The characteristics of two-phase flow resistance and bubble behavior had been experimentally investigated in a microchannel heat sink containing 14 parallel 0.15 mm×0.25 mm rectangular microchannels with hydraulic diameter of Dh=187.5 μm by using deionized water as working fluid. The experimental results revealed that vapor phase gradually increase with the increase of heat flux under constant inlet temperature and mass flux

leading to the increase of pressure drop in parallel multiple microchannel. When keeping the heat flux and inlet temperature constant

the pressure drop exhibited a trend of first decrease and then increase with the increase of mass flux. In addition

the lowest point in the curve of pressure drop vs mass flux (Δpexp vs G) is considered to be the starting point of the static flow instability (namely

OFI point). The characteristics of four different bubble behavior are studied by basing on the help of visual image and bubble dynamics theory. It is found that due to the effects of the interaction of buoyancy and flow inertia force

the bubble was rapidly cleared at the initial stage of bubble nucleation. When the bubble size is approximately equal to the height of the channel

the bubble enter the local confined growth phase

leading to the increase of viscous resistance between the bubble and the surface of wall. And under the influence of heat flux

the bubble continues to grow and is limited in the width direction

so that the bubble's growth was completely confined both at height and width direction. Thereafter

the bubble gradually grew and elongated as well as flowed downstream. Moreover

when the surface tension is insufficient to maintain the liquid bridge between two adjacent bubble

the liquid bridge broke and resulted in bubbles merging. It is more interesting that due to the influence of the downstream bubble reversal flow and the flow inertia force

The upstream bubble is gradually compressed

leading to the velocity of the bubble tail lower than that of the bubble head. Finally

by using the current pressure drop data to verify the prediction accuracy of Kim & Mudawar correlation

it is found that the Kim & Mudawar correlation can only predict the variation trend of pressure drop

but can not predict the pressure drop value.