Test Section

The flow boiling for refrigerant flow in an annular duct with a small clearance between the inner and outer pipes is explored. As schematically shown in Fig. 2.3, the test section of the experimental apparatus is a horizontal annular duct with the outer pipe made of Pyrex glass to permit the visualization of boiling processes in the refrigerant flow. The outer Pyrex glass pipe is 160 mm long and 4 mm thick with inside diameter of 20 mm.

Both ends of the pipe are connected with a copper tube of the same size by means of flanges and are sealed by O-rings. The inner smooth pipe has 16.0, 18.0, 19.0 or 19.6 mm nominal outside diameter (the pipe wall thickness is 1.5, 2.5, 3.0 or 3.3 mm) and is 0.41 m long, so that the hydraulic diameter of the annular duct D_h is 4.0, 2.0, 1.0 or 0.4 mm

( corresponding to the gap size of 2.0, 1.0, 0.5 or 0.2 mm for the duct). In order to insure the gap between the ducts being uniform, we first measure the average outside diameter of the inner pipe and the mean inside diameter of the Pyrex glass, with the accuracy of the measurement in the range of ±0.01 mm. Then we photo the top and side view pictures and measure the average distance between the inside surface of the Pyrex glass pipe to the outside surface of the inner tube, with the accuracy also in the range of ±0.01mm. From the above procedures the duct gap is ascertained and its uncertainty is estimated to be 0.02 mm. An electric cartridge heater of 160 mm in length and 13.0 mm in diameter with a maximum power output of 800W is inserted into the inner pipe. Furthermore, the pipe has an inactive heating zone of 10 mm long at each end and is insulated with Teflon blocks and thermally nonconducting epoxy to minimize heat loss from it. Thermal contact between the heater and the inner pipe is improved by coating a thin layer of heat-sink compound on the heater surface before the installation of the heater. Then, 8 T-type calibrated thermocouples are electrically insulated by covering their beads with the electrically nonconducting thermal bond before they are fixed on the inside surface of the inner pipe so that the voltage signals from the thermocouples are not interfered by the DC current passing through the cartridge heater. The thermocouples are positioned at three axial stations along the smooth pipe. At each axial station, two to four thermocouples are placed at top, bottom, or two sides of the pipe circumference with 180° or 90° apart. The outside surface temperature Tw of the inner pipe is then derived from the measured inside surface temperature by taking into account the radial thermal conduction through the pipe wall.

Figure 2.4 shows the detailed thermocouple locations at each axial station and the arrangement of the cartridge heater.

The test section for the new refrigerant in small tubes is described next. Due to the tubes tested being relatively small, the refrigerant flow rates in them are very low and

direct measurement of heat transfer and pressure drop in the tubes is difficult and is subject to large error. Thus 28 small tubes all made of copper, each having the same diameter and length, are put together to form the test section, as shown in Fig. 2.5. Each small tube has an inside diameter of 2.0 mm, outside diameter of 3.0 mm and length of 150 mm.

Specifically, these 28 tubes are placed together side by side forming a plane tube bundle. In order to allow the refrigerant to flow smoothly into the small tubes, a section including divergent, convergent and straight portions is connected to the inlets of the tubes. Besides, another section including straight and convergent parts is attached to the exits of the tubes.

Both the inlet and exit sections are formed by the stainless steel plates. At the middle axial location of the small tubes 14 thermocouples are soldered onto the outer surface of the tubes. Specifically, these thermocouples are soldered onto 14 selected tubes at the circumferential position of from the top of the tube or from the bottom of the tube, as shown in Fig. 2.6. Two copper plates of 5-mm thick are respectively soldered on the upper and lower sides of the tube bundle shown in Fig. 2.7. Obviously small crevices exist between the tube outside surface and copper plates. Due to the good thermal contact of the copper plates and the tubes, there is no need to fill the crevices with conducting grease.

Instead, the crevices provide the space for the thermocouple wires leading to the data logger. The copper plates contacting with the small tubes are heated directly by an electric-resistance heater of 2.6-mm wide, 0.5-mm thick and 2.5-m long. The heater is connected to a 500W DC power supply. Mica sheet is placed in the narrow space between the heater and copper plates to prevent leak of electric current to the copper plates. The power input to the heater is measured by a power meter with an accuracy of ±0.5﹪. In order to reduce the heat loss from the heaters, the whole test section is wrapped with a 10-cm thick polyethylene layer.

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