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  • 1.
    Blomqvist, Claes
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Air Movements through Horizontal Openings in Buildings: A Model Study2004In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, no 1, p. 1-10Article in journal (Refereed)
    Abstract [en]

    A building contains a number of large openings like doors and staircases. When the temperature of the spaces connected by these openings differs, the difference in density will cause air movements through them. Horizontal air movements through vertical openings in buildings like doors and windows are well investigated while studies of air movements through horizontal openings like stairwells are less frequent and therefore this work is focusing on this case.

    The paper reports on an experimental study of the possibility of using buoyancy forces to distribute air and heat through horizontal openings. The experiments have been carried out in a scale model with water as the operating fluid.

    The result of the study shows that the flow rate through a horizontal opening is roughly half of the flow rate through a vertical opening for the same conditions, probably caused by the more complex flow pattern in the horizontal opening. A staircase below the horizontal opening will guide the flow somewhat and will cause a small increase of the fluid exchange through the opening.

  • 2.
    Cehlin, Mathias
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building Engineering, Energy Systems and Sustainability Science, Energy Systems and Building Technology.
    Mapping tracer gas concentrations using a modified Low Third Derivative method: numerical study2019In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 18, no 2, p. 136-151Article in journal (Refereed)
    Abstract [en]

    In indoor applications, computed tomography is the process of transforming a network of intersecting attenuation measurements into a spatially resolved two-dimensional concentration map. In this study the Low Third Derivative method (LTD) was numerically evaluated and optimized for different conditions. A modified version of the LTD algorithm (LTDm) was proposed and evaluated against the original version. Eight test maps were reconstructed under different conditions, such as weight ratio, pixel resolution, beam density and measurement noise. Performance of both LTD algorithms was found to be intimately related to the number of peaks and complexity in the test map and the steepness of the peaks. The LTDm algorithm improved the quality, especially for concentration maps including steep gradients and regions with very low concentrations. The LTDm method heavily lessened aliasing distortions and efficiently minimized the effects of noise.

  • 3.
    Cehlin, Mathias
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Computed tomography for indoor application2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, no 4, p. 349-364Article in journal (Refereed)
    Abstract [en]

    This paper deals with tomographic techniques for two-dimensional spatially resolved concentration measurements indoors. This represents a significant advance over the traditional point measuring method for mapping tracer gas and pollutants. Methods for recording of data are stressed as well as different types of tomographic reconstruction algorithms such as the Smooth Basis Function Minimization (SBFM) and the modified Low Third Derivative (LTDm) methods. Among the reconstruction algorithms available today, SBFM and LTDm are among the most promising. These algorithms show potential for reconstruction of gas concentration in rooms, since they are regularized to converge towards smooth concentration distributions. Using the LTD method and ‘snapshot’ configuration enables the examination and real-time monitoring of transient flows.

  • 4.
    Cehlin, Mathias
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för energi- och maskinteknik.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Computed Tomography for Indoor Applications2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, no 4, p. 349-364Article in journal (Other academic)
    Abstract [en]

    This paper deals with tomographic techniques for two-dimensional spatially resolved concentration measurements indoors. This represents a significant advance over the traditional point measuring method for mapping tracer gas and pollutants. Methods for recording of data are stressed as well as different types of tomographic reconstruction algorithms such as the Smooth Basis Function Minimization (SBFM) and the modified Low Third Derivative (LTDm) methods. Among the reconstruction algorithms available today, SBFM and LTDm are among the most promising. These algorithms show potential for reconstruction of gas concentration in rooms, since they are regularized to converge towards smooth concentration distributions. Using the LTD method and ‘snapshot’ configuration enables the examination and real-time monitoring of transient flows.

  • 5.
    Claesson, L
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Etheridge, D
    Unsteady Flow Reversal in Natural Ventilation Stack: Model Scale Tests2005In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, no 1, p. 25-36Article in journal (Refereed)
    Abstract [en]

    Measurements of the unsteady flow in a ventilation stack of a school have been made at model scale in an environmental wind tunnel. The results confirm the expectation that the stack is well-sited, in that it was difficult to induce flow reversal except over a narrow range of wind directions and the amount of reversal was small (reversal percentage < 5 %). The results also show that the technique is capable of reliably detecting such small reversal percentages.

