Knowledge concerning the complicated changes of mass and heat transfer is desired to improve the performance and durability of unitized regenerative fuel cells (URFCs). In this study, a transient, non-isothermal, single-phase, and multi-physics mathematical model for a URFC based on the proton exchange membrane is generated to investigate transient responses in the process of operation mode switching from fuel cell (FC) to electrolysis cell (EC). Various heat generation mechanisms, including Joule heat, reaction heat, and the heat attributed to activation polarizations, have been considered in the transient model coupled with electrochemical reaction and mass transfer in porous electrodes. The polarization curves of the steady-state models are validated by experimental data in the literatures. Numerical results reveal that current density, gas mass fractions, and temperature suddenly change with the sudden change of operating voltage in the mode switching process. The response time of temperature is longer than that of current density and gas mass fractions. In both FC and EC modes, the cell temperature and gradient of gas mass fraction in the oxygen side are larger than that in the hydrogen side. The temperature difference of the entire cell is less than 1.5 K. The highest temperature appears at oxygen-side catalyst layer under the FC mode and at membrane under a more stable EC mode. The cell is exothermic all the time. These dynamic responses and phenomena have important implications for heat analysis and provide proven guidelines for the improvement of URFCs mode switching. Full article

A two-dimensional, single-phase, isothermal, multicomponent, transient model is built to investigate the transport phenomena in unitized regenerative fuel cells (URFCs) under the condition of switching from the fuel cell (FC) mode to the water electrolysis (WE) mode. The model is coupled with an electrochemical reaction. The proton exchange membrane (PEM) is selected as the solid electrolyte of the URFC. The work is motivated by the need to elucidate the complex mass transfer and electrochemical process under operation mode switching in order to improve the performance of PEM URFC. A set of governing equations, including conservation of mass, momentum, species, and charge, are considered. These equations are solved by the finite element method. The simulation results indicate the distributions of hydrogen, oxygen, water mass fraction, and electrolyte potential response to the transient phenomena via saltation under operation mode switching. The hydrogen mass fraction gradients are smaller than the oxygen mass fraction gradients. The average mass fractions of the reactants (oxygen and hydrogen) and product (water) exhibit evident differences between each layer in the steady state of the FC mode. By contrast, the average mass fractions of the reactant (water) and products (oxygen and hydrogen) exhibit only slight differences between each layer in the steady state of the WE mode. Under either the FC mode or the WE mode, the duration of the transient state is only approximately 0.2 s. Full article

Background: Commercial sex plays an increasingly important role in China’s growing HIV and sexually transmitted infection (STI) epidemics. In China, street-based sex workers (SSWs) are a subgroup of female sex workers with a particularly high risk of HIV/STI infections but are neglected in responses to HIV. This study assesses changes in HIV voluntary counseling and testing (VCT) utilization and high-risk sexual behaviors following a three-month HIV preventive intervention among SSWs in Chongqing, China. Methods: A three-month intervention was conducted by a team of peer educators, outreach workers from community-based organizations and health professionals. It mainly included distribution of free pamphlets and condoms and delivery of onsite and clinic-based VCT. Cross-sectional surveys were conducted prior to (n = 100) and immediately following (n = 112) the intervention to assess its impact. In-depth interviews were conducted among 12 SSWs after the intervention to further explore potential barriers to HIV prevention. Results: The intervention significantly increased SSWs’ participation in VCT (from 2.0%–15.2%, P < 0.001). Despite participants’ improved HIV-related knowledge level (from 24.0%–73.2%, P < 0.001), there were minimal changes in the levels of condom use with clients. Qualitative research revealed that fear of police arrest and stigma were the main barriers to VCT utilization. Low condom use was associated with family financial constraints, inadequate power in condom negotiation, low awareness and misconceptions of HIV infection risks. Conclusion: HIV intervention improved VCT utilization and knowledge but we did not observe an increase in condom use after this short intervention. SSWs faced substantial economic, social and environmental barriers to VCT utilization and condom use. Full article

The circulation pump in an organic Rankine cycle (ORC) increases the pressure of the liquid working fluid from low condensing pressure to high evaporating pressure, and the expander utilizes the pressure difference to generate work. A portion of the expander output power is used to offset the consumed pumping work, and the rest of the expander power is exactly the net work produced by the ORC system. Because of the relatively great theoretical pumping work and very low efficiency of the circulation pump reported in previous papers, the characteristics of the expander power used for offsetting the pumping work need serious consideration. In particular, the present work examines those characteristics. It is found that the characteristics of the expander power used for offsetting the pumping work are satisfactory only under the condition that the working fluid absorbs sufficient heat in the evaporator and its specific volume at the evaporator outlet is larger than or equal to a threshold value. Full article