Modelling of cowl performance in building simulation tools using experimental data and computational fluid dynamics

Abstract

Exhaust cowls are used in conjunction with hybrid ventilation systems to efficiently convert wind energy into negative pressure and thus minimize the electrical energy required by the extract fan. Yet the fact that cowl performance is largely dictated by operating conditions imposes particularly stringent demands on modelling. This paper demonstrates, by way of a concrete example, the need for and potential benefits of a new methodological approach to the modelling of cowls. The study focuses on a specific modelling strategy, applied within a building simulation program, for a cowl used in a hybrid ventilation system. The method is progressively simplified to produce four variants, which chiefly vary according to their level of detail and, hence, the associated modelling effort. Wind pressure coefficients at facade, above roof and in the cowl are needed for all model variants. Some of the investigated variants rely on CFD computations of airflow around the building to determine these values. This study uses the example of a single-family house (SFH) to identify those criteria requiring particular attention in the performance of CFD numerical flow analyses. All four variants are examined on the basis of this example to determine which simplifications to the model are appropriate and permissible without unduly compromising the accuracy of the results.

Introduction

Ventilation losses and fan operation account for almost 10% of total energy use in the EU. Hybrid ventilation systems are expected to yield significant energy savings in the long term. A hybrid ventilation system is defined as a two-mode system that is controlled to minimize energy consumption while maintaining acceptable indoor air quality and thermal comfort. The two modes refer to natural and mechanical driving forces.

In order to implement hybrid ventilation in the residential sector, a fuller knowledge of the working principles and techniques of these systems is vital. The RESHYVENT project [1], which ran from January 2002 to December 2004 as part of the EU Fifth Framework Programme, focused on the investigation and development of demand-controlled hybrid ventilation systems in residential buildings. The cluster project brought together four industrial consortia with a multidisciplinary scientific group. Each of the industrial consortia had developed, constructed and evaluated complete prototypes of a hybrid ventilation system for a specific climate. A multidisciplinary group comprising 12 partners from research institutes, consultancy companies and universities carried out the scientific research for the development of these systems.

Roof cowls are used to shelter the ventilation exhaust duct from rain- and wind-induced flow reversal. Wind acting on the end of a cylindrical duct produces a suction effect through the locally accelerated airflow around the duct. This effect can be more or less enhanced by the roof cowl, depending on its shape.

One of the many technical issues warranting further investigation and examined in this project was the suction performance of exhaust duct cowls for hybrid ventilation systems. As hybrid ventilation systems operate with low driving pressures, the pressure augmentation and suction performance of the cowl must be determined more carefully than for purely mechanical exhaust systems.

The ventilation system design tools presently developed by Standards Committee CEN/TC 156 WG7 consider the suction effect of cowls in terms of their continuous and monotonous pressure difference curves. This approach has not, however, been shown to be adequately sophisticated for the dimensioning and for performance assessments of hybrid ventilation systems using transient building and ventilation system computer simulations. This has prompted the development of a more sophisticated approach, based on the following three elements: (a) cowl performance data, determined in wind tunnel tests in accordance with ventilation component test standards, (b) local data on wind airflow above the roof, determined by CFD, (c) an interface between CFD and the building and ventilation simulation design tool.

This paper presents four methods for determining cowl performance in function of the flow rates in the duct and the wind-induced flow field above the roof. It further describes how these methods can be integrated and used in building simulation tools. Typical airflow patterns over a pitched roof, with the associated parameters air velocity (direction, speed) and pressure coefficient, are shown for several wind directions. The paper also examines the impact of simplifications in cowl modelling on the results of hybrid ventilation system performance assessment studies.