<p> The objective of this research is to investigate the effect of DC, AC and pulse wave
applied voltage on two-phase flow patterns, heat transfer and pressure drop during
tube side convective condensation of refrigerant HFC-134a in an annular channel. Experiments
were performed in a horizontal, single-pass, counter-current heat exchanger
with a rod electrode placed along the center of the tube. The electric field was applied
across the annular gap formed by the electrode connected to the high-voltage source
and the grounded surface of the inner tube of the heat exchanger. The electric field
between the two electrodes was established by applying a high voltage to the central
electrode. The high voltage was generated by amplifying the voltage output from a
function generator. The flow was visualized at the exit of the heat exchanger using
a high speed camera through a transparent quartz tube coated with an electrically
conductive film of tin oxide.</p> <p> The effect of a 8 kV DC applied voltage was investigated for mass flux in the range 45 kg/m^2s to 160 kg/m^2s and average quality of Xavg= 45%. 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/m^2s. Increasing the mass flux decreased the effect of electrohydrodynamic forces on the two-phase flow heat transfer and pressure drop.</p> <p> The effect of different AC and pulse wave applied voltage parameters (e.g. waveform, amplitude, DC bias, AC frequency, pulse repetition rate and duty cycle) on heat transfer and pressure drop was investigated. Experiments were performed
with an applied sine and square waveform over a range of frequencies (2 Hz < f < 2 kHz), peak-to-peak voltages (2 kV < Vp-p < 12 kV) and DC bias voltage (-10 kV < VDc < 10 kV), and with an applied pulse voltage of amplitude 12 kV and duty cycle from 10% to 90%. These experiments were performed for a fixed mass flux of 100 kg/m^2s, inlet quality of 70%, and heat flux of 10 kW /m^2. For the same amplitude and DC bias, the pulse wave applied voltage provides a larger range of heat transfer and pressure drop control by varying the pulse repetition rate and duty cycle compared to the sine waveform.</p> <p> The effect of a step input voltage on two phase flow patterns, heat transfer and pressure drop was examined and analyzed for an initially stratified flow. The flow visualization images showed that the step input voltage caused the liquid to be extracted from the bottom liquid stratum toward the center electrode and then pushed to the bulk flow in the form of twisted liquid cones pointing outward from the central electrode. These transient flow patterns, which are characterized by high heat transfer compared to the DC case, diminish in steady state. The effect of the
amplitude of the step input voltage and the initial distance between the electrode and
liquid-vapour interface on the liquid extraction was investigated experimentally and
numerically. At sufficiently high voltages, the induced EHD forces at the liquid-vapour
interface overcame the gravitational forces and caused the liquid to be extracted
towards the high voltage electrode. The extraction time decreased with an increase
of the applied step voltage and/ or decrease of the initial distance between liquid
interface and the high voltage electrode. The numerical simulation results were, in
general, in agreement with the experimental results.</p> <p> The effect of pulse repetition rate of pulse applied voltage on two phase flow patterns, heat transfer and pressure drop can be divided into three regimes. At the low pulse repetition rate range, f < 10 Hz, the two-phase flow responded to the induced EHD forces, and liquid was extracted from the bottom stratum to the center electrode and then pushed back to the bulk flow in the form of twisted liquid cones. Increasing the pulse repetition rate in this range increased the repetition of the extraction cycle and therefore increased heat transfer and pressure drop. In the mid pulse repetition rate range, 10 Hz < f < 80 Hz, the extraction was not completed, which led to lower heat transfer compared to the lower pulse repetition rate range. In this range, the
two phase patterns were characterized by liquid-vapour interface oscillations between
the center electrode and the bottom stratum and liquid droplet oscillations which
increased the momentum transfer and therefore pressure drop. Increasing the pulse
repetition rate in this range decreased heat transfer and increased pressure drop. In
the high pulse repetition rate range, f > 80 Hz, increasing the pulse repetition rate
decreased both the interfacial and droplet oscillations and therefore decreased the
heat transfer and pressure drop till the two phase flow patterns resembled that for
an applied DC voltage. For the same pulse repetition rate, increasing the mass flux
decreased the effect of EHD forces on heat transfer and pressure drop. The heat
transfer enhancement ratio and pressure drop ratio increased with an increase of the
duty cycle for the same pulse repetition rate of the applied voltage.</p> <p> Different combinations of pulse repetition rate and duty cycle of applied pulse
wave voltage can be used to achieve different values of heat transfer and pressure drop.
This can be very beneficial for heat transfer control in industrial applications. An
advantage of such control is that it eliminates various measurements devices, control
and bypass valves, variable speed pumps, fans and control schemes used in current
technology for heat transfer and pressure drop control. The range of control of the
ratio of the heat transfer coefficient to the pressure drop is from 8.24 to 20.56 for mass flux of 50 kg/m^2s and it decreased with increasing mass flux untill it reached
1.63 to 3.81 at mass flux 150 kg/m^2s.</p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/17422 |
| Date | 12 1900 |
| Creators | Sadek, Hossam |
| Contributors | Ching, Chan Y., Cotton, James S., Shoukri, Mamdouh, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0018 seconds