The Use of Ionic Liquids in Refrigeration

Ionic liquids are salts, usually with organic cations and inorganic anions, that are liquid at room temperature. There are a wide variety of ionic liquids that can be synthesized with different properties for different applications. They are generally non-volatile, non-toxic, and non-flammable with high heat capacity, high density, high thermal and chemical stability. We propose its use as an absorbent in an absorption refrigeration cycle. The refrigerant in this case would be a gas such as carbon dioxide. The present work deals with the desirable properties of ionic liquids for this application. For example, the absorbent must have a high solubility, and the heat and mass transfer coefficients of the absorbent-refrigerant solution must be large. The viscosity of the mixture, on the other hand, should not be so large as to make its pumping difficult.

Surrogate Models for Predicting the Performance of a Liquid-Liquid Cylindrical Cyclone Separator

Liquid-Liquid Cylindrical Cyclone (LLCC) separators are devices used in the petroleum industry to extract a portion of the water from the oil-water mixture obtained at the well. The oil-water mixture entering the separator is divided due to centrifugal and buoyancy forces in an upper (oil rich) exit and a bottom (water rich) exit. The advantages in size and cost compared with traditional vessel type static separators are significant. The use of LLCC separators has not been widespread due to the lack of proven performance prediction tools. Mechanistic models have been developed over the years as tools for predicting the behavior of these separators. These mechanistic models are highly dependent on the inlet flow pattern prediction. Thus, for each specific inlet flow pattern a sub-model has to be developed. The use of surrogate models will result in prediction tools that are accurate over a wider range of operational conditions. We propose in this study to use surrogate models based on a minimum-mean-squared-error method of spatial prediction known as Kriging. Kriging models have been used in different applications ranging from structural optimization, conceptual design, multidisciplinary design optimization to mechanical and biomedical engineering. These models have been developed for deterministic data. They are targeted for applications where the available information is limited due to the cost of the experiments or the time consumed in numerical simulations. We propose to use these models with a different framework so that they can manage information from replications. For the LLCC separator a two-stage surrogate model is built based on the Bayesian surrogate multistage approach, which allows for data to be incorporated as the model is improved. Cross validation mean squared error measurements are analyzed and the model obtained shows good predicting capabilities. These surrogate models are efficient and versatile predicting tools that do not require information about the physical phenomena that drives the separation process.

Comparison of Subcooling Effect of Water as a Refrigerant With the Current Refrigerants

The performance comparison of water as a refrigerant (R718) with some prevailing refrigerants including R717, R290, R134a, R12, R22, and R152a is presented. A computer program simulating an actual vapor compression refrigeration cycle including subcooling was developed to calculate the coefficient of performances (COPs) for the different refrigerants. Evaporator temperatures above which water yields a better COP over the other refrigerants are investigated for subcooling case. The effect of degree of subcooling on the COPs is elaborated. For most of the refrigerants (R290, R134a, R12, R22, and R152a) the COP increases by around one percent (1%) per one Kelvin (1K) subcooling, while the COP for R718 and R717 increases by around 0.2 % and 0.5 % per one Kelvin (1K) subcooling. At constant evaporator temperature, increasing the degree of subcooling results in decrease of the relative COP gain of R718. R718 gives the highest relative COP increase at constant condenser temperature and polytropic efficiency. The effect of polytropic efficiency on the performance is also investigated. It is observed that the evaporator temperature range at which R718 presents a better COP than other refrigerants increases with increasing values of polytropic compressor efficiency if the degree of subcooling is kept constant.

A Numerical Study Concerning the Influence of the Grashof Number in the Indirect Ice Storage Tanks Performance

