The article the influence of changes of combustion gas temperature during flow around of horizontal cylinder on local Nu number was presented. In order to test an influence of effect waste gas temperature cycle of experimental investigations were conducted. Experimental tests were carried out on a properly designed measuring cylinder furnished with a number of thermocouples embedded along the cylinder perimeter. The cylinder was made from stainless steel of known thermal conductivity, and was cooled on the outer side through a water cooling system. The cylinder was placed horizontally in a heating chamber equipped with an axially positioned gas burner fired with natural gas. Gas and air feeds were regulated with control valves, based on combustion gas analyzer data.U članku se prezentira utjecaj promjena temperature plinova izgaranja tijekom strujanja oko horizontalnog cilindra na lokalni Nusseltov broj. U svrhu ispitivanja utjecaja temperature otpadnih plinova izvedena je serija eksperimenata. Eksperimenti su izvršeni na propisno izvedenom mjernom cilindru na kojeg su po opsegu postavljeni termoparovi. Cilindar je načinjen od nehrđajućeg čelika poznate toplinske provodnosti i s vanjske strane hlađen pomoću rashladnog sustava s vodom. Cilindar je smješten horizontalno u komoru za zagrijavanje opremeljnu s aksijalno postavljenim plinskim plamenikom na prirodni plin. Dovod plina i zraka regulira se pomoću kontrolnih ventila, temeljen na podacima iz analizatora dimnih plinova

