| 1. |
- Romanov, Pavel
(författare)
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Hardening of Carbon Steel by Water Impinging Jet Quenching Technique : Differential Cooling of Steel Sheets and Quenching of Cylindrical Bars
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Austenitization followed by quenching is a well-known conventional heat-treating procedure which is widely used on carbon steels with the aim to obtain high strength in as-quenched condition. Such quenching is usually done by immersing a steel product into the cooling medium which provides a uniform cooling of the surface. The cooling rate can be adjusted to a certain degree on a “component” length-scale by using different cooling mediums such as water, oil, polymer solution, etc. However, certain steel products such as beams, pillars in automobile industry or different machinery parts in agriculture require a proper and controllable cooling gradient and thus mechanical property gradient within the product. It is difficult to control the cooling rates locally on the length-scale smaller than the product only by replacing the quenching medium. In addition, quenching by immersing the product into the cooling medium is accompanied by thermal stresses due to the different cooling rates of the surface and the core, and also accompanied by transformation stresses due to the volume change during phase transformations. These stresses may lead to negative effects such as undesired residual stresses or even cracks. Therefore, cooling must be properly optimized and controlled to eliminate these drawbacks. Such a controllable cooling can be performed by several impingements of the water jets onto a hot austenitized surface at certain locations. By controlling the water flow, number of jets, their locations and other parameters, the global and the local cooling rates can be optimized for a specific industrial application later on. This thesis demonstrates the potential and capability of the water Impinging Jet Quenching Technique (IJQT) to provide a flexible and controllable cooling for both differential and for uniform quenching cases. The test rig of IJQT was developed in the University of Gävle and was used to perform quenching experiments in this study: differential cooling of thick sheets and uniform quenching of bars to different depths. Differential cooling was performed on square-shaped carbon steel sheets with thickness of 15 mm, and the uniform quenching with different flow rates was performed on carbon steel cylindrical bars with 100 mm in diameter. Along with the physical experiments, Comsol Multiphysics 5.6 software was used to solve a 1D heat transfer problem to estimate the cooling rate profile along the radius of the bar. The experiments were verified by observations and characterization of the microstructure using light optical microscopy (LOM), and by examining the mechanical properties through tensile tests and hardness measurements. The results of the quenching experiments and verifications showed a high potential and flexibility of the IJQT in differential cooling case as well as in the uniform quenching case.
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| 2. |
- Jahedi, Mohammad, 1984-
(författare)
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Experimental and Numerical Investigation of the Quenching Process on Rotary Hollow Cylinder by Multiple Impinging Jets
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The worldwide competitive market on metal products with higher quality in industry has increased the need to implement more advanced and controllable quenching techniques in the hardening stage of the heat treatment process. Moreover, sustainability and energy efficiency are key factors to consider in the development of advanced quenching techniques. Among various cooling methods that are used in industry, a water impinging jet quenching system is one of the few that offers wide flexibility to adjust cooling rate based on the chemical composition and proper phase transformation in the continuous cooling transformation diagram (CCT) to achieve desired material properties. On an industrial scale, a large number of water impinging jets are placed in the cooling configuration introducing multiple array of jets in the quenching system. In the literature study by the author, there has been interest to study the quenching heat transfer by single water jet in various applications. Even so, little scientific attention has been paid to the multiple array of water impinging jets and the importance of various quenching parameters on the quenching heat transfer with multiple array of jets. This thesis deals with a study of quenching rotary hot hollow cylinder with multiple configurations of water impinging jets. The aim of this investigation is to obtain better understanding of boiling heat transfer phenomena in application of multiple array of water impinging jets and quenching parameters. An experimental test rig was designed to control most influential parameters in quenching experiments. The results of experimental study contained recorded temperature data beneath the quenching surface of a hollow cylinder. A heat conduction inverse solution based on the GMRES method was developed for application of quenching hollow cylinder. This model used the recorded temperature data of quenching experiments to predict surface temperature and heat flux. A thorough parametric study investigated the effect of various quenching parameters and multiple configuration of jets in terms of local and area-averaged heat transfer over surface as well as in the solid material.The local surface boiling curve captured clear effect of multiple array and cyclic variation of heat transfer caused by rotation of hollow cylinder. The delay in onset of wetting front flow growth over the surface, collision of adjacent wetting front flows and