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|Title:||Electrohydrodynamic Control of Convective Condensation Heat Transfer and Pressure Drop in a Horizontal Annular Channel|
|Advisor:||Ching, Chan Y.|
Cotton, James S.
|Keywords:||voltage, electric field, electrohydrodynamic, condensation, pressure, annular, channel|
|Abstract:||<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>|
|Appears in Collections:||Open Access Dissertations and Theses|
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