  • 6.
    Hayati, Abolfazl
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building Engineering, Energy Systems and Sustainability Science, Energy Systems and Building Technology.
    Mattsson, Magnus
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building Engineering, Energy Systems and Sustainability Science, Energy Systems and Building Technology.
    Sandberg, Mats
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building Engineering, Energy Systems and Sustainability Science, Energy Systems and Building Technology.
    A wind tunnel study of wind-driven airing through open doors2019In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, p. 113-135Article in journal (Refereed)
    Abstract [en]

    Temporarily enhanced natural ventilation of indoor environments can be achieved by opening windows and/or doors, i.e. airing. In this study, wind driven airing rate through doors was measured by tracer gas at a building model in a wind tunnel. Both single opening and cross flow airing was investigated, with doors placed in centrally on the long side of an elongated building model. It was found that cross flow airing yielded 4–20 times higher airing rate than single opening airing; lowest value occurring with opening surfaces perpendicular to wind direction. At single opening airing, windward positioned door yielded about 53% higher airing rate than leeward positioned. Inclusion of a draught lobby (extended entrance space) lowered airing rate by 27%, while higher wind turbulence increased it by 38%. Advection through turbulence appeared a more important airing mechanism than pumping. At cross flow, however, turbulence and draught lobby had practically no effect.

  • 7.
    Heiselberg, P.
    et al.
    Hybrid Ventilation Centre, Department of Civil Engineering, Aalborg University, Aalborg, Denmark.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Evaluation of Discharge Coefficients for Window Openings in Wind Driven Natural Ventilation2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 5, no 1, p. 43-52Article in journal (Refereed)
    Abstract [en]

    This paper describes the classical approach for calculation of wind driven airflow through large openings in buildings and discusses the fulfilment of the limiting assumptions. It is demonstrated that the limiting assumptions are not fulfilled for large openings in buildings for cross ventilation, and therefore, the classical approach is not appropriate for prediction of airflow through large openings in buildings in the cross ventilation case. Using the approach for real openings and estimating the discharge coefficient for window openings has also not been very successful. The discharge coefficient cannot be regarded as a constant and it is very difficult to estimate correct values resulting in less accuracy of prediction of natural ventilation.

  • 8.
    Iqbal, Ahsan
    et al.
    Danish Building Research Institute, Aalborg University, Copenhagen, Denmark .
    Wigö, Hans
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system.
    Heiselberg, Per
    Indoor Environmental Engineering, Aalborg University, Aalborg, Denmark.
    Afshari, Aliresa
    Danish Building Research Institute, Aalborg University, Copenhagen, Denmark .
    Effect of opening the sash of a centre-pivot roof window on wind pressure coefficients2014In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 13, no 3, p. 273-284Article in journal (Refereed)
    Abstract [en]

    This paper describes the effect of outward opening the sash of a window on local and overall windpressures. Wind tunnel experiments were used for the purpose of evaluation. A centre-pivot roofwindow on a pitched roof in a modelled scaled building was used in the analysis of wind pressures.The wind pressures were defined in terms of wind pressure coefficients. Traditionally, wind pressurecoefficients are extracted from the analysis of a sealed plane surface. These wind pressurecoefficients are used to estimate the natural ventilation rate through windows/openings due to windeffect. Surface averaged wind pressure coefficients do not accurately estimate the airflow rates. Therefore, local wind pressure coefficients are needed, especially for dynamic calculation of airflow rates. From the wind tunnel experiments, it is concluded that outward opening the sash can significantly affect the wind pressure distribution near to the window. The use of wind pressurecoefficients from the analysis of a sealed plane surface may lead to erroneous estimation of airflow rate.

  • 9.
    Janbakhsh, Setareh
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy engineering. Linköping University, Department of Management and Engineering, Division of Energy Systems, Sweden.
    Moshfegh, Bahram
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy engineering. Linköping University, Department of Management and Engineering, Division of Energy Systems, Sweden.
    Ghahremanian, Shahriar
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy engineering. Linköping University, Department of Management and Engineering, Division of Energy Systems, Sweden.
    A Newly Designed Supply Diffuser for Industrial Premises2010In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 9, no 1, p. 59-67Article in journal (Refereed)
    Abstract [en]

    The results of this investigation revealed the airflow distribution from a new design of supply diffuser under non-isothermal conditions. To illustrate the indoor climate parameters in the occupied zone, for both the heating and cooling seasons, an experimental investigation was carried out in industrial premises. The indoor climate was explored at ankle, waist and neck levels for a standing person at different positions, to determine the variation of the thermal comfort indexes and draught rating (DR) with position in the facility. The observed PPD and DR values indicate acceptable levels of thermal comfort in the facility for both summer and winter cases. The conclusion can be drawn that well-distributed airflow saves energy by removing the need for an additional heating and cooling systems during cold and hot weather seasons.

