José Alberto Reis Parise

The Role of Heat Transfer in Refrigeration

Refrigeration plays a key role in today’s society, providing human comfort and food preservation. From ice and large foodpacking plants to oil refineries and chemical manufacturing, industrial processes rely heavily on refrigeration. According to the International Institute of Refrigeration [1], approximately 15% of the world’s electricity consumption goes to refrigerating and air conditioning systems. The importance of refrigeration is also reflected in the ever-growing number of specialized literature and conferences held worldwide. No doubt, there is much room for improvement, especially by reducing the impact of refrigeration on the environment and improving equipment design (thereby improving energy utilization). The systematic application of the science of heat transfer to refrigeration engineering will certainly contribute to achieve these ends. The efficiency of a heat pump cycle is primarily dictated by the temperature levels at which the cycle operates. In the vapor compression cycle, this means that the pressure levels that affect compressor behavior are ultimately determined by the performance of the heat exchangers that establish the interaction between the working fluid and the heat source and heat sink. In the vapor compression refrigeration cycle, poor heat exchanger efficiency will result in greater temperature differences across both the condenser and evaporator, given heat sink (ambient) and source (refrigerated medium) temperatures. Thus, the thermodynamic cycle would operate under increasingly separated temperature levels. Because the heat extraction (evaporator) and heat rejection (condenser) occur with phase change,

A visual study of R-404A/oil flow through adiabatic capillary tubes

The present study explores the potential of using visualization techniques to investigate refrigerant/oil flow through adiabatic capillary tubes. A literature review shows that these techniques have been used before for capillary tube investigations, but none of these studies focused on the refrigerant/oil phenomena. Therefore, the main objective is to investigate the flow of a refrigerant/oil mixture through a glass capillary tube, with special emphasis on the behavior of the vaporization point. The test fluids are R-404A (a near azeotropic blend) and a polyolester-type oil. Experimental data cover oil concentrations ranging from 5.6 to 6.9% (by mass), degrees of subcooling ranging from 6.2 to 21.5 °C (11.2 F to 38.7 F), and a condensing pressure of 1825 kPa (250 psig). The results show trends of mass flow rate, and give some useful insights about the location of the vaporization point for various oil concentrations and operating conditions.

Simulation of multistream plate-fin heat exchangers of an air separation unit

Hot and cold reversible heat exchangers of an air separation unit are simulated. Five fluid streams exchange heat with six fluid streams in parallel and counter flow. The numerical method employed divides the heat exchanger in a number of sections, for which fluid properties, capacity rates and heat transfer coefficients are considered constant. Single and two-phase streams are taken into account. Results obtained from the model are compared with field data.

NUMERICAL SIMULATION OF AN ETHYLENE RE‐LIQUEFACTION PLANT

The present paper is concerned with the numerical simulation of an ethylene re-liquefaction plant. The plant is part of a petrochemical complex, set up in the city of Aratu, state of Bahia, Brazil. The cooling effect is obtained through a two-stage cascade cycle, involving two circuits with different working fluids: ammonia in the high pressure side, and ethylene is the low pressure. The main components of the system include: two-stage double acting reciprocating compressors, shell-and-tube heat exchanger, store and flash tanks and control valves. The simulation model was developed by considering the energy and mass conversion laws, the empirical heat transfer coefficient relations, heat exchanger and compressor theories and the thermophysical properties of the working fluids. The result was a set of nonlinear equations solved by a sequential modular technique. Predicated thermodynamic parameters (temperature, pressure and mass flow rate), at different points of the systems, were compared with the available plant operational data, with food agreement being obtained. A parametric anaylsis of the plant was also carried out.

Nucleate Boiling in Large Arrays of Impinging Water Sprays

The nucleate boiling of subcooled water, under 100 cm2 square arrays of impinging sprays, was experimentally investigated. Three types of commercially available full cone pressure nozzles, of distinct flow capacities, allowed for runs where the average impinging coolant mass flux spanned the 0.3–7.2 kg/m2-s range. Array geometry was varied adjusting nozzle-to-nozzle and nozzle-to-impingement surface distances. Experimental construction allowed for good drainage of spent coolant and unrestricted air entrainment to spray cones. The average heat flux through the heated, upward-facing, copper impingement surface was found to be equal to the sum of single-phase and nucleate boiling heat flux components. The phase-change component was experimentally observed to depend upon wall excess temperature only. The proposed heat transfer correlation reproduces original experimental data with a mean absolute error of 10.6%. Non-critical-heat-flux (non-CHF) cooling capacity and efficiencies of up to, respectively, 2000 kJ/kg and 83% were observed.

Comparing Single Phase Heat Transfer to Arrays of Impinging Jets and Sprays

The single phase heat transfer characteristics of square arrays of impinging water sprays were investigated experimentally. A total of 230 experimental points, covering a wide range of flow rates and different spray array geometries were obtained with three different models of commercially available full cone spray nozzles. Results were compared to an available correlation for the area average heat transfer coefficient of impinging arrays of submerged jets. It is shown that spray impingement techniques may provide the same heat transfer coefficient obtained with impinging jets under much smaller coolant flow rates per unit heated area. Coolant pumping power required by both techniques was also compared. It is shown that spray arrays require, for the establishment of a given area average heat transfer coefficient, more pumping power than submerged jet arrays.Copyright

Fuel Cells and Cogeneration

Although historically grown as independent energy technologies, fuel cell and cogeneration may adequately work to each other’s benefit. Some fuel cells deliver heat at sufficiently high temperatures, which can be certainly used as heat sources for cogeneration or trigeneration schemes. The paper presents an overview of the innumerable combinations of the simultaneous production, with fuel cells, of (i) heat and power, (ii) cold and electricity, and (iii) cold, heat and electricity, in its multiple varieties. The survey included combined power cycles (also called hybrid systems) where the fuel cell works together with other thermodynamic cycles to produce, with a high fuel-to-electricity efficiency, electricity alone. A large number of cogeneration arrangements are mentioned. Some are described in detail. A brief analysis of benefits and drawbacks of such systems was undertaken. The review was limited to articles published in archival periodicals, proceedings and a few technical reports, theses and books.Copyright

Steady-State Simulation of an Automotive Air Conditioning System Running on CO2

This work presents a semi-empirical simulation of an automotive climate control system equipped with a transcritical vapor compression cycle running on carbon dioxide. The cycle components (a compressor, a throttling valve, an evaporator, a gas cooler, a suction accumulator and a suction line heat exchanger) were modeled to study the operation of the system, in the steady-state regime, under high ambient temperatures. The model took into account the severe conditions of tropical climates since the temperature at the inlet of the gas cooler is one of the predominant factors in the transcritical cycle performance. To assess the performance of the cycle, the thermodynamic model, reduced to a set of non-linear algebraic equations, was solved by a modified Newton-Raphson method. Reasonable agreement was found when results predicted by the model were compared with experimental data available in the literature.Copyright

Simulation of a Direct Evaporative Cooler and a Cooling Tower by the Effectiveness—NTU Method

A direct evaporative cooler and a counter-flow cooling tower were simulated by the effectiveness–NTU method. One-dimensional energy and mass balance as well as heat and mass transfer equations were applied. The resulting algebraic equations were solved numerically. To validate the simulation model, numerical results of the evaporative cooler and cooling tower were compared with available experimental data. This simple application of the effectiveness–NTU method to such important heat transfer devices may prove pedagogically useful to undergraduate mechanical engineering students attending heat transfer courses.