Kevin M. Traeger. Predrag S. ..... visualized for CO2 and R410A in the 6 and 3 mm glass tubes, and they are compared with the Weisman et al. and the Wojtan et ...
ACRC TR-251
Charge Minimization of Microchannel Heat Exchangers Kevin M. Traeger Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign June 2005
ABSTRACT This thesis focuses on microchannel heat exchangers, and the methods used to try and reduce the charge contained in them. First, four condensers were designed and manufactured with the same air side characteristics and same tube design (with the exception of some blocked ports in one design) so that any difference in charge, capacity, pressure drop etc. can be attributed to flow geometry. These designs were a two circuit serpentine design, a two pass parallel flow design, and two one pass parallel flow designs. The difference between the two one pass designs were the number of microchannel ports in the tubes, one with 19 ports and the other with 10. The one pass design with 10 ports contained the least amount of charge, on average about 20 g, while the other one pass design contained the most charge, about 28 g. The reason for this difference was not only because the smaller tube volume, but an increase in mass flux also decrease the amount of charge in the exit header by 2030%. The serpentine condenser also had very low charge amounts, but pressure drop was about ten times higher than in the parallel flow designs. This caused a 4% lower COP than what was given with the one pass design. Two serpentine evaporators have also been examined. The evaporators had similar core volumes of about 870 cm3. One evaporator had double the fin length by using a “splitter” fin, which reduced the length of the evaporator tube and therefore reduced the internal volume. Both evaporators performed almost exactly the same, but the splitter fin evaporator had less of a pressure drop and contained 15% less charge. Finally, the models developed for this thesis were used to theoretically compare the performance and charge of six different refrigerants, propane (which was the refrigerant used for experiments), R22, R134a, isobutane, ammonia, and R410A in a condenser. A pressure drop based on a 1% decrease of Carnot COP was found for each refrigerant. Ammonia was found to be able to have same capacity as the other refrigerants, but able to achieve the capacity with a much smaller cross sectional area and a very small amount of charge.
ACRC TR-252
Experimental Investigation of Viscous Two-Phase Flow in Microchannels Jason D. Burr Ty A. Newell Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign June 2005
ABSTRACT 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.
ACRC TR-253
Experimental and Modeling Investigation of Two Evaporator Automotive Air Conditioning Systems Steffen Peuker Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign December 2006
ABSTRACT This document presents results of experimental and model investigations of two evaporator automotive air conditioning systems using R134a and R744 as refrigerants. The R134a system investigated originated from the vehicle air conditioning system used in the U.S. Army HMMWV (High Mobility Multi-purpose Wheeled vehicle). The results from the HMMWV R134a breadboard system investigation are used as a baseline to compare the experimental results from the investigation of the U.S. Army HMMWV R744 two evaporator prototype system. The subject of different hardware setups (e. g. choice of expansion devices) for the HMMWV R744 two evaporator system and their implications on performance is addressed. For the case of two controllable expansion devices, the iterative process which was used to derive an ambient temperature dependent high side pressure correlation for the HMMWV R744 two evaporator system is presented. The optimized HMMWV R744 system shows higher cooling capacity (up to 57%) and higher coefficient of performance (up to 18%) compared to the HMMWV R134a system. In addition, further general issues related to R744 two evaporator systems are investigated. Different system configurations are explored to investigate where to split and reunite the two refrigerant streams and how this affects the system stability. Several expansion device combinations are investigated with the focus on fixed area versus controlled area expansion devices. The role of an accumulator in an R744 two evaporator system is explained. A control strategy for an R744 two evaporator system using two controlled area expansion devices is introduced and validated against transient experimental data. Dymola and the AirConditioning Library are used to simulate an R744 two evaporator automotive air conditioning system. The model results are validated against experimental data in steady state and transient conditions. The predicted performance at steady state is within 10% of the experimental results. For the investigated transient scenario the model prediction shows some discrepancy but the overall trends are well predicted.
ACRC TR-254
Investigation of Transient Two-Phase Flow during Refrigeration and Air Conditioning System Startup Michael J. Hedrick Emad W. Jassim Ty A. Newell Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign December 2006
ABSTRACT The purpose of this investigation is to develop models for transient slug flow under refrigeration system startup conditions. Slug flow is generated for two-phase air-water and R134a with pressure differences of 69-520 kPa; slug volumes of 40-400 ml; and 6.35, 10.2, and 13.4 mm tube diameters under high and low pressure side conditions. Flow visualization images are recorded with a high speed camera and digital web camera. A film thickness sensor design is described for use in the once through flow loop. The trends of the film thickness sensor output, pressure difference, slug location, and slug breakdown distance are discussed. Four separate models are generated to describe the slug acceleration and breakdown. The models are compared to experimental data and simplified further. The simplified model predicts the acceleration and breakdown of the slug with four inputs: fluid, applied pressure difference, slug volume, and tube diameter. The performance and uncertainty of the model are discussed. Models are also generated to determine the tube length required for slug breakdown.
