Refrigerant distribution effects on the performance of microchannel evaporators.
The performance of air to refrigerant heat exchangers can be affected greatly by both refrigerant and air flow distribution. Maldistribution of either fluid stream can lead to reduced heat transfer effectiveness of the heat exchanger and thus lower system efficiency and capacity. Microchannel evaporators are especially susceptible to refrigerant maldistribution as they typically have many parallel channels fed by a common header or manifold. This paper outlines general trends in refrigerant flow distribution in such evaporators taking into account various heat exchanger orientations. In addition, links between heat exchanger performance and refrigerant flow distribution characteristics in microchannel evaporators are made using both analytical and experimental methods. These results show that unification of the liquid refrigerant flow distribution in the evaporator can lead to quantifiable improvements in evaporator performance.
Effect of channel geometry on flow reversal in microchannel evaporators
Abstract Boiling instability and periodic flow reversal are commonly seen in microchannel heat exchangers, but have been studied in air conditioning systems only recently. The amount and frequency of reverse flow largely depends on the geometry of the channels. This paper presents results obtained in a R134a automotive air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). Results for multiple microchannel evaporators with different channel diameters and lengths are presented. The mass flow rate and frequency of reversed vapor are measured for each geometry. Meanwhile, a mechanistic model is developed to simulate the bubble dynamics inside of a single microchannel tube. This model is validated against experimental data and employed to explain the effect of channel length and diameter on flow reversal. Both experimental and simulation results show that larger channel diameter and longer channel length result in less flow reversal with a lower frequency.
Impacts of climate and land use changes on flood risk management for the Schijn River, Belgium
Abstract Flooding is the most common natural disaster in Europe. Modern flood risk management relies not only infrastructure development but also on governmental and non-governmental actors applying legal, economic and communicative water management instruments. Within the European Union (EU), flood management closely relies on policy set at the EU and national levels. It is now recognized that a sound understanding of climate change is required in addition to current management by taking into account land use change and socio-political context, as climate and land use changes have major impacts on hydrological responses. This paper investigates the hydrological behavior due to urbanization under current and future climate scenarios of high summer and high winter rainfall for 20 sub-catchments of the Schijn River, located in the Flanders region near Antwerp, Belgium. As urbanization increases and existing rainfall-runoff models neglecting the specific behavior of urban runoff, a hydrological model was developed based on a basic reservoir concept and applied to the existing rainfall-runoff model (PDM) flow to examine the specific urban contribution. Results revealed that peak flow for urban runoff and the total peak flow (i.e. rural and urban runoff) were significantly higher (i.e. ranges from 200% to 500%) than the existing rainfall-runoff model (PDM) flows, because of faster and more peaked urban runoff response. The impact of climate change on current and future conditions was also assessed by estimating peak flows with respect to return periods from the flood frequency curve. The predicted peak flow of high summer future climate scenario was significantly higher (i.e. ranges from 200% to 250%) than that of the current climatic condition for this region. Furthermore, hourly peak flow and daily volume ratios of 100-year return period for the highest, lowest and average impervious area were projected for the time horizon of the year 2100. It is concluded that climate change impacts contribute the most in producing peak flow in coming years, while increased urbanization takes the second place for both hourly and daily values. Results on urbanization effect and climate change impact assessment are useful to the water managers for spatial planning, emergency planning and insurance industry.