L. Szecówka

M. Górska

R. Budzik

English

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M. GÓRSKA, L. SZECÓWKA, R. BUDZIKTHE INFLUENCE OF CHANGES OFCOMBUSTION GAS TEMPERATURE DURING FLOWAROUND THE HORIZONTAL CYLINDER ON LOCAL NU NUMBERReceived – Prispjelo: 2008-01-02Accepted – Prihva}eno: 2008-10-25Preliminary Note – Prethodno priop}enjeINTRODUCTIONThe temperature of flowing combustion gas has agreat influence on the heating rate, but this is chiefly ow-ing to the increasing share of radiation in relation to con-vection 1, 2.In addition, it should be remembered that the in-crease in the temperature of flowing combustion gas in-creases its volume, which results in an increase in flowvelocity 3. By establishing the relationship betweenthe change of the temperature of combustion gas flush-ing the cylinder and the obtained Nu number, informa-tion about the effect of the velocity and temperature ofcombustion gas on the cylinder heating rate can be ob-tained 4.THE EXPERIMENTAL STANDThe heating chamber used in the tests enables thelaboratory-scale simulation of conditions close to theconditions prevailing in industrial heating units 5, 6.Before proceeding to measurements, the heating cham-ber was properly prepared (Figure 1) by installing a testcylinder and measuring equipment in the form of ther-mocouples, a radiation flux measuring device and an an-alyzer probe. Then, a gas burner was operated. Thequantity of air and gas supplied was controlled usingrotameters. The temperature of inflowing combustiongas upstream the cylinder was measured using a thermo-couple.Knowing the distribution of temperatures on theouter surface of the cylinder and on its inner waterstream-cooled surface, it was possible to calculate thedistribution of the local unit heat flux distribution as afunction of the combustion gas inflow angle. The con-trol of the combustion process, with emphasis on assur-ing a constant combustion gas velocity, was carried outusing rotameters and a secondary air orifice in the cham-ber based on the values computed by a computer pro-gram written for this purpose. In the computation, theprogram allowed for the type of gas combusted, as wellas temperature, pressure, and the volumetric fluxes ofmedia introduced, as red out from the rotameters.METALURGIJA 48 (2009) 2, 121-124 121The article the influence of changes of combustion gas temperature during flow around of horizontal cylinderon local Nu number was presented. In order to test an influence of effect waste gas temperature cycle of expe-rimental investigations were conducted. Experimental tests were carried out on a properly designed measuringcylinder furnished with a number of thermocouples embedded along the cylinder perimeter. The cylinder wasmade from stainless steel of known thermal conductivity, and was cooled on the outer side through a water co-oling system. The cylinder was placed horizontally in a heating chamber equipped with an axially positionedgas burner fired with natural gas. Gas and air feeds were regulated with control valves, based on combustiongas analyzer data.Key words: Flow around, horizontal cylinder, local Nu number.Utjecaj promjena temperature plinova izgaranja tijekom strujanja oko horizontalnog cilindra nalokalni Nusseltov broj. U ~lanku se prezentira utjecaj promjena temperature plinova izgaranja tijekom struja-nja oko horizontalnog cilindra na lokalni Nusseltov broj. U svrhu ispitivanja utjecaja temperature otpadnih plino-va izvedena je serija eksperimenata. Eksperimenti su izvr{eni na propisno izvedenom mjernom cilindru na kojegsu po opsegu postavljeni termoparovi. Cilindar je na~injen od nehr|aju}eg ~elika poznate toplinske provodnosti is vanjske strane hla|en pomo}u rashladnog sustava s vodom. Cilindar je smje{ten horizontalno u komoru za za-grijavanje opremeljnu s aksijalno postavljenim plinskim plamenikom na prirodni plin. Dovod plina i zraka regulirase pomo}u kontrolnih ventila, temeljen na podacima iz analizatora dimnih plinova.Klju~ne rije~i:strujanje oko, horizontalni cilindar, lokalni Nusseltov brojISSN 0543-5846METABK 48(2) 121-124 (2009)UDC – UDK 66.045.1:532=111M. Gorska, L. Szecowka, R. Budzik; Faculty of materials ProcessingTechnology and Applied Physics, Czêstochowa University of Technol-ogy, Czestochowia, PolandThe program also enabled the computation of the ve-locity of combustion gas in the chamber, as dependenton its temperature and the excess air number, as mea-sured with a combustion gas analyzer.THE MEASURING CYLINDER AS ANINSTRUMENT FOR THE DETERMINATIONOF THE LOCAL CONVECTIVEHEAT-TRANSFER COEFFICIENTThe measuring cylinder simulating round steelcharge was main measuring apparatus, which allowedlocal and average surface film conductance. The cylin-der was made of non-rusting steel 2H13 that was 64 mmin external diameter. The thickness of cylinder wall had10 mm (Figure 2). The measurement methodology wasbased on the application of doubling measurements. Theverifying measurement was based on the thermal energybalance. The principal measurement, on the other hand,involved taking the temperatures on the inner and theouter