查看更多>>摘要：Measurement uncertainty has significant negative impacts on the operation and control of heating, ventilation and air conditioning systems. It is a big challenge and should be solved urgently. Existing studies focus on reducing the impacts of measurement uncertainty by developing uncertainty tolerant methods without quantifying the measurement uncertainties themselves. They therefore fail to fundamentally solve them. This study aims to directly quantify the measurement uncertainties of water flow meters in multiple water-cooled chiller systems using a Bayesian approach. A measurement uncertainty quantification strategy is proposed based on Bayesian inference and energy balance models, and the Markov chain Monto Carlo method is used to achieve the strategy. The site data collected from a chiller system are used to test the strategy. Four simulation tests with different levels of measurement uncertainty are conducted to further test and systematically validate the strategy. Test results show that the measurement uncertainties (both systematic and random uncertainties) of the water flow meters in the chiller systems can be quantified effectively and with acceptable accuracy. The strategy performs very well in quantifying random uncertainties of flow meters, and the relative errors range from 0% to 12.8%. The performance of the strategy in quantifying systematic uncertainties is also satisfactory, and the relative errors range from 0.1% to 36.57%. The proposed strategy is able to quantify measurement uncertainties and can be used to optimize the control of chiller systems and improve the reliability of chiller systems.
查看更多>>摘要：Designing air-water systems for industrial applications requires a fundamental understanding of mass accommodation at the liquid-vapor interface, which depends on many factors, including temperature, vapor concentration, and impurities that vary with time. Hence, understanding how mass accommodation changes over a droplet's lifespan is critical for predicting the performance of applications leveraging evaporation. In this study, experimental data of water droplets on a gold-coated surface evaporating into dry nitrogen is coupled with a computational model to measure the accommodation coefficient at the liquid-vapor interface. We conduct this measurement by combining macroscopic observations with the microscopic kinetic theory of gasses. The experiments utilize a sensitive piezoelectric device to determine the droplet radius with high accuracy and imaging to measure the droplet contact angle. This setup also quantifies the trace amounts of non-volatile impurities in the droplet. For water droplets evaporating in a pure nitrogen stream, the accommodation coefficient directly relates to vapor flux over the droplet's surface and is affected by the presence of impurities. We obtained a surface-averaged accommodation coefficient close to 0.001 across multiple water droplets evaporating close to room temperature. This quantification can aid in conducting a more accurate analysis of evaporation, which can assist in the improved design of evaporation-based applications. We believe the modeling approach presented in this work, which integrates the kinetic theory of gases to the macroscale flow behavior, can provide a basis for predicting evaporation kinetics in the presence of extremely dry non-condensable gas streams.
Sbaity, Ahmad AlamirLouahlia, HasnaLe Masson, Stephane
15页
查看更多>>摘要：This article presents experimental and analytical studies of thermosyphon loop with hybrid condenser for cooling data center under various climatic constraints. Two cooling modes are investigated: the passive cooling mode using only the thermosyphon loop without electrical consumption and the hybrid cooling mode using the pumped sea water to cool the condenser of the thermosyphon. Experiments are conducted under various operating conditions to show the influence of the selected parameters on the cooling capacity. Several cooling systems are compared: two different condensers, phase change materials are added in the form of internal plates fixed on the interior walls of the data center and finally the cooling mode of the condenser is also changed. The results show that the passive thermosyphon loop cooling capacity is 1750 W for ambient temperature at 23 degrees C and it increased to 2900 W at 10 degrees C. The cooling capacity is extended to 3000 W by using the thermosyphon with the condenser cooled by the sea water at 23 degrees C. The thermosyphon loop with a hybrid cooling condenser is investigated annually as the cooling system of a chosen typical data center in seven French coastal cities. The results show that the passive thermosyphon loop is generally more powerful for cooling data center in winter. In summer, the use of the sea water cooling of the condenser is required. Taking into account the climatic constraints, the annual energy saving can reach 54% using the combination of the passive and the hybrid thermosyphon condenser.