creation of upwash flow were captured on the surface heat flux contour plot. Higher heat flux was obtained around stagnation and upwash flow zones over the quenching surface. The relation between jet flow rate and multiple array configuration revealed a trade-off between these two parameters in terms of optimizing the water resource usage and desired cooling rate with this cooling technique.Comprehensive parametric study revealed effect of various quenching parameters in the local heat transfer in the boiling regimes. The results show improvement of heat flux in the film and nucleate boiling is more difficult than transition boiling regime. In the study of area-averaged heat transfer in 1-row array, higher subcooling and jet flow rate enhance the surface heat flux. In contrast, smaller rotation speed, jet-to-jet spacing and initial wall-superheat temperature increase the area-averaged surface heat flux of hollow cylinder. An extra row of nozzles in the array (2-row) also enhanced the area-averaged surface heat flux significantly. The results from comprehensive parametric study of 4-row in-line and staggered configurations have been used to propose correlation for surface area-averaged Nusselt number. In the local heat transfer, two correlations of average and maximum local heat flux at stagnation point of water impinging jet were proposed.The result of this study and the proposed correlations may provide a road map for engineers to design hollow cylinder quenching system with multiple array of water impinging jets based on cooling rate for proper phase transformation and optimized water resource and energy usage in the quenching process.
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| 3. |
- Milic, Vlatko, 1992-
(författare)
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A techno-economic system approach for the energy renovation of residential districts built before 1945
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A key factor in the quest for sustainable development worldwide is reducing the energy use and greenhouse gas emissions from residential buildings. The residential and service sector represent 39% of the final energy use in Sweden. The housing stock in Sweden is fairly old with approximately 25% of the residential buildings built before 1945, many of which possess heritage values. Considering the generally poorer thermal performance in older buildings compared to newer ones, it is important to investigate the techno-economic energy efficiency potential in this part of the built environment. The aim of this thesis is to develop a bottom-up approach and to analyze energy renovation of residential districts built before 1945 from a system perspective with regard to targets of life cycle costs (LCC), energy use and preservation of building heritage values. The developed approach includes a combination of economic and environmental impacts from a building owner and energy utility point of view. The approach includes analysis on four different levels, i.e., building level, cluster level, district level and city level.The results show that the developed approach is successful in integrating targets of LCC and energy use, as well as preservation of heritage values, during techno-economic energy renovation. By a further development of the change-point model, data related to building thermal power characteristics, such as Qtotal and balance temperature, can be calculated and used for analysis of a residential district. Moreover, the cluster with the initial poorest thermal performance, i.e., the single-family houses in stone, account for the highest decrease in specific energy use (70–78%) and LCC (34–37%) during energy renovation at LCC optimum. The corresponding figures for the buildings with the best thermal performance initially, i.e., the cluster with multi-family buildings in wood, are 23–24% and 14%, respectively. Furthermore, it is concluded that the cost-effective energy efficiency potential is highly correlated with initial building properties and preservation requirements, which significantly affects the stone buildings. This is because insulation on the inside of the external walls is cost-effective in these buildings, but not in wooden buildings, which consequently decreases the energy savings potential from 46–69% in a balanced energy renovation scenario to 8–30% in a restricted energy renovation scenario.The findings also show that the environmental performance of the building district is closely linked with the selected energy system boundary. This can be exemplified by CO2 emissions of 0.7–1.1 kg CO2 eq./(m2·year) at LCC optimum for multi-family buildings when considering biomass an unlimited resource, compared to 28.9–40.0 kg CO2 eq./(m2·year) when considering biomass a limited resource with condensing coal power plants as the marginal user. Furthermore, on a city level it is concluded that the environmental performance of the district heating (DH) system is improved as a result of techno-economic energy renovation of a district, and that the net income is decreased (8%) despite a lower system cost (12–13%) due to less DH sold to end users. The global CO2 emissions are decreased by 3,545–3,737 tonnes/year and the primary energy use is decreased by 5.0–5.2 GWh/year. Apart from the developed bottom-up approach for analysis of the energy renovation of residential districts built before 1945, this thesis has provided valuable results to the research community, building sector and authorities in terms of (1) the further development of the change-point model, which enables time-effective analysis of the thermal performance of residential districts; (2) the environmental benefits with techno-economic energy renovation of residential districts from a DH producer perspective and (3) the need to develop packages of EEMs that are profitable for both DH producers and end users of DH.
