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TRÁVNÍČEK, P., KUKLA, R, VÍTĚZ, T., MAREČEK, J.: Experimental determination of temperatures of the inner wall of a boiler combustion chamber for the purpose of verifi cation of a CFD model. Acta univ. agric. et silvic. Mendel. Brun., 2011, LIX, No. 1, pp. 235–242 The paper focuses on the non-destructive method of determination of temperatures in the boiler combustion chamber. This method proves to be signifi cant mainly as regards CFD (Computational Fluid Dynamics) simulations of combustion processes, in case of which it is subsequently advisable to verify the data calculated using CFD so ware application with the actually measured data. Verifi cation of the method was based on usage of reference combustion equipment (130 kW) which performs combustion of a mixture of waste sawdust and shavings originating in the course of production of wooden furniture. Measuring of temperatures inside the combustion chamber is – considering mainly the high temperature values – highly demanding and requires a special type of temperature sensors. Furthermore, as regards standard operation, it is not possible to install such sensors without performing structural alterations of the boiler. Therefore, for the purpose of determination of these temperatures a special experimental device was constructed while exploiting a thermal imaging system used for monitoring of the surface temperature of outer wall of the reference boiler. Temperatures on the wall of the boiler combustion chamber were determined on the basis of data measured using the experimental device as well as data from the thermal imaging system. These values might serve for verifi cation of the respective CFD model of combustion equipment. temperature on the inner wall of a combustion chamber, combustion equipment, thermal imaging measurement, CFD model of the combustion process, heat transmission coeffi cient Verifi cation of measured data by the means of their comparison with data calculated using CFD so ware and their validation is essential in connection with creation of CFD models in various types of so ware applications. Numerous methods (Grace et al., 2004; Stamou et al., 2006) may be used for this purpose – depending on the type of matter to be resolved (Miltner et al., 2007; Lorente et al., 1996). The method presented in this paper may be exploited primarily in case of combustion equipment in case of which it is diffi cult to determine the actual temperature inside the combustion chamber, mainly due to its rather high values. Depending on the type of combustion equipment and combustion conditions, the respective temperature might reach up to 750°C (Lavric et al., 2004; Novozhilov et al., 1996). However, the temperature of the outer surface of the boiler reaches max. 100 °C and it is easily measurable. Should the outer surface temperature be known, it is also essential to know the heat transmission coeffi cient. It is a material constant which is, however, dependent on the temperature, i.e. it varies in connection with the temperature (Tomba et al., 2000). The heat transmission coeffi cient values form the content of material data sheets (value range: 20 °C–1000 °C). Nevertheless, two unknown factors 236 P. Trávníček, R. Kukla, T. Vítěz, Jan Mareček remain in the calculation – the heat fl ow rate and the surface temperature of lining inside the combustion chamber. Due to this fact it was essential to construct an experimental device using which it would be possible to determine relation between the boiler outer surface temperature and the temperature of the inner wall of a combustion chamber. Measuring of the surface temperature of the boiler outer surface was based on the method of contactless measurement using a thermal imaging system, which uses energy of infrared radiation for temperature measurement purposes. Infrared radiation is subdivided into three ranges (short-wave, medium and remote infrared range) within the wave length range between 0.78 m and 1000 m (Sakai et al., 1994). Increasing temperature of the source results in increasing of the energy of infrared radiation and the radiation maximums shi towards shorter wave lengths; for example, the maximum radiation of a human body is 9,3 m (Vaško, 1963). The applied thermographic system conducts measurements within the range of 7.5 to 13 m (wave length) and −20 °C to 500 °C (reference temperature range). Following measurement of the outer surface temperature and determination of temperature diff eren ces in the external device, each measured point is subsequently allocated to an algorithm used for calculation of the thermal fi eld of the inner wall of the combustion chamber. The aforementioned thermal fi eld may serve for verifi cation of CFD results based on simulation of combustion processes. MATERIAL AND METHODS Determination of Material Emissivity A measuring point, at which the temperature was measured using a OMEGA HH11 contact thermome ter (accuracy of temperature measurement: ± 0.1 °C), was created on the casing of the