Electrohydrodynamic control of convective condensation heat transfer and pressure drop in a horizontal annular channel

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.

The effect of a 8 kV DC applied voltage was investigated for mass flux in the range 45 kg/m²s to 160 kg/m²s and average quality of x_avg= 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/m²s. Increasing the mass flux decreased the effect of electrohydrodynamic forces on the two-phase flow heat transfer and pressure drop.

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 < V_p–p < 12 kV) and DC bias voltage (-10 kV < V_DC < 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/m²s, inlet quality of 70%, and heat flux of 10 k\Vjm2. 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.

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.

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.

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/m²s and it decreased with increasing mass flux untill it reached 1.63 to 3.81 at mass flux 150 kg/m²s. (Abstract shortened by UMI.)