查看更多>>摘要：Variable refrigerant flow (VRF) systems faults are inevitable due to installation errors, degradation, and other reasons. It is of great value to quantitatively understand the impact of faults on VRF systems performance and to clarify the changing trends of variables under different types of faults through experiments. In particular, the experimental analysis of simultaneous faults situations is helpful to improve the fault detection and diagnosis technology of VRF systems. There have been some previous experimental studies on the impact of faults, but none of them concerns modern VRF systems and their simultaneous faults. This paper presents results from a laboratory study of a VRF system with different types of faults. It provides the first published results of combinations of triple simultaneous faults, in addition to previously untested types of double simultaneous faults. The quantitative impact of the three crucial performance parameters, e.g. cooling capacity, system power, and COP, of the system under different faults has been analyzed. In addition, the quantitative influence and variation trend of system parameter variables during single fault and simultaneous fault are summarized. Results show that the outdoor fouling fault has the greatest impact, which can cause a 47.6% COP drop and 80.27% cooling capacity reduction. The influence of the simultaneous fault on the variable trend is superimposed and offset, but the trend influence of some faults also has a dominant characteristic.
查看更多>>摘要：In this study, state-of-the art deep neural networks to train and predict the heat transfer in building structures were proposed. Today, many of studies analyze thermal energy performance of buildings by analytical or numerical methods. Although building energy performance can be predicted effectively by finite element method, it is still time-consuming to calculate and solve the heat transfer problem. Moreover, expert engineer is required and complicate process to set simulation model is essential. In this work, a novel deep-learning method, which was pre-trained by the thermal simulation data, was developed to predict the thermal behavior of building structures in a fast time without complicated process. Heat transfer simulations of the slab wall building structure depending on its thermal properties and geometries were carried out to get training datasets for deep learning. The database of thermal simulation results was used for deep learning training. The image of temperature and heat flow distribution was trained by convolutional encoding-decoding network and the value of total heat loss through building and thermal bridge coefficient was trained by multi-layer perceptron. After train completed, the thermal behavior could be predicted in a second by just feeding information such as blueprint image and thermal properties of constructions into deep-learning architecture. There was no need to set a new simulation model at each time which consumes time and effort for modeling, meshing and calculating. With the developed network, the prediction of thermal behavior with high accuracy was possible in a super-fast time.
查看更多>>摘要：Throttle low-temperature closed-cycle Joule-Thomson refrigerators with low performance (compressor volumetric flow rate less than 25 m3/h) are one of the promising directions of development of refrigeration equipment. These refrigerators operate with mixtures of refrigerants and are capable of thermostatting different samples at the temperature below -160 degrees C. Such refrigerators have several special features so that it would take an integrated approach to design new types of refrigeration equipment based on Joule-Thomson refrigerators. This paper proposes such a method for mixtures with more than three components. The method comprises a design calculation stage and an experimental stage; the stages are interlinked. As a part of considering these design stages, we describe an original method for calculating the multi-component mixed refrigerant charge and the original algorithm of solving the problem of ensuring the "estimated circulated composition" in the steadystate operation mode in 3-5 iterations. Also, we present the results of experimental verification of the integrated approach to the design for the case of a low-temperature medical refrigerator created based on the original patented layout.