ACRC TR-255
Quantitative Visualization of CO2-Oil Mixtures in CO2 Expansion Flows Matthew A. Scott Dimitrios Kyritsis Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign May 2007
ABSTRACT Quantitative visualization of CO2-oil mixture flows through channels that simulate CO2 expansion devices was pursued. The existence of traces of lubricant oil (specifically polyolester (POE)) in the working medium makes the flow amenable to laser-induced fluorescence (LIF) diagnostics. This technique is used to visualize the flow of CO2-oil mixtures through an optically accessible test section under various operating conditions. The results of the measurements provide data on the concentration of the lubricant that is entrained by CO2 in the expansion device as well as information about the form with which the oil is transported through the ejector (liquid films, droplets, mist, etc.). Further flow visualization was accomplished using high-speed cinematography, which revealed information about the oil film structure as a function of oil circulation rate and CO2 flow rate. Further experiments using laserbased shadowgraph were conducted and the need for a square cross-section was established due to refraction in the round tubing. Results from these experiments will guide the design of practical ejector geometries and will also indicate the extent to which various flow models may be employed in the investigation of CO2 refrigeration systems with ejectors.
ACRC TR-256
An Analytical and Empirical Study of Frost Accumulation Effects on Louvered-Fin, Microchannel Heat Exchangers Yanping Xia Predrag S. Hrnjak Anthony M. Jacobi Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign December 2006
ABSTRACT The thermal-hydraulic behavior of folded-louvered-fin, microchannel heat exchangers is explored under conditions of air-side frosting, defrosting, and refrosting. The temperature distribution within a two-dimensional composite fin is analyzed. A parametric analysis shows that for some conditions, such as those typical to frostcoated fins, the problem can be approximated as a two-dimensional slab on a one-dimensional fin. Under this approximation, an exact solution to the heat diffusion equation is obtained through an eigenfunction expansion. The analytical solution and a one-term approximation to the full solution have broad applicability in addition to their use for calculating fin efficiency for frost-coated fins. Valid HA-LMED and UA-LMTD methods for wet- and frosted-surface heat transfer are formulated. The UALMTD method is shown to provide the best results for dry, partially-wet/frosting, and fully-wet/frosting conditions. Without area partitioning, the HA-LMED method is only applicable to fully-wet/frosting conditions. For all the conditions considered in a parametric study to mimic the experimental range of this work, the UA-LMTD method provides the value of the air-side convective heat transfer coefficient within 3% and is more accurate than the HALMED method. Heat transfer and pressure drop data for nine different fin geometries are presented, and a decrease in the overall heat transfer coefficient and an increase in the pressure drop are observed as frost accumulates on the surfaces. A reduction in air-side flow rate and bridging of louver gaps by frost are identified as the factors most important to the reduced heat transfer performance. Correlations are presented for predicting the thermal performance of these heat exchangers under frosting conditions. A numerical model is developed to predict the time-varying performance of folded-louvered-fin, microchannel heat exchangers. The model utilizes the correlations developed from the experimental data and incorporates a sub-model for frost properties. The model successfully predicts the heat transfer performance of the heat exchangers studied, but its ability to predict the pressure-drop behavior needs further improvement. The model can be used to evaluate geometry effects on the frosting behavior of the louvered-fin, microchannel heat exchangers, and can be easily generalized to other applications with simultaneous heat and mass transfer.
ACRC TR-257
Experimental Investigation of an Environmental Control Unit Utilizing Carbon Dioxide for Heating and Cooling Scott S. Wujek Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign September 2006
ABSTRACT This report details the tests performed on R22 box and R744 breadboard versions of the US Army Environmental Control Unit (ECU) with a nominal cooling capacity of 9,000 BTU/hr (0.75 Tons or 2.64 kW). An R22 ECU box currently used by the Army was tested and used as the baseline for comparison with the later R744 testing. This baseline system is generally used where portable air conditioning is needed in the field; the box is also equipped with electric resistance heaters. The baseline box was found to outperform its rated capacity. An R744 system was tested in a breadboard format, this system was comprised of components similar to what would be used in an ECU box if it were converted from R22. This breadboard system was capable of operating as both an air conditioner and as a heat pump. A cooling COP approximately equal to the box system was attained with the breadboard. The breadboard did provide a HPF greater than unity for all heat pump tests, making it is more efficient than the electric resistance heaters found in the box system. Detailed comparisons were made between several R744 heat exchangers in different orientations under varying operating conditions. Evaporators were tested in horizontal and vertical tube orientations. The flat top fin evaporator was found to work well in both horizontal and vertical orientations. The round top fin evaporator, which may have contained a manufacturing defect, performed much better with the tubes vertical, than with the tubes horizontal. Overall the best heat exchanger was the round top fin evaporator with the tubes vertical; however this heat exchanger performed the worst with the tubes horizontal. In air conditioning mode, both gas coolers performed similarly. In heat pump mode, the six-port tube heat gas cooler performed better in almost every regard in comparison to the four-port tube with the only exception being the air side pressure drop.