surfaces of the cylinder using a set of thermo-couples. In the verifying measurement, a cooling waterstream (5) was delivered to a measuring cylinder, withits temperature being measured with a flow counter (2),whereas a temperature sensor (3) installed enabled acontinuous measurement of water temperature at themeasuring device entrance. The cooling water cooledthe inner side of the cylinder body and kept the tempera-ture at a constant level. After flowing through the set oftubes, the cooling medium left the measuring cylinder,while its temperature was measured by a temperaturesensor installed at the exit (4). The knowledge of cool-ing medium temperature at the entrance and the exit, aswell as the volume flux that flowed through the measur-ing cylinder enabled, using the a thermal energy counter(1), allowed the quantity of energy being carried away tobe determined.The thermal energy counter (1), through an incorpo-rated microprocessor, performed a continuous computa-tion of the quantity of thermal energy carried away withwater, and then showed the obtained results as a meanvalue, averaged for a 3-minute time interval.Simultaneously with conducting the thermal energybalance, the cylinder surface temperature was measuredat eleven measurement points on the cylinder perimeter,using jacket-type thermocouples (7), which was assu-med as the principal measurement (Figure 2). The valueof the thermal electromotive force from the thermo-couples was transmitted to a measuring card (8), thusproviding the recording and archiving of the data on thecomputer at a frequency of approx. 200 Hz per channel(9). The measurement of the inner cylinder surface wastaken using two jacket-type NiCr-NiAl thermocouples.The results of investigation received through twomethods were compared every time after every measur-ing cycle. On these base the validation was conductedand uncertainty of method was estimated. The differ-ence between main and checking measurementamounted to 10 %.The measuring cylinder placed in the heating cham-ber is presented in the photograph below (Figure 3).RESULTS OF EXPERIMENTAL TESTSThe tests were carried out in the range of combustiongas temperature of 250÷750 °C, with the change in thistemperature having a considerable effect on the physical122 METALURGIJA 48 (2009) 2, 121-124M. GÓRSKA et al: THE INFLUENCE OF CHANGES OF COMBUSTION GAS TEMPERATURE DURING FLOW AROUND...Figure 1. Schematic diagram of the heating chamber withthe measuring cylinder and the radiation sensorinstalled. Figure 2. Schematic diagram of the measuring system forthe determination of the local convective he-at-transfer coefficient.Figure 3. View of the measuring cylinder with a set ofthermocouples and a heat counter.parameters of this combustion gas. The graphic interpre-tation of convective heat-transfer computation resultsrequires inclusion of additional information on the tem-perature and velocity, at which the measurement wastaken. Using heat transfer similarity in the form criterionnumber Nu, the test results were enabled to be utilizedand applied to other conditions of round charge heating.The summary of test results is made for three velocityranges: 0,5; 0,6 and 0,7 m/s. The average combustiongas velocity was calculated on the strength of substratecombustion and through their temperature measurementnear measuring cylinder. The investigation was exe-cuted by undisturbed flow (0 Hz) and disturbed throughpulsation range from 14 to 74 Hz.The distribution of the Nu number as a function ofthe combustion gas inflow angle and velocity is illus-trated in Figures 4 and 5.Results proved that received the value of the Nunumber strongly depends on the velocity of flowingcombustion gas. The increase in combustion gas veloc-ity from 0,5 to 0,7 m/s at some points on the cylinderperimeter caused an increase in the Nu number value byas much as 50 %.An increase in the Nu number was also observedupon the introduction of pulsatory disturbance to thecombustion gas at a specific frequency. The air, origi-nally delivered to burner was put to the pulsations. It as-sured maintenance of constant velocity amplitude intro-duced to combustion flow. Velocity amplitude of airamounted about 40 % of average value.The pulsations caused first and foremost the changeof flow cylinder velocity through combustion. It con-tributes to removing wall layer around cylinder and itimproves heat exchange, and as a result the change ofvalue heat transfer stream.With increasing pulsation frequency, the Nu numberincreased (the point of detachment should be isolatedfrom this assumption), until it reached a frequency valueof 50 or 62 Hz. At low combustion gas flow velocities of0,5 m/s, the frequency value that most greatly increasedthe heat exchange was 62 Hz. Whereas, with the flow athigher velocities of 0,6 and 0,7 m/s, the dominant fre-quency was 50 Hz, causing the occurrence of the great-est Nu number.The change of combustion gas temperature has aquite different effect on the heat exchange process, as itcauses an inversely proportional change of the Nu num-ber (Figure 6), whereby with increasing the