  • 10.
    Kabanshi, Alan
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system. Center for the Built Environment, University of California, Berkeley, Berkeley CA, USA.
    Wigö, Hans
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system.
    Keus van de Poll, Marijke
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Environmental psychology.
    Ljung, Robert
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Environmental psychology.
    Sörqvist, Patrik
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Environmental psychology.
    The influence of heat, air jet cooling and noise on performance in classrooms2015In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 14, no 3, p. 321-332Article in journal (Refereed)
    Abstract [en]

    The quality of indoor environments influences satisfaction, health, and work performance of the occupants. Additional understanding of the theoretical and practical value of individual indoor parameters in relation to health and performance aids indoor climate designers to obtain desired outcomes. This also results in expenditure savings and increased revenue: health care and improved productivity. Here, we report two experiments that investigated how heat, cooling strategy and background noise influence performance in a full-scale classroom mockup setting. The results show that heat and background noise are detrimental to logic-based tasks and to writing, whilst cooling manipulations can protect performance. Implications for indoor environment design are discussed.

  • 11.
    Kobayashi, T
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Sakara, K
    Yamanaka, T
    Kotani, H
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Wind driven flow through openings: analysis of the stream tube2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 4, no 4, p. 323-336Article in journal (Refereed)
    Abstract [en]

    Wind approaching a building provided with openings on the windward and leeward sides has a choice, either it flows through the openings or flows around and above the building. This choice gives rise to a dominant stream tube containing the fluid flowing through the openings. In this paper the stream tube is analysed based on wind tunnel measurements and CFD simulation. A house model with dimensions 120 mm (Width)×120 mm (Height)×180 mm (Length) was provided with rectangular openings of equal size located opposite each other. The end walls were thin giving rise to a sharp edged opening. The size of the openings expressed as the porosity (opening area divided by the façade area) was 1.3 %, 5.2 %, 11.6 %, 20.7 % and 46.5 %. In the wind tunnel, velocity including velocity fluctuations and pressure were measured along the centre line through the openings. In the CFD prediction it was possible to visualize the stream tube by the method of “flying particles”. This made it possible to explore the change in shape of the stream tube and to calculate the cross-sections of the stream tube at different positions and to know the total pressure distribution within the stream tube cross section. Finally, the discharge coefficient based on stream tube analysis was compared to that from a conventional chamber method.

  • 12.
    Kobayashi, Tomohiro
    et al.
    Department of Architectural Engineering, Osaka University, Osaka, Japan.
    Sagara, Kazunobu
    Department of Architectural Engineering, Osaka University, Osaka, Japan.
    Yamanaka, Toshio
    Department of Architectural Engineering, Osaka University, Osaka, Japan.
    Kotani, Hisahi
    Department of Architectural Engineering, Osaka University, Osaka, Japan.
    Takeda, Shogo
    Department of Architectural Engineering, Osaka University, Osaka, Japan.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Stream Tube based Analysis of Problems in Prediction of Cross-Ventilation rate2009In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 7, no 4, p. 321-334Article in journal (Refereed)
    Abstract [en]

    The airflow rate of a building ventilated by wind is usually predicted by using the wind pressure coefficients obtained for a sealed building and discharge coefficients based on measuring the airflow characteristics through an opening in a sealed chamber (chamber method). This can result in the underestimation of wind driven flow through large openings located on opposite sides of a room. In this paper, the discharge coefficient, based on the chamber method, and the actual condition of cross-ventilation are calculated and compared with each other by means of stream tube analysis. The driving pressure based on wind pressure coefficients obtained from a sealed building are also compared with those based on pressures inside the stream tube of the actual flow field representing a porous rather than sealed building. A building model of dimensions 120 mm (width)×120 mm (height)×180 mm (length) was used for the analyses. The size of openings, expressed as the porosity (opening area divided by façade area), was 11.6 %, 20.7 % and 46.5 %. These models were analyzed by CFD simulation and the stream tubes caught by the opening were determined. From the analysis the errors in discharge coefficient and wind pressure coefficient were identified. Finally, the flow rate based on these discharge coefficients and driving pressures were calculated and compared. It is shown that the effect of the underestimation of the discharge coefficients by the chamber method is significant for all cases of porosity studied in this paper. Moreover, it is shown that the use of wind pressure coefficients is not appropriate for the case of extremely large openings.

