Experimental Evaluation and Simulation Research on Novel Variable Refrigerant Flow System
by Xiaojie Lin
Variable refrigerant flow (VRF) system is a popular building air conditioning system which could provide cooling or heating to individual rooms independently. The system is called “variable refrigerant flow” system due to its capability of regulating the refrigerant flow via the precise control of variable speed compressors and electronic expansion valves in each indoor unit. In this dissertation, an advanced VRF system which could provide space cooling, heating and water heating is experimentally evaluated in cooling and heating season for both heat recovery operation and water heating operation. The VRF system is simulated in EnergyPlus and validated with experimental data. Based on the deviation analysis and literature review, it is found that the existing VRF model could not fully reflect the operation characteristic of VRF systems, leading to a high uncertainty in cooling/heating energy and energy consumption. A new VRF model is thereafter proposed, validated in this research and resulted in a model uncertainty less than 5%. Based on the new model, the seasonal performance of an energy saving control strategy and the concept of chilled water storage are investigated. Meanwhile, to solve the mismatch between the building’s thermal load and cooling/heating capability of the VRF system, a new VRF system with phase change material (PCM) based thermal energy storage (TES) is proposed. The new VRF system utilizes single TES device to support both cooling and heating season operation. The performance of new VRF system with PCM based TES is investigated and compared to that of the baseline VRF system. It is found that the new VRF system with PCM based TES could achieve both energy efficiency and demand response goals in cooling and heating season. Based on the comparison, the effect of operation strategies and grid incentive program are discussed. Finally, the economic analysis of the new VRF system with PCM based TES based on annual performance is carried out.
Doctoral Dissertation
http://hdl.handle.net/1903/19769
Simulation and Analysis of Energy Consumption for Two Complex and Energy-Intensive Buildings on UMD Campus
by Dana Savage
The Microbiology Building and Hornbake Library are two multi-purpose and complex buildings, and are among the highest energy-intensive buildings on the University of Maryland College Park Campus. This thesis details the energy analysis and energy consumption models developed to identify energy savings opportunities for these two buildings. Three reports are given per building: one – a comprehensive summarization of relevant building information; two – a utility analysis, including an energy benchmarking study, evaluating the relative performance of each facility; three – a detailed energy model to replicate current operation and simulate potential energy savings resulting from no-and-low cost energy conservation measures. In total, 11 of the 12 measures simulated are strongly recommended for implementation. The predicted combined energy and utility savings are respectively 18,648.4 MMBtu and $436,128 annually. These actionable proposals to substantially reduce the buildings’ energy consumption contribute to the University’s commitment to achieve greater energy efficiency throughout campus.
Doctoral Dissertation
http://hdl.handle.net/1903/20043
Thermal and Hydraulic Performance of Spine Fin Tube Heat Exchangers at Low Reynolds Number Conditions
by Carlos Herrera
The goal of this work is to present the air-side thermal and hydraulic performance of heat exchangers with spine-fin surface augmentation. Although not as common as plain / plate fin, spine-fin heat exchangers have been used for decades in household refrigeration evaporators and in the outdoor coils of household air-conditioning systems. Of particular interest in this study, was the performance at low air-side Reynolds numbers (500–900). Heat transfer coefficients for this geometry were evaluated for samples of varying fin pitch, fin height and tube diameter in both parallel and angled bank arrangements. Water was selected as the hot fluid operating in the turbulent regime with mass flow rates varying at each airflow rate test point. Static cold and hot stream temperatures were maintained for all tests. Air-side heat transfer coefficient (AHTC) is highest for the lower diameter tube heat exchangers and increases in fin pitch lowered the AHTC. This behavior is not seen in plain fin, microchannel and other heat exchangers.
Master's Thesis
http://hdl.handle.net/1903/19766
Determining the Air-Side Performance of Small-Diameter, Enhanced Tube-Fin Heat Exchangers through Numerical and Experimental Methods
by Dennis Nasuta
New correlations for the air-side pressure drop and heat transfer coefficient (HTC) of slit and louver fin heat exchangers with 3-5 mm outer diameter tubes were developed based on Computational Fluid Dynamics (CFD) simulations of small, symmetric fin sections using Design of Experiments (DOE) techniques. The prediction accuracy of these CFD-based correlations was validated by experimental testing of 16 unique 5 mm slit and louver fin heat exchangers under a range of air velocities. The experimental results indicate that the proposed CFD-based correlation with correction factors for air-side pressure drop can predict 100% of the experimental observations with 20% error or less. After a new data reduction procedure accounting for fin conduction was implemented and a single correction factor applied, the HTC correlations could predict 98% of accepted test data with 20% error or less regardless of fin type.
Master's Thesis
http://hdl.handle.net/1903/20033
Modeling of Mobile Air Conditioning System
by Zhenyuan Mei
Since the conventional refrigerant R-134a is being phased out due to its high global warming potential, finding a suitable replacement refrigerant and a system design is of great importance. However, most of the alternatives are either flammable or expensive. Therefore, to ensure the safety of passenger and reduce the refrigerant charge, a secondary loop system with coolant loop on both condenser and evaporator side was proposed. The performances of this system using R-134a, R-152a, and R-1234yf were evaluated and compared to that of conventional direct expansion system using R-134a under the US06 driving cycle condition. The results show that the coefficient of performance of the secondary loop system is significantly lower than that of direct expansion system due to high pressure ratio and high compressor revolution speed. For the secondary loop system, the R-152a has better performance than R-1234yf and is a more suitable alternative refrigerant.
Master's Thesis
http://hdl.handle.net/1903/20399
Development of a Compact Heat Exchanger With Bifurcated Bare Tubes
by Zhiwei Huang
Heat transfer enhancement of air-to-fluid heat exchangers by novel surface or geometry design and optimization is a major research topic. The traditional way of reducing airside thermal resistance is to extend airside heat transfer area by adding fins and the conventional method of reducing fluid side thermal resistance is to use enhanced inner surfaces. These approaches have limitations in further reducing the thermal resistance.
Small diameter (4 and 5 mm) fin-and-tube heat exchangers, louvered fin mini-channel heat exchangers (MCHX), newly studied round bare tube heat exchangers (BTHX) and shape optimized bare tube heat exchangers (sBTHX) with diameter of 0.8~1.0 mm were experimentally investigated using air and water to gain the fundamental understanding of heat transfer and the current technology limitations. Correlations of air-side heat transfer coefficient and pressure drop were then developed for BTHX and sBTHX.
To improve current technologies, a novel bifurcated bare tube heat exchanger (referred as bBTHX, hereafter) was proposed in this study. It was numerically investigated and optimized using Parameterized Parallel Computational Fluid Dynamics (PPCFD) and Approximation Assisted Optimization (AAO) techniques. The most unique feature of bBTHX is the addition of bifurcation, which enhances airside heat transfer by creating 3D flow and waterside heat transfer by boundary layer interruption and redevelopment. The airside and waterside pressure drop can also be reduced by proper design and optimization, resulting in smaller fan and pumping power. Compared to MCHX with similar capacity and frontal area, the optimal bBTHX design has 38% lower total power and 83% smaller volume and 87% smaller material volume. Compared to BTHX with similar capacity and frontal area, the optimal design has 28% lower total power and 11% smaller volume and 10% smaller material volume.
Doctoral Dissertation
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