Carbon dioxide (CO2) is a common working fluid for refrigeration systems. The triple point of CO2 (about −56 °C and 0.51 MPa) is often regarded as the lower operating limit for the con-ventional CO2 vapor compression systems, because below this temperature and pressure, solid CO2 could occur and block the system components. However, if the technical issue could be solved and a stable operation of a vapor compression cycle for heat absorption be-low the triple point pressure (or sublimation cycle) could be realized, there would be a great potential for CO2 to replace the common refrigerants with a very high environmental impact such as R-23 for refrigeration applications below −50 °C. The focus of this work is on the dis-cussion of the feasibly of the sublimation cycle regarding the energy efficiency and the block-ing issues.
Seven different two-stage and three-stage CO2 sublimation systems are theoretically evalu-ated and compared to a two-stage R-23 system, which serves as a baseline. A calculation model for the systems is developed. The optimum intermediate pressures for each system as well as the high pressure for the systems in transcritical operations are calculated within the given temperature and pressure constraints. Multiple influence factors, such as the ambient temperature, compressor efficiency, are considered in determining the operating limit and evaluating the performance for each system.
In order to find out the cause of the blockages in the sublimation system due to the solid CO2, the solid-gas flow is visualized through experiments. Different throttling devices are investi-gated under various inlet conditions. As the sublimator, a heated sight glass assembly is used. It is found that besides the inlet temperature and pressure condition, the tube wall in the down-stream section of the throttling devices has a great influence on the blockages. A larger heat flux also helps to reduce the blockage in the sublimator.
Based on the knowledge gained from the theoretical investigation of the cycle variant and preliminary experiments, a cascade sublimation system is designed, constructed and tested. Despite the fact that the system still requires optimization in terms of energy efficiency and operation stability, it is capable of long continuous operation, and thus the basic feasibility of the sublimation cycle is verified. Finally, the further issues and improvement potentials for the heat transfer and sublimator are discussed.:Acknowledgment
Abstract
Contents
Index of figures
Index of tables
List of abbreviations and symbols
1 Introduction
1.1 Background and Motivation
1.2 Objective and procedure
2 Fundamentals and state of the art
2.1 The R-744 sublimation cycle
2.2 Expansion into solid-gaseous region and critical flow
2.3 Sublimator and solid-gas two-phase flow
2.4 Summary
3 Thermodynamic analysis of sublimation systems
3.1 Definition of the cycle variants
3.1.1 The baseline system
3.1.2 R-744 cascade systems
3.1.3 R-744 booster systems
3.2 Boundary conditions
3.3 Description of the models
3.3.1 Compressor
3.3.2 Heat exchangers
3.3.3 Other components
3.3.4 Fluid properties
3.4 Process calculation and optimization
3.5 Results and discussion
3.5.1 General boundary conditions
3.5.2 Variable temperatures
3.5.3 Variable compressor efficiency
3.5.4 Variable pressure loss and superheating in the sublimator
3.6 Evaluation of the system variants
4 Experimental visualization of the solid-gas flow
4.1 Throttling below the triple point
4.1.1 Experimental setup - test rig I
4.1.2 Results and discussion
4.2 CO2-Sublimation in a horizontal channel
4.2.1 Experimental setup - test rig II
4.2.2 Results and discussion
4.3 Summary
5 Experimental investigation on the performance of a cascade sublimation system
5.1 Experimental setup – test rig III
5.1.1 The refrigerant cycles
5.1.2 The sublimating unit
5.2 Methodology
5.2.1 The measuring procedure
5.2.2 Data evaluation and uncertainty analysis
5.3 Results and discussion
5.3.1 Transient behavior
5.3.2 System performance
5.3.3 Compressor performance
5.3.4 Long period measurements
5.4 Summary
6 Existing issues and optimization potentials
6.1 Blockage-free operation at low wall temperatures
6.1.1 Supplementary experiment
6.1.2 Outlook
6.2 Heat transfer
6.2.1 Supplementary experiment
6.2.2 Outlook
7 Summary
Literature
Appendix A. Differential evolution
A.1 Basics of differential evolution
A.2 Convergence of the results for different system variants
Appendix B. Mass flow rate from the capillary tubes
B.1 Measurement of the mass flow rate
B.2. Comparison of the results with the numerical model and correlations
Appendix C. Supplement to the measurements of the test rig III
C.1 Exemplary measurement of the R-23 operation
C.2. Measurement of the air velocity for the sublimator
Appendix D. Supplement to the measurements at low wall temperatures
D.1. Calculation of the heat transfer coefficients for the airside
D.2. Determination of the local sublimation heat transfer coefficients
Publications during the PhD study
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:86030 |
| Date | 15 June 2023 |
| Creators | Xu, Yixia |
| Contributors | Hesse, Ullrich, Langebach, Robin, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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