  • 13.
    Larsson, Ulf
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system. Division of Energy Systems, Department of Management and Engineering, Linköping University, Linköping, Sweden.
    Moshfegh, Bahram
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system. Division of Energy Systems, Department of Management and Engineering, Linköping University, Linköping, Sweden.
    Comparison of ventilation performance of three different air supply devices: a measurement study2017In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 16, no 3, p. 244-254Article in journal (Refereed)
    Abstract [en]

    People today spend a significant part of their time in an indoor environment, whether it be home, school, vehicle or workplace. This has put greater demands on indoor environment, in terms of both air quality and thermal comfort. The main objective of building ventilation is to take care of pollutants and lower their concentration, but it is also used to cool or heat indoor air. The aim of this paper was to study the behavior of three different ventilation supply devices, i.e. mixing supply device, displacement supply device and confluent jet supply device, in an office room. Different cases have been studied experimentally with different airflow rates, supply air temperature and supply devices. The results shows that mostly that we can expect, but the results show a very small difference in ventilation efficiency between the different systems and in theory there should be a larger difference. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

  • 14. Li, Y
    et al.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Hui, S
    Robustness of Air Distribution in Plenum-Based Ductless Ventilation Systems2004In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, no 2, p. 105-118Article in journal (Other academic)
    Abstract [en]

    This paper introduces a concept of robustness of an air distribution method, which is defined as being capable of meeting the ventilation requirements during varying operational conditions. The robustness performance may be particularly important when the system allows individual control of the supply air parameters. As a preliminary example, plenum-based (ductless) air distribution methods are studied using computational fluid dynamics. Among the four basic air distribution methods in plenum-based systems, it is found that the floor supply and ceiling return system does not always produce the conventional displacement ventilation system performance when the heat sources are not concentrated. The ceiling supply with ceiling return produces the best robustness performance over a wide range of supply velocities.

  • 15.
    Nyman, Hans
    et al.
    Mälardalen University.
    Sandberg, Mats
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Building science - installation technology.
    The Influence of External Wind in Tunnels2011In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 10, no 1, p. 31-47Article in journal (Refereed)
    Abstract [en]

    A model tunnel (approximately ten hydraulic diameters) with different designs of the tunnel mouth has been placed in a wind tunnel and has been subjected to the effects of external wind by varying the wind direction at the mouth of the tunnel. In the experimental oriented study pressures have been measured and the airflow has been made visible with smoke and by the sand erosion method (semolina). The relation between the flow ratio and the direction of the wind has been explored. When the wind is blowing parallel to the tunnel, the tunnel flow is about 70% of the reference flow (the undisturbed flow due to the wind through an area corresponding to the tunnel cross section). This result holds for the length of this tunnel. For longer tunnels it will decrease due to increased friction. When the angle at which the wind is blowing increases, the tunnel flow decreases. The pressure measurements made it possible to quantify some of the phenomena which were observed in the visual trials. A large under-pressure was measured just outside the mouth of the model tunnel on the side of the tunnel corresponding to the separation in the visual trials. In front of the tunnel, a pressure increase due to the braking of the airflow was measured. The position of the pressure increase moved depending on the internal resistance in the tunnel. If the resistance in the model tunnel was high, the over-pressure in front of the mouth of the tunnel was higher and further from the opening. When the internal resistance was reduced (corresponding to e.g. a very short tunnel) the over-pressure decreased and moved closer to the tunnel opening.

