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Recent studies have reported that thermal aging of insulation paper affects its moisture absorption property, which can affect moisture equilibrium in oil-paper insulation. In engineering practice, Fessler's model is one of the most widely used methods to estimate the moisture content of paper through measurement of moisture content of oil. However, this model lacks consideration of the thermal aging effect on equilibrium moisture. This study comprehensively investigated the effect of thermal aging on moisture equilibrium in oil-paper insulation. Based on the accelerated ageing and moisture measurements, the equilibrium moisture content of insulation paper with different aging and temperatures were obtained. The effects of aging condition and the temperature on equilibrium moisture are discussed and Fessler's model was modified accordingly. The results show that: 1) the decreasing of specific surface area is the dominant factor that affects the moisture equilibrium in oil-paper insulation, which is caused by thermal aging; 2) the modified Fessler model is reasonable and suitable to estimate the equilibrium moisture content of insulation paper with different aging.

Thermal aging of insulation paper has obvious effects on its moisture absorption property. The presented studies aim to investigating the effects of aging on moisture diffusion in oil-immersed insulation paper. Based on the experimental designs and preparation of experimental materials, the transient moisture contents of insulation paper with different aging status were obtained at different conditions. Then, the experimental results were discussed. Furthermore, the diffusion factors and equilibrium time constants were extracted based on the model of moisture diffusion in oil-immersed insulation paper, which are applicable to our studies. Finally, the effects of thermal aging on the diffusion factor and equilibrium time constant were analyzed. The results show that: 1) With the aging of insulation paper, the rate of moisture absorption increases, and the equilibrium moisture content of insulation paper decreases at the same temperature. Besides, the equilibrium time constant decreases with aging at the same temperature. 2) Considering the effects of aging, thickness of insulation paper and temperature, the empirical expression of diffusion factor was obtained and verified.

Oil-immersed bushing is one of the most important accessory equipment of a transformer, whose insulation status is crucial. Moisture is an important factor that affects the insulation status of oil-immersed bushing. It has a very important significance to estimate the moisture content of oil-immersed bushing accurately. Here, the 72.5 kV bushing samples with different moisture contents were made. Afterwards, the frequency domain dielectric spectrum (FDS) of samples were tested at 15°C. The Havriliak-Negami equation was modified based on the analysations of complex planes, then it was used to analyse the experimental results. Moreover, the characteristic parameters and method to estimate moisture content were obtained and testified. Finally, the field application of the proposed estimation method was also discussed. The results of the studies shown here indicate that the moisture content of oil-immersed bushing can be estimated based on the proposed method effectively; however, in order to estimate the moisture content of field oil-immersed bushing, the elimination of temperature effects should be considered.

This paper discusses the modeling of round tube plate fin heat exchanger (RTPF) and microchannel heat exchanger (MCHX) under wet air conditions. Â The heat exchanger models are based on finite volume method. In each control volume, the empirical heat transfer and pressure drop correlations for refrigerant and air are adopted and the effectiveness-NTU method is applied for heat transfer calculation. For the round tube heat exchanger, the tube circuiting is considered. For microchannel heat exchanger, both uniform distribution and maldistribution among parallel microchannel tubes are investigated and compared. When the air dew point temperature is higher than the tube wall temperature, dehumidification occurs. Two methods are compared to simulate the wet air condition: (1) the air side is simulated based on the total enthalpy method; (2) the air side is simulated based on simultaneous heat and mass transfer method. The heat exchanger models are validated against the experimental results of a 2.5 ton residential air-conditioning system in Air-Conditioning and Refrigeration Center at University of Illinois. The experiment was conducted based on AHSI/AHRI standard 210/240 at Conditions A, B (wet coil) and C (dry coil). The baseline system contains round-tube evaporator, while the round-tube evaporator is later replaced with a microchannel evaporator. The microchannel heat exchanger is tested at both direct expansion and flash gas bypass conditions. The models show good accuracy compared to the experimental results. The capacities are within 2% while the saturation temperatures are within 2oC.

Flash Gas Bypass with venting of reversed vapor is implemented in a R134a automotive air conditioning system. The mass flow rate of reversed vapor is measured by a flow meter placed on the venting line. The effects of microchannel geometries including channel length and channel diameter on the amount and frequency of flow reversal are investigated. A mechanistic model is developed to simulate the bubble dynamics inside of single microchannel tube. This model is validated against experimental data and used to explain the effect of channel length and diameter on flow reversal.

A system model is developed in Engineering Equations Solver (EES) to simulate the performance of a residential air-conditioning system. The system model includes detailed component models of evaporator, compressor, condenser and expansion valve, while the components are linked through refrigerant mass flow rate, pressure and enthalpy. The evaporator and condenser models are based on finite volume method. In each control volume, the empirical heat transfer and pressure drop correlations for refrigerant and air are adopted and the effectiveness-NTU method is applied for heat transfer calculation. If the humidity ratio of air is high and dehumidification occurs, the air side is simulated based on the total enthalpy method. For round-tube heat exchanger, the tube circuiting is considered. For microchannel heat exchanger, it is assumed uniform distribution among parallel microchannel tube. The compressor model is based on the manufacturer’s 10-coefficient map method. The expansion valve is simulated as an isenthalpic process. The inputs into the system model are air inlet conditions and refrigerant superheat and subcooling. The system model is validated against the experimental results of a 2.5 ton residential air-conditioning system in Air-Conditioning and Refrigeration Center at University of Illinois. The experiment was conducted based on AHSI/AHRI standard 210/240 at Conditions A, B (wet coil) and C (dry coil). The baseline system contains round-tube evaporator and condenser, while the round-tube evaporator is later replaced with a smaller microchannel evaporator. The system model shows good accuracy compared to the experimental results. The capacity and COP are within 2% while the saturation temperatures are within 2oC. The system model is applied further to investigate the benefits of replacing the baseline round-tube heat exchangers with microchannel heat exchangers. If the round tube evaporator and condenser are replaced with a respectively equal size microchannel evaporator and condenser, the capacity increases by 12% while the COP increases by 20%. To match the capacity, the compressor speed may be reduced or a smaller compressor may be used. Then, the COP may increase by 28%.

The performance of two evaporators (round tube plate fin and microchannel) and their effect on system performances were experimentally investigated in R410a residential air conditioning systems. Both systems share identical components except evaporators and system performance were measured under AHRI A, B and C conditions. Under all test conditions, the system with the microchannel evaporator outperformed the baseline system with the round tube plate fin evaporator. Up to 10.6% increase of capacity and 13.1% increase of COP were achieved if both systems were operated under the same compressor speed. Up to 23.8% improvement of COP was obtained if the cooling capacities of both systems were kept the same. The refrigerant charge was reduced by 13.4% compared with the baseline system, due to the smaller internal volume of the microchannel evaporator. The air side heat transfer area of microchannel evaporator was just ½ of the baseline round tube plate fin evaporator, and so was the face area.

This paper presents a method to quantify the distribution of liquid refrigerant mass flow rate in parallel flow microchannel heat exchanger from the infrared images. This method can be used for several types of heat exchangers: evaporators, condensers, gas-coolers and etc., also it can be applied to various heat exchanger designs: different inlet/outlet locations, different flow configurations and etc. After being implemented in a heat exchanger model, this method is validated against experimental data, and it is used to examine the validity of quality distribution function in operating heat exchangers. Also, heat exchanger model equipped with this method provides quantitative information of capacity degradation due to refrigerant maldistribution.