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| 4. |
- La Fleur, Lina, 1985-
(författare)
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Energy renovation of multi-family buildings in Sweden : An evaluation of life cycle costs, indoor environment and primary energy use, and a comparison with constructing a new building
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Residential buildings account for 27% of the final energy use in the European Union. In cold climates, space heating represents the largest proportion of the energy demand in residential buildings. By implementing energy efficiency measures (EEMs) in existing buildings, energy use can be significantly reduced. The Energy Performance of Buildings Directive states that renovations of buildings offer an opportunity to improve energy efficiency. Renovations that include measures implemented with the specific purpose of reducing energy use are referred to as energy renovations. In addition to improving energy efficiency, an energy renovation can also improve the indoor environment. Sweden, like many other European countries, faces the challenge of renovating an ageing building stock with poor energy performance. Improving energy efficiency and performing energy renovations in a cost-effective manner is central, and optimization approaches are often used to identify suitable EEMs and energy renovation approaches. New buildings usually feature better energy performance compared to older buildings, and one approach for reducing energy use in the building sector could be to demolish old buildings with poor thermal performance and build new buildings with better thermal performance.The aim of this thesis is to evaluate energy renovations of multi-family buildings with regard to space heating demand, life cycle costs, indoor environment and primary energy use. The choice between energy renovation of a multi-family building and the demolition and construction of a new one is also investigated with regard to life cycle costs (LCCs). A Swedish multi-family building in which energy renovation has been carried out is used as a case study. The building was originally constructed in 1961 and has a lightweight concrete construction. The renovation included improving the thermal performance of the building envelope and replacing the exhaust air ventilation system with a mechanical supply and exhaust air ventilation system with heat recovery.The methods used in the studies include dynamic whole building energy simulation, life cycle cost analysis and optimizations, and a questionnaire on indoor environment perception. Extensive field measurements have been performed in the building prior to and after renovation to provide input data and to validate numerical predictions. In addition to the studied building, the analysis of the choice between energy renovation and the demolition and construction of a new building includes three other building construction types, representing common Swedish building types from the 1940s, 1950s and 1970s.The analysis shows that the energy renovation led to a 44% reduction in space heating demand and an improved indoor environment. The indoor temperature was higher after the renovation and the perception of the indoor temperature, air quality and noise in the building improved. The EEMs implemented as part of the energy renovation have a slightly higher LCC than the optimal combinations of EEMs identified in the LCC optimization. It is not cost-optimal to implement any EEMs in the building if the lowest possible LCC is the objective function. Attic insulation has a low cost of implementation but has limited potential in the studied building with its relatively good thermal properties. Insulation of the façade is an expensive measure, but has a great potential to reduce heat demand because of the large façade area. Façade insulation is thus required to achieve significant energy savings. Heat recovery in the ventilation system is cost-effective with an energy saving target above 40% in the studied building. The primary energy factors in the Swedish Building Code favor ground source heat pumps as a heat supply system in the studied building.The LCC of renovation is lower compared to demolishing and constructing a new building. A large proportion of the LCC of demolition and new construction relates to the demolition of the existing building. In a building with a high internal volume to floor area ratio, it is not always possible to renovate to the same energy performance level as when constructing a new building. A more ambitious renovation approach is also needed compared to a building with a smaller volume to floor area ratio.
