Luis Rosario

Thermodynamic Analysis of Magnetic Refrigerators

An analysis of a magnetic refrigeration cycle was carried out. The system consists of heat exchangers and beds of magnetic materials. The analysis considered that the system operates near room temperature in a magnetic field between 1 and 5 T and uses 3 kg of gadolinium (Gd) spheres packed in two magnetocaloric beds. The heat transfer fluid is water. The beds are periodically magnetized and demagnetized and the fluid flows are arranged to meet the cycle requirements. Sensitivity analysis has been performed. Cooling power, magnetic field, and temperature span trends are simulated. The cooling and heating effects were estimated based on the magnetocaloric effect of gadolinium. Findings indicate that the higher the magnetic field is the higher the cooling power with the same temperature span. It was also observed that the cooling power decreases with the increase in the temperature span for various magnetic fields. COP vs. temperature span was also considered. The trend indicates that COPactual / COPCarnot decreases with an increase in the temperature span. These trends agreed with those shown by experimental data.Copyright

Thermodynamic Analysis of Transcritical Carbon Dioxide Cycles

Research on natural environmentally friendly refrigerants is very important especially because the world-wide agreement about restriction in the use of ozone depleting refrigerants. Carbon dioxide is a natural refrigerant that has been considered for certain refrigeration and air conditioning applications. The aim of this paper is to present a thermodynamic analysis of carbon dioxide cycles in order to evaluate the potential performance of a refrigeration cycle using carbon dioxide. A thermodynamic model for the cycle is developed which can simulate the operation of a carbon dioxide refrigeration/AC cycle. This model takes into account the practical effects of the thermo-physical properties of carbon dioxide as refrigerant in a trans-critical cycle. A sensitivity analysis has been conducted so that cycle performance is estimated. The cooling load, compressor power, and coefficient of performance (C.O.P.) were evaluated. One and two stages of compression were also considered for comparison purposes. Cycle performance was evaluated based on variation of important parameters such as evaporator, intermediate, and discharge pressures. The effects of cycle components on system capacity and cycle performance are also investigated. Comparison between performances of the CO2 cycle and the standard vapor compression cycle is performed. Thermodynamic calculations showed reasonable agreement with available experimental data based on the general assumptions made.Copyright

Predictions of Relative Humidity and Temperature in an Operating Room

This paper uses airflow simulations to evaluate different ventilation systems on an operating room (OR). This study compares air distribution systems for an operating room by use of computational fluid dynamics (CFD) modeling. The air supply distribution and exhaust arrangements were modeled for a directional air flow system where air moves across the space from the high-pressure supply area to the low pressure exhaust area. Calculations were done to model a typical operating room in a steady-state condition with inclusion of object such as surgical lights, operating table, heat sources such as surgical staff and a patient, side-wall supply grille and exhaust air grilles. The discharge angle for the side-wall supply grille was varied from 0 to 45 degrees. Air return locations were also studied. One and two air exhaust outlet sites inside the surgical suite were considered. In the two-exhaust outlet configuration, one position was close to the floor and the other position was high on the wall. Simulations with combinations of 1:0, 1:0.343, 0.343:1, and 1:1 flow rates between the two return locations were performed. Predictions for the air movement, room temperature, room relative humidity, and concentration of contaminants within the operating room are shown. Analysis of these predictions is discussed. The supply and exhaust conditions of the ventilation air flow are shown to play an important role in the control of air quality. Results show good agreement with experimental data.Copyright

Numerical Analysis of Thermal Behavior in a Refrigerated Warehouse

This paper presents a study of thermal behavior in a refrigerated warehouse equipped with ceiling type refrigeration unit. A two-dimensional steady-state problem was solved by using computational fluid dynamics modeling and simulation. The computational model includes a refrigerated space at the center plane of an array of 4 piles by 3 stacks of palletized product packages with a evaporator fan refrigeration unit, in which the fan pull air through cooling coil and blow out into the space. A parametric analysis was done based on fourteen simulation cases of conditions including different locations of the refrigeration unit as well as different normal forced air velocities produced by the fan. Typical distributions of velocity and temperature are presented. From the results, it is found that better cooling effectiveness and uniformity of the refrigerated space can be achieved by using higher normal supply air speed or locating the refrigeration unit lower and closer to the array of stacks of product packages.Copyright