ACRC TR-258
Carbon Dioxide and R410a Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern in Horizontal Tubes at Low Temperatures Chang Yong Park Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign January 2007
ABSTRACT Carbon dioxide (CO2) has been seriously considered as an alternate refrigerant for HCFC and HFC fluids, due to the increasing interest of environmentally safe refrigerants in air-conditioning and refrigeration systems. In this study, CO2 flow boiling heat transfer coefficients and pressure drop are measured in macro-scale (6.1 and 3.5 mm) tubes at evaporation temperatures of –15 and –30 °C. The measured results show that the nucleate boiling is a main heat transfer mechanism in the 6.1 mm tube and the contribution of convective boiling becomes greater with the decrease of tube diameters and the increase of mass fluxes. The surface roughness of the 6.1 and 3.5 mm tube are presented by SEM and AFM images and surface profiles, and it is shown that the rougher surface of the 6.1 mm tube can affect the flow boiling heat transfer. The CO2 heat transfer coefficients and pressure drop are measured in a mini-scale (0.89 mm) multi-ported tube at the evaporation temperature of –30 °C. Also, R410A and R22 flow boiling heat transfer coefficients and pressure drop in a macro-scale (6.1 mm) tube were measured, and they are compared with CO2. This comparison presents that the CO2 flow boiling heat transfer coefficients are higher than R410A and R22 at low vapor qualities, and CO2 pressure drop is significantly lower than R410A and R22. This advantageous characteristic for CO2 could be explained by properties such as surface tension, reduced pressure, and the density ratio of liquid to vapor. The prediction of heat transfer coefficients and pressure drop was performed by general correlations and the calculation results are compared with measured values. Two-phase flow patterns were visualized for CO2 and R410A in the 6 and 3 mm glass tubes, and they are compared with the Weisman et al. and the Wojtan et al. flow pattern maps. The flow pattern maps can determine the flow patterns relatively well, except the transition from intermittent to annular flow.
ACRC TR-259
Integrating HVAC Equipment with Other Building Subsystems Ben Yannayon Clark W. Bullard Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign May 2007
ABSTRACT Data now becoming available from prototype low energy residences confirm that peak heating and cooling loads can be reduced by factors of 3 to 5, using currently available technologies. This paper explores the range of candidate HVAC systems capable of dealing with such fundamentally different load profiles as 8°C balance points, spatially non-uniform loads, and non-negligible thermal capacitance. Results reveal surprisingly small differences in overall efficiency among well-designed centralized (ducted) and decentralized systems providing either mechanical or desiccant dehumidification. Centralized systems offer opportunities for integration of domestic hot water heating, and some types of decentralized systems offer the long-term possibility of being integrated into wall panels or other structural elements. Since efficiencies are comparable, system selection is therefore likely to be driven by such factors as initial costs, complexity and reliability.
