Results 1 - 10 of 3643
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[en] A model of compressible flow through an orifice, in the region of transition from free molecular to isentropic expansion flow, has been developed and tested for accuracy. The transitional or slip regime is defined as the conditions where molecular interactions are too many for free molecular flow modeling, yet not great enough for isentropic expansion flow modeling. Due to a lack of literature establishing a well-accepted model for predicting transitional flow, it was felt such work would be beneficial. The model is nonlinear and cannot be satisfactorily linearized for a linear regression analysis. Consequently, a computer routine was developed which minimized the sum of the squares of the residual flow for the nonlinear model. The results indicate an average accuracy within 15% of the measured flow throughout the range of test conditions. Furthermore, the results of the regression analysis indicate that the transitional regime lies between Knudsen numbers of approximately 2 and 45
[en] Stratum ventilation has been proved to be more energy-efficient than conventional ventilation methods. Thermal comfort of stratum ventilation can be further improved. Compared with steady airflows, dynamic airflows have potentials to improve thermal comfort due to distinct dynamic characteristics. This study aims to investigate and compare the dynamic characteristics of airflows produced by steady and pulsating air supply under stratum ventilation. Experiments were conducted, in which steady and pulsating air supply were used to condition a classroom served by stratum ventilation, respectively. Five test conditions including one with steady air supply and four with pulsating air supply were designed. Analysis on skewness and kurtosis showed that the air velocity distributions of steady air supply were closer to normal distributions than pulsating air supply. Multi-scale decomposition of instantaneous air velocity signals was performed by using wavelet analysis. The one-dimensional continuous wavelet transform (CWT) showed that the airflows produced by the two supplies had differentiated CWT coefficients with time. In conclusion, the dynamic characteristics of airflows created by pulsating air supply were more similar to that of natural winds, which may provide better thermal comfort. (paper)
[en] Highlights: • Two new models of an UFAD system with a floor cooling are developed and validated. • A new correlation for the convection heat transfer coefficient is proposed. • The maximum error of the detailed model for the thermal decay is 8.4%. • The simplified model is simple/accurate enough for use in energy simulation tools. • A nomographic design chart for quick sizing of the system. - Abstract: This paper analyses the thermal behaviour of an underfloor air distribution system (UFAD) combined with a floor cooling, focusing on the thermal decay (TD). Two new models were implemented: a detailed model based on the convective heat transfer coefficient inside the plenum, and a simplified model based on the bypass factor. To validate the models we performed three experiments. Both, detailed and simplified models, reproduce well the measured TD. The maximum relative errors between the predicted and measured TD were 8.4% (transient state) and 1.3% (steady state). Although differences were higher at transient regimen, the terminal unit is designed to operate with smooth changes of the boundary conditions. As the error obtained by detailed and simplified models was near identical, the model based on the bypass factor is recommended because of its simplicity. A sensitivity analysis was performed. We evaluated: (1) the TD as a function of the supply air temperature and air flow rate, (2) the cooling capacity of the floor, and (3) the cooling capacity of the ventilation air. The paper concludes with a nomographic design chart that brings together all the features of the terminal unit, allowing a quick selection of the operating parameters.
[en] Highlights: • The infiltration is responsible for more than half of the total cooling load. • A direct method is developed for measuring the transient infiltration airflow rates. • The method is validated by the experiments of tracer gas decay technique. • The measurement errors of infiltration airflow rates are distributed between ±10%. • The practical application of the proposed method is discussed. The measurement of the infiltration airflow rates can support the calculation of the infiltration cooling load for the better understanding and optimizing the energy consumption of cold stores. However, the large temperature difference and the intense transient features make it difficult and complex to measure the airflow rates accurately. In this paper, a simple and practical method to measure the transient infiltration airflow rates is developed by using the local air velocity linear fitting. The proposed method is validated by the measurement results of the tracer gas decay method. It is concluded that the proposed method shows a good performance on the transient infiltration airflow rates measurement. The measurement errors are between ±10%. To enhance the application of this method, the layout of the measuring points of the air velocities are analyzed. The results show that, along the vertical layout direction, air velocity measuring points around the neutral level (where the cold and the warm air separate, about the middle height of the door) are not preferred when using this method. What’s more, the calculation of the infiltration cooling load by using this measuring method is also discussed.
[en] We report the creation of a low flow rate sensor from PEDOT micro-hairs. The hairs are printed as pipette-defined depositions using a nanopositioning system. The printing technique was developed for fabricating structures in 2D and 3D. Here micro-hairs with diameters of 4.4 μm were repeatedly extruded with constant heights. These hairs were then applied to produce a prototype flow rate sensor, which was shown to detect flows of 3.5 l min−1. Structural analysis was performed to demonstrate that the design can be modified to potentially observe flows as low as 0.5 l min−1. The results are extended to propose a practical digital flow rate sensor. (fast track communication)
[en] A mathematical model of an automotive air conditioning (AAC) system with a variable displacement compressor (VDC) is developed in order to simulate and analyze its steady-state performance. A test system is established to validate the system model, and the simulated results agree well with the experimental data. The simulation results show that there is a performance band for the system parameter relationship due to the frictional forces between the moving components of the VDC within which all the steady-state points fall. Different from the one-one parameter relationship in an AAC system with a fixed displacement compressor, it is a multiple-one parameter relationship in the AAC system with a VDC. The influence of the compressor rotary speed, air temperature at the condenser inlet and air flow rate through the evaporator on the performance band is simulated. The evaporating pressure performance band moves in the direction of increasing evaporator inlet air temperature and increasing evaporating pressure when the compressor rotary speed increases. The evaporating pressure performance band moves in the direction of decreasing evaporating inlet air temperature and decreasing evaporating pressure when air temperature at the condenser inlet or air flow rate through the evaporator increases
[en] With the increasing share of renewable power resources turbomachines need to be operated under a wider range of operating conditions including highly off-design regimes. Under such regimes an undesirable phenomenon of blade flutter might occur and possibly destroy the machine. To prevent this, intensive research is conducted by research teams worldwide. Blade flutter research program at the Institute of Thermomechanics of the Czech academy of sciences (IT CAS) mainly aims to advance experimental techniques for investigation of sonic and transonic blade flutter. For this purpose, the new sophisticated test facility was designed and manufactured. As part of the design process, the CFD computations were conducted in order to investigate the flow field in the test facility. This paper presents results of these computations with detailed analysis of flow structures occurring during the air flow through the stationary blade cascade.