置换通风热力分层高度的数值研究

根据置换通风系统的工作原理,介绍了置换通风热力分层高度的定义及数值计算方法,并对送风温度、送风速度等因素对热力分层高度的影响作了定性分析.在此基础上拟合出量纲一热力分层高度的经验公式.     

Theoretical and experimental investigation of impinging jet ventilation and comparison with wall displacement ventilation

This paper focuses on evaluating the performance of a new impinging jet ventilation system and compares its performance with a wall displacement ventilation system. Experimental data for an impinging jet in a room are presented and non-dimensional expressions for the decay of maximum velocity over the floor are derived. In addition, the ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom ventilated with the two systems. The internal heat loads from 25 person-simulators and lighting were used in the measurements and simulations to provide a severe test for the two types of ventilation systems. In addition to a large number of experimental data CFD simulations were used to study certain parameters in more detail. The results presented here are part of a larger research programme to develop alternative and efficient systems for room ventilation.     

Numerical modeling of a complex diffuser in a room with displacement ventilation

A micro/macro-level approach (MMLA) has been proposed which makes it possible for HVAC engineers to easily study the effect of diffuser characteristics and diffuser placement on thermal comfort and indoor air quality. In this article the MMLA has been used to predict the flow and thermal behavior of the air in the near-zone of a complex low-velocity diffuser. A series of experiment has been carried out to validate the numerical predictions in order to ensure that simulations can be used with confidence to predict indoor airflow. The predictions have been performed by means of steady Reynolds Stress Model (RSM) and the results have good agreement both qualitatively and quantitatively with measurements. However, measurements indicated that the diffusion of the velocity and temperature was to some extent under-predicted by the RSM, which might be related to high instability of the airflow close to the diffuser. This effect might be captured by employing unsteady RSM. The present study also shows the importance of detailed inlet supply modeling in the accuracy of indoor air prediction.     

Experimental investigation of thermal and ventilation performances of stratum ventilation

Stratum ventilation has been proposed to cope for elevated indoor temperatures. Air speed, temperature and CO₂ concentration of a stratum ventilated office are investigated experimentally. The data obtained under well defined conditions and therefore can be used for validating numerical models. Thermal comfort conditions and ventilation efficiency are studied based on the experimental results of four experimental cases. Thermal comfort indices, i.e. PMV, PPD and PD are calculated from measured data. The values of these indices are found to satisfy the requirements of ISO 7730, CR 1752-1998 and ASHRAE 55-2010. In terms of thermal comfort, the two cases with supply air temperature of 21 °C are found to perform better compared with the two cases with supply air temperature of 19 °C. For all the cases, the ventilation effectiveness is close to 1.5. This ventilation method could therefore be expected to provide indoor air quality in an efficient way.     

自然通风作用下中庭建筑热环境的数值模拟

以上海地区一实际建筑为例,应用K-ε两方程湍流模型、零方程模型和数值模拟方法对双层玻璃幕墙中庭建筑内的温度场进行了模拟,计算了不同室内负荷下适用自然通风的时间。研究结果表明,自然通风在过渡季节能有效改善室内热环境,较好地满足人体热舒适要求。     

The thermal environment in offices

The reactions of 26 subjects to the thermal environment in a commercial building throughout the working day have been monitored for one year. This paper describes the instrumentation and the results obtained from the survey. The results show that each category of the Bedford comfort scale does not cover equal temperature intervals; the central, comfortable, category being more than twice as wide as the others. The preferred temperature of the occupants of the building was 22.0 °C which is in fairly good agreement with that predicted by Fanger's comport equation (22.6 °C). Away from the region of the optimum thermal environment the difference between the comfort equation and the survey results was much greater. If, as a design criterion the ,aximum number of complaints that would be acceptable from the occupants of a multi-occupancy office is 10%, the survey shows that the temperature throughout the office should be within the range 20–23 °C. The paper compares the reaction of the subjects during the summer and winter and morning and afternoon and that between males and females. Also investigated was the difference in clothing habits between males and females throughout the year.     

Field study on occupant comfort and the office thermal environment in rooms with displacement ventilation

A field survey of occupants' response to the indoor environment in 10 office buildings with displacement ventilation was performed. The response of 227 occupants was analyzed. About 24% of the occupants in the survey complained that they were daily bothered by draught, mainly at the lower leg. Vertical air temperature difference measured between head and feet levels was less than 3 degrees C at all workplaces visited. Combined local discomfort because of draught and vertical temperature difference does not seem to be a serious problem in rooms with displacement ventilation. Almost one half (49%) of the occupants reported that they were daily bothered by an uncomfortable room temperature. Forty-eight per cent of the occupants were not satisfied with the air quality. PRACTICAL IMPLICATIONS: The PMV and the Draught Rating indices as well as the specifications for local discomfort because of the separate impact of draught and vertical temperature difference, as defined in the present standards, are relevant for the design of a thermal environment in rooms with displacement ventilation and for its assessment in practice. Increasing the supply air temperature in order to counteract draught discomfort is a measure that should be considered carefully; even if the desired stratification of pollution in the occupied zone is preserved, an increase of the inhaled air temperature may have a negative effect on perceived air quality.     

Pollutant flushing with natural displacement ventilation

We examine the time taken to flush pollutants from a naturally ventilated room. A simple theoretical model is developed to predict the time taken for neutrally-buoyant pollutants to be removed from a room by a flow driven by localised heat inputs; both line and point heat sources are considered. We show that the rate of flushing is a function of the room volume, vent areas (A^*$A^{*}$) and the distribution, number (n) and strength (B  ) of the heat sources. We also show that the entire problem can be reduced to a single parameter (μ$\mu$) that is a measure of the vent areas, and a dimensionless time (τ$\tau$) that is a function of B, V   and μ$\mu$. Small-scale salt-bath experiments were conducted to measure the flushing rates in order to validate our modelling assumptions and predictions. The predicted flushing times show good agreement with the experiments over a wide range of μ$\mu$. We apply our model to a typical open plan office and lecture theatre and discuss some of the implications of our results.     