ACRC TR-260
Using Anisotropic Micro-Scale Topography to Manipulate the Wettability of Aluminum and Reduce the Retention of Water Andrew D. Sommers Anthony M. Jacobi Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign August 2007
ABSTRACT A method is described for fabricating controlled micro-scale, topographical features on aluminum surfaces for the purpose of exploiting those features to affect the surface wettability. Using a photolithographic approach, a photoresist-masked surface is subjected to a plasma etch in a mixture of gaseous BCl3 and Cl2. Parallel grooves, microns to tens of microns in width, depth and spacing are studied, because this geometry is scaleable for mass production by roll-to-roll micro-embossing, and because the anisotropic nature of these features provides a directional change in wettability that can reduce the retention of water on the surface. Aluminum was studied because it is naturally hydrophilic and widely used in wet-surface heat exchanger applications, because of its low cost and excellent mechanical and thermal properties. Water droplets placed on a micro-grooved aluminum surface using a micro-syringe exhibit significantly increased apparent contact angles, and for water condensed onto an inclined, micro-grooved surface, the droplet volume at incipient sliding is reduced by more than 50% compared to droplets on a surface without micro-grooves. No chemical surface treatment is necessary to achieve this water repellency; it is accomplished solely through the anisotropic surface topography. The droplet geometry shows an elongated base contour relative to a surface without micro-grooves, and discontinuities in the three-phase contact line are also introduced by the grooves. A mechanistic model is presented for predicting the critical droplet size on micro-grooved surfaces. This model extends earlier work by accounting for the droplet geometry and contact-line changes caused by the micro-grooves. The model is validated through comparisons of predicted to measured critical droplet sizes, and it is then used to provide guidance for the development of surfaces with enhanced water drainage behavior. In a broad range of air-cooling applications, water retention on the air-side surface of metallic heat exchangers is problematic, because it can reduce the air-side heat transfer coefficient, increase core pressure drop, and provide a site for biological activity. In refrigeration systems, the accumulation of frost on metallic fins requires periodic defrosting and reduces energy efficiency. When water is retained on these surfaces following the defrost cycle, ice is more readily formed in the subsequent cooling period, and such ice can lead to shorter operation times before the next defrost is required. Thus the management and control of water droplets on heat-transfer and air-handling surfaces is vital to energy efficiency, functionality, and maintenance in air-cooling systems. The micro-structured surfaces introduced in this work are proposed for use in air-cooling and dehumidifying applications, but they may have other applications where the management of liquids on a surface is important.
ACRC TR-261
Two-Phase Pressure Drop and Flow Regime of Refrigerants and Refrigerant-Oil Mixtures in Small Channels Brandon S. Field Predrag S. Hrnjak Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign September 2007
ABSTRACT As microchannel heat exchangers have become more sophisticated in their design, more exact understanding of the flow inside them is necessary. A decrease in diameter enhances the heat transfer (which takes place at the inner walls of the tubes), but also increases the pressure drop (as the diameter decreases, it becomes like drinking a milkshake through a coffee stirrer). The inclusion of even small amounts of oil in circulation can have a significant effect as well. Historical correlations and studies of two-phase flow have been shown to be insufficient for predicting pressure drops in the smaller channels, due to the different fluid physics that are relevant in flows of small diameter. This study is aimed at understanding the fluid property effects that contribute to pressure drop and flow regime. Two-phase pressure drop data for four refrigerants (R134a, R410A, R290 and R717) were measured in a channel with hydraulic diameter of 148 μm. These data were combined with previous two-phase data of R134a in small channels (hydraulic diameters ranging from 70 to 300 μm) to generate a separated flow model that spans a wide variety of fluid properties. Refrigerant was then mixed with two different viscosities of oil at concentrations ranging from 0.5 to 5% oil, and two-phase pressure drop measurements were taken of those mixtures. Flow visualizations of three of these refrigerants (R134a, R290 and R717) and several concentrations of a R134a-oil mixture were made in a channel with 500 μm hydraulic diameter, and flow regime classifications and comparisons with previous flow maps were made. Finally, a mechanistic description of the two-phase flow that occurs in small channels is put forth, based on the pressure drop measurements and the flow visualizations.
ACRC TR-262
Condensation and Freezing Front Propagation on Surfaces with Topographic and Chemical Modifications Yongfang Zhong Anthony M. Jacobi John G. Georgiadis Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign February 2008
ABSTRACT The growth of condensate and the propagation of a freezing front are investigated on surfaces with special wetting behavior. Topographic and chemical modifications are applied on silicon substrates to obtain these wetting properties. Micro-grooves and micro-posts are fabricated through classical photolithography, and chemical modification is achieved by using the FDTS (heptadecafluoro-1, 1, 2, 2-tetrahydrodecyltrichlorosilane) coating with the molecular-vapor-deposition method. The evolution of condensate on super-hydrophobic surfaces (usually contact angles ≥ 150°) is presented. For micro-grooved surface, condensate, which is initially in troughs, is observed to flow into the droplets above crests during the growth of a single droplet or as a result of coalescence. The top view of the base contour of droplets remains circular during the growth for micro-grooved surfaces but not for the samples with micro-posts. A small inclination angle is observed for the condensed droplets on samples with deep grooves. This drainage behavior is most likely associated with the morphology of condensate in the troughs due to the entrapped air. A hypothesized explanation is proposed for the sliding behavior of condensed droplets, and a model based on energy minimization is developed to understand the condensate morphology in the troughs of micro-grooves. Experimental data on propagation were reported for a freezing front crossing on micro-grooved substrates in a 200μm-to-400μm-thick water layer. The speed of a freezing front oscillates locally over the grooves, with propagation retarded in troughs and accelerated on crests. When the water freezing point moves at a constant speed, the speed of a freezing front on crests can be more than 8 times the speed in troughs. A numerical estimation shows that the difference of the propagation speeds on crests and in troughs is mainly caused by thermal effects.
