David A. McNeil

Small-scale modelling of the physiochemical impacts of CO2 leaked from sub-seabed reservoirs or pipelines within the North Sea and surrounding waters

A two-fluid, small scale numerical ocean model was developed to simulate plume dynamics and increases in water acidity due to leakages of CO2 from potential sub-seabed reservoirs erupting, or pipeline breaching into the North Sea. The location of a leak of such magnitude is unpredictable; therefore, multiple scenarios are modelled with the physiochemical impact measured in terms of the movement and dissolution of the leaked CO2. A correlation for the drag coefficient of bubbles/droplets free rising in seawater is presented and a sub-model to predict the initial bubble/droplet size forming on the seafloor is proposed. With the case studies investigated, the leaked bubbles/droplets fully dissolve before reaching the water surface, where the solution will be dispersed into the larger scale ocean waters. The tools developed can be extended to various locations to model the sudden eruption, which is vital in determining the fate of the CO2 within the local waters.

A comparison between HIGHFLUX and plain tubes, boiling pentane in a horizontal kettle reboiler

Abstract An experimental study has been undertaken into the enhancement obtained in the heat-transfer coefficients when HIGHFLUX tubes are used in preference to plain tubes while boiling pentane. The study involved two experimental facilities, a single-tube pool boiler and a 241 tube, 17 row by 17 column, thin slice kettle reboiler. The pool boiling results show that the HIGHFLUX tubes produce heat-transfer coefficients that are up to five times larger than their plain tube counterparts. In flow boiling the enhancement is 3–6 times. In both cases, HIGHFLUX tube performance is shown to deteriorate when small degrees of subcooling are present in the liquid. The deterioration still leaves the HIGHFLUX tubes with a significantly higher heat-transfer coefficient than the plain tubes. Existing flow boiling design methodologies are shown to produce performance characteristics that HIGHFLUX tubes do not follow.

Dropwise condensation of steam on a small tube bundle at turbine condenser conditions

Application of dropwise condensation to utility turbine condensers is investigated by comparing the thermal performance of dropwise and filmwise bundles at industrially relevant conditions. Steam and steam-air mixtures were condensed on bundles of in-line, titanium tubes. The row-by-row heat transfer coefficients are presented against bundle position. They show the expected behavior for filmwise condensation but demonstrate a different one for dropwise. In air-free steam, the dropwise heat transfer coefficients are much larger and do not vary significantly with bundle position. In air-steam mixtures the dropwise values decrease similarly to their filmwise equivalents. The findings are in accord with those found for other geometries. The findings indicate that significant reductions in condenser size can be obtained if permanent dropwise condensation can be produced at industrially relevant conditions.Application of dropwise condensation to utility turbine condensers is investigated by comparing the thermal performance of dropwise and filmwise bundles at industrially relevant conditions. Steam and steam-air mixtures were condensed on bundles of in-line, titanium tubes. The row-by-row heat transfer coefficients are presented against bundle position. They show the expected behavior for filmwise condensation but demonstrate a different one for dropwise. In air-free steam, the dropwise heat transfer coefficients are much larger and do not vary significantly with bundle position. In air-steam mixtures the dropwise values decrease similarly to their filmwise equivalents. The findings are in accord with those found for other geometries. The findings indicate that significant reductions in condenser size can be obtained if permanent dropwise condensation can be produced at industrially relevant conditions.

An experimental study of viscous flows in contractions

Abstract The need to improve the methods used when designing emergency, pressure-relief systems on polymerisation reactors, has made the flow of highly viscous fluids in pipeline fittings highly topical. This paper investigates the flow processes involved in single-phase, viscous flows in nozzles and orifice plates. These fittings were chosen because they would give an insight into the behaviour of highly viscous flows in other geometries, such as the flow upstream of the seat in a pressure relief valve. Experimental data are presented for a pipe, two conical nozzles and a sharp-edged orifice plate for laminar flows in the Reynolds number range 50–400 and for turbulent flows. The volume flow rate—pressure drop characteristics are presented for both nozzles and the orifice plate. The discharge momentum flow rate for the pipe, a nozzle and the orifice plate are also given. Analysis of the data shows that nozzles and orifice plates that are geometrically similar have a similar resistance to flow. It is also shown that the contraction coefficient for an orifice plate tends to unity at low Reynolds numbers.

