The comparison of design airflow rates with dynamic and steady-state displacement models in varied dynamic conditions

A temperature-based method is usually applied in displacement ventilation (DV) design when overheating is the primary indoor climate concern. Different steady-state models have been developed and implemented to calculate airflow rate in rooms with DV. However, in practical applications, the performance of DV depends on potentially dynamic parameters, such as strength, type and location of heat gains and changing heat gain schedule. In addition, thermal mass affects dynamically changing room air temperature. The selected steady-state and dynamic models were validated with the experimental results of a lecture room and an orchestra rehearsal room. Among the presented models, dynamic DV model demonstrated a capability to take into account the combination of dynamic parameters in typical applications of DV. The design airflow rate is calculated for the case studies of dynamic DV design in the modelled lecture room in both dynamic and steady-state conditions. In dynamic conditions of heavy construction in 2–4 hours occupancy periods, the actual airflow rate required could be 50% lower than the airflow rate calculated with the steady-state models. The difference between steady-state and dynamic multi-nodal model is most significant with heavyweight construction and short occupancy period (17%–28%). In cases with light construction, the dynamic DV model provides roughly the same airflow rates for four-hour occupancy period than the Mund’s model calculates. The dynamic model can significantly decrease the design airflow rate of DV, which can result in a reduction of investment costs and electrical consumption of fans.

Water Flow Characteristics and Related Effects in PEMFC

Water management in proton exchange membrane fuel cells(PEMFC)is a topic of great importance for the optimization of these systems.Effective proton conductivity calls for moderate moisture content in the membrane,while uneven water distribution can lead to instability of the whole flow field,thereby decreasing the performance of the fuel cell.In the present study,a simplified two-tier hybrid structure is used to investigate the impact of the dynamic behavior of liquid water on the current density of the PEMFC.Simulation results show that water droplets attached to wall sides tend to increase current density.Visualization experiments confirm the existence of liquid droplets and the enhancement of current density,while indicating that the best performance and stability of fuel cell are attained for a cathode air flow rate of 300 ml/min.

Experimental study on the operating performance of the air source heat pump(ASHP)with variable outdoor airflow rate under the standard frosting condition

Frosting is a common phenomenon of the ASHP under the heating mode in winter,and the outdoor air flow rate flowing through the evaporator of the ASHP was always thought to be a major contributor.In order to validate its contribution,effects of outdoor fan airflow rate on the performance of air source heat pumps(ASHPs)were investigated under the winter heating condition.The experiment was conducted in a laboratory at the standard 2℃ air dry bulb temperature(DB)/1℃ air wet bulb temperature(WB)frosting condition,which enabled the analysis of the operating performance,frosting performance,and heating performance of the ASHP unit by changing the airflow rate of the outdoor fan.Results showed that as the airflow rate of the outdoor fan reduced from 100%to 36%,the operating performance decline and the elevated frosting-defrosting loss were observed.Meanwhile,both the frosting rate and the operating efficiency during frosting-defrosting cycles showed an increasing trend then followed by decreasing tendency.The maximum frosting rate and operating efficiency were 0.92 g/m^(2).min and 2.92,respectively,which were observed at 74%airflow rate of the outdoor fan of the ASHP unit.The observation implied the existence of the“minimum frosting suppression airflow rate”.At 36%airflow rate of the outdoor fan of the ASHP unit,however,the performance of the ASHP unit was attenuated greatly,with the frosting-defrosting efficiency loss coefficient of 0.47,the heating capacity and COP reduction by 51.5 and 38.8%,respectively.These findings provided significant references to the optimization of ASHPs performance with variable airflow rate of the outdoor fan under frosting conditions.

Evaluation of ventilation performance and energy efficiency of greenhouse fans

In order to investigate fan performance in fan-ventilated greenhouses(Urbana,USA),the effects of guard screen and loose belts on fan ventilation airflow and power consumption in greenhouse operations were examined with four belt-driven fans as trial subjects.The Fans Assessment Numeration System was used to measure the airflow rate.Temperature,relative humidity and power consumption were also monitored.Results show there were significant differences in the airflow rate between the fans with a cleaned and uncleaned guard screen(P<0.05).Power consumption also differed significantly even with the same cooling effect in greenhouse.When fan belts were adjusted to the proper tension,the fan speed and airflow rate were 13.1% and 30.1% higher than those of original belts,respectively.The daily average power consumption for the fan with the original loose belts was 20.4% higher than that with the adjusted belts when the pad was not working and 24.2% higher with pad working.The ventilation performance of fans with identical specifications showed a variation by up to 13.0% in terms of the ventilating efficiency ratio.These results demonstrated that fans should be cleaned routinely,and belt tension should be checked to ensure that fan performance meets specifications.This can reduce the power consumption in greenhouses for environmental control.Moreover,reordering fan staging,so that the most efficient fans are used in areas of greatest demand,can also reduce ventilation energy costs.