Search Results

A traditional method of controlling evaporator superheat in a vapor compression air conditioning system is the thermostatic expansion valve (TXV). Such systems are often used in automotive applications. The TXV depends on superheat to adjust the valve opening. Unfortunately, any amount of superheat causes that evaporator to operate at reduced capacity due to dramatically lower heat transfer coefficients in the superheated region. In addition, oil circulation back to the compressor is impeded. The cold lubricant almost devoid of dissolved refrigerant is quite viscous and clings to the evaporator walls. A system that could control an air conditioner to operate with no superheat would either decrease the size of its existing evaporator while maintaining the same capacity, or potentially increase its capacity with its original evaporator. Also, oil circulation back to the compressor would be improved.

The paper presents a semi-empirical model to predict refrigerant and lubricant inventory in both evaporator and condenser of an automotive air conditioning (MAC) system. In the model, heat exchanger is discretized into small volumes. Temperature, pressure and mass inventory are calculated by applying heat transfer, pressure drop and void fraction correlations to these volumes respectively. Refrigerant and lubricant are treated as a zeotropic mixture with a temperature glide. As refrigerant evaporates or condenses, thermophysical properties are evaluated accordingly with the change of lubricant concentration. Experimental data is used to validate the model. As a result, refrigerant and lubricant mass is predicted within 20% in the evaporator. However, in the condenser, lubricant mass was consistently under-predicted while refrigerant mass was predicted within 15% error. Moreover, the lubricant under-prediction becomes more significant at higher Oil Circulation Ratio (OCR).

This paper presents a method of utilizing infrared images to quantify the distribution of liquid refrigerant mass flow rate in microchannel heat exchangers, which are widely used in automobile air conditioning systems. In order to achieve quantification, a relationship is built between the liquid mass flow rate through each microchannel tube and the corresponding air side capacity calculated from the infrared measurement of the wall temperature. After being implemented in a heat exchanger model, the quantification method is validated against experimental data. This method can be used for several types of heat exchangers and it can be applied to various heat exchanger designs.

This paper presents a simulation model for a reversible air conditioning and heat pump system for electric vehicles. The system contains a variable speed compressor, three microchannel heat exchangers, an accumulator, and two electronic expansion valves. Heat exchangers are solved by discretizing into cells. Compressor and accumulator models are developed by fitting data with physical insights. Expansion valves are modeled by isenthalpic processes. System performance is calculated by connecting all parts in the same way as the physical system and solved iteratively. The model is reasonably validated against experimental data from a separate experimental study. Future improvement is needed to take into account maldistribution in outdoor heat exchanger working as an evaporator in HP mode. Charge retention in components also requires further study.

This paper presents experimental study of periodic reverse flow and induced boiling fluctuations in a microchannel evaporator and their impacts on performance of R134a mobile A/C system. Simultaneous flow visualization and pressure measurements revealed that reverse flow due to confined bubble longitudinal expansion caused periodic oscillations of the evaporator inlet pressure and the pressure drop, and their oscillation magnitude and frequency increase with ambient air temperatures because of higher average refrigerant mass flux and heat flux. Three potential impacts of vapor reverse flow reversal on evaporator performance are identified: 1) mild liquid maldistribution; 2) increased the evaporator pressure drop; 3) reduced heat transfer coefficient. Finally, to mitigate vapor reverse flow impacts, revised flash gas bypass (FGBR) method is proposed: vent and bypass backflow vapor trapped in the inlet header.

Reductions of noise in vehicle passenger compartments in recent years have made some previously undetectable noises audible. Expansion devices used in automobile air conditioning systems are known producers of noise. The fact that these devices are mounted very close to the passengers increases the problems associated with the reduction of this noise. The understanding of the propagation mechanisms from the noise generated in the refrigerant by the expansion device, through the tube and evaporator walls, and finally to the outside air is important. This paper will focus on how noise from expansion devices is transmitted through tube walls downstream of the expansion valve.

Automotive air conditioning compressor produces an annular-mist flow consisting of gas-phase refrigerant flow with oil film and oil droplets. This paper reports a method to calculate the oil retention and oil circulation ratio based on oil film thickness, wave speed, oil droplet size, oil droplet speed, and mass flow rate. Oil flow parameters are measured by high-speed camera capture and video processing in a non-invasive way. The estimated oil retention and oil circulation ratio results are compared quantitatively with the measurements from system experiments under different compressor outlet gas superficial velocity. The agreement between video result and sampling measurement shows that this method can be applied in other annular-mist flow analysis. It is also shown that most of the oil exists in film from the mass point of view while oil droplets contributes more to the oil mass flow rate because they travel in a much higher speed.

Automotive air conditioning evaporators are known to emit whistling sounds briefly during the startup phase of operation. These sounds have led to customer complaints. Work at the University of Illinois shows that this problem develops when single phase vapor refrigerant flows through the evaporator plate at high velocity. It was further discovered that nitrogen could be used to simulate the phenomenon leading to simpler experimental methods. This paper discusses the development of a technique to use inexpensive rapid prototyping as a tool in the design of new plate evaporators. Some preliminary results along with some limitations of the new technique are discussed.