Analysis of Thermal Comfort and Contaminant Removal in an Office Room With Underfloor Air Distribution System

The study of human thermal comfort requires detailed information about distributions of air velocity, air temperature and relative humidity in an occupied zone. Air quality is related to the contaminant distributions and contaminant removal effectiveness in indoor environment. This paper presents an evaluation of thermal comfort and contaminant removal for an office setting with underfloor air distribution system by the use of computational fluid dynamics modeling. A typical single cubicle in a large office floor in steady state condition of airflow as well as heat and mass transfer is investigated for both cooling and heating scenarios. The model includes a typical cubicle in a large office floor with a chair, a desk with a personal computer on its top, and heat sources such as seated people, computer monitor and CPU, and lights. Air enters the occupied zone through an inlet located at the floor level supplying a vertical upward inflow. Five different locations of the inlet diffuser, three different inlet air speeds, and four different loads of heat loss through the floor slab in heating case scenario were considered. Distributions of velocity, temperature, relative humidity, and contaminant concentration in such cases were computed. The results were compared among various simulation cases and showed reasonable agreement with experimental data taken from related literature.© 2005 ASME

Thermodynamic Analysis of a Magnetic Liquefier for Hydrogen

The aim of this paper is to present a thermodynamic analysis of a magnetic liquefier for hydrogen. A hydrogen liquefaction cycle is examined. A magnetic refrigerator, a liquid-nitrogen pre-cooling system, an expansion valve and a liquid-hydrogen separator integrate the system. The magnetic refrigerator consists of two heat exchangers and two stages of beds of magnetic materials. The analysis considered that the system operates with gaseous hydrogen entering at high pressure and ambient temperature. A fraction of this incoming flow is liquefied by the system. One magnetic material GdNi2 is used in the upper stage and another magnetic material GdPd is used in the lower stage. The heat transfer fluid for the magnetic refrigerator is helium. The beds in both stages are periodically magnetized and demagnetized and the fluid flows are arranged to meet the cycle liquefaction requirements. Sensitivity analysis has been performed to study the thermodynamic behavior of the magnetic liquefier cycle. Liquefaction efficiency, the nitrogen boil-off rate per unit mass of hydrogen, the fraction of the hydrogen gas that is liquefied and the magnetic refrigerator performance trends are evaluated. Simulations indicate that the higher the performance of the magnetic refrigerator is the higher the liquefaction efficiency of the system is with the same intermediate temperature. It was also observed that the liquefaction efficiency increases with the decrease in the nitrogen boil-off rate per unit mass of hydrogen. Magnetic liquefier exhibits a great potential by showing a very high efficiency when compared to small and large scale commercial liquefiers for hydrogen.Copyright

Experimental Measurements of an Air Conditioner Performance

The performance of the air conditioner was tested in an extensive experimental program using the environmentally controlled chambers in a test facility. Two psychometric rooms provided constant ambient temperature and humidity conditions for a test unit using ASHRAE standard procedures [1]. The indoor and outdoor units were placed into separate environmental chambers, which provided precise temperature, humidity, and airflow conditions for simulation of various operating conditions. The first goal of the experimental program was to define the range of conditions over which the test unit should be tested. The second goal of the experimental studies was to determine the performance of the test unit under the defined conditions. All air conditioner performance data has been collected with air side instrumentation only. Experimental tests were performed using the test unit over a range of outdoor temperatures between 22.4°C (80°F) and 40.6°C (105°F) and indoor temperatures between 18.3°C (65°F) and 35°C (95°F). Analysis of the experimental data was performed by studying air conditioning parameters such as heat rejection rate qc , compressor power W, system capacity qe , and coefficient of performance COP. The analysis was accomplished with the variation of a boundary condition. The sensitivity analysis of experimental data gave expected results when compared to those shown by air conditioning units similar to our test unit.Copyright