THE HEAT TRANSFER AND RESISTANCE OF TUBES DURING HIGH TEMPERATURE HEATING OF AIR
An experimental study was made of heat transfer and resistance when heating air under the conditions met in high-temperature recuperators. Tests were made in heat-resistant steel tubes of 1 mm wall thickness. Clean dry compressed air was passed through the tubes. Details are given of the experimental rig and of the measurements that were made. Various corrections that were necessary are explained. Local heat-transfer and friction coefficients were calculated. The maximum relative error in determining heat transfer was plus or minus (3 to 8) % and in determining the coefficient of friction plus or minus (4 to 15) %, depending on the experimental conditions. Two sizes of tube were used, 14 and 22 mm internal diameter. When long tubes were used the air was heated to between 800 and 850 deg C, on reducing the length of the heated part of the tune, the air temperature fell to 700 and 600 deg C. The rate of flew of air by weight ranged ton 14 to l44 kg/m/sup 2/ sec and the linear velocity from 3 to 190 m/sec. The hydraullc resistance under isothermal conditions was studied in the Reynolds number range of 17,000 to 175,000 and under heat exchange conditions from 7000 to 105,000. Heat transfer was studied in the Reynolds number range 7000 to 108,000. The heat tranger and resistance relatioships were found to be the same for both sizes of tube and for different air pressures. Undr isothermal conditions the tubes behaved as though smooth for Reynolds numbers below 30,000; the coefficient of friction then followed the wellkmown relations for smooth tubes. For values of Reynolds number greater than 30,000 the coefficient of friction followed the law for rough tubes. However, in both region the coefficient of friction for Reynolds numbers between 17,000 and 175,000 may be calculated. Under heat-transfer conditions the coefficient of friction behaved just as under isothermal conditions, provided the Reynolds number is related to the flow temperature. If it is related to the wall temperature, the experimental points lie considerably lower. Uhder heat-exchange conditions the coefficient of friction may be calculated as for isothermal conditions, using a value of Reynolds number related to the wall temperature, but in this case a correction must be introduced for the temperature factor. The curves indicate that no single-valued simple relationship exists between the criteria of thermal and hydraulic similarity. It appears that heat transfer also depends on the temperature conditions, which may be alloyed for by a temperature factor. The influence of the temperature factor is particularly marked when determining the Nusselt criterion from the wall temperature and the Re or Pe numbers tom the flow temperature. The heat exchange relatioship corresponds well for the relationship between the criteria of thermal and hydraulic similarity with allowance for the temperature factor. Values of the criteria of thermal similarity calculated from mean experimental data are in good agreement with the calculated curves of local heat-transfer. Thus the formulas are suitable for heattransfer calculations on individual sections of ihe tubes and on tubes as a whole. The results are briefly compared with previously published data and are stated to be intermediate between them. A formula for the relationship between the criteria of thermal similarity is in comparatively good agreement with experimental data only over the Reynolds number range of 20,000 to 60,000. (TCO)
- Research Organization:
- Originating Research Org. not identified
- NSA Number:
- NSA-13-022857
- OSTI ID:
- 4201826
- Journal Information:
- Teploenergetika, Vol. Vol: 6; Other Information: Orig. Receipt Date: 31-DEC-59
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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