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Heat Transfer Under Convective Boiling of Refrigerants R-404A and R-407C In a Horizontal Copper Tube Electrically Heated
| Content Provider | Semantic Scholar |
|---|---|
| Author | Filho, Enio Pedone Bandarra Jabardo, José Maria Saiz Lima, Cíntia Da Silva |
| Copyright Year | 2000 |
| Abstract | Convective boiling of refrigerants R-404A and R-407C in a horizontal copper tube of 12.7 mm internal diameter has been experimentally investigated. A 2 m long test section electrically heated has been used for that purpose. Tests have been performed preferentially at the average test section temperature of s"c. Effects over the heat transfer coefficient of such parameters as quality, mass velocity and heat flux have been considered. Kandlikar' s correlation has been evaluated against experimental results for values of the fluid parameter obtained in this investigation. INTRODUCTION Convective boiling designates the liquid to vapor change of phase that takes place under forced flow of a heated fluid. In this study major attention has been focused on conditions prevailing in direct expansion evaporators, especially covering a region from the evaporator inlet to the section where dryout occurs. This is by large the most important region in the evaporator, and that where thermal equilibrium conditions prevail. Post dryout conditions encompass the misty two-phase flow and the superheated vapor regions. Regarding heat transfer, misty flow has been generally considered as a single phase saturated vapor region. Nucleate boiling and evaporation at the liquid-vapor interface are the mechanisms associated to convective boiling heat transfer, occurring either isolated or simultaneously. The latter is designated as convective boiling by some authors, more restrictive than the one used in this study. Throughout the paper this condition will be designated as "strictly convective boiling", Saiz Jabardo et al (1999). Some of the current heat transfer correlations for convective boiling actually assume a superposition of those effects, after Chen (1966), see, for example, Jung et al (1989), Jung & Radermacher (1991), Gungor & Winterton (1986), Wattelet (1994), and others. Heat transfer is affected by the flow regime, as one should expect, due to changes in the topology of the liquid/vapor interface, as suggested by Collier & Thome (1996). Three major physical parameters affect the flow regime transition in convective boiling: mass velocity, heat flux, and quality. For a given heat flux and reduced mass velocity, typically lower than 100 kg/(s m), stratified (wavy) regime occurs over the range of qualities encompassing those prevailing in a typical evaporator (say x>5%), Kattan et al (1998a), Wattelet et al (1992). Nucleate boiling might occur on the wall in contact with the liquid filling the bottom of the horizontal tube, specially at lower qualities. As the liquid layer gradually turns thinner, bubble nucleation might be suppressed. The heat transfer coefficient is not significantly affected by quality and remains essentially constant up to the complete dryout of the wall, though, at very low mass velocities, it has been observed that it gradually diminishes as the liquid layer at the bottom of the tube evaporates. Eighth International Refrigeration Conference at 25 Purdue University, West Lafayette, IN, USAJuly 25-28,2000 Higher mass velocities prompt the transition from a bubbly and intermittent, mostly slug, nucleate boiling dominated regime, to the annular one. The heat transfer coefficient presents a typical behavior, being strongly dependent upon the heat flux in the nucleate boiling region. As the annular regime sets in, the heat transfer coefficient no longer depends upon the heat flux but increases gradually as the film thickness at the wall becomes thinner as a result of intense evaporation at the liquid/vapor interface, Jung et al (1989). The heat transfer coefficient behavior along the evaporator described in previous paragraphs will be carefully cross examined in subsequent sections on the basis of experimental results obtained under convective boiling conditions of refrigerants R-404A and R-407C flowing in a horizontal copper tube electrically heated. EXPE~ENTALBENCH An schematic circuit diagram of the experimental bench used in present study is shown in Fig. 1. The refrigerant is pumped from the condenser through a filter drier and a sight glass (SG) to the mass flow meter and preheater before reaching the entrance of the test section (TS). In order to allow for the flow development a 1.5 m length, 12.7 mm internal diameter copper tube is interposed between the preheater exit and the TS entrance. The results reported herein were obtained in a 2m long, 12.7 mm internal diameter copper tube test section. The preheater and test section are heated by tape electrical resistors, uniformly wrapped on the external surface of the tube in such a way to guarantee a uniform heat flux. Refrigerant bulk temperature is measured at the TS inlet and outlet through type T sheathed thermocouples, whereas surface temperature is measured at four equally spaced cross sections along the TS by type T A WG#30 thermocouples. These thermocouples are nested in longitudinal grooves of a couple of centimeters long in such a way to reduce possible fin effects in temperature readings. At each measuring cross section, the surface temperature is read at three locations, 90" spaced, from the bottom to the top of the tube. The heaters arc wrapped around the tube and covered by successive layers of fiber glass and foam thermal insulation. Details of installation of the test section electrical heaters and surface thermocouples are shown in Fig. 2. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1460&context=iracc |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
