In the present study, flow boiling experiment using Ni-resistance method of temperature determination and numerical simulation by means of lattice Boltzmann method (LBM) are carried out to confirm the effect of bubble departure and bubbles merging on subcooled flow boiling heat transfer in pure water and CTAC/NaSal(1:1) solutions. Tests are performed with the same cross-section channel of 6.0 mm × 3.5 mm, inlet fluid temperature of T_in = 80 ℃, and outlet pressure of P_out = 101.3 kPa. With microlens and high-speed camera, the bubble departure process and flow patterns are recorded at different concentrations, gravity directions, and mass velocities. The improved Shan–Chen model with large density ratio (T_S = 0.8T_C, ρ_l/ρ_v = 36.55) is applied to simulate the bubble departure and bubbles merging process with changing gravity. The present results show that the superheat of CTAC/NaSal solution has a positive relationship with the bubble departure time and heat transfer coefficient (HTC) of pure water is almost unaffected by surfactant additives without bubble departure. Without bubble departure, the dry spot at the bottom of bubble causes the higher superheat, and the increasing velocity of liquid phase is the main reason of subcooled flow boiling heat transfer.

The foaming characteristic of surfactant solution is rarely discussed in surfactant enhancing pool boiling heat transfer and the critical heat flux (CHF). In the present work, saturated pool boiling experiment using transparent heating plate of indium tin oxide (ITO) coating and lattice Boltzmann method (LBM) simulation are conducted to get further insights into the effect of bubble merging on the wettability characteristics and CHF. With a high-speed camera, the gas–liquid phase and nucleate site distribution at the heating surface are recorded. The improved Shan–Chen model is applied in LBM simulation to investigate the interaction between nucleate sites. The experimental and simulation results show that vapor film appears at CHF. The vapor film inhibits nucleate sites to be effective and other new bubbles to emerge. Bubble merging decreases the contact line of gas–liquid–solid interface. The irreversible dry spot appears in the center of large bubbles due to CHF. It implies that preventing bubble merging is another factor of surfactant enhancing pool boiling heat transfer and CHF.

The liquid emitting from capillary subjected to a high electric voltage could be dispersed in different ways depending on applied potential and flow rate. The electro-spraying of deionized water was experimentally studied adopting the high-speed camera under different operating parameters. The time resolved images and evolution of drop or/and jet initiation, pulsation, deformation, and separation from capillary tip or meniscus were captured and analyzed. The known electro-spraying modes were identified according to the geometrical forms of meniscus, jet, or/and drop. A spherical droplet with diameter larger than the outer diameter of capillary could be generated in dripping mode at low potential, while a spherical droplet with diameter smaller than outer diameter of capillary could be observed in micro-dripping mode with relative high potential. Spindle-like droplet could be found in spindle mode and usually further disperse into a few of drops with different size for larger flow rate. In the range of applied potential and flow rate, oscillating jet and simple jet could be found, where a long jet could be elongated by electric stresses and further break up into finer drops with highly charged drops. In addition, the operating parameters and drop sizes for different modes are also discussed. Meanwhile, the spraying modes with double capillaries were also observed and discussed.