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Experimental facility for the study of liquid overfeed fin-and-tube evaporators: validation of numerical models.

Article Excerpt
INTRODUCTION

The introduction of new refrigerant fluids and increasing energy-efficiency requirements drives changes in the design of refrigeration systems and their components and requires the assistance of mathematical modeling and simulation to predict complex phenomena. However, before mathematical models are presented as reliable tools for design optimization, they should first be validated experimentally.

Most of the recently reported studies in refrigeration are related to single-stage systems and direct-expansion evaporators. The work of Liang et al. (2001) presented numerical and experimental studies of the refrigerant circuitry of evaporator coils. Jangho Lee et al. (2003) presented an experimental setup and results for validation of a method for analyzing fin-and-tube evaporators using a zeotropic mixture. Rigola et al. (2004) studied the two-phase flow in double-pipe condensers and evaporators and compared numerical results with results from an experimental unit.

Liquid overfeed evaporators and their systems, though of great importance for industrial refrigeration, have received less attention in recent years; a bibliographic reveals that more of the studies in this area are from the 1960s. Stoecker (1998) included in his handbook an interesting chapter on liquid recirculation systems, with references to some of those 1960s studies. Wile (1962) measured the influence of the circulation ratio in tests of finned-tube ammonia evaporator cooling air. Lorentzen (1965) conducted experiments similar to those of Wile, introducing the additional parameter of heat flux. Lorentzen and Baglo (1969) conducted an experimental study of a gas pump recirculation system. For liquid overfeed evaporator design, ASHRAE (2006) also references studies from these years (Lorentzen and Gronnerud [1967]; Lorentzen [1968]). A more recent experimental study of liquid overfeed systems is the work of Giuliani et al. (1999), which focuses on the composition shift of a zeotropic mixture through a refrigeration system with a concentric tube counter-flow evaporator.

A detailed mathematical model of fin-and-tube heat exchangers called CHESS (Compact Heat Exchanger Simulation Software) has been developed by the authors of this paper for the fluid-dynamic simulation of this element (Oliet et al. [2002]; Oliet [2006]; Perez-Segarra et al. [2007]). The refrigerant-side resolution strategy is similar to that presented in Garcia-Valladares et al. (2004). The model can simulate evaporators, condensers, and single-phase refrigerant heat exchangers with complex geometry produced with expanded, galvanized, and brazed manufacturing technologies. In order to approach the complex physical phenomena and the available geometries, a strategy of resolution is adopted based on discretization of the heat exchanger into macro control volumes around the tubes. Over these macro control volumes, the equations of conservation of mass, momentum, and energy are applied to both fluids, and the energy conservation equation is applied to the solid elements. The model requires only fundamental empirical information about the heat transfer coefficients and the friction factors.

The objective of this work is to present an experimental facility designed and constructed with the objective of validating the fin-and-tube evaporator model, CHESS, in conditions of refrigerant overfeed, emphasizing the experimental methodology and procedures indispensable to the correct validation process.

Numerical results for the evaporator from the detailed CHESS model were compared with the experimental data. Six different correlations for the two-phase flow heat transfer coefficient were compared, and their influence on the global performance prediction of the evaporator was studied. The comparisons embrace evaporator cooling capacity, refrigerant pressure drop, and outlet vapor quality.

EXPERIMENTAL FACILITY

The experimental facility is designed for the study of liquid overfeed fin-and-tube evaporators in a wide range of working conditions with varying refrigerant flows and airflows and their temperatures.

The facility is formed by three instrumented circuits: the liquid overfeed refrigeration system circuit, a closed air circuit through the evaporator, and a liquid coolant circuit for the condenser (each discussed in detail below). The latter two circuits are dedicated to providing stable controlled conditions for the testing of the liquid overfeed evaporator and the refrigeration system.

Refrigeration System Circuit

The refrigeration system circuit consists of two subcircuits that share common elements and permit two different testing modes. The first subcircuit is a complete vapor-compression system with a mechanical liquid overfeed evaporator. It is used for experiments, with the evaporator working as part of the operating refrigeration system, and for studies of the refrigeration cycle. This mode will be abbreviated in this article as COMP MODE. The second subcircuit is a closed refrigerant loop between the evaporator and the condenser that does not include the compressor and is intended for testing the evaporator with cooling provided from the secondary fluid in the condenser. It allows the possibility of decoupling the operation of the evaporator from the compressor's limitations, expanding the range of the facility for testing evaporators. This mode will be abbreviated as NOCOMP MODE.

The switching from one subcircuit to another is carried out with a combination of manual shut-off valves. A scheme of the refrigeration system circuit that indicates the measuring instruments' positions is shown in Figure 1.

[FIGURE 1 OMITTED]

Vapor-Compression System Circuit (COMP MODE). The experimental system uses refrigerant R-134a, and the circuit is constructed in stainless steel tubes. The main components are a semihermetic reciprocating compressor, a fin-and-tube evaporator, a brazed-plate liquid-cooled condenser, high-and low-pressure receivers, and a gear pump (see Figure 1). The electronic expansion valve is pulse-width controlled in a function of the liquid refrigerant level in the low-pressure receiver and allows for a range of cooling capacities from 1.2-8.5 kW.

The circulation of the refrigerant through the evaporator is assured by means of a magnetically coupled gear pump with a variable-speed drive that permits refrigerant flow rates up to 700 kg/h.

The system was designed for continuous operation, and the different operating points are...

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