combustion equipment. The most signifi cant prerequisite was to prevent fl uctuation of temperature in the course of time. The aforementioned point was also monitored using FLIR E320 thermal camera. In case that the temperature values proved to diff er, the temperature in the thermal camera was calibrated by the means of setting up the emissivity value in the user interface of this device. The fi nal emissivity value was determined at the time when the temperature values on both the devices were balanced. Thermal Imaging Measurement In addition to emissivity of the monitored material, it is also essential (for thermal imaging measurement purposes) to measure the air temperature, air humidity and distance from the monitored object. The air temperature and humidity were measured using OMEGA RH81 thermo-hygrometre featuring the temperature measurement accuracy of ± 1°C and humidity measurement accuracy of ± 4 % (at the temperature of 25 °C and relative humidity within the range of 10–90%). The temperature and humidi ty were measured in the close vicinity of the thermal camera and combustion equipment, and the arithmetic mean was subsequently calculated on the basis of these values. The thermal screening measurement was conducted at a constant distance from the combustion equipment. Three photographs were created in the course of one hour. The distance of the camera from the combustion equipment was determined using Leica DISTOtm A5 laser EDM device (measurement accuracy: ± 1.5 mm at a distance between 0.2 and 200 m). The thermal imaging measurement as such was conducted using FLIR ThermaCAM E320 thermal camera (FOV: 25°). The average temperature of the surface was calculated using ThermaCAM QuickReport so ware in which each pixel of the video recording was allocated to one temperature value. An arithmetic mean was subsequently created on the basis of all values. Experimental Measurement The basis for the experimental device was a layer of lining material (fi re clay) and insulation layers consisting of Peril, a porous material. Both these materials are used in the reference source. This layer of material was subsequently insulated using mineral wool and placed on a resistance heater (1200 W). Temperatures of the inner and outer surfaces of the layer were read at the time interval of 1 second by two T-type thermoelectric couples (measurement range: −200 °C to 400 °C) which were connected to ADAM, an external data collection module. The typi cal deviation was ± 1 °C while the maximum deviation was ± 1.5 °C. Measured data were subsequently evaluated using ControlWeb so ware. For a picture of the experimental device see Fig. 1. Calculation A network of 188 elementary areas was created on the photograph from thermal screening. As regards each of the elementary areas, ThermaCAM QuickReport so ware was used to determine the respective average temperature. The surface temperature of the inner section of the combustion chamber pertaining to each respective elementary area was subsequently calculated using the following formula derived from the experimental measurement:  t2  ln      22.256  t1 =  [ ̊C], (1) 0,0019 where: t1 .... surface temperature of inner part of combustion chamber t2 .... surface temperature of outer part of boiler. This formula is valid for boiler surface temperatures in the range between 23 °C and ∞. In case of calculation of a temperature lower than 23 °C, the following formula is to be used: Experimental determination of temperatures of the inner wall of a boiler combustion chamber for the purpose 237 t1 = 504.92ln(t2) − 1553.8 [ ̊C], (2) where: t1 .... surface temperature of inner part of combustion chamber,S t2 .... surface temperature of outer part of boiler. RESULTS AND DISCUSSION Thermal Imaging Measurement For an overview of measured limit conditions that are essential for correct evaluation of the thermal imaging measurement see Tab I. Based on the thermal imaging measurement it was determined that the average temperature of the monitored area was 60.9 °C. The maximum temperature in the area was 90.9 °C and the minimum temperature in the area was 16.4 °C. The minimum temperature in the area was lower than the air temperature due to the fact that a small part of the boiler surface was cooled by the fl oor. However, this anoma ly did not aff ect any additional calculations. For the temperature distribution fi eld see Fig. 2. Experimental Measurement The heat transmission coeffi cient dependence on the temperature was converted into a chart on the basis of the data sheets pertaining to individual materials used for lining of the boiler. For the heat transmission coeffi cient dependence on the tempera ture in case of fi re clay see Fig. 3; for the heat transmission coeffi cient dependence on the tempera ture in case of the insulation material (Peril) see Fig. 4. Both the fi gures documents the fact that the function characterizing the r

Experimental determination of temperatures of the inner wall of a boiler combustion chamber for the purpose of verification of a CFD model