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The present paper describes the results of experimental investigations of heat transfer during condensation of R134a, R404A and R407C in pipe minichannels with internal diameters 0.31-3.30 mm. The results concern investigations of the local heat transfer coefficient. The results were compared with the correlations proposed by other authors. Within the range of examined parameters of the condensation process in minichannels made of stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al., Mikielewicz and Shah correlations within a limited range of the mass flux density of the refrigerant and the minichannel diameter. On the basis of experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient.

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The heat removal process of a compact condenser in a refrigeration compressor system, constructed on the basis of pipe minichannels, occurs in three regions: the removal region of superheat, the two-phase proper condensation region, and the liquid subcooling region. The intensity of heat removal in the two-phase region depends on the heat efficiency of the condenser. The length of this region is determined by the process parameters and the external influences. This paper analyzes the influence of periodically generated hydrodynamic disturbances on the length of the condensation region for R134a and R404A refrigerants. Experimental tests were conducted for six diameters of pipe minichannels: 0. 64, 0.90, 1.44, 1.92, 2.30, and 3.30 mm. A range of frequencies from 0.2 to 5 Hz of periodically generated disturbances showed an explicit and unfavorable influence on the decrease of the length of the proper condensation region. This influence reduced the efficiency of the process. A computational model was developed to calculate the length of the two-phase region under disturbance conditions. The model proposed was verified by the results of the experiment, and compliance was obtained in the range of ±15%.

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