temperatureof flowing combustion gas, while maintaining its con-stant velocity, the Re number decreases.This results from the fact that as the combustion gas,which is a mixture of several two- or three-atom gases,heats up, changes in its parameters, i.e. density, coeffi-cient of dynamic viscosity, specific heat and thermalconductivity, occur. The increase of temperature causesa decrease in combustion gas density, with a simulta-neous increase in dynamic viscosity. These two factorsvery substantially influence the behaviour of the Nunumber. Heat exchange by convection is also dependenton the parameters of gas flowing and flushing an objecttested.The tests carried out have shown that the value of thelocal Nu number is also dependent on the flushing fluegas temperature, and decreases with increasing tempera-METALURGIJA 48 (2009) 2, 121-124 123M. GÓRSKA et al: THE INFLUENCE OF CHANGES OF COMBUSTION GAS TEMPERATURE DURING FLOW AROUND...Figure 4. The distribution of the local Nu number as a fun-ction of the combustion gas inflow angle  andvelocity for undisturbed flow.Figure 5. The distribution of the local Nu number as a fun-ction of the combustion gas inflow angle  andvelocity for introduced disturbances of a fre-quency of 50 Hz.Figure 6. Variation of the Re number as a function tempe-rature and combustion gas velocity.ture (Figure 7). Similarly, decreasing trend accrued incase of another value of inflow angle.Sample results obtained from the experimental testsare shown for an inflow angle of =0° with the combus-tion gas flow velocity at a level of w = 0,6 m/s. The in-crease in combustion gas temperature under the realconditions of carrying out the tests caused a more rapidheating up of the cylinder surface, but this was not at-tributable solely to the convection, but also the increas-ing share of thermal radiation. To this end, correctingcomputations were made, which were aimed at isolatingthe heat flux delivered by convection from the overallheat flux supplied to the cylinder surface. The computa-tion was made based on the knowledge of the overallheat flux delivered to the cylinder, as calculated fromthe primary measurement made by the set of thermo-couples arranged on the cylinder perimeter, and the heatflux, as measured by the radiation flux measurement in-strument. From these data, the percentage radiationshare of the overall heat flux was computed, which wasreduced in the computation by the value of the local sur-face film conductance. The value of the overall heat fluxwith the isolated heat flux delivered by radiation, asshown in Figure 8, is related to the combustion gas tem-perature of 550 °C and the velocity of 0,6 m/s in thepulsatory undisturbed flow.SUMMARYAfter computation of the values of the convectiveheat-transfer coefficient and the Nu number (Figure 7),it was noticed that the temperature increase reduced thevalue of the local Nu number. A similar reduction wasobserved during the application of pulsatory combus-tion gas disturbance, where the frequency of introduceddisturbances did not have any significant influence ofchanging the declining behaviour of the curve. Thevalue of the Nu number decreases with increasing com-bustion gas temperature, which results from the increaseof the thermal conduction of the combustion gas, whichgrows rapidly with the increase of combustion gas tem-perature.REFERENCES1 Kondjoyan A., Daudin JD.: Effects of free stream turbulen-ce intensity on heat and mass transfers at the surface of a cir-cular cylinder and an elliptical cylinder, axis ratio. 4. Inter-national Journal Heat Mass Transfer 38(1995), 1735-1749.2 Lee D.H., Chung YS., Kim DS.: Turbulent flow and heattransfer measurements on a curved surface with a fully de-veloped round impinging jet. International Journal HeatFluid Flow 18(1997) 160-169.3 Addessio F.L., Baumgardner J.R., Dukowicz J.K., JohnsonN.L., Kashiwa B.A., Rauenzahn R.M., Zemach C.:CAVEAT: A computer code for fluid dynamics problemswith large distortion and internal slip. Los Alamos – Natio-nal Laboratory, Mexico 1990.4 Dumouchel F., Lecordier J.C., Paranthoen P.: The effectiveReynolds number of a heated cylinder. International JournalHeat Mass Transfer 41(1998) 1787-1794.5 Szecówka L., Górska M.: The Numerical Modelling of Pul-sation Disturbance Flow Around a Circular Steel Charge.Acta Metall. Slovaca R. 11(2005)1, 321-325.6 Szecówka L.: An influence of pulsation on combustion gasof fuel gas and pollutants emission. Seria: Metalurgia nr 17,Czêstochowa 2001.Note: The rensposible for English language are the Authors.124 METALURGIJA 48 (2009) 2, 121-124M. GÓRSKA et al: THE INFLUENCE OF CHANGES OF COMBUSTION GAS TEMPERATURE DURING FLOW AROUND...Figure 7. Dependence of the local Nu number on the tem-perature of flowing combustion gas for a com-bustion gas velocity of w=0,6 m/s.Figure 8. The shares of radiation and convective heat flu-xes of the overall heat flux delivered to the cylin-der surface.

Utjecaj promjena temperature plinova izgaranja tijekom strujanja oko horizontalnog cilindra na lokalni Nusseltov broj

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