Real-time measurement of mixing ratio of refrigerant/refrigeration oil mixture

The mixing of refrigeration oil with refrigerant in a refrigeration cycle has great influence on cycle performance. A sampling method is the most general way to measure the mixing ratio of refrigerant and refrigeration oil. Since the sampling method is time-consuming and reduces the amount of refrigerant and oil in the cycle, a real-time measurement is desirable. In this study, a refractive index measurement was applied to measure the mixing ratio of refrigerant/oil mixture. A laser displacement sensor was used to detect any change in optical path which results from changes of the refractive index of refrigerant/oil mixture. For the practical application of real-time measurement of the oil circulation ratio (OCR) in the refrigeration cycle, a correlation between the refractive index and the mixing ratio was developed. In addition, the changes of the refractive index in a range of a few percentages of the oil concentration and under subcooled conditions were measured. Finally, a transient measurement of the OCR in a practically operating refrigeration cycle was carried out successfully.     

Flow boiling of non-azeotropic mixture R32/R1234ze(E) in horizontal microfin tubes

Flow boiling of a potential refrigerant R32/R1234ze(E) in a horizontal microfin tube of 5.21 mm inner diameter is experimentally investigated. The heat transfer coefficient (HTC) and pressure drop are measured at a saturation temperature of 10 °C, heat fluxes of 10 and 15 kW m^?2, and mass velocities from 150 to 400 kg m^?2 s^?1. The HTC of R1234ze(E) is lower than that of R32. Degradation in the HTC of the R32/R1234ze(E) mixture is significant; the HTC is even lower than that of R1234ze(E). The HTC is minimized at the composition 0.2/0.8 by mass, where the temperature glide and the mass fraction distribution are maximized. A predicting correlation based on Momoki et al. (1995) associated with the correction methods of Thome (1981) to consider the mass transfer resistance and Stephan (1992) to consider the additionally required sensible heat is proposed and validated with the experimental results.     

Basic study on new cryogenic refrigeration using CO2 solid–gas two phase flow

In this paper, a cryogenic refrigeration method is described, which utilizes CO₂ solid–gas two phase flow and the dry ice. The CO₂ solid–gas two phase flow is achieved by expanding liquid CO₂ and thus refrigeration process less than CO₂ triple point −56.6 °C can be available. The experimental work is divided into two parts and two experimental set-ups were designed, constructed and tested. The interest of the first experiment test is the feasibility of expanding liquid CO₂ into CO₂ solid–gas flow in a horizontal circular tube by expansion valve. The second experiment focuses on the feasibility of the refrigeration of liquid CO₂ expanding into solid–gas two phase flows used in a prototype CO₂ heat pump system. The results show that solid–gas two phase flows can be achieved by expanding liquid CO₂ by expansion valve in a closed CO₂ heat pump system loop and low temperature refrigeration below −56.6 °C is achieved in the experiments, which give greater possibility to create a cryogenic refrigeration process below −56.6 °C for food industries, bio-medical engineering, etc.     

Effects of flash gas generation at the expansion device inlet on the dynamic characteristics of a refrigeration system

The flash gas generation at the expansion device inlet in multi-air-conditioners causes rapid reduction of refrigerant mass flow rate and irregular distribution of refrigerant into multi-indoor units. The objective of this study is to evaluate the influence of the flash gas generation on the dynamic characteristics of a refrigeration system. The dynamic characteristics of a refrigeration system with an electronic expansion valve (EEV) were measured with time at various levels of flash gas generation, which were expressed as flash gas ratio. In addition, the averaged operating parameters and system COP were investigated with the variation of flash gas ratio. As the EEV inlet condition changed from the subcooled to the two-phase state, the refrigerant flow rate decreased rapidly due to the flash gas generation at the EEV inlet. At two-phase inlet conditions, the system operating parameters, such as mass flow rate, suction and discharge pressures, fluctuated periodically with time. As the flash gas ratio increased, the average COP decreased and the discharge temperature increased, degrading the system performance and reliability more severely.     

A review of hydrocarbon two-phase heat transfer in compact heat exchangers and enhanced geometries

Hydrocarbons are considered as alternative fluids for refrigeration, air-conditioning and heat pump applications. Pure butane, propane or their mixtures can be adopted, but due to their flammable properties, the systems have to be designed in such a way that the refrigerant charge is minimized. Therefore, compact heat exchangers and enhanced geometries are adopted in such systems. In this paper, the current state of the art for two-phase heat transfer calculations for pure hydrocarbons and their mixtures is reviewed and analysed. Recommendations are proposed for estimating evaporation and condensation heat transfer in various geometries including enhanced tubes as well as compact heat exchangers.     

