Jader R. Barbosa

Experimental mapping of the thermodynamic losses in vapor compression refrigeration systems

The present study introduces a methodology for mapping the thermodynamic losses of vapor compression refrigeration systems which consists of a theoretical model that splits the coefficient of performance into four terms: (i) the coefficient of performance of an ideal cycle (Carnot), (ii) the efficiency of a real cycle running with an ideal compressor, (iii) the compression efficiency of a real compressor, and (iv) the cycling efficiency. In addition, measurements of the relevant variables at several positions along the refrigeration loop are also required, generating performance data not only for the whole unit but also for each one of the system components. The proposed methodology points out the thermodynamic losses of the refrigeration system, identifying opportunities for energy performance improvement. In addition, the methodology is suitable for comparing different refrigeration systems with respect to the same thermodynamic baseline. Albeit the methodology was originally developed for household refrigerators, it can be easily extended to any kind of vapor-compression refrigeration systems.

Forced convective boiling of steam–water in a vertical annulus at high qualities

Abstract Boiling experiments were conducted in a vertical annulus in which heat was applied to the inner surface of the channel. The design of the test section is such that it allows a direct observation of the flow field by means of a high-speed video system. The heater is made of a 0.56 mm thick stainless steel pipe, the heated length is 0.32 m and the outer diameter is 19.1 mm. The equivalent diameter of the annulus section is 12.9 mm. Local measurements of the heater wall temperature, and hence of heat transfer coefficient, were carried out using a radiation equilibrium thermocouple which could be traversed along the heated section. The total mass flux ranged from 23.6 to 43.3 kg / m 2 s , the heat flux from 9.3 to 326.9 kW/m 2 and the inlet quality from 0.48 to 0.79. The performance of six correlations is assessed based on the experimental results.

Modeling the Stiction Effect in Automatic Compressor Valves

Abstract The stiction effect is a source of thermodynamic losses in compressor suction and discharge systems. Deformation of the lubricant film between the valve and the seat delays the valve opening, as larger pressure differences between the cylinder and the discharge or suction chambers are needed to overcome the stiction force. A model is presented for the dynamic behavior of a ring-shaped lubricant oil film between a discharge valve and the seat. The valve is allowed to move under the action of an external force due to the time-dependent pressure difference between the cylinder and the discharge chamber. The main contributions of the model are the consideration of a finite oil amount between the valve and the seat and relationships for calculating the film thickness initial condition. At typical conditions of discharge systems of domestic refrigeration compressors, viscous effects are the dominant component in the oil stiction force under dynamic conditions.

Two-phase non-equilibrium models: the challenge of improving phase change heat transfer prediction

This lecture addresses some recent developments in modelling of macroscopic thermodynamic and hydrodynamic non-equilibrium phenomena in convective phase change (boiling and condensation) of pure fluids and mixtures. Proper accounting of such phenomena may hold the key to explain and predict deviations from the classical (equilibrium) phase change convective heat transfer behaviour reported in the literature and yet not fully understood. In the first part of the paper, a detailed qualitative description of the classical heat transfer coefficient behaviour is presented together with two examples of departure from macroscopic equilibrium largely supported by experimental evidence. The second part of the paper reviews successful attempts to model the non-equilibrium phase change phenomena taking place in the two situations. The first example is a thermodynamic non-equilibrium slug flow model (one in which saturated Taylor bubbles become separated by slugs of subcooled liquid) that predicts the peaks in heat transfer coefficient at near-zero thermodynamic quality observed in forced convective boiling of some pure liquids. The occurrence of such peaks is typical of low latent heat, low thermal conductivity systems and of systems in which the vapour volume formation rate for a given heat flux is large. The second example is a comprehensive annular flow calculation methodology that predicts the decrease in the heat transfer coefficient with increasing quality observed in convective boiling of binary and multicomponent mixtures. In this case, as will be seen, coupled mass transfer resistance and hydrodynamic non-equilibrium effects generate concentration gradients between the liquid film and entrained droplets that are responsible for the heat transfer deterioration. In addition, it will be shown that for condensation of mixtures the methodology predicts a heat transfer intensification which has been subsequently confirmed by independent experimental results.

Modeling of state and thermodynamic cycle properties of HFO-1234yf using a cubic equation of state

Thermodynamic property data including specific enthalpy, specific entropy and specific volume were generated for the new refrigerant HFO-1234yf (2,3,3,3-tetrafluoroprop-1-ene) using the well-known Peng and Robinson cubic equation of state. A general approach applicable to any fluid based on the concept of departure functions has been applied. Data for saturated vapor pressure, saturated liquid density, ideal gas heat capacity at constant pressure and critical properties were obtained from the open literature. The predictive capability of the proposed calculation methodology has been validated with thermodynamic property data for HFC-134a (1,1,1,2-tetrafluoroethane), showing average deviations lower than 0.6%. A thermodynamic analysis of the new refrigerant in the light of thermodynamic properties of an idealized cycle was carried out so as to measure its performance with respect to that HFC-134a under the same evaporating and condensing pressure conditions.

