Zhengdong Chen

An experimental investigation of a solar chimney model with uniform wall heat flux

Abstract Experiments were carried out using an experimental solar chimney model with uniform heat flux on one chimney wall with a variable chimney gap-to-height ratio between 1:15 and 2:5 and different heat flux and inclination angles. Results showed that a maximum airflow rate was achieved at an inclination angle around 45° for a 200 mm gap and 1.5 m high chimney, and the airflow rate is about 45% higher than that for a vertical chimney at otherwise identical conditions. It was found that the prediction method available in the literature can substantially overpredict the airflow rate for the chimney geometry investigated in this work, especially for vertical chimneys with large gaps. The main reason for the overprediction of airflow rate was shown due to the underestimation of the pressure losses at the chimney outlet by using loss coefficients obtained for normal forced flows.

Some examples of solution multiplicity in natural ventilation

Abstract This paper shows that under certain conditions, multiple solutions for the flow rate exist in a natural ventilation system, induced by the non-linear interaction between buoyancy and wind forces. Under certain physical simplifications, the system is governed in steady state by a non-linear algebraic equation or a system of equations. Three examples are given here: a single-zone building with two openings, a channel with two end openings, and a two-zone building with two openings in each zone. Analytical and numerical solutions are presented. It is shown that in all three cases the flow rate exhibits hysteresis. These results have significant implications for multi-zone modelling of natural ventilation and smoke spread in buildings. An experimental investigation using a small-scale model in a water tunnel confirms that two steady-state solutions exist for a single-zone building.

Buoyancy-driven displacement natural ventilation in a single-zone building with three-level openings

Abstract Theoretical solutions for thermal stratification and airflow rates are obtained for buoyancy-driven displacement ventilation in a single-zone building with three-level openings. Three displacement natural ventilation modes are identified: (I) the middle opening (MO) is below the stratification interface; (II) MO is above the stratification interface with inflow through MO; (III) MO is above the stratification interface with outflow through MO. It was found that for a wide range of building geometries, the ventilation mode is uniquely determined by the building geometry and the type of buoyancy sources. However, there are certain ranges of building geometry for which displacement ventilation may occur with either mode (I) or mode (II) depending on initial conditions. The significance of this finding is that numerical simulation packages for predicting displacement natural ventilation of buildings need to have the ability to cope with multiple solutions. It was also found that for a given ventilation mode, the location of the stratification interface is a function of the geometrical parameters only and independent of the strength of buoyancy sources. Some general guidelines for optimising natural ventilation in a building with three-level openings are provided.

Experimental modelling of buoyancy-driven flows in buildings using a fine-bubble technique

Abstract A new technique has been developed for demonstration and experimental modelling of buoyancy-driven ventilation airflows in buildings by using electrolytically generated fine hydrogen bubbles. Experiments for displacement natural ventilation in a single-zone building induced by two types of buoyancy sources, i.e. a point source and a line source, showed that the ventilation and stratification phenomena are successfully modelled by the fine-bubble technique. The experimental results for stratification interfacial positions are in good agreement with both the experimental data and theoretical predictions available in the literature. An analysis has also been carried out on similarities between the natural ventilation flows due to temperature difference and that due to concentration difference.

Fine bubble modelling of smoke flows

This short note presents a simple experimental modelling technique for smoke spread and movement in enclosures and sloping tunnels by using fine hydrogen bubbles, which are generated by electrolysis in a water tank. Results obtained from two independent experiments agree very well with each other, demonstrating the consistency of the new experimental method. The experimental results for the stratification interface height are also in good agreement with both the experimental data and theoretical predictions available in the literature. The developed method is then used to study smoke spread in a tunnel fire, and the effect of tunnel slopes on smoke movement is visualised.