Floor heating and cooling combined with displacement ventilation: Possibilities and limitations

Design guidelines envisage that floor heating can be used together with displacement ventilation (DV), provided that the supply air is not overly heated before it can reach heat and contaminant sources. If this is not controlled a mixing flow pattern could occur in the room. The use of floor cooling with DV is also considered possible, although draught risk at ankle level and vertical air temperature differences must be controlled carefully, because they could increase.Few studies on these topics were found in the literature.An indoor environmental chamber was set up to obtain measurements aimed at analysing the possibilities and limitations of combining floor heating/cooling with DV. Air temperature profiles, air velocity profiles, surface temperatures and ventilation effectiveness were measured under different environmental conditions that may occur in practice. These values were compared to equivalent temperature measurements obtained using a thermal manikin.The measurements show that floor heating can be used with DV, obtaining high ventilation effectiveness values. A correlation between the floor heating capacity and the air temperature profile in the room was found. Measurements showed that floor cooling does not increase draught risk at ankle level, although it does increase vertical air temperature differences.     

Control of Airborne Particle Concentration and Draught Risk in an Operating Room

The influence of location of airborne particle source, ventilation rate, air inlet size, supply air velocity, air outlet location, and heat source on the dkributiuns of airborne particle concentration and draught risk in an operating room is investigated. The investigation is carried out by using a flow program with the k-E mdel of turbulence. Based on a standard case, five cases, each with one changed parameter, are computed, and the detailed field distributions of air velocity, temperature, airborne particle concentration, and draught risk are presented. The parametric study concludes that, for a better air quality and thermal comfort, it is desirable to use a higher inflow rate, a larger inlet area, and a uniform velocity profile of supply air. Outlet location and heat source have little influence on the disrributions of the particle concentration in the room. It has also been found that the distributions of particle concentration in the recirculating zone are very sensitive to the location of the particle sources.     

A new formula for determining a minimum recommended value of inlet air velocity from UFAD system to prevent occupants from draught risk

During the winter period the natural convection flow along a cold window surface can be the cause of thermal discomfort. Warm air blown into the room near the window prevents occupants from draught risk. The paper presents a new formula for determining a minimum recommended value of inlet air velocity. A balance of a momentum flux of flow of the down draught currents and the warm air jet inflow from the UFAD (under floor air distribution) system was a base for the analytical solution. In order to estimate air movement in a typical room, numerical simulations are executed. A computational model of the room with the UFAD system is constructed within the CFD environment developed by Fluent Inc. Ten cases with varying inlet air parameters and a window U$U$-value are considered. The analytical estimation of minimum inlet air velocity is confirmed by simulation results of temperature and velocity distributions.     

空调机制冷运行时室内温度场及温降速率的实验研究

本文以中间侧送风下回风的流场作为实验研究模型,研究柜式空调制冷机在不同送风速度和不同送风角度运行时,室内温度场和温度下降速率,从而确定其主要影响因素。实验结果表明:室内温差主要在高度方向上;送风角度对空调房温度下降速率和温度场有明显影响,送风速度次之;导风板角度大时,温度场的温差较小;当导风板位于90°,送风速度为高速时,室内温度下降最快。另外,本文对其机理也进行了初步探讨。     

The effect of false ceiling on the cooling capacity of passive chilled beams

Passive chilled beams are often used to provide cooling or additional cooling when the ventilation system cannot cope with the whole cooling load. The advantage of passive cooling is that it is a silent cooling. Often the chilled beams are installed above a false ceiling and thereby the room is subdivided into two compartments. From the chilled beam a plume is generated. Make-up air (return air) needs to flow into the upper compartment to substitute the airflow generated by the chilled beam. Therefore openings for this purpose are installed in the false ceiling. Small openings constitute a resistance to the flow and the locations of the openings affect the flow pattern. The overall performance was studied in a mock-up of a real office by changing both the size and position of the openings for the make-up air. A uniform heating source was arranged by covering the floor with a heating foil. The best location and size of the openings were explored by both recording the heat absorbed by the beam and the temperature in the room. Minimum temperature attained in the room is the signature of the most efficient cooling. To achieve efficient cooling with a uniform floor-based heating source, two conditions must be fulfilled: a) the return opening area must be at least equal to the horizontal area of the chilled beam; b) the return air openings must be located at the perimeter of the room.     

