Parametric performance of circumferentially grooved heat pipes with homogeneous and graded-porosity slab wicks at cryogenic temperatures

A recently developed, potentially high-performance nonarterial wick has been extensively tested. This slab wick has an axially varying porosity which can be tailored to match the local stress imposed on the wick. The purpose of the tests was to establish the usefulness of the graded-porosity slab wick at cryogenic temperatures between 110 K and 260 K, with methane and ethane as working fluids. For comparison, a homogeneous (i.e., uniform porosity) slab wick was also tested. The tests included: (1) maximum heat pipe performance as a function of fluid inventory, (2) maximum performance as a function of operating temperature, (3) maximum performance as a function of evaporator elevation, and (4) influence of slab wick orientation on performance. The experimental data was compared with theoretical predictions obtained with the computer program GRADE

Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies

On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the ability to precisely position nanosized samples in antinodes of the electro-magnetic field for maximal magnetic interaction. A method is developed to entrap micro-crystals containing spins in a well defined location on a substrate's surface. Traditional cavity ESR measurements are then performed on a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr$^{5+}$ spins were entrapped as well and measured while approaching the lower limit of the ESR sensitivity. This method suggests the feasibility of on-chip ESR measurements at dilution refrigerator temperatures by enabling the positioning of samples atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic

Multi-photon Rabi oscillations in high spin paramagnetic impurity

We report on multiple photon monochromatic quantum oscillations (Rabi oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of Mn$^{2+}$ (S=5/2) impurities in MgO. We find that when the microwave magnetic field is similar or large than the anisotropy splitting, the Rabi oscillations have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model. We show that these new frequencies come from multiple photon coherent manipulation of the multi-level spin impurity. We develop a model based on the crystal field theory and the rotating frame approximation, describing the observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10

Validation of a Charge-Sensitive Vapor-Injected Compression Cycle Model with Economization

In recent years, research on economized vapor injected (EVI) compression systems showed potential improvements to both cooling capacity and coefficient of performance (COP). In addition, the operating range of compressors can be extended by reducing the discharge temperature. However, the optimum operation of such systems is directly related to the amount of refrigerant charge, which often is not optimized. Therefore, an accurate charge estimation methodology is required to further improve the operation of EVI compression systems. In this paper, a detailed cycle model has been developed for the economized vapor injected (EVI) compression system. The model aims to predict the performance of EVI systems by imposing the amount of required refrigerant charge as an input. In the cycle model, the EVI compressor was mapped based on the correlation of Tello-Oquendo et al. (2017), whereas evaporator, condenser and economizer heat exchanger models were constructed based on the available ACHP models (Bell, 2010). With respect to charge inventory, the 2-point regression model from Shen et al. (2009) was used to account for inaccurate estimation of refrigerant volumes, ambiguity in slip flow model, solubility of refrigerant in the lubricating oil, among others. The cycle model has been validated with experimental performance data taken with a 5-ton Environmental Control Unit (ECU) that utilizes EVI technology. The developed cycle model showed very good agreement with the data with a MAE in COP of less than 4%. Furthermore, the estimated charge inventory has been compared to the one-point regression model. Results showed that the former method allowed to predict the charge inventory with an MAE of less than 0.5%

Development and Validation of a Mechanistic Vapor-Compression Cycle Model

Detailed models are crucial tools for engineers in designing and optimizing systems. In particular, mechanistic modeling of vapor compression systems for accurate performance predictions at both full- and part-load conditions have been improved significantly in the past decades. Yet, fully deterministic models present still challenges in estimating charge inventory in order to optimize the performance. In this work, a generalized framework for simulating vapor compression cycles (VCC) has been develvoped with emphasis on a charge-sensitive model. In order to illustrate the capabilities of the tool, a direct–expansion (DX) cycle has been considered. In the cycle model, the compressor was mapped by employing the ANSI/AHRI 540 10-coefficient correlation, the evaporator and the condenser were constructed based on the ACHP models (Bell, 2010). Furthermore, a TXV model was implemented based on Li and Braun (2008) formulation. With respect to the charge inventory estimation, the two-point regression model proposed by Shen et al. (2009) was used to account for inaccurate estimation of refrigerant volumes, ambiguous flow patterns for two-phase flow, and amount of refrigerant dissolved in the oil. The solution scheme required manufacturer input data for each component as well as the amount of refrigerant charge. Hence, the degree of superheating at the evaporator outlet, the subcooling at the condenser outlet and the perfromance parameters of the VCC system can be predicted. The model was validated with available experimental and numerical data available in literature. The simulation results demonstrated that the proposed model is more accurate and more generic than other methods presented in the literature