ACRC TR-263
Comparison of Two Refrigerants Blends R404A and R410A in Commercial Refrigeration System in Low Temperatures Application Radko Brock Anthony M. Jacobi Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign date
ABSTRACT text
ACRC TR-264
Tribology of Carbon Dioxide Including Instrumentation for Testing at Extreme Pressures and Characterization of Advanced Protective Tribological Materials Nicholaos G. Demas Andreas A. Polycarpou Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign April 2008
ABSTRACT The refrigeration industry has shown an inclination towards the use of carbon dioxide as a refrigerant in some applications due to the fact that it is an environmentally friendly, non-flammable, nontoxic and economical solution to the harmful hydrofluorocarbon refrigerants widely used today. Even though carbon dioxide has long been a proven refrigerant from a thermodynamic perspective and has been used in the past for refrigeration applications, its applicability to refrigeration systems has been limited because carbon dioxide systems have to be operated at very high compressor pressures. Advances in compressor designs have eliminated concerns regarding design criteria and safety due to the thermodynamic properties of carbon dioxide and the range of pressures required for the carbon dioxide refrigeration cycle for typical applications in refrigeration. However, the development of knowledge of tribology related to carbon dioxide environment is vital since reliable operation is essentially a tribological issue as it determines whether the interacting surfaces in motion will simply wear over prolonged operation or lead to sudden failure, which is known as scuffing, a common failure for compressors. Tribology of carbon dioxide is a new field and research on this topic in the open literature is scarce. Traditionally, addressing the tribological needs (friction reduction, wear control, life increase) of modern compressors has been primarily achieved and focused on the development of lubricants. As compressors are designed to operate under increasingly severe conditions of higher speeds and loads for increased efficiency, their contacting and sliding surfaces are subjected to extreme conditions. Such severe conditions lead to excessive wear and premature failures, thus becoming a major liability issue for compressor manufacturers. With current needs to advance compressor design and performance (new refrigerants, higher temperatures, limited or no liquid lubrication, advanced alloys and manufacturing processes) the capabilities of liquid lubricants and additives are limited and the development of advanced surface materials, surface treatments and coatings will be necessary. The research includes tribological experiments using carbon dioxide as a refrigerant at high pressures under a wide range of loading and temperature conditions, examination of the effects of lubricants in the tribological performance of the material interfaces, microscopy analysis and chemical studies of the tribologically tested surfaces, and investigation of protective materials for advancing the tribological performance of interacting surfaces.
ACRC TR-265
Flow-Boiling of R-134a-Based Nanofluids in a Horizontal Tube Kristen R. Henderson Anthony M. Jacobi Department of Mechanical Science and Engineering University of Illinois at Urbana-Champaign December 2008
ABSTRACT Nanofluids, heat transfer fluids containing suspensions of nanoparticles, can be considered to be the nextgeneration heat transfer fluids as they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids. In this study, the influence of nanoparticles on the flow-boiling of R-134a and R-134a/polyolester mixtures is quantified. With direct nanoparticle dispersion of silica nanoparticles in R-134a, the heat transfer coefficient is decreased by as much as 55% in comparison to pure R-134a. This degradation is, in part, due to the moderate dispersion quality of the mixture; dispersion tests for various nanoparticle/halocarbon combinations reveal the difficulty in directly dispersing nanoparticles in refrigerants. Moreover, the influence of copper oxide (CuO) nanoparticles on the flow-boiling of R-134a/polyolester mixtures is measured. A mixture of copper oxide particles stably suspended in a synthetic ester (POE) to a 4% volume fraction is prepared. For a mixture of R-134a/POE/CuO with a particle volume fraction of 0.02%, the nanoparticles have no apparent effect on the boiling heat transfer coefficient. However, for a CuO volume fraction of 0.04%, an average enhancement of 52% in the heat transfer coefficient is manifest, over that of the same refrigerant-oil mixture without nanoparticles. A further increase in the CuO volume fraction to 0.08% results in a still larger average enhancement of 76% in the heat transfer coefficient. In addition to heat transfer enhancement, it is found that the presence of nanoparticles has an insignificant effect on the system pressure drop (within the experimental uncertainty) when compared to baseline data.
