Psychrometric Properties of Humid Air from Multi-Fluid Helmholtz-Energy-Explicit Models

Psychrometric properties of humid air are widely used in the analysis and modeling of thermal systems. In this work we present a method for obtaining these properties from the multi-fluid mixture formulation of the GERG mixture model. This mixture model was originally developed to model the thermodynamics of natural gas mixtures, and now has been extended to model thermodynamic properties relevant for carbon capture and storage. The primary advantage of this formulation is that the dry air composition is not fixed, and can be adjusted to suit the application, for instance in submarines, for Martian atmospheres, etc. We present an algorithm that can be used to calculate the saturated vapor water composition in vapor-liquid equilibrium, and other properties that arise out of this equilibrium calculation, such as relative humidity and dewpoints. Solid-vapor equilibrium is not considered, and neither is the calculation of wet-bulb temperatures

Oil retention and its effects on pressure drop and heat transfer in microchannel heat exchangers of air conditioning and refrigeration system

In vapor compression, a small portion of the compressor oil circulates with the refrigerant through the cycle components, while most of the oil stays in the compressor. The presence of oil increases the pressure losses and results in an additional thermal resistance to the heat exchange process. The goals of this study were to investigate the oil retention and its effects on heat transfer and pressure drop of refrigerants and oil mixtures in microchannel type condenser and evaporatorTwo different louvered-fin aluminum microchannel heat exchangers set as condenser and evaporator were tested. The experiments were conducted in a custom-made test facility built ad-hoc for this study that controlled the amount of oil released to the heat exchangers and measured the corresponding oil retention, the heat transfer rates, and the pressure drops. The refrigerants used were R410A and R134a in combination with synthetic polyol ester (POE) oil. The saturation temperatures for condenser applications varied from 85 to 130 F (29 to 54C) while for evaporator applications, the range was from 33 to 48F (0.5 to 9C). The oil mass fraction (OMF) were varied from 0 to 5 wt.%. For microchannel type condenser, the results from the present work indicated that the oil retained in the condenser strongly depended on the OMF of the mixture. The oil retention volume increased if the OMF increased and it was measured up to 11% of the total condenser internal volume. The oil retention volume for high mass flux conditions were higher than those for low mass flux conditions and the effect of mass flux on the oil retention was small for low OMFs but it became more evident for OMFs of 3 wt.% and higher. Oil affected the heat transfer rate of the microchannel condenser and it penalized the heat transfer capacity by as much as 10 percent if the oil mass fraction was 3 wt.%. For both refrigerant R410A and POE mixture and refrigerant R134a and POE oil mixture, the heat transfer rate at low saturation temperature increased slightly if the OMF increased up to about 3 wt.%; then the heat transfer rate started to decline at higher OMFs. Oil also increased the refrigerant-side pressure losses of the microchannel condenser up to 19 percent with respect to oil free conditions.The oil retention volume in the microchannel evaporator was measured up to 13 % of total internal volume of evaporator. Oil affected the heat transfer rate of the microchannel evaporator and it penalized the heat transfer capacities by as much as 11% if the oil mass fraction was 3 wt.%. For air-conditioning and refrigeration systems, when OMFs were equal to or less than 1 wt. %, the decrease in heat transfer rates were within 4 %. The oil decreased the heat transfer rate and its impact was also depended on the mass flux. The refrigerant-side pressure drop across the microchannel evaporators increased by 10 to 25 percent when oil was present inside the heat exchangers and the OMF was in the range of 1 wt.%

TRANSIENT PERFORMANCE EVALUATION OF AUTOMOTIVE SECONDARY LOOP SYSTEMS

Automotive air-conditioning is a high impact technology where improvements in energy consumption and environmental performance can make a significant difference in fuel efficiency and comfort. The mandatory phase out of R134a as refrigerant in the European Union has set the stage for new systems and alternative refrigerants. While some of these refrigerants, such as R152a or R290, have a low Global Warming Potential, their flammability requires secondary loop systems to be used. The added thermal mass of such systems may increase power consumption and delay cool down while benefitting thermal comfort during start/stop operation. The recent revival of electric vehicles, as well as the associated focus on air-conditioning energy consumption, provides new challenges and opportunities. This research focuses on the performance evaluation of refrigerants R152a and R290 during transient operation in secondary loop systems, quantification of thermal storage benefits for start/stop operation, and investigation of energy saving potentials in electric vehicles through the use of advanced air-conditioning system controls and cabin preconditioning. A test facility was built to dynamically test secondary loop systems over a wide range of pull down conditions and drive cycles using a passenger cabin model and associated controls. It was shown that R290 is a viable alternative in secondary loop systems and system performance may be on par or better compared to R134a direct expansion systems. The preservation of cooling capacity and thermal comfort during off-cycle periods were quantified for a secondary loop system, as well as a combined ice storage system. System efficiency increases with longer off-cycle periods compared to direct expansion systems. Advanced compressor control strategies and the use of cabin preconditioning can make use of this characteristic and improve energy efficiency by more than 50%. Ice storage may be used in combination with cabin preconditioning to preserve comfort for an extended driving time with reduced use of the vapor compression cycle. A Modelica model of the secondary loop system was developed and validated with experimental data. The model enables dynamic simulation of pull-down and drive cycle scenarios and was used to study the effects of coolant volume and coolant concentration on transient performance

