2019

S.M. Abolarin, 2019, " Heat transfer and pressure drop characteristics in the transitional flow regime with twisted tape inserts"

Studies on heat transfer and pressure drop characteristics in a tube with twisted tape inserts have been receiving research attention in the laminar and turbulent flow regimes since 1921. However, several gaps exist in the transitional flow regime. The purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in a smooth circular copper tube with conventional twisted tape (CTT) inserts, alternating clockwise and counter clockwise twisted tape (CCCTT) inserts and peripheral u-cut twisted tape (PUCTT) inserts without and with ring (PUCTTR) inserts.

An experimental set-up was designed and constructed in this study. The set-up was validated by comparing the results of the heat transfer and pressure drop characteristics in a smooth tube (without twisted tape inserts) with literature. The smooth circular copper tube had a wall thickness, an inner diameter, and a length of 1.5 mm, 19 mm, and 5.27 m respectively. The twisted tape inserts considered in this study were fabricated from 1 mm thick and 18 mm wide copper strips. The strips used for the CTT inserts were twisted to form tapes with twist ratios of 3, 4 and 5. A total of five 900 mm long CTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m. The strips used for
CCCTT inserts were twisted to obtain a twist ratio of 5 and 12 tapes were joined longitudinally so that a clockwise direction twisted tape insert was connected to a counter clockwise direction twisted tape insert. The assembling was at connection angles of 0°, 30° and 60°, to form CCCTT inserts with an overall length of 5.27 m. For the PUCTT inserts, the peripheries of the strips were cut to achieve depth ratios of 0.105 and 0.216. The strips were twisted to form tapes with a twist ratio of 5 and ring inserts were soldered on the PUCTT inserts to form PUCTTR inserts with ring space ratios of 1.25, 2.5 and 5. A total of five 900 mm long PUCTT inserts were connected longitudinally to an additional 770 mm insert to form a tape with overall length of 5.27 m.

Water was circulated as test fluid and experiments were conducted at constant heat flux boundary condition, at Reynolds numbers of 300 – 11 404. This Reynolds number range covered the transitional flow regime, as well as sufficient parts of the laminar and turbulent flow regimes. This study focused on the identification of the transitional flow regime with the CTT, CCCTT, PUCTT and PUCTTR inserts. With the CTT inserts, it was the influence of twist ratio and heat flux on the transitional flow regime. For the CCCTT inserts, it was the influence of connection angle and heat flux on the transitional flow regime. Modified Grashof number which is a function of heat flux was used to describe free convection effects in the CTT and CCCTT inserts. The PUCTT and PUCTTR inserts it was the influence of depth ratio as well as ring space ratio on the transitional flow regime.

When twist ratios and heat fluxes of the CTT inserts were compared, a reduction in twist ratio and heat flux caused the transitional flow regime to occur earlier. When the CCCTT inserts were compared it was found that both the start and end of the transitional flow regime were influenced by the connection angle and heat flux. When different connection angles of the CCCTT inserts were compared it was found that an increase in connection angle enhanced the heat transfer in the transitional flow regime. An increase in heat flux significantly enhanced the heat transfer in the laminar flow regime and delayed transition. When depth ratios of the PUCTT inserts were compared, an increase in depth ratio caused the transitional flow regime to occur earlier. Furthermore, the transitional flow regime occurred earlier with PUCTTR inserts than with
PUCTT inserts and transition occurred even earlier as the ring space ratio was reduced. An increase in depth ratio and reduction in ring space ratio significantly enhanced heat transfer in the transitional flow regime. It can be concluded that when the CTT, CCCTT and PUCTT inserts were compared, transition first occurred with the CCCTT inserts and delayed the most with the CTT inserts.

Heat transfer and pressure drop correlations were developed to predict the experimental data in the laminar, transitional and turbulent flow regimes. Where applicable the correlations were developed as a function of Reynolds number, twist ratio, modified Grashof number, connection angle, depth ratio and ring space ratio.

