Refrigerator Systems Analysis
C. W. Bullard,* Y. Liu, R. Srichai, R. Woodall
UIUC Air Conditioning and Refrigeration Center

The objectives of these experimental and analytical investigations are to (1) quantify performance tradeoffs associated with using dual evaporator systems as an alternative to mixing cabinet airstreams as a means of controlling compartment temperatures; (2) analyze strategies for minimizing charge inventory to reduce cycling losses; (3) identify system and component performance implications of designing for multi- or variable-speed compressors; (4) identify the scope of performance improvements obtainable through capillary tube-suction line heat exchanger design.

This project is motivated by the emergence of new technologies for stationary air conditioning systems: (1) making heat exchangers more compact, (2) varying compressor and fan speeds, and (3) modulating refrigerant flow. The goal is to use our existing simulation model and experimental facilities to: (1) quantify the benefits of ultracompact heat exchanger on the performance of split and unitary systems; (2) explore design implications of on-off control and multi- or variable-speed compressor/fan control in combination with fixed throttling devices and/or electronic expansion valves; (3) examine the effect of evaporator design on system performance and noise; and (4) develop a tool to expedite the capillary-tube/charge optimization process for room air conditioners.

Over the past few years, it has been demonstrated conclusively that chlorofluorocarbons, or CFCs, contribute to the depletion of the ozone layer. For this reason, new ozone-safe refrigerants are being developed to be used in air conditioning and other refrigeration systems. This conversion to new refrigerants will require the establishment of a reliable database for the identification of the various heat transfer regimes and for the design of heat transfer equipment. Currently, two experimental apparati are used to determine the condensation and evaporation heat transfer coefficient characteristics and pressure drops associated with these new refrigerants. Analytical and numerical methods of modeling these phenomena are also under development.

This project is directed toward the design of a new generation of heat exchangers for residential refrigerators. In particular, research is being conducted to develop new condenser configurations and exploit new air-side heat transfer enhancements. Heat transfer diagnostics, system simulation, and simple optimization methods are being used to identify promising new directions for heat exchanger development and to estimate the potential benefits of improved component performance in this application.

An investigation of the thermal performance of the wire and tube condensers that are typically used in domestic refrigerators is being conducted. The emphasis of this investigation is on the air-side heat transfer characteristics. Both forced flow and the natural convection limit are be- ing studied experimentally. Simultaneously, a computer model for predicting the thermal performance of wire and tube condensers for a variety of configurations is being developed.

This research is directed at developing a fundamental understanding of frost formation and growth in air-conditioning and refrigeration applications. This deeper understanding will be used to develop models of frost de position that account for the microstructure. Detailed experiments to study frost initiation on conventional surfaces and on surfaces coated with hydrophilic and hydrophobic materials are complemented by careful measurements of mature frost growth on flat-plates and on surfaces with louvers and vortex generators. The data and resulting models will help guide the development of frost-tolerant design methods, frost mitigation techniques, and defrost strategies.

To examine the full-scale implementation of heat transfer augmentations, an evaporator calorimeter has been designed and constructed. This apparatus will provide data for modeling and correlating the overall air-side heat transfer performance of liquid-to-gas heat exchangers. A pilot study of spine-fin heat exchanger performance will be undertaken in this calorimeter. The spine fine geometry is common in HVAC/R applications, and a generalized correlation will be useful to design engineers.

In many applications, water retention and shedding affect heat exchanger performance. Unfortunately, there are no general models available for predicting condensate retention and its effects on heat transfer. The purpose of this project is to develop and validate a model that can predict condensate retention in these applications and to provide design correlations for the wet performance of heat exchangers. This research will provide the first geometrically generalized model of condensate retention. Special attention will be directed at condensate management for enhanced surfaces.

This project addresses heat transfer in an unsteady, developing, channel flow. Controlling the unsteadiness of the flow may provide significant heat transfer enhancements. This research focuses on the application of this idea to the low Reynolds number flows associated with refrigeration and air conditioning applications. Experiments in rectangular and triangular channels will quantify the heat transfer enhancement as a function of the acoustic or mechanically induced pulse frequency and amplitude, and the boundary layer structure will be investigated in detail using laser diagnostics. The physical mechanisms for the enhancement will be determined with a focus on exploiting these effects.

In refrigeration and air conditioning systems, oil circulating with the refrigerant provides compressor lubrication. Although very little is known about real oil circulation rates, component-level heat transfer measurements have demonstrated that the oil concentration has a profound effect on heat transfer performance. Using a new ultrasonic technique, in situ measurements of oil concentrations and circulation rates under transient and steady-state operating conditions will be made, with simultaneous system performance measurements. These data will provide new insights into oil circulation rates in real refrigeration and air conditioning systems, and will provide an evaluation of its influence on thermal system behavior.