Oil Retention and Its Effects on Pressure Drop and Heat Transfer in Microchannel Evaporators of Air Conditioning and Refrigeration Systems

In Air-conditioning and Refrigeration systems, a small portion of the compressor oil circulates with the refrigerant through the cycle components. The oil circulating with the refrigerant flow retains in heat exchangers and affects its performance in terms of increased pressure losses and decreased heat transfer capacity.The purpose of this thesis is to experimentally investigate the oil retention and its effects on heat transfer and pressure drop of refrigerants and oil mixtures in microchannel type evaporators. In this work two different louvered-fin aluminum microchannel heat exchangers were tested in evaporator mode. The refrigerants R-410A and R-134a with Polyolester (POE) were considered with oil mass fraction ranging from 0.5 wt.% to 5.5 wt.%. The tests were run at a range of saturation temperatures which was divided into two sets; one set for air-conditioning application ranging from 33 to 48�F (0.5 to 9�C) and the second set for coolers and refrigeration systems with evaporation temperature ranging from 0 to 33�F (-18 to 0.5�C). Refrigerant entered the evaporator as saturated liquid and exited at as superheated vapor ranging from 5o F to 25o F (2.8?C to 13.9� C). Refrigerant mass flow rates were varied from 150 lbm/hr to 400 lbm/hr (0.019 kg/s to 0.05 kg/s). The results showed that the oil retention was strongly depended on the OMF and, at same OMF and saturation temperature, the oil retention increased if the refrigerant mass flux decreased. In addition, at same inlet conditions oil retention in microchannel evaporators increased if the superheat on the outlet side increased. The oil retention volume in the microchannel evaporators was measured up to 13 % of estimated internal volume of the evaporators. The oil decreased the heat transfer rate and increased the pressure drop. Its impact was also depended on oil mass fraction and the refrigerant mass flux. The effect of oil on heat transfer capacity was insignificant for OMF less than 1 wt. %. The refrigerant-side pressure drop across the microchannel evaporators increased by 10 to 25 percent when oil was present inside the heat exchangers and the OMF was in the range of 0 to 1 wt.%.Mechanical & Aerospace Engineerin

Predicting and testing the performance of next generation heat exchangers with low-GWP refrigerants

Energy efficiency regulations are forcing air conditioning (AC) and heat pump (HP) manufacturers to replace existing high global warming potential (GWP) refrigerants in their systems with more climate friendly alternatives. Air-to-refrigerant fin-and-tube heat exchangers (FTHXs), and water-to-refrigerant coaxial heat exchangers are commonly found in residential and commercial AC and HPs. To adapt to these new refrigerants, heat exchanger manufacturers need to make design changes in their equipment, in order to ensure that the next generations of their products is energy efficient, climate friendly, and cost effective. This thesis aims to develop a set of guidelines for pseudo-optimal design of FTHXs in order to accommodate the next generation of low-GWP refrigerants, with the goal to obtain near optimal performance under current manufacturing constraints.A novel pumped refrigerant loop was developed, and coupled with an airside setup and a psychrometric chamber facility, to test FTHXs. The experimental facility was used for validating a segment-by-segment heat exchanger model called cross-fin (Xfin), by comparing model predictions against data collected from three custom designed FTHXs with distinct refrigerant circuitries. The mean absolute percentage error (MAPE) between the experimental, and model predicted capacities was found to be 1.0%, 2.4%, and 0.9%, for the interleaved, vertical, and block circuited FTHXs, respectively. A preliminary simulation study was performed using a four-component HP model to investigate the change in system performance metrics of an R410A based water-to-water HP, using R454B and R452B as low-GWP 'drop-in' alternatives. A four-component heat pump model, validated against the performance datasheet, was used for simulations and showed that some design changes may be necessary to existing equipment, in order to adapt them for near optimum performance with low-GWP fluids. Then, Xfin model predictions were compared against experiments performed on the block circuited FTHX with R1234ze(E). The MAPE between the experimental and model predicted capacities was found to be 1.4%. A parametric simulation study was done by modifying the refrigerant circuitry and fin pitch, showing that FTHX performance metrics such as capacity (Qevap) and refrigerant pressure drop ([Delta]Pref), are influenced by changes in geometry. Finally, simulations were executed to compare the performance of R1234yf and R1234ze(E) on the R410A based block circuited indoor evaporator FTHX. It was found that the performance, in terms of the ratio Qevap/[Delta]Pref, was most sensitive to the number of circuits, followed by the tube diameter, number of tubes in the FTHX, and the fin density. Based on the simulation results, two customized FTHX designs were suggested, with different changes to the FTHX slab size. One design prioritized increased FTHX capacity, and the other prioritized reduction in refrigerant pressure drop to acceptable limits

