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Computational simulation of thermal-fluid processes in a barometric mixing unit
California State University, Northridge has installed a 1 MW Direct Fuel Cell® (DFC) power plant and the plant has been in operation since January 2007. A Heat Recovery Unit (HRU) has been designed and installed to extract the thermal energy contained in hot exhaust gases to heat up campus heating hot water and domestic hot water loops and as well as to warm up an existing campus swimming pool. In order to combine the exhaust gases and pass them through the HRU a Barometric Thermal Trap (BaTT) was designed and installed. An inline fan is installed downstream of the BaTT unit. The bottom of the BaTT is open to atmosphere and therefore ensures that pressure remains at nearly atmospheric and does not affect the operation of fuel cell units at any time. In order to determine the characteristic factors affecting the flow into and out of the BaTT and finding optimized flow conditions, a model of the BaTT was created using software suite, FLUENT®. The analysis of turbulence is based on the standard k-ε turbulent model. The results matched field measured data and showed that under slight negative atmospheric pressure -20 Pa at the top outlet of the BaTT there will be no outflow from the bottom opening. The most desirable condition for maximum heat recovery appears to be at a pressure of -40 Pa at the top outlet. Under these conditions, the net outdoor air flux into the BaTT is about 700 SCFM from the bottom outlet. Results show that heat conduction from walls of the BaTT is about 1 to 2% of total heat flux which can be improved by adding insulation. Results also showed that temperature varies from 350K to 650K within the BaTT at about -40 Pa at the top outlet pressure but it is constant at about 650K for pressures above -20 Pa at the top outlet. Pressure profiles showed a linear variation with height from zero gauge pressure at bottom to the selected pressure specified at the top outlet.