Assessment of thermal environment using a thermal manikin in a field environment chamber served by displacement ventilation system

This paper presents a thermal comfort study using a thermal manikin in a field environment chamber served by the Displacement Ventilation (DV) system. The manikin has a female body with 26 individually heated and controlled body segments. The manikin together with subjects was exposed to 3 levels of vertical air temperature gradients, nominally 1, 3 & 5 K/m, between 0.1 and 1.1 m heights at 3 room air temperatures of 20, 23 and 26 °C at 0.6 m height. Relative humidity at 0.6 m height and air velocity near the manikin and the subjects were maintained at 50% and less than 0.2 m/s, respectively. The aims of this study are to assess thermally non-uniform environment served by DV system using the manikin and correlate the subjective responses with measurements from the manikin. The main findings indicate that room air temperature had greater influence on overall and local thermal sensations and comfort than temperature gradient. Local thermal discomfort decreased with increase of room air temperature at overall thermally neutral state. The local discomfort was affected by overall thermal sensation and was lower at overall thermally neutral state than at overall cold and cool sensations.     

置换通风几个问题的讨论

分析了讨论下部送风与置通风的关系,热力分层高度的计算财面空气层温升与工作区温升的确定,送风量计算等问题。     

Numerical analysis of tunnel thermal plume control using longitudinal ventilation

A CFD model of the 4th Beijing subway line was used to study the effect of longitudinal ventilation on heat and smoke plume movement in the tunnel. The critical ventilation velocity is correlated with the heat release rate for both a simplified heat fire source model and a complete combustion fire source model with methane gas as fuel. The influences of the heat source length and the fuel gas inlet geometry on the critical velocity are investigated for both fire source models. The results show that the influences of the combustion process and fire source area variation are not included in models based on Froude number preservation theory. Thus, Ri is no longer suitable as a dimensionless number for the critical ventilation velocity when the fire geometry or combustion conditions influence the results. The back-layering air temperature above the front of the fire source can be used to explain the different critical velocity variation regimes for all the simulation conditions.     

置换通风式空调房间内速度场和温度场数值模拟

应用计算流体力学的理论和方法模拟了三维空调房间的空气的湍流流动,研究强迫对流作用下的置换通风式空调室内气流的速度和温度分布及换热规律,并利用CFD软件FLUENT对模型进行具体的计算,得出了一些有益的结论。     

单、双热源置换通风系统流场及温度场的数值模拟

利用计算流体力学(CFD)方法,分别对单、双热源置换通风系统进行了数值模拟。分析和比较了两种情况下室内空气的速度和温度分布,指出室内温度场在垂直方向上存在三个温升层;单、双热源形成的垂直温度分布有较大差别;双热源送风参数的计算不能简单按单热源的计算参数进行,应考虑热源分散的影响。     

四种不同座椅送风形式热环境的数值模拟

给出了座椅送风的简化模型和计算方法，将座椅送风工况下的风速和温度的模拟结果与实验小室的实测数据进行对比,二者吻合较好。利用简化模型模拟了相同设计条件下四种不同座椅送风形式的送风效果，结果表明，座椅下送风＋椅背送风形式的热舒适性最好，座椅下送风形式热舒适性最差，另外两者居中。建议工程设计时应综合考虑热舒适性和经济性来确定座椅送风形式。     

置换式通风房间室温分布的分类表达

置换通风及其衍生形式的发展,提供了改善室内环境及系统性能的可能性。这些性能的评价指标多基于实际的室温竖直分布。然而现有预测室温变化的经验模式又往往存在一些功能上的局限性,难以避免出现较大误差。提出了以特定的近似分析解作为基本分析架构的分类表达法,它有助于辨析室内存在不同类型热源分布及流率条件的室温分布问题。     

Experimental and computational investigation into suppressing natural convection in chilled ceiling/displacement ventilation environments

The combination of chilled ceiling and displacement ventilation systems can cause destruction of the displacement flow pattern in some circumstances. This paper reports on the performance of a new technique for achieving stable conditions for displacement airflow in the presence of a chilled ceiling system. The technique is based on the attachment of a honeycomb slat system to the underside of a chilled ceiling, thereby suppressing downward cool natural convection. Investigations were carried out using both computational and experimental methods for a range of typical office environment conditions. The results showed that a slat depth to width ratio of 10 could suppress the natural convection by more than 80% when the Rayleigh number reached 7 × 10⁶. This confirms that the technique is capable of minimising downward cool air currents, resulting in preservation of the displacement flow pattern in the presence of the chilled ceiling. The proposed slat system can raise the general air temperature in the space allowing some displacement flow pattern to occur. The outcome of this study is the emergence of a honeycomb slat-based approach for improving the performance, together with provision of general advice for designers as regards the combination of radiant cooling/displacement ventilation systems.     

Numerical investigation on a grouting mechanism with slurry-rock coupling and shear displacement in a single rough fracture

Bulletin of Engineering Geology and the Environment - A slurry diffusion model of a single random-roughness fissure is established to consider the slurry and geological fracture coupling based on...     

Evaluation of thermal comfort and indoor air quality in offices

Professor Derek Croome and his colleagues in their CIB Montreal paper present the results of an investigation into the indoor environment of a naturally ventilated office and conclude that data from laboratory tests are insufficient. A field test method is established which allows an investigation to take into account window and door opening patterns and peoples reactions to air temperature, fresh air and movement.