A Study of Heat Pump Fin Staged Evaporators Under Frosting Conditions

This dissertation provides a detailed description of the research work completed on fin staged heat exchangers. The effects of staging fin on the frosting performance of heat pump evaporators and the whole heat pump system have been studied experimentally and theoretically. Frost degrades the performance of fin-and-tube outdoor coils as well as the whole heat pump system. The objective of the experimental part of this study was to investigate the effects of the staging fin on the frost/defrost performance of heat pump outdoor coils under different operating conditions. To accomplish this objective, a series of frosting tests was conducted on an off-the-shelf heat pump system with five (three two-row and two three-row) evaporators over a range of outdoor temperatures and humidities and a range of airflow rates typical of those found in residential sized heat pumps. Performances of the heat pump unit with baseline or fin staged outdoor coils at either frosting or steady-state test conditions are compared and analyzed. Experimental data showed that for a given tworow heat pump outdoor coil operating at the standard ANSI/ASHRAE 35 °F (1.7 °C) frosting conditions, fin staging increased cycle time and COP. There was a small decrease in peak capacity at lower initial airflow rates. At a lower temperature of 28 °F (2.2 °C), cycle time continued to be enhanced with fin staging, and cyclic COP was within 5% of the base case when fin staging was used. In the second step of this work, an analytical model to simulate the performance of both the baseline and fin staged heat pump coils under frosting conditions was developed based on fundamental heat and mass transfer principles. The transient performance of the frosted evaporator was analyzed with the quasi-steady state approach. The section-by-section evaluation scheme was combined with the tubeby- tube approach to model the mass transfer process in the frost formation module. The two-dimensional fin surface was divided into a number of parallel non-overlapping sections. Each of the sections was the calculation unit for the mass transfer. Methods for calculating the airside heat transfer coefficient and friction factor were developed and applied to the simulation model of the fin staged coil. To verify the validity of the frosted evaporator model, the frosting performance of three two-row coils at the same test conditions was simulated and compared with experimental data. The frosted evaporator model appeared to provide satisfactory simulation of the fin-and-tube heat exchanger during the frost buildup process. Comparisons with the test data indicated that the model could capture the trends of the coil capacity, pressure drop, airflow and frost growth. The model also provided a variety of other simulation results including frost mass accumulation, air velocity inside coil, air and refrigerant outlet state, and so on. Overall, the numerical results were in reasonable agreement with the test data under different frosting operation conditions

Feasibility Study: Vertical Farm EDEN

Hundreds of millions of people around the world do not have access to sufficient food. With the global population continuing to increase, the global food output will need to drastically increase to meet demands. At the same time, the amount of land suitable for agriculture is finite, so it is not possibly to meet the growing demand by simply increasing the use of land. Thus, to be able to feed the entire global population, and continue to do so in the future, it will be necessary to drastically increase the food output per land area. One idea which has been recently discussed in the scientific community is called Vertical Farming (VF), which cultivates food crops on vertically stacked levels in (high-rise) buildings. The Vertical Farm, so it is said, would allow for more food production in a smaller area. Additionally, a Vertical Farm could be situated in any place (e.g. Taiga- or desert regions, cities), which would make it possible to reduce the amount of transportation needed to deliver the crops to the supermarkets. The technologies required for the Vertical Farm are well-known and already being used in conventional terrestrial greenhouses, as well as in the designs of bioregenerative Life Support Systems for space missions. However, the economic feasibility of the Vertical Farm, which will determine whether this concept will be developed or not, has not yet been adequately assessed. Through a Concurrent Engineering (CE) process, the DLR Institute for Space Systems (RY) in Bremen, aims to apply its know-how of Controlled Environment Agriculture (CEA) Technologies in space systems to provide valuable spin-off projects on Earth and to provide the first engineering study of a Vertical Farm to assess its economic feasibility

Evaluating the reliability of point estimates of wetland reference evaporation

The Penman-Monteith formulation of evaporation has been criticised for its reliance upon point estimates so that areal estimates of wetland evaporation based upon single weather stations may be misleading. Typically, wetlands comprise a complex mosaic of land cover types from each of which evaporative rates may differ. The need to account for wetland patches when monitoring hydrological fluxes has been noted. This paper presents work carried out over a wet grassland in Southern England. The significance of fetch on actual evaporation was examined using the approach adopted by Gash (1986) based upon surface roughness to estimate the fraction of evaporation sensed from a specified distance upwind of the monitoring station. This theoretical analysis (assuming near-neutral conditions) reveals that the fraction of evaporation contributed by the surrounding area increases steadily to a value of 77% at a distance of 224 m and thereafter declines rapidly. Thus, point climate observations may not reflect surface conditions at greater distances. This result was tested through the deployment of four weather stations on the wetland. The resultant data suggested that homogeneous conditions prevailed so that the central weather station provided reliable areal estimates of reference evaporation during the observation period March–April 1999. This may be a result of not accounting for high wind speeds and roughness found in wetlands that lead to widespread atmospheric mixing. It should be noted this analysis was based upon data collected during the period March-April when wind direction was constant (westerly) and the land surface was moist. There could be more variation at other times of the year that would lead to greater heterogeneity in actual evaporation.</p> <p style='line-height: 20px;'><b>Keywords: </b>evaporation, Penman-Monteith, automatic weather station, fetch, wetland</p