Nucleate boiling experiments have been performed using water at saturated or near-saturated flow boiling conditions. Both the thermal boundary condition and the approaching flow (inlet) condition have been prescribed. Bubble substrate temperature and approach velocity were both were constant. A microscale heater and a Wheatstone bridge have been used to maintain a constant wall temperature around an artificial cavity. The power delivered by the heater is instantaneously monitored, see Figure 1. This bubble generator has been given a special shape to minimize the velocity boundary layer thickness at the location of the artificial cavity. The bubble generator intrudes a pipe and positions the artificial site at the centre of the pipe. As a result, a (nearly) homogeneous liquid velocity profile approaches the bub¬ble at the boiling site. This profile was also measured with LDA. Accurate heat flow rate data have been obtained by controlling and measuring the surface condition at a high temporal resolution. The bubble growth has been measured from two sides simultaneously using a high-speed CCD camera synchronized with the heat flow rate measurements. Stereoscopic measurements are a necessity since the bubbles are created at a vertical wall and are deformed by the liquid flow.The heat flow rates required for the volume change observed for the bubble are compared with the instantaneous heat flow rates measured, see Figure 2. At the time of bubble detachment, the heat flow rate from the wall is only found to instantaneously increase in the case of zero liquid flow, i.e. in the case of pool boiling. No notable change in heat flow rate is observed at the time of bubble detachment in forced convection boiling. Measured bubble growth rate will be compared with previous results for boiling at a site that is embedded at a wall and with results for other temperature boundary conditions.
|Title of host publication||Two-phase systems for ground and space applications, 19-21 September 2006, Brussels, Belgium|
|Editors||E. Gatapova, J.-C. Legros, o. Kabov|
|Place of Publication||Belgium, Bruxelles|
|Publication status||Published - 2006|