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Dust in ventilation ducts : accumulation , measurement and removal - Report A9 -

This thesis is based on studies undertaken during the period 1998-2003 at the HVAC laboratory of Helsinki University of Technology.The thesis focuses on dust accumulation in, and removal from, recently installed supply air ducts and on the bristle behaviour of rotating duct cleaning brushes. The results of dust accumulation, measured using three different methods, were compared and the amount of dust in newly installed air ducts was evaluated. The vacuum tests was found to be an efficient method of collecting dust samples on the duct surface.

Rotating brush behaviour in duct cleaning

A simple model of a rotating duct cleaning brush was presented using large deformation elastic theory. The results obtained by the model were compared with the results obtained by a laboratory test. Especially, the effect of air drag on the brush behaviour was considered.

Large defromation analysis of a rotating bristle in brush duct cleaning

Rotating brushes are commonly used in cleaning air ducts. A very simple model simulating the behaviour of a typical brush bristle is presented. The model consists, in effect, of a combination of a polynomial trial solution, the point collocation method, Simpson's integration rule and a Mathcad code. The main interest from the cleaning point of view is in determining the contact force and the contact angle between the bristle tup and the duct surface. The effect of the degree n of the polynomial trial solution - n=4, n=6, n=8 - is studied.

Modelling bristle behaviour in rotating brush duct cleaning

The bristle of a rotating cleaning brush for air ducts was modelled using large deformaion elastic theory. The point collocation method with a trial solution consisting of undertermined parameters was employed to discretize the resulting non-linear problem. The main interest was in determining the value of the bristle tip contact normal force N, the bristle tip contact angle B and the torque T needed to rotate a brush. The results obtained using the simple model were compared with the results obtained from a laboratory test.

Evaluation of Prediction Accuracy by Local Dynamic Similarity Model for Cross-Ventilation Flow Rate

The proposed local dynamic similarity model can select an adequate discharge coefficient to match the approaching flow angle. This is an improvement over the conventional orifice flow model where the discharge coefficient is set to a fixed value. The accuracy of predicting ventilation flow rates for an isolated cross-ventilation model is greatly improved when the discharge coefficients actually decrease with change of wind direction.

On the night purging of naturally ventilated enclosures - the effect of the relative area of openings

We consider the overnight evolution of an initial two-layer thermal stratification (a warmupper layer and a cooler lower layer) in an enclosure ventilated via openings at high and low levels.Results of our laboratory experiments show that an efficient displacement flow is not always established and four distinct ventilation flow regimes are observed depending on the ratio R (= at /ab ) of the upper opening area, at , and lower opening area, ab ( > 0). For a given initial stratification, displacement flow is established only if R is sufficiently small (i.e. at

CFD Modelling of Atrium-assisted Natural Ventilation

Using computational fluid dynamics (CFD) techniques to model buoyancy-driven airflows hasalways proved challenging. This work investigates CFD modelling of buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and when warmed by internal heat gains producing a column of warm air in the atrium and connect space drives a ventilation flow. Results of CFD simulations are compared with predictions of an analytical model and small-scale experiments [1].

Thermal effects on the dispersion of a gaseous contaminant in a naturally-ventilated room

The influence of thermal effects on the dispersion of a gas in a naturally-ventilated room is investigated using CFD in conjunction with measurements. The gas dispersion inside the room, with and without thermal effects, is characterised by a statistical analysis of the CFD-predicted gas concentrations at a large number of points across the room with a view to quantifying the thermal effects. It is concluded that even small temperature differences can lead to significantly different cross flow behaviour and rates of gas concentration decay at the relatively low air change rate considered.

Natural Ventilation with Pre-cooling

We investigate two different natural ventilation regimes of an auditorium or other occupied open-plan space which is equipped with pre-cooling. The room has a low-level vent and a high level vent. The occupants act as a source of heat, which if without pre-cooling, can lead to the room becoming too warm for comfort. In the first ventilation regime, a chiller is fitted to the low-level vent, and the occupants provide positive buoyancy which drives the ventilation upwards and draws fresh air through the pre-cooling system.

Numerical Evaluation of Wind Pressure Distributions of Buildings by Means of a Modified k-e Model

A modified k-e model that restricts turbulent time scale using Durbin’s limiter was applied for numerical prediction of wind pressure distributions of building models. It is known that the standard k-e model tends to overestimate wind pressure of windward

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