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| 5. |
- Steen Englund, Jessika, 1983-
(författare)
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Prediction of Energy Use of a Swedish Secondary School Building : Building Energy Simulation, Validation, Occupancy Behaviour and Potential Energy-Efficiency Measures
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Residential and public buildings account for about 40% of the annual energy use in Europe. Many buildings are in urgent need of renovation, and reductions in energy demand in the built environment are of high importance in both Europe and Sweden. Building energy simulation (BES) tools are often used to predict building performance. However, it can be a challenge to create a reliable BES model that predicts the real building performance accurately. BES modelling is always associated with uncertainties, and modelling occupancy behaviour is a challenging task.This research presents a case study of a BES model of a school building from the 1960s in Gävle, Sweden, comprising an example of a validation strategy and a study of energy use and potential energy-efficiency measures (EEMs). The results show that collection of input data based on evidence, stepwise validation (for unoccupied and occupied cases), and the use of a backcasting method (which predicts varying occupancy behaviour and airing) is an appropriate strategy to create a reliable BES model of the studied school building. Several field measurements and data logging in the building management system were executed, in order to collect input data and for validation of the predicted results. Through the stepwise validation, the building’s technical and thermal performance was validated during an unoccupied period. The backcasting method demonstrates a strategy on how to predict the effect of the varying occupancy behaviour and airing activities in the school building, based on comparisons of BES model predictions and field measurement data. After applying the backcasting method to the model, it was validated during an occupied period. The annual predicted specific energy use was 73 kWh/m2 for heating of the studied building. The distribution of heat losses indicates that the best potential EEMs are changing to efficient windows, additional insulation of the external walls, improved envelope airtightness and new controls of the mechanical ventilation system.
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| 6. |
- Andersson, Harald, 1987-
(författare)
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Numerical and experimental study of confluent jets supply device with variable airflow
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, application of confluent jets for design of ventilation supply devices has been studied. Similarly, numerus studies have been made on the potential and application of variable air volume (VAV) in order to reduce the energy demand of ventilation systems. This study investigates the combination of supply devices based on confluent jets and VAV, both in terms of the nearfield flow behavior of the device and the impact on thermal comfort, indoor air quality and energy efficiency on a classroom-level space when the airflow rate is varied.The method used in this study is an experimental field study where the confluent jets-based supply devices were compared to the previously installed displacement ventilation. The field study evaluated the energy efficiency, thermal comfort and indoor air quality of the two systems. In the case of the confluent jets supply devices, airflow rate was varied in order to see what impact the variation had on the performance of the system for each airflow rate. Furthermore, the confluent jets supply devices were investigated both experimentally and numerically in a well insulated test room to get high resolution data on the particular flow characteristics for this type of supply device when the airflow rate is varied. The results from the field study show nearly uniform distribution of the local mean age of air in the occupied zone, even in the cases of relatively low airflow rates. The airflow rates have no significant effect on the degree of mixing. The thermal comfort in the classroom was increased when the airflow rate was adapted to the heat load compared to the displacement system. The results lead to the conclusion that the combination of supply devices based on confluent jets can reduce energy usage in the school while maintaining indoor air quality and increasing the thermal comfort in the occupied zone.The results from the experimental and numerical study show that the flow pattern and velocity in each nozzle is directly dependent on the total airflow rate. However, the flow pattern does not vary between the three different airflow rates. The numerical investigation shows that velocity profiles for each nozzle have the same pattern regardless of the airflow rate, but the magnitude of the velocity profile increases as the airflow increases. Thus, a supply device of this kind could be used for variable air volume and produce confluent jets for different airflow rates.The results from both studies show that the airflow rate does not affect the distribution of the airflow on both near-field and room level. The distribution of air is nearly uniform in the case of the near-field results and the room-level measurement shows a completely uniform degree of mixing and air quality in the occupied zone for each airflow rate. This means that there is potential for combining these two schemes for designing air distribution systems with high energy efficiency and high thermal comfort and indoor air quality.