Experimental investigation of the velocity field and airflow pattern generated by cooling ceiling beams

In the modern office environment there are numerous heat generating equipment, heat loads from solar radiation and heat produced by people. The loads will often exceed the load the ventilation system can cope with. To meet this demand on extra cooling capacity the commercial market provides cooling ceiling panels and chilled beams. A chilled beam is a source of natural convection, creating a flow, vulnerable to disturbances, of cold air into the occupied zone. Experiments have been conducted in a mock up of an office room; qualitative information has been obtained by visualisation. Instantaneous velocity profiles of the airflow generated by the chilled beam has been registered. In addition, the temperature field below the chilled beam has been measured with a whole field measuring technique. The results show that the airflow from the chilled beam has behaviour similar to a two-dimensional plume but exhibits strong oscillation both sideways and along the chilled beam. These oscillations (intermittence) might cause a sensation of draught but in order to clarify this further investigations are required. Furthermore, airflow generated by heat sources in the room may reverse the flow generated by the chilled beam.     

Must cold air down draughts be compensated when using highly insulating windows?

Rooms with high windows are likely to have comfort problems caused by cold air down draught, which are usually solved by placing heating appliances underneath the windows. In the city of Zug, Switzerland, a highly insulated educational building with a concrete core system for heating and cooling is planned. The purpose of our investigation was to find out whether any measures are necessary in this building to compensate the effects of down draught in the occupied zone. Special attention has been paid to the effect of passive measures like air flow obstacles or openings in the window sill. Experiments were carried out in a room climate laboratory on 1:1 scale to measure the down draught and the thermal comfort in the occupied zone. Several configurations of window sizes, insulating standards and outside temperatures were investigated. In addition, the results were compared to values obtained from analytical calculations and from literature. The conclusion was that the window frame, not the window pane, is the critical element and that there will be no unacceptable draught risk in the occupied zone, as long as the window frame is also highly insulating. Therefore, no additional measures to compensate the down draught were planned for the building in Zug. Furthermore, it was found that the down draught is not only dependent on the window temperature but also on the heat load in the room, i.e. a heat load increases the boundary layer thickness and decreases its peak velocity.     

Airflow and air temperature distribution in the occupied region of an underfloor ventilation system

Measurements were carried out in an office-type experimental room ventilated by a floor return-type underfloor ventilation system to investigate the distributions of airflow velocity and air temperature. A fan-powered floor air unit (FAU) with rectangular supply and return air outlets covered by straight-profile linear bar-type air diffusers was installed to deliver the conditioned air in the experimental room. Turbulence intensity and draught rate distributions inside the room were also calculated by using the measured data. From the experimental results, it is found that undesirable high air velocities and high draught rates were created within a small region near the supply outlet of the FAU. Temperature differences between different height levels were maintained within an acceptable comfort level under the tested supply air conditions and heat loads. The results indicated that the temperature stratification could be maintained at an acceptable comfort level by designing the supply air conditions properly. A clearance zone is suggested as a design consideration for locating the FAUs and occupants to avoid undesirable draught discomfort to the occupants.     

Experimental parametric study of forced and mixed convection in a passenger aircraft cabin mock-up

Forced and mixed convection has been investigated experimentally in a full scale passenger aircraft cabin mock-up. The mock-up represents a generic cabin section of the A380 upper deck. Large scale particle image velocimetry (PIV) and temperature field measurements were conducted in a cross sectional plane of the cabin mock-up. The flow fields for two different air inlet configurations were measured and analysed under isothermal and cooling conditions. Furthermore the flow rates at the different air inlet positions were varied while keeping the air exchange rate constant. Our extensive experimental parametric study demonstrates that the flow field in aircraft cabins is affected by various fluid mechanical phenomena. Interaction between the supplied air jets, negative buoyancy forces acting on these air jets and interaction of thermal plumes with the supplied air jets, all influence the flow field inside the cabin. The impact of these effects differs considerably depending on the ventilation configuration and relative mass flow settings at the supply inlets.     

