Development of a Dynamic Test Facility for Environmental Control Systems
by Amr El-Sayed Alaa El-Din Gado
Passenger cars and light trucks consume 80% of the total oil imported by U.S.A. Mobile air conditioners (MACs) increase vehicle fuel consumption and exhaust gas emissions. They operate most of the time in a transient state. It is currently impossible to test the performance of an air conditioner during transient operation without it being associated with its intended conditioned space, the car cabin. In this research work a new smart test facility is designed, built, and verified. This facility makes it possible to test the MAC independent of the vehicle, but yet under realistic dynamic conditions. The facility depends on simulation software that measures the conditions of the air supplied by the MAC and subsequently adjusts the conditions of the air returning to the MAC depending on the results of a thermal numerical model of the car cabin that takes into consideration sensible and latent loads, as well as passengers' control settings. It was successful in controlling the temperature and relative humidity within ±0.9°C and ±5% of their respective intended values. The test facility is used to investigate the dynamic performance of a typical R134a MAC system. The tests include pull-down, drive cycle, and cyclic on/off tests. The analysis focuses on the latent capacity and moisture removal due to the difficulty in measuring these variables during field tests. The results show that the most energy efficient method to pull-down the air temperature inside a hot-soaked cabin is to start with fresh air as long as the temperature in the cabin exceeds that of the ambient and then switch to recirculated air. The effect of re-evaporation is illustrated by showing the off-cycle latent capacity. Cyclic tests show that the net moisture removal rate has a minimum at around a 2 minute duty cycles. This implies a means of controlling the coil latent heat factor by varying duty cycle. The automotive air conditioning system is numerically modeled and used in cooperation with the cabin model to conduct numerical tests. The numerical simulation results are compared to the experimental results and the error is less than 1.5 K of cabin air temperature.
Doctoral Dissertation
http://hdl.handle.net/1903/3391
An Object Oriented Simulation Framework for Steady-State Analysis of Vapor Compression Refrigeration Systems and Components
by David Hallam Richardson
Thermo fluid energy system simulation has shown to be a useful tool for engineers; encompassing component design, system design and with increasing interest, system optimization. Thermo fluid energy systems, be they for comfort cooling, comfort heating, power generation, or any other purpose typically possess a unique composition and function. This has resulted in simulations for individual rather narrowly defined energy systems, each customized for the particular system of interest. However, it is impossible to ignore that the majority of thermo fluid energy systems share, among others, the common characteristics of fluid flow, mechanical work input/output and energy input/output via heat transfer. This dissertation exploits this similarity, and develops an object oriented methodology for modeling components and solving systems created from such components, operating in steady-state. The technique is novel in that it discriminates between systems, and their sub-systems, referred to as components. This methodology serves as a functional starting point which will appeal to the objectives of individual research groups, such as industrial sponsors, academic professionals, and students. The dissertation then presents several examples highlighting the major points in the analysis, and a complex example that demonstrates where such a tool may be usefulness in a product design environment. Lastly, the dissertation presents a component based, user-friendly interface specifically for vapor compression refrigeration systems. Several examples are used to validate the component models, reproducing experimental data reasonably well within a range of 5% for most performance variables.
Doctoral Dissertation
http://hdl.handle.net/1903/3753
Heat Transfer and Mechanical Design Analysis of Supercritical Gas Cooling Process of CO2 in Microchannels
by Guohua Kuang
An extensive review of the literature indicates a lack of systematic study of supercritical CO2 gas cooling and no prior work on CO2/oil mixture in supercritical region, suggesting a lack of fundamental understanding of supercritical gas cooling process and a lack of comprehensive data that would help quantify the performance potential of CO2 microchannel heat exchangers for engineering applications.
This dissertation presents a systematic and comprehensive study on gas cooling heat transfer characteristics of supercritical CO2 in microchannels. Semi-empirical correlation is developed for predicting heat transfer performance of supercritical CO2 in microchannels. The effect of oil addition on heat transfer performance has been experimentally investigated as well. It is shown that presence of lubricant oil mixed with supercritical CO2 in the heat exchangers can substantially affect heat transfer and pressure drop coefficients.
Because of the outstanding performance of supercritical CO2 and its promising potential as a substitute for current refrigerants, attention has been paid to the design of CO2 microchannel heat exchangers. The extensive review of the literature also indicates no previous study in systematically developing a simulation model for structural design of microchannel heat exchangers. The dissertation extends the research to the mechanical design analysis of microchannel heat exchangers. A finite-element method (FEM) based mechanical design analysis of tube-fin heat exchangers is carried out to develop a simulation model of the heat exchangers. The solid modeling and simulation scheme can be served as a guide for mechanical design of CO2 heat exchangers. Experiments are conducted to validate the developed models as well.