查看更多>>摘要：Organic Rankine cycle (ORC) has been widely used to utilize low-grade waste heat from low, medium, and hightemperature heat sources. Meanwhile, the research of the ORC operates in the cryogenic temperature has also attracted much attention due to the increasing requirement of liquid natural gas (LNG). However, there is still a lack of experimental research on the cryogenic ORC. In this paper, the experimental study of a cryogenic ORC using a semi-hermetic scroll expander and propane (R290) has been investigated. Liquid nitrogen is used as the cryogenic cold source in this system while the circulation water is used to simulate the low-grade waste heat. The system performance of utilizing the cold energy of the liquid nitrogen and thermal energy of the circulation water has been investigated. First, the effects of the evaporation pressure and pressure drop on system performance are examined. Moreover, the system performance between the regenerative organic Rankine cycle (RORC) and basic organic Rankine cycle (BORC) has been compared. The experimental results indicate that evaporation pressure has a significant effect on system performance, and different optimum evaporation pressure will be obtained under different liquid nitrogen mass flow rates. The maximum electrical power 673.59 W, system thermal efficiency 6.78%, and cold energy utilization efficiency 12.32% can be obtained, while the evaporation pressure and liquid nitrogen mass flow rate are 1.36 MPa and 120 kg/h, respectively. What's more, the results also indicate that the pressure drop exhibits a high sensitivity on system performance, and there is a clear linear proportional relationship between pressure drop and electrical power. Furthermore, the comparison results show that RORC has a better system performance than BORC. The experimental results provide design and optimal operation strategy for the cryogenic organic Rankine cycle.
查看更多>>摘要：Experimental research was conducted to observe the frost growth under forced convection and cryogenic conditions (-180 degrees C). A laser was installed as a light source to investigate the frost microscopically, and the frost film and frost seed were observed on the test surface. The process of frost growth under cryogenic conditions was classified into four periods and it was compared with those under general-low temperature conditions. The frost coverage factor was defined based on the pixel intensity of the frost image, and the shape of the frost layer could be visualized using the frost coverage factor. The heat flux of the test surface was compared with the frost coverage factor, and it decreased by 40% when the frost coverage factor was 1. Therefore, the frost coverage factor can be used as a useful index to evaluate the heat transfer performance of the heat exchanger operating under cryogenic conditions.
查看更多>>摘要：In this work, a segmented converging thermoelectric generator is proposed and optimized by designing a variable converging angle in each segment of the heat exchanger. A mathematical model of thermoelectric generator is developed to determine the corresponding converging angles. Also, the effects of air temperature and mass flow rate on the segmented converging structure are explored. Results show that the output power of thermoelectric generator can be apparently improved by using the segmented converging heat exchanger that can make the temperature differences of thermoelectric modules in all segments approximately identical. Compared to the conventional one, the output power can be increased by 12.5% at the air temperature of 500 K and air mass flow rate of 20 g/s, where the converging angles of the segments of heat exchanger are 0 degrees, 2 degrees, 1 degrees, and 1.8 degrees respectively. It seems that the converging angles between the segments change like a sine wave, and the wave range rises with increasing the air temperature and decreasing the air mass flow rate. In addition, the backpressure power loss induced by the segmented converging heat exchanger is lower than the conventional plate-type one due to an apparent decrease of the head loss of outlet cover. This novel optimization structure of the heat exchanger can provide a global performance improvement for the thermoelectric generator system.
查看更多>>摘要：In medium-frequency heat treatment, it is difficult to thoroughly heat the interior of welds of heavy-wall pipes, and blind zones of heating or overheating zones with latent defects are very readily present. To solve the electromagnetic-heating-induced latent defects, we researched the mode of heat transfer inside the welds with heat sources under motion conditions to determine the temperature field evolution, spatial magnetic field distribution, and spatial matching relationship in cases of dual heat sources interfering with each other, tangential to each other, and not interfering with each other. With the increase in the axial spacing between the internal and external heat sources, the magnetic field intensity at the middle of the thickest part of the welding seam on the steel pipe increased and then decreased. When the heat sources interfered with each other, the temperature rise at the weld center from electromagnetic heating was no longer fast after the Curie point was reached. The induced magnetic field heating concentration positions shifted toward both sides of the weld center. The latent defect area was reduced by 18.7 mm2 when the spacing between the internal and external heat sources was 19 mm. This study explores the dual-heat-source interaction mechanism for the first time in the heat treatment of pipes to minimize the blind zones of heating and overheating zones.
NSTL
Elsevier