An Extra Mode of Enhanced Heat Transfer by Oscillating Bubbles in Minichannels and Microchannels

Experimental investigations on heat transfer in tubular micro- or minichannel arrangements more often report on two-phase flow instabilities, pulsations or oscillations, which result in a remarkable influence on heat transfer efficiency. In order to explain the piston-like oscillations of the steam-plugs and water-slugs (-columns), the authors studied the somehow similar process which occurs in the worldwide known toy steam boat. Experiments have been performed which used a demonstration plant made of glass. By controlled electrical heating, high-speed video, pressure and local temperature measurements, the paths of energy have been disclosed. The results are as surprising as the effect of making gold from sand with respect to an equivalent axial heat-conductivity of the water-filled glass tube. Initiated by these results, an abstracting model is presented that analytically quantifies this new regenerating (oscillating and conducting) heat transfer mode e.g. concerning the combination of a heat recharging tube wall and an oscillating water column in a field of diminishing temperatures between the temperature of the boiler surface and the subcooled bulk water. By introducing these heat transfer details, the steam boat can give an answer, not only on frequency and amplitude of the oscillations, but on the steady state conditions for — or time-dependency of — the location of zero-crossing as well. Experimental results and model calculations are in good agreement and need no fitting factors. This is the base to discuss that process along with its physical parameters and compare it to the above mentioned observations in flow-boilers or pulsating heat pipes etc. which use microchannels or minichannels.