Manufacturing, space, and efficiency demands precipitated the development and wide application of louvered-fin heat exchangers. The relative impact of boundary-layer development and flow oscillations on heat transfer performance is not well understood in this geometry. This project uses a numerical and experimental approach to develop a detailed understanding of the heat transfer enhancement mechanisms, their relative contributions, and the conditions under which they prevail. After numerical and experimental agreement is obtained, the numerical studies will be used to develop optimal louver designs, and the experiments will examine three-dimensional effects.

Heat exchanger performance is critical in air conditioning and refrigeration applications, but is often limited by the air-side heat transfer coefficient. This project will explore a novel method for enhancing the air side using stream-wise vortices. These vortices will be generated and controlled by flow manipulation. Experiments using naph thalene sublimation and liquid crystal thermography will be conducted to measure the local and average heat transfer and pressure drop impact of three vortex generation techniques: leading-edge irregularities, surface protuberances, and winglets. Vortex interaction with the surface will be studied in detail, and the results applied to enhancing exchanger performance.

When a liquid film flows over a vertical column of horizontal tubes, it falls from tube to tube as droplets, jets, or a sheet, depending upon the flow rate, tube geometry, and liquid properties. If the gas surrounding the liquid film is also flowing, it may affect this falling film mode. The mode influences the heat transfer rate from the tube to the liquid, but its behavior is not well understood. This project is an experimental study directed at understanding the falling film mode and its impact on heat transfer.

When an oil is in solution with a liquid halocarbon refrigerant, the mixture has thermophysical properties different from those of the pure refrigerant. Changes in the saturation state and transport properties can have an important impact on heat transfer and thermal system performance. The thermophysical properties of conventional refrigerants with commonly used oils have been measured; however, environmental concerns have prompted the consideration of many new refrigerants and refrigerant/oil combinations several of these candidates are blends of refrigerants. This research project is directed at measuring and modeling the thermophysical properties of new refrigerants and refrigerant blends with synthetic and natural oils.

The implications for public safety, down time, repair costs, and product loss make hydraulic shock a crucial issue for the refrigeration industry. Dangerous pressure excursion incidents have been attributed to the initiating mech anisms of condensation-induced shock and vapor-propelled liquid slugging. The objectives of this study are to identify the critical flow regimes in refrigeration piping and to analyze and model the initiating mechanisms of hydraulic shock within these regimes. Along with advancing our understanding of two-phase transients, knowledge of the generic causes of these transients will allow engineers to avoid them through proper system design.

This work integrates thermal design principles with modern control techniques to provide the basis for developing optimal systems for transient operating and environmental conditions. An experimental facility has been constructed to develop and evaluate alternative control techniques and hardware for mobile air conditioning systems. Alternative control methods that involve the use of advanced electronic devices and novel types of actuators and control inputs are being investigated. The project will determine which combinations of sensors, actuators, and control devices work best.

Heat transfer and pressure drop models are being formulated for refrigerant mixtures. A unique method for liquid film thickness has been developed.

Recent concerns about the impact of certain CFCs on the stratospheric ozone concentrations have resulted in the need to replace R-12 as the refrigerant in automotive air conditioning systems. An automotive air conditioning research facility is being used for studying the impact of alternative refrigerants on the overall steady-state and transient performance of the system. Component and overall system models are being developed.

This project provides user support and program enhancement for the BLAST public domain building energy analysis program which is widely used throughout the world. Enhancements under development include computer graphics interfaces, mechanical system design capability, lighting analysis capability, and expanded mechanical system simulation options.

Current building energy calculation schemes decouple the building simulation from the simulation of its HVAC systems. This was done originally because of a lack of computer memory and power. These limitations no longer exist, and it is now possible to eliminate simulation problems that arise because of this decoupling. This project is developing new energy calculation methods that use combined simulation of the building and its systems.

The conversion to ozone safe refrigerants necessitates developing new understanding of their heat transfer and pressure drop characteristics in refrigeration system components. This project, one of several undertaken by the ACRC, is aimed at understanding the behavior of new refrigerants in complete condensers. The study involves a full-scale test apparatus with both air and refrigerant loops and a companion computer-modeling effort.

As the global community attacks the ozone problem, even refrigerants with low ozone depletion potential, such as R-22, are being targeted to be phased out. This refrigerant is used widely in residential air conditioning systems. The purpose of this project is to build a testing facility to evaluate possible system performance improvements of residential AC systems using zeotropic alternatives. This considers both efficiency, through heat exchanger design, and comfort, through system control strategies.

An application guidance for advanced and available cooling and heat pump systems has been developed with a matrix including fuel sources, cooling process, load types, thermal source, and sink. Optimization includes analyzing system energy efficiencies and developing cost estimates for owning and operating the systems, and present values over the life cycles of these systems.

This project is a study of the airside heat transfer and pressure drop impact of wavy passages on heat exchanger performance. It has been observed that unsteadiness, either forced or self exciting, significantly increases heat transfer. Self-induced unsteadiness is generally well understood as the enhancement mechanism for wavy channels; however, the very large design parameter space has made it difficult for engineers to exploit this mechanism. This study will provide a fundamental understanding of the flow of the potential and proper application of wavy fin enhancements.