Refrigerant and Oil Migration and Retention in Air Conditioning and Refrigeration Systems

Air Conditioning and Refrigeration Project 16

A Study of Evaporation Heat Transfer Coefficient Correlations at Low Heat and Mass Fluxes for Pure Refrigerants and Refrigerant Mixtures

An average R12 refrigerant correlation has been developed for the mass flux range of 25-100 kg/m2-s and the heat flux range of 2-10 kW/m2. Refrigerant mixtures of 80% R22/lO% R141b and 65% R22/35% R123 have also been tested over asjrniJar range of conditions. Mixture heat transfer coefficients have been detennined and correlations for each mixture pair are presented. The R221R141b average correlation may have a strong dependence on changes in surface tension. The heat transfer coefficient of R22JR141b compares well with that of R12. The heat transfer coefficient of R22/R123 severely under performs R12.Air Conditioning and Refrigeration Center Project 0

Investigation of Adiabatic Refrigerant Pressure Drop and Flow Visualization in Flat Plate Evaporators

Adiabatic pressure drop and flow visualization in chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate heat exchangers were investigated for vertical upward flow with R134a. Qualities ranging from subcooled liquid to superheated vapor were investigated. Mass fluxes ranged from 16 kg/m2-s (for superheated vapor) to approximately 300 kg/m2-s (for sub-cooled liquid). The pressure drop experiments were conducted for 10o C and 20o C inlet temperatures. The flow visualization experiments were conducted at a 10o C inlet temperature. The following is the order of highest to lowest pressure drop geometries on both a mass flux and mass flow bases: chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate. These trends are more pronounced on a mass flow basis. Four flow regimes were observed for the flat plate geometries investigated and are mapped out on a mass flux versus quality basis for each geometry. The chevron geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the bumpy plate geometries. The kinetic energy per unit volume of the flow was found to have a strong linear relationship with pressure drop for both single-phase and two-phase flow, suggesting that inertial effects are the dominant mode of pressure drop in flat plate heat exchangers. Vapor pressure drop prediction models based on the kinetic energy of the flow are presented, which predict pressure drop within 20%. A two-phase pressure drop model is developed, also based on kinetic energy per unit volume of the flow. A pseudo void fraction is defined in order to correlate the two-phase pressure drop to the single-phase pressure drop. The two-phase pressure drop model predicts two-phase pressure drop to within 15% of experimental measurements. A description of and modifications to the experimental test facilities are provided. In addition, the geometries and construction of the plates are provided.Air Conditioning and Refrigeration Project 12

Probabilistic Flow Regime Map Modeling of Two-Phase Flow

The purpose of this investigation is to develop models for two-phase heat transfer, void fraction, and pressure drop, three key design parameters, in single, smooth, horizontal tubes using a common probabilistic two-phase flow regime basis. Probabilistic two-phase flow maps are experimentally developed for R134a at 25 ??C, 35 ??C, and 50 ??C, R410A at 25 ??C, mass fluxes from 100 to 600 kg/m2-s, qualities from 0 to 1 in 8.00 mm, 5.43 mm, 3.90 mm, and 1.74 mm I.D. horizontal, smooth, adiabatic tubes in order to extend probabilistic two-phase flow map modeling to single tubes. An automated flow visualization technique, utilizing image recognition software and a new optical method, is developed to classify the flow regimes present in approximately one million captured images. The probabilistic two-phase flow maps developed are represented as continuous functions and generalized based on physical parameters. Condensation heat transfer, void fraction, and pressure drop models are developed for single tubes utilizing the generalized flow regime map developed. The condensation heat transfer model is compared to experimentally obtained condensation data of R134a at 25 ??C in 8.915 mm diameter smooth copper tube with mass fluxes ranging from 100 to 300 kg/m2-s and a full quality range. The condensation heat transfer, void fraction, and pressure drop models developed are also compared to data found in the literature for a wide range of tube sizes, refrigerants, and flow conditions.Air Conditioning and Refrigeration Project 18

