Abstract
An experimental study was carried out to explore the heat transfer characteristics on the outside of smooth and enhanced tubes, during evaporation and condensation of R134A in the annulus of a tube-in-tube heat exchanger. The three-dimensional enhanced surface tube consisted of primary enhancement patterns and secondary patterns; results were compared to the performance of an equivalent smooth tube. The equivalent external diameter of the inside horizontal copper tubes used in this study was 19.05 mm, while the outer tube varied in size, allowing a comparison of heat transfer for different annulus dimensions. Tests were conducted with a fixed inlet/outlet vapor quality and a constant saturation temperature for varied mass velocities in the range of 30 to 100 kg/(m2∙s). For condensation, the ratio of heat transfer coefficient enhancement (enhanced tube/ smooth tube) was up to 1.78; this can be attributed to the turbulence increase, as well as liquid film re-distribution, produced from the dimples. Furthermore, the condensation heat transfer coefficient increased rapidly with increasing mass flux. For flow boiling in the annulus between the 1EHT tube and outer tube, the heat transfer coefficient during boiling was 11–36% higher when compared to the smooth tube at xave = 0.35, while the performance of the 1EHT tube was not as good as the smooth tube at xave = 0.5. The heat transfer deterioration can be explained by decreased effective nucleate flow boiling heat transfer area and the flow pattern transition between a slug/wavy-stratified flow to wavy-stratified flow.
Highlights
The application of heat transfer enhancement technologies provides the potential to minimize cost, produce smaller units, and increase the reliability of heat exchangers
The relationship of the condensation heat transfer and mass velocity is given in Figure 6a; the corresponding heat flux values are given in Figure 6b for a comparison
These results differ from similar experimental investigations reported by Tang and Li [18] and Chen et al [26]; the results of the present study show that the heat transfer coefficient increased with increasing mass flux, implying that the convective condensation component occupied an important role in the overall heat transfer for the present test conditions
Summary
The application of heat transfer enhancement technologies provides the potential to minimize cost, produce smaller units, and increase the reliability of heat exchangers. Enhanced tubes with passive enhancement can enhance the thermal performance while producing a small increase (when compared to a smooth tube) to pressure drop for most conditions. According to Webb and Kim [1], three-dimensional enhanced tubes are good choices for heat transfer argumentation; three-dimensional enhanced tubes are considered here. When compared to traditional two-dimensional enhancement techniques, the enhanced three-dimensional tubes provide better condensate drainage effects, and an increase in nucleate site densities can be achieved using the unique characteristic structures that make up the enhancements (i.e., dimples, grooves, etc.)
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