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The article presents method and results of experimental study of the intensified heat transfer for the single-row bunch consisting of bimetallic pipes with spiral knurled ribs with the following parameters: outer diameter of a rib d = 56 mm; rib height h = 15 mm, rib pitch s = 2.5 mm, average thickness of a rib D = 0.5 mm; diameter of a rib at the bottom d0 = 26 mm, coefficient of fins j = 21. The pipes cross pitch S1 in a single-row bunch makes 58, 61, 64 and 70 mm. The intensification of a heat transfer has been arranged in a stream of heated air over an experimental bunch by means of two types of exhaust shaft i.e. the one with adjustable height and the one with the adjustable section through passage. The aim of the work was to perform an experimental study and to summarize the data of a heat transfer of the single-row bunches consisting of bimetallic finned tubes under the intensified (mixed) air convection and also to develop an engineering method of calculation of single-row recirculation air heaters. The results of experimental study of the intensified convective heat transfer of the single-row bunch consisting of bimetallic finned tubes in a stream of heated air are presented in the form of dependences of number of Nusselt on Grashof number and Reynolds number. As a result of generalization of experimental data, the generalized criteria equation for calculation of heat transfer of the single-row bunch consisting of bimetallic finned tubes when cross pitches of installation of tubes, the areas of exhaust outlets and heights of the exhaust shaft are various, has been obtained. The engineering technique for design calculation of the single-row recirculation air heater has also been developed.

The technique and results of experimental study of heat transfer of a single bundle consisting of bimetallic tubes with helically knurled edges, in natural and mixed convection of air are presented. Mixed convection, i.e. a heat transfer, when the contribution of free and forced convection is comparable, was created with the help of the exhaust shaft mounted above the heat exchanger bundle and forced air movement was created by the difference in density of the air in the shaft and the environment. The experimental dependence of the heat transfer of finned single row of bundles in the selected ranges of Grashof and Reynolds numbers has been determined. It is demonstrated that heat transfer in the mixed convection is 2.5−3 times higher than in free one and the growth rate of heat transfer with increasing Reynolds number is more than in the forced convection. Different forms of representation of results of experiments were analyzed and it was determined that the Nusselt number has a single power dependence on the Reynolds number at any height of the exhaust shafts. A linear dependence of the Reynolds number on the square root of the Grashof number was determined as well as the proportionality factors for different shaft heights. It is noted that the characteristics of the motion of air particles in the bundle in free convection is identical to the motion of particles in forced convection at small Reynolds numbers, i.e. a free convection flow smoothly flows into a forced convection one without the typical failures or surges if additional driving forces arise.