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This paper analyses the mixed convection process in a fanless evaporator of an air heat pump. The text of the paper shows the authors’ experimental studies results of the temperature distribution and the local values of heat transfer coefficients on the outer surface of vertical tubes with longitudinal fins for the case of mixed convection and fins of a specific shape of their cross-section (prismatic, wavy fins). The experimental studies include the air velocities wa=2,3 m/s and the temperature differences between air and the refrigerant inside the heat exchanger tubes which is ΔT=24-40K. The results obtained were used for verification of CFD modeling of the heat transfer process for the discussed case of heat transfer and the geometry of the finned surface. The numerical analysis was performed for: the temperature distribution along the fin height, the tube perimeter and height, the distribution of local heat transfer coefficients on the finned tube perimeter and along its height. The simulated calculations were used to verify the method of determination of fin efficiency.

The paper presents thermal and flow analyses of the boiling process of R507, R410 and R407 C refrigerants inside vertical tubes (21 mm) with coiled-wire inserts and various coil diameters (20; 20.5 mm), coil pitches (26; 44 mm) and wire diameters (1.5; 2 mm). The study differs from other publications as regards the conditions under which the experiment was conducted. It focuses on the boiling process in two long vertical tube sections (2 m), paired in an in-line arrangement. The study was conducted within a moderate range of mass flux densities 80–240 kgm−2s−1 and at low heat flux densities 5–11 kWm−2, corresponding to the operating conditions of air coolers. The study examined the influence of vapour quality, mass flux density, geometrical parameters of the inserts and the impact of temperature glide on heat transfer coefficient and flow resistance increases as compared with a plain tube. The obtained increase ratios of heat transfer coefficients amounted to 1.1-1.7 for an azeotropic agent and to 1.1-1.3 for zeotropic agents, with the relative increase in flow resistances amounting to 1.8-4.5. New equations are proposed in the paper for the calculation of heat transfer coefficient and flow resistance values for boiling inside vertical tubes with spiral inserts. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Machine Construction & Maintenance is the property of Institute for Sustainable Technologies - National Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Technical Transactions / Czasopismo Techniczne is the property of Sciendo and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Technical Transactions / Czasopismo Techniczne is the property of Sciendo and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Technical Transactions / Czasopismo Techniczne is the property of Sciendo and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents a mathematical model of the heat and mass transfer processes for a rotary-spray honey dehydrator with a heat pump and a closed air circuit. An analytical calculation model, based on the energy balance equations of the dehydrator and heat pump, was used to model the transient dehydration process of honey in a dehydrator. The presented article includes a different approach to modelling both the dryer and the heat pump assisting the drying process. The novel quality of this study lies in the use of original equations to determine the heat and mass transfer coefficients between honey and air and using an actual model of a cooling unit to model the honey dehydration process. The experimentally verified calculation algorithm enables an analysis of the effects of air flow rate, mixer rotation speed, and cooling unit power on the efficiency of the drying process. The dehydrator calculation model was used to minimize the drying time by selecting the optimal evaporative temperature values of the cooling unit. For fixed mixer speed and air flow rates, optimal values of evaporation temperatures allow for 8–13% reduction in honey drying time and an increase in the specific moisture extraction rate (SMER) by 4–32%. [ABSTRACT FROM AUTHOR]