Doctoral Dissertation
http://hdl.handle.net/1903/4278
Investigation of Performance Degradation of Evaporator for Low Temperature Refrigeration Applications
by Jan Muehlbauer
An evaporator test stand has been designed, sized, constructed, calibrated and operated to investigate the evaporator performance degradation for low temperature refrigeration applications. Measurements have been taken of air- and refrigerant-side temperatures, refrigerant-side pressures, air- and refrigerant-side differential pressures, air- and refrigerant-side mass flow rate and the power consumption of the electric defrost heater. The system is designed to work as a commercial refrigeration system for low temperature applications including freezing and defrost cycles. A manual and automatic defrost control has been designed and tested. The calculated values are the air- and refrigerant-side capacities. The tests have shown that the system is able to accomplish all desired test conditions. The performance of the evaporator was evaluated continuously during testing. Its performance degraded constantly by accumulation of frost during each frosting cycle, which resulted in the loss of superheat towards the end of each cycle. The defrost operation could not restore the full capacity of the evaporator. The defrost cycle consists out of two phases, melting the ice and re-cooling the coil. The first phase melts all the ice of the coil but the drainage of the resulting water is incomplete. The re-cooling phase freezes the amount of water residue on the evaporator coil, which causes an accelerated frost formation in the following freezing cycle. All over this behavior causes an accelerated reduction in evaporator capacity and therefore earlier superheat loss. It has been investigated for two air flow rates. The comparison of cycles with different air flow rates showed that with an increased air flow rate the frost density increases and the free flow area of the air passage in the evaporator decreases more slowly.
Master's Thesis
http://hdl.handle.net/1903/3687
Investigation of Two Performance Improvement Options For Household Refrigerators
by Ahmet Ors
Due to environmental concerns, the refrigeration industry is facing the challenge of developing more efficient and environmental friendly refrigerators. Environmentally harmful refrigerants, CFC's and HCFC's, have already been shifted toward environmental friendly refrigerants such as HFC's and hydro carbons. However HFC's have a significant global warming potential. Accordingly, new policies have taken effect which are forcing the refrigerator industry to develop refrigerators that will reduce energy consumption and refrigerant emissions to reduce energy bills and the global warming effects of refrigerators.
This study presents the research conducted on the condenser improvement of one of the commercially available household refrigerators and designing and applying a so-called Alternating evaporator duty cycle (AED) with a two step capacity modulated compressor.
In the condenser improvement study, a household refrigerator's condenser configuration was changed from the cross-flow configuration to the counter-flow configuration without changing other components and cabinet structure. Ten experiments for different refrigerant charges were conducted and it was experimentally proved that the refrigerator with counter flow condenser consumes 1% less energy compared to the one with cross-flow condenser.
To study the potential of the AED cycle, a side-by-side household refrigerator equipped with a conventional cycle was converted into the AED cycle. First of all, the performance of the refrigerator with the new cycle was simulated and then cycle components were designed. Two different kinds of evaporators were used for food (R) cabinet such as forced convection fin-and-tube type evaporator (FCE) and natural convection tube-and-plate type evaporator (NCE), to investigate the humidity control improvement.
Experimental results shows that, average humidity ratios of freezer (F) and R compartments during the cyclic operations are 0.5 gH2O/(kg of dry air) and 2.5 gH2O/(kg of dry air), respectively using the FCE and 0.6 gH2O/(kg of dry air) and 2.4 gH2O/(kg of dry air), respectively with NCE. Therefore, the humidity ratio of R compartment is maintained at 4-5 times higher level than that of F compartment for the AED cycle, and also 4 - 5 times higher than that R and F compartments for the base cycle. In addition to the better humidity control, AED cycle provides separate and more efficient cabinet temperature control."
Master's Thesis
http://hdl.handle.net/1903/3494
Investigation on Refrigerant Distribution in Evaporator Manifolds
by Dae-Hyun Jin
To provide essential design information of microchannel evaporators, an experimental study was conducted on the effects of geometry, operating conditions and fluid properties on the distribution of refrigerant and pressure drop in horizontal heat exchanger manifolds. An experimental facility with a visualization section for mimicking a real heat exchanger manifold geometry was developed. Under realistic operating conditions, measurements of refrigerant distribution were conducted by measuring mass flow rates and vapor quality of all branch tube groups (individual adjacent heat exchanger tubes were grouped in groups) for a total of 60 test cases. The flow direction within the heat exchangers is vertically upward.
