OCR Text |
Show (sensing diameter 2 mm). Atmospheric temperature in the furnace was measured by a sheathed K type thermocouple {O.D. 1.6mm) located above the work (about 100mm) . It was set approximately at the center of the working space. The intake temperature was measured at the inlet of the blower, and this value was considerd the fresh air temperature. Also, in order to estimate the radiant heat transfer rate to the work, wall temperature was measured on every visible wall from the work by K type thermocouples {O.D. 0.5mm). For the indirectly heating furnace, we examined the heat response of the furnace system during a transient heating process from room temperature (288-ftT) up to a target temperature (1173/0- Uniformity of furnace temperature was confirmed before each trial. Two burners were controlled to drive at maximum input until the furnace temperature reached the target temperature. In this condition, the fuel gas flow rate, the supplying pressure and the air ratio were set 2.0 m3//i, 196lPa and 1.2 respectively (higher calorific value was 25.6/cV^, lower calorific value was 22>.\kW). In order to measure the transient work temperature, a carbon steel brick was set in the furnace working space ( 500W x 500.D x 500// mm), and its temperature was measured by a sheathed K type of thermocouple {O.D. 1.6mm). At the exit of the radiant tube, the flue gas flow rate and temperature were monitored using a sheathed K type thermocouple {O.D. 1.6mm) and a Pitot tube (2mm in sensing diameter ). Data from each type of furnace were recorded simultaneously at a sampling rate 1 Hz using a data logger (ADVANTEST-R7326B), and were processed by a personal computer. 2.4 CFD model To predict the transient heat performance of indirect heat furnaces, a commercial C F D code FLUENT/UNS was used. The standard k - e model was used as a turbulence model and D S M for radiative heat transfer. Combustion of the fuel gas was not calculated. Alternatively, heat input of combustion was modeled by hot air flows in which air has quantities of heat equivalent to the real flue gas. The furnace model include properties of the furnace elements to the extent possible. In particular, because of temperature dependencies of the thermal properties of the furnace elements strongly affect predicted furnace performance, thermal properties such as specific heat and thermal conductivity were approximated by appropriate polynomials as functions of the temperature in order to improve the accuracy of prediction. |