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Experimental validation of simulations on flow through venturis and diffusers

Drenth, B. (2016) Experimental validation of simulations on flow through venturis and diffusers.

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Abstract:The R&D department of Bosch Thermotechnology B.V. in Deventer focuses its research on wall mounted boilers. Wall mounted boilers are used in houses for central heating and the supply of hot water. Heat is generated by burning a mixture of air and gas. The hot gases pass a heat exchanger with metal pins which transfer the heat from the hot gases to the water. Mixing of the air and gas is done in a so called venturi pipe. A venturi is a section of pipe with a contraction. The velocity in this contraction is increased, which causes the static pressure to drop. This section of low pressure can be used to suck in the gas. The venturi will also ensure the right ratio of gas to air which is needed to ensure safe combustion and the right amount of emissions. Venturi pipes will induce a certain amount of pressure loss, caused mostly in the diverging part of the venturi. The EHC5 department in Deventer does simulations on the flow through venturis to try and minimize the amount of pressure loss and optimize venturis. Minimizing the amount of pressure loss will increase the modulation range of the boiler. The goal of this research is to validate these simulations by experiment by performing pressure measurements on diffusers and venturis. A number of experiments will be performed to make sure the experimental set-up is reliable. The main set-up will consist of a fan to create an airflow and an orifice meter to measure the amount of mass flow. The challenge is to create a fully developed flow in the test section. This requires a certain amount of entry length of pipe. The accuracy of the pressure measurement will first be tested by measuring the pressure distribution on a section of PVC and copper pipe. Results showed variations in the pressure drop over small sections of pipe, which were accredited to defects in the manufacturing of the pressure taps. During the next experiments the pressure distribution was measured over a set of diffusers with and without gas inlet and with different diverging angles. For these experiments better care was taken in the manufacturing of the static pressure taps. Results of the pressure distribution of the diffusers without a gas inlet showed good agreement with simulations. The only large difference in static pressure between measurements and simulations occurred at the entry of the diffusers. This difference was due to a deviation in the diameter of the entry pipe, which caused a higher velocity and thus lower static pressure than expected. When the gas inlet was open, the results of the measurements showed large deviations with respect to the simulations. Simulations predicted a larger amount of pressure loss than observed during the measurements. For the final experiment a WB6 venturi was used. The measured static pressure was different from the theoretical value, as a far lower static pressure was observed in the throat of the venturi. Also, the amount of pressure loss was larger than expected.
Item Type:Internship Report (Master)
Bosch TT, the Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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