Associated production of weak bosons at LHC with the ATLAS detector

The study of the associated production of weak vector bosons at LHC allows to search for New Physics through the measurement of possible deviations of the weak boson self-couplings from the expectation within the Standard Model. The sensitivity of the ATLAS experiment to Standard Model diboson (W+W-, W± Z, Z Z, W± gamma, and Z gamma ) production in pp collisions at sqrt(s) = 14 TeV, using final states containing electrons, muons and photons, is presented. These studies use Monte Carlo data sets (with full detector simulation) from the ATLAS Computer System Commissioning, which furthermore include detailled trigger information as well as effects of detector calibration and alignment corrections. The influuence of backgrounds on diboson detection is assessed using further dedicated large samples of fully simulated background events. The sensitivity of the ATLAS experiment to anomalous triple gauge boson couplings is determined. Even for small integrated luminosities (about 0.1 fb-1) the sensitivity to anomalous triple gauge boson couplings at the LHC can be significantly improved by ATLAS, when comparing to results from Tevatron that use 1.0 fb$^{-1}$ of data

Performance Testing of a Vapor Injection Scroll Compressor with R407C

Current studies indicate that the method of economized vapor injection (EVI) increases both cooling capacity and coefficient of performance (COP) of vapor compression systems and enlarges the operating range of compressors by reducing the discharge temperature. The design and analysis of EVI systems require comprehensive and comparable performance data of the compressor. In this work, a thermodynamic model was developed to simulate the potential benefit of EVI systems. Furthermore, the performance of a vapor injection (VI) scroll compressor has been experimentally investigated using a modified compressor calorimeter and the refrigerant mixture R407C. During the experiments, the injection flow was regulated by controlling the injection superheat. The experimental results confirm the predicted tendencies of the EVI model. The investigation also reveals that the injection pressure affects the VI compressor performance and needs to be included in the compressor performance evaluation

Stratified Flow Model to Predict Oil Retention in Horizontal Refrigerant Gas Lines of Unitary Split Systems Running R410A and POE32

Most air conditioning and refrigeration systems that employ the vapor compression cycle rely on oil circulating with refrigerant to lubricate the bearings and other contact surfaces in the compressor. The lubricant acts as a sealant to reduce leakage losses during the compression process and it also helps to absorb some of the excess heat that is generated in the compression chamber. However, this oil circulation results in oil retention in various other components outside the compressor depending on the physical interaction between lubricant and refrigerant and their transport properties. Other factors, such as geometry and orientation of connecting lines and system operating conditions (e.g., refrigerant flow rate and oil circulation ratio), also impact the oil retention. As a result of oil retention, the oil level in the compressor reduces, which may ultimately affect its efficiency and life span. In addition, the pressure drop across the system increases and the efficiency of heat exchangers (evaporators and condensers) decreases with oil retention. The current line sizing rules reported in the ASHRAE Handbook Refrigeration have only limited consideration of the effects of oil in the system. With the increasing development of variable speed systems as well as future use of newer refrigerants, there is a need in the industry for upgrading the line sizing recommendations to consider the effects of oil retention, especially the connecting gas lines of unitary split systems. To address this issue, a physics based model has been developed to predict oil retention in horizontal lines. The model is validated using experimental data collected for R410A-POE32. The developed model will be a backbone of a design tool, which will provide more information on oil retention in refrigerant gas lines of the commonly used refrigerant-lubricant combinations in the HVAC&R industry