AIR-SIDE HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS OF PERIPHERAL FIN HEAT EXCHANGERS

We present an experimental evaluation of the peripheral finned-tube heat exchanger. In this novel compact evaporator geometry, the air-side is composed by an arrangement of open-pore cells formed by radial fins whose bases are attached to the tubes and whose tips are connected to peripheral fins. Each fin arrangement is made up of six radial fins and six peripheral fins forming a hexagon-like structure. The air-side fin configuration is composed of three levels of fin arrangement, each characterized by the length of radial fin and mounted with a 30° offset from its neighboring level. Experimental data on the air-side heat transfer and pressure drop were generated in an open-loop wind tunnel calorimeter. A one-dimensional theoretical model based on the theory of porous media has also been developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny-Carman model with the particle diameter definition due to Whitaker and the friction factor correlation due to Ergun. The model overpredicts the air-side thermal conductance by less than 15% for air flow rates higher than 14 L/s. The air-side pressure drop is underpredicted by the model, but still within the limits encountered in the literature. The analysis is complemented with an entropy generation minimization analysis in order to demonstrate the procedure for obtaining an optimized configuration of the heat exchanger.Copyright

A thermodynamic nonequilibrium slug flow model

We present a calculation methodology to predict the peaks in heat transfer coefficient at near zero equilibrium quality observed in forced convective boiling in vertical conduits. The occurrence of such peaks is typical of low latent heat, low thermal conductivity systems (such as refrigerants and hydrocarbons), and of systems in which the vapor volume formation rate for a given heat flux is large (low-pressure water). The methodology is based on a model that postulates that the mechanism behind the heat transfer coefficient enhancement is the existence of thermodynamic nonequilibrium slug flow, i.e., a type of slug flow in which rapid bubble growth in subcooled boiling leads to the formation of Taylor bubbles separated by slugs of subcooled liquid

Entropy Generation Minimization Analysis of Active Magnetic Regenerators

A performance assessment of active magnetocaloric regenerators using entropy generation minimization is presented. The model consists of the Brinkman-Forchheimer equation to describe the fluid flow and coupled energy equations for the fluid and solid phases. Entropy generation contributions due to axial heat conduction, fluid friction and interstitial heat transfer are considered. Based on the velocity and temperature profiles, local rates of entropy generation per unit volume were integrated to give the cycle-average entropy generation in the regenerator, which is the objective function of the optimization procedure. The solid matrix is a bed of gadolinium spherical particles and the working fluid is water. Performance evaluation criteria of fixed cross-section (face) area (FA) and variable geometry (VG) are incorporated into the optimization procedure to identify the most appropriate parameters and operating conditions under fixed constraints of specified temperature span and cooling capacity.

Dynamics of gas bubble growth in oil-refrigerant mixtures under isothermal depressurization

This paper proposes a numerical model to predict the growth of gaseous refrigerant bubbles in oil-refrigerant mixtures with high contents of oil subjected to isothermal depressurization. The model considers an Elementary Cell (EC) in which a spherical bubble is surrounded by a concentric and spherical liquid layer containing a finite amount of dissolved liquid refrigerant. The pressure reduction in the EC generates a concentration gradient at the bubble interface and the refrigerant is transported to the bubble by molecular diffusion. After a sufficiently long time, the concentration gradient in the liquid layer and the bubble internal pressure reach equilibrium and the bubble stops growing, having attained its stable radius. The equations of momentum and chemical species conservation for the liquid layer, and the material balance at the bubble interface are solved via a coupled finite difference procedure to determine the bubble internal pressure, the refrigerant radial concentration distribution and the bubble growth rate. Numerical results obtained for a mixture of ISO VG10 polyolester oil and refrigerant HFC-134a showed that the bubble growth dynamics depends on model parameters such as the initial bubble and liquid layer radii, the initial refrigerant concentration in the liquid layer, the initial pressure in the liquid phase, the decompression rate and the EC temperature. Despite its simplicity, the model demonstrated to be a potential tool for predicting bubble growth and foaming that may occur as a result of cavitation in oil-lubricated bearings and refrigerant degassing from the oil sump during compressor start-up. Keywords: refrigeration compressor, oil-refrigerant mixtures, bubble growth, numerical modeling

Viscosity behavior of mixtures of CO2 and lubricant oil

Experimental data on the viscosity of mixtures of CO2 and lubricant oil were acquired and correlated using an excess-property approach based on the classical Eyring liquid viscosity model. Three oils of different types and viscosity grades (alkylbenzene AB ISO 32, mineral MO ISO 50 and polyol ester POE ISO 68) were evaluated at temperatures ranging from 36.5 to 82oC. The excess activation energy for viscous flow was successfully correlated as a function of temperature and concentration using Redlich-Kister polynomial expansions with up to three terms. Large departures from the ideal solution viscosity behavior have been identified in all mixtures. The nature of the observed deviations has been explored in the light of their dependence on temperature, refrigerant concentration and oil type. The Katti and Chaudry (1964) model of the activation energy of viscous flow displayed the best correlation of the experimental data, with RMS deviations of 4.6% (AB ISO 32), 3.3% (MO ISO 50) and 2.8% (POE ISO 68).