Piotr A. Domanski

Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO2 and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration,which consisted of a compressor, condenser,expansion device,and evaporator. The CO 2 system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems,an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO2 with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM,the COP of CO 2 was lower by 21% at 32.2 � C and by 34% at 48.9 � C. At higher speeds and ambient temperatures,the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO2. # 2001 Elsevier Science Ltd and IIR. All rights reserved.

Evaluation of Suction Line-Liquid Line Heat Exchange in the Refrigeration Cycle

Abstract The paper presents a theoretical evaluation of the performance effects resulting from the installation of a liquid-line/suction-line heat exchanger (LLSL-HX). It examines cycle parameters and refrigerant thermodynamic properties that determine whether the installation results in improvement of COP and volumetric capacity. The study shows that the benefit of application of the LLSL-HX depends on a combination of operating conditions and fluid properties — heat capacity, latent heat, and coefficient of thermal expansion — with heat capacity being the most influential property. Fluids that perform well in the basic cycle are marginally affected by the LLSL-HX, and the impact on the coefficient of performance and volumetric capacity may be either positive or negative. Fluids performing poorly in the basic cycle benefit the from LLSL-HX installation through increase of the coefficient of performance and volumetric capacity.

A simplified cycle simulation model for the performance rating of refrigerants and refrigerant mixtures

Abstract A simulation program, CYCLE11, which is useful for the preliminary evaluation of the performance of refrigerants and refrigerant mixtures in the vapour compression cycle is described. The program simulates a theoretical vapour-compression cycle and departures from the theoretical cycle that occur in a heat pump and in a refrigerator. The cycles are prescribed in terms of the temperatures of the external heat-transfer fluids with the heat exchangers generalized by an average effective temperature difference. The isethalpic expansion process is assumed. The program includes a rudimentary model of a compressor and a representation of the suction line and liquid line heat exchange. Refrigerant thermodynamic properties are calculated by using the Carnahan-Starling-DeSantis equation of state. Refrigerant transport properties are not included in the simulations. The program can generate merit ratings of refrigerants for which limited measured data are available. An example of simulation results stresses the need for careful application of simplified models and consideration for the assumptions involved.

Glide matching with binary and ternary zeotropic refrigerant mixtures Part 1. An experimental study

Abstract An improvement of the coefficient of performance (COP) of the refrigeration cycle can be realized when temperature profiles of the refrigerant mixture and the heat transfer fluid (HTF) are matched. For the same temperature lift, the benefit of glide matching increases as the application glide increases. High-glide binary mixtures composed of components far apart in boiling points tend to have a non-linear relationship between temperature and enthalpy in the two-phase region. The introduction of an intermediate boiler as a third component can linearize this relationship and, theoretically, increase the cycle COP when heat-source and heat-sink fluids are substantially linear (e.g. water, brines, dry air). The research described in this paper was directed at exemplifying this characteristics of ternary mixtures by experimental evaluation of the performance of an R23/142b binary mixture and an R23/22/142b ternary mixture in a generic laboratory breadboard refrigeration system.

An improved correlation for two-phase pressure drop of R-22 and R-410A in 180° return bends

A correlation for two-phase flow pressure drop in 180° return bends is proposed based on a total of 241 experimental data points for R-22 and R-410A from two independent studies. The data span smooth tubes with inner diameters (D) from 3.3 mm to 11.6 mm, bend radiuses (R) from 6.4 mm to 37.3 mm, and curvature ratios (2R/D) from 2.3 to 8.2. The correlation consists of a two-phase pressure drop for straight-tubes and a multiplier that accounts for the bend curvature. The Buckinham-PI Theorem was used to formulate the curvature multiplier in terms of refrigerant properties, flow characteristics, and bend geometry. The correlation predicts all data with a RMS deviation of 25%, and 75% of the data fall within ±25% error bands.