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| 7. |
- Carlander, Jakob, 1985-
(författare)
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On the Effect of Occupant Behavior and Internal Heat Gains on the Building’s Energy Demand : A case study of an office building and a retirement home
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- About 12% of the greenhouse gas emissions and 40% of the total energy use in the EU derive from the buildings. User behavior, construction, and HVAC systems has a significant impact on a building’s energy use. If a building is to be energy-efficient it is important to understand how all these parameters are connected. This study is motivated by the need to decrease the energy use in buildings to reach the goals of energy use and greenhouse gas emissions. In this thesis, measurements of indoor climate and electricity use, together with time diaries was used to create input data for an energy simulation model of a retirement home. A parametric study was conducted to simulate how energy demand was affected by changes in five different parameters in an office building. Also, two different energy-efficiency indicators were used to see how indicators can affect the perceived energy-efficiency of buildings.High amount of airing and low electricity use had the most impact on the heating demand in the retirement home, and electricity use had the highest impact on the total energy demand in the office building. The model of the retirement home using data gathered on-site had 24% higher energy use than the model using standard user input data. In the office building, total energy demand for heating and cooling could be lowered with 12-31% by lowering the electricity use with 30% compared to standard user input data.For office buildings the most important thing to lower total energy demand seems to be lowering the electricity use. Using today’s standard user input data does not correspond well to using on-site gathered data in a retirement home and it is therefore important to develop the standard user input data further. The indicator kWh/m2, seems to promote buildings with low occupancy. This could lead to buildings being utilized in an in-efficient way. The indicator kWh/m2 should either be replaced or combined with an indicator that takes occupancy into consideration.
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| 8. |
- Liu, Linn, 1985-
(författare)
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A systematic approach for major renovation of residential buildings
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In Sweden, buildings are responsible for about 40 % of total energy use and about 10 % of total CO2 emissions Today more than 60 % of existing Swedish residential buildings are over 40 years old and are in need of major renovation. In addition, 15 % of all multi-family buildings and 27 % of all single-family houses were built before 1945. The increased energy use and threat from CO2 emissions of the building sector create a need for energy efficiency. The important role that renovation of residential buildings will play in reducing the total energy used by the Swedish building sector as well as in reducing primary energy use and CO2 emissions on both the national and global levels has been the impetus for the studies included in this thesis.The aim of the current research is to develop a methodology from a system perspective which can be used to analyze the energy use, optimal life cycle cost (LCC), energy efficiency measure (EEM) package, indoor environment, CO2 emissions, and primary energy use of a building or a community during major renovation. The developed methodology accomplished at three different levels, i.e. building level, cluster level and district level. The methodology considers both energy efficiency and economic viability during building renovation and will also play an important role in overall urban planning. The studied buildings include both non-listed and listed residential buildings and the tools used include building energy simulation (BES), survey, technical measurements, LCC optimization and building categorization.The results show that the combination of BES, technical measurements and surveys provides a holistic approach for evaluation of energy use and indoor environment of the studied residential buildings. The results from the current study also show that the 2020 energy target, i.e., reduction of energy use by 20 %, for the building sector can be achieved by all the studied building types and that the total LCC of these buildings are below the cost-optimal point. In comparison, the 2050 energy target, i.e., reduction of energy use by 50 %, for the building sector may be achieved by the non-listed buildings, but when the constraints relevant to listed buildings are added the cost-optimality changes as some EEMs in direct conflict with the building’s heritage value may not be implemented.The investigation of primary energy use and CO2 emissions by the residential buildings show that the higher the energy saving, the lower the primary energy use becomes, and vice versa. With the same energy saving, the heating system with higher primary energy factor results in higher primary energy use. From a CO2 emissions point of view, EEM packages proposed to help buildings connected to a CHP based district heating system, to reduce the energy use or LCC are not consistently effective. Since these EEM packages will reduce district heating demand, the electricity produced in the CHP plant will also decrease. When the biomass is considered a limited resource, measures such as investment in a biofuel boiler are not favourable from the CO2 emissions point of view. The current study has also shown that combining building categorization method and LCC optimization method will help the community to reduce its energy use, primary energy use and CO2 emissions in a systematic and strategic way.
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