Sung Tack Ro

Experimental investigation of the performance of R22, R407C and R410A in several capillary tubes for air-conditioners

Abstract The objective of this study is to present test results and to develop a dimensionless correlation on the basis of the experimental data of adiabatic capillary tubes for R22 and its alternatives, R407C (R32/125/134a, 23/25/52 wt.%) and R410A (R32/125, 50/50 wt.%). Several capillary tubes with different length and inner diameter were selected as test sections. Mass flow rate through the capillary tube was measured for several condensing temperatures and various degrees of subcooling at the inlet of each capillary tube. Experimental conditions for the condensing temperatures were selected as 40, 45 and 50°C, and the degrees of subcooling were adjusted to 1.5, 5 and 10°C. Mass flow rates of R407C and R410A were compared with those of R22 for the same test conditions. The results for straight capillary tubes were also compared with those of coiled capillary tubes. A new correlation based on Buckingham π theorem to predict the mass flow rate through the capillary tubes was presented based on extensive experimental data for R22, R407C and R410A. Dimensionless parameters were chosen considering the effects of tube geometry, capillary tube inlet conditions, and refrigerant properties. Dimensionless correlation predicted experimental data within relative deviations ranging from −12% to +12% for every test condition for R22, R407C and R410A. The predictions by the developed correlation were in good agreement with the results in the open literature.

Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump system

Abstract Performance of a heat pump system using hydrocarbon refrigerants has been investigated experimentally. Single component hydrocarbon refrigerants (propane, isobutane, butane and propylene) and binary mixtures of propane/isobutane and propane/butane are considered as working fluids in a heat pump system. The heat pump system consists of compressor, condenser, evaporator, and expansion device with auxiliary facilities such as evacuating and charging unit, the secondary heat transfer fluid circulation unit, and several measurement units. Performance of each refrigerant is compared at several compressor speeds and temperature levels of the secondary heat transfer fluid. Coefficient of performance (COP) and cooling/heating capacity of hydrocarbon refrigerants are presented. Experimental results show that some hydrocarbon refrigerants are comparable to R22. Condensation and evaporation heat transfer coefficients of selected refrigerants are obtained from overall conductance measurements for subsections of heat exchangers, and compared with those of R22. It is found that heat transfer is degraded for hydrocarbon refrigerant mixtures due to composition variation with phase change. Empirical correlations to estimate heat transfer coefficients for pure and mixed hydrocarbons are developed, and they show good agreement with experimental data. Some hydrocarbon refrigerants have better performance characteristics than R22.

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32R134a, R290R600a, and R32R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m−2 and mass fluxes are set to the discrete values in the range of 424–742 kg m−2 s−1 for R22, R32, R134a, R32R134a, and R32R125; 265–583 kg m−2 s−1 for R290, R600a, and R290R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.

Power augmentation of combined cycle power plants using cold energy of liquefied natural gas

The feasibility of using inlet air cooling by virtue of the cold energy of liquefied natural gas (LNG) to increase power output of gas/steam combined cycle power plants during warm seasons is analyzed. Air cooling capacity and power augmentation for a combined cycle system based on a high performance gas turbine are demonstrated as a function of the ambient temperature and humidity. It is shown that the relative increase in power of the combined cycle plant amounts to 8% for dry air condition (relative humidity lower than 30%) and 6% for usual humidity conditions (60% relative humidity). The system appears feasible even with a realistic air pressure loss in the inlet air cooler if humidity is not too high.

Analysis of the dynamic characteristics of a combined-cycle power plant

Gas/steam combined cycle has already become a well-known and substantial technology for power generation due to its numerous advantages including high efficiency and low environmental emission. Many studies have been carried out for better performance and safe and reliable operation of combined-cycle power plants. A power plant is basically operated on its design conditions. However, it also operates on the so called off-design conditions due to the variation in a power load, process requirements, or operating mode. Therefore, the transient behavior of the system should be well-known for the safe operation and reliable control. In this study, dynamic simulation is performed to analyze the transient behavior of a combined-cycle power plant. Each component of the power plant system is mathematically modeled and then integrated into the unsteady form of conservation equations. Transient behavior was simulated when rapid changes and periodic oscillations of the gas turbine load are imposed. Time delay characteristic caused by the thermal and fluid damping is analyzed and overall time-response of the combined power plant system is shown.