The present paper deals with typical chiller-based ice storage tanks. Natural convection of water (Phase Change Material-PCM) near its density maximum leads to a peculiar nature of the flow pattern in the liquid phase of the PCM, giving rise to a multi-cellular regime that affects drastically the heat transfers within the tank. So, this work intends to examine numerically how the flow pattern affects qualitatively the performance of such thermal storage devices. This is done by investigating the influence of the non-dimensional parameter Grashof number during the charging operation step of such devices that corresponds to the ice making process occurring within the storage tank. Besides, the tank is assumed to be vertically positioned, as well as their internal tubes through which the secondary fluid flows. In order to analyze the heat transfer between the PCM and one internal tube during the growth of an ice layer around it, one selected a vertical annulus as the physical model. The inner vertical wall represents one of the tubes packed into a typical storage tank, while the outer vertical wall represents the thickness of formed ice around the tube. Regarding the annulus, the top and bottom walls, as well as the outer vertical wall were considered thermally insulated. The thermal analysis is focused in the heat transfer at the inner wall for different values of Grashof, keeping unchanged all other parameters that govern the natural convection problem with phase change. An overview of the cooling process is analyzed through streamlines and isotherms, for specific instants of the physical process. Further, a heat transfer analysis for the total charging stage is presented.

Preliminary Design Considerations for Integrating a Composite Impeller in a Brushless Permanent Magnet Motor

Traditional turbomachine design is generally characterized by a shaft driven impeller that was created using common manufacturing processes. However, a cutting edge and innovative impeller design involves the manufacturing of the impeller on a winding machine. Through the use of such a machine, a lightweight and high-strength impeller with carbon fibers or other fibers can be prototyped quickly and easily while also produced in large quantities. The impeller will not only be a composite of multiple parts, but a single solid piece. One of the most interesting attributes of this impeller however, is the ability to integrate it as the rotor for a permanent magnet electric motor. Using the concepts of permanent magnet motor design, current can be applied to the housing of the impeller, which would generate forces that would serve the purpose of both rotating the impeller, and securing it into place. This has the advantage of eliminating most of the moving parts in a turbomachine design, but also creating a durable, lightweight and cheap design to prototype and reproduce. Another key advantage to this design is where the torque on the impeller is applied. Traditional turbomachines generally have the torque generated from forces applied from the shaft, which requires higher tangential forces and hence shear stresses due to a much smaller moment arm. But the integration of magnetics into the outer shroud of the impeller allows for the forces to be applied to the outer edge, which requires less force due to a much greater moment arm. Furthermore, this type of motor inherently allows for all of the electrical components to be outside of the fluid flow, which reduces the need for extensive sealing and insulation. In this paper, basic concepts behind the design of electric motors are outlined, as well as how they can be integrated with a rotor impeller, such that together they could act as a turbomachine.

An Analysis of the Loss Mechanisms in Low-Speed Axial Fans Using Scaling Arguments and Their Effects on a Proposed New Definition of Efficiency

Low-speed axial fans are used extensively for ventilation purposes in industrial and commercial buildings. In agricultural applications, such as a greenhouse, the ventilation is critical, since entire crops can be damaged or destroyed if a clean air supply is not maintained. The cost-marginal nature of these businesses demand that operating costs be kept to a minimum, hence there is a strong motivation to develop higher efficiency ventilation fans. An analysis of a low-speed axial fan has been developed using a control volume-based energy balance. The specific fan is an axial ventilation fan that is commonly found on agricultural facilities such as green-houses or livestock buildings. These fans induce an airflow from a large building into the open atmosphere at very low (or often effectively zero) system restriction or pressure rise. The definition for static efficiency, which is commonly used by the axial fan community, is examined and its implications are discussed. Since static efficiency yields a zero-percent efficient fan at a zero pressure rise operating condition, the ventilation fan industry has developed an alternate definition of efficiency. This alternate definition of efficiency, along with other proposed definitions, are described and their limitations are discussed. A new definition of efficiency is introduced and its basis in the integral energy equation is identified. The primary loss mechanisms of low-speed axial turbomachinery are discussed and scaling arguments are developed and used in the integral energy equation analysis. The results of this analysis yield an expanded expression of efficiency in which the loss mechanism terms can be empirically determined. When analyzed with values for a particular fan system, these results can further be used as the basis for an optimization study of that fan system.