Pressure drop measurements in condensing steam over a horizontal bundle of staggered tubes

Pressure drop measurements in a 15 row staggered configuration condenser, p/D=1.33, are described. Heat flux densities of up to 90 kW·m−2 were used. The results are compared with the those of another investigation using a condenser with much less closely packed tubes. At relatively low suction, pressure drop in the closely packed condenser could be predicted by conventional single phase correlations. At higher values of suction the condensing pressure loss was considerably lower in this condenser than the single phase value. In the more loosely packed condenser this difference occurred at all values of suction. It is concluded that more experimental and theoretical work is required to explain these effects in view of their importance in turbine condenser design.

A comparison between a small in-line and a staggered tube bank condensing steam filmwise at low-pressures

Abstract Data have been produced for filmwise condensation of steam, and steam–air mixtures, flowing downwards across two tube bank, a 15 row, in-line bank containing 75 tubes and a 15 row staggered bank containing 82 tubes. Both banks were tested at conditions typical of those found in the UK electricity generating industry. Steam was supplied at pressures of 50, 75 and 100 mbar, at velocities of 10, 20 and 33 m/s and with air concentrations of 0 and 10,000 ppm. Steam to cooling water temperature differences of 5, 10 and 15 K were used to generate heat fluxes of up to 90 kW / m 2 . The data and a mathematical model were used to investigate the effect of geometry difference on the heat transfer and pressure difference characteristics. For this particular configuration, the staggered and the in-line tube banks gave the same performance. This is not consistent with similar studies done by other researchers and indicates that tube spacing is important.

PRESSURE DROP IN DOWNWARD FLOW OF CONDENSING STEAM OVER A HORIZONTAL BUNDLE OF SQUARE PITCHED TUBES

Pressure drop measurements in a 15-row steam condenser configured with in-line tubes, p/D < 1.33, are described. Pressures from 50 to 100 mb and Remax in the range 1,000-18,000 were imposed. It is shown that pressure loss coefficients for the bundle and for two-row pairs were lower than predictions for equivalent single-phase tests except near the bottom of the bundle. There was some evidence that increase in suction parameter increased this effect in the top rows. Taken together with previous investigations [3, 4], a falling trend of suction effect is evident the more closely packed the tubes are. The discrepancy between these findings and the results of simulation experiments [7] is noted.Pressure drop measurements in a 15-row steam condenser configured with in-line tubes, p/D < 1.33, are described. Pressures from 50 to 100 mb and Remax in the range 1,000-18,000 were imposed. It is shown that pressure loss coefficients for the bundle and for two-row pairs were lower than predictions for equivalent single-phase tests except near the bottom of the bundle. There was some evidence that increase in suction parameter increased this effect in the top rows. Taken together with previous investigations [3, 4], a falling trend of suction effect is evident the more closely packed the tubes are. The discrepancy between these findings and the results of simulation experiments [7] is noted.

Two-Phase Flow on the Shell Side of a Shell and Tube Heat Exchanger

Two-phase flow on the shell side of a shell and tube heat exchanger is complex. Several studies have produced flow pattern maps that show surprising differences in flow regime boundaries for data sets that contain relatively small variations in fluid and flow properties. Despite this, correlations for void fraction and pressure drop are sufficiently accurate to allow the thermal-fluid design of heat exchangers to be completed. However, these correlations are based on experimental data taken from tube bundles containing tubes with diameters less than 20 mm. This study examines their applicability to tube bundles containing tubes with a diameter of 38 mm. Results for void fraction and pressure drop are presented for air-water flows near atmospheric pressure. The results were obtained for flows through a thin-slice, in-line tube bundle containing 10 rows. The tube bundle contained a central column of tubes with half tubes placed on the shell wall to simulate the presence of other columns. The tubes were 38 mm in diameter and 50 mm long with a pitch to diameter ratio of 1.32. Previous studies have shown that the void fraction in a shell-side, gas-liquid flow becomes constant after only a few rows. Thus, the void fraction was only measured at one location. A single-beam, gamma-ray densitometer was used to measure void fractions near row 7 in the maximum gap between the rows. Corresponding pressure drops were obtained between rows 3 and 10. Data are presented for a mass flux range of 25-688 kg/m2s and a gas mass fraction range of 0.0005-0.6. The measurements are shown to compare reasonably well with predictions from correlations available in the open literature. This shows that these methods can be used for tube-bundles containing larger diameter tubes. Some elements of a heat-exchanger design require a more complex analysis. For example, tube vibration calculations require the distribution of void and phase velocity along the tube length. Such analysis can be provided by multiphase computational fluid dynamic (CFD) simulations. CFD approaches to modelling these flows require empirical inputs for the drag coefficient and the force on the fluid by the tubes. These are deduced from the measured data. The wall forces are shown to scale well with increased tube diameter, however, caution is required when selecting the drag coefficients.