Friction factor correlation and pressure loss of single-phase flow inside herringbone microfin tubes

Pressure drop of single-phase turbulent flow inside herringbone microfin tubes of different fin dimensions has been measured experimentally to develop a general correlation of single-phase friction factor for the herringbone tubes. Water has been used as a working fluid and the mass flow rate has been varied from 0.03 to 0.2 kg/s, where the Reynolds number range is 10⁴ to 6.5 × 10⁴. Comparison of experimental data of the herringbone microfin tubes with those of helical microfin and smooth tubes shows that pressure drop of the herringbone tube is significantly higher than the helical and smooth tubes depending on the fin geometric parameters and mass velocity of the working fluid. Through semi-analytical approach and using the present experimental data, a new correlation of single-phase friction factor for the herringbone microfin tubes has been proposed incorporating the effects of fin geometric parameters. The proposed correlation can predict the experimental data within ±10%.     

Predicting heat pump performance by thermodynamic generalizations of fluid flow and heat transfer data

Thermodynamic generalizations based on reduced pressure proposed in the 1960s are reviewed and updated to reflect the current state of the art. The application of the method is illustrated by analytical and numerical examples and an assessment made of its value in heat exchanger design practice with special emphasis on two-phase forced convection refrigeration cycle applications. It is shown that this thermodynamic approach provides the heat exchanger designer, and to some extent the system engineer with an additional tool which is simple, effective and above all more reliable, particularly in evaporator and condenser design practice, than current conventional semi-empirical correlations.     

Ébullition par Convection Forcée des Mélanges Eau‐Ammoniac dans un Tube Vertical

An experimental study has been conducted on the forced convective boiling heat transfer of ammonia‐water mixtures flowing inside a 6 mm inner diameter vertical smooth tube. Using a water‐heated double pipe type generator, the local heat transfer coefficients are measured inside the inner tube for a range of heat flux density (29.93 — 99.79 kW/m²), mass flux density (35.36 — 99.04 kg/m²·s), mass flow rate (0.001 — 0.03 kg/s) and ammonia mass concentration (49%, 55% and 61%). The effect of the experimental parameters on the heat transfer coefficients is analysed. Three methods are used to predict the boiling heat transfer coefficients. Experimental data were compared with the available correlations. The obtained results confirm the good performance of the Mishra et al. (1981) and Bennett‐Chen's (1980) correlations in predicting the convective boiling heat transfer coefficient of NH₃‐H₂O mixtures. These methods are able to predict the boiling heat transfer data within an average accuracy of ± 20 %.     

Régimes d'écoulement et mesure de l'épaisseur d'un film liquide ruisselant autour d'un tube horizontal

The flow characteristics and film thickness have been studied experimentally for the case of a falling liquid film flows on a horizontal cylinder. The optical method based on laser‐induced fluorescence is presented in this paper. All tests have been conducted with the adiabatic water/fluorescence mixture at atmospheric pressure. Five falling‐film modes are observed for different liquid mass flow. The wavelengths have been measured for each film Reynolds number value for droplet and column modes. The transitions between different falling‐film modes are studied and the hysteresis phenomenon is observed. The measured dimensionless wavelengths are compared to the results of Hu and Jacobi (1998). The experimental results show the local evolution of film thickness around the cylinder. They are compared to the results of Rogers and Goindi (1989), Rogers (1986) and Chyu and Bergles (1987). The local measurements of film thickness are correlated by including the effects of film Reynolds number.     

Mathematical modeling and thermodynamic investigation of the use of two-phase ejectors for work recovery and liquid recirculation in refrigeration cycles

This paper presents the results of a numerical investigation on the performance of ejector cycles in which the work recovered is used to recirculate liquid through the evaporator. The ejector recirculation cycle, in which the ejector is only used to recirculate liquid and improve evaporator performance, and the standard ejector cycle, in which the ejector can be used to both recirculate liquid and directly unload the compressor, are investigated. The analysis uses a microchannel evaporator and refrigerants R134a, R410A, and CO₂. It is seen that fluids that have large throttling loss but gain little benefit from liquid recirculation (CO₂) should use the ejector to directly unload the compressor, while fluids that have lower throttling loss but gain significant benefit from liquid recirculation (R134a) should use the ejector to improve evaporator performance through liquid recirculation. It is also seen that the ejector recirculation cycle is better suited for ejector off-design operation.