Simulation and performance enhancement of the air cooling system in the valve halls of a DC/AC power converter station

The present article reports on a ventilation system that uses impinging air jets to remove a portion of the heat generated in DC/AC converter towers. Each tower is a vertical stack of electronic components used in the power conversion. Airflow from the ventilation system enters the room through inlet ports located on the ground at two opposite sides of the DC/AC converter towers. The existing ventilation system circulates sufficient airflow in terms of the total heating load; however, elevated temperatures were reported within the towers due to poor air circulation. A numerical study has been conducted to investigate steady three-dimensional (3-D) turbulent mixed (combined free and forced) convection air cooling of vertical stacks of heat-generating blocks simulating typical towers in two valve halls of a DC/AC power converter station. The simulation results include the magnitudes of the net airflows for all the inter-block gaps, the maximum temperature in each gap, and the flow structure represented by streamlines at various locations of the 3-D domain. The location of the inlet, the inlet size and aspect ratio, and the location on the tower at which the airflow was aimed were varied parametrically to improve the ventilation relative to the existing design. These results demonstrate that, for fixed inlet mass flow rate, all the towers’ heat generating surfaces can be kept under 60°C via simple modifications of the variables used in the parametric study.     

Numerical and experimental study of velocity and temperature characteristics in a ventilated enclosure with underfloor ventilation systems

Airflow and temperature distributions in an enclosure with heat sources ventilated by floor supply jets with floor or ceiling air exit vents were investigated using experimental and numerical approaches. These ventilation configurations represent the floor return or the top return underfloor ventilation systems found in real applications. Experiments and numerical simulations were performed on a full-sized environmental chamber. The results reveal that the temperature stratification in the enclosure highly depended on the thermal length scale of the floor supply jets. When the thermal length scale of the supply jet was >1, temperature stratification was minor for all tested heat densities and air distribution methods. Significant vertical temperature gradients occurred when the jet thermal length scale was <1. Changes in air distribution methods also became significant for temperature stratification at small supply jet thermal length scales. Temperature stratification also affected the terminal height of the momentum-dominant region of the vertical buoyant supply jets. The applicability of these results to underfloor ventilation design was also discussed. PRACTICAL IMPLICATIONS: In designing underfloor ventilation systems, supply jet conditions and heat load density have to be considered to avoid thermal discomfort because of excessive temperature stratifications. This study demonstrated, by both numerical simulations and experiments, that thermal length scale can be used as a design indicator to predict thermal stratifications under a floor return and a top return underfloor ventilation setting.     

Flow patterns and thermal comfort in a room with panel,floor and wall heating

Thermal comfort aspects in a room vary with different space heating methods. The main focus in this study was how different heating systems and their position affect the indoor climate in an exhaust-ventilated office under Swedish winter conditions. The heat emitters used were a high and a medium–high temperature radiator, a floor heating system and large wall heating surfaces at low temperature. Computational fluid dynamics (CFD) simulations were used to investigate possible cold draught problems, differences in vertical temperature gradients, air speed levels and energy consumption. Two office rooms with different ventilation systems and heating needs were evaluated. Both systems had high air exchange rates and cold infiltration air.     

Measurements of room air distribution in a ventilated office environment

Results of measurements of the room air flow properties in an office space environment (4.74 m × 4.87 m × 2.87 m) with and without partitions (acoustical screens) are presented. The measurements included air flow pattern, air speed, and turbulence intensity. The profiles of the mean speed and the turbulence kinetic energy at the supply air opening are also presented. The data show the effect of using acoustical screens (with various heights and a gap between the floor and the bottom of the screen) on the air flow pattern and the air flow properties within the room. These data are intended for use in the evaluation of the existing numerical and scale models, and to aid in furthering the understanding of the behaviour of room ventilation airflows.     

Experimental investigation of PCM cold storage

This article presents an experimental analysis of cooling buildings using nighttime cold accumulation in a phase change material (PCM), otherwise known as the “free-cooling principle”. Studies of the ceiling and floor free-cooling principle, as well as passive cooling, are presented. The free-cooling principle is explained and some of the types of PCMs suitable for summer cooling are listed. An experiment was conducted using paraffin with a melting point of 22 °C as the PCM to store cold during the nighttime and to cool hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time are presented for different air velocities and inlet temperatures.     

