Singh, Rohitendra K (2009) A study of air flow in a network of pipes used in aspirated smoke detectors. Research Master thesis, Victoria University.

Abstract

A Very Early–warning Smoke Detection Apparatus (VESDATM) detects the earliest traces of smoke by continuously sampling the air from a designated area. Air sampling is achieved by use of a system of long pipes containing numerous small inlet orifices termed as sampling holes. The air samples are drawn to the detector by means of an aspirator. In spite of the high sensitivity of the detector, much of this advantage can be lost if the smoke transport time within the pipe network is excessive. Consequently there has been a legislation introduced by Standards such as AS 1670 and BS 5839 stating the maximum transport time to be within 60 seconds of entering that extremity of a pipe system of 200 meters aggregate length, and the suction pressure was to be no less than 25 Pascals. Once the pipe network is installed, it is impractical and often impossible to test the transport time and suction pressure drop of every sampling hole in a complex network of pipes. Therefore, a software modelling tool is required to accurately predict these parameters to 90% of measured value with high accuracy. The flow regimes within the sampling pipes proved complex, involving frequent transitions between laminar and turbulent flows due to disturbances caused to the main flow by jet flows from the sampling holes. Consequently, the published equations to determine friction factors does not predict pressure loss and transport time results to an acceptable accuracy for this thesis. Computational Fluid Dynamics simulations were carried out at various magnitudes of disturbances similar to the effects in VESDA pipe network. The data from the CFD were analysed and the results were used as a guide to develop mathematical models to calculate the friction factor in flow regimes where jet disturbances are present. The local loss coefficients of fittings such as bends and couplings were experimentally determined for all types of fittings used in VESDA pipe networks. The local loss coefficients that were determined made significant improvements in calculating pressure losses compared to the results obtained when commonly used loss coefficient values were used. The characteristics of the VESDA aspirators of all models were determined. The experiments were carefully set up to ensure the apparatus did not have any influence on the aspirator performance. Mathematical models were developed for each VESDA model. A relationship between the magnitude of disturbance and the delay it caused for the smoke to travel from one segment to the next was established. From this relationship, a new transport time mathematical model was developed. Validations of all mathematical models were carried out in different pipe configurations. In all cases the results calculated were within 90% or better compared to the measured results.

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