Analysis of thermal stress evolution in the steam drum during start-up of a heat recovery steam generator

A program to predict the transient characteristics of the heat recovery steam generator (HRSG) is constructed and the start-up behavior of the high-pressure part of an HRSG is analyzed with a special focus on the estimation of the thermal stress in the steam drum. Three HRSG start-up procedures (steady gas turbine without gas bypass, steady gas turbine with gas bypass, start-up gas turbine) are simulated. The gas bypass is conducted during the initial stage of the start-up. Estimation of the maximum thermal stress makes it possible to optimize the gas bypass mode. It is shown that bypassing part of the gas flow lowers the peak stress much in case of the steady gas turbine exhaust condition. Examples of the scheduling of the gas flow increase using the step and ramp modes are demonstrated. In case of the simultaneous start-up, the peak stress is considerably low due to the gradual increase of gas temperature and mass flow and bypassing just a small portion of gas flow is enough to keep it under the allowable limit.

An experimental study of frost formation on a horizontal cylinder under cross flow

Abstract Frost layers formed on the front and rear surfaces of a horizontal cylinder during cross flow are found to be thicker than those at the top and bottom surfaces where the flow separation is nearly initiated. This observation was obtained in an experimental study carried out to examine frost formation on a horizontal cylinder given a cross flow condition. The thickness of the frost layer and the temperature distribution in the cylinder were measured for various experimental conditions. The local heat flux around the cylinder and the effective thermal conductivity of the frost layer were likewise evaluated, while thickness and surface temperature of the frost layer around the cylinder were measured periodically. These measurements were obtained by varying the Reynolds number, temperature, and humidity. The dew point temperature of the inlet air, however, was kept below the freezing point throughout the experiment. Results also reveal that inlet air velocity, temperature, and humidity affect thickness and thermal conductivity of the frost layer.

Experimental Study on the Flow Characteristics of Streamwise Inclined Jets in Crossflow on Flat Plate

Experimental study has been conducted to investigate the flow characteristics of streamwise 35 deg inclined jets, injected into a turbulent crossflow boundary layer on a flat plate. Flow is visualized by schlieren photographs for both normal and inclined jets to determine the overall flow structure with the variation of the velocity ratio. A three-dimensional velocity field is measured for two velocity ratios of 1.0 and 2.0 by using a five-hole directional probe. The visualization study shows that the variation of the injection angle produces a significant change in the flow structure. It is recognized that the jet flow is mainly dominated by the turbulence for a small velocity ratio, but it is likely to be influenced by an inviscid vorticity dynamics for a large velocity ratio

Evaporation heat transfer of R-32, R-134a, R-32/134a, and R-32/125/134a inside a horizontal smooth tube

Abstract Evaporative heat transfer coefficients of R-32, R-134a, R-32/134a, and R-407C (R-32/125/134a: 23/25/52 wt%) have been measured in a horizontal smooth tube. Experiments were conducted for test section average temperatures of −12.0–17.0°C, with a mass flux of 240–1060 kg m−2 s−1, and a heat flux of 4.1–28.6 kW m−2. Evaporative heat transfer characteristics of R-407C as a possible alternative to R-22 have been compared with those of R-22. A new correlation based on superposition model for pure refrigerants and refrigerant mixtures was presented. Experimental results were compared with several correlations which predict evaporative heat transfer characteristics. Comparison with the experimental data in the open literature showed that our correlation gave a satisfactory result.

The thermal conductivity of R22, R142b, R152a, and their mixtures in the liquid state

An experimental apparatus for measuring the thermal conductivity of liquids by the transient hot-wire method was constructed and tested with toluene as a standard liquid. Measurements were performed on R22, R142b, and R152a. The thermal conductivities of mixtures of R142b and R152a with R22 were also measured by varying the weight fraction of R22. Experiments were performed in the range from −50 to 50°C and from 2 to 20 MPa and the measured data are analyzed to obtain a correlation in terms of temperature, pressure, and composition of the mixture. While the thermal conductivity of R22 + R152a mixtures varies monotonously with composition, that of R22 + R142b mixtures turned out to go through an extremum value. The accuracy of our measurements is estimated to be within 2%.