Friction and Heat Transmission Coefficients

Abstract

The work described in the paper was carried out to determine simultaneously the friction and heat transmission coefficients of metal pipes of various cross-sections. A technique has been worked out and apparatus constructed for the simultaneous determination of the friction and heat transmission coefficients of the central portion of a pipe through which a fluid is flowing. With this apparatus, and with water as the working fluid, tests have been carried out upon a series of drawn copper pipes whose cross-sections varied from circular through square, to rectangular with sides in the ratio of 8/1; in addition two drawn brass pipes, one of which had had its internal surface artificially roughened, have been tested. The extreme values of velocity and Reynolds number are in the ratio of about 300/1; the maximum values being about 340 cm. per sec. (11 ft. per sec.) and 30,000 respectively. The friction coefficient of the rough pipe under isothermal conditions has been determined for values of the Reynolds number up to 50,000, using air as the working fluid. The observations made have been reduced by the method of dimensional analysis and the following conclusions drawn:— (1) That the results of the heat transmission tests on the various pipes can be correlated on the basis of the hydraulic diameter. (2) That an increase in friction coefficient is not necessarily accompanied by an increase in heat transmission coefficient. (3) That the friction coefficient under heat transmission conditions is the same as that under isothermal conditions if the viscosity in the former case is that pertaining to the wall temperature of the pipe.