Time and Frequency Characteristics of Pressure Fluctuations during Subcooled Nucleate Flow Boiling

Time and frequency analyses methods including PDF (Probability Density Function), CDF (Cumulative Probability Function), FFT (Fast Fourier Transform) and PSD (Power Spectral Density) were used to investigate the pressure fluctuation characteristics during subcooled nucleate flow boiling and the effects of various parameters. The experimental conditions were heating currents to a high frequency induction heater of 200–720 A, liquid temperatures of 50–85°C, and flow rates of 0–290 L/h. The CDF and PSD methods were more effective than the PDF and FFT methods for the time-frequency analyses of the pressure fluctuations. Higher heating currents, lower liquid temperatures and lower flow rates result in wider frequency distributions and larger pressure fluctuation amplitudes in the time domain and wider frequency distributions and higher PSD of the pressure fluctuations in the frequency domain. The effects of the operating conditions on the Onset of Nucleate Boiling (ONB) were analyzed qualitatively by correlating the pressure fluctuation signals with visual images obtained from the experiments. A pressure drop occurs at the ONB regardless of the conditions with the pressure drop appearing earlier at higher heating currents and liquid temperatures and lower flow rates.