Experimental Investigation of Viscous Two-Phase Flow in Microchannels

Multi-port microchannel tubes are increasingly popular for use in a variety of heat transfer applications, primarily for automotive condensers and radiators, but also in a variety of refrigeration and air conditioning applications. These channels offer a greater surface area to volume ratio, providing for enhanced heat transfer over a conventional tube in many applications. Previous research has focused on characterizing the performance of such tubes for two-phase refrigerant flow. Most studies have focused on pure refrigerant flow, but in most applications, as a third viscous ???phase??? will be present in the form of lubricating oil. Much research has been done to account for the effects of increased viscosity due to the presence of oil in the flow, but the effects of viscosity in microchannels rather than larger conventional tubes remain largely unexplored. The goal of this study is to investigate the qualitative and quantitative effects of the presence of oil within the refrigerant for two-phase flow in multi-port, extruded aluminum microchannel tubes. Three techniques are used to characterize these effects. Flow visualization experiments, using a transparent test section, demonstrate the flow configuration between the ports and flow regime within individual ports. Additionally, experimental adiabatic pressure drop and void fraction measurements ??? performed for a variety of fluids and flow conditions ??? quantitatively characterize the behavior of the refrigerant-oil mixture in two-phase flow. Experimental results demonstrate a stark change in the flow when viscosity of the liquid phase is increased. These are noted by a change in the observed flow patterns, increased pressure drop, and depressed void fraction as compared to less viscous conditions. These trends cause significant departures from the behaviors characterized in many existing predictive correlations, and present a challenge to incorporate viscosity into modified correlations.Air Conditioning and Refrigeration Project 15

Characterization of Two-Phase Flow in Microchannels

Aluminum multi-port microchannel tubes are currently utilized in automotive air conditioners for refrigerant condensation. Recent research activities are directed toward developing other air conditioning and refrigeration systems with microchannel condensers and evaporators. Three parameters are necessary to analyze a heat exchanger performance: heat transfer, pressure drop, and void fraction. The purpose of this investigation is the experimental investigation of void fraction and frictional pressure drop in microchannels. A flow visualization analysis is another important goal for two-phase flow behavior understanding and experimental analysis. Experiments were performed with a 6-port and a 14-port microchannel with hydraulic diameters of 1.54 mm and 1.02 mm, respectively. Mass fluxes from 50 to 300 kg/s.m2 (range of most typical automotive applications) are operated, with quality ranging from 0% to 100% for two-phase flow experiments. R410A, R134a, and air-water mixtures are used as primary fluids. The results from the flow visualization studies indicate that several flow configurations may exist in multi-port microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Flow mapping of the fluid regimes is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. Experimental analysis and flow observations suggest that pressure drop and void fraction in microchannel is dependent on the most probable flow regime at which the two-phase mixture is flowing. In general, correlations for void fraction and pressure drop predictions are based in a separated flow model and do not predict the experimental results in the range of conditions investigated. A flow regime based model is developed for pressure drop and void fraction predictions in microchannels.Air Conditioning and Refrigeration Project 10

Investigation of an R134A Refrigerant/Iso 32 Polyol Ester Oil Mixture in Condensation

Air Conditioning and Refrigeration Project 12

An Investigation of Void Fraction in the Stratified/Annular Flow Regions in Smooth, Horizontal Tubes

Refrigerants R134a and R410A have been used for void fraction measurements in smooth horizontal tubes with diameters between 4mm and 7mm. Quality and mass flux were varied from 5% to 90% and 75 kglm2-s to 700 kglm2-s, respectively. Two test loops, one for condensing flows at 35C and the other for evaporating flows at 5C, were used in the investigation. Results show that near the transition from annular to stratified flow void fraction changed from viscousinertial dependence to gravitational-inertial dominated dependence. An important feature observed is the annular flow region's relative insensitivity to mass flux while the border region between annular and stratified flows is characterized by strong mass flux dependence.Air Conditioning and Refrigeration Project 7

Determining the economic costs and benefits of conservation actions: A decision support framework

The need for conservation action to be cost-effective is widely accepted, resulting in increased interest and effort to assess effectiveness. Assessing the financial and economic costs of conservation is equally important for assessing cost-effectiveness, yet their measurement and assessment are repeatedly identified as lacking. The healthcare sector, in contrast, has made substantial progress in identifying and including costs in decision-making. Here, we consider what conservation can learn from this experience. We present a three-step framework for identifying and recording the relevant economic costs and benefits of conservation interventions where the user (1) describes the costing context, (2) determines which types of cost and benefit to include, and (3) obtains values for these costs and benefits alongside metadata necessary for others to interpret the data. This framework is designed to help estimate economic costs but can also be used flexibly to record the direct costs of interventions (i.e., financial costs) and calculate financial and economic benefits. Although recording data on economic costs and benefits is deceptively complex, this framework facilitates improved recording, and indicates how collating this data could enhance the assessment of cost-effectiveness across conservation contexts using a range of decision-making tools. © 2022 The Authors. Conservation Science and Practice published by Wiley Periodicals LLC on behalf of Society for ConservationWe thank Alec Christie, Ashley Simkins, and Anthony Waldron for helpful discussions and Arcadia, MAVA, and the David and Claudia Harding Foundation for funding. We thank two anonymous reviewers, and Gwen Iacona for detailed comments that helped improve the manuscript. The work was completed by Thomas White as part of a PhD supported by a Balfour studentship at the Department of Zoology, University of Cambridge