炭企业图书资料馆知识的有效转移,需加强对知识转移保障机制的研究和建设.图书资料馆知识转移的保障机制可分为管理保障、信息资源保障、服务保障和安全保障等.应据此构建图书资料馆的知识转移保障机制.     

Optimum cycle control of a two-stage injection heat pump with a double expansion sub-cooler

The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures, and various control methods for a heat pump with injection have been presented. However, most studies on cycle control have been theoretical, and practical control methods for cycle optimization based on experimental results hardly have been presented. In this study, an optimum cycle control method was proposed for a refrigerant injection heat pump with a double expansion sub-cooler based on the intermediate pressure and the injection ratio. The optimum sub-cooler pressure ratio was proposed from 0.4 to 0.7 in view of the heating capacity, and from 0.7 to 0.8 in view of the COP. The optimum injection ratio increased from 0.1 to 0.3 with an increase in the compression ratio.     

Effects of external humidification on the performance of a polymer electrolyte fuel cell

The dryness of the membrane of a polymer electrolyte fuel cell (PEFC) decreases the ionic conductivity, resulting in performance reduction. In this study, the effects of external humidification to the membrane were investigated by varying the humidification side such as anode humidification, cathode humidification, and both anode and cathode humidification (called as both-side humidification). The amount of required water vapor into the gas was increased rapidly to maintain the relative humidity constant with the increase of cell temperature. The best performance of the cell was achieved by both-side humidification. However, as the humidity condition approached saturation state, anode humidification yielded comparable performance to both-side humidification. In anode humidification, the increase of the cell temperature degraded the performance, even though the amount of water supply to the membrane remained constant. At constant relative humidity conditions with anode humidification, the polarization curves of the PEFC were almost the same, regardless of the cell temperature when the relative humidity was higher than 60%.