- Author
- Ingason, H.
- Title
- Thermal Response Models for Glass Bulb Sprinklers: An Experimental and Theoretical Analysis. BRANDFORSK Project 618-911.
- Coporate
- Swedish National Testing and Research Institute, Boras, Sweden
- Report
- SP REPORT 1992:12; CIB W14/92/18 (S), 1992, 110 p.
- Keywords
- sprinklers | thermal analysis | wind tunnels | tests | laser doppler velocimetry | sprinkler heads
- Identifiers
- wind shadow-effects; glass bulb sprinkler; plunge test; ramp test; response parameter; growing fire
- Abstract
- Thermal response models for glass bulb sprinklers have been studied considering the following thermal response parameters; the Rate of Time Index (RTI), the Conduction parameter (C) and the Change of Phase Parameter (CHP). The RTI parameter reflects the thermal time constant of the glass bulb and the C parameter the heat conduction loss to the sprinkler fitting. The CHP parameter is thought to be a factor accounting for a time delay shortly before activation caused by an endothermic phase change of the operating medium or work needed to shatter the glass bulb. Combinations of one, two and three parameters have been used to predict response times for various plunge and ramp test conditions, as well as for a wide range of growing fire conditions. These calculations indicate that the test results can not be fully explained in the framework of the thermal response models applied. Firstly, consistently different C values were obtained for ramp tests with different rates of temperature rise. Secondly, tests with precondition temperature of the sprinkler close to the operation temperature resulted in higher RTI values compared with cases with normal precondition temperatures. A thermal analysis with the finite element computer program SUPER-TASEF shows that a plausible reason for the different C values is uneven heating of the sprinkler frame arms during different ramp tests while the higher RTI values can be attributed to a time delay caused by temperature gradients within the glass bulb. This indicates that the CHP parameter found necessary to explain some earlier test data in terms of endothermic phase change or work needed to shatter the glass bulb, more likely reflects an affect of thermal time delay. Calculated heat release rate at sprinkler response in a growning fire condition was used to evaluate the thermal response models. For the fast, medium and slow fire growth rates the heat release rates at sprinkler response were found to be similar, whether based on calculations with two parameters (RTI and C) or with all three parameters (RTI, C, CHP). However, when using only the RTI value, the predicted rate of heat release at sprinkler response increased significantly. The velocity around the frame of one of the tested sprinklers has been measured at different orientations using Laser Doppler Velocimetry (LDV). The velocity has also been calculated with a field model code and a comparision between measured and calculated velocities show good agreement. Calculated convective heat transfer to the sprinkler bulb with the frame arms orientated parallel with the air flow indicate that the heat transfer varied with the square root of the velocity as assumed in the original RTI theory. The RTI theory is therefore deemed to be valid for the simulated sprinkler irrespective of the orientation but depdneing on the orientation.