Stratified flow is observed for the end inlet case of the dividing manifold due to the gravitational effect. The liquid level increases along the dividing manifold because the liquid is traveling farther than the vapor due to inertia difference. Near the end of the manifold, the liquid level is almost constant. For the side inlet case, it is observed that the incoming refrigerant impinges on the inner side wall of the manifold, and is divided symmetrically near the inlet, and the interface between the vapor and the liquid has a V-shaped form near the inlet.
Based on the measurements, it is observed that for the end inlet case, the profile of the branch tube inlet vapor quality is of a ""stepwise"" shape. There exist two almost constant value regions, one of about 100% vapor quality near the inlet and the remainder of about 12% vapor quality with a very short transition region. For the end inlet case, as the manifold inlet mass flow rate increases, the number of branch tube groups having almost 100% tube inlet vapor quality increases also because the vapor-liquid interface is moving farther towards the end of the manifold due to the increased momentum. However, for the side inlet case, there is no such region having 100% branch tube inlet vapor quality. For the side inlet case, the profile of the branch tube inlet vapor quality is symmetric. Near the inlet, the branch tube inlet vapor quality is about 60 ~ 70%, and near the end of the manifold, the branch tube inlet vapor quality is about 20%. In between two regions, the branch tube inlet vapor quality decreases monotonously along the manifold. The flow distribution is strongly affected by the manifold inlet location and/or manifold inlet geometry and manifold inlet vapor mass flux.
Correlations are proposed using the T-junction concept in a modified form from Watanabe et al.'s method (1995). For R-410A and R-134a tests with both inlet cases, 90% of measured vapor inlet quality data and 90% of measured liquid fraction of taken off data are within predicted values ± 0.1. To investigate the effect of refrigerant maldistribution on the performance of the tested heat exchangers, heat exchanger simulations were conducted. Based on the heat exchanger simulation results using test results for the refrigerant distribution, for the side inlet case, the capacity degradation based on the uniform distribution at the tested inlet manifold mass flow rates (at 30, 45 and 60 g/s) is 5 ~ 8%. For the end inlet case, as the inlet manifold mass flow rate increases, the capacity degradation based on the uniform distribution ranges from 4% to 15% as a function of the manifold inlet mass flow rate. Therefore, the side inlet is preferred for a wide range of mass flow rates compared to the end inlet.
Doctoral Dissertation
http://hdl.handle.net/1903/3845
Microscale Study of Nucleation Process in Boiling of Low-Surface-Tension Liquids
by Saeed Moghaddam
A novel MEMS device has been developed to study some of the fundamental issues surrounding the physics of the nucleation process intrinsic to boiling heat transfer. The study was focused on boiling of FC-72 liquid. Over the past 50 years, scientists have developed several competing mechanistic models to predict the boiling heat transfer coefficient. Although the developed models are intended to predict the heat transfer coefficient at macroscales, their fundamental assumptions lie on complex microscale sub-processes that remain to be experimentally verified. Two main unresolved issues regarding these sub-processes are: (1)bubble growth dynamics and the relative importance of different mechanisms of heat transfer into the bubble and (2)vapor/liquid/surface thermal interactions and the bubble's role in heat transfer enhancement during the nucleation process. The developed device generates bubbles from an artificial nucleation site centered within a radially distributed temperature sensor array (with 22-40 μm spatial resolution) while the surface temperature data and images of the bubbles are recorded. The temperature data enabled numerical calculation of the surface heat flux. Using the test results, the microlayer contribution to the bubble growth was determined to increase from 11.6% to 22% when surface temperature was increased from 80°C to 97°C. It was determined that the transient conduction process occurs predominantly at the bubble/surface contact area, and before the bubble departure, contrary to what has been commonly assumed in classical boiling models. For the first time, the convection heat transfer outside the contact area (often known as microconvection) and transient conduction within the contact area were differentiated. The microconvection heat flux was found to be relatively close to that of the equivalent natural convection produced by the same geometry, but becomes significantly stronger than natural convection at higher surface temperatures. Test results under saturation conditions showed that when surface temperature is increased from 80°C to 97°C, the contribution of the different mechanisms of heat transfer within a circular area of diameter equal to that of the bubble changes from: (1)28.8% to 16.3% for microlayer, (2)45.3% to 32.1% for transient conduction, and (3)25.8% to 51.6% for microconvection.
Doctoral Dissertation
http://hdl.handle.net/1903/3878
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