In this paper, we report an experimental investigation of subcooled and saturated pool boiling of water around a heated wire in a confined space. The experiments were carried out in a transparent miniaturized channel submerged in a transparent water container. Boiling in the miniaturized channel was brought about by an electrically-heated platinum wire (0.127mm in diameter) that was placed along the centerline of the channel. Both ends of the channel were open to the water in the container. Triangular and square channels with the hydraulic diameters ranging from 0.87mm to 10mm were tested under a wide variety of subcooling conditions ranging from 0 to 55℃ at the atmospheric pressure. Experiments were also performed by changing the channel inclination angles ranging from 0 degree (horizontal) to 90 degree (vertical). The experimental results show that the channel size has a tremendous effect on the boiling characteristics. For instance, the CHF decreased drastically as the channel size shrank from the unconfined space. The boiling curves further demonstrate that both the CHF and the heat transfer rate for the triangular channel are higher than those for the square channel having the same hydraulic diameter. It is further found that as the subcooling was increased, both the CHF and the heat transfer coefficient increased. In addition, the increase of the inclination angle (from horizontal to vertical) led to an increase in both the CHF and the heat transfer coefficient.

Experimental investigations on the flow patterns of air-water two-phase flow in a vertical circular channel and a vertical equilateral triangular channel were carried out. Instinct understandings of two-phase flow regimes affected by the shapes of the cross-sections of the channels were gotten by flow visualization with the aid of a digital video camera. The following flow patterns at different air and water superficial velocities were recognized for both channels: bubble flow, slug flow, churn flow, and annular flow. For the air-water flow at high Reynolds numbers in the equilateral triangular channel, two new flow patterns were found besides the above-mentioned flow patterns: one is called "Secondary Flow Creep Flow", and the other is named as "Capillary Creep Flow". Secondary flow creep flow takes place in the zone of SG V=35-95m/s, SL V=0.0004-0.016m/s. Three liquid streams driven by secondary flow exist in the middle of the sidewall of triangular channels. Capillary creep flow is found in the zone of SG V=15-30m/s, SL V=0.0004-0.016m/s. In this case, secondary flow is not strong enough to drive liquid to the center of sidewall, instead, liquid gathers in the corner due to the capillary force. The observation of creep flows in the equilateral triangular channel proves that secondary flow will affect two-phase flow patterns in noncircular channels at high Reynolds number.

Experiments of steam-water two-phase flow frictional pressure drop in a vertical helical coil were carried out in the high-pressure water test loop of Xi'an Jiaotong University. The coil is made of stainless steel tube with an inner diameter of 16mm, the helix diameter measured from tube axis to tube axis is 1.3m, and helix angle of the coll is 3.65℃. The experimental conditions are: pressure p=4～18MPa, mass velocity G=400～1400kg/(m2s), inner wall heat flux q=100～700kW/m2. Based on these data, a correlation for predicting the steam-water two-phase flow frictional pressure drop was derived, it can be used for the design of steam generator of HTGR.

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10mm and 1.55mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.

Characteristics of co-current upward air-water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters (dh=2.886 and 1.443mm). However, for the smallest channel (dh=0.866mm), dispersed bubbly flow pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of dh=0.866mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally, flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.