  • 16.
    Sandberg, Mats
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    An alternative View on the Theory of Cross Ventilation2004In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 2, no 4, p. 409-418Article in journal (Other academic)
    Abstract [en]

    The mechanism of cross ventilation is dealt with in this paper. The results are obtained by a combination of wind tunnel studies and CFD predictions using a Reynolds stress model as the turbulence model. All buildings have been exposed to a uniform velocity field and therefore the reference flow rate for an opening is equal to the velocity multiplied by the opening area. The openings were located at or close to the position of the stagnation point on the corresponding sealed building. The view taken in the paper is that the flow is generated by an interaction between the atmospheric boundary layer flow approaching the building and the building itself. The process starts with the undisturbed boundary layer flow and ends when the boundary layer has been fully restored. The paper begins with scrutinising the basic assumptions behind the orifice equation, the most common model for relating a pressure differential to a flow. This pressure differential is usually taken from the pressure coefficients recorded on a sealed building. The applicability of pressure coefficients from sealed bodies, as a way to predict the flow rate, is dealt with. An important difference between a sealed building and one provided with an opening is that in the latter case the flow has a choice i.e to flow through the opening or pass around the building. This choice leads to the formation of a flow tube passing through the opening. It starts in the undisturbed boundary layer, with the catchment area and, close to the building, occurs the retardment area. The size of the retardment area and it’s state (i.e. relation between static and dynamic pressure) convey information about the type of flow. A new phenomenon i.e. a catchment effect or an attractor effect has been identified, which, at certain conditions, generates a flow through an opening which is larger than the reference flow rate.

  • 17.
    Sattari, Amir
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system. KTH, Installations- och energisystem, Stockholm.
    Particle Image Velocimetry Visualization and measurement of Air Flow over a Wall-Mounted Radiator2015In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 14, no 3, p. 289-302Article in journal (Refereed)
    Abstract [en]

    A common room-heating technique involves the use of a wall-mounted radiator without forced convection. The cold surrounding air passes adjacent to the warm surfaces of the radiator where it absorbs heat and gains momentum to rise along the wall surface and finally circulate in the entire room. Understanding the properties of heated airflows is important for several purposes. To understand the flow process it is important to identify where the transition from laminar to turbulent flow occurs and to quantify the turbulent fluctuations. With the objective to characterize the airflow in the vicinity of wall surfaces, the local climate over the radiator was visualized and measured using a two-dimensional particle image velocimetry technique. The PIV technique yields 2D vector fields of the flow. The resulting vector maps were properly validated and post-processed using in-house software to provide the average streamlines and other statistical information such as standard deviation, average velocity, and covariance of the entire vector field. The results show that, for a room with a typical heating power, the airflow over the radiator becomes agitated after an ordinate of N = 5 - 6.25 over the radiator upper level, in which N is the dimensionless length based on the thickness of the radiator. Practical problems encountered in near-wall PIV measurements include generating a homogeneous global seeding that makes it possible to study both plume and entraining region, as well as optical problems due to near-surface laser reflection that makes the measurement process more complicated.

  • 18.
    Sattari, Amir
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Building science - installation technology.
    Sandberg, Mats
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Building science - installation technology.
    PIV Study of Ventilation Quality in Certain Occupied Regions of a Two-Dimensional Room Model with Rapidly Varying Flow Rates2013In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 12, no 2, p. 187-194Article in journal (Refereed)
    Abstract [en]

    The use of supply jet flows is the most common type of air distribution for general ventilation. Usually the supply flow rate is constant or slowly varying (VAV-systems) to cope with a varying load. A novel air distribution method, with the potential to reduce stagnation and to increase the ventilation efficiency, is to introduce rapid flow variations (pulsations). This paper reports on a fundamental study of this type of air distribution. The purpose of the study was to explore the effect of flow variations on stagnant zones and the levels of the turbulent kinetic energy and the relative turbulence intensity. A small scale room model is used that consists of an enclosure with a ventilation supply at the bottom and an extract at the top of the opposite wall. Water was used as an operating fluid and the model had a design which mainly generated a two-dimensional flow. The size of the model made it possible to investigate the two-dimensional velocity vector field using the Particle Image Velocimetry (PIV) method in regions corresponding to occupied regions. Further post processing was conducted from the resulting vector fields. The comparison between cases of constant inflow and pulsated inflow (flow variations with frequency of 0.5 Hz) was conducted for three domains: two belonging to the far-field occupied zone and one belonging to the near-field, downstream of the supply wall jet.

