تجزیه و تحلیل عملکرد حرارتی مبدل حرارتی تحت میدان الکتریکی
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Energy Conversion and Management, Volume 44, Issue 7, May 2003, Pages 1163–1175
This paper presents a numerical method to analyze the thermosyphon heat exchanger with and without the presence of electrohydrodynamics (EHD). The proposed model is capable of handling both balanced and unbalanced thermosyphon heat exchangers. For the balanced thermosyphon heat exchanger, the calculated results of heat transfer rate for water and R-134a agree well with experimental data. For the unbalanced thermosyphon heat exchangers, it is found that the performance improvement increases with the ratio of when EHD is applied at the condenser alone. Conversely, the performance improvement decreases with the ratio of when EHD is applied at the evaporator alone.
At present, waste heat recovery is an important subject for energy conservation. The thermosyphon air preheater is one of the major equipments for energy recovery because of its high thermal conductivity, low cost and easy construction. A schematic of the working principle of the heat pipe heat exchanger is shows in Fig. 1. When heat is added to the evaporator section, the working fluid inside the tube vaporizes and carries heat from the high temperature heat source to the low temperature heat sink, where the working fluid releases the heat to the condenser section. The condensate then returns to the evaporator by gravitational force. Full-size image (9 K) Fig. 1. Schematic of thermosyphon heat pipe. Figure options In a typical application of the thermosyphon heat exchanger, the dominant resistance is usually on the air side. Thus, to improve the overall performance of thermosyphon heat exchangers effectively, it is essential to enhance the associated air side performance. Passive enhancement methods via various extended fin patterns such as louvers and slits are often employed. However, due to the severe operating ambient conditions, utilization of the highly interrupted surfaces is not practical. Therefore, an active enhancement method, such as electrohydrodynamic (EHD), becomes very attractive for air side enhancement. Accordingly, it is the purpose of this study to examine the applicability of a thermosyphon heat exchanger in cooperation with the EHD technique. The advantages of this technique are, using only a small electric input from a high voltage supply and electrodes (e.g. needle, wire and mesh), appreciable heat transfer improvements and rapid thermal control. The enhancement mechanism of the EHD can be described from the following equation : equation(1) where ρc is the electric charge density, is the electric field strength, ε is the permittivity of a dielectric fluid medium, and ρ is the density of the fluid. The first term on the right-hand side of Eq. (1) is the Coulomb force due to the net free charges in the fluid. Fig. 2 shows the basic principle of EHD for an air side application. As seen in the figure, with sufficiently high voltage into the wire electrode, the corona wind is generated. The ions wind is produced by the ionization of a gas near the wire and the drift of positive ions to the outer surface tube electrode. During movement of the ions, they give energy from their momentum to the neutral molecules by collisions, and a change of the flow direction of molecules occurs. In this study, efforts are paid on the development of a simulation model of thermosyphon heat exchangers with and without the presence of EHD. Validation of the proposed model with experimental data is conducted. Full-size image (6 K) Fig. 2. Characteristics of corona wind.
نتیجه گیری انگلیسی
The primary purpose of this paper is to present a numerical method to study thermosyphon heat exchangers with or without the presence of EHD. This proposed method is capable of handling both balanced and unbalanced thermosyphon heat exchangers. For the balanced thermosyphon heat exchanger, the calculated results of heat transfer rate for water and R-134a agree well with experimental data. For the unbalanced thermosyphon heat exchangers, it is found that the performance improvement of the thermosyphon heat exchanger increases with the ratio of when the EHD is applied at the condenser alone. Conversely, the performance improvement of the thermosyphon heat exchanger decreases with the ratio of when the EHD is applied at the evaporator alone. With EHD supplied both at the condenser and evaporator, the performance improvement of the thermosyphon heat exchanger decrease slightly with the ratio of .