A quasi-steady state mathematical model of an integrated ground source heat pump for building space control

This paper is concerned with the development of a mathematical model, capable of describing the quasi-steady state performance of an integrated ground source heat pump, which is used for heating and cooling of an institutional building located in a Mediterranean climate. The model is structured on functional basis according to the heat pump vapour compression or primary circuit, a secondary ground loop circuit and a secondary building loop circuit. Heat pump heating and cooling capacities, as well as COP, are considered to be dependent variables and are estimated in the model using performance fitted maps. Independent variables include: compressor speed, circulation pump speeds, ground loop return temperature and building circuit return temperature. The model is validated using data from a full-scale ground source heat pump installation. The validated model is used to examine system capacity and performance sensitivity to different control optimisation strategies, including set-point control of room air temperature, room air bandwidth temperature, building loop return water temperature and building loop return bandwidth temperature.     

地铁用间接蒸发冷却器换热性能影响因素

为解决地铁站冷却塔设置难题,提出了一种采用低速电机驱动旋转布水装置的间接蒸发冷却器,在两种布置方式下,对其换热性能进行了单因素实验,并运用正交实验法对较优布置方式下影响换热器换热的因素进行了分析。结果表明:两种布置方式下,喷嘴与蒸发冷却器的间距、两组换热管束间距均存在最佳值,喷嘴双侧旋转布水优于单侧旋转布水;换热器平行气流布置且喷嘴双侧旋转布水为较优布置方式,此时,换热器换热量随喷水量、转速、空气速度、冷却水进口温度的增加以及喷水温度、空气温度的降低而增大,其中,冷却水进口温度对换热器换热影响最为显著,其他因素对其换热的影响从主到次顺序为:喷水量、空气温度、空气速度、喷水温度、转速、冷却水流量。     

水平送风工况下CRAC机组间距对机架散热的影响

本文研究架空地板数据中心CRAC水平送风方式下四种CRAC机组送风间距对机房内机架散热和穿孔地板气流分布的影响,分析四种CRAC送风间距的机房机架入口温度和通过穿孔地板的体积流量,比较四种工况的机架设备散热情况和热循环,为今后数据机房设计以及机房热管理和节能改造提供了依据。     

地板辐射供冷-置换通风的实验研究

为了研究地板辐射供冷的热工性能,测试了北京地区不同室外气温下地板辐射供冷系统的运行工况,得到了该系统的制冷量,地板表面温度,室内温度场分布等参数,并且把单独地板辐射供冷系统的运行参数与地板辐射供冷-置换通风复合式系统进行了对比,提出了将地板辐射供冷与置换通风配合用于夏季空调室内供冷除湿的新型空调方式,置换通风系统在近地面处形成干燥空气层,可有效防止夏季热湿天气在地板表面出现结露现象,并且使这种新型空调系统条件下地面与室内的换热得到强化.通过理论分析和实验研究指出这是一种舒适、节能的空调方式.     

Perception and sensitivity to horizontal turbulent air flows at the head region

This work deals with experimental investigations on human reaction to local air movements of people in global thermal comfort, performing light activity. An analysis on draught risk was developed comparing the results with previous research findings on human response to draught. The intensity of air velocity, in terms of mean value and relative turbulence, was referred to the level at which normally clothed people could perceive and feel air movements behind the neck, in global neutral thermal condition. This work provides evidence of how the exposure duration to air movements plays a fundamental role on air flow sensitivity. The human reaction to an air flow was observed to vary with exposure duration: the feeling changes in intensity while the air flow persists blowing constantly. Moreover, different reactions have been observed between female and male test persons. Although these results were observed in the typical situation of horizontal air jet flows blowing from behind, they could apply in climatically controlled environments, where air flow is supplied horizontally at low speed, and the occupants are sitting far from the inlet section.     

Impact of heat load location and strength on air flow pattern with a passive chilled beam system

A passive chilled beam is a source of natural convection, creating a flow of cold air directly into the occupied zone. Experiments were conducted in a mock-up of an office room to study the air velocities in the occupied spaces. In addition, velocity profiles are registered when underneath heat loads exist and the cool and warm air flows interact. Experimental laboratory study revealed that in the case of the underneath heat gains, even no upward plume was generated and the dummy only acted as a flow obstacle, having a significant effect on the velocity profile. Furthermore, in an actual occupied office environment, the thermal plumes and the supply air diffuser mixed effectively the whole air volume. The maximum air velocity measured was still below 0.25 m/s with the extremely high heat gain of 164 W/m². The results demonstrate that analysis methods were the interaction of convection flow and jet are not taken into account could not accurately describe air movement and draught risk in the occupied room space.