Supervisor: Prof JP Meyer

D.R.E. Ewim, 2019, "Condensation inside horizontal and inclined smooth tubes at low mass fluxes"

Condensation has been extensively investigated from as early as 1914. However, there are several gaps in the literature, especially for in tube condensation at low mass fluxes and inclined tubes. Until now, no study has systematically investigated the influence of temperature difference, vapour quality, and inclination on the heat transfer coefficients and pressure drops during condensation inside horizontal and inclined smooth tubes at low mass fluxes. Thus, the purpose of this study was to increase the fundamental understanding of two – phase flow behaviour at low mass fluxes by experimentally investigating the heat transfer, flow pattern, and pressure drop characteristics during condensation inside horizontal and inclined smooth tubes at low mass fluxes.

An existing experimental set‐up was modified to accommodate the “low mass flux” needs of this study and the initial results were successfully validated against literature. A smooth circular copper tube in tube test condenser with an inner tube 1.49 m long, an inner diameter of 8.38 mm and an outer diameter of 9.54 mm was designed and built. The annulus had an inner diameter of 14.5 mm and an outer diameter of 15.88 mm. Heat transfer and pressure drop experiments were conducted for mass fluxes of 50, 75, 100, 150, and 200 kg/m 2 s, at 15 different inclination angles from −90° (vertically downwards) to +90° (vertically upwards). The temperature differences (differences between the average refrigerant saturation temperature and
tube wall temperature) were varied from 1°C to 10˜C, while the average saturation (condensation) temperature was maintained at 40°C. The mean vapour qualities were varied between 0.1 to 0.9. R134a was used as the test fluid while water was used in the annulus to cool the test section. A total of 2 178 videos, 2 920 mass flow rate measurements, 56 301 temperature measurements, and 1 536 pressure drop measurements were taken. The flow patterns were recorded in grey levels with two high-speed video cameras installed at the inlet and outlet of the test section through sight glasses made from borosilicate. To improve the image quality and ensure uniformity in the distribution of the light, a uniform (LED) backlight
was used. This LED backlight was a 99% uniform, 50 by 50 mm red light. An uncertainty analysis showed that the maximum uncertainties of the pressure drops, heat transfer coefficients and vapour qualities presented in this study were 9%, 12%, and 5% respectively.

For horizontal flow, it was found that the flow patterns were predominantly stratified and stratified wavy. It was also found that the heat transfer coefficients were dependent on the temperature difference between the temperature of the wall on which condensation occurs and the temperature of the condensing refrigerant. Furthermore, it was found that the heat transfer coefficient decreased with an increase in the temperature difference. When comparing the heat transfer results at low mass fluxes to the literature, it was found that the absolute mean deviation varied by up to 42%. An amendment was suggested in a stratified heat transfer coefficient term from literature. It was found that with this amendment, the heat transfer coefficients of low mass fluxes could be estimated with errors of an average of ± 5%.

For inclined flow, six flow patterns namely ─ stratified, stratified wavy, annular, annular wavy, intermittent, and churns flows were observed. Bubbly flow was not observed on its own, but was observed during intermittent flows. These flow patterns were adopted using the descriptions of flow regimes as prescribed by Thome. It was found that the inclination angles significantly influenced the flow patterns and the heat transfer coefficients. Downwards flows accounted for an increase in heat transfer coefficient with the maximum heat transfer coefficient found at inclinations of −15° and −30° at the corresponding minimum temperature difference tested for in each case. The maximum inclination effect was approximately
60% and was obtained at the lowest mass flux of 50 kg/m 2 s. In general, it was concluded that the heat transfer coefficientswere more sensitive to the temperature difference for downwards flows than for upwards flows. Furthermore, there was no significant effect of the temperature difference on the heat transfer coefficients for upwards flows. It was also found that thevertical downwards (-90 o ) and upwards (+90 o ) orientations were almost independent of the temperature difference. With respect to the inclination effect, it was found that in general, they decreased with increase in temperature difference, but increased with a decrease in mass flux and vapour quality.

With respect to pressure drops in smooth and inclined tubes, it was found that they increased with an increase in mass flux, temperature difference and vapour quality. Furthermore, the lowest and highest measured pressure drops were obtained during the downward and upward flows respectively. On the other hand, the opposite was found for the frictional pressure drops.

Supervisor: Prof JP Meyer