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TECHNICAL REPORTS Prototype and Performance Evaluation of Refrigerant-Refrigerant Microchannel Heat Exchanger Authors: Susumu Yoshimura* and Shinichi Wakamoto* 1. Introduction We developed a refrigerant-refrigerant microchan- nel heat exchanger, in which microchannel tubes con- sisting of several fluid paths through which low-temperature fluid and high-temperature fluid flow are joined, for the purpose of reducing the size and improving the performance of heat exchangers. From the results of performance evaluation of a prototype, we confirmed that the promotion of mixing of gas and liquid in the header could control the performance deteriora- tion caused by maldistribution of a gas and liquid two-phase refrigerant, which was a major technological challenge in this type of heat exchanger. 2. Specifications of the Prototype and Test Method 2.1 Specifications of the Prototype Figure 1 shows the prototype. Flat single tubes con- sisting of paths for the flow of low-temperature and high-temperature fluids, produced by aluminum extrusion, having a width of 25 mm and a thickness of 2 mm, are joined together. Two ends of the respective tubes are connected to the headers with a resultant configuration that provides five parallel flow units of low-temperature and high-temperature fluids. A single tube consists of 12 round microchannels having a cross-section with a 1 mm bore. The joints between singles tubes and between a single tube and the header are brazed together. The header bore is 6 mm and the effective length for heat exchange is 600 mm. Fig. 1 Schematic view of the Prototype Heat Exchanger 2.2 Test conditions and test method The operating fluids were R410A for cold fluid and water for hot fluid and these were counterflowed for heat exchange. Table 1 shows the test conditions. The cold fluid temperature Tci and volume flow rate F of the hot fluid were fixed, while the hot fluid inlet temperature Thi, mean mass velocity G of the cold fluid in the mi- crochannel, and inlet quality Xi were changed. The header was positioned as shown in Fig. 2: (a) in a horizontal position and (b) in a vertical position. In a horizontal position, the heat exchanger was inclined at 50 degrees as shown in figure (a) and the fluid was introduced into the header horizontally and distributed downward vertically. On the other hand, in a vertical position, the fluid was introduced vertically and distrib- uted horizontally. In the distribution section, the inser- tion δ of a single tube into the header was set at 0 and 2 mm. The inlet length on the low-temperature side through which two-phase fluid flow was 200 mm. The thermal conductance of the heat exchanger AK was determined from the measured values of the inlet and outlet temperatures Tci and Tco of the cold fluid, inlet and outlet temperatures Thi and Tho of the hot fluid, and volume flow rate F, using equation (1) below. The Cp and [LMTD] used in the equation are the specific heat at constant pressure and logarithmic mean temperature difference, respectively. The temperatures and flow rates mentioned below, unless otherwise specified, are those on the cold fluid side. Table 1 Experimental conditions G F Thi Tci Xi kg/m2s L/min °C °C 450 to 1300 2 37, 43 26.7 AK = ρFCp (Thi −Tho ) [LMTD] - 0.08 to 0.55 (1) 600mm Header Microchannel (ID 6mm) 25mm 4mm *Advanced Technology R&D Center 20 3. Results of the Test and Consideration 3.1 Single tube characteristics First of all, to identify the characteristics without the influence of distribution, single tube characteristics were examined by removing one parallel flow unit from the 155mmPDF Image | Heat Pump with Natural Refrigerants 3041
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