Radial Ultra-Micro Wave Rotors (UµWR): Design and Simulation

It has been shown that the wave rotor technology has the potential of improving the performance of gas turbine cycles. Moreover the radial wave rotor is an additional innovation for this technology. Unlike the commercialized axial-flow wave rotor (Comprex®), a radial one has the benefit of using centrifugal forces to improve the compression process or flow scavenging. The geometry of the rotor is much simpler and is ideal for microfabrication, which is relying mainly on two-dimensional processes to create three-dimensional features. This paper is presenting several radial ultra-micro wave rotors (UμWR) configurations and numerical analysis of these rotors. In a radial placement, the wave rotor has four possible configurations: two - general configuration, through-flow and reverse-flow, and each of these could have the low pressure air port positioned at inside or outside of the rotor. Results have been obtained using FLUENT, a Computational Fluid Dynamics (CFD) commercial code. The vast information about the unsteady processes occurring during simulation is visualized.

Numerical Analysis of Blade Geometry Generation Techniques for Centrifugal Compressors

It is a known fact that machined impellers result in improved compressor performance compared to cast impellers of the same design. The performance improvements can be attributed to better surface finish, more accurate geometric definition (tighter dimensional tolerances), well defined edges, and the lack of blade tip fillet on shrouded impellers. In addition, it has been observed through experimental investigations that the construction method of the impellers has an impact on performance. For flank-milled machined impellers, a hub and shroud blade profile is connected by pre-determined straight-line-elements (SLE) - which would correspond to a tool path - to generate the blade surface according to the design intent of the compressor engineer. For cast impellers, the method of connecting hub and shroud blade profile points leads to an arbitrary surface definition and is dependent upon a designer's interpretation of blade profile data and/or the solid model, as well as the CAD software. Although the shape of the hub and shroud profiles are preserved, the resulting blade surface defined by connecting these two profiles may not correspond to the design intent of the compressor engineer. Because the blade surface deviates from the design intent, the compressor performance can deteriorate. Foundries rely on a full 3D design model to create tooling for cast impellers, as opposed to hub and shroud profiles typically required of a 5-axis machining program. Therefore, these construction differences become significant for cast impellers. This paper presents computational fluid dynamic investigations of two types of impellers - with blade surfaces generated using SLE and using CAD arbitrary definitions. Because there are many different mathematical definitions that CAD tools employ for curves, the resulting arbitrary blade surface is not unique. The numerical results will help understand the causes of the performance difference as well as the effects of SLE blades to the flow through the impeller. Input conditions for computational dynamic simulations are based on experimental results. All references to experimental data in the present paper are for cast impellers. Therefore the differences in performance are attributed to blade definition (SLE vs. other) and not to differences resulting from manufacturing methods.

Comparing Water (R718) to Other Refrigerants

Even though water (R718) is one of the oldest refrigerants, state of the art technology is required to use water as a refrigerant in compression refrigeration plants with turbo compressors. To compare water (R718) to other refrigerants, a code is developed in which all refrigerants can be compared in a single p-h, T-s, or p-T diagram. Using the code, the COP isolines of water (R718) and any refrigerant can be generated in a graph to determine which refrigerant has a better COP for a certain evaporation temperature and temperature lift. In regard to using water (R718) as a refrigerant, some specific features complicate its application in refrigeration plants with turbo compressors. Because the cycle works at very low pressure, the volumetric cooling capacity of water vapor is very low. Hence, huge volume flows have to be compressed with relatively high pressure ratios. Therefore, the use of water (R718) as a refrigerant, compared to classical refrigerants, such as R134a or R12, requires approximately 200 times the volume flow, and about twice the pressure ratio for the same applications. Because of the thermodynamic properties of water vapor, this high pressure ratio requires approximately a two to four times higher compressor tip speed, depending on the impeller design; while the speed of sound is approximately 2.5 times higher. Reynolds numbers are about 300 times lower and the specific work transmission per unit of mass has to be around 15 times higher. Two factors are introduced to compare the irreversibilities of R718 and other refrigerants and the main source of irrevercibility in R718 cycle is identified. Finally, the current state-of-the-art R718 is reviewed.

Thermal Analysis of Plate Condensers in Presence of Flow Maldistribution Using Refrigerant R134a

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit vapor quality of a refrigerant, namely R-134a. A wide range of parameters are studied and these show the effects of the mass flow rate ratio of cold fluid (water) and two-phase refrigerant fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.