An Optimized Design of Finned-Tube Evaporators Using the Learnable Evolution Model

Optimizing the refrigerant circuitry for a finned-tube evaporator is a daunting task for traditional exhaustive search techniques due to the extremely large number of circuitry possibilities. For this reason, more intelligent search techniques are needed. This paper presents and evaluates a novel optimization system called ISHED1 (intelligent system for heat exchanger design). This system uses a recently developed non-Darwinian evolutionary computation method to seek evaporator circuit designs that maximize the capacity of the evaporator under given technical and environmental constraints. Circuitries were developed for an evaporator with three depth rows of 12 tubes each, based on optimizing the performance with uniform and nonuniform airflow profiles. ISHED1 demonstrated the capability to generate designs with capacity equal or superior to that of best human designs, particularly in cases with non-uniform airflow.

Mathematical model of an air-to-air heat pump equipped with a capillary tube

Abstract This paper described in general a computer model for simulation of steady-state performance of a split, residential, air-to-air heat pump. Organization of the model is discussed and approach to modelling of main heat pump components is explained. The modelling effort emphasis was on the local phenomena to be described by fundamental thermodynamic, heat transfer and fluid mechanic relationships. The model has been verified in a wide range of operating conditions from high temperature cooling to low temperature heating.

Glide matching with binary and ternary zeotropic refrigerant mixtures Part 2. A computer simulation

Abstract The glide-matching study presented in Part 1 was a laboratory investigation which demonstrated the evaporator performance in detail. However, since it was not possible to instrument the condenser sufficiently, some computer simulation work was conducted using a semi-theoretical model cycle -11, which has been under continual development at NIST for the past five years. As in the experimental effort, R22, R142b, R22/142b, R23/22/142b and R23/142b working fluids were investigated, but the simulation work did not include heat pump operation with liquid-line/evaporator heat exchange. By utilizing the model to quantify entropy generation at various state points within the cycle, it was possible to locate the likelihood of temperature profile pinch points in both the condenser and evaporator. This information clarified the impact of non-linearities on the system performance.

Limited options for low-global-warming-potential refrigerants

Hydrofluorocarbons, currently used as refrigerants in air-conditioning systems, are potent greenhouse gases, and their contribution to climate change is projected to increase. Future use of the hydrofluorocarbons will be phased down and, thus replacement fluids must be found. Here we show that only a few pure fluids possess the combination of chemical, environmental, thermodynamic, and safety properties necessary for a refrigerant and that these fluids are at least slightly flammable. We search for replacements by applying screening criteria to a comprehensive chemical database. For the fluids passing the thermodynamic and environmental screens (critical temperature and global warming potential), we simulate performance in small air-conditioning systems, including optimization of the heat exchangers. We show that the efficiency-versus-capacity trade-off that exists in an ideal analysis disappears when a more realistic system is considered. The maximum efficiency occurs at a relatively high volumetric refrigeration capacity, but there are few fluids in this range.

An experimental and computational study of approach air distribution for a finned-tube heat exchanger

The distribution of air flow approaching a finned-tube heat exchanger is one of the predominant factors influencing the heat exchangers performance. This article describes a method for measuring and predicting the inlet air flow distribution using particle image velocimetry (PIV) and a computational fluid dynamics (CFD) model, highlighting the source and magnitude of air side maldistribution. The studied case was a single-slab, four-depth-row, louvered-fin heat exchanger installed vertically in a horizontal duct. The measured data showed that the air approaching this very simple test case generally maintained velocities of 1.25 ms−1 (4.1 fts−1) to 1.35 ms−1 (4.4 fts−1), but certain portions of the coil were completely obstructed, resulting in no air flow, and other portions realized velocities of over 1.7 ms−1 (5.6 fts−1). A CFD model of the air flow through this heat exchanger was developed based on a momentum resistance modeling approach. The CFD results agreed well with the PIV measurements, predicting the local velocities within 3% over most of the domain and within 10% in areas with the largest velocity gradients.