C.Y. Ching

Heat Transfer Enhancement in Axial Taylor-Couette Flow

An experimental investigation was performed to determine the effect that surface roughness has on the heat transfer in an axial Taylor-Couette flow. The experiments were performed using an inner rotating cylinder in a stationary water jacket for Taylor numbers of 106 to 5×107 and axial Reynolds numbers of 900 to 2100. Experiments were performed for a smooth inner cylinder, a cylinder with two-dimensional rib roughness and a cylinder with three-dimensional cubic protrusions. The heat transfer results for the smooth cylinder were in good agreement with existing experimental data. The change in the Nusselt number was relatively independent of the axial Reynolds number for the cylinder with rib roughness. This result was similar to the smooth wall case but the heat transfer was enhanced by 5% to 40% over the Taylor number range. The Nusselt number for the cylinder with cubic protrusions exhibited an axial Reynolds number dependence. For a low axial Reynolds number of 980, the Nusselt number increased with the Taylor number in a similar way to the other test cylinders. At higher axial Reynolds numbers, the heat transfer was initially independent of the Taylor number before increasing with Taylor number similar to the lower Reynolds number case. In this higher axial Reynolds number case the heat transfer was enhanced by up to 100% at the lowest Taylor number of 1×106 and by approximately 35% at the highest Taylor number of 5×107 .Copyright

Measurement of Local Mass Transfer Distribution in a Large Diameter S-Bend at High Reynolds Number

The local mass transfer in a 203mm diameter back to back bend arranged in a S-configuration was measured at a Reynolds number of 300,000. A dissolving wall method using gypsum dissolution to water at 40°C was used, with a Schmidt number of 660. The experiment was performed in a flow loop by flowing water through the test section. The topography of the unworn and the worn inner surface was quantified using nondestructive X-ray Computed Tomography (CT) scans. The two scanned surfaces were aligned to a common coordinate system using commercial software and in-house routines. The local mass transfer rate was obtained from the local change in radius over the flow time. Two regions of high mass transfer were present: (i) along the intrados of the first bend near the inlet and (ii) at the exit of the extrados of the first bend that extends to the intrados of the second bend. The latter was the region of highest mass transfer in the S-bend.Copyright

Flow Accelerated Corrosion in Single Bends Under Annular Two Phase Flow Conditions

The distributions of the mass transfer coefficient in horizontal 90 degree bends were measured under a range of two phase annular flow conditions. A dissolving wall technique at a high Schmidt number (Sc = 1280) is used for the measurements. The maximum mass transfer occurred on the centerline of the bend outer wall at an angle of approximately 50 degrees from the bend inlet under all tested conditions. The area of maximum mass transfer was found to span approximately 30 degrees in the circumferential direction. A second region of enhanced mass transfer occurred on the latter part of the bend with a local maximum occurring slightly off the bend centerline in some cases. Changing the air and water superficial velocities (Jν = 20 to 30 m/s, JL = 0.17 to 0.41 m/s) showed that the air velocity had a larger effect on the mass transfer than the water velocity; however the effect of the water velocity on the mass transfer was not insignificant.Copyright

Transient Two-Phase Flow Patterns by Application of a High Voltage Pulse Width Modulated Signal and the Effect on Condensation Heat Transfer

The objectives of this study are (i) to determine the transient phase redistributions of a two-phase flow in a smooth horizontal annular channel by applying high voltage pulses to induce electric fields and (ii) to quantify the resultant changes in the condensation heat transfer. The experiments were performed using refrigerant R-134a flowing in a tube that was cooled on the outside by a counter-current flow of water. The electric fields are established by applying high voltage to a concentric rod electrode inside a grounded tube. The effect of the electrohydrodynamic (EHD) forces on the changes to the initial stratified/stratified wavy flow pattern was visualized using a high speed camera. The EHD effect results in the redistribution of the liquid-vapour phase within the channel and unique flow structures, such as twisted liquid cones and entrained droplets, are observed. These structures only appear during the initial application of EHD and are absent in the steady state flow pattern. Experiments were performed using a 8kV pulse width modulated (PWM) signal with duty cycles ranging from 0–100% to evaluate the heat transfer and pressure drop characteristics of the transient EHD flow patterns. The resultant heat transfer increased with the duty cycle to approximately 2.7-fold at a low mass flux (45–55kg/m2 s) and 1.2-fold at a high mass flux (110kg/m2 s). The enhancement was higher as the pulse width was increased.Copyright

Horizontal Tube Side Convective Condensation Under an Applied DC Voltage

An experimental study was performed to investigate the tube side convective condensation under an applied DC high voltage. Experiments were performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed along the centre of the tube. A 8 kV DC voltage was applied across the annular gap between the central electrode and the pipe wall. The experiments were performed for mass flux in the range 45 to 156 kg/m2 s and average quality of 45 percent. The application of the high voltage electric field results in electric body forces at the liquid-vapor interface which extracts the liquid from the bottom stratum towards the vapour core. The application of the 8 KV DC voltage increased heat transfer and pressure drop by factor 3 and 4.5 respectively at the lowest mass flux of 45 kg/m2 s. The results show that liquid extraction from the bottom liquid stratum changed the mode of heat transfer at the bottom of the tube from convective condensation into film condensation.Copyright

Effect of Frequency on Electrohydrodynamic Enhanced Tube-Side Condensation

The effect of high voltage AC electric fields on two-phase flow regime redistribution for flowing refrigerant HFC-134a has been investigated. In particular, the effect of the frequency of an applied square wave between 0 and 8 kV was studied. The flow visualization test section at the exit of the heat exchanger was made of quartz tube coated with an electrically grounded transparent conductive film of tin oxide. The flow regime transitions under the applied AC electric fields were recorded using a high speed camera at frames rates of 2000 frames per second for frequencies in the range of 4 Hz to 2 kHz. The experiments were performed at a mass flux of 55 kg/m2s, inlet and outlet qualities of 45% and 30% respectively, which correspond to stratified flow with the liquid level below the electrode (without EHD). At the low frequency range, both the heat transfer and pressure drop increased with an increase of frequency due to the periodic extraction of the liquid stratum. In the intermediate range of frequencies, the time period of the applied signals was less than the time needed to complete the extraction cycle, and therefore the heat transfer decreased while the pressure increased with an increase of frequency. In the high frequency range, the flow regimes approach those for the DC case where the effect of frequency is negligible on both heat transfer and pressure drop. This is mainly because the fluid medium cannot respond to the high frequency of the applied signals