  • 19.
    Shakeri, Amid
    et al.
    Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC, Canada .
    Dolatabadi, Ali
    Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC, Canada .
    Haghighat, Fariborz
    Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, QC, Canada .
    Karimipanah, Taghi
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Impact of occupant modelling on the prediction of airflow around occupants in a ventilated room2007In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 6, no 2, p. 129-144Article in journal (Refereed)
    Abstract [en]

    Localized ventilation systems typically create highly asymmetric or non-isothermal environments around occupants with significant vertical temperature gradient and highly non-uniform airflow regimes that could be directed toward a segment of the body. These effects may have pronounced impact on occupant's thermal comfort. The airflow field and temperature distribution near the occupant can be determined either by performing full-scale measurements or by simulation methods. Usually, human subjects or manikins are used in field studies involving measurement techniques. However, as an alternative to full-scale measurement, Computational Fluid Dynamics (CFD) has been proven to be a practical and valuable tool for predicting the airflow field. At the same time, the accuracy of the predictions of the local airflow within the microclimate of the occupant is highly dependent on the proper modelling of the occupant itself. The human body not only has a complicated physical shape, but also has complex thermo-physiological properties. Modelling of all these aspects is a formidable challenge and an extremely time-consuming task. Therefore, various simplifications have been made in order to decrease the level of complexity so that the computation may be performed with the available computer resources. This paper reports the results of a detail numerical simulation to study the impact of occupant modelling on the airflow and temperature distribution and their influences on the occupant's thermal comfort. First, the predictions made by the CFD model were compared with experimental data that were measured in a specially designed experimental chamber. Good agreement was observed. Four type of configuration were used to model the occupant: a conventional block form, three-node, six-node and finally eight-node configurations. Further simulations were carried out to investigate the assumption of uniform heat distribution. An assessment of uniform and non-uniform heat distribution scenarios for various occupant configurations and ventilation systems showed that the assumption of uniform heat distribution is valid for a wide range of operating conditions.

  • 20.
    Stymne, Hans
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Emenius, Gunnel
    Boman, CarlAxel
    Passive Tracer Gas Measurement of the Long Term Variation of Ventilation in Three Swedish Dwellings2006In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 5, no 3, p. 333-343Article in journal (Refereed)
    Abstract [en]

    The objective of this study is to investigate how measured ventilation rates in dwellings vary over the heating season in a Nordic climate. The aim is to draw conclusions about the possibility to transform a measurement result obtained during a relatively short period of time into one which would have been expected as an average over a whole season. If such normalisation of measurement data is not possible, dwellings may be misclassified as under- or over-ventilated, a matter which may dilute a possible relationship between health and air quality in epidemiological studies.

    Passive tracer gas measurements of ventilation were performed in a detached single-family house and in a flat (apartment) in Stockholm during four consecutive winter seasons. Measured averages of air change rate data are reported for 47 two-week periods for those two naturally ventilated dwellings. Another measurement using two different tracer gases was performed in an airtight, extract ventilated detached house over one year. The variation of ventilation is discussed in terms of variation in the ventilation driving forces induced by inside-outside temperature differences. The naturally ventilated house shows a slightly better correlation between air change rate and indoor-outside temperature difference than the town flat. It is concluded that the correlations are not good enough for predictive use for either dwelling. Therefore it does not seem possible to “normalise” ventilation measurement data. A slightly better possibility to predict the weather influence exists for the airtight, extract ventilated house. A possible reason for the lack of good correlation between air change rate and natural driving forces is a highly variable influence from occupant behaviour.

  • 21.
    Taghi, Karimipanah
    et al.
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system.
    Sandberg, Mats
    The confinement effects on jet kinetic momentum flux quantified by measuring the reaction force2014In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 13, no 3, p. 285-298Article in journal (Refereed)
    Abstract [en]

    A turbulent jet is the most important flow element in mechanical ventilation. Mixing ventilation is basedon the properties of turbulent jets. By entrainment into the jet the ambient air is set into motion. For ajet supplied within a room the enclosure may affect the jet in several ways, through: a) Coanda effect which is the tendency of a fluid to be attracted to a nearby surface. A free jet is turned into a wall jet and the momentum flux of the jet decreases by friction against the room surfaces.b) The jet collides with the opposing wall and the jet is transformed into a wall jet. c) The size of the cross sectional area relative to the supply opening will affect the flow pattern withinthe enclosure. One can expect the direction of the inflow (entrainment) to the jet to be affected. d) Location of supply and extract. The location of the supply is a factor that influences the pressure gradient within the room. This paper considers the items b), c) and d). The main characteristic of a jet is its momentum flux, but determining the momentum flux is not an easy task and has lead to contradicting results. Standard methods require velocity field measurements which have their restrictions and uncertainties. To overcome these problems a direct and more reliable method was used by recording the flow force, caused by an impinging jet, with a digital balance. Thetests were carried out both for unenclosed (free jet) and enclosed cases. In the latter case tests were conducted with supply and extract both located on the same wall and located on opposite walls. Detailed pressure measurements were conducted to describe the details of the reaction force. There was a clear effect of the confinement on the reaction force and a Reynolds number dependence.

  • 22.
    Wigö, Hans
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Building science - installation technology.
    Effects of intermittent air velocity on thermal and draught perception: A field study in a school environment2013In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 12, no 3, p. 249-255Article in journal (Refereed)
    Abstract [en]

    Air movement in an indoor space may be experienced in very different ways. For persons feeling cool, air movement tends to be perceived as draught, whilst when feeling warm air movement may provide a desired cooling effect. In the transition zone it therefore seems difficult to use constant air velocity as a tool for cooling without creating draught problems. One possible way to use air movement as a method to improve thermal comfort, without resultant draught problems, could be to use intermittent air velocity instead of constant velocity. This new cooling method was implemented in a high school in Sweden and evaluated during spring (April) and autumn (September). The present paper reports results from two field experiments where subjects were exposed to velocity variations. The analysis shows significant effects of velocity condition on thermal comfort and air quality. In summary, people exposed to velocity variation perceived the air as cooler and fresher compared with those exposed to constant low velocity and very few classified the air movement as draught. A further conclusion is that even the pupils who were exposed to velocity variation wanted slightly more air movement.

  • 23.
    Wigö, Hans
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Effects of intermittent air velocity on thermal and draught perception during transient temperature conditions2008In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 7, no 1, p. 59-66Article in journal (Refereed)
    Abstract [en]

    Previous research has shown that air movement has a significant influence on humans' thermal comfort. For persons feeling cool, air movement tends to be perceived as draught, whilst when feeling warm air movements may provide a desired cooling effect. In the transition zone it therefore seems difficult to use constant air velocity as a tool for cooling without creating draught problems. Nevertheless, from an energy saving perspective it appears to be far more efficient to use enhanced convective cooling, induced by the air movement, to cool only the occupants instead of the entire building. One possible way to use air movement as a method to improve thermal comfort without resultant draught problems could be to use intermittent air velocity instead of constant velocity. The present paper reports results from three experiments where subjects have been exposed to velocity variations, showing support for the hypothesis that it is possible to cool humans and reduce the percentage of occupants who are dissatisfied with the room temperature, without creating draught problems, through intermittent cooling.

  • 24.
    Wigö, Hans
    et al.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Nilsson, Håkan O.
    University of Gävle, Department of Technology and Built Environment, Ämnesavdelningen för inomhusmiljö.
    Application of a thermal manikin to evaluate heat loss rates from people caused by variations in air velocity and air temperature2004In: The International Journal of Ventilation, ISSN 1473-3315, E-ISSN 2044-4044, Vol. 3, no 3, p. 219-225Article in journal (Refereed)
    Abstract [en]

    Heat loss monitoring from a thermal manikin was undertaken representing an occupant in a classroom during a lesson period of 80 minutes in which the room temperature was increased from 21 to 24°C for various airflow velocity configurations. A group of subjects was exposed to various conditions of temperature and airflow rate so that the impact of these variations on their surface/skin temperature could be determined. It was found that skin temperature remained stable and close to 34°C for all conditions of exposure. Thus, over the temperature and air velocity range considered, these new findings verified the suitability of using a thermal manikin, set to steady uniform surface temperature, to determine the heat loss characteristics from occupants subjected to intermittent velocity variation. When the manikin was exposed to a high velocity pulse, the heat loss from the whole body increased by 10% while the heat loss from exposed areas (hands and face) increased by 20 % (when compared to no velocity pulse). After the 80 minutes monitoring period, the total energy loss from a manikin exposed to velocity variations was 2% higher than when exposed to constant low velocity.

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