The majority of heat-exchange apparatus operate in a regime where the parameters of one flow differ considerably from those of another (pressure, temperature, etc.); A marked difference in the coefficients of convective heat exchange from both sides of the separating wall of the heat exchanger is observed in that case. Relatively lower coefficients of convective heat exchange can be partially compensated by development of the heat-exchange surface due to finning (it may be increased 10-25 times as compared with the surface of smooth tubes [1]).One promising method of improving the effectiveness of finning is the method of gas-dynamic spraying [2], which consists in the deposition of particles of a powder formed from a high-speed cold or negligibly heated slightly dust-laden flow on the surface of the fin. The method makes it possible to use not only metals, but also alloys and cermets (ceramic-metal mixtures) for spraying, and, therefore, to form surfaces with different properties. An element of a heat exchanger formed from finned tubes using the method of gas-dynamic spraying is shown in Fig. 1.Let us examine the possibility of improving the effectiveness of finning. The ratio of the average difference in the temperatures of the fin Tr and surrounding medium 1",.., to the difference in the temperatures of the surface supporting the finning (the surface of the base) Tb.= and the surrounding medium is understood as finning effectiveness (f'm efficiency OF): owhere Ft is the surface area of the fro.Computed values [3] of the efficiency of circular fins of trapezoidal shape are presented in Fig, 2a. These relationships are obtained on the assumptions that the coefficient of convective heat exchange is constant, the vector of the thermal flux in the fm is normal to the surface on which it is mounted, and the contact resistance at the base of the I'm is negligibly small.In practice, the contact resistance at the base of the fm is one of the factors restricting use of tube finning. Aluminum fins with an interference fit to the tube have high contact resistances, which increase rapidly with rising temperature. They can therefore be used to temperatures of 100~ since attachment of the f'ms to the tubes is weakened at higher temperatures as a result of the high thermal expansion of the aluminum. Fins set in grooves are suitable for tubes, the wall thickness of which is increased by the depth of the groove [I]. With wire firming, the overall length of the unwelded segments may be 30% [4]; this makes itself felt on the characteristics of the heat exchangers.Computed and experimental data on the efficiency of the wire f-tuning of tubes are presented in Fig. 2b. The pronounced difference in the values is explained by the effect of contact resistance.To assess the effect of contact resistance near the base of the fins on T/F, we performed an experimental investigation of the elements of heat exchangers formed from tubes finned using gas-dynamic spraying.A schematic diagram of the experimental stand is shown in Fig. 3. The stan...
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