Taghi Karimipanah

Impact of occupant modelling on the prediction of airflow around occupants in a ventilated room

Abstract 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.

The confinement effects on jet kinetic momentum flux quantified by measuring the reaction force

Abstract A turbulent jet is the most important flow element in mechanical ventilation. Mixing ventilation is based on the properties of turbulent jets. By entrainment into the jet the ambient air is set into motion. For a jet supplied within a room the enclosure may affect the jet in several ways, through: 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. The jet collides with the opposing wall and the jet is transformed into a wall jet. The size of the cross sectional area relative to the supply opening will affect the flow pattern within the enclosure. One can expect the direction of the inflow (entrainment) to the jet to be affected. 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. The tests 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.