This paper presents the development of a CO2 gas cooler model using the moving-boundary (MB) method. The model aims to separate the gas cooler into two regions, supercritical and supercritical liquid, to predict the steady-state thermal heat transfer rate for air-type CO2 heat exchanger. The model uses the latest correlations for refrigerant and air-side heat transfer coefficients and pressure drops. The experimental results of fin-and-tube type and micro-channel type gas coolers were used for model validation. The mean absolute error (MAE) of the gas cooler heating capacity predictions was approximately 4%, while the predictions of the outlet temperature of the refrigerant side were within  ± 3 K. The present MB model also showed an improved computational time of up to 10 times faster compared to a discretized model, which can reduce the overall computational effort in the simulation of detailed transcritical cycle model.

A generalized moving-boundary algorithm to predict the heat transfer rate of transcritical CO2 gas coolers

The present study focused on the thermal hydraulic performance improvement of a CO2 gas cooler with the principal goal of reducing its volume, and consequently the necessary charge of the working fluid. To achieve this compactness, the leading methods for supercritical CO2 cooling (including oil effects) were taken from the literature and programmed into a very detailed simulation code, together with the leading air-side flow and heat transfer methods. The effects of tube size, volumetric flow of air, type of air-side fin, miscible oil concentration, etc. were addressed. Compared to an existing design, the volume and refrigerant charge of the CO2 gas cooler were reduced by at least 14%. The simulations with oil showed that there was up to 6% adverse influence on the size of the gas cooler and its CO2 side pressure drop increased up to 2.65 times when oil concentrations of up to 3% were considered.

Working fluid charge reduction. Part II: Supercritical CO2 gas cooler designed for light commercial appliances

Improvement of heat transfer performance and unmatched characteristics of a water-cooled carbon dioxide gas cooler

This paper presents the new ternary non-azeotropic mixture of R-32/R-125/R-161 as an alternative refrigerant to R-407C. The physical properties of the ternary mixture are similar to those of R-407C, and it is environmental friendly, that is, it has zero ozone-depletion potentials (ODP) and lower global warming potentials (GWP) than R-407C. Theoretical cycle performances of R-32/R-125/R-161 and R-407C are calculated and analyzed firstly. Based on the theoretical study, experimental tests are performed on a vapour-compression refrigeration system with a rotor compressor which was originally designed for R-407C (without any modifications to system components for R-407C). Experimental results under different working conditions indicate that the pressure ratio and power consumption of the new refrigerant are lower than those of R-407C, and its refrigerating capacity and COP are superior to those of R-407C, respectively, and its discharge temperature is slightly higher than that of R407C. Therefore, the new refrigerant R-32/R-125/R-161 could be considered as a promising refrigerant to R-407C. [Reprinted with permission from Elsevier. Copyright, 2007].

Cycle performance study on R-32/R-125/R-161 as an alternative refrigerant to R-407C.

Towards the introduction of environmentally friendlier refrigerants, CO2 cycles have gained significant attention in cooling and air conditioning systems in recent years. In this context, a design procedure for an air finned-tube CO2 gas cooler is developed. The analysis aims to evaluate the gas cooler design incorporated into a CO2 air conditioning system for residential applications. Therefore, a simulation model of the gas cooler is developed and validated experimentally by comparing its overall heat transfer coefficient. Based on the model, the evaluation of different numbers of rows, lengths, and diameters of tubes, as well as different ambient temperatures, are conducted, identifying the most suitable design in terms of pressure losses and required heat exchange area for selected operational conditions. The comparison between the model and the experimental results showed a satisfactory convergence for fan frequencies from 50 to 80 Hz. The absolute average deviations of the overall heat transfer coefficient for fan frequencies from 60 to 80 Hz were approximately 10%. With respect to the gas cooler design, a compromise between the bundle area and the refrigerant pressure drop was necessary, resulting in a 2.11 m2 bundle area and 0.23 bar refrigerant pressure drop. In addition, the analysis of the gas cooler’s performance in different ambient temperatures showed that the defined heat exchanger operates properly, compared to other potential gas cooler designs.

https://www.mdpi.com/1996-1073/13/10/2428/pdf

Design Evaluation for a Finned-Tube CO2 Gas Cooler in Residential Applications

/pdf/numerical-simulation-of-fin-and-tube-gas-cooler-for-49zzpw44eb.pdf

Numerical Simulation of Fin and Tube Gas Cooler For Transcritical CO2 Air Conditioning System

A theoretical investigation is considered to discover possible alternatives on behalf of R134a, R12 and R22 for traditional vapor compression refrigeration system. For the same two azeotrope blends, R404A and R407C are nominated. The theoretical analysis is conceded for the refrigerating system at a continual condensation temperature of 50 ℃ and −10 ℃ to 10 ℃ evaporating temperature. Results to investigate alternative refrigerant have slightly lower performance coefficient than HFC134a, CFC22 and CFC12 respectively. Further, it is noticed the average performance coefficient of alternative refrigerants is 7.26% for R404A and 1.41% for R407C enhanced for the degree of superheating and sub cooling. The experimental setup is developed for R404A and R407C to observed different parameters such as pull down time for evaporator temperature, discharge pressure, power consumption, performance coefficient and energy efficiency ratio and compared with R134a. Smaller pull down time, lesser power consumption and isentropic work done improves the performance coefficient of the system, similar or maybe higher behavior like R134a proved that the nominated refrigerants used as an alternative to R12 and R22.

Investigation and Experimental Evaluation of Vapor Compression Refrigeration System by Means of Alternative Refrigerants

This paper aims at the study of heat transfer coefficient, effectiveness and capacity of trans critical CO2 for plain fin and tube gas cooler to investigate the effect of fin density, fin thickness, transverse and longitudinal tube spacing, gas cooler width and air inlet temperature. For this purpose, the Indian climate conditioned condenser model was used which have 40 °C DBT and the condenser pressure 90 bar. The thermo-physical and transport properties of CO2 have a monitor for the physical behavior at state point of the fluid or mixture using REFPROP software. Refrigerant and air side heat transfer coefficients were estimated and compared with the available correlations. It has been observed that the heat transfer coefficient and effectiveness for 830–1030 m3/h flow rate shows intimacy with rich correlation compared with other correlations.

Nilam P. Jadhav

Papers

Numerical Simulation of Fin and Tube Gas Cooler For Transcritical CO2 Air Conditioning System

Investigation and Experimental Evaluation of Vapor Compression Refrigeration System by Means of Alternative Refrigerants

Performance and Geometrical Parametric Evaluation of Plate Finned Tube